Spiral and Multislice Computed Tomography of the Body ( THIEME )

Mathias Prokop Michael Galanski Co-Editors

Aartj. van der Molen Cornelia M. Schaefer-Prokop

'Thieme

Abbreviation Meaning AAA MO-HNS AAST ABPA ACC ACE ACKD ACS ACTH ACVB

abdominal aortic aneurysm American Academy ofOtolaryngology-Head Neck Surgery American Association for the Surgery ofTrauma allergic bronchopulmonary aspergillosis N-acetylcysteine acetylcholinesterase acquired cystic kidney disease anterior cervical space adenocorticotropic hormone aortocoronary venous bypass graft

AFLP AFP ACS AHA AlP AICC ALL AML APKD APUD ARDS ASD ASH ASNR ASSR ATAI ATS AUA AUSPV AV

acute fatty liver of pregnancy alpha-fetoprotein adrenogenital syndrome American Heart Association acute interstitial pneumonia American Joint Committee on Cancer acute lymphocytic leukemia acute myeloic leukemia acquired polycystic kidney disease amine precursor uptake and decarboxylation (cell line) adult respiratory distress syndrome atrial septal defect asymmetric septal hypertrophy American Society ofNeuroradiology American Society of Spine Radiology acute traumatic aortic injury American Thoracic Society American Urological Association anomalou~ unilateral single pulmonary vein atrioventricular

AVM BAL BALT BlP BMT BO BOOP BPH bpm BS CAD CBV CCC CCD CHF CLL CM CML CMN CMP CMV COLD COP COPD CPR CRP CS CSF CT CTA CTAP CTC CTm CTEPH CTHA CTP

arteriovenous malformation bronchoalveolar lavage bronchus-associated lymphoid tissue bronchiolitis obliterans interstitial pneumonia bone marrow transplant bronchiolitis obliterans bronchiolitis obliterans with organizingpneumonia benign prostatic hyperplasia beats per minute buccal space coronary artery disease cerebral blood volume cholangiocellular carcinoma charge couplet device chronic heart failure chronic lymphatic leukemia contrast medium chronic myeloid leukemia contrast medi urn nephropathy cardiomyopathy cytomegalovirus chronic obstructive lung disease [~COPDl ~togenic organizing pneumorna chronic obstructive pulmonary disease curved-planar reformation C-reactive protein carotid space cerebrospinal fluid computed tomography CT angiography CT during arterial portography CT cholangi ography CT dose index chronic thromboembolic pulmonary hypertension CT hepatic arteriography cr perfusion imaging

7 Principles ofCT, Spiral CT, and Multislice CT M. Prokop

2 Scanning

Principle

(p. 2)

Image

Reconstruction

Image

Display and

Scanning

(p .. 3) Documentation

Parameters

(p. 5)

(p. 7)

11

Spiral CT I Helical CT

Multislice CT

.. .. .. .... .. .. .. .. .. .... .. .. .. .. .. .... .. ~....

Scanning

(p. 18)

Principle

Detector Types (p. 20) System

Performance

Image

Reconstruction

Scanning

(p. 24) (p. 24)

Parameters

Workflow,

(p. 32)

Image Review,

Display

and Documentation (p. 36)

Cardiac Multislice CT Prospective

ECG Triggering

Retrospective

(p. 38)

ECG Gating (p. 39)

CT Fluoroscopy Principle Technique Radiation

and Image

. Reconstruction

(p. 42) Safety (p. 42)

(p. 42)

. ~.~~.~~~~~~.

1 Principles ojCT Spiral CT, and Multislice CT Computed tomography (CT) has been one of the biggest breakthroughs in diagnostic radiology. The first clinical CT scanner was developed by Godfrey N. Hounsfield for examinations of the head and was installed in 1971 at Atkinson-Morley's Hospital in Wimbledon, England. The first body CT scanner was installed in 1974, and before the end of the 1970s the basic technical evolution of CT was complete (Table 1.1). Technical details were refined during the 1980s, and CT technology remained on a plateau until the early 1990s, when the advent of spiral (helical) CT scanning sparked a further, rapid evolution leading to improved diagnostic capabilities, 3D imaging techniques, and CT angiography. The latest innovation is the introduction of multislice CT in 1998. This new technology is vastly expanding the performance of CT scanners: it truly transforms CT from a trans axial imaging modality to a 3D technique that yields high quality images in arbitrary planes and forms the basis for an expanding variety of 3D visualization techniques, including virtual endoscopy. In addition, these scanners have the potential to revolutionize cardiac imaging with CT.

Table 1.1 phy

Historical milestones in computed tomogra-

1929

Radon: basic mathematical principles

1963

Cormack: image reconstruction

1971

Hounsfield (EMI Laboratories): technology

1971

Head scanner (EMI Mark I)

1974

Whole-body scanner (ACTA)

1974

Third-generation scanner (Artronix)

1977

Fourth-generation scanner (AS&E scanner)

1979

Nobel Prize awarded to Hounsfield and Cormack

1980s

Technical refinements

1983

Dynamic spatial reconstructor

1983

Electron beam CT scanning

1987

Scanners with a continuously rotating tube

1989

Spiral CT (helical CT)

1991

Dual-slice spiral CT (Elscint)

1991

CTangiography

1995

Real-time reconstruction (CT fluoroscopy)

1998

Multislice CT (4 detector rows _~~~

1999

Multislice cardiac imaging

2001 /2

Multislice CT (8/16 detec~to"l'!u r--ow-~"tr

Future

Cone beam CT (a 128 detectorrows)

Computet Tomograpfy Scanning Principle

Computed tomography is an x-ray tomographic technique in which an x-ray beam passes through a thin axial section of the patient from various directions (Fig. 1.1).Parallel collimation is used to shape the x-ray beam to a thin fan, which defines the thickness of the scan plane. Detectors measure the intensity of the attenuated radiation as it emerges from the body. A mathematical image reconstruction (inverse Radon transformation) calculates the local attenuation at each point within the CT section. These local attenuation coefficients are translated into "CT numbers" and are finally converted into shades of gray that are displayed as an image. With conventional CT scanners the volume of interest is scanned in a sequential fashion, usually proceeding one section at a time. The first two generations of CT scanners (Table 1.2) were superseded in the late 1970s by third- and fourth-generation scanners, which are still in use today. In third -generation scan-

Fig. 1.1

Principle of CT scanning.

Image Reconstruction Table 1.2

Comparison of CT scanner generations

Type

First generation

and types

Second generation

Third generation

Translation-rotation

Principle

Single

Series

Arc (30°_60°)

Active detector rows

2

1-2

1-16

3-52

256-1000 0.4-10 s

Scan time

135-300 s

5-150 s

ners, tube and detector array rotate synchronously around the patient. The detector array covers the full width of the fan beam (Fig.1.2). In fourth-generation scanners, the detector elements cover a full circle around the scanner opening and remain stationary during the scan, while only the x-ray tube rotates around the patient (Fig. 1.2). Third-generation scanners, however, offer better scatter suppression and require less detector elements, which is the reason why all multislice CT scanners use third-generation technology. Attempts to speed up the imaging process led to the development of a multi-tube CT scanner called the dynamic spatial reconstructor (the "Mayo monster" equipped with 28 tubes, able to scan up to 240 sections, each of 1 mm thickness, in one 360° rotation), electron beam CTscanning, spiral CT, and, recently, multislice CT. Of these procedures, only spiral and multislice CT have achieved large-scale clinical impact.

Image Reconstruction The detector signals registered during a scan are preprocessed to compensate for inhomogeneities in the detector system and to correct for beam-hardening effects within the patient. After various correction steps and transformation from signal intensities into x-ray attenuation values these data are called the CT raw data (Fig.1.3).The raw data sets for third- and fourthgeneration scanners each consist of the attenuation profiles of some 500 to 1500 projections for each 360° rotation of the x-ray tube. Each projection in turn is composed of some 500 to 1500 single attenuation values. Image reconstruction from the raw data sets finally yields the image data set.

Electron beam scanning

Rotation

Detectors

Detector elements/row

Fourth generation

Electron beam deflection

Ring (360°)

Semicircular

(210°)

4 600-4000

432/864

1-5 s

~50 ms

Scanning principle

Image reconstruction

3rd Generation

Fig.1.2 Comparison of scanning principle and image reconstruction in third-generation (a, b) and fourthgeneration CT scanners (c, d). The fourth generation uses a stationary detector ring, and the data acquired by one detector are assembled into a projection for the various tube positions.

Image reconstruction starts with the selection ofthe desired field of view. Each ray from the tube to the detector that passes through this field of view is used for reconstruction. The attenuation coefficient for each image point is determined by averaging the attenuation values for all rays that cross this point (back projection). But this type of unfiltered back projection yields a very unsharp image with blurred edges, so multi-

3

I

1 Principles oICT, Spiral CT, and Multislice CT

Scan Data

-+

<:> Fig. 1.3

Id

Prepro-

Convolutionj -+ filtering

cessing

~

,...~~ , •. ~J

Imaae data Back projection

~~\~J.j

Processes involved in CT image reconstruction.

pie rays are assembled into a projection, and the resulting attenuation profile is subjected to an edge-enhancing mathematical filtering (convolution) process. The so-called "convolution kernel" determines the type offiltering. Back projection of the filtered attenuation profiles then yields a sharp image. The convolution kernel (reconstruction algorithm) used for the/itered back projection determines the properties of the reconstructed CT sections in terms of spatial reso-

a

Filtered raw data -+

b

Fig. 1.4 Effect of the convolution kernel on spatial resolution and image noise. In the lung parenchyma (high intrinsic contrast), the use of a sharp kernel (b) instead of a standard kernel (a) increases image sharpness. In

lution and image noise, and can vary from soft or smooth to sharp or edge-enhancing (Fig. 1.4). Third and fourth-generation scanners differ in the way that the attenuation values are assembled into a fan-shaped projection. The third generation uses the fan from a single tube position to the detector, while the fourth generation takes the data acquired by a single detector for various tube positions and assembles them into a projection (Fig. 1.2).

c

d

the liver (low intrinsic contrast), a soft kernel (c) is preferable to a sharp kernel (d) because the increased noise could otherwise obscure low-contrast structures.

Image Display and Documentation Fig. 1.5 The pixels in the CT matrix actually represent volume elements (voxels) in the scanned body region. Note that for a standard section thickness of 10 mm, each voxel has a matchstick shape.

I 0.5mm:10mm

I

FOV=25 em

Matrix= 5122

Image Display and Documentation •

Image Matrix and Field of View

ACT image is composed ofa square image matrix that ranges in size from 256 x 256 to 1024 x 1024 picture elements, or pixels. Since a CT section has a finite thickness, each pixel actually represents a small volume element. or voxel. The size of this voxel depends on the matrix size, the selected field of view (FOV) , and the section thickness (Fig. 1.5).In most CT examinations the voxel has a matchstick shape, i.e., the pixel size measured in the plane of the section, the x-y plane, is 10 to 20 times smaller than the section thickness, measured along the z-axis. This anisotropy (nonuniform shape) of the voxels can be decreased only by greatly reducing the section thickness. Only with multislice CT is it possible to obtain nearly isotropic (cube-shaped) voxels for larger body areas. (See also p. 46 for a more complete discussion of the issue.) There may be a difference between the actual image matrix that is reconstructed from the raw data and the display matrix that is shown on the viewing monitor or is printed out to film. Although the display matrix is usually identical to the image matrix, a larger display matrix may be selected (e.g., 1024 x 1024 instead of 512 x 512) to improve the image quality. It is not usually necessary to use data from the entire body cross section for image reconstruction, and afield of view (FOV) of more restricted size can be reconstructed from the raw data. This field of view is characterized either by its size in mm or by the zoomfactorrelative to the maximum field of view possible on that scanner. Depending on the scanner it can either be round or square. Some manufacturers call this the reconstruction field of view (RFOV) to distinguish it from the display field of view (DFOV) that can be

selected from that field and magnified for display on the monitor. Usually such a magnified image is less sharp than one directly reconstructed from the raw data because it uses only a portion of the image data rather than all the information contained in the raw data set. The reconstruction and display field of view have to be distinguished from the scan field of view (SFOV) available on some scanners. The SFOV is a reduced, centrally located area from which data are acquired, which can increase the sampling rate and thus improve spatial resolution. Usually this technique is applied in examinations of the extremities (calcaneus). spine or in the head and neck region. Since a reduced SFOV uses a fan beam with a narrower angle, the technique also reduces radiation exposure to those portions of the patient that are outside the SFOV. For this reason, it is used on some scanners to reduce radiation exposure for cardiac multislice CT imaging. H

CT Numbers

During image reconstruction, a numerical value (CT number) is assigned to each voxel according to the degree of x-ray attenuation in that voxel. To reduce dependence on the energy of the radiation and to obtain numerical values of convenient size, the CT number is defined as follows: CT=

1000

X

(u-U.water)/U-water

The unit for CT attenuation is called the Hounsfield unit (HU). The numbers are set on a scale in which -1000 represents the attenuation of air and 0 is the attenuation of water. Note that there is no upper limit to the scale. The available range of CT numbers varies between scanners and available bits per pixel (e.g., from -1024 to 3071 HU with 12 bits or up to 64,500 HU for 16 bits) .

5

pending on viewing conditions). Consequently, there is no point in assigning the complete diagnostic range of CT numbers (some 4000 HUl to the available range of gray levels (from white to black) because discrimination between structures with small differences in CT numbers would no longer be possible. It is therefore better to display just a portion of the CT scale. This so-called window is defined by its width, which affects image contrast. and by its level (center). which determines image brightness. Reducing the window width increases image contrast: lowering the window level brightens the image, and raising the window level darkens the image (Fig. 1.7). Examples of window settings shown in Table 1.3.

3000

2000 Cortical bone 1000 Cancellous bone Soft tissues Water

'ot ~Lung .1000~Air

•

Fig.lo6 The scale of CT attenuation numbers is defined by the values for air (-1000HU) and water (0 HU). Soft tissues occupy a narrow range around 50 HU.

The scale of IT attenuation numbers is shown graphically in Fig. 1.6. Except for fat, lowprotein fluids, and fresh blood, there are no "typical" values that would permit the specific characterization of soft [issues by their CT numbers (see also Fig. 7.5). •

Window

Settings

The human eye can distinguish only a limited number of gray levels (from about 40 to 100, de-

Image

Processing

and Analysis

The software used in CT scanners provides various options for processing and manipulating the cr sections. The features of greatest practical importance are the measurement of lengths and angles and the analysis of CT numbers within a selected region of interest (ROI). This ROJ may be selected interactively, allowing the operator to choose between predefined shapes (circle, ellipse, rectangle) or freely outlined regions of arbitrary shape. A computer program can then calculate mean CT numbers and standard deviations within the ROJ and can generate histograms. The scanned volume (an also be manipulated to produce reformatted images in any secondary plane of section ("multi planar reforms-

:::~:-I~j,

-1000 10

0

",~ •••••••••••• 1500/-650

-1000 . 1300 Fig. 1.7 Window settings. For optimum contrast, the gray levels available for vlewinq or filming are assigned to a designated portion of the CT scale. The window is

lung wIndow

.

Bone window

defined by its width, which determines contrast. and by itslevel,whlchdetermlnesbrightness.

Scanning Parameters Table13

Window

lung Emphysema

Scanning Parameters

settings Width

level center

1500

-650 -800

800

Soft tissues,

noncontrest uver.ncrcontrast Softtis5ues+CM Uver+CM Neck+CM

a Angiography

400 200 400 300 300

40 40 70 60-100 50

500

Bone

Gantry Angulation The gantry, or frame that houses the scanning equipment, can be tilted about the x-axis to perform oblique scans. The range of gantry angulation may be ::: 30°, depending on the scanner. Gantry angulation is used mainly for scanning the head, neck. or spine. It is unnecessary for most other applications and is increasingly being abandoned with multislice scanning and reconstruction of tilted sections from a volume dataset.

Section Thickness • tion." MPR) and various types of three-dimensional reconstruction (see Chapter 2, Image Processing). •

CT

Documentation

images from conventional scanners are usually printed on film for image review. It is importent to document all the window settings that are relevant for the examined body region. For example, chest scans should be documented in both the lung window and soft-tissue window settings. A bone window setting may be added to screen for skeletal metastases. Attempts to display two non-overlapping window settings simultaneously (e.g., mediastinum and lung) have been abandoned, because structures whose CT numbers lie between the two windows (e.g.. pleural lesions) are too easily missed. This dual window technique is no longer used. With modern spiral CT or multislice scanners, or with the introduction of picture archiving and communication systems (PACS), printing of CT images is increasingly abandoned in favor of direct viewing on CRT or flat screen monitors and storing image data in a digital archive.

Section

Profile

The beam collimation setting determines the thickness of a CT section. But as in conventional radiography. the x-ray tube emits a conically divergent beam rather than parallel rays. To obtain sections of reasonably uniform thickness, it is necessary to use collimators located directly behind the tube. Some scanners use additional post-patient collimators in front of the detectors for further optimization of the section profile. Despite these measures, a truly planeparallel section is never obtained in practice, and adjacent portions of the scanned object are always included. In addition, the finite size of the focus of the x-ray tube causes an area outside the primary beam to receive radiation of less intensity. This region is called the penumbra (Fig.l.8a). The effect on the acquired section can be described in terms of the section sensitiviry profile (SSP) or slice profile {Fig. 1.8), which shows how much a point in the object contributes to the image as a function of its distance from the center of the section. The ideal section profile is a rectangle whose width equals the desired section thickness, so that all points outside the section contribute nothing to the measured attenuation while all points within the section contribute uniformly to the CT number. Real section profiles have rounded "edges". meaning that adjacent regions also contribute slightly to the image. While the profile of thick IT sections (7-lOmm) closely approximates to an ideal rectangular shape, thin sections exhibit a more bell-shaped profile (Fig. 18b).

7

•

8

1 Principles

of CT, Spirol CT, and Multislice

IT

Sensitivity profiles Focal spot

lOmm 100%

50 %

~~

5mm

3mm

slice

2mm slice

t mm slice

'~'

I-+--+-+------f--H-H-\---I

---++--tt-

O%4.~~~-4~~~~~--~~~~ b

FWHM

FWTA

-4.0

4.0 8.0 Table position (mrn)

Hq. U:I Because of the x-ray beam geometry, regions located outside the selected section thickness are ineluded in the scan (a). This results in a rounded section sensitivity profile, which approximates to an ideal rectilinear shape only in thicker sections (b). The effective

section thickness (* section width SWj can be measured by the width of the section profile at half of its peak value, called the full width at half maximum (FWHM), or the width that encompasses 90% of the area under the curve, called the full width at tenth area (FWTA)(c).

In clinical practice, the increasingly large "tails" of thinner section profiles do not substantially affect image quality because the decreased width of the profile is more important and leads to improved resolution along the z-axis.

A more rigorous measure of the width of the section profile is the width that encompasses 90% of the area under the curve, called the full width at tenth area (FWTA). The FWTA indicates the width at which object elements located outside the section contribute just 10% to the cr number. FWHM and FWTA yield similar values in conventional cr when thick sections are chosen, but they differ markedly in spiral cr and when thin collimation is used in conventional cr. The slice profile quality index (SPQI) is another, more recent measure. It describes the percentage of the area under the section sensitivity profile that is bounded within an ideal rectangular profile of the same section width.

•

Effective

Section

Thickness

The width of the section sensitivity profile is usually quantified by measuring the width of the curve at 50% of its peak value, called thefull width at haJfmaximum (FWHM) (Fig.1Sc). This value is also known as the effective section thickness or section width (SW). It is by definition equal to the section collimation (SC) or nominal section thickness in conventional CT. It should be noted, however, that the effective section thickness is not equivalent to the section collimation in spiral cr. It is the commonest measure for describing the spatial resolution along the patient axis (a-axis),

Scanning Parameters 9 Partial Volume

Effect

The CT number of a pixel is determined by the x-ray attenuation that occurs in the corresponding voxel. If tissues with different attenuation properties occupy the same voxel (e.g., blood vessels and lung), the resulting CT number willwith good approximation-represent the sum of the different attenuation values (partial volume

effect or partial volume averaging):

tal.

C T =viXC Ti + V2 XC T 2 + ... , where the partial volume elements vi add

Given the much greater extent of a voxel along the z-axis than in the x-y plane, the section collimation contributes a great deal more to this partial volume effect than the field of view or pixel size (Fig. 1.9). • Section Collimation A number of anatomic structures (portions of the aorta, chest wall, or liver borders) are oriented parallel to the long axis of the body. The usual transaxial section orientation in CT means that the scan will cut the corresponding tissue boundaries at a perpendicular angle, which tends to minimize partial volume effects. For this reason, a section collimation of 7-10 mm is most commonly used in conventional body CT examinations. Partial volume effects are particularly troublesome in scans that are oblique or parallel in relation to tissue boundaries (diaphragm, apex

of lung, pole of kidney) and in the evaluation of small structures (small vessels, bronchi, adrenals). A collimation of3-5 mm can be used to evaluate structures that are parallel to the scan plane (e.g., the pancreas) or for evaluating small organs such as the adrenals. Thin collimation of 1 -2 mm is preferred for the lung, where a detailed structural analysis is required for the diagnosis of interstitial lung disease. With multislice CT scanning thin collimations have become standard (see below). Table Feed

In conventional CT, a tissue volume is scanned section-by-section. This is accomplished by moving the patient table by a designated amount, called the table feed (table increment), between consecutive scans. Contiguous imaging is generally performed, meaning that the section thickness and table feed are equal. Overlapping Scans

Reducing the table feed produces an overlapping scan pattern that increases the radiation dose to the patient. While overlapping scans have been recommended to improve the 3D imaging of skeletal structures with conventional CT, the introduction of spiral scanning has made them obsolete. Discontinuous

Respiratory

Fig. 1.9 Partial volume effect. Because of the matchstick-like configuration of the voxels, the CT number of a voxel is affected not only by the object of interest (e.g., a round lesion) but also by adjacent structures (e.g., lung parenchyma). This creates an averaging effect that distorts the CT number of the voxel.

Scanning

When the table feed is increased, intersection gaps are produced. This may be useful in selected cases where it is necessary only to search for gross pathologic changes that extend over larger ranges. Misregistration

Respiratory misregistration in CT occurs when structures that move with respiratory excursions are missed due to variations in the depth of respiration between sections. However large the efforts to reproduce a consistent depth of breath holding for 5 to 20 respiratory cycles, some gaps along the z-axis will generally occur. The thinner the collimation and the smaller the lesion, the greater the risk of respiratory misregistration will be (Fig. 1.10).This has only a moderate effect on the detection of hepatic lesions or the evaluation of the kidneys and adrenals, but it can seriously hamper the search for pul-

1 Principles ofCT, Spiral CT, and Multislice CT

.~ 15l· Fig. 1.10 Respiratory misregistration: different depths of breath holding can cause a small lesion (e.g., a pulmonary nodule) to be missed in sequential scans.

monary metastases. To obtain seamless coverage, a section collimation of less than 5mm should be avoided in areas that move with respiratory excursions. This creates conflicting demands in conventional Cl, where it is not possible both to minimize partial volume effects and avoid respiratory misregistration in the same scan. Reconstruction Algorithm (Convolution Kernel) The convolution kernel used in reconstructing an image from the raw data determines the relationship between spatial resolution and image noise. Noise limits contrast resolution, and thus the ability to differentiate objects that show very little attenuation difference from their surroundings. High contrast resolution is important for the detection of lesions in parenchymal organs like the liver and pancreas. High spatial resolution is necessary for the detection of very fine morphologic changes in lung or bone. Highresolution convolution kernels [HR kernels, sharp kernels) improve spatial resolution but disproportionately increase noise as well. Conversely, soft or smooth kernels lead to a concomi-

Image noise Fig. 1.11 The use of hiqher-resolution convolution kernels can improve spatial resolution but leads to a disproportionate increase in image noise.

tant reduction in noise and spatial resolution (Fig. 1.11). Standard kernels are designed as a compromise for good spatial resolution and reasonably low image noise for most body applications. Partial Scan Because attenuation of an x-ray beam is identical in both directions (from the tube to the detector and from the detector to the tube), enough data for reconstructing a cr image can be obtained from less than a full 360° rotation. Instead, data from a partial rotation of 180° plus the fan angle of the x-ray beam (usually some 60°) is sufficient for this purpose. This partial scan or half scan reconstruction can be used to reduce the scan time per CT section and thus reduce motion artifacts. In particular, it is employed for cardiac imaging using electron beam CT or multislice Cl, when temporal resolution is of utmost importance.

VIII

Abbreviations CTSI CUP CVS CVS CWP DAD DCF DEQCT DES DIP DLDCF DLP DORV DPB DRE DSA D-TGA DVT EAA EAP EBCT EBT ECA ECG ECST EHE ELCAP EMLA ERC ERCP ESR FBI FBSS FDG FIGO FLC FNAC FNH FOV FSH FWHM FWTA Gd-MRA GFR

CT severily index carcinoma of unknown primary calcium volume score continuous volume scanning (ECBn coal worker's pneumoconiosis diffuse alveolar damage deep cervical fascia dual-energy QCT diethylstilbestrol desquamative interstitial pneumania deep layer of the deep cervical fascia dose - length product double outlet right ventricle diffuse panbronchiolitis digital rectal examination digital subtraction angiography complete transposition of the great arteries deep venous thrombosis extrinsic allergic alveolitis early arterial phase electron beam CT electron beam tomography external carotid artery electrocardiogram European Carotid Surgery Trial epithelioid hemangioendothelioma Early Lung Cancer Action Project eutectic mixture oflocal anesthesia endoscopic retrograde cholangiography endoscopic retrograde cholangiopancreatography erythrocyte sedimentation rate fat - blood interface failed back surgery syndrome f1uorodeoxyglucose Federation Internationale de Gynecologie et d"Obstetrique fibrolamellar carcinoma fine needle aspiration cytology focal nodular hyperplasia field of view follicle-stimulating hormone full width at half maximum full width at tenth area Gadolinium-enhanced MRA glomerular filtration rate

GI GIP GIST GSD GTD HAE HAP HBV HCA HCC HD HDL HIV HL HMG-CoA HNP Ho:YAG HOCM HRCT HSV HVP ICA ICFT ICU IGCCCG IHE IHSS

gastrointestinal giant ceU interstitial pneumonia gastrointestinal stromal tumor glycogen storage diseases ~estational trophoblastic disease epatic alveolar echinococcosis hepatic arterial phase hepatitis B virus hepatocellular adenoma hepatocellular carcinoma Hodgkin's disease high density lipoproteins human immunodeficiency virus Hodgkins lymphoma hydroxymethylglutaryl coenzyme A

herniated nucleus pulposus ho lmiu m:yttriu m:alurninu rn:ga rnet hypertrophic obstructive cardiomyopathy high-resolution thin-slice CT herpes simplex virus hepatic venous phase internal carotid artery intracavitary fibrinolysis

intensive

care unit

International Germ Cell Cancer Collaborative Croup infantile hemangioendothelioma idiopathic hypertrophic subaortic stenosis

IIAC ILP IMA INR IPF IPH

idiopathic infantile arterial calcification interstitial laser photocoagulation inferior mesenteric artery international normal ratio idiopathic pulmonary fibrosis idiopathic pulmonary hernosiderosis

IPMT IV IVC IVF IVP KUB LA LAA LAD

intraductal papillary mucinous tumor

intravenous inferior vena cava

in vitro fertilization intravenous pyelogram kidney abdomen pelvis (plain abdominal radiograph) left atrium left atrial appendage left anterior descending coronary artery

Scanning Principle Spiral (Helical) CT

During the past decade, spiral (= helical) CT has become the standard technology for the maj ority of clinical indications for CT scanning.

(see Fig. 1.15). The higher the pitch, the lower the radiation dose to the patient and the greater the available range of scan coverage. Advantages

Scanning Principle

Spiral CT requires a scanner with a continuously rotating x-ray tube. A tube with a large heat capacity is needed that can operate continuously for the duration of the scan. More than 100 seconds scan duration are available with current scanners. In contrast to conventional CT, the patient is not scanned section-by-section but is translated through the scan plane at a uniform table speed during acquisition of the raw data (Fig. 1.12). The technique is named spiral or helical CT after the spiral or helical pattern that is traced by the scan. A CT image can be generated from any segment within the scanned volume, so the table feed is unrelated to the site of image reconstruction. Sectional images can be produced at arbitrary levels, and individual images can be overlapped as desired without increasing radiation exposure. The spacing between the reconstructed sections is called the reconstruction interval, increment or index. The section collimation (nominal section thickness) and table feed can be varied independently of each other in spiral CT. The pitch defines the ratio of the table feed (table increment) per gantry rotation to the section collimation

Position of

The advantages of spiral CT arise from continuous data acquisition and short total scanning time. With conventional CT, small lesions such as pulmonary or hepatic metastases may be missed due to respiratory misregistration (see Fig. 1.10). But spiral CT can acquire data in a seamless volume during one breath hold, eliminating respiratory motion and interscan gaps. The use of overlapping reconstruction intervals allows for optimum visualization of small lesions while eliminating partial volume effects. When thin collimation is used, 2 0 reformatted images can be generated in arbitrary planes of section, and 3D reconstructions of good quality can be produced. Owing to the short scan time, most spiral examinations can be performed during a single breath hold. Intravascular contrast medium can be used more effectively, providing either higher contrast or a reduction in the volume of contrast material. Arterial phase imaging has become available only with spiral scanning. These advantages can Significantly improve the detection of hepatic and pancreatic lesions compared with conventional scanning. High vascular contrast is the foundation for CT angiography (CTA), which is a technique that is not available with conventional CT. Volume acquisition and short scan times are utilized in this technique to capture the arterial enhancement and generate angiogram-like vascular images (see Chapter 24).

tube focus Disadvantages

Fig. 1,12

Principle of spiral CT.

Most disadvantages of spiral CT result from the use of older scanners. In these scanners the continuous tube operation necessitates a lower radiation dose per rotation and may lead to a marked increase in image noise. Modern scanners employ an improved tube technology, so noise is no longer a limiting factor. While the scan duration is shorter with spiral CT, there are more sections to be processed. This may increase the time required for image reconstruction, especially when overlapping

11

Abbreviations LAM LAO LAP LCA LCX LDH LDL LH LHA LHV LI LIMA LIP LITI LOCM LPV L-TGA LV MAC MALT MEN MFH MHV MIBC MinIP MIP mIP MLDCF MLI mLV Mn-DPDP MPNST MPR MRA MRCP MRI mRV MS MSAD MTF MTT NASCET NASH NASS Nd:YAG

lymphangioleiomyomatosis left anterior oblique late arterial phase left coronary artery left circumflex [coronaryartery[ lactate dehydrogenase low density lipoproteins luteinizinghormone left hepatic artery left hepatic vein linear interpolation left internal mammary artery lymphocytic interstitial pneumania

laser-induced thermotherapy low-osmolar contrast media left portal vein corrected transposition of the great arteries left ventricle Mycobacterium avium complex mucosa-associated lymphoid tissue

multiple endocrine neoplasia mal~nant fibrous histiocytoma mid e hepatic vein meta-iodobenzylguanidine minimum intensity projection maximum intensity projection minimum intensity projection middle layer of the deep cervical fascia multislice linear interpolation morphologic left ventricle mangafodipir-trisodium malignant peripheral nerve sheath tumor multiplanar reformation magnetic resonance angiography magnetic resonance cholangiopancreatography magnetic resonance imaging morphologic right ventricle masticator space multiple slice average dose modulation transfer function mean transit time

North American Symptomatic Endarterectomy Trial nonalcoholic steatohepatitis North American Spine Society neodymium: yttrium-aluminumgarnet

NHL NIPF n.p.o. nr-MIP NSCLC NSIP OLT OM 00 OP OR PA PACS PAI-l PAN PAP PAPVR PCP PCS PDA PDA PDV PE PEEP PEl PET pi PID PIE PLC

non-Hodgkin"s lymphoma nonspecific interstitial pneumania and fibrosis nothing by mouth (nil per as) noise-reduced MIP non-small cell lung cancer nonspecific interstitial pneumania

orthotopic liver transplantation obtuse marginal branch osteoid osteoma organizing pneumonia operating room posteroanterior picture archiving and com-

ill unication

system

plasminogen activator inhibitor-! polyarteritis nodosa pulmonary alveolar proteinosis partial anomalous pulmonary venous return Pneurnocysris carinii pneumonia

posterior cervical space patent ductus arteriosus posterior descending coronary artery pancreaticoduodenal vein pulmonary embolism positive end-expiratory pressure percutaneous ethanol injection positron emission tomography post injection pelvic inflammatory disease pulmonary interstitial emphyserna

pulmonary lymphangitis carcinornatosa

PLDD PMF PMMA PMS PNH PPH PPS PS PSA PT PTA PTC

percutaneous laser disk decom-

pression

progressive massive fibrosis polymethylmethacrylate pharyngeal mucosal space paroxysmal nocturnal hemoglobinuria primary pulmonary hypertension parapharyngeal space parotid space prostate specific antigen prothrombin time percutaenous transluminal angioplasty percutaneous transhepatic cholangiography

IX

1 Principles ofCT, Spiral CT, and Multislice CT sections are used. While this was a time-consuming process with older scanners. new scanners provide fast reconstruction and often require less than one second per image. An increased number of images needs to be viewed. documented and archived. which again may increase time and costs. There is a basic tradeoff in spiral CT between a large scan volume and a high spatial resolution in the z-direction. Short scan ranges. such as the inner ear, can be covered with thin collimation but long ranges, such as in thoracoabdominal CT examinations, require a thicker collimation. The short scan time also complicates the administration of contrast medium, and new types of artifact may occur. A faulty contrast technique can lead to suboptimum examinations or misleading findings (see Figs. 7.39-7.41).

Image •

Reconstruction

Interpolation

Algorithm

and

Section Profile

The table movement during the scan will produce motion artifacts if the raw data acquired during a 360° rotation are used directly for image reconstruction. This is because the first and last projections in the 360° rotation sample different data (due to the table motion during tube rotation). To avoid these artifacts, interpolation of the raw data before image reconstruction is required. The goal of the interpolation is to obtain a complete set (360°) of projections at the desired z-axis position in the scanned volume. The simplest linear interpolation of the projection data is called 360° LJ (Fig. 1.13a). At every angular position of a 360° rotation it interpolates between the two projections in the spiral data set that are closest to the chosen position along the z-axis. This interpolation from 720° of data results in a complete (360°) set of projections for the chosen z-position. The 360° Ll interpolation provides the least image noise but substantially broadens the section profile (Fig.1.l3b). More advanced interpolation algorithms exploit the fact that x-ray attenuation is independent of direction, i.e., the attenuation along a ray between the tube and detector is equal in both directions. This makes it possible to com-

pute a virtual second spiral (conjugated data) for the attenuation values along a ray from the detector to the tube, and to interpolate the projections at corresponding angles between the real and virtual spiral. This algorithm is called 180° LI (Fig. 1.13a) but actually uses data from 360° plus the fan angle of the x-ray beam. The resulting section profile is substantially narrower (Fig.U3b) because the distances between corresponding projections in the real and virtual spirals are less than between corresponding projections in the real spiral alone. Differences between the 360° and 180° interpolations are best appreciated in multiplanar reformations and are most pronounced when pitch factors> 1 are used (Fig. 1.14). The 180° Ll algorithm, however, results in a larger image noise since only half the data are used for interpolation as compared to 360° LI. In fact, the noise with 180° Ll is as high as it would be with a 360° Ll and half the exposure dose. Higher-order interpolation algorithms use not only two points from adjacent (real or virtual) spirals but instead, apply a more complex weighting function (longitudinal filtration, z-filtering) to the spiral projection raw data. This zfilter function defines how much each projection contributes to the final image depending on its distance to the reconstructed section. Such algorithms can be optimized to obtain more rectangular section profiles at the cost of more image noise (e.g., 180° HI). or they can reduce noise (and thus dose requirements) at the expense of a slightly broadened section profile (e.g., HRLF -10, SmartHelical, GE). Effective Section Thickness (Section Width)

While the width of the section profile in conventional CT is equal to the section collimation (nominal section thickness), the bell-shaped section profile in spiral scanning must be described in terms of the effective section thickness or section width (SW). The effective section thickness depends on the section collimation and a number of other factors that include the table feed and interpolation algorithm. The most frequently used measure for the effective section thickness is the full width at half maximum (FWHM), i.e., the width of the profile at one-half ofits peak value. For a pitch of 1 the section width, given as FWHM, is identical to the section collimation if

X Abbreviations PTCA PTLD FIT PV PVOD PVP PVP PVS QCT RA RAO RAS RBF RB-ILD RBV RCA RCC REAL RF RHA RHV RIMA RLD RN ROI RPS RPV RSV RV RVOT SAPHO SBO SC SCCa SCF SCLC SEQCT SFOV SI SLAP SLDCF SLE SLS SMA SMS SMV

percutaenous transluminal coronaryangioplasty post-transplantation lymphoproliferative disorder partial thromboplastin time portal vein puhnonary veno-occlusive disease portal venous phase percutaneous vertebroplasty prevertebral space quantitativeCT right atrium right anterior oblique renal artery stenosis regional blood flow respiratory bronchiolitis associated interstitial lung disease regional blood volume righ t coronary artery renal cell carcinoma Revised European-American Lymphoma (classification) radiofrequency right hepatic artery right hepatic vein right internal mammary artery right lateral decubitus regenerative nodule region of interest retropharyngeal space right portal vein respiratory syncytial virus right ventricle right ventricular outflow tract synovis tis-acn e-pustulosis- hyperostosis-osteitis (syndrome) small bowel obstruction slice collimation squamous cell carcinoma superficial cervical fascia small cell lung cancer single-energy QCT scan field of view sacroiliac

superior labral anterior - posterior (tear) superficial layer of the deep cervical fascia systemic lupus erythematosus sublingual space superior mesenteric

artery

superior

vein

submandibular space mesenteric

SPECT SPIO SSD SSP SIT SVC SVS TACE T-ALL TBC TEE TF TGA THAD THPE TIA TIPS TNB TNM TNMS

single photon emission computed tomography superparamagnatic iron oxide (particles) shaded surface displays section sensitivity profile scaphoid - trapezium - trapezoideum superior vena cava

step volume scanning (EBCT) trans catheter arterial chernoembolization T-cell acute lymphoblastic leukemia tuberculosis transesophageal echocardiography table feed transposition of the great arteries transient hepatic attenuation differences transient hepatic parenchymal enhancement transient ischemia attack transjugular intrahepatic portosystemic shunt transthoracic needle biopsy tumor, node, metastasis (staging classification tumor, node, metastasis,

serum

(staging classification) tubo-ovarian abscess TOA TRUS transrectal ultrasound thyroid stimulating hormone TSH time to peak TIP transurethral resection TUR ultrafastCT UFCT Union Internationale Centre Ie UICC Cancer DIP usual interstitial pneumonia UP) ureteropelvic junction US ultrasound ultrasmall superparamagnatic USPIO iron oxide (particles) V/Q, scanning ventilation-perfusion scintigraphy VATS video-assisted thoracic surgery VOl volume of interest VRT volume rendering technique VS visceral space VSD ventricular septal defect WHO World Health Organization ZEEP zero end-expiratory pressure 13-hCG P-human chorionic gonadotropin

Image Reconstruction

13

Image plane

.r ,

"

,, ,, ,

\'

' '

a

o

4

2 3 Z-position/collimation

a

360' L1:Linear interpolation between projectional data atzundz:» TF 180' L1:Linear interpolation between conjugated data at z" and projectional data atz' + TF

b Fig. 1.14 Quality comparison of coronal reformatted images using 360' LI (a) and 180' LI (b). The examination was performed with 3mm collimation, 6mm table feed, and a 2mm reconstruction increment.

measured spiral data set calculated conjugated data set Image plane

a 180 LI interpolation is used (see Fig.1.l3b). With a 360 LI interpolation, the section width is 28% larger. The same 28% larger section width is obtained if a 180 LI interpolation with a pitch of 2 is used (see Fig. 1.15b). 0

0

270

0

Image Noise

Image interpolation always includes a projection whose position corresponds precisely to the center of the section (Fig. 1.13a). This projection is not interpolated in the algorithms described, whereas its 180 counterpart undergoes maximum interpolation. This leads to a position-dependent discrepancy of spatial resolution and noise in the scan plane, as image areas near the tube show slightly better sharpness due to the lack of interpolation while opposite image areas show a reduction in noise (see Fig. 7.48). Newer interpolation algorithms (e.g., 180 adaptive interpolation) can correct for these discrepancies.

90

0

z"

3

4

Z-position/collimation

b % 100

"

- -,

, , ,

80 60

1800 LI 3600 LI

r'

u:

" "" ",

"

;, "

,.

_ 1__

"

" " " ,r , ,'' , , , ,

40 20

Pitch

p";l

0

, , , "

r

-5.0

, "" "

" ", , , ,

, , 0 -10

Collimation SC=Smm

,, ,'' , ,'

Standard CT ~ ,

.A

0

~5.010.0 Table position (mm)

Fig. 1.13 Principle of raw data interpolation, (a) Conventional diagram, (b) angluar diagram, (c) comparison of section profiles for 360' and 180' linear interpolation (L1).

XI Contents

Principles of cr, Spiral cr, and Multislice cr Image Processing and Display Techniques

p.45

Patient Preparation

p.83

Optimization

and Contrast Media

of Scanning Technique

p. 131

cr-Guided

p. 161

Interventional

Procedures

Image Analysis

p.187

Neck

p.223

Mediastinum,

system

Pleura, and Chest Wall

p.279 p.373

The Liver

p.405

The Biliary Tract

p.477

The Spleen

p.497

The Pancreas

p.513

The Gastrointestinal

Tract

p.541

Peritoneal Cavity and Retroperitoneum

p.595

Adrenal Glands

p.625

The Kidneys

p.639

Urinary Bladder

p.683

Female Pelvis

p.699

Male Pelvis

p. 725

Lymphatic System

p. 741

Heart

p. 759

Vascular System

p.825

Musculosceletal References Index

'.

p.109

Radiation Dose and Image Quality

Lungs and Tracheobronchial

4'"

p.1

System

p.929 p.998 p. 1052

1 Principles ofCT, Spiral CT, and Muitislice CT As mentioned above, a comparison of 360 LI and 180 LI interpolation shows that 360 LI yields a 28% larger section width (less spatial resolution along the z-axis) and a 29% reduced image noise, Variants such as SmartHelical (GE) lead to a 10% wider section profile than 180 LI but only suffer from 8-16% less noise (depending on the pitch), 0

Acquisition

Section Collimation

0

0

Scanning Parameters

The variable scan parameters in spiral CT examinations are reviewed in Table L4. Three basic parameters for spiral CT scanning are userselectable, In most scanners these are section collimation (SC), table feed per rotation (TF), and reconstruction interval (RI), In some scanners the table feed TF is substituted by the pitch factor P. All the other parameters are varied only in exceptional cases, For this reason, the basic scanning parameters provide an excellent idea of how a spiral CT scan was performed, In this book, we use a triplet of numbers {SCITFlRlj to describe these basic parameters, Section collimation, table feed and pitch are the most important acquisition parameters, while the reconstruction increment is the most important parameter for image reconstruction.

Table

1.4

Scan

parameters

Acquisition

parameters

SC

=

collimation

TF

=·Table

P P*

81.

=

Slice

Scan

RI

=

Reconstruction

width

Table Feed and Pitch

nominal

section

thickness

=N X PX SC=P' X SC = TF /[N x SC] = beam pitch = TF /S C = N X P slicepitch 'TI XTS= TI x N X PX SCIRT e

(mm) increment

(mm)

=

effective

=

reconstruction

section

thickness interval

parameters

=

Table

=

Scan time

"Equipment

•

The table feed per tube rotation (TF), also called the table increment, can be selected independently of the section collimation in spiral scanning, Pitch (P) is defined as the ratio ofthe table feed per tube rotation to the collimation (Fig, US), The following rules should be noted, Scanning at a pitch less than 1 produces an overlapping scan pattern that increases the radiation dose to the patient Overlapping scans may offer slight advantages for the 3D reconstruction of contours that are roughly parallel to the scan plane (e.g, the calvaria), but there are

parameters

Section

Derivative

The section collimation (SC) determines the spatial resolution that is achieved along the zaxis (the direction of table travel), SC can be varied in fixed increments, depending on the scanner. Usually the manufacturer predefines these increments, but with some scanner units they can be modified at installation, The following settings can be recommended as a good compromise between clinical requirements and number of collimation settings: SC=O,Smm, 1 mm, 2mm, 3mm, Smm, 7 mm, or 10mm,

CT

o

(mm)

(em)

=

=

rotation

pitch

length

Reconstruction

RT

and multislice

Pitch

-Volume =

i1 spiral

(mm)

feedltube

SW

N

Parameters

0

speed

(mm/s)

=

(s)

IF/RI

=

scan ciJration

=

duration

=

L1TS

=

(L X RD/(N

X P x S

parameters

Rotation

"Number

time

(s)

of detector

rows

Single

slice:

of one tube N

=

rotation

1; dual slice:

N

=

2; multislice:

Scanning Parameters 15

Pitch=2

Pitch=l

-SC

"

,I r

Pitch= 1 TF=Smm

j

\

/

Pitch = 2 TF= 10 mm

,

\

I I

80

I'

"

"

40

1\

1\ 1\ \

,, ,' I

\

, ,

20

,, ,,

:

o

-S.O

S.O

"

" ,_.• - -, .!I-

_..•.

.

,

I

1\

"

Table feed TF=Smm

'

" "

. - ....•

I,

"

TF

Pitch = 1 ----"--' : " SC=Smm ::

60 1180 LlI----

o~~~~~-,--~~~~-,--10

,

, ,

\'

-l-·-···_-\,

% 00

"\'

f

"

" I

se

Collimation SC=Smm

I

',

rf

60

20

I

••..1 I...•• 14-~

TF

SC

I

40

Pitch=2

--

TF

% 100 80

Pitch-'l

Pitch =2 SC= 2.Smm

J

:

"

"" ,, '' "

, ',

O~-r~/r'~,-,,-,~,"~,--,-10

-10

-S.O

o

S.O

10

Fig. 1.15 Increasing the pitch while keeping the section collimation SC constant has the effect of stretching outthe spiral (a). This widens the section profile (b). The resulting profile with 180 LI at pitch 2 is identical to the section profile with 360 LI at pitch 1 (compare Fig. 1.13 c). Thus, using a pitch of2 instead of 1 will yield

an only 30% wider section width but cover twice the scan length. Increasing the pitch by reducing the section collimation SC covers the same scan length (c). With 180 LI, this leads to a 35% narrower section width than with a pitch of 1 (d).

very few indications in which the minimal improvement of image quality justifies the increased exposure. Moreover, there is little if any effect on image noise, and the higher radiation dose essentially goes unutilized. The only exceptions are scanners with dual detectors (see below, p. 20), which have special z- filtering algorithms that can utilize the higher applied dose to improve image quality (useful in examinations of the intervertebral disks, for example). If the pitch is greater than 2, the volume of interest will be under-sampled, leading inevitably to artifacts. This may be acceptable in some instances, such as trauma cases where priority is placed on the rapid assessment of large volumes rather than on high image quality. But as a general rule, it is better to avoid scanning at pitch values greater than 2. One exception is scanning with very thin collimation, where even conventional CT sections have a bell-shaped profile. Due to broadening of the section profile,

a pitch as high as 3 can be used without causing serious artifacts if the collimation is 1 mm or less (Fig. 1.16).

0

0

0

•

Effective Section Thickness and Pitch

The most Widely used interpolation algorithms at present (180 U) do not increase the section width (effective section thickness) over the section collimation when a pitch of 1 is used (Fig. 1.15 b). Increasing the pitch to 2 reduces the radiation dose by half and doubles the scan length but does not increase the effective section thickness to a proportionate degree. The section profile obtained with 180 LI and a pitch of 2 equals the profile obtained with 360 LI and a pitch of 1, i.e., the scan length is increased and radiation exposure reduced, each by a factor of 2, with identical resolution along the z-axis (Fig.USb). 0

0

0

16

1 Principles oICT, Spiral CT, and Multislice CT reduces the radiation dose but also keeps the effective section thickness as small as possible. A pitch of 2 would be most favorable on theoretical grounds but is more vulnerable to motion and spiral artifacts. A pitch between 1.5 and 2 is the most reasonable tradeoff for most clinical investigations. Rotation Time Fig. 1.16 For narrow collimation, the pitch can be increased to 3 without markedly compromising image quality. Note the excellent quality of multiplanar reformations with a SCITF /RI = 1/3/1 protocol for visualization of subsegmental pulmonary emboli (arrow). Note that the vertebral end plates are well delineated but suffer from minor horizontal interpolation artifacts (arrow head).

Increasing the pitch may produce an increase or decrease in the section width. If the section collimation is kept constant while the table feed is increased, the section width increases (Figs. 1.15a, b). Conversely, the section width is reduced if the section collimation is decreased while the table feed remains unchanged (Figs. LISc, d). In clinical practice, the length L of the scan range and the available scan duration Tl (usually a maximum of 30 seconds for breath-hold scanning) determine the selection of the table feed (see formula in Table 1.4). In this situation it is generally best to use a pitch greater than 1 to maximize the spatial resolution along the z-axis for a given scan length (Table 1.5). This not only

Table 1.5 Effect of pitch on z-axis resolution (effective slice thickness) for a constant table feed

Scan parameters Pitch SC TF

1800 LI

3600 LI

3mm 4mm Smm 6mm

3.9mm 4.6mm 5.3mm 6.0mm

6.9mm

sc=

6mm 6mm 6mm 6mm

2.0 1.5 1.2 1.0

Section width (FWHlvlp

7.2mm 7.5mm 7.8mm

slice collimation, TF = table feed, FWTA full , width of section profile at half maximum Eo

The rotation time (RT) of the x-ray tube (duration of one revolution) is 1 second in most scanners but can vary from 0.42 to 2 seconds, depending on the scanner type. The table speed (TS), or scan length per unit time, can be calculated by dividing the table feed TF by the rotation time (see formula in Table 1.4). The IS is one-third higher in 0.75 second scanners and one-third lower in 1.5second scanners than in standard 1 second scanners. With a shorter rotation time, the z-axis coverage within a given scan time can be increased. All scanning protocols (basic parameters) for spiral CT presented in this book are based on scanners with a rotation period of 1 second. To achieve the same coverage with a slower scanner, the scan time must be increased by the factor RT compared with a 1 second scanner. This is feasible only in regions that do not show respiratory motion (usually skeletal examinations). In all other cases, slower scanners require wider section collimation. Conversely, faster scanners can apply narrower collimation. Because of the detector properties in some scanners, the number of projections available for image reconstruction must be reduced at the shortest rotation time available with these scanners. This leads to a degradation of image quality (loss of sharpness) in the scan plane. If the rotation time is user-selectable (not available in all scanners), it is advantageous to select a longer rotation time for studies that require higher in-plane spatial resolution (e.g., high resolution lung or skeletal examinations). Scan Time

The maximum scan time (scan duration) available in a spiral scanner depends on the x-ray

Image Review, Display and Documentation tube. The higher the dose requirement. the shorter the available scan time will be. Old scanners were limited to a scan time of 24 seconds. but curren t units can achieve scan times up to lOOseconds. In practice, scan times longer than 30 seconds are feasible only for body regions that can be examined without breath holding-the neck, the musculoskeletal system, and in some cases the pelvis. Thoracic and abdominal examinations generally require breath holding to ensure optimum image quality. In some regions (e.g., the lower abdomen), scanning can proceed while the patient slowly exhales after holding his or her breath and continues breathing shallowly. This will cause no substantial loss of quality and will enable scan times longer than 30 seconds.

Reconstruction •

Parameters

Raw Data Interpolation

Most modern scanners use the 180 LI raw data interpolation algorithm, but there are variants available among different units (e.g., SmartHelical on GE scanners, slim2 on Siemens scanners). Fourth-generation scanners (Marconi/Philips) use an interpolation (z-filtering) in which the effective section thickness is largely independent of the pitch (for a constant collimation), but image noise increases with the pitch. This differs from standard interpolations (360 LI. 180 LI), in which image noise is independent of pitch for a given collimation. 0

0

0

Reconstruction Algorithm (Convolution KerneO

The convolution kernels in spiral CT are -no different from those in conventional CT (see above).

The reconstruction increment has nothing to do with section collimation or section width. Rather, it defines the degree of overlap between the axial sections. For the detection of small structures (e.g., pulmonary nodules) it is important to choose Rl so that at least a 30% section overlap is generated. For optimum multiplanar reformations or 3D reconstructions, the reconstruction increment should not exceed half the section width (effective section thickness). This provides a 50% degree of section overlap and generates more than twice as many CT images as conventional scanning. Non-overlapping reconstruction wastes an important advantage of spiral CT, and small lesions located at the boundary between two sections may be missed due to partial volume effects. Overlapping reconstruction ensures optimum lesion display. The theoretic optimum for RI is even smaller than half the section width (see Chapter 4, p. 124), but the added value in clinical practice is generally low.

Image

Review,

cr

Axial

Increment

One of the main advantages of spiral scanning is the continuous acquisition of data, which makes itpossible to generate axial CTsections retrospectivelyat arbitrary z-positions within the scanned volume. The reconstruction increment (RI) defines the spacing of the reconstructed images.

Images

Spiral CT images differ very little in appearance from conventional CT sections obtained with a similar radiation exposure, an identical convolution kernel, and an identical number ofprojections. This is because the raw data interpolation eliminates image artifacts, and the spatial resolution in the axial scan plane corresponds to the spatial resolution of conventional CT images. Compared with conventional CT, however, the image noise for the same scan dose may be 18% lower (360 LJ) or 15% higher (180 LJ), depending on the type of interpolation algorithm used. Spiral CT with a small reconstruction increment generates a large number of images. For example, spiral CTofthe abdomen with 1 s rotation time, a table feed of8 mm, a scan time of 30 seconds, and image reconstruction at 2 mm intervals (as in CT angiography) will generate (30s x 8mm/s/2mm=) 120 images. This large number of images cannot be conveniently documented and reviewed with traditional methods. 0

Reconstruction

Display and

Documentation

0

Principles «ct. Spiral CT, and Muitislice CT •

Cine Mode and Image Documentation

As a rule, not all images can be documented on film One solution is to document every second to fourth image on film, depending on the type of examination, Abnormalities must be adequately documented, however, and a suspicious or abnormal finding must be reproducible, For this reason, selected overlapping sections or multiplanar reformations (MPRs) should also be documented if required, Image quality (noise) can be improved by retrospectively increasing the section width (approximately 7-10mm) for documentation instead of using the original sections, In some scanners, this can be done directly from the raw data by fusing projections from multiple tube rotations (similarly to multislice CT), These thicker sections reduce noise, which is particularly beneficial in scans taken with a low dose or a thin collimation, If this option is not available, thicker sections can be produced by averaging the data from several overlapping images (e.g, using thick axial reformations;' This is usually too time-consuming

for routine

examinations

but may be appropriate in selected cases where image noise is excessive, It is, however, the preferred technique for image documentation in almost all multislice CT applications,

If not all the reconstructed images are printed, all available images should be viewed in an interactive cine display so that an accurate evaluation can be made, This mode displays the individual images in the scanned volume on the monitor in a movie-like sequence, providing a three-dimensional impression of complex, nonlinear structures that have multiple intersections with the image plane, The radiologist can control the speed and direction of the image display sequence interactively by manipulating a computer mouse or trackball, Image Processing

A data set composed of overlapping axial spiral CT images is well suited for generating multiplanar reformatted images (MPRs) in arbitrary sectional planes through the scanned volume (see pp,48), In cases with equivocal findings, MPRs can assist the diagnosis by providing information in an extra viewing plane, When thin sections have been used, reformatted images of good quality can be produced, Three-dimensional images (see pp.Gtl) of the skeleton or vascular system improve anatomic orientation and can enhance the presentation of findings to referring colleagues,

Multislice CT

Multislice CT, or multidetector-row CT, multidetector CT, or volume CT, as it is also called, represents the next breakthrough in CT technology, Multislice CT scanners provide a huge gain in performance that can be used to reduce scan time, reduce section collimation, or to increase scan length substantially, Multislice CT transforms CT from a trans axial into a truly three-dimensional imaging technique, Multislice CT has gained a rapid acceptance by the radiological community, There is an almost exponential growth of the number of scanners: in 1998 there were 10 scanners installed, in 1999it was 100by the middle of the year, and by the end of 2000, over 1000 scanners were installed worldwide,

Scanning Principle

Unlike standard systems that use a single detector arc or detector ring, multislice CT systems are equipped with two or more parallel detector arrays and always utilize a third-generation technology with synchronously rotating tube and detector array as well as solid state detectors (Fig, 1.17),Although the very first CT scanners in the 1970s also employed a split detector, the first dual or split detector systems with continuously rotating tubes becarue available only in the early 1990s, Systems with four active detector arrays were introduced in 1998, and systems with 8, 10 or 16 active detector arrays are now available, The performance of many of these systems is further improved by a faster rotation time, As a result, a four-detector-row scanner with 0,5 s

Scanning Principle 19 Table 1.6 Advantages of higher scanner performance with multislice CT or subsecond spiral CT Shorter scan duration Reduced motion artifacts: • Children • Trauma patients • Acutely ill patients Improved scanning of parenchyma! organs • Well-defined phase of contrast enhancement Reduced volume of contrast medium Perfusion imaging Longer scan ranges Trauma

II»

Thoraco-abdominal

spine/blunt trauma

CT angiography: • Aorta and peripheral run-off • Thoraco-abdominal aorta • Carotids from arch to intracerebral circulation Thinner sections Fig. 1.17 Principle of multislice CT scanning. More than two detector rows are exposed simultaneously.

rotation has an about 8 times higher performance than a conventional 1 s single-detectorrow scanner. Multislice CT does not always involve spiral data acquisition, although spiral acquisition is implied in this book unless stated otherwise. A sequential mode (step-by-step scanning) as in conventional CT with possibilities of image fusion is also available, although it will be used only in a minority of applications, such as HRCT of the lungs or interventional procedures. Advantages

The performance of multislice CT is at least 4 times, with modern scanners 8-20 times higher than that of a conventional spiral CT scanner. This enormous increase in performance can be used for shorter scan duration, longer scan ranges, and thinner sections (Table 1.6). In general, the performance gain will be used to improve all of the above to a substantial amount. In consequence, multislice CT has overcome one of the most severe limitations of spiral CT, namely the inverse relation between scanning range and section collimation.

IMear-isotropic imaging: • • • • •

Temporal bone imaging Musculoskeletal imaging Arbitrary imaging planes Multiplanar reformats 3D rendering

Shorter scan duration will reduce the danger of motion artifacts, especially in children or critically ill patients. Marked improvements can be seen for trauma patients or in dyspneic individuals with suspected pulmonary embolism. Shorter scan duration also will allow for scanning of the liver or other parenchymal organs in a more well-defined phase of contrast enhancement, which again improves lesion detection and characterization. Shorter scans will also allow for substantial reduction of the volume of contrast material to be administered as long as only an arterial phase of contrast enhancement is required. Longer scan ranges are especially important for CT angiography. CTA-examinations become feasible that include the abdominal aorta and the peripheral run-off vessels down to the feet. Alternatively, the whole aorta can be scanned with high spatial resolution, or the carotids can be examined form the aortic arch to the intracranial circulation. Long scan ranges such as in thoraco-abdominal studies no longer pose a

1 Principles «ct. Spiral CT, and Multislice cr problem, even for indications that require a high spatial resolution. Finally, thinner sections and even isotropic imaging will become feasible yielding almost identical spatial resolution in all directions. This makes near-isotropic multiplanar imaging possible with a spatial resolution in any arbitrary plane that equals, or often exceeds, the resolutionofMRI. Disadvantages

The downside is a markedly increased data load, especially if near-isotropic imaging is performed. A scan of the chest and abdomen (60cm) can be performed with a 4 x 1 mm collimation in 50s and produces up to 500-800 images, depending on the degree of overlap. A chest scan with an identical collimation will produce as many images if reconstructed in a smoothing convolution kernel for the mediastinum and an edge-enhancing kernel for the lungs. ACTA of the aorta and peripheral arteries may produce 1000 images and more. The only way to avoid this enormous amount of data is to (acquire and) reconstruct thicker sections. This, however, will forgo many of the advantages of multi slice CT. Current scanning protocols frequently are modified versions of standard spiral CT protocols with a somewhat thinner section collimation. Most present scanners and workstations can easily handle the data created by such protocols. However, if near-isotropic imaging is performed, image reconstruction speed and data handling become limiting factors for some of the present scanners and workstations and make this technique cumbersome for clinical routine use. There is not yet an in-depth experience with near-isotropic imaging, nor do standardized protocols exist. This book suggests protocols optimized for clinical routine work as well as specialized protocols for more advanced applications and multiplanar imaging. Image processing adds to the increased time requirements for data analysis if a full-resolution scan is performed. No standardized protocols are available yet but manufacturers are working on techniques to include 3D processing in the standard workflow. Image noise grows as section collimation is reduced. For this reason, it is important to reconstruct thicker sections (MPR or axial sections) to keep image noise low. With very thin

collimation the geometric efficiency of the detector deteriorates (see Fig. 5.5). It varies between manufacturers and depends on scanner geometry, the implementation of beam collimation and image interpolation algorithms. With 16-slice scanners, the geometric efficiency approaches that of single slice CT. An increase in patient dose is only necessary if thin-section images of high quality are required. In all other cases, multislice CT requires less dose than conventional CT or a similar dose as spiral CTwith a pitch of2. Detector Types Dual Detector Systems

Dual or split detector systems are based on a detector array that is twice as wide as a conventional CT detector and is split in half (Fig. 1.18). Elscint was the first company to use this concept on a spiral CTsystem (ElscintTwin). Now, most vendors offer such dual-slice systems. Multidetector-row

Systems

True multidetector-row or multislice systems are able to acquire at least four simultaneous sections. To be able to choose between various section collimations, the detector arrays have to be subdivided into multiple detector rows. Data from each of the scanned sections is recorded by a data acquisition system (DAS), which consists of one detector row or a combination of detector rows, depending on the chosen section collimation (Figs. 1.19-1.21).The basic types of detectors available are described below. The width of each detector row is not given as its real width but instead as the width of the corresponding xray beam at the center of the scan field. Matrix Detector

Matrix detectors consist of multiple detector rows of identical width (Fig.1.l9a). The GE matrix detector is a typical example. It uses 16 parallel detector arrays with a width of 1.25mm each. One has to note that the detector width mentioned in this text (as well as most publications) is not the "real" width of the detector elements but the width of the detector-row beam

Detector Types

~~'.~.x-ray tube ..~.~~ Beam collimators

. X-ray beam ... Central trajectory

I I I I I Single-slice

Fig. 1.18 Comparison

of single and dual (split) detector systems.

collimation, measured in the center of the scan field. The real width of the elements of the x-ray detector is about twice as large, depending on the scanner geometry and the distance from the center of the scan field to the actual detectors. By exposing only the innermost four detector rows, a 4 x 1.25 mm collimation can be produced. For wider collimator settings the signals of groups of two or more adjacent detector rows have to be added to obtain 4 x 2.5 mm, 4x3.75 mm and 4x5 mm collimations (Fig. 1.19b). By partially covering the innermost two detector rows, also 2 x 0.63 mm sections can be generated. The same principle of grouping adjacent arrays holds true with 8-detector-row scanners, which will allow for 8 x 1.25 mm and 8x2.5 mm section collimation (Fig.1.l9c). The total collimation (e.g., 8 x 1.25 mm = 10 mm, or 8x2.5 rnrn Zfl mm) is limited by the total width of the detector array. e

•

Adaptive

Multislice

Dual-slice

Array Detector

Adaptive array detectors consist of detector rows that grow in width from the center of the section to the periphery (Fig.1.20a). Philips (Asymmetrix detector) and Siemens (AAD) have adopted this concept and use the same detector in their scanners. By collimating half of the two innermost 1 mm-wide detector arrays of 4-detector -row scanners, a section collimation of 2 x 0.5 mm can be obtained. By partially collimating the

next arrays, 4 x 1 mm sections can be gained. Adding the innermost two detector rows will yield 4 x 2.5 mm collimation (Fig. 1.20b). The rationale behind this approach is based on the fact that the x-ray beam hits the more peripheral detectors at an oblique angle. Any perpendicular septa in the more peripheral portions of the detector will block some of these oblique rays and will thus decrease the detection efficiency. Reducing the number of septa in the peripheral regions of the detector array should thus increase geometric dose efficiency. Indeed, the detector has a dose efficiency that is only slightly less than that of a single-slice scanner, if a section collimation s2.5mm is used (for more detailed discussion on geometric efficiency see Chapter 5 and Fig. 5.5). The detector geometry is optimized only for a particular number of active detector rows. A new detector configuration is needed as the number of detector rows progresses from 4 to 16 (Fig. 1.20 c). With more rows, also more septa have to be added. Hybrid

Detectors

Hybrid detectors are similar to matrix detectors with the exception that the innermost detector rows are thinner than the outer ones (Fig. 1.21a). This concept is embraced by Toshiba (selectable slice-thickness multirow detector, SSMD). These hybrid detectors are based on a detector array of a total width of 32 mm that con-

21

22

1 Principles of CT, Spiral CT, and Mu/tislice CT _16x1.25mm

__ 5mm

1.5mm 1 2.5mm :1 ':l.m

mm 2.5mm

.

Srnm

Center of scan plane ___

20mm_

20mm

2xO.63mm 4xl.25mm 4x2.5mm 4x3.75mm 4x5mm

2xlOmm

8xl.25mm 8x2.5mm 16xO.63mm 16xl.25mm Fig. 1.19 Matrix detectors consist of detector rows of identical widths. For the GE HiLight detector this width is chosen so that each detector row covers a 1.25 mm-thick section in the center of the scan field (a). Collimating the x-ray beam, or combining the signal from adjacent detector rows, or both, will yield various section collimations (b). An 8-row scanner provides 8 x 1.25 mm and 8 x 2.5 mm collimation. The 16-row scanner uses a hybrid detector with 16 x 0.63 mm and 16 x 1.25 mm collimation (c).

sists of 1 mm-wide detector rows, With 4-detector-row scanners the innermost 2 mm of this array are further subdivided into four 0.5 mm wide detector rows. With this detector configuration there is a multitude of possible collimations, ranging from 4 x 05 mm, 4 x 1 mm to

2xO.5mm

4x1 mm

4x2.5mm 4x5mm

2x8mm

•

Mll

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i ••.

iiWIiI

1111t.tIIIIII!l%~

16xO.75mm 16x1.5mm

8x3mm

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Fig. 1.20 Adaptive array detectors have outer detector rows that are wider than the innermost detector rows (a). The detector used by Philips and Siemens covers a total section width of20 mm and allows for section collimations ranging from 2 x 0.5 mm to 4 x 5 mm (b). A new detector configuration is chosen for the new 16row scanner, which will be ableto provide 16 X 0.75 mm to 16 x 1.5 mm collimation (c). Philips will additionally offer a 8 x 3 mm collimation.

4 x 8 mm (Fig. 1.21b; Table 1.7). With 8- and 16detector-row scanners the innermost 8 or 16 rows of the 32 mm-wide detector array have a width of 05 mm (Fig. 1.21c). The Toshiba detector uses dividers between the various detector rows that are opaque to the

Detector Types

~-

4xO.5mm

15xl mm

15xl mm--

Fig. 1.21 Hybrid detectors are matrix detectors with thinner innermost detector rows (a). Toshiba uses a detector that covers 32 mm and allows for 4 x 0.5 mm to 4 x 8 mm collimation (b). The new detector configuration for a 16-row scanner will be able to provide 16 x 0.5 mm to 16 X 2 mm collimation (c).

32mm

scintillation light produced by each detector element (thus avoiding cross-talk) but which are transparent to the incident radiation. This effect

Cone Beam Systems (Volumetric

increases dose efficiency but also makes the de-

on cone beam technology in which a wide x-ray beam is used to cover a large range along the zaxis. Prototypes by various vendors include a

tector slightly more vulnerable radiation. The 16-detector-row

to scattered

CT)

Future generations of CT scanners will be based

scanners of all other

2S6-row detector and a 1024' flat panel detec-

vendors (Figs. 1.19c and 1.20c) also use thinner

tor. Such scanners may provide enormous spatial resolution and high-speed data acquisition.

innermost detector rows and therefore

be considered hybrid detectors.

should

Cone beam systems that are already on the market are reserved for rotational

angiography.

23

•

1 Principles ofCT, Spiral CT, and Multislice CT They use image intensifiers or (CCD) flat panel detectors as the radiation receptor and require a num ber of seconds for acquiring one (single-rotation) data set. The present systems are only able to depict high-contrast structures such as vessels during intra-arterial contrast injection. The reason is the large amount of scattered radiation that is not blocked from hitting the detector-as opposed to multislice CT where the relatively narrow beam and the various septa between the detector rows achieve adequate scatter suppression. This scattered radiation destroys low-contrast resolution if no adequate scatter-reduction techniques are developed. Unless this major obstacle is solved, true cone beam CTwill not become clinically feasible. Another major problem with cone beam detectors is the bandwidth of the signal transmission system that has to transfer the various detector signals out of the gantry to the image reconstruction computer. The data throughput in these systems will become enormous and will grow by a factor of 64 relative to a 4-detector row scanner if 256-detector rows are employed. At present there is no system that can transmit and process the acquired data with sufficient speed. The third and biggest problem is image noise, which will increase substantially. Image noise for isotropic resolution with voxels of a size of x3 is theoretically proportional to IIx'. This means that image noise will grow by a factor of 2*~ 16 if the size of an isotropic voxel is reduced from (Irnm}' to (0.5mm)3, and by 10,000 if the voxel size is decreased to (0.1 mm)3 As a consequence, sophisticated noise suppression and data reconstruction techniques will be necessary to create clinically useful images. For the reasons mentioned above, it will be several years until cone beam scanners become available for clinical use.

1a'~

System Performance The performance of multislice CT scanners increases with the number of active detector rows, the speed of x-ray tube rotation, and with the available pitch factor. This increased performance can be used to reduce scan time, reduce section collimation, or to increase scan length (Table 1.6). At the same time, increase in

performance has to be counterbalanced by the amount of artifact induced and by increased image noise with thinner sections. Rotation Speed

The rotation speed of the x-ray tube is commonly described by the time required for one tube revolution, the so-called rotation time RT. All multislice scanners have a rotation time of 0.8 seconds or less. Most high-end scanners even allow for a 0.5 s rotation. With the newest generation of scanners, rotation times of 0.42 s are available. Pitch

There are two definitions ofpirdi factor available with multislice CT scanners, depending on whether a single section collimationSC or the total cOiKmatwnofthedetectorarray(N x SC)ischosen as the reference. To distinguish between them, an asterisk is used to indicate the definition used by most manufactures for 4-slice scanners (volume pitch P*, also called sh'ce pitch or detector pitch), while P denotes the definition agreed upon internationally (lEC) and preferred by most physicists (pitch P), also called beam pitch:

P=TFI(NxSCj

P*~TF/SC

Pitch Volume pitch

While P is independent of the number of detector rows, P* increases as the number of detector rows grows. As with single-slice spiral CT, the pitch P can be increased up to 2, independent of the number of detector rows N. In multislice scanners with four active detector rows this corresponds to P*=8. Some manufacturers have limited the maximum pitch P to 1.5 (corresponds to P* = 6 for four-slice scanners). Relative System

Performance

System performance is proportional to the number of detector rows N and improves with a shorter rotation time RT of the x-ray tube. This concept can be applied to standard spiral CT as well as to dual detector scanners or multislice CT. Table 1.7 gives an overview of the relative performance of various scanner types and scanning protocols. It has to be noted that the quality of axial images may deteriorate when the highest possible pitch factors are chosen.

Image Reconstruction Table1.7 Examples of system performance relative to a 1 sec single-slice spiral CT scanner with pitch System

Rotation time

(5) 1s-scanner 1s-scanner 0.8 s-subsecond scanner 0.5 s-subsecond scanner 1s-dual detector scanner 0.5 s-dual detector scanner 0.8 s-multislice scanner 0.8 s-multislice scanner 0.5 s-multislice scanner 0.5 s-multislice scanner 0.5 s-multislice scanner 0.42 s-multislice scanner

1s 1s 0.8s 0.5 s 1s 0.5s 0.8s 0.8s 0.5 s 0.5 s 0.5 s 0.42s

Image Reconstruction

Image reconstruction in 2- and 4-detector-row (T systems does not have to use sophisticated interpolation if a section-by-section mode is used, but even then, the cone beam geometry of the system will lead to artifacts for regions outside the center of the gantry. With spiral acquisition, the situation is becoming more complex because the trajectories of the various detector rows may overlap and yield "redundant" data, depending on the pitch factor employed. For this reason, more sophisticated raw data interpolation schemes have to be used. •

The Cone Beam Problem

Due to the nature of the x-ray beam, which arises from a small focal spot, all (T scanning relies on cone beam geometry. With single-slice CTthese effects are hardly visible but they play a major role in multislice scanning because the same structure may be depicted on different detector rows during one revolution of the x-ray tube (Fig.1.22a). Only in the center of rotation are structures always captured by identical detector rows. The effect becomes more prominent the further a structure is from the isocenter (rotational axis), and the more detector rows are used (Fig. 122c). Simple raw data interpolation algorithms make the assumption that all beams are parallel (Fig.1.22 b), which yields reasonable results up to four active detector rows but fails with a higher number of rows. While adaptation for

Detector rows (N)

Pitch P

Pitch p.

=

2

Rei. performance

0.5 2 2 2 2 2 0.75 1.5 1.5 2 1.35

1 2 2 4 4 4 4 8 16

2 2 2 4 4 3 6 6 8 10.8 32

2

1.25 2 2 4 1.875 3.75 6 8 10.8 38

thin section and 8-slice scanners may still be possible, 16-slice scanners require more sophisticated cone beam algorithms for image reconstruction. •

Multislice Linear Interpolation

Algorithms for 4-slice systems that are analogous to 180 Ll and 360 LI from spiral (T are called 180 MLI (multislice linear interpolation) and 360 MLi (Fig. 1.23). For each projection angle, they use the projection data from the two detectors that are closest to the scanning pane (3600 MLI only real trajectories, 1800 MLI also virtual trajectories from the detector to the x-ray tube). Section profiles with these algorithms vary between those from conventional 180 Ll and 360 LI spiral (T interpolation, but the dependence on the pitch factor is more complex because of the sampling patterns described below. It is difficult to represent these effects using the standard diagrams for single-slice spiral (T. A diagram that displays the rotational angle of the x-ray tube for varying z-positions of each DASchannel (active detector row) is more suitable for this purpose (Fig. 123 b, compare also Fig. 113a). 0

0

0

0

0

•

0

The Sampling Problem

Depending on the pitch factor, the spiral trajectory of the first detector row of a 4-slice scanner may overlap with that of the second, third or fourth row for P* = 1, 2, and 3, respectively (Fig. 1.24). For a pitch of 0.5 (P* = 2), even the

25

•

7 Principles aICT, Spiral CT, and Multislice CT

Assumption for reconstruction: parallel rays

a

FOV z-axis

c

4 detector rows

8 detector rows

Fig. 1.22 Cone beam geometry. Peripheral structures will be seen by different detector rows during one revolution of the x-ray tube (a). Simple raw data interpolation algorithms make the assumption that all beams are parallel like in convention spiral CT scanning (b). Such algorithms will fail with more than four detector rows be-

cause the cone beam effect will become more prominent when more detector rows are used (c). The distance ASC traveled by the outermost rays within the field-of-view (FOV) becomes larger than the section collimation SC and will cause artifacts in standard reconstructions.

conjugate data from the virtual spiral overlap the real spiral at the isocenter (Fig. 1.24b). This makes data sampling particularly inefficient and only allows for an interpolation between data samples that are one section collimation apart. As a consequence, the section width is widened Similar to a 360 LI interpolation. Because of

varying degrees of overlap with changing pitch factors, the sampling density and thus the section width varies (Fig. 1.25). In principle this sampling problem applies also to 8- and 16-slice scanners but, because of the cone beam effect, the projection rays for the overlapping detector rows are not completely

0

Image Reconstruction Fig. 1.23 Multislice linear interpolation for a 4-slice scanner and a pitch P = 1.5 (volume pitch P* =6). The conventional representation (a) and the angular diagram (b) are equivalent but the angular diagram provides a better overview. Note that the trajectories of the original spiral (detector row 4) and the conjugated data (from detector row 1 to x-ray source) overlap.

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Fig. 1.24 Overlapping data sampling occurs when trajectory of one detector row superimposes another, e.g. for a pitch of 0.75 (P* = 3) (a). In the center of rotation, even the conjugated data from

b the on isothe

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virtual spiral overlap the real spiral trajectory for a pitch of 0.5 (P* =2) (b). Note, however, that the cone beam geometry ameliorates this effect, especially for image points farther away from the isocenter.

27

28

1 Principles ofCT, Spiral CT, and Multislice CT

SW/SC 300%

200%

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f-----_.__---t-------,-------\

1.5

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redundant outside the isocenter. Still, there are constellations in which the sampling is particularily advantageous or disadvantageous. While Philips and Siemens have made their interpolation algorithms independent of pitch, Toshiba and GE provide preferred pitch factors to ensure particularly advantageous data sampling for their 4-, 8- and 16-slice systems. Siemens has chosen another approach, in which a z- filtering for 4-slice systems and cone-beam reconstruction for 16-slice systems is chosen in a way that makes section width, image noise and radiation dose independent of the sampling pattern with varying pitch (see Figs. 1.25c and 1.28c). •

150% 360 MLI

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Fig. 1.25 Effective section thickness (section width) as a function of pitch for 180 LI and 360 LI with single-detector-row CT (a) compared to 180 MLI and 360 MLI with 4-slice scanners (b). Adaptive interpolation schemes (c) keep section width independent of the pitch. HQ = high quality mode, MS = high speed mode on GE 0

0

0

scanners.

0

Z-Filter Interpolation

Z-filter interpolation is reserved for 4- and 8slice scanners uses a similar concept as higherorder interpolation algorithms for conventional spiral CT. Not only the two projections from the detectors that are closest to the scan plane are used but also adjacent projections [multipoint interpolation). These projections are weighted according to their distance from the scan plane (Fig. 1.26).This filter function may even contain negative portions that result in "edge enhancement" along the z-axis and thus can ensure that the section width is equal to the collimation even at unfavorable pitch factors. This, however, will come at an increased image noise. By using a wide filter function, noise is reduced and the section width becomes larger. Z- filtering controls the width of the slice profile (section width SW) of the reconstructed images. While most manufacturers (Philips, Siemens, Toshiba) always display the correct section width SW on their user interface, GE has chosen to display only multiples of SC as potential section widths SW on their scanners. This is correct for the vast majority of parameter combinations. For the thinnest section widths at pitch P> 1, however, the true width of the section profile at half maximum (FWHM) is approximately 30% larger than SC (for 4- and 8slice scanners but also for 16-slice units) and than the displayed value for SW. This widening of the section profile is also present with other manufactures but is correctly represented in the displayed section width SW. Depending on the manufacturer, various combinations between chosen section collimation and reconstructed section width are available (Tables 1.8-1.10). In principle, the only re-

Image Reconstruction

360

Fig. 1.26 Z-filtering is a technique that weights projection data according to distance from the reconstructed imaging plane.

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straint is that the section width must be larger than or equal to the collimation. The main difference between manufacturers is how they treat the minimum available section width. If standard 180 MLI or 360 MLI algorithms (or variants) are used, the section width varies between 100% and" 128% ofthe section collimation, depending on the pitch factor. The minimum section width with the GE high speed mode (P *= 6) on 4-slice scanners is some 30% wider than the collimation SC (see Table 1.8). It is only in this mode and in the UF (P*=10.8, P=1.35) and US (P*=13.4, P= 1.675) modes in 8-slice scanners that the "reconstruction slice thickness' indicated on the user interface is not approximately equal to the section width. Philips provides the real section width on theiruserinterface. The available minimumnumbers vary with pitch (between SC and SC+30%). 0

0

Siemens varies their z- filter (adaptive array interpolation) with the pitch in such a way that section width, image noise, and dose requirements remain constant independent of pitch (Fig.1.25c). As a standard, they offer a section width that is some 30% wider than the section collimation. An edge-enhancing z- filter is also available for the 4 xl mm detector configuration that provides 1 mm section width independent of the pitch. The resulting images, however, suffer from a substantially higher noise Oust like the difference between 180 LI and 360 LI in spiral CT). Toshiba uses a z-filter of varying width (MUSCOT algorithm) that provides a given section width independent of the chosen pitch factor. The minimum section width is identical to the collimation and can be increased in steps of 0.5 mm. If SW is chosen equal to SC, the noise is substantially increased (like with all other vendors). 0

0

29

1 Principles ofCT, Spiral CT, and Multislice CT

•

Cone Beam Interpolation

Z-filter interpolation still relies on the assumption that rays are parallel (Fig. 1.22c). Real cone beam corrections require more complex calculations. Such algorithms are mandatory with 16 or more detector rows because cone beam artifacts (see Fig. 7.47) increase substantially with wider detector arrays. Most of these algorithms are still being refined, and use various types of compensation techniques for the cone beam geometry. Variants of 3D back projection (e.g., COBRA, Philips or ConeView, Toshiba) theoretically should yield the least artifacts. They rely on reb inning of projection data and back projection along the correct cone angles. For each voxel, the projectional rays that are next to it are actually used for backprojection. The Adaptive Multiple Plane Reconstruction (AMPR, Siemens) shifts the plane of interpolation from an axial orientation to an oblique position with a maximum angulation determined by the cone angle (Fig. 1.27). This yields a set of oblique planes (as an intermediate step) that rotate with different z-positions. Interpolation between these oblique planes then creates axial, coronal, or arbitrarily oriented sections of any desired section width without necessarily having to go through a real reconstruction of an orthogonal 3D data set. GE uses a combination of approaches (Crossbeam correction, Hyperplane and Conjugate Ray reconstruction) to address the various problems of cone-beam scanning, i.e. cone-beam artifact reduction and thin-slice, high-pitch scanning. At present, artifact behavior of these various algorithms has not yet been studied.

The cone beam interpolation used in the 16-slice scanners of Philips, Siemens and Toshiba allow for arbitrary pitch factors up to 1.5 to 2. However, Toshiba suggests preferred pitch values (at present P*=ll, 15 and 23; P= 0.6875, 0.9375, and 1.4375, respectively). Similar to their 4-slice and 8-slice systems, GE supports fixed pitch factors (P*=9, 15, 22 and 28; P= 0.5625, 0.9375, 1.375 and 1.75, respectively), for which their cone beam algorithms have been specifically optimized. •

Image Noise

Image noise in multislice CT depends on the zfilter (or cone beam algorithm) and the chosen section width. To understand how, we have to look back at conventional spiral CT. There, the noise was independent of the pitch but varied with the chosen interpolation algorithm. In fact, it was much smaller with 360 LI as compared to 180 LI; for identical noise, the dose with 180 LI would have to be doubled. The 180 LI algorithm had an identical performance to the 360 LI only at a pitch of 2 and twice the mAs setting (same section profile, identical noise at identical dose). This should have made the 360 LI algorithm the interpolation of choice for low pitch factors, and is another argument for using 180 LI only in combination with high pitch factors. The effects becomes clear when the noise data are normalized to identical patient exposure (Fig. 1.28 a), which can be done by increasing the mAs setting proportional to the pitch so that the effective mAs (=mAs/Pitch) remain constant. Multislice CT overcomes the limitation of single-slice spiral CT because of the higher number of detector rows and, thus, faster coverage. With 4-slice scanners, the 360 MLI algorithm has similar advantages in terms of image noise and dose requirements over 180 MLI as the corresponding algorithms from single-slice CT. When considering the pitch-dependence of the section width (Fig. 1.25) and normalizing the noise behavior to identical patient exposure (Fig. 1.28b), it becomes clear that a 360 MLI algorithm should be preferred for pitches up to 1 (slice pitch up to 4), and the 180 MLI algorithms should be used for pitches up to 2 (P" up to 8). Since a variant of 180 MLI is used for both the HO. (P*=3) and HS (P*=6) modes ofGE scanners, the noise per dose is higher for the HQ. mode than with the HS mode. 0

0

0

0

0

0

0

0

o

0

0

0

0

Fig. 1.27 Cone beam interpolation by interpolating raw data to obtain multiple angulated imaging planes that are then interpolated to form a 3D volume.

Image Reconstruction ReI. dose requirements for constant noise

0

lS0%

100 % i-------''<:----~---'''o,_=__----

i ~~~ ..···..··'··········" ..·····i~

180 LI

360 LI

:

0

Pitch

i------r-----r---r---,----,---,--

a

0.25

0.5

1.25

0.75

1.5

1.75

2 P

ReI. dose requirements for constant noise 180MLI

lS0%

"''''--·'f'''·--'' . --.--..'--'.----..+.~.--." --"."..

133 % 100%

Pitch

b

0.25 1

0.5 2

0.75 3

1 4

1.25 5

1.5 6

2 P 8 P*

1.75 7

ReI. dose requirements for constant noise ISO% SW=S --

133 %

--.--1': --

100%

: SW=1,2~=~ SW=l.Sa==------

67 %

--

SO%

,

--.. ,,-- ,---- .. 1':

-- --.-- ..

Conv. CTl::;=;;;;;::;;;::~::::::=---

--.----

t-v~

--.--.-- .. --, .. ----

~

SW=2xSC:,

Pitch

f----~------'-----

c

More complex z- filtering may improve the relation between noise and section width (e.g., SmartHelical) and can reduce the noise compared to a 180 LI algorithm (GE). Siemens use adaptive z- filtering to keep the noise and the section profile independent of the pitch at identical patient exposure (Figs.1.25c and 1.28c). The noise with this algorithm is 12-16% lower than the noise with conventional CT but the section profile is always some 30% wider than the collimation (as with 360 LI). Since 4-slice CT allows for using very thin collimation (4 xl mm), this broadening (to 1.3mm) is of little practical importance. With the Toshiba approach, the user selects a certain section width (in 0.5 mm steps) and the system chooses an appropriate zfilter width. Noise will increase substantially if the smallest section width (identical to the collimation) is used but decreases with the next larger section width. When choosing an even wider section width, more data are included in the reconstruction and image noise decreases further. The noise behavior with 16-slice scanners has not yet been published but in general, a similar behavior as for -slice units can be expecdet. If the section width SW is chosen identical to the section collimation SC, image noise should be substantially increased, while it should rapidly diminish if at least a 30% wider section width is employed.

0.25 O.S 123

0.75

1

4

1.25 567

1.5

1.75

2 P 8 P*

Fig. 1.28 Relative dose requirements for constant image nois as a function of pitch. Comparison of 180 LI to 360 LI for sinqle-slice CT (a), and 180 MLI for -l-stice CT (b). Adaptive interpolation keeps image noise fairly independent of the pitch (c). 0

0

0

31

1 Principles act. Spiral CT, and Multislice CT Scanning Parameters As with spiral CT scanning, section collimation (SC), table feed per rotation (TF) , and pitch (P) are the most important acquisition parameters in multislice CT In addition to the reconstruction increment (Rl), however, there is the effective section thickness or section width (SW) of the reconstructed images that contributes to the most important reconstruction parameters, All the other parameters are varied only in exceptional cases, Together with the number of active detector rows N, the acquisition parameters can be given as (N x SC ITF), and the reconstruction parameters can be given as (SW / Rl). Because more than one set of reconstructions is possible with multislice CT, it is reasonable to separate acquisition and reconstruction parameters,

Acquisition Parameters As has been painted out above, the section collimation is determined by the available detector configuration, In general, thinner sections (4x 1 to 4 x "1.25mm, and 8 x 1 to 8 x 1.25 mm) are used for near-isotropic volumetric imaging with the option to reconstruct arbitrary cut planes, Thicker sections (4 x 2 to 4 x 2,5 mm or more) are employed with fast spiral scanning for routine applications, and when data acquisition has to be particularly fast With 16-slice scanners, thin sections become standard even for routine fast spiral scanning, GE offers 16 x 1.25 nun: Philips and Siemens provide 16 x 1.5 mm, and Toshiba gives the choice between 16 x 1 mm and 16 x 2 mm for this purpose, For isotropic volumetric imaging, the scanners offer 16 x 0,62 mm (GE), 16 x 0,75 mm (Philips, Siemens), and 16 x 0.5 mm (Toshiba), There is a continuing discussion as to whether to use a high or low pitch factor, Lower pitch factors (e.g. , the HQ, mode in a 4-slice GE scanner) yield axial images with less cone beam artifact, but good results of a similar (and often better) quality can be obtained when images are acquired with a thin collimation and reconstructedwith a larger section width, In addition, low pitch factors require a higher patient dose for identical image noise, Using a thin collimation and a large pitch factor has the additional advantage that it is possible to go back to the

raw data and reconstruct it again with a thin section width, From such a data set, it will be possible to obtain superior quality multiplanar reformations (see Fig, 2,8), Such an approach is especially useful when findings on axial sections are not clear and require further evaluation by another imaging plane, With 8- and 16slice scanners this difference becomes less apparent because only thin sections are available for data acquisition, One has to bear in mind, however, that the geometric efficiency in 4-slice scanners (see Fig, 5,5) decreases with thinner sections, which will lead to an increase in image noise that may require using higher exposure settings, This limitation is overcome with 8- and 16-slice detector systems because of substantially higher geometric efficiency, There is a limit to such an approach of using thin collimation, however, on those scanners that do not allow for increasing the reconstructed section width arbitrarily, This is for example the case for current four-slice GE systems (see Table 1.8), A 4 x 1.25 mm collimation will not allow for reconstructing 5mm axial sections, In practice, many users will therefore not use such a thin collimation, but instead resort to 4x2,5 mm or 4x3,75 mm collimation, With the GE 8- and 16-slice scanners the maximum section width is 5mm even with the thinnest collimation, For the four-slice system there is a workaround described below that uses thick MPR from an overlapping thin-section secondary raw data set

Reconstruction Parameters The secrion width SW (effective section thickness) has to be larger than or equal to the section collimation Sc. Apart from this constraint, SW can be chosen independently from SC for most multislice CT units, The available choices for section width depend on the manufacturer and the type ofz-filtering (see Tables 1.8-1.10), Note that a section width that is identical to the collimation SC will result in a substantially increased image noise, and therefore should be reserved for only those applications with the maximum possible z-axis resolution (e.g, pulmonary or skeletal imaging), For most routine applications, and for reasons of image noise, a section width of 5-8 mm

Scanning Parameters 33 will be chosen. Only for special indications such as HRCT, skeletal imaging. imaging of the pancreas and adrenals, or preoperative staging of liver tumors will smaller section widths be necessary. The reconstruction increment RI can be chosen in a similar fashion as in spiral CT. For most routine applications, a moderate overlap of some 20% of the section width will suffice. For optimum quality of 3D reconstructions, at least a 50% overlap should be chosen unless the reconstruction increment is already as small as the pixel size. This will depend on the chosen field of view (see Table 4.4). For most body applications with an FOV of 30-40 cm, the pixel size is between 0.6 and O.S mm. Thus, a reconstruction increment of exactly the same size will yield an isotropic grid of data points. Note, however that this is not necessarily identical with isotropic data sampling (see also pp. 46). Chapter 4 gives an overview of recommended scan and reconstruction parameters for various clinical requests and various vendors (Tables 4.4-4.6). •

Secondary Raw Data Set

Whenever possible, a thin collimation should be preferred, because this will allow for creating a 3D data set of near isotropic resolution. We suggest calling such a set of overlapping axial images a secondary raw data set, because it can serve as the basis for reconstructing images in any desired plane and any desired section thickness using simple multiplanar reformatting (MPR)functions. Such a secondary raw data set typically consists of 0.5-1.5 mm-thick images reconstructed every 0.4 to 1 mm. The MPR function of the scanner or a workstation can be employed to create axial sections of arbitrary thickness and with arbitrary reconstruction increments. Such an approach can also be used with scanners that have no direct reconstruction of thicker sections from the raw data available (e.g., GE).ln addition, these sections can be anatomically adapted to compensate for improper positioning of the patient. Such a technique is especially helpful for symmetrical structures such as the inner ear or the cervical spine.

Vendor-specific Approaches In multislice CT, reconstruction algorithms are rather vendor-specific. All vendors use 2D fanbeam z- filtering algorithms for 4-slice scanners that neglect the cone-beam effect. These z-filtering variants largely determine which parameters the user may select and how. In the following paragraphs, the approach of the various vendors to 4-slice scanning is discussed. The approach to S- and 16-slice scanning is covered in chapter 4, optimization of scanning technique (p. 122, Table 4.6). General Electric Scanners

General Electric noticed that only at certain pitches the measurements from different detectors are relatively unique and complementary. From this the preferred helical pitch concept was developed, which made only two distinct pitch factors and corresponding section widths available. Scanning with P*=3 is called HQ (high quality) mode. while P*=6 is called HS (high speed) mode. The HO. mode provides good contrast resolution, low (cone beam) artifact levels and matches the quality of pitch 1 in single-slice CT. The HS mode is designed for high volume coverage and thin slices, comparable with pitch 2 in single-slice CT. For both pitch levels, image reconstruction is based on a variant of ISO° MLl. On GE scanners, the user chooses the section width SW (reconstruction slice thickness) first, depending on the clinical requirements. This choice is much as in single-slice helical CT. However, only multiples of the width of a single detector row (1.25 mm) are available (Table I.S). The user then has to decide whether to scan with 4 x 1.25 mm, or thicker collimation. Images can be reconstructed at arbitrary intervals, just as in conventional spiral CT. lfmultiplanar reformations are required, the data set can be reconstructed a second time with thinner section width. Consequently, quality of MPR is best if thin sections are chosen for acquisition. There is presently a constraint as to which combinations of section collimation and section width are actually available: for example, the largest section width that allows for using a 4 x 1.25 mm detector configuration is only 2.5 mm (Table I.S). This makes it necessary to use axial multiplanar reformations if section widths shall be reconstructed that are thicker than the maximum section width allowed by the scanner interface.

1 PrinciplesofCT, Table

1.8

Spiral CT, and Multislice CT

Selectable

Detector Configuration

options

in the GE Lightspeed

Available

table feed TF

system

(HU,

20(0)

Available

section

SW

interface)

(user

width

Measured

section

(FWHM') HQ_Mode2

width

HQ_Mode

HS_Mode2

4X 1.25mm

3.75mm

7.5mm

1.25 mm 2.5 mm

1.3mm 2.5mm

7.6mm 2.5mm

4x 2.S0mm

7.5mm

15.0mm

2.5mm 3.75mm

2.6mm 3.8mm S.Omm

3.2mm 3.8mm 5.0mm

3.9mm S.Omm 7.5 mm

S.Omm

7.5mm

5.0mm 7.Smm*

S.2mm 7.5 mm

2

S.Omm

4 x 3.75mm

11.25

mm

22.5mm

3.75mm S.Omm

4 x S.OOmm

15mm

30.0mm

IO.Omm* FWHM = full width at half maximum; section width SW is called face HQ = high quality, HS= high speed .• cnly available as 2 x 7.5 or 2 x 10 mm; with a special technique

1

10.0mm 'reconstruction

slice thickness'

HS_Mode2

NA 7.5mm 6.4mm 7.5mm 10.0mm on

user

inter-

2

For axial scans (step-and-shoot). raw data from multiple detectors can be averaged (1 i, 2i and 4i modes) before image reconstruction to reduce partial volume effects (e.g.. in the posterior fossa). Because of the lower pitch the patient dose (CTDlvoi. see Chapter 5) is twice as large for the HQ. mode compared to the HS mode if rnA settings are kept unchanged. Even if the scanner is allowed to change the rnA to provide equal image quality, HQ. requires about 50% more dose than HS.With the SmartHelical protocol. image noise can be reduced for both modes as compared to conventional helical CT. When switching from a GE single-slice scanner to amultislice unit, the patient dose increases dramatically if mAs settings are kept unchanged. The reason for this is the shorter scanner geometry, which yields a higher dose per mAs and problems with focal spot tracking. which were solved shortly after the first release of the LightSpeed scanner. For these reasons, a new user should not apply identical mAs settings but adjust the mAs downwardly to obtain a similar CTDlvoi with the two systems. Siemens Scanners

Siemens makes use of the adaptive array detector system and has developed an optimized fan beam reconstruction algorithm. called Adaptive Array Interpolation (AAI or SureView). This al-

2 x 0.625

mm is also available

gorithm ensures that the user-selected section width SW remains constant. independently of the chosen pitch factor (Fig. 1.25 c). Slice pitches P* between 2 and 8 are available for routine use. and even smaller values for cardiac CT. Available combinations of section collimation and section width are given in Table 1.9. In practice. pitch factors P* between 5 and 8 will be used in most situations. Siemens allows the user to prescribe multiple reconstruction protocols from one data set. thus speeding up workfiow if both a thick section for image review and thin sections for MPR and 3D imaging are required. To keep both image noise and patient exposure constant independent of the pitch (Fig.1.28c). the system raises the mAs settings automatically in proportion to the pitch factor. To make life easier for the user, the user interface on the scanner console does not display the real mAs but values for effective mAs, which are defined as mAseff= mAs I P = mAs x N I P*. These effective mAs correspond to the settings that would occur with a conventional step-and-shoot CT technique. The definition of mASeff can also be applied to other scanners and is valid even for conventional spiral CT. However. it also carries the risk that. should the user switch to a multislice scanner and apply the same mAs settings as on the single-slice unit. the change in definition of "mAs" on the

Scanning Parameters Table 1.9

Selectable options in the Siemens Volume loom; the Marconi MX 8000 is comparable

Detector

configuration 2 x O.5mm 4x 1.0mm 4x 2.5mm 4x Smm 2x8mm

FWHM

=

Available table feedTF

Available section width SW (FWHM)

0.5-2 mm 1.25-8 mm

0.5; 1.0; 3.0; 6.0; 8.0;

2.5-20mm 5-40mm 8-64mm

0.75; 1.0; 1,25; 1.5; 2.0 mm 1.25; 1.5; 2.0; 3.0; 4.0; 5.0; 6.0; 7.0; 8.0; 1O.0mm 4.0; 5.0; 6.0; 7.0; 8.0; 10.0 mm 7.0; 8.0; 10.0 mm 10.0 mm

full width at half maximum

multislice unit may not be accounted for and a substantially higher radiation dose would be applied. For example. a setting of 200 mAs at a pitch of 2 on a single-slice unit would correspond to a setting of 100 mAs(eff)on the multislice scanner. Keeping the 200 mAs setting would double the patient dose. Philips Scanners

Philips employs the same detectors as Siemens. The image reconstruction algorithm is termed Multislice Interpolation (MSlIMSSl). Exact details for this algorithm. however, are not yet public. User-definable options on the four-slice system are very similar to the Siemens system with collimations ofO.5-5.0mm, slice widths of 0.5-lO.0mm and pitch factors up to p~ 2 (see Table 1.9). Philips also employs the concept of effective mAs on most oftheir scanners. Toshiba

Scanners

Toshiba has developed a z-filtering fan-beam algorithm called multislice cone beam tomography reconstruction (MUSCOT) . Toshiba suggests preferred pitch values of P*~2.5, 3.0, 3.5, 4.5, 5.0, 5.5, 6.0, and 6.5. Toshiba prefers non-integer

Table

1.10

Selectable

options

in the Toshiba

Aquillion

Detector configuration

Available table feed TF

4xO.5mm 4 x 1.Omm 4X2.0mn 4x 3.0mm 4x4.0mm 4 x S.Omm 4x 8.0mm

1.2S-3mm 2.5-6 mm 5-12mm 7.5-18mm 10-24 mm 12.5-30 mm 20-48 mm

FWHM

=

full width at half maximum

values of P* to optimize sampling density and shift the position of the conjugate data. According to their specifications, however, best image quality is obtained with P*~3.0 and 5.5. The zfiltering is a multipoint interpolation using different filter widths (FW) that are automatically chosen by the system depending on which section width (SW) is selected by the user. The section width can be increased in 0.5 mm steps from a minimum identical to the collimation SC to a maximum of five times SC (Table 1.10). As with all other manufacturer, image noise is substantially increased for SW~SC. As with Siemens scanners, section width is independent of the pitch. Inpractice, apitchfactor P* ~ 5.5 (6.5 forCT A) will be used for most applications since it combines few multislice artifacts with a large coverage per time. For a constant rnA setting, patient dose decreases but image noise increases with larger pitch factors. For this reason, rnA settings have to be proportionately increased with larger pitch (constant effective mAs or CTDlvoi). Under such conditions of constant patient dose, noise is relatively independent of pitch.

Multi

"gAvailable

Section

width

SW (FWHM)

0.5-2.5 mm in 0.5 mm increments 1.0-5 mm in 0.5 mm increments 2-10 mm in 0.5 mm increments 3-15 mm in 0.5 mm increments 4-20 mm in 0.5 mm increments 5-20 mm in 0.5 mm increments 8-20 mm in 0.5 mm increments

7 Principles DICT, Spiral CT, and Multislice CT

Workflow, Image Review, Display and Documentation Multislice CT can be used as a technique for fast spiral scanning or as a true volumetric imaging modality (see also Chapter 4). With fast spiral scanning, thicker sections are reconstructed and viewed, very much like spiral CT. With volumetric imaging, a secondary raw data set of thin overlapping axial sections is created first. This data set is then used to reconstruct (anatomically adapted) thick axial sections as well as thick multiplanar reformations or various types of 3D images. The workflow of image processing, display, and documentation may change substantially. Fast Spiral Scanning

Fast spiral scanning is a technique that is excellently suited for routine imaging tasks. It provides at least equivalent, and often superior results to single-slice spiral CTwhile retaining the same basic imaging workflow. Display and documentation can remain similar to spiral CT (see above, p. 17). The thicker sections are used for making a diagnosis (preferably using cine displays for viewing) and can be printed on film. They are used to compare findings to older spiral-CT examinations, and are excellent for communicating findings to referring physicians. If there are remaining diagnostic problems, thin overlapping images can still be reconstructed (if thin-section scanning was performed) and further processing is done according to the volume imaging procedure described below. Volumetric

Imaging

Volume imaging takes full advantage of the three-dimensional capabilities of multislice CT. Reconstruction of a secondary raw data set of thin overlapping axial images gives the user full control over the available information contained in the data volume. However, such a secondary raw data set consists of several hundred images, depending on the covered scan range and the chosen reconstruction increment. This puts a lot of s train on image recons truction, image processing, data transfer, image reviewing techniques, archiving, and image demonstration. The imaging workflow has to be completely changed as compared to current scanners. With 4- and some 8-slice scanners, most of these

processes are not yet optimized because of substantial hardware and software constraints. As a consequence, the volumetric imaging mode will probably not be used as often as it should. Table 1.11 gives an overview of hardware and software requirements for optimum data handling when using a volumetric imaging technique withmultisliceCT. Image reconstmction from the raw data set can take a substantial time, even with sub-second reconstruction, if some 300-400 images are reconstructed for the chest or 400-500 images are reconstructed for one abdominal series. For this reason, very fast reconstruction (multiple images per second) is important to allow for a high patient throughput. Standard image reviewing is best performed on thick axial or multiplanar reformations that were reconstructed by the CT technician according to a predefined protocol that depends on the clinical imaging task (see Chapter 4, and the various organ chapters). The technicians can also perform various 3D reconstruction tasks according to specific protocols, such as curved planar reformations along the pancreatic duct, volume-rendered or MIP images of the vascular system, or exarticulated 3D views of joint surfaces (see Chapter 2). This allows for substantial time savings in a setting where imaging workstations are not real-time interactive. Interactive image reviewing is ideal if the imaging workstation is (near) real-time interactive. Mostworkstations on the market, however, suffer from substantial constraints because they are usually optimized for only a few specific processing tasks or they are too complex or too slow to be useful in a routine setting. Most important for interactive reviewing is a thick-section MPR mode that is truly real-time and allows for fast scrolling through the data set as well as for interactive change of the section thickness. Most software is real-time interactive for one-pixel-thin MPRs and only updates the image to the desired thick MPR as soon as the interaction is stopped. This may be sufficient for data sets obtained with high radiation exposure and thus little image noise. However, it is suboptimum for the vast majority of standard examinations, especially in the abdomen, where noise is very disturbing and can be reduced only by thick MPR. Interactive volume rendering is another option that gains increasing importance as a primary tool for image interpretation. Short data loading times, easy han-

Image Review, Disp/ayand Documentation Table 1.11

Hardware and software requirements for optimum workflow with multislice CT Volume imaging Minimum

Optimum

100-300

200-400

300-1200

> 1 image/sec > 10 patients 1 >6000

> 2 images/sec > 5 patients > 10.000

> 6 images/sec all pts. from one day > 40.000

CTworkstation RAM Hard disk" Volume accelerator board

512MB >40GB

1 GB >40GB

>2GB >80GB

Data transfer rate

100 Mbitlsec

Fast spiral scanning

Typical number of images Hardware

CTscanner Reconstruction speed Storage capacity of raw data Storage capacity for images2

100 Mbit/sec

+

III

;":

,1 1 111

/\ 1 Gbit/s

Software

CTscanner

Predefined scan protocols Predefined ~rocessing protocols (Thick) MPR MIP VRT

CTworkstation Real-timethick MPR Semi-automated segmentation MIP Volume rendering 1

2

3

depending on workflow. Raw data should remain available until it is decided whether additional thin sections are necessary depending on workflow. If further processing is done on the scanner, substantially larger storage capacities are necessary depending on workflow, Processing (MPR MIP, VRT) by the technicians can be done on the scanner or on a separate workstation

dling, proper opacity presets (see Table 2.4), and true real-time interaction are important for timeefficient image review and reporting. Processing workstations need to be equipped with sufficient computing power and memory. For multislice CT with several hundred images, the RAM should be more than 1 GB in size. For interactive volume rendering, accelerator boards are available that make real-time interaction possible (several frames per second). They differ with respect to the available matrix size for real-time interaction, and whether they allow for perspective rendering (important for virtual endoscopy) . Data networking may become another bottleneck. Fast networks are mandatory (at least IOOMbit/s, more for 16-slice units). Shared

databases and picture archiving and communication systems (PACS) for data archiving can help substantially to reduce data traffic due to multislice CT data sets. Multislice CT studies at present represent the largest data sets to be transferred over PACS networks. Transferring all necessary data (which may be well over 1000 images, if images are transferred to various workstations) from a multislice CT examination must not take longer than the time slot assigned to each patient on such a scanner. Appropriate planning prior to setting up a multislice CT system is essential to avoid clogged networks and suboptimum workflow. Image documentation will still require printing out of thick axial sections, especially when a patient is transferred to another institution. This

1 Principles oiCT, Spiral CT, and Multislice CT

is important as long as there are no generally

available standards for image reviewing outside of radiology departments in the offices of referring physicians. Most vendors now offer DICOM viewers that can be put on a CD-ROM together with the patient data. There is the additional

need to document critically important axial, multiplanar, or three-dimensional images on film or even on paper prints (color, black and white). Paper prints are generally well received by referring physicians because they can be more easily added to the patient record.

CardiacCT

Electron Beam CT (EBCT) is an established imaging modality for the noninvasive diagnosis of coronary artery disease (see also Chapter 23, Heart). Although cardiac CT is possible to some degree on single-slice or dual detector CT units (calcium scoring), only the introduction of multislice CT opened up cardiac CT for clinical practice. Multislice CT allows for morphologic imaging of the heart, calcium scoring, and coronary CTA. Phase-selective cardiac imaging adds the option offunctional heart studies. A high temporal resolution is needed to freeze cardiac motion and avoid artifacts. To eliminate breathing artifacts the complete heart should be scanned within one breath-hold. ECG synchronization of data is necessary to capture the heart in a (relative) motionless phase. EBCT uses non-mechanic beam rotation with acquisition times between 30 and 100 ms. Multislice CT is based on mechanically rotating CT units that are limited currently to 420-500 ms rotation time, but in the future may provide faster rotation of the order of 300ms. With current fourdetector-row systems with 0.5 s rotation time, a temporal resolution of 100-250 ms can be accomplished, depending on the heart rate, the

20

250-320

pitch, the rotation time and the segmental reconstruction capabilities. Even these systems are at their limit in terms of spatial and temporal resolution. These problems, however, probably will be solved with 16-slice cardiac scanners with faster rotation times and narrower section collimation which may reach a temporal resolution of 50-65 ms, comparable to current EBCT. Prospective ECG Triggering

Prospective ECG-triggering is used for sequential acquisition of four simultaneous sections. Temporal resolution is achieved by a partial scan technique. A prospective trigger is derived from the ECG to initiate scanning at a user-selectable time after the preceding R-wave or before the next R-peak. This delay is defined to coincide with the end-diastolic phase of the heart cycle. It is usually chosen between 40% and 80% of the RR interval. Due to limitations in scan cycle times (table movement), every other heart beat is used for data acquisition (Fig. 1.29). In order to

ms

15 c

o

:p

'is CL

10

N 5

Fig. 1.29 Prospective ECG gating. A partial scan is triggered by the R-wave of the ECG, Alternate heart beats are used for scanning and table incrementation, respectively.

Cardiac CT

capture the heart within one breath hold, in 4slice scanners a 4 x 2-3 mm collimation is employed, depending on the available detector configuration and the scan length, The table feed should be identical to the total collimation width (10-12 mm) or slightly less (6-10 mm) for overlapping sections, With 16-slice units collimation comes down to 1-2 mm with table feeds in the 12-32 mm range, Standard partial scan techniques require acquisition of data from half a rotation plus the fan angle of the x-ray beam, This will result in a temporal resolution of some 320-360 ms for a 0,5s rotation time, Some manufacturers offer optimized reconstruction that is achieved by rebinning the fan beam data to parallel beam geometry and using a reduced field of view of 240 mm or less, This results in an effective scan time of 50-55% of the tube rotation time, yielding a temporal resolution only slightly above 250ms, The exposure time should be less than 40% ofthe RR interval, i.e. the heart rate should be below 96 beats per minute (bpm) for 250 ms exposure time, and below 75 bpm for 320 ms exposure time, Administration of a beta blocker is advisable only ifthe heart rate is too high, and the diastole needs to be prolonged, Almost motion- free images of the heart are obtained up to aheart rate of70 bpm. Since all data are used for image reconstruction, no increase in radiation dose to the patient is necessary as compared to a conventional CT At identical noise levels, dose is similarto or smaller than thatofEBCT In 16-slice scanners, the decreased rotation times of 0.40-0.42 s results in routine temporal resolutions in the order of 80-200 rns, Also, the critical value of the heart rate for optimal temporal resolution has shifted to higher values (8085 bpm) , which is important in clinical scanning,

Retrospective

ECG Gating

In retrospective cardiac gating, a continuous spiral scan is acquired with simultaneous ECG-recording, In order to obtain enough projectional raw data during each part of the cardiac cycle, overs amp ling with a low pitch factor (P= 0,2-0.4) is required, The pitch is adapted to the heart rate, with higher pitch for higher heart rates, The oversampling will necessarily increase radiation exposure to the patient

The user can prospectively or retrospectively select a gate during the RR interval of the ECG for which data for image reconstruction are to be accessed, Gating can be performed with a relative approach (time delay as a fraction of the heart cycle) or with absolute approaches (either a fixed delay after the preceding R-peak or before the next R-peak). More sophisticated approaches take the relative duration of systole and diastole into account Overlapping reconstruction further improves image quality on MPR and 3D images, Retrospective gating reduces the sensitivity to cardiac arrhythmia, Multiphase reconstructions during various time points within the RR-interval allow for phase-selective or functional cardiac imaging, Single Sector Reconstruction

With single sector reconstruction, only data from the prescribed time range during one cardiac cycle are used for partial scan reconstruction of images, This yields a temporal resolution of about 250ms for 0,5s rotation time and 200 ms for 0.4 s rotation time, Images can be reconstructed using no interpolation at all by including only those data from the various detector rows that are closest to the desired plane of reconstruction, This works best for very low slice pitch factors P < 0,25 but, depending on the heart rate, pitch can be increased up to P = 0.4 without major artifacts (see below), Multislice Cardiac Volume Reconstruction (MSCV, Siemens) is an algorithm that combines partial scan reconstruction with multislice spiral weighting, For each projection angle in the multislice data set a two-point linear interpolation is performed between data acquired during the desired heart phase (Fig, 1.30), The z-range can be extended if nearest-neighbour instead of linear interpolation can be used, This algorithm produces a constant relation of the average slice width to the chosen slice collimation: SW -1.25 SC (lJ-L4 SC), Also, retrospective generation of thicker slices from the same dataset with SW= 1.5, 2 or 3 times the chosen collimation SC is possible, resulting in slices with lower noise and better low-contrast resolution at the expense of reduced z-rcsolution. Multisector

Reconstruction

Multisector reconstruction (e.g, ACV, Siemens or Snapshot Burst, GE) can increase the temporal

39

7 Principles orCT, Spiral CT, and Multislice

CT Fig. 1.30 Retrospective ECG gating

by multislice cardiac spiral interpolation. A two-point interpolation be-

15

c

o

:-e

tween corresponding projections acquired during the desired heart phase is performed. Oversampling with low pitch factors is a prerequisite.

10-

Vl

o

CL

N

Time

resolution of the above algorithm by using scan data from more than one heart cycle for image reconstruction (segmented reconstruction, Fig. 1.31a). The partial scan data set then consists of a number of projection sectors and a maximum temporal resolution of RT/2M (M = number of heart cycles used). For a 0.5 s 4slice scanner, a temporal resolution of up to 125ms can be reached with two sectors, and less than 80ms with up to four sectors. The more sectors that are used, the higher the overlap during data acquisition has to be. This requires lower pitches and results in an increased patient dose. The maximum number of sectors is limited by the number of detector rows. Due to phase-coherence between the tube rotation and the heart cycle, multiple sector reconstruction can only lead to improved temporal resolution if the patient's heart rate and the tube rotation are appropriately desynchronized so that the projection angles of the start and endpoints of each subsegment fit together and form a complete partial scan data segment. For data reconstruction using m sectors, the temporal resolution will depend on the heart rate and lie between RT/2 and RT/2 m. Temporal resolution will vary during the scan together with the patienf's heart rate. On a 4-slice scanner, the highest temporal resolution with 2-sector reconstruction is achieved for 65-70 bpm and 90-100 bpm, while 80 bpm leads to synchronization and therefore reduced temporal resolution. Spatial resolution will be decreased if data from two consecutive heart beats do not precisely match. At the same time, the diastole is long enough to allow for 250 ms reconstructions

for heart rates below 65. Thus, heart -rate-sensitive algorithms are being developed by most vendors, which use two or more sectors for higher heart rates and the conventional single sector reconstruction for slow rates (Fig. 1.31b). This gives an optimum compromise between temporal and spatial reconstruction. The relation between SW and SC remains fairly independent of heart rate: SW - 1.3 (± 0.2) SC. The first results with 16-slice scanners indicate that retrospective reconstruction techniques that neglect cone-beam geometry can be applied to the heart since it is sufficiently centered and does not contain very high contrast structures. Due to smaller section collimation (16xO.5-0.75mm) and improved axial sampling there is higher spatial resolution and less geometric artifact in coronary stents. Cone beam corrections, however, are able to further improve image quality. •

Pitch

Continuous volume coverage with single sector reconstruction requires limiting the pitch dependent on the heart rate. For optimum spiral weighting with two interpolation partners for every projection the following restriction applies: Pitch* < (N -1) x RT I (TRR + RToO (R'L-full rotation time: 500ms and RTA= partial scan rotation time" 360 ms). If faster volume coverage is needed and 50% of proj ections are generated with nearestneighbor interpolation, this restriction is reduced to P* < N x RT/TRR. Heart rates of 45-1 00 bpm correspond to pitch factors of P =0.3750.825 (PA 1.5-3.3 for 4-slice scanners) for 0.5s

Cardiac CT 41 Fig. 1.31 Retrospective EGG gating with multisector interpolation. The same phase of more than one heart cycle is used to collect data for interpolation, thus effectively improving the temporal resolution (a). This is only possible if data from each heart cycle provide different projection angles. Thus the temporal resolution varies with the heart rate (b).

125 ms 15

c

o

:e

~

10

Vl

o 0...

N 5

Time

a

600 -

500

Diastole Exposure time

400 Vl

Problematic Heart rates

E

300

/

I

-,

200 125 ms

100 Half scan reconstruction

Heart rate

o~--~--~--~--~--~--~--~--~--~--. 40 100 120 60 80

rotation time. Temporal resolution is in the order of 250 ms. If this type of interpolation is also used for muitisectorretrospective ECG-gating techniques, the pitch should be adapted as follows to the heart rate to achieve full coverage: P*£[l+(N-l)/mJxRT/TRR TRR is the maximum RR interval that is expected during the scan, N is the number of active detector rows, and m is the number of sectors for multisector reconstruction (m /\N) . However, fairly good results with this approach have only been seen with 4-slice scanners, which require a pitch of P= 0.2-0.375 (P *= 0.8-1. 5), depending on the heart rate. Lower pitches result in better image quality but are limited by the scan range that can be covered with the scanner. In practice, covering a 12cm scan range with 4x 1 mm collimation

within 40s on a 0.5s scanner requires a table feed of 1.5mmls (p*= 1.5; P= 0.375). Pitch limitations remain with 16-slice scanners but scanning speed increases substantially. In practice, the table speed (TF/RT, mm/s) is the limiting factor, which means that lower pitch factors have to be used with faster rotation times RT. Best results for 0.5-1 mm collimation are obtained with frequency-dependent multisector reconstruction and a table speed between 4 and 10 mmls, depending on collimation and manufacturer. •

ECG-gated

Reconstruction

with

Extended Coverage

Some clinical applications do not require complete suppression of motion artifacts but profit from reduced pulsation effects. Such applications include imaging of coronary artery

42

7 Principles

act. Spiral ct. and Multislice Acquired

projection

CT

data Half scan reconstruction

Fig. 1.32 Principle of CT fluoroscopy. Half scan reconstruction improves temporal resolution. Temporal overlap of images provides a fluoroscopic effect.

Overlapping half scan reconstruction

1.5

2

Tube revolutions

bypass grafts, cardiac morphology, or imaging of the central vessels. For such applications, ECGgating can be modified to allow faster scan speeds and reduced radiation exposure. This is done by excluding only those phases of the heart cycle from reconstruction that contain most motion artifacts (systole or phase of systolic pulse wave). The excluded time window ATs can be varied but usually some 250ms are used for this propose. The resulting temporal resolution is RT/2. The approach is phase-inconsistent but yields images of substantially better quality than a non-gated approach. The reconstructed image stacks should overlap in the zdirection giving a pitch limitation of: P* ss (N-l) -RTf (RTf2 + ATs). In the chest for ATs = 250ms, the resulting pitch P=0.75 (P'/\3.0) allows for covering 220mm in 36s with 4 x 1 mm collimation. Different values of ATs used for other applications will lead to different preferred pitch values. With 16-slice scanners, the whole chest and abdomen can be covered with such a technique in 16 x 1.5mm resolution. The technique is still a prototype application. •

Radiation

Exposure

Retrospective gating uses only a small portion of the acquired raw data for image reconstruction. As a result, the patient exposure required to obtain a given image quality increases substantially. The dose increases as the time window used for reconstruction (e.g., 125ms) gets shorter relative to the duration of the RR inter-

val (e.g., 750 ms for a heart rate of 80 bpm). This increase is larger for smaller pitch factors. The dose is usually increased by a factor of 3-4 as compared to ECG triggered techniques or nontriggered multislice scanning. Various dose reduction techniques are presently being developed by the manufacturers. By positioning the heart close to the center of the scan field (patient slightly shifted to the right of the examination table), the scan angle may be reduced, thus excluding the more peripheral portions of the chest. Such a technique will cause artifacts close to the periphery of the exposed scan field (see Fig. 7.45c) but these artifacts only involve the very periphery of this region and can be excluded from the display field of view. By choosing a smaller field of view « 24ern) centered on the heart such artifacts are no longer visible but the radiation dose to the skin and breasts is substantially reduced. ECG-controlled modulation ofthe tube output (ECG-pulsing) holds substantial potential for further dose reduction. With this technique, the rnA is substantially (80%) decreased during the (systolic) phases of the heart cycle that are usually not used for image reconstruction (see Fig. 23.15). Functional imaging, however, still remains possible because image quality generally suffices for volumetric assessment of the ventricles. Other new dose-reduction developments include edge-preserving image filtering and low-kvp scanning. In edge-preserving image filtering special postprocessing filters can reduce image noise while anatomical edges are pre-

CTF/uoroscopy

served. The technique may be' used to lower radiation exposure or to improve low-contrast resolution, alone or in combination with ECGpulsing. Scanning at 80 to 100kVp can increase contrast-enhancement in coronary CTA and coronary calcium density at the expense of a little increase in noise. The resulting dose reduction can be as high as 30%, while contrast-to-

noise ratios will be preserved. Reducing the scan field of view can be used to decrease the dose to outside the region of interest (heart). Similar effects can be generated by special bowtie filters placed behind the x-ray tube that reduce the radiation dose to the periphery of the body outside the region of the heart.

CT Fluoroscopy CT fluoroscopy is a technique that produces constant updates of the scanned data and therefore provides fluoroscopic cross-sectional images.

Principle and Image Reconstruction To gain a high temporal resolution despite amuch longer rotation time of the x-ray tube, images are generated in a temporarily overlapping fashion. For this purpose, partial scanning is employed. Data from onlyslightlymore thanhalfatube rotationare reconstructed (180 + fan angle). In addition, the 360 rotation is subdivided into sectors, and after the tube has scanned a new sector, the old one is dropped from the image and the new one is incorporated into the image. To speed up reconstruction, each sector may be reconstructed separately by filtered backprojection, and the final image is assembled simply by superimposing each sector. High refresh rates, up to 8 ormore images per second, are possible with this technique (Fig. 1.32). 0

0

Technique Fluoroscopy is employed for real-time monitoring of interventional procedures. A foot switch or table switch together with a monitor close to the gantry are necessary in order to follow the procedure. A moving table helps to find the region of interest or the needle tip interactively. However, it can sometimes be quite cumbersome to reposition the table if the needle leaves the scan plane because it is not always evident in which direction it does so. CT fl uoro scopyusing am ultislice scanner can be used to acquire multiple sections simultaneously and to see more easily in which direc-

tion a needle is deviating from the prescribed path. Some vendors offer the option to fuse the inner two sections (target plane), and simultaneously display and update the sections above and below this target plane (see Fig. 6.3). This way it is much easier to determine in which direction the needle or table have to be repositioned.

Radiation Safety When identical mAs settings are used as for diagnostic scanning, radiation dose to the scanned section increases substantially. As compared to a 1 s conventional section, a lOs fluoroscopy will cause 10 times the radiation exposure. For this reason, fluoroscopy always employs low mA settings, and-if possible-also low kVp settings. Too low an exposure, however, will substantially decrease image quality and may hamper the detection of the target lesion or a safe path towards the lesion. For this reason, the dose has to be adapted to the patient size (higher kVp settings in obese patients) and the organ region (higher mAs for the abdomen than for the lungs). Still, radiation dose in fluoroscopy is usually substantially higher than in a conventional CT-guided biopsy. Usually conventional step-and-shoot techniques may be almost as fast as fluoroscopyguided procedures, provided that there is a foot switch and display monitor in the examination room. Fluoroscopy should be reserved for difficult cases, in which conventional techniques are less safe or substantially slower (e.g., pulmonary nodules close to the diaphragm, difficult approaches in the abdomen). Protective gloves and special instrumentation (e.g., needle holders) should be used that protect the hands of the examiner from the radiation beam.

2 Image Processing and Display Techniques M. Prokop

"Data Volume

46

Cine Viewing

48

Multiplanar Reformations

__________________________ __________________________

Principle (p. 48) Artifacts (p. 51) Applications (p. 52) Image Analysis and Documentation

48

(p. 53)

Maximum Intensity Projection (MIP) and Minimum Intensity Projection (MinIP)

53

Principle (p.53) Thin-Slab MIP (p. 57) Minimum Intensity Projections (MinIP) (p. 57) Artifacts and Pitfalls (p. 59) Applications of MIP and MnlP (p. 60) 3D Surface

Rendering (Shaded Surface Display)

60

Principle (p. 60) Applications (p. 63) Volume Rendering Techniques

63

Principle (p. 63) Special Techniques (p. 67) Artifacts and Pitfalls (p. 67) Applications (p. 69) Segmentation Principle (p. 72) Automated Techniques, Applications (p. 74)

•... Computer-assisted

Vinual Endoscopy Principle (p. 78) Alternative Visualization Techniques Artifacts and Pitfalls (p. 81) Applications (p. 81)

.••...

72

Diagnosis (p. 74)

78 (p. 79)

46

2 Image Processing

and Display Techniques

Computed tomography is primarily a crosssectional imaging technique. Even with conventional scanning. a three-dimensional body region is mapped by CT but information along the patient axis (z-axis) is only available in discrete steps. With the advent of spiral and multislice scanning. CT has moved from a transaxial to a truly volumetric imaging tool. All techniques of two-dimensional (2D) and three-dimensional (3D) image processing rely on a set of individual axial sections acquired by CT scanning. These sections are stacked together in the computer to form a data volume that can be further processed and manipulated. Spiral and m ultislice CT have revolutionized data acquisition. With proper selection ofthe scan parameters. 2D and 3D images of exceptional quality can be generated. This contrasts with the relatively poor quality of many 2D and 3D reformatted images in conventional CT. With multislice CT data sets. near isotropic resolution becomes standard but images suffer from increased image noise. For this reason. new noise-suppression techniques have to be developed to take full advantage of the improved spatial resolution. There is a multitude of 2D and 3D processing techniques which are summarized in Table 2.1. Multiplanar reformations (MPR) are the most important 2D tools to create images in arbitrary. even curved planes through the volume. Maximum intensity projections (MIP) are popular for display of CT angiographic data sets. Shaded surface displays (SSD) have been used to visualize complex vascular anatomy and

Data •

Table 2.1

Overview

of selected

2D and 3D processing

tools 2D Display

Tools

Cine displays Multiplanar reformation Thick MPR Curved planar reformation Vessel

tracking

3D Display

Tools

Maximum intensfy projection Minimum intensity projection Shaded surface display Volume rendering technique S Tissue transition prqjection Raysum

MIP minlP/mlP

SSD VRT TTP

projection

Segmentation

Tools

Cutting functions Region growing Dilation/erosion Closing functions Removal of flying Watershed Automated Automated

MPR CPR

(removal of holes) pixels (floaters)

algorithms bone removal volume analysis

skeletal structures. Volume rendering techniques (VRT) are the most versatile rendering methods that allow for transparency as well as 3D effects. Interactive manipulation of data sets becomes more and more important the larger the data volumes become and the more complex and subtle the diagnostic questions to be answered.

Volume

Voxel-based vs. Grid-based Models

In conventional CT the data volume is subdivided into volume elements (voxels) in which the voxel cross-section (in the axial x-y-plane) equals the pixel cross-section. while the voxel height (along the z-axis) is defined by the section collimation (Fig. 2.1 a). With spiral CT. and with some conventional CT protocols as well. overlap of sections becomes possible. In addition. the section sensitivity profile is bellshaped. and a voxel may actually contain information from adjacent voxels. This makes a voxel-based model of 3D image data sets hard to handle: voxels no longer have sharply defined

upper and lower boundaries (due to the section profile). and voxels overlap substantially along the z-axis. Thus. a grid-based model is more helpful: here the center of each former voxel is used to define a 3D grid. The spacing along the z-axis then corresponds to the reconstruction increment. and the spacing in the x-y-plane corresponds to the pixel size (Fig. 2.1 b). Each grid intersection is assigned the CT number of the corresponding pixel in an axial CT image. 3D Sampling Function

The CT number samples information from the surrounding volume according to a sampling

Oata Volume

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Overlapping sections SW = 3mm, RI = 1.5mm

Grid of center points superimposed over voxels

Grid-based model with sampling function (PSF)

Fig. 2.1 Representations of a 3D data volume. The voxel-based model relies on box-shaped voxels (a) but has a problem dealing with overlapping image reconstruction and non-rectangular section profiles. A grid-

based model (b) assigns a node to the center of each voxel. Each node samples information along the z-axis (according to the section sensitivity profile) and within the scan plane (according to the point spread function).

function defined by the section profile in the zdirection and by the point spread function in the x-y-plane. This sampling function can be directly visualized by scanning a tiny lead bead and looking at the resulting image in the x-yplane anel the x-z- or y-z-plane (Fig. 2.2). As can be seen, the sampling function is not a voxel-

shaped box but instead an ellipsoid with a short axis that is determined by the spatial resolution in the x-y-plane (i.e., by the convolution kernel) and a long axis that is determined by the section profile. Fig. 2.2 b demonstrates that the sampling function is spherical for 1 mrn collimation, i.e., there is no distortion along the z-axis with

a

b

Fig.2.2 Sampling function (point spread function). The information sampled by each grid point in the data volume can be visualized by scanning a tiny lead bead (represented as a red dot in these images). Note that there is already blurring within the scan plane (x-y-

c plane) (a). Longitudinal distortion occurs along the zaxis for SC/TF = 3/4 (b). Resolution is isotropic with SCfTF = 1/1. The distortion effects along the z-axis are identical to those in the x-y-plane (c).

47

•

2 Image Processing and Display Techniques

the results that there is isotropic resolution with such a scanning protocol. Note, however, that results change if a different convolution kernel (e.g.) a high resolution algorithm is used, Secondary

Raw Data Set

The more isotropic the data set, the better the resulting image quality of 2D and 3D reconstructions, For multislice reconstruction of a

cr

secondary raw data set consisting of thin overlapping axial sections forms the basis of nearisotropic imaging, Image noise, however, may pose a problem with very thin sections, Increased noise may introduce artifacts in various rendering protocols and therefore should be kept low by using a sufficient radiation dose, a smoothing convolution kernel, or by applying noise-reducing prefiltering techniques,

Cine Viewing Cine viewing is an excellent tool for evaluating large numbers of sections, be they axial or multiplanar. It is important that the user has full control of the direction and speed of such cine displays, Such mouse- or trackball-controlled scrolling can be further improved if the system allows for fast scrolling through large ranges as well as finely controlled scrolling though just a few sections of interest It has been demonstrated that interactive cine viewing speeds up the evaluation of large data sets, allows for more accurate assessment of complex structures that cross the sectional plane multiple times (e.g. vessels or bowel loops), improves lesion detection (e.g. in the lungs or the liver), and increases productivity in picture archiving and communication systems

lanar

Pitfalls

In the initial phase, when moving from film reading to interactive cine viewing, there may be an increased risk of missing findings, The reason is that film reading is more patternbased than interactive viewing: any finding that lies outside the normal will be detected by an experienced viewer, even if it is located in usual sites (e.g, , cutaneous metastases), With cine viewing, the radiologist has to evaluate each organ system at a time, which means that many radiologists will have to change their normal search pattern.

Reformations

Principle Multiplanar reformations (MPRs) are two-dimensional reformatted images that are reconstructed secondarily in arbitrary planes from the stack of axial image data (Fig, 2,3), Coronal or sagittal reformations are generated by extracting and displaying only those voxels from the data volume that are positioned one above the other within the coronal or sagittal plane, Oblique or curved reformations are constructed in an analogous fashion, but the image data must be interpolated between adjacent voxels. Defining

(PACS) environments, Cine viewing becomes the primary mode of evaluation with multislice CT because of the large number of images to be viewed,

the Plane of Image Reformation

The image plane for a MPR is defined interactively on a workstation using a suitable refer-

ence image, which may consist of an axial image, another MPR, or a 3D image such as a maximum intensity projection (MIP) or a shaded surface display (SSD), By drawing a "cut line" on the reference image, the radiologist defines a section that is perpendicular to the reference image, The cut line may be drawn in any direction (orthogonal, oblique, or curved), Curved-planar reformations (CPR) are generally needed to depict structures that pass through multiple axial planes of section (e.g, a bronchus or blood vessel), In these cases it is necessary to have a workstation that allows the user to define the reformation plane on multiple reference images because the structure of interest usually leaves and enters the reference images repeatedly, For depicting tubular struc-

MPR-Principle

coronal sagitta

Fig.2.3 Principle of multiplanar reformation The images are reconstructed from a "stack" slices.

(MPR). of axial

tures, software is currently available that provides semiautomatic positioning of the cut line and keeps the line centered on the structure of interest (Fig. 2.4). Ideally only a few reference points have to be dropped on the structure of interest (usually a vessel), and the software should automatically generate a centered path through these regions {vessel tracking}. •

Thick MPR

As a rule, MPRs have a width equal to one voxel. Averaging a series of adjacent data points perpendicular to the reformatted plane can produce reformations with a greater section thickness (thick MPR). This is advantageous because it leads to reduced image noise and improved image quality (Fig. 2.5). This is an efficient tech-

nique to keep image noise low without losing in-plane spatial resolution, especially with thinsection multislice CT data sets. As with MRI, high-quality sections of arbitrary angulation can be obtained with a section width of 1-7 mm, depending on the clinical indication. The effect of noise-reduction by creating thicker sections is direction-dependent. Along the z-axis, image noise is highly correlated: even with highly overlapping reconstruction, the CT numbers on adjacent sections will not change much if a large section collimation was used. This will result in a suboptimal noise suppression along the z-axis when thick axial MPR are reconstructed (Fig. 2.6). In the axial (x-y) plane, image noise is hardly correlated between adjacent pixels unless very small fields-of-view have been used for image reconstruction. Thus, CT numbers in adjacent pixels change due to variations in noise. On MPRs perpendicular to the axial plane, noise therefore cancels out rapidly if multiple pixels are averaged and thicker MPRs are created (Fig.2.6c). With thin collimation, however, the difference between axial and coronal or sagittal reformations becomes smaller because correlation of noise along the z-axis decreases substantially. In summary, the image quality in coronal or sagittal reformations (perpendicular to the axial scan plane) can be greatly improved by increasing the section thickness to several millimeters. Thick axial reformations (parallel to the scan plane) can only be recommended when the CT data has been acquired with thin sections. Thick axial reformations are therefore mainly used in multislice CT. Note that excessive section thickness in the MPR leads to troublesome partial volume effects.

Fig. 2.4 Vessel tracking uses seed points that are dropped on a vessel (e.g., using MPR or MIP) to find the center of the vascular structure that connects these points (a). The resulting image can then be rotated around this centerline, allowing for detailed analysis of the vessel wall (b).

a

b

50

2 Image Processing and Display Techniques

a Fig. 2.5 If image noise is high (especially for thin-sectioruimaqinq) .the quality of MPRcan be improved by increasing the width of the section: (a) 1 pixel wide (0.6mm). (b) 5 pixels wide (3 mm) coronal sections from a 4 x 1 mm multislice CT data set. Note that low-

a

b

contrast resolution improves and the gastric folds can be better delineated (arrow) while the partial volume effects through the imaging plane also increase and small detail may vanish (arrow head).

c

Fig. 2.6 Thick axial MPR. Because section width (zaxis) is usually larger then the pixel size (x-y-plane). axial sections have to be thicker than coronal sections to efficiently reduce image noise unless very thin collima-

tion has been used. Primary 3 mm thick axial section from a 4x 2.5 mm multislice CT data set (a). Image noise on a 7.5 mm thick axial MPR (b), is identical to noise on a 3 mm thick coronal MPR (c).

•

Summed projections from an imaging volume in which the bony structures were removed result in DSA-like images. This effect can be enhanced if a threshold value above the softtissue range (e.g., > 80-100 HU) is used as well.

Ray-Sum Projection

For a ray-sum projection, the CT numbers encountered in the viewing direction are averaged or added together. Thus, a simple ray-sum projection is identical to a (very) thick MPR. The result simulates a conventional radiograph of the selected imaging volume (Fig. 2.7). Ifa threshold range is preassigned, only voxels with CT numbers within that range are averaged.

MPR-Arti facts 51

Fig. 2.7 Ray-sum projection (SO mm thick MPR) of the C02-filled colon (CT colonography) in a patient with Crohn's disease.

Artifacts

The quality of MPR images is best in imaging planes that deviate little from the scan plane. At the same time, the resolution along the z-axis affects all sections that are perpendicular to the scan plane. The smaller the section width and

the smaller the reconstruction interval, the better the image quality (Fig. 2.8). Section-by-section scanning and large section thickness in conventional CT generally lead to stair-step artifacts in reconstructions that are perpendicular to the axial plane. Spiral or multislice CT are much less vulnerable to such step artifacts because of overlapping image reconstruction. Good-quality MPRs require an overlap of some 30-50% of the section width. Thin collimation (for scanning) and thin section width (for data reconstruction) yield reformatted images of outstanding quality in any plane of section desired. Thicker collimation leads to loss of sharpness along the z-axis (Fig. 2.8). For objects outside the center of the gantry or interfaces that are oriented obliquely but almost parallel to the scan plane (such as the calvaria, tibia plateau or wrist bones) step artifacts and serrations of contours may also occur with spiral or multislice CT data sets if larger pitch factors are used (Fig. 2.9). These artifacts can be caused by the data interpolation process and by undersampling of data in z-direction.

Fig. 2.S Using thinner collimation (e.g.,4x1 mm instead of 4 x 2.5 mm) improves the quality of reformatted images, (a) SW/RI=3/2.5, (b)SW/RI =1.25/ 0.7. Note the improved delineation ofthe choledochal wall.

a

b

a

b

Fig. 2.9 Step artifacts and serration artifacts occur in multislice CT data sets at high-contrast interfaces oblique to the scan plane if higher pitch factors are used. MPR of the wrist bones scanned with 2 x 0.5 mm collimation at a pitch P= 1.5 (a) and ata pitch ofO.S (b).

2 Image Processing and Display Techniques

Applications

Multiplanar reformations are seldom worthwhile in conventional CT, because generally the image quality is too poor to yield substantial additional information. Spiral and multislice CT, however, open up many new potential applications for multiplanar imaging. The fact that MPRs have been underutilized in the past was due primarily to a lack of user-friendly software. •

Anatomically

Corrected

Data Sets

MPRs can compensate for faulty positioning and can provide symmetrical images for improved side-to-side comparisons (of the inner ear, neck, pelvis or shoulders, for example). Especially for coronal reformations, the MPR should be positioned parallel to the most important anatomic structures (e.g., the posterior ribs or the pelvis) to improve anatomical orientation. Tilted MPR may be helpful for the chest (parallel to the trachea or parallel to the sternum) or for the abdomen (parallel to the abdominal aorta). Appropriate tilting will ensure that important anatomic detail are sectioned longitudinally. In general, similar cut planes can be used as in MRI. •

Problem-solving

MPRs can be used as a problem-solving tool in cases where it is necessary to trace pathologic structures through multiple planes and in cases

a Fig. 2.10 Examples of applications of MPR: (a) The relation of this tumor recurrence to the muscles of pelvic floor is better appreciated on the coronal MPR.

that require imaging in a second plane. In spiral CT this applies consistently to skeletal investigations, frequently to CT angiography, and occasionally to investigations of tumors involving the liver, kidneys, or lower abdominal organs (Fig.2.10a). With thin-section multislice CT the role ofMPR is vastly increased. MPRs are important for defining the relation of focal lung lesions to the pulmonary fissures, chest wall and mediastinal structures, for assessing lymph nodes in all three planes, for defining tumor infiltration into adjacent structures, and for evaluating the bowel and structures of the small pelvis. Curved planar reformations are essential for delineation of soft plaques in CTA (see Fig. 2.4b) , evaluation of the pancreatic duct in one image (Fig.2.1Cb), or for the display and differentiation of ureteral stones from paraureteral calcifications. •

Noise Reduction

in Standard

Displays

Thick MPRs can be employed as the most important technique for noise reduction when thin collimation has been used to acquire a data set (see Fig. 2.5). The technique can be applied to spiral CT as well as to multislice CT data. Thick axial MPR can substitute for reconstructing thick axial sections directly from the raw data set. Thick MPR are a way to increase image quality substantially in low-dose applications of multislice CT (see Figs. 5.16-5.18).

b (b) Excellent overview ofthe pancreatic duct in a patient with chronic pancreatitis and stents in the pancreatic and common bile ducts.

MIPIMinlP-Principle

Image Analysis and Documentation

When MPR is used selectively to answer a question that cannot be resolved in the axial plane, it is usually best to perform the reformation interactively on a workstation. Software should be used that provides a "soft" transition between the reformatted sections when the imaging plane is changed. Abrupt transitions may cause lesions to be missed. In situations where it is important to document one or more additional planes, such as sagittal and coronal reformatted images of the calcaneus, a standard protocol can be established for computing and documenting the sections. To ensure the best image quality, the width of the MPR should be appropriate for the region under study (Table 2.2).

Table 2.2 Recommended section width (SW) of axial, coronal, and sagittal reformations Suggested width (SW) of MPR (mm)

I:Region

Neck Routine Larynx Chest Routine Bronchial system Peripheral tumors Chest wall Mediastinum

1.5/10 1.5 3

1.5 5

3

5

3-4 3 3-4 3-4 3-4

3-5 3-5 3-5 3-5 3-5

1

1 3 3 1 0.75

Abdomen 1 1

I 1

I. 1

Routine Liver (tumors):' Pancreas (tumors)" Kidne¥s (turners)" Bowel Small pelvis"

c

iMusculoskeletal system Cervical spine Thoracolumbar spine" I' Pelvis3 I., Foot Ifc Hand!

I

2 2

0.75 0.75

I ® depending on patient size

Maximum Intensity Projection (MIP)/Minimum Principle

Maximum intensity projection (MIP) and minimum intensity projection (MinIP or mIP) are volume-rendering techniques in which suitable editing methods are used to define the volume of interest (VOl). All of the CT image data set may be used, or the volume may be confined to a region of interest. In the most difficult case, only selected organ systems are included or excluded from the VOL The actual images are generated by projecting the volume of interest into a viewing plane and displaying the maximum CT numbers (for MIP) or the minimum CT numbers (for MinIP) that are encountered along the direction of the proj ection, called the viewing angle (Fig. 2.11). Both techniques ensure that optimum contrast

Intensity Projection (MinIP)

is produced between small, high-contrast structures and surrounding tissues. MIP views are used for CT angiography and for specialized pulmonary studies, while MinIP views are used mainly for visualizing the central tracheobronchial system. The following discussion of the use ofMIP in CT angiography also applies to other MIP applications and to MinIP views. •

Attenuation

Information

MIP preserves the attenuation information of structures with maximum CT numbers, so an anteroposterior viewing angle in most body regions would display skeletal structures rather than contrast-enhanced vessels. Thus, unlike MR angiography, some view angles in CTA make

53

54

2 Image Processing and Display Techniques Fig. 2.11 Principle of the maximum intensity projection (MIP). This technique displays the maximum CT numbers that are encountered along the view angle of the projection. Vascular imaging requires editing of the data volume to eliminate bony structures.

Projected VOl

Viewing direction

it necessary to edi t bony structures out of the CT image. This editing process may be done manually, semiautomatically, or automatically, depending on available workstation software (see Segmentation, p. 73).

a Fig. 2.12 MIP of the abdominal aorta, (a) When the projected volume encompasses the whole body, all vessels are superimposed over each other. Calcifications are well displayed, but thrombi are not directly visualized. There are linear artifacts in the periphery of the image (due to variations in image noise related to spiral

A major advantage of the MIP view is that contrast -enhanced vessels can be distinguished from wall calcifications by their different attenuations (Fig.2.12a). Even small vessels (1 mm or less in diameter) will be visualized as

b interpolation), (b) Reducing the width of the VOl (curved thin slab MIP) improves the contrast between small vessels and the background. Superimposing mesenteric vessels are removed and the thrombus becomes visible. Note that there are small bilateral renal artery aneurysms.

M/P-Pr'incip/e

long as they have a higher CT number than their surroundings within the VOL •

Image Background

The image background depends on the voxels with the highest CT numbers in the structures surrounding the vessels. If the surroundings are homogeneous, the MIP will display voxels that have the highest CT numbers based on statistical fluctuations (i.e., image noise). Because of this, an increase in image noise in the original data set is associated with an increase in background attenuation in MIP images. Arterial vessels generally have heterogeneous surroundings consisting of fat, unenhanced soft tissues, contrast -enhanced organs, and venous vessels. As the CT number of the surrounding structures increases, so does the background attenuation. It is therefore important to exclude as much contrast -enhancing surrounding structures from the VOl as possible in order to optimize vessel-to-background contrast (Fig.2.12). •

a

b

c

Image Contrast

Image contrast in MIP images is determined both by the CT numbers of the vessels of interest and by the difference between these CT numbers and the background attenuation. Accordingly, MIP image contrast increases with the level of intravascular contrast, and it decreases as a result of partial volume effects (which reduce the attenuation of small in-plane vessels) and as a result of higher background attenuation. For the evaluation of small blood vessels such as the renal arteries, the scans should be performed with a thin collimation, optimum intravascular contrast should be achieved, and background attenuation should be kept as low as possible. Background attenuation can be minimized by selecting a thin VOl in the direction of the projection and by eliminating all higher-attenuating superimposed structures such as contrast -enhanced organs or venous vessels by proper segmentation techniques.

Fig. 2.13 Image noise on MIP. (a) Axial section after bone removal in a very obese patient, (b) Noise in small vessels remains unchanged but noise in the aorta and the background is reduced on MIP images (here, 1 cm slab width), (c) Background attenuation on MIP images increases and the background noise decreases as the projected VOl becomes wider (here, 5 cm slab width)

IFor optimum image quality in the MIP (orl /\MinlP) view, the volume of interest (VOl) should be as thin as possible (Figs. 2.12 and g2.13). •

Viewing Angle

Given the different degrees of spatial resolution within the scan plane and along the z-axis, better image quality is obtained with axial viewing angles (i.e., nearly parallel to the z-axis) than with perpendicular view angles (e.g., anteroposterior or lateral).

2 Image Processing and Display Techniques

Where feasible, it is advantageous to render the MIP in a craniocaudal or slightly oblique direction. For evaluation of the renal arteries, the projection can be limited to the few sections that actually contain the vessels. An oblique projection can separate the aorta from the vertebral column, thereby eliminating superimposed structures and avoiding the need for a more complex segmentation process (see Fig.24.75a). •

Cine Loop

A single MIP conveys only two-dimensional information; it does not encode depth relationships. Consequently the foreground and background are not differentiated (Fig.2.12a), and a superimposed structure can obscure a less attenuating (usually smaller) vessel. Generating multiple images at constant angular intervals and viewing the images from various perspectives in a cine loop can improve 3D orientation. •

Image Noise

Image noise in the axial data set will propagate to MIP images in a more complex fashion than noise in thick MPR. In a homogenous region, noise leads to random deviation of CT numbers from the "real" CT attenuation is this region. MIPs display the pixels with maximum CT num-

a

b

Fig. 2.14 Noise-reduced MIP. High image noise (due to an high resolution filter kernel) on the MPR (a) leads to MIP of suboptimum quality with losss of bony detail (c).

bers, i.e., those pixels with maximum deviation from the real CT numbers. Because of noise, the CT numbers on MIP images of a homogenous region grow, the wider this region is (and thus, the more pixels are traversed). The bell-shaped histogram of CT numbers in a homogenous region indicates that the probability of encountering a higher CT number decreases the more this number deviates from the real attenuation. Thus, the wider the homogenous region that is projected, the smaller the resulting image noise on a MIP. If only a small homogenous region (such as a small vessel) is projected, image noise is largely unaffected by MIP. • Noise in small vessels remains unchanged I but background noise is reduced on MIP images (Fig. 2.13b). • Background attenuation on MIP images increases and the background noise decreases as the projected VOl becomes wider (Fig. 2.13c). • Contrast between small vessels and the background is improved if a thin slab is used (Fig. 2.12). Only if noise on the original image is excessive, such as for low dose applications of multislice CT, image noise on MIP images remains high

c

d

Reconstructing 3mm thick coronal sections (b) and using those for MIP substantially improves image quality (d).

Minimum Intensity Projections (MinIP)

and may obscure diagnostically relevant information. In such a situation, we suggest improving image quality by creating an overlapping set of thick MPR first (in the same orientation as the final MIP; e.g., coronal MPR for anterior-posterior MIP), and then reconstructing an MIP from this noise-reduced data set. Such a technique (noise-reduced MIP, nr- MIP) is excellently suited for large vessels but may reduce the contrast of small structures (Fig. 2.14). Thin-Slab MIP

As noted above, a thin VOl can improve the quality of the MIP image. In a technique called thin-slab MIP, this is accomplished by dividing the imaging volume into multiple thinner subvolumes, called "slabs," that have a specified thickness ranging from a few millimeters to several centimeters. A maximum intensity projection is then performed on each slab. An axial view angle is preferred with spiral CT because of better spatial resolution. In multislice CT, thinslab MIPs in arbitrary planes become feasible. By generating overlapping thin-slab MIPs and surveying the images interactively [sliding thin-slab MIP) one can improve spatial orientation and can cover the whole scan range. Comparing a thin-slab MIP view of designated thickness with an axial section or MPR of the same thickness, we note that the spatial resolution is identical. Small structures of high contrast (e.g., pulmonary vessels) decrease in contrast with increasing width of the axial section or MPR as a result of partial volume effects, but the contrast on MIP images remains fairly constant regardless of the width of the slab used for the MIP (Fig. 2.15). Minimum Intensity Projections (MinIP)

The imaging characteristics and artifact behavior of minimum intensity projections (MinIPs) are analogous to those of MIPs. They will be discussed for imaging of the tracheobronchial system. As a rule, only the central portions of the tracheobronchial system can be evaluated with this technique.

b

a

Fig.2.15 Comparison of a thin slab MIP (a) and a thick CPR (b) each of 10 mm width. Note the better contrast with MIP and the lowernoisewith MPR.

•

Image Contrast,

Noise and VOl

The greater the attenuation difference between air- filled bronchi and surrounding tissues, the more clearly the bronchi will be visualized. While MinIPs will clearly define bronchi that are surrounded by structures of soft -tissue density (mediastinum or consolidated lung), they will rarely delineate bronchi that are surrounded by aerated lung parenchyma or emphysematous lung (Figs. 2.16 and 2.17). MinIP created from high resolution data sets will suffer from the increased noise, which will cause a reduction in background density (similar to the increase in background density with MIP) and will obscure bronchi. This effect increases with wider VOl (Fig. 2.16) As a rule, intrapulmonary bronchi or bronchi obscured by superimposed lung parenchyma are depicted only if they have an attenuation value of approximately -1000 HU. The attenuation values of smaller bronchi are increased due to partial volume effects. Especially when oriented roughly parallel to the scan plane, small and even medium-size bronchi will not be visualized in sections that are more than 3 mm thick. The thinner the sections, the better the image quality.

57

2 Image Processing and Display Techniques

a

c

b

Fig. 2.16 Minimum intensity projection (MinIP). (a) A narrow VOl (1 cm) in this patient with pulmonary fibrosis and centrilobular emphysema displays the central bronchi and the distribution of the small bullae to good advantage. Note that the fibrotic changes cannot be securely detected, (b) Increasing the width to 3cm sub-

a

b

Fig. 2.17 Comparison of MPR and MinlP in a patient with a tracheal diverticulum (arrow), multiple tracheobronchial polyps (arrowheads), mucus in the trachea (wide arrow) and marked emphysema, (a) 1 pixel thick MPR (b) 10mm thick MPR (c) 10mm thick MinlP, (d)

•

Imaging Volume and Viewing Angle

A basic concern in the segmentation ofMinlPs is to eliminate superimposing air from the imaging volume. This air may be extracorporeal or be any air present in soft tissues or in a pneumothorax. Otherwise the extremely low attenuation value of the air will obscure the bronchi.

stantially decreases the distinction of bullae and the display of the upper lobe bronchi, (c) Using a noisy highresolution data set for MinlPdecreases background density and makes evaluation of the lung parenchyma virtually impossible.

c

d 3 cm thick MiniP. Note that the polyps and the mucus are easily missed on MinlP, and that the emphysema makes evaluation of more peripheral bronchi impqssible.

When an anteroposterior viewing angle is used, the imaging volume should be selected so that the central portions of the tracheobronchial system are displayed while the VOl is kept as thin as possible (Figs. 2.16 and 2.17). An attempt should be made to include a minimal amount of paramediastinal lung tissue in the VOl, as this would cause an unacceptable reduction in bronchial contrast.

MIPIMinlP-Artifacts and Pitfalls

When a craniocaudal (axial) viewing angle is used, a somewhat broader imaging volume (2-5 ern) can be selected. Any greater thickness of the VOl would generally lead to excessive superimposition of bronchial structures. A lateral viewing angle is rarely feasible. Artifacts and Pitfalls •

CT Angiography

If the selected VOl is too wide or if surrounding structures of higher attenuation are present, smaller vessels with CT numbers less than or equal to the background attenuation may be missed. This is often a result of partial volume effects or insufficient arterial contrast. Intravascular lesions such as mural thrombi or soft plaques often cannot be directly visualized with MIP. However, partial volume effects between a soft plaque and the opacified vessel lumen can cause an apparent decrease in luminal attenuation in the affected region. Intimal flaps due to dissection can be detected only if they are precisely parallel to the MIP viewing angle (Fig. 2.18a, b). If the true lumen and false lumen show different degrees of contrast enhancement, the width of the higher attenuating lumen will generally be overestimated unless the projection is parallel to the intimal flap. Calcifications have a higher attenuation and therefore superimpose on vascular structures. They are optimally detected on MIP images. At the same time it may become impossible to estimate the degree of a stenosis caused by a hard plaque (Figs. 2.18c.d). For optimum evaluation

of stenoses caused by hard plaques, curved planar reformations are necessary. •

Pulmonary MinlP and MIP

Contour irregularities can be appreciated only when they are defined in a profile view. Thus, minimum intensity projections tend to underestimate the extent of endobronchial lesions, or miss them entirely if they are small, due to the obscuring effect of superimposed endobronchial air (Fig. 2.17). Consequently, MinIPs are not useful for the detection of intraluminal tumors. Vascular pulsations transmitted to the lung tissue and respiratory artifacts can cause areas of low attenuation to appear at the point of vascular branching. This creates multiple focal hypoattenuating lesions on MinIP images, especially in the retrocardiac space, that may simulate emphysema. Pulsation of the retrocardiac vessels can lead to artifactual beading of the vessels on pulmonary MIP, even when individual sections are reconstructed in an overlapping fashion. When relatively thick slabs are used for pulmonary MIP, there may be rare cases in which small structures (e.g., pulmonary nodules) are obscured by larger ones (e.g., pulmonary vessels). If the selected imaging volume is too broad, MinIP can only visualize the central potions of the bronchial system (Fig.2.17 d). Failure to eliminate extracorporeal air from the imaging volume results in "black images".

Fig.2.18 Artifacts in CT angiography. (a, b) Intimal flaps in aortic dissection are only displayed when the flap is parallel to the projecting direction, (c. d) Calcifications may superimpose on vascular structures and can make the evaluation of stenoses impossible. Compare MIP (c) to CPR (d).

a

b

c

d

59

2 Image Processing and Display Techniques

a

~ b

Fig. 2.19 Sliding thin-slab MIP for nodule detection. Compare (a) 1.25 mm thick axial section, (b) 5 mm thick axial section, and (c) 5 mm thick MIP. Note that the high

Applications of MIP and MinlP

MIPs are mainly used to present findings of CT angiography. They are best suited for visualizing abdominal arteries (see Fig. 2.12) and the pelvic and lower limb vessels. MIPs are useful for thoracic studies only if the anatomic relationships in the imaging volume are relatively simple. MIPs are not recommended for the evaluation of complex vascular malformations, aortic dissections, central pulmonary emboli, or free- floating clots. Thin-slab MIPs are useful for imaging small pulmonary lesions in cases where thin collimation has been used for data acquisition. They improve the detection of small pulmonary nodules (Fig. 2.19). Diffuse lung diseases can be detected more accurately and at an earlier stage

130 Surface

c contrast of the 1.25 mm thick section is retained in the MIP while differentiation between vessels and nodules is improved.

than with other methods. The images combine the high contrast of a thin section with the superior anatomic orientation of a thicker section. Minimum intensity projections are used in evaluations of the central tracheobronchial system. They may be useful for the localization ofextrabronchial air collections or bronchial abnormalities (Fig. 2.17) and for demonstrating strictures, concentric stenoses, and dilatations, but MinIPs have very limited applications in the detection of tumor-associated changes. Minimum intensity proj ections are a sensitive tool to disp lay parenchymal density, in particular obstructive pulmonary disease and emphysema (Fig. 2.16). Thin-slab MinIP can be applied for evaluation of the intra.hepa.tic bile ducts and the pan-

creatic duct.

(Shaded Surface Display)

Principle

A shaded surface display (SSD) is a surface-rendered image that provides a realistically looking three-dimensional view of the surface of a structure of interest within the acquired volume data set. When creating a SSD, it is first necessary to define the "3D object" of interest, such as the

bony pelvis or abdominal aorta. This process of separating the object from the background, called segmentation, may be quite simple or extremely difficult depending on the object contrast. The simplest segmentation process is to define a 3D object by selecting a suitable range of CT numbers (e.g., all voxels whose attenuation exceeds a threshold value of 150 HU).

Threshold Selection and Size Representation

A three-dimensional display of the object surface is created by illuminating the object with one or more virtual light sources and then computing and displaying the intensity of the light that is scattered back into the plane of observation (Fig. 2.20). At each point in this plane, the data points of the object that are encountered first in the viewing direction are used to generate a 3D surface image. The distance of the surface from the light source and the surface gradient of the CT densities are used to create shadowing effects and heighten realism of the 3D display. Most programs allow only one virtual light source to be used. By varying the position of the light source details of the object surface can be high-lighted differently. With some programs, multiple objects can be displayed simultaneously and encoded in various colors. The projection of the object into the viewing plane can be done either by casting parallel rays (orthographic rendering) or by casting rays from a virtual eye on to the object [perspective rendering). While orthographic rendering is excellent for gaining an overview of the spatial relationship of structures, perspective rendering with various viewing angles (usually 15°-90°)

allows for maneuvering through the data volume and is the prerequisite for techniques such as virtual endoscopy. The quality of the SSD depends on the scan parameters, as in a 2D reformatted image, but generally the effect is less obvious because of the considerable amount of information that is lost in the thresholding process. A more important determinant of SSD image quality is object segmentation, especially the selection of the segmentation threshold. Threshold Selection and Size Representation

The optimum threshold value for segmentation is the value that depicts a structure in its true, complete size. In theory, this value is midway between the CT number of the object (e.g., the aorta) and that of its surroundings (Le., surrounding fat or other soft tissues). We know from practical experience that the theoretical value should be reduced by about 10%, depending on various imaging parameters. However, only structures that are larger than the section width SW will be portrayed at their true size.

Virtual light source Scattered

light --------..

Reflected light -----,. Vector perpendicula to object surface

Fig. 2.20 Shaded surface display (SSD).A virtual light source illuminates the 3D object, and software calculates the light intensities that are scattered and reflected toward the observer.

Eye of the virtual observer

ILightthat is scattered in the direction of the virtual observer. The length of the vector determines the gray level of the object surface.

61

2 Image Processing and Display Techniques

The larger the section width, the more object elements will be falsely represented in the display. Relatively small or thin details that are oriented parallel to the scan plane will lose contrast due to partial volume effects, with smaller objects appearing progressively diminished, and finally vanishing completely, as their diameter decreases. The choice of threshold, therefore, is always a compromise (Table 2.3). When SSD is used in skeletal examinations, this diminishing effect can lead to bony pseudodefects or the non-visualization of thin bony plates (e.g., the orbital floor). In CTA, small vessels that course horizontally may appear narrowed, may show pseudo-stenoses or pseudoocclusions, or may not be visualized (Fig. 2.21). •

Lowering

the Threshold

When the threshold is lowered, more voxels contribute to the displayed object, and the apparent diameter and volume of the object increase. This can partially compensate for partial volume effects, enabling smaller structures to be portrayed more realistically. Lowering the threshold also leads to longitudinal distortion of larger structures of higher contrast. If the threshold value is lowered even further, the CT number of single pixels will exceed the threshold due to image noise. This will cause flying pixels to appear in the 3D display (see Fig. 7.32), and useful image details will be obscured. Noise can also cause "stalactite" artifacts to appear on the upper and lower surfaces of low-contrast structures. Soft tissues whose CT numbers lie above the threshold value (e.g., after in-

Table 2.3

Fig. 2.21 The selected threshold value profoundly affects the appearance of the SSD. In this patient with a high-grade renal artery stenosis, no apparent pathology is seen with a threshold of 120 HU (a), while a threshold of 200 HU simulates a vascular occlusion (b).

travenous contrast administration) may also obscure structures of interest. The remedy is either to raise the threshold value or use more sophisticated segmentation techniques to separate the object of interest from other structures that also lie above the selected threshold value (see Fig. 2.40). •

Raising the Threshold

When the threshold is raised, fewer voxels contribute to the object display, and the apparent diameter and volume of the object decrease. This can eliminate flying pixels and superimposed structures of lower attenuation, but it

Threshold values for segmentation in 3 D surface rendering Threshold

Application

Comments

Bone

>150

Skeletal studies

Lower values are used in osteoporosis

Vessels

>150

eTA

Lower values can be used after smoothing

I Tissue

Soft tissues

Feasible only in exceptional cases

Fluid

Feasible only in exceptional cases

Fat

Not feasible

Lung Air

>-600 <-200 >-500 <-500 <-500 AA __§1CY';

Bronchogenic carcinoma Lung surface

Relation of tumor to vessels and pleura (Sub)pleural abnormalities, lung volume

Skin Colon, larynx Central bronchi Peripheral bronchi

Skin surface "Luminal cast" "Luminal cast" "Luminal cast"

VRT-Principle

also increases artifacts such as the pseudo-stenoses noted above. Frequently the optimum threshold value in skeletal applications is quite low. especially in patients with osteoporosis. but higher values must be selected to eliminate superimposed structures despite the fact that more "holes"appearin the bone.

Applications Shaded surface displays yield impressive images that can define surface structures and clarify complex three-dimensional relationships, for example among bone fragments or vascular structures (e.g., see Fig. 24.42 or Fig. 25.15). Objects rendered by SSD can be rotated and viewed from any desired angle, and SSDs can be used preoperatively to give the surgeon a three-dimensional impressionofthe operative situs.

Volume

Rendering

a tool for the presentaused as an aid to diagnosis in complex anatomic or pathologic situations (e.g., acetabular fractures, complex vascular relationships). As a result. SSDs are used predominantly in skeletal studies (see Chapter 25) and eTA (see Chapter 24) and they have occasional applications in bronchial examinations (e.g., see Fig. 9.14). The rendering of other types of soft-tissue structures is a laborious process that cannot be recommended for routine clinical investigations. SSD have a role in virtual endoscopy (see p. 70) because the rendering process is much faster than with volume rendering techniques. SSD is therefore well suited for interactive navigation though a virtual endoscopic data set (see Fig. 2.50). SSDs are primarily

tion offindings and are only occasionally

Techniques

Principle Maximum and minimum intensity projections are simple examples of volume rendering techniques. The image is generated by casting rays through the volume of interest (ray tracing), and projecting anumeric value that is derived from the data encountered along each ray according to prescribed rules (e.g., the maximum CT number in MIP). Generally, however, volume rendering technique (VRT) is the term applied to a complex procedure that is very versatile and can combine characteristics of surface rendering and MIP. VRT assigns a range of opacity values to CT numbers and thus yields better definition of object contours or semitransparent display of structures (Fig. 2.22). VRT is quickly becoming an established standard technique for rendering of spiral CT or multislice CT data sets. Image quality is high and the user has a whole range of rendering effects at his disposal. VRTcan best be understood when comparingitto shaded surface displays (SSD). Such surface rendering is a "binary' process in which all CT numbers belonging to the 3D object (within the chosen threshold range) have maximum

opacity, while all CT numbers outside the range have zero opacity and do not contribute to the image. Since all the voxels within the CT range have maximum opacity, only the surface of the object is depicted in the SSD. In volume rendering, opacity values are continuous and can vary between 0% and 100%. The behavior of VRT is determined by the opacity curve. This curve may be drawn man ually, a procedure that is tedious and not very reproducible. A very comfortable way of creating the curve is to use preset curve forms and to control its precise location on the CT number scale by window and level operations (Figs.2.22a and 2.23, Table 2.4). Alternatively, opacity curves may be produced by a number of (partially overlapping) trapezoids that can be altered in form, height and position on the CT scale (Fig.2.22b). These trapezoids can be chosen to represent the attenuation range of various tissues, e.g., fat, other soft tissues, contrast-enhanced blood vessels, and bone. A different color can then be attributed to each of these trapezoids to be able to differentiate between these tissues on the final image. VRT can either create a transmission display (similar to a ray-sum projection or MIP) or

63

64

2 Image Processing and Display Techniques

Opacity(%) 100r

Opacity(%)

100.---------------~--------~--~

50 _ _

__ i WI L = 40Q1100 •.

O+---~~--~----~,----._--~----~ -200

-100

o

100

200

300

-200

400

-100

CT number (HU)

Fig. 2.22 Principle of volume rendering (VRT). During the creation of a volume rendered image, an opacity curve determines the opacity of various tissues depending on the CT numbers of each voxel. For most instances, a simple technique based on a single opacity ramp from full transparency to full opacity yields good results (a). This ramp can be determined similar to the

o

100

200 300 400 CTnumber(HU)

window/level operation of normal CT images. Alternatively, multiple trapezoids with various colors that are assigned to tissues of different CT numbers can be used as an opacity function (b). Note that structures with low opacities (e.g. fat) will appear semitransparent with both techniques. No gradient shading (reflectivity =0) was used in these examples.

a Fig. 2.23 The opacity curve and the reflectivity determine the resulting volume rendered image. A narrow opacity ramp (W/L= 400/400) yields an excellent surface display of the abdominal vasculature (a). Note the

calcified gallstone (arrow). Shifting the opacity curve towards lower values yields semitransparent displays of soft tissues (b. W /L= 400/200). Additional gradient shading was used on these images.

a surface display (similar to the SSD) depending on the amount of gradient shading that is used to simulate reflectivity. This "reflectivity constant" may be determined either by using presets or by adjusting its numeric value.

•

Color Coding

Color coding may be performed on a per-trapezoid basis as described above. Ideally, various tissues are colored in a different way, thus allowing for visual differentiation. In practice, however, there is substantial overlap between

VRT-Principle Ta~e

2.4

Opacity

settings

Application

for volume Opacity

rendering s<sttings

W/L Skeletal

studies

CT angiography Diagnostic viewing Softtissue

Comments Range

Curve

300/200

100 ..400

Ramp

Lower values

400/300

100 ..500

Ramp

Values

100 ..300 -1050 .. 450 -900 .. 300

Ramp Ramp Ramp

MRP-like

400/100 1500/-300 1200/-300

depend

are used

500/-500

-750 .. -250

Spike

Air/soft

tissue

interfaces

projection Luminal casts

500/-500

-750 .. -250

Inv. ramp

Air/soft

tissue

interfaces

1200/-300 1200/-300 1200/-300 200/150

-900 -900 -900 100

Ramp Ramp Ramp Inv. ramp

Double

spike

depending

on contrast

Angioscopy Opooty

settings

In this table, Inv.

may vary a single

.. -300 ..-300 ..-300 .. -300

enhancement

and

opacification

rendering

Tissue-transition

Virtual endoscopy Bronchoscopy Laryngoscopy Coionoscopy

in osteoporosis

on vessel

implementation

for calcifications of volume

rendering

is used for most applications (which is sufficient for most imaging tasks) full opacity for lower CT numbers and zero opacity for hgh CT numbers

opacity function, maximum best achieved by fixed color

opacity schemes,

for center of range but may require use of multiple

trapezoids

on some

.workstations

various tissue types, e.g., between contrast-enhanced organs and blood vessels, and between vessels and bone, and no secure discrimination is possible (Fig. 2.22 b). Alternatively, a color scale can be assigned to the CT number scale. This color scale may be fixed relative to the CT numbers thus providing an absolute reference color depending on the attenuationortissue type (Fig. 2.24a). This technique is ideal for displaying non-contrast-enhanced structures. Because contrast enhancement may vary substantially between patients, a flexible assignment of colors and CT numbers is advisable, depending on the actual setting of the opacity curve. This can be done by assigning the color scale to the slope of the opacity curve (Fig.2.24b) . •

Spatial

Resolution and Matrix Size

Fast response is usually obtained by reducing the spatial resolution of the VRT image during user interaction. A 128-matrix will only give a rough orientation but a 256-matrix already provides enough detail to find the optimum viewing angle for demonstrating a structure ofinterest. A 512-matrix, however, further improves spatial resolution and has a positive effect on

the display of small vessels or bony surface detail (Fig. 2.25). A 1024- or 2048-matrix is also used in some systems, at least as an intermediate step during the rendering process. Such large matrices are especially helpful if structures have to be greatly enlarged from the original data set (such as the environs of a cerebral aneurysm), or ifvery long ranges have to be displayed (e.g., chest and abdomen or abdomen and legs). Interactive

Rendering-Movies

Presets make it easy to choose a proper display setting, and in most of the recent software. selection of parameters can be performed interactively close to real time. With specific volume rendering boards that can be installed in standard PCs, real-time interaction with> 8 frames per second becomes feasible. Movie sequences can be generated that allow for an off-line viewing of VRT data without having real-time interactive VRT software available. If standard digital movie formats (e.g. AVI, MPEG) are used, these sequences may be displayed on any Pc. Alternatively, images can be transferred to videotapes and be displayed on any videocassette recorder. Such

65

66

2 Image Processing and Display Techniques

Air I skin

Fat

Vessels / bone Soft tissues

o~~~~~~~~~~~~~~~~~ -1000 -500 o

500

relative color scale assigns colors to the opacity window but not to the CT number scale (b). Additional gradient shading was only used in (a).

Fig. 2.24 Color coding. An absolute color scale assigns colors to various soft tissues, depending on their CT numbers but independent of the opacity setting (a). A

a Fig. 2.25

b 2

Matrix size and image quality, (a) VRT with 256 resolution, (b) VRTwith 5122 resolution.

VRT-Artifacts and IPitfalls

a

c

b

Fig. 2.26 VRT of air-containing structures in the paranasal sinuses, pharynx and larynx, (a) Display of surface (cast). (b) Tissue transition projection of the airway wall. (c) Virtual endoscopy of the larynx.

movies are especially helpful for displaying complex anatomy to referring physicians. They already are being used for the display of renal cell carcinomas prior to nephron-sparing surgery, for the display of the anatomy in the evaluation of living renal donors, and for surgical planning of cere bral aneurysms. Special Techniques •

Air Casts

Inverted opacity curves that provide a high opacity for low-attenuation regions and low opacity for high-attenuation regions can be used to selectively display a "cast" of the tracheobronchial system, the larynx, or the colon (Fig.2.26a, see also F.igs.2.36a and 2.50a). •

Tissue Transition Projections

Tissue transition projections selectively render the walls of the structure of interest. With VRT this can be reached by using an opacity curve that has the shape of a "spike" that is centered on the CT number between the two structures of interest (e.g., -500 HU for imaging of soft tissue-air interfaces such as in the colon or the trachea). This display technique, for example, produces images that simulate a double-contrast examination (Fig. 2.26b, see also Figs. 2.36 and 2,50).

•

MPR-like

Rendering

MPR-like rendering is a technique that can substitute for traditional MPR. With VRT such images can be obtained by using a linear opacity curve with a slope that covers a range similar to conventional window settings. No reflectivity should be employed, to allow for optimum evaluation of cut surfaces. By using interactive cut planes, the radiologist can section through the data volume in a fashion similar to interactive MPR but has the additional advantage of depth information provided by these images (Fig. 2.27, see also see Fig. 2.22a). Table 2.4 provides suggestions for proper opacity settings, depending on the clinical imaging task. Artifacts and Pitfalls

--------

VRT is less susceptible to pseudo-stenoses or pseudo-occlusions compared to SSD, although suboptimum opacity settings also lead to image artifacts (Fig. 2.28). Venetian blind artifacts depend on the viewing angle and may be found incertain implementations of volume rendering (Figs.2.29a, b).

67

68

2 Image Processing and Display Techniques

b

a Fig. 2.27 MPR-like VRT. Cutting into a volume rendered image (with reflectivity set to 0) creates MPR-like views that add the advantages of volume rendering

(depth information) to that of MPR (evaluation of cut planes), (a) Soft tissue setting (W/L= 400/100), (b) lung setting (W/L= 15001-300).

a Fig. 2.28 Volume rendering uses the whole 3D data set for image creation. However, only parts of the 3D data are displayed, depending on the opacity function. With an opacity window ofW/L=400/250 (a) there is

Image noise may cause irregularities of the

object surfaces (Fig.2.29c) or may even obscure underlying pathology. Depending on the opacity function, noise may also cause a veil-like shadowing of structures that are deeper inside the displayed volume.

superimposition of the right renal vein (arrows). With an opacity window of 400/300 (b) portions of the accessory renal arteries on the left are lost (arrow heads) but the lower pole artery on the right is visualized (arrow).

It has been stated that VRT does not lose information because the whole data volume is projected. However, VRT does not display all the information in the data volume but, like all 3D rendering techniques, selectively displays portions of the scanned object, depending on its CT attenuation and position relative to the viewer (Fig. 2.28).

VRT-Applications

a

c

b

doscopic views I (upper IPortion of image), (c) Ilmage noise is especially disturbing in VRT with gradient shad2 ing (reflectivity) and a 512 resolution.

IFig..2.29 Artifacts.rfajVenetiari blindartifactsaretypical for some software implementations of volume rendering. (b) They are especially disturbing on virtual en-

a

b

Fig. 2.30 Differentiation between calcified plaques and vessel lumen is simplified on color displays (a) compared to black-and-white rendering (b). In this patient

Applications

Volume rendering can be used for CTA, skeletal imaging, tracheobronchial imaging, display of the lungs, the colon, abdominal organ and even as a primary tool for image analysis. •

CTAngiography

In CTA, volume rendering is becoming the new standard for the displayofthe pulmonaryvessels, the aorta, and the abdominal vessels. Colorcoded VRT provides display of vessel lumen and

c

d with a superior vena cava occlusion, color display (c) improves the visualization of the collaterals in the mediastinum relative to black-and-white displays (d).

calcifications in separate colors, thus making it easy to localize calcified plaques (Fig. 2.30). Color coding also may allow differentiation between arterial and venous vessels, as well as between organs with different contrast enhancement (Fig. 2.31). When the aorta is eliminated by editing procedures, excellent displays of the celiac and mesenteric vessels become available (Fig. 2.32). Segmentation can be used to eliminate superimposing soft tissues and make it possible not only to display the vessel lumen but

69

70

2 Image Processing and Display Techniques

also thrombosed portions of the vessel (e.g., of an abdominal aneurysm) as well (Fig. 2.33). •

Fig. 2.31 Differentiation between arteries, veins, and contrast-enhanced organs is possiblewith VRT ifthere is differential enhancement.

Skeletal Imaging

For skeletal imaging, VRT is an excellent tool for classifying complex fractures (e.g., of the acetabulum), for demonstrating the spatial position of fracture fragments, and for planning of complex or minimally invasive surgery. In orthopedic applications, VRT is superior to SSD because it is less susceptible to partial volume effects with pseudo-defects in thin bony laminae or osteoporotic patients. In addition, VRT allows for simultaneous display of bones as well as tendons or muscles, and even allows for assessment of the skin contours (Fig. 2.34), e.g., in patients prior to corrective surgery. In the spine, the vertebrae as well as the intervertebral disks can be displayed, making it an ideal tool for assessing and demonstrating pathology of the spinal column (Fig. 2.35).

Fig. 2.32 Removal of the aorta allows for excellent display of the celiac and mesenteric vessels. In this patient the relationship of the aneurysm to the mesenteric side branches can best be demonstrated on a PA view (b).

a

b

Fig. 2.33 With a soft tissue setting forVRT, and elimination ofsuperimposing soft tissue by image segmentation, direct display of thrombi is possible.

a

b

VRT-Applications

a

b

c

Fig. 2.34 VRT not only allows for a selective reconstruction of the skin (a) or bone (b), but also semitransparent displayof skin that make the underlying bone visible (c). Note the bilateral obstruction of the brachiocephalic veins.

•

Tracheobronchial

Imaging

For imaging ofthe tracheobronchial system, VRT provides either casts or semitransparent walls of the tracheobronchial tree (Fig. 2.36). VRTis especially helpful when it comes to virtual bronchoscopy (see p.81) because even segmental bronchi may be evaluated if a thin-section multislice CT data acquisition was used. •

Lungs

For the lungs, volume (or surface) rendering can be employed to create images of the lung surface (see Fig. 2.47) and to assess the spatial relationship of focal lung lesions to the chest wall or mediastinum (Fig. 2.37). It can be used to create color-coded density maps that help detect and

a

b

Fig. 2.36 The tracheobronchial system can be visualized as an "intraluminal cast" using SSD or VRT (a) or using a tissue transition projection (b). Similar effects

Fig. 2.35 Volume rendering of the spinal column allows for demonstration of the vertebrae as well as the intervertebral disks.

c are possible using a 20 mm thick MPR after removal of the mediastinal tissues by region growing (c).

71

2 Image Processing and Display Techniques

quantify regions with focal air trapping or perfusion abnormalities (Fig. 2.38). • Colon In the colon, VRT is an excellent technique for virtual colonoscopy but also for surveying the data with the help of sliding thin slabs (see below) that combine the advantages of axial, coronal, or sagittal MPR with the depth information provided by VRT. Again, casts or semitransparent displays of the colon become available, some very similar to conventional barium enema studies (see Fig. 2.50). Fig. 2.37 Volume rendering allows for excellent displays of the location of focal lesions relative to the chest wall and the pulmonary vessels. Note that this metastasis is due to a tumor thrombus spreading within a peripheral pulmonary artery.

Fig. 2.38 Color-coded density maps can be created when using a rainbow spectrum and a lung-centered opacity curve (W/L= 500/-750). Note the wedgeshaped perfusion defects (arrowheads) in this multislice CT A of chronic pulmonary embolism.

•

Routine Reporting

In the chest and abdomen, VRT with proper opacity settings provides semitransparent or color-coded displays of soft tissues (see Fig. 2.27). In the abdomen, in particular, VRT allows for excellent display of the liver, portal vessels, bowel structures, the pancreas, and even the suprarenal glands. VRT with opacity settings that mirror those of conventional CT imaging have been suggested as a primary viewing mode for making the diagnosis. By analogy with a soft -tissue window, the opacity settings should be chosen in a way that makes normal fatty tissue transparent but is able to demonstrate increased density in the fat on the rendered images (see Fig. 2.27). For the purpose of using VRT as the primary diagnostic tool, however, cut planes have to be available that expose cross-sections of organs that would otherwise be rendered opaque. In addition, interaction has to be real-time to allow for fast maneuvering through the data volume. There is still no hard data, however, that such a procedure is safe and does not miss important findings.

Segmentation Principle

Both 3D volume rendering (e.g., MIP, VRT) and 3D surface rendering (SSD) require a segmentation process that defines the volume of interest and separates it from structures that should not be represented in the 3D image. The process of manipulating the data set to control what structures are included or excluded in the rendering

is termed "editing". Positive editing is based on marking the structures of interest that will be retained in the 3D image. Negative editing involves marking the unwanted structures that will be removed. A distinction is drawn between 2D editing functions, which are applied section by section, and 3D editing functions, which are applied to the 3D data set as a whole.

Segmentation-Principle

•

Cutting Functions

Cutting functions may be used on the original data volume before 3D rendering or on the 3Drendered object itself. Cutting can be performed in the original data volume on a section-by-section basis, a technique that is very time consuming and can only be recommended in exceptional and complex cases in which a small set of sections are edited. It is more sensible to perform cutting functions on 3D-slabs, i.e., stacks of images that are a few mm to several cm wide. This can substantially reduce the effort ofediting butis still comparatively time consuming iflarger ranges, such the whole chest or abdomen, have to be processed. The most efficient technique is based on cut lines that are drawn on a number of unevenly spaced reference sections. The program then performs a three-dimensional interpolation between these cut lines and thus defines the volume to be included or excluded from the volume of interest that is chosen for further processing (Fig. 2.39). This technique allows the lungs or the abdominal vessels to be edited in a few minutes, and even allows for complex procedures in which portions of organs are removed for better visualization of neighboring structures (see Figs. 2.31-2.33). Cutting in a 3D-object first requires a view in which the cut plane can be drawn. This plane will be perpendicular to this view, so the user

a

b

Fig. 2.39 Cutting functions using a "rubber sheet" algorithm are highly efficient for complex editing tasks. A number of arbitrary regions of interest (ROI) are drawn at a few levels throughout the chest (a). Display of reo

has to make sure that the structures to be separated do not overlap. The cutting functions are best used to exclude unwanted objects but they can also be employed to focus on certain regions and include the structures of interest. •

Threshold Techniques

Threshold techniques are simple processes that use a threshold value or range of CT numbers to define the volume that will undergo segmentation (see Table 2.3). Threshold techniques are generally used in the rendering of SSDs. When threshold techniques are used alone, it is common for an image to include different anatomic structures that need to be displayed separately. This would apply, for example, to opacified vessels and the vertebral column, or to the femoral head and acetabulum. For this reason, threshold techniques are often combined with other methods that can separate objects within the same threshold range. •

Connectivity

Connectivity algorithms are used to identify contiguous image regions that have a certain property in common. Such a "region-growing" algorithm starts at a seed point and detects all voxels that satisfy the specified condition and are interconnected to the seed point. The usual condition is that the voxels lie within a prescribed range of CT numbers.

c maining volume with indicated ROllevels (b). A volumerendered vascular display (W/L= 600/200) demonstrates multiple AV -shunts in either lung.

2 Image Processing and Display Techniques

Connectivity algorithms can be applied to single axial sections, to slabs consisting of multiple sections, or to the 3D volume as a whole. The larger the volume, the faster the theoretical rate of segmentation, but there is a greater risk that artifactual "bridges" will form between image regions that are actually separate. Various methods can be used to eliminate these bridges (Fig. 2.40). Frequently, object separation can be achieved simply by changing the threshold range. Other methods involve the use of manually drawn separating lines or "morphologic operators" to erase the unwanted connections. Watershed algorithms are very helpful for separating two structures that are connected by a bridge. They determine the plane with the lowest CT number within that bridge and cut at that level. In practice this means that the operator only has to place a seed point on the structure to be included and others on the structures to be excluded and let the watershed algorithm do the separation. In many cases such a procedure will be sufficient; in some more complex cases, a repeat of this procedure at another level of the data volume will become necessary. •

Morphologic

Operators

Morphologic operators depend on the morphology of a structure rather than its attenuation values. The most simple but useful operators are those that remove rows ofvoxels from the surface of an object (erosion) and those that attach rows of voxels to the surface of an object {dilation}. Erosion operators can be used to remove bridges between adjacent objects. Dilation can then be per-

a

b

c

Fig. 2.40 Segmentation. Connectivity algorithms can be used to mark contiguous regions that have common characteristics. "Bridges" that persist between structures to be separated (a) can be eliminated by the use of

formed to restore the object to its approximate original size, although surface details will be lost (Fig. 2.40). Thesequenceoferosionfollowedbydilation is called "opening"because it opens up connections between structures, while the combination of dilation followed by erosion is called "closing" because it removes holes within an object. After successful detection of objects (e.g., the skeletal structures) to be excluded by region growing, it is advisable to perform a dilation procedure on these objects to include and eliminate also voxels at the object boundary that contain higher CT numbers than the surrounding structures. If these voxels are not excluded (Fig. 2.41), ghosting artifacts will result in 3D rendered images. Other morphologic operators can recognize small groups ofvoxels and can therefore be used to erase "flying pixels" from an image (Fig. 2.42). Automated Techniques, Computer-assisted Diagnosis

Thereisanincreasingamountoffullyorsemiautomated techniques being developed that require no or only minimal input (such as positioning of various seed points) for complex editing tasks. Such tools allow for automated extraction of the lungs, or bone removalin CTA (Fig. 2.43), or auromated vessel analysis (Fig. 2.44). Computer-aided diagnosis tools are able to automatically detect, extract, and measure the volume of pulmonary nodules for 1ung cancer screening. Similar tools are being developed for colon cancer screening.

d

e

cutting functions (b), erosion operators (c), or by increasing the threshold (d). Subsequent dilatation can be performed to remove the higher-attenuating areas that remain around bony structures (e).

Automated

Techniques, Computer-assisted

Fig. 2.41 Bone removal by region growing. Region growing with a threshold above 200 HU allows separation of bones from vessels but leaves high-attenuation areas around bony structures that lead to ghosting artifacts on MIP (a). This ghosting may completely obscure smaller vessels. Dilatation of the bone and removal of holes in the vertebral body using "closing operators", yields high quality MIP (b).

a

b

a

b

a

b

Fig. 2.42 Flying pixels ("floaters") due to image noise are especially disturbing on low dose data sets (a) but can be removed by size-dependent filter functions or regiongrowing of the structures of interest (b).

Fig. 2.43 Automated bone removal is now feasible on some workstations. By clicking on the structures to be removed or, in this case, retained (a), automatic removal is quite effective (b). Note the type II endoleak after aortic stent grafting (arrow).

Diagnosis

75

76

2 Image Processing and Display Techniques

Fig. 2.44 Automated vessel analysis is based on automatic vessel tracking but adds measurements perpendicular to the vascular course. Depending on the software, local cross-sectional area, and the minimum and maximum diameter of the vessel of interest are calculated and a rubber-band image ofthe vessel is displayed.

There are programs that include some form of artificial intelligence to account for anatomic variations. Many of these tools have just entered the market or are about to be introduced soon, pending FDA approval. Applications

Segmentation is a prerequisite for eTA with MIP images in an anteroposterior direction. Positive editing is generally sufficient if there is a focused question. For the presence of a renal artery stenosis, for example, best results are obtained if a thin curved slab is included that just encompasses the renal arteries from the origin

to the intrarenal branches and excludes the renal veins as well as portions of the anterior and posterior renal cortex (see Fig. 2.13b). Negative editing with exclusion of the skeletal structures can be recommended if a survey of the thoracic, abdominal, or peripheral vasculature is required (Table 2.5). Segmentation can improve visualization of complex vascular anatomy or pathology (posterior-anterior views) if SSD or VRT are used (see Fig. 2.31). In addition, it can provide selective views of certain vascular territories (e.g., mesenteric vessels) by eliminating the aorta or other superimposing structures (see Fig. 2.32). For visualization of the pulmonary vasculature with MIP, SSD, or VRT, prior removal ofthe chest wall is necessary (see Fig. 2.39). Segmentation of soft tissues is only useful for VRT. Using such a technique, visualization of complex anatomic relationships becomes possible (see Fig. 2.33). For the lungs, segmentation can be employed to selectively display the lung tissue in order to obtain density maps or to depict the pulmonary surface (see Fig. 2.47). Removal of all soft tissue structures in the chest that are connected to the mediastinum (connectivity with a threshold> -200 HU followed by a 2-3 pixel dilatation) will yield an overview of the aerated lung. Thick MPR from such a data set will create images that are similar to tissuetransition projections ("tissue transition MPR", Fig. 2.45). In the skeleton, segmentation can be used for bone exarticulation and provide selective views of the joint surface, e.g., of the acetabulum, but also of the glenoid or the calcaneus (Fig. 2.46, see also Fig. 25.41).

Fig. 2.45 Tissue-transition MPR can be obtained by removing all extracorporeal air as well as all soft tissues (> -200 HU), followed by a 2-pixel dilation, and then using a thick MPR to visualize interfaces of air-filled structures. Note that also the pulmonary vessels appear translucent (a). If no dilation is performed, the resulting image will selectively display the aerated lung

(b). a

b

Segmentation-Applications Table 2.5

Segmentation procedures for various clinical tasks

Application

Display goal

eTA

Bone removal Chestwall

Pulmonary vessels

Abdomen

Abdominal vessels

Neck

Carotid arteries

Bone exarticu- joint surface lation

technique

Automated bone removal or Cutting (rubber sheet): include all except chest wall Mark bone (RG/WS > 180 HU), dilate bone (2-3 pixels), remove bone Cutting (rubber sheet): include all except abdominal wall Mark carotids (RG/WS = 150-400 HU) using lower + upper threshold, create two VOl roughly including skull and neck/cervical spine, mark bone (RG/WS > 500 HU - skull, RG/WS > 180 HU spine), dilate bone (2-4 pixels) remove bone Draw center line through carotids (manualfsemiautomated) , produce 3-5 mm thick slab curved MPR/MIPIVRT Mark bone of interest (RG/WS > 180 HU), dilate 2-3 pixels, mark bone to be removed (RG/WS), dilate 2-3 pixels, remove bone

Lung extraction

Volume measurements

Mark trachea + large bronchi (RG <-900HU), dilate 2-4 pixels, extract each lung (RG <-200 HU)

Colon extraction

Selective display

Block ileocecal valve if necessary (0 HU line), mark colon (RG<-500HU) use multiple seeds if necessary, dilate 3 pixels for SSD orVRT

Tracheobronchial extraction

Selective display

Mark tracheobronchial tree (RG/WS <-900HU), increase threshold until overflow into lung parenchymajust does not yet occur, dilate 2-3 pixels forVRT/SSD

Tissue-transition MPR

Transparent wall of air-filled organ (larynx, trachea, bowel)

Mark air-filled organ (RG <-200HU), dilate 2-3 pixels, remove rest

Surface display

RG= region growing; WS = watershed algorithm; suggested thresholds in brackets have to be individually ,§4
Fig. 2.46 Bone exarticulation can be used to create an > unobscured view of articular surfaces (here: complex fracture ofthe acetabulum).

77

2 Image Processing and Display Techniques For volume quantification, of organs or lesions, the structure has to be isolated first by proper editing procedures. The techniques may be fully automated (pulmonary nodule detection and quantification, see Fig. 9.18) or will require substantial user input (e.g., quantification of hepatic or splenic volume). Often, a combination of connectivity (seeding) and manual cutting functions will provide the quickest results (Fig. 2.47). Fig. 2.47 Quantification of the lung volume may be helpful prior to lung transplantation (especially for living donors) or for follow-up after surgery. The total volume of the aerated lung including the tracheobronchial tree or an isolated lung are easily determined using region growing techniques.

Virtual Endoscopy Principle

Virtual endoscopy is a 3D-rendering technique that simulates endoscopic views. The first step is to select suitable threshold ranges (SSD) or opacity settings (VRT) that define the internal surfaces of interest, such as the bronchi, paranasal sinuses, blood vessels, or the gastrointestinal tract (Table 2.6). The endoscopic effect is produced by perspective rendering along a path within the data set. This provides an "endoscopic" view of internal objects (Fig. 2.48 and 2.49) . •

Maneuvering through the Data Volume

The viewer can interactively change his or her location, viewing direction, and the visual field of the virtual endoscope. Good viewing angles Table 2.6

that come close to those of real endoscopes vary between 60° and 90°. Orientation can be established from multiplanar reconstructions and from the virtual endoscopic image itself. In some systems the operator can preprogram a "flightpath" through the organ system or define the path interactively. A good first approximation of the flight path through tubular organs can be gained by calculating a centerline using so-called skeletonization algorithms. Techniques to view the walls are helpful, and back-mirror techniques that inverse the viewing direction are essential for some applications such as virtual colonoscopy. Still, at present, maneuvering through the data sets is a tedious and time-consuming effort. For this reason, virtual endoscopy is mainly used as a problem-solving tool, for example to

Threshold values for virtual endoscopy

Application

Interface

Opacity settings 1,

Threshold" W/L

Bronchoscopy Laryngoscopy Colonoscopy Angioscopy 3

Mediastinum/air Bronchial wall/air Soft tissue/air Soft tissue/air Soft tissue/contrast material

surface rendering (SSD), b volume rendering (VRT)

>-soo >-900 >-SOO >-SOO < 1S0

1200/300 300/700 1200/300 1200/300 200/1 SO

Range -900 -950 -900 -900

so

... 300 ... -550 ... 300 ... 300 ... 2S0

/\

Curve Ramp Ramp Ramp Ramp Inv. Ramp

Alternative

Visualization

Techniques

Fig. 2.48 Virtual endoscopy yields perspective views by using a central projection instead of the parallel projection used in other techniques. Internal surfaces can be displayed using SSD (a) or volume rendering (b).

a

b

a

b

Fig. 2.49 Virtual endoscopy relies on a high contrast between lumen and surrounding tissues. Most applications such as virtual cystoscopy (a) rely on air as a negative contrast material. For virtual angioscopy ( b) or cholangioscopy, the positive contrast in the structure of interest requires an inverted opacity curve. Note the small entry tear in this type B.

differentiate a colonic fold from a real polyp in CT colonography. Alternative Visualization Techniques

Casts ofa luminal organ are known from CTA and there represent the standard display technique with SSD or VRT. Similar techniques are possible with the tracheobronchial system, and yield a tree-like structure. Distal branches are displayed best if the data was acquired with thin-section multisliceCT. Forthecolon, these casts resemble a monocontrast barium enema, but they are superior because they provide a three-dimensional display of the colonic surface (Fig.2.50a). Casts, however, have an intrinsic drawback: they are less sensitive for displaying small endoluminal protrusions such as polyps orwall-adherent tumors. These lesions appear as small indentations on the surface of the cast and may easily be overlooked.

Tissue transition projections render the walls of the structure of interest in an opaque or translucent fashion. They can be created either by using SSD or VRT with a narrow range ofCT numbers that correspond to the transition zone between luminal contents (air or contrast medium) and the surrounding tissues. This display technique, for example, produces images that simulate a double-contrast barium enema (Fig.2.50b). Alternatively, a tissue transition MPR (see also Fig. 2.36) can be chosen that provides similar results (Fig. 2.50c). ThickMPRis useful for displaying small vessels (thickness 3-5 mm) or the central tracheobronchial system (thickness 7-20 mm). They provide an overview of anatomy and pathology (see Fig. 2.36) and make it easier to detect focal lesions because these will alter the attenuation of the vessel or bronchus. Sliding thin-slab VRT is a method similar to sliding thin-slab MIP but uses volume rendering

79

80

2 Image Processing and Display Techniques

a

b

d

e

9 Fig. 2.50 Alternative visualization techniques may provide a helpful adjunct to virtual endoscopy, as demonstrated in this patient with a colonic lipoma and small polyps in the proximal transverse colon, (a) Intraluminal cast. (b) Tissue transition projection using VRT. (c)

Tissue transition MPR. (d) Thin-slab VRT. (e) Coneddown VRT. (f) Virtual colonic dissection ("virtual gross pathology"), (g) Virtual colonoscopyusing a panoramic view ("unfolded cube"), (h) MPR demonstrating fat attenuation in the lesion, proving it to be a colonic lipoma.

Virtual Endoscopy-Applkations

instead of maximum intensity projections. This technique can be employed in CTA. tracheobronchial imaging as well as CT colonography (Fig.2.50d). It provides a good overview of the anatomy even in complex and spatially contorted cases. Coned-down VRT allows for in-detail analysis of a subvolume of interest and is helpful for vascular structures as well as CT colongraphy (Fig.2.50e). Virtual dissection (virtual gross pathology) is a new technique for CT colonography that finds the centerline of the colon. stretches it out and performs a virtual longitudinal cut in order to obtain a fiattened view of the inner colonic surface (Fig.2.50f). In order to avoid spatial distortion. only slim stripes of the colon wall are displayed. A panoramic view (Fig.2.50g) yields endoscopic projections in and against the direction of movement as well as views of the sides of the colon. This allows for almost complete endoscopic evaluation of the colonic surface and improves detection of polyps hidden behind larger folds. Other techniques have been tried as an alternative to virtual colonoscopy and include Mercator's projection, bull's eye views and others, but they have not yet galned clinical acceptance. Finally, multiplanar reformations (Fig. 2.50 h) must not be neglected because they are able to yield information about CT attenuation, contrast enhancement as well as transmural extent of a lesion.

Artifacts and Pitfalls Threshold-based techniques (SSD) are inherently more susceptible to artifacts than volume rendering techniques. Even with optimum thresholding, SSDs are degraded by pseudo-occlusions and pseudo-stenoses in small vessels (virtual angioscopy) and by pseudo-defects in the bronchial or colonic wall (virtual bronchoscopy and colonoscopy, respectively). These effects are less pronounced in volume rendering when suitable parameters are selected. With VRT even smaller vessels and airways can be evaluated. In virtual colonoscopy, it is often difficult to distinguish between intraluminal contaminants and polyps. In virtual colonoscopy and gastros-

copy, residual intraluminal fluid can obscure the surface of the stomach or bowel and prevent their evaluation. Optimum preparation is critically important. Thin bowel walls, especially when two bowel loops are adjacent to each other, or haustra may appear translucent and require readjustment of threshold or opacity values. In virtual bronchoscopy, breathing and pulsation may lead to ring-like distortion of the bronchial walls. Mucous can mimic polypoid lesions, although it often appears elongated and is therefore easily distinguished from real pathology. Measurements of luminal diameters and degrees of stenosis are strongly dependent on the viewer's perspective and, especially, on the selected threshold value.

Applications

Virtual colonoscopy is the most widely used virtual endoscopic technique (see also Figs. 15.1618). It is part of a CT colonography study and has been proposed for colon cancer screening as well as for detection of concomitant lesions in stenosing colon cancers. However, very few people use it to examine the whole colon. Instead it is used mainly as a tool to differentiate colonic folds that mimic polyps from real polypoid lesions. With further refinement of the technique, however, virtual colonoscopy may also become a method for screening the whole colon because it should have a higher sensitivity for detecting small polyps that are easily missed on cross-sectional images alone. Virtual bronchoscopy at present has very few clinical indications because most diagnostic decisions can also be made based on alternative display techniques such as thick MPR. Because of the strong enlargement of structures, however, it might gain a role with multislice scanning of the tracheobronchial system. Virtual bronchoscopy can more readily detect small polypoid protrusions into the bronchial lumen and is able to depict lymph nodes that bulge the bronchial wall at the level of a bronchial bifurcation, a sign considered indicative of malignant disease by bronchoscopists. What remains is a gutding tool for fiberoptic bronchoscopy that

81

2 Image Processing and Display Techniques helps in biopsies of more peripheral lesions, or for determining the safest point for transbronchial biopsies in a central location close to a main vessel. Virtual cystoscopy has the potential to detect very small polypoid bladder lesions, even in regions that may be hard to inspect by real cystoscopy, For best results, the technique requires filling of the bladder with contrast material or air. Virtual angioscopy has provided almost no additional information over other visualization

techniques, The only indication may be the display of the relative location of the struts of aortic stent grafts relative to the ostium of branching vessels, A major disadvantage of this techniques is the overestimation of stenoses due to calcified plaques (blooming effect),

Virtual cholangioscopy, virtual gastroscopy, virtual enteroscopy, or endoscopy of other organ

systems have been suggested as well, but most techniques fail to provide helpful information over other, less complex visualization tools,

3 Patient Preparation M. Prokop, A. J.

Preparation

and Contrast Media Application

van der Molen

for CT Examinations

.

Preparation of Patients with Impaired Renal Function (p. 84) Preparation of Patients with Allergy to Contrast Media (p. 86) Preparation of Patients with Hyperthyroidism (p. 87) Bow •• Preparation (p. 87) Positioning (p, 88) Vascular Access (p. 88) Planning the Scan (p. 89) Gastrointestinal Contrast Me
(p. 91)

Parenteral Contrast Media Types of Contrast Media (p. 93) Complications of Contrast Material Injection (p. 93) Treatment of Adverse Reactions (p. 95) Contrast Injection Parameters (p. 96) Intravenous Contrast Administration (p. 96) Basic Principles (p. 96) Injection Parameters and Organ Enhancement (P, Contrast Injection (p. 98) NaCi Bolus (Saline Flush) (p. 99) Contrast. Material Concentration (p. lC Scan D••ay (p. 101) Individualized Contrast Material Inject Perfusion Studies (p. 105) Direct Arterial, Portal, or Venous Contrast Administration Intrathecal Contrast Administration (p. 107) Intracavitary Contrast Administration (p. 107)

3

Patient Preparation and Contrast Media Application

The strategies applied in CT examinations are determined by the technical characteristics of the CT scanner and by the nature of the clinical question. Examinations without contrast medium are suitable only for intrinsically highcontrast structures (bone. stone detection. and lung parenchyma) and for the detection of he-

Preparation

for CT Examinations

When a patient is scheduled for CT examination. it must be decided whether special patient preparation is required (Table 3.1). In patients who have significant renal function impairment or other relative contraindications to contrastenhanced CT. other imaging modalities (ultra-

Table

3.1

Patient

preparation

Vv'hen the examination the

I

is schedulded,

determine

following:

• Renal function

status

(serum

creatinine>

130

u.moi/l)

Plasmacyto,m,a or amyloidosis? • Allergy to contrast materials? • Hyperthyroidism? Papillary or follicular thyroid carcinoma? • Bowel preparation (CT-Sellink prep, virtual colonoscopy)? Fasting (for CT of stomach or pancreas)? • Coordinate with anesthesiologist or pediatrician Intensive care patient on ventilatory support? Small child requiring sedation? • Determine

scan

When patient

protocol

arrives

for

(If not already

done).

examination:

Review the points listed above. Informed consent: Contrast administration Hypotensive agent (Buscopan lnterventional procedure • Bowel opacification • Hydration necessary? • Place needle for CM

orglucagon)

[positioninq • Radiation shielding) Comfortable

protection supine,

of the gonads prone,

or lateral

(apron,

other

decubitus

position • Padding

beneath

head, knees,

• If necessary, gastrointestinal tration is repeated • Buscopan

matomas. Almost all other types of soft-tissue examination will profit from the parenteral administration of contrast medium. Newer techniques rely on the analysis of multiple perfusion phases in contrast-enhanced scans. Bowel opacification is necessary for most abdominal CT examinations.

or glucagon

or lower legs contrast

administration?

adminis-

sound. MRl) should be considered as an alternative. The patient should come in advance if «al contrast will be administered. For CT examination of the upper abdomen. patients should be scheduled some 30minutes before the planned scan. and 60-90 minutes before the scan if the whole abdomen is to be examined with good bowel opacification. When the patient arrives on the day of the examination. the radiologist should review the contra indications to the procedure and inform the patient of potential side effects from the examination (contrast administration. use of antispasmodics or Hi/Hz blockers) or from any CTguided intervention. Gastrointestinal contrast material is administered orally if required. lfthe definitive protocol for the examination has not already been prescribed. it is done at this time. Preparation of Patients with Impaired Renal Function Iodinated contrast media (CM) can induce renal injury through a combination of changes in renal hemodynamics (initial vasodilatation followed by prolonged vasoconstriction) and direct tubular toxicity. Contrast nephropathy (CMN) can occur in patients with risk factors and is generally defined as an increase in serum creatinine >25% or >O.5mg/dl (45[Lmolll) within 48-72 hours compared to baseline. Established risk factors are pre-existing renal insufficiency, diabetic nephropathy, large contrast volumes, and dehydration. Possible risk factors include congestive heart failure, recurrent contrast procedures, and multiple myeloma patients with dehydration. For patients with normal renal function « l.Smg/dl or < 130Amolll) there are no renal

Preparation

contraindications for contrast -enhanced cr. In patients with risk factors. the serum creatinine levels provide an indicator ofrenal function. The creatinine clearance (CCr) as a better indicator of the glomerular filtration rate (G FR) can be estimated using the Cockroft-Cault formula: CCr(mllmin) ~ (140- age) x body weight (kg)/ (serum creatinine 0jimolll) x 0.81) For females a correction factor of 0.85 is usually used. Based on this formula. patients can be adequately stratified in risk categories (Table 3.2). As large volumes of CM are considered an independent risk factor. it is advisable to keep the contrast volume as low as reasonably possible. A useful rule was described by Cigarroa (1 mg/dl = 88 iimol/l): 5 ml CM per kg body weight (max. 300 ml) /

serum creatinine (mg/dl)

Hydration is an important part of prevention of CMN (Table 3.3). The inpatient can be prepared by infusing "100-150 ml/h of 0.45% saline in 5% glucose starting 4-12 hours before the examination. In outpatients. 1000ml can be infused starting approximately 30-60 minutes prior to contrast administration. Intravenous or oral hydration should be continued for 12 to 24 hours after the examination. Urine output should be monitored if possible. There is no associated benefit from using mannitol or diuretics. Nonionic low-osmolar contrast media. often already used routinely for all intravascular injections. are always indicated in patients at risk

of Patients with Impaired Renal Function

for CMN. The use of iso-osmolar contrast medium has not shown additional advantages. It is important to reduce other exogenous risks by withholding nephrotoxic medications such as non-steroidal anti-inflammatory drugs.

Table 3.3 phropathy

Suggestions for prevention of contrast (according to Waybill and Waybill, 2(01)

8» Identification

of patients

High risk patients - Patients < 25m - Patients

with stable

travenous administration. • Estimate creatinine clearance

clearance

creatinine

clearance

with stable

25-50 ml/min and risk factors" Moderate risk patients - Patients

with stable

creatinine

clearance

creatinine

clearance

25-50 ml/min - Patients

with stable

50-75 ml/min and risk factors" - Risk factors - Diabetic nephropathy - Congestive heart failure Recent administration of eM - High volume of eM required

risk

']

and identify

patients who are at moderate (c.f. Table 3.2). pPreparation

of patients

or high risk

at moderate

or high

risk

• Choose alternative imaging modality if possible. • Discontinue non-steroid anti-inflammatory drugs and Dipyridamole 48-72 hours before the procedure. • Withhold diuretics or before the procedure.

ACE inhibitors

for

24

hours

• Hydration in 0.45% saline

moderate risk patients: + 5 % glucose @ 1.0-1.5 ml/kg/hr (based on volume status) beginning 4 hours before procedure, and continuing 8-12 hours after the procedure.

• Hydration in 0.45% saline

high risk patients: + 5 % glucose @ 1.0-1.5 ml/kg/h (based on volume status) beginning 12 hours before procedure, and continuing 12-24 hours after the procedure. of patients

• Use low osmolar • Minimize volume Follow-up

creatinine

11m in

increased

• Measure serum creatinine in patients with (suspected) decrease in renal function, diabetes mellitus, or other risk factors scheduled for any in-

Examination Table 3.2 Stratification of contrast material-induced risk in patients with impaired renal function

with

ne-

at moderate

or high

risk.J

contrast material. of contrast material.

of patients

at moderate

or high

risk

• Discontinue Metformin 48 hours following the procedure, and restart only if creatinine levels are normalized. • Closely monitor urine output; and increase intravenous fluid rate to maintain input greater than output. The goal is to maintain positive fluid balance with hi~ urine flow rate. • If patient develops decreased urine output or progressive increase in creatinine, consult a nephrologist. • Check blood urea nitrogen and creatinine 24 hours after the procedure. Ifthere is any increase, admit the patient,

con-

tinue hydration and observation, and recheck daily until serum creatinine levels have returned to baseline (this may also be done on an outpatient-basis

in selected

cases).

85

86

3 Patient Preparation and Contrast Media Application scheduled shortly before the patient's ular dialysis session.

ACE inhibitors, diuretics, or the antiplatelet agent dipyramidole for 48 hours prior to the examination ifclinicallypossible. Metformin (e.g., Glucophage), an oral antihyperglycemic agent, has attracted interest recently because of the risk of causing lactic acidosis (pH <7.Z5, lactate >5mmol) in diabetic patients with impaired renal function. Current guidelines (e.g., European Society of Uroradiology, ESUR) advise the following: if the study is elective and renal function is normal, metformin should be discontinued for 48 hours following the examination. It can be reinitiated if renal function (serum creatinine) remains normal. If renal function is impaired (> l30'molll), alternative diagnostic modalities should be considered first. If CT is still required, metformin should be stopped and the exam should be postponed for 48 hours. Metformin can be reinitiated 48 hours after the study if renal function has not deteriorated. If the study is urgent and imperative, metformin should be stopped, patients should be hydrated as described above and renal function should be closely monitored. Patients with preterminal renal disease may need an individualized regime for preventive hydration because of an increased risk of developing renal failure. A recent publication suggests that a twoday regimen of hydration combined with acetylcysteine 600 mg twice daily-one day before and on the day of the examination-can protect the kidneys from CMN, even in patients with already decreased renal function. The role of other agents, like theophylline, prostaglandin El, dopamine, and the newer endothelin antagonists, in the prevention of CMN is still unclear. Patients on chronic dialysis can receive a normal contrast dose but the examination should be Table

3.4

Preparation

of high risk patients

140-50

Time

mg (methyl)prednisolone'

300 mg cimetidine2 50 mg

in 20-50 ml

diphenhydramine"

p.o. or i.v.

saline:'

I'

use nonimic

i.v.

contrast

• prophylactic short-term infusion of Hi (and HZ) antihistamine agents • prophylactic treatment with corticosteroids. When LOCM are used routinely for CT, only those patients with a history of a proven moderate to severe adverse reaction to contrast media should be premedicated if there are no alternative options (MRl, US). When corticosteroids are used in elective studies, it is important that the first dose is given at least "lZ hours prior to the CT examination. The chemotoxic effects are probably not improved by these agents. In most strategies the steroids are combined with Hi-antihistamines. There is conflicting evidence about the proved benefit of adding l-h-antihistamines, but at least a theoretical benefit is suggested. When LOCM are only used selectively, they should be used in all patients with known risk factors for adverse reactions: proven history of multiple allergies, bronchial asthma, severe congestive heart failure or recent heart attack, in children,

slim or obese

patients)

Category

of administration 2 h before the examination

Glucocorticoidsteroid

2 h before the examination

Hz-antihistamine

directly

Hi-antihistamine

during low-osmolar

The necessity to prepare patients depends whether or not non ionic low-osmolar contrast media (LOCM) are routinely used. There is controversy as to whether this risk can be reduced pharmacologically in the case of non ionic contrast media, but this has been proven for ionic media. Two basic precautionary measures are available:

12 hand

i.v.

18 G i.v. line Always

Preparation of Patients with Allergy to Contrast Media

(dose has to adapted

Ei~dicatipn

prior to the examination examination

media

A dose of 50 mg prednisolone is equivalent 12 Alternative: 50 mg ranitidine (e.g. Zantace') 13 Dilution at the discretion ofthe radiologist, 14 Alternative: 2 mg clemastine (e.g. Taveqile)

next reg-

to 250 mg hydrocortisone either

as a slow

injection

cr 10 mg dexamethasone or short infusion

Bowel Preparation

diminished renal function, and proven history of any adverse contrast reaction, For patients with a moderate or severe reaction in the past, proceed as above, A practical protocol is outlined in Tab Ie 3.4.

from these medications may be as high as the risk of llT in unselected patient groups, Premedication (Table 3.5) can be useful, however, in high-risk patients with known hyperthyroidism caused by Graves' disease, an autonomous adenoma in multinodular goiter, or a papillary or follicular thyroid carcinoma, Patients who are scheduled for diagnostic thyroid scintigraphy, or who are in the course of therapy with radioactive iodine should not undergo CT studies with iodinated contrast media without physician consultation, Injecting iodinated contrast material in non-premedicated patients with papillary or follicular thyroid cancer will make radioactive iodine treatment ineffective for a number of months and may seriously influence the overall prognosis of the disease,

Preparation of Patients with Hyperthyroidism Iodinated contrast media may contain free iodide in concentrations up to 20 [ig/ml apart from the organic iodine content During a routine CT examination there may be an administration of 2-3 mg free iodide, which corresponds to 10-40 times the minimum required dailyintake. In iodine-deficient areas (e.g. , parts of central Europe) there is ahigher incidence of multinodular goiter and autonomous thyroid tissue that may result in (non overt) hyperthyroidism especially in the elderly, When such patients are subjected to an excessive iodine load, this may result in iodine-induced hyperthyroidism or thyrotoxicosis (Il'T): which is often difficult to treat There are few controlled studies on the risk of developing 1lT after exposure to iodinated contrast media, One such study in unselected patients showed a very low incidence of llT of 0,03-0,2% with cases of llT occurring both in euthyreotic and hyperthyreotic patients, Prophylaxis is a matter of controversial discussion, even in patients from iodine-deficient regions, because the incidence of side effects

Table 3.5 cording

Possible

prophylactic

strategies

for

patients

Bowel Preparation The contents of the gastrointestinal tract can hamper or prevent evaluation of the stomach, duodenum, and colon on CT scans, Unless there is special emphasis on these organs, no special preparation is needed but it can be recommended to discontinue solid foods some 3-4 hours prior to the examination, In special investigations of the stomach and pancreas, solid foods should be discontinued the evening before the examination, The patient may continue to take oral medications, however. This makes it possible to achieve optimum dis-

at increased

risk for

iodine-induced

hyperthyroidism

to Hehrrnann. 1996)

Patients at risk Hyperthyroidism, Graves Latent hyperthyreodism Papillary thyroid cancer Follicular thyroid cancer Elective

CT

Sodium

Perchlorate

disease in patients

with

autonomous

adenoma

studies:

Thiamazol

3 dd 300 mg 1 dd 30 mg

Start 1 day before CT and continue for 8-14 days Start 1 day before CT and continue for 28 days

EmergencyCTstudies: Sodium

Thiamazol

perchlorate

1 dd 800 mg 1dd30mg

Therapy should be clinically Barium 2% should be used

controlled for bowel

Directly Directly

priorto CT and continue prior to CT and continue

with measurements opacification

of serum

with 3 x 300 mg for 8-14 days for 28 days

free T sand

TSH

(ac-

87

3

Patient Preparation

Table

3.6

'Stomach,

and Contrast Media Application

Bowel cleansing duodenum,

reduce motion artifacts. One should consider protecting highly radiosensitive areas such as the breasts and gonads with lead shielding in those examinations in which they are outside the scanned field. A wrap-around lead apron may be used in place of a gonadal shield. Keeping a lead shield in place within the scan range will cause major artifacts and mandates a repeat examination of that region. It has been reported, however, that partially radio-opaque shielding with bismuth garments (for the breasts) allows the shield to be kept within the scan field. To avoid artifacts, at least one arm (preferably both) should be removed from the scanning field (e.g., extended above the head). All metallic objects (zippers, wallets, ECG electrodes, metalreinforced ventilation tubing, etc.) should also be removed from the field.

for CT examinations

pancreas

No solid food for 12 h No food or liquids for 6 h Small

intestine

(CT enteroclysis)

No solid food for 24 h Low-residue diet High fluid intake (31/d) Mild laxative on day before

examination

Colon No solid food for 2 days, Laxative medication preparations) Mini-enema

bu.

copious

for 2 days

on morning

liquids

(preferably

'dry'

of examination

tension of the stomach and duodenum following the administration of butylscopolamine or glucagon and (negative) oral contrast medium. For special CT examinations of the small bowel (CT enteroclysis), solid foods are discontinued 24 hours before the examination. Although multiple methods will provide good results, a regimen of a low-residue diet, high fluid intake (3000 ml/day) and a combination of laxative medications (magnesium sulfate and bisacodyl) for one day is simple and works well. (Table 3.6). Special CT examinations of the colon (CT colonography) require the same kind of bowel preparation that would precede a barium enema. The regimen suggested above (low-residue diet, high fluid intake, and combination of laxative medications) can be used for two days, but prepackaged kits (e.g., Fleet) are also available (Table 3.6). "Dry" preparations, typically given for barium enemas, are superior to "wet" preparations (e.g. Golytely), typically given for colonoscopy, because there is less fluid left within the colon, and air can be used as a negative contrast agent.

Vascular Access Peripheral Venous Access

Intravenous access is established with a sufficiently large cannula (18-20 G), preferably placed in an antecubital vein. The vein should be substantial enough to accommodate the 3-5 mlls injection rate that is used in most protocols. Higher flow rates mandate larger cannulas(l6-17G). Central Venous Access

_

A central venous catheter (triple-lumen catheter or other catheter with a 6 G lumen) is suitable for contrast injection only at relatively low flow rates of 1.5-2.5 mlls (see manufacturer's guidelines). Depending on the type and length of the catheter it may become ruptured or dislodged due to excessive flow rates. If doubt exists, a trial NaCI injection should be performed manually to test catheter function, and the power injector should be coupled to a pressure-limiting device. Large central venous catheters such as an introducer sheath, a dialysis catheter, etc. allow for higher flow rates of 4 mlls or more, again depending on the catheter type used.

Positioning includes placing the patient in a comfortable, symmetrical position and providing adequate radiation protection. Padding should be placed beneath the head, knees, or lower legs to keep the patient comfortable and

Femoral venous access may be useful for CT angiography of the thoracic aorta but it is generally avoided where possible because of the higher risk of extravasation associated with

Positioning

Femoral Venous Access

Planning the Scan such a technique when performed on the CT table. It should be noted that many ordinary antecubital venous cannulas are not long enough to provide secure access in obese patients. In these cases a small insertion set can be used to thread a somewhat longer catheter safely through the femoral vein into the iliac vein (i.e.. using the Seldinger technique). If femoral access is required or if contrast medium must be injected through a pedal vein (especially in children) for abdominal scanning. it should be noted that high-contrast streak artifacts can persist in the inferior vena cava for up to 10 seconds after the end of the injection and can hamper the evaluation of adjacent structures. For this reason. lower contrast material concentrations can be recommended in these situations. •

Planning the Scan Planning the examination should be done as early as possible so that the patient can be adequately prepared and any missing information can be acquired prior to the examination. A scanning protocol that is appropriate for the scanner and for the actual requirements of the examination should be formulated and documented. In most cases. one will chose from a set of standard protocols. which may be modified according to individual requirements (e.g., in slim or obese patients, patients with impaired renal function or bad venous access). Such standard protocols should include information about patient preparation, data acquisition and image reconstruction technique as well as 3D postprocessing and filming. We recommend taking a written record of the individual scanning protocol on the request form, using an abbreviated code that simplifies the adaptation of standard protocols. We suggest using the notations listed in Table 3.7. This information should also be included in the patient comment on the CT scanner so that any subsequent problems can be investigated retrospectively to find explanations for unexpected effects in a particular examination.

Arterial Access

Certain examinations. such as CT during arterial portography (CTAP). require the placement of an intra-arterial catheter under fluoroscopic guidance in an angiography room. On the CT table a scanogram should be taken prior to contrast administration to recheck the catheter position.

Table 3.7 Scan

Suggested

notation

for examination

Spiral CT

parameters

Example

parameters

SCiTF/RI f /

5/8/4 f

Acquisition:

NXSCITFfi

Reconstruction:

SWIRl ax/cor/sag

4X1/5.5 T 5/4 ax, 1.5/3

Multislice CT:

sc=

TF = table feed direction, SW section width (mm), ax/cor/sag section

collimation

(mm),

A

Contrast

cor

(mmlrotation), =

RI

thin section

acquisition

5 mm axial,

1.5 mm coronal

=

reconstruction

interval

reformations

(mm),

f /\

«scan

section orientation

Example

parameters

Standard:

V/FID

100/2/100

Conrasr dilution:

V:NiFID

60:60/4/30

1: 1 dilution

= 150 mg/ml

NaCI bolus

V+NiFiD

100+50/4/20

SOml saline

flush

Bolus triggering:

V!FIDX

120/4/5A

bolus

Biphasic injection:

Vi/Fi+V2/Fz/D

50/5+50/2/20

fast injection

Biphasicscan:

V/F/Di+Dz

150/4/20+

arterial

70

triggering

S s after aortic

followed

and portal

threshold

by slower

phase

injection

scans

V= contrast volume (ml), F -flow rate (mils), De s can delay (s), Ne volume (ml) of dilution medium bolus, DX" delay after reaching trigger level at position X, contrast concentration 300 mg iodine/ml

or NaCI

Patient Preparation

and Contrast Media Application

To optimize image quality and to minimize radiation exposure of the patient either automated programs can be employed (most of which are just about to enter the market), or estimates of the required mAs settings based on the patient's size and weight may be used, Most of these approaches are still in their infancy, and no general recommendations can be made (see Chapters, p, 149), Documentation ofkVp, mAs, scanning mode, pitch and section collimation is

Gastrointestinal

Contrast

Media

Bowel opacification is necessary for the adequate evaluation of the gastrointestinal tract and for differentiating it from other structures of soft-tissue density, With good bowel opacification, pathologic changes in the bowel wall and lumen can be appreciated on the opacified scans.

Types of Gastrointestinal Contrast Media Contrast media for the bowel are classified as positive or negative, depending on whether the material is hyperattenuating or hypoattenuating relative to the walls of the gastrointestinal tract (Table 3,8), Water or methyl cellulose preparations are negative contrast media that facilitate evaluation of the mucosa following intravenous administration, They also do not superimpose on abdominal vessels and are therefore becoming a new standard for multislice CT of the abdomen,

Table

3.8

Types

of gastrointestinal

Medium contrast

(non-carbonated)

Methyl

cellulose

Paraffin

media

vegetable

Air

oils

pancreas

Tumor

diagnosis

Small

bowel,

colon

CT enteroclysis

Small

bowel,

colon

Recta!

Colon,

suspension

Applications

Stomach,

preparations

suspension,

Positive contrast

For use in CT examinations water or juice should not be carbonated, Methyl cellulose preparations can be administered orally (given adequate additives for taste) and have the advantage of a higher viscosity, The higher viscosity makes methyl cellulose ideally suited for CT enteroclysis or colonography. Water-based negative contrast media, however, provide insufficient detail if the bowel is not fully distended, and can hamper the detection of cystic lesions in the female pelvis, For this reason they are often used in conjunction with a spasmolytic compound (n-butylscopolamine, e,g" Buscopan) or are administered by enema or pump injection, Oily or fat-containing contrast media are not recommended for oral administration because of their unpleasant taste, which may lead to noncompliance, However, (full fat) milk has been described as a useful contrast medium for the upper abdomen, For administration by enema (colon) or pump injection (small bowel

media

Water

BaS04

contrast Region

Negative

required to be able to determine the patient's individual radiation exposure, If available, it is easier to use the volume CT dose index (CIDlYoi) instead, This number is an indicator of the locally applied dose and is displayed on the scanner console of most modern CT units for the specific protocols selected by the user. However, some manufacturers only display the CIDlYoiif explicitly asked by their customer.

stomach

application,

Only for virtual

poor oral compliance

endoscopy

media Stomach

+ bowel

Stomach

+ bowel

Universal

(contraindicated

if perforation

(contraindicated

in hyperthyroidism)

may be

present)

lodinated

solutions

Universal

Technique

enteroclysis) an oily contrast preparation improves the contrast between the bowel wall and the lumen, and makes it easier to distinguish between extra-enteric fluid and bowel, Air or COA as contrast media produce a very high negative contrast, which is particularly useful in virtual endoscopy of the colon (or stomach), C02 has the advantage over air that it is readily absorbed by the body and is eliminated by respiration, It induces less spastic response of the bowel wall and therefore is better tolerated by most patients, In addition, colon distension was reported to be superior with C02 as compared to air. Bowel distension can be further improved with injection of 20-40 mg of n-butylscopolamine (e.g. Buscopan) as soon the first symptoms (bowel colics) occur, Barium sulfate (BaS04) suspensions and l'odinated solutions are positive contrast media that have universal applications, Intestinal structures are clearly identifiable as such even when they are poorly distended, One disadvantage of positive contrast media is that they make mucosal surfaces more difficult to evaluate following IV administrationofcontrastmateriaL Barium suspensions have an agreeable taste and reportedly produce somewhat better opacification of the upper gastrotntesttnal tract They are contraindicated, however, in patients who may have a perforated bowel (clinical suspicion, immediate postoperative period, or after endoscopic intestinal biopsy), Iodinated contrast media should be avoided in hyperthyroidism and are contraindicated when patients are scheduled for scintigraphy or radiotherapy with iodine 131. Technique of Gastrointestinal Administration

Contrast

With few exceptions, oral contrast medium should be used routinely forCTexaminations of the abdomen. With spiral CT the bowel is opacified with a positive contrast medium, in which case an iodinated solution or BaS04 suspension may be used with equal success, Care should be taken that the medium is sufficiently diluted (3-4% for agents such as Telebrix Castro or Castrografin, 2% for barium suspensions), Spasmolytic agents are administered only if adequate distension of the duodenum is required,

of Gastrointestinal

Contrast Administration

Iodinated contrast media produce better opacification of the stomach and duodenum if the last cup of oral contrast is more highly concentrated (5%~IOml in 200ml water) and is ingested in the examination room, This will correct for the frequent increase in gastric secretions that occurs before the examination, For examinations of the upper abdominal organs, only the proximal small bowel requires opacification, This is accomplished by having the patient drink 500ml of contrast medium over a 30 minute period, Examinations covering the entire abdomen also require opacification of the distal ileum and perhaps the colon (for pelvic imaging), achieved by having the patient drink I to LSI of contrast medium, The doses should be fractionated to distribute them evenly over the ingestion period of 60 to 90 minutes prior to the examination, Starting the scan too early leads to insufficient opacification of the distal bowel segments, while an excessive delay between the last ingested portion of contrast medium and the CT scan leads to contrast problems in the proximal small bowel. Optimum bowel opacification requires accurate timing and adequate patient instruction, Excellent colon opacification is achieved when oral contrast administration is started the evening before the examination day, With their appointment, patients receive a bottle of 20 ml of contrast agent that they have to dilute to 600mL This can be taken with food in two portions and gives a good large bowel delineation, With multislice CT, especially when thinsection imaging is performed, delineation of the abdominal vessels becomes more important and requires use of negative contrast agents, Distinction between lymph nodes, masses and nonopacified bowel loops is less of a problem because of the increased spatial resolution of multisliceCT For the stomach and pancreas we recommend methyl cellulose preparations because of their increased viscosity, while water is sufficient for the upper abdominal examinations, The contrast material (500-1000 ml) should be given over a short period of time to ensure proper distension of the stomach and duodenum, For examinations of the whole abdomen, larger amounts of negative contrast material are required (1-1.51), and should be given 30-60 minutes prior to the examination, For the lower abdomen, some authors suggest a

91

92

3 Patient Preparation

and Contrast Media Application

combination of 500-1000 ml of positive contrast material given over a period of 30-90 minutes prior to the examination, followed by 500-1000 ml of negative contrast material given during the last 15 minutes prior to the scan, Sufficient time between ingestion of the two types of contrast material is required to avoid mixing, Even then, some portions of the small bowel or even the stomach may retain a

Table

3.9

Gastrointestinal

Region

contrast

luminal opacification that is identical to the attenuation of the bowel wall, which makes it impossible to evaluate mucosal or intraluminal abnormalities, Special techniques to achieve optimum opacification of the stomach, small and large bowel are summarized in Table 3,9 and are discussed in Chapter 15, Gastrointestinal Tract

administration

Anti

Technique

Comments

spasmodics Standard

protocols

Upper abdomen

500 mI positive

contrast

medium

Taken orally in fractionated over 30min period

amounts

Abdomen

1-1.51

contrast

medium

Taken orally in fractionated over a 60-90 min period

amounts

medium

Done occasionally

Special

positive

GI protocols 200 ml positive

Esophagus

contrast

to mark the lumen

on examination table or thick barium paste on exam table Stomach

500 ml water

before

250 ml water

on table

Tumor detection/staging In left lateral decubitus

exam

repeat for contrast Small

bowel

CT enteroclysis

1.5 I methylcellulose or 1.51 highly dilute BaS04

Rectum

500 ml positive

contrast

rectally

staging

in the small

1-1.51 positive contrast rectally or 1-1.51 negative contrast

Inflammatory disease Tumor staging/inflammatory

Stomach

3 packets

N.p.o. for 6 h

Coon

Rectal

Virtual

catheter

Done only to mark the rectum tumor

Colon

position,

series

via duodenal

for

pelvis disease

endoscopy

Parenteral

of effervescent

powder

air or C02 insufflation

Contrast

Bowel preparation

essential

Media

Intravascular contrast medium is necessary for most CT examinations of blood vessels and parenchymal organs, The contrast volume and mode of contrast administration vary with the nature of the study, CT scanning with intravascular contrast may have any of six objectives:

• interstitial enhancement (inflammatory lesions, tumors) • Urinary enhancement (excretion, pyelocalyceal system) • perfusion analysis (lesion characterization, tissue perfusion),

• CT angiography (vessel evaluation) • vascular opacification (to differentiate vessels from lymph nodes in nodal staging) parenchymal enhancement (lesion detection and characterization)

The same contrast media can also be used for the opacification of anatomic and pathologic spaces, like the spinal canal, lower urinary tract, pleura, peritoneal cavity, abscesses, or fistulas,

Complications of Contrast Material Injection

Number

Total

population

Known allergy Known

contrast

allergy

Severe adverse

reactions:

of cases

All adverse Ionic

169,284/168,363 12,913115,058 5785/9667

12.7%

3.1%

0.22%

0.04%

23.4%

6.9% 11.2%

0.53%

0.10%

severe

dyspnea,

Contrast

fall in blood

Media

Ionic contrast media are moderately well tolerated, depending on the injection rate used for IV administration (Table 3.10). Some patients experience warmth, pain, or burning at injection rates higher than 2mIls. For this reason, ionic contrast media are not recommended for use in spiral or multislice CT. They are still used in intracavitary applications such as fluoroscopic studies of the bladder and bowel. In many Western countries, most radiological departments have completely switched to nonionic media for all (intravascular) indications and therefore only few ionic media are still on the market. •

Nonionic

low-osmolar

Contrast

IMonionic

44.0%

Typesof Contrast Media Ionic High-osmolar

reactions

(ioniclnonionic)

Media

Nonionic contrast media are generally preferred over ionic media because of their lower osmolarity and significantly lower rate of adverse events (Table 3.10). There is also a lower incidence of reactions related to chemo- or osmotoxicity during contrast injection, making it safe to use rapid injection (flow) rates. The main disadvantage of nonionic media is their higher cost. Most contrast media used for intravenous application are monomeric molecules. They are in general better tolerated than dimeric molecules despite the fact that the latter are nearly iso-osmolar. Dimeric agents have a substantially higher viscosity and different hydrophilicity, and need to be heated up to body temperature prior to intravenous injection. Dimeric contrast agents are preferentially used for intrathecal and intra-articular applications (eT arthrography).

pressure,

cardiac

Complications Injection Contrast

0.72% arrest.

0.18% ansciousness

of Contrast

Material

Extravasation

The commonest complication of intravascular injections is contrast extravasation. Most often, this involves small volumes « l Oml), but when large volumes extravasate serious injury to skin and subcutaneous tissues may be produced. A number of risk factors have been identified: non-communicative patients (children, elderly), severely debilitated patients, multiple punctures in the same vein, and injections on the dorsum of the hand and foot. Treatment guidelines are outlined in Table 111.

Adverse Reactions to Contrast Media Parenteral contrast media can incite a variety of adverse reactions (Table 3.12), which may occur early (usually < 20 minutes) or late. The reactions

Table

3.11

Treatment

of contrast

extravasation

of the affected extremity heart • Ice packs - 15-60 min applications,

• Elevation

above the three

times

dally (day 1-3) • Close observation for 2-4 hours • Inform referring physician Plastic

surgery

consultation

when

• Extravasated volume >30 ml of ionic highosmolarCM • Extravasated volume >100 ml of nonionic lowosmolarCM • Skin blistering • Altered tissue perfusion (capillary refill) or change in sensation in hands

93

3 Patient Preparation and Contrast Media Application Table media

3.12

Symptoms

K Early reactions

of adverse

reactions

to contrast

(up to 60 min postinjection)

Nausea or vomiting Urticaria Diffuse erythema, angioedema Bronchospasm

pulmonary

edema

Respiratory arrest arrest

(

reactions

• Muk.pie

Most

(60 rrm to 3 days post-

irjection) Skin reactions

• Skin rash, itching, Systemic

risk factors

for adverse

reactions

risk factors

Chemotoxic

shock

Cardiac Delayed

General

• Previous • Bronchial

(requiring treatment)

Anaphylactic

media

Anaphylactoid

Vasovagal reaction Laryngeal edema, Hypotension

Selected

to contrast

• Ionic contrast media • Overt hyperthyroidism (contraindication) • Female gender (especially delayed reactions)

Mild to moderate

Severe

Table 3.13

• • • • •

reactions

serious contrast reaction asthma requiring treatment allergies (atopy) requiring treatment reactions

important

ones

include:

Cardiovascular instability Congestive heart failure grade III-IV Diabetic nephropathy Autonomous thyroid nodules in elderly Liver and kidney disease

swelling

reactions

• Headache, dizziness • Nausea, diarrhea • Chills, rigors

• Flu-like symptoms

Delayed Arm pain " Other reactions

may have an anaphylactoid etiology but can also result from osmolar effects (e.g .. direct irritation of the venous wall) or the number of carboxylic and hydroxylic side chains (chemotoxicity). The incidence of adverse reactions depends on whether the medium is ionic or nonionic and on the specific product that is used. It is also influenced by the concentration and volume of the contrast medium. the rate of contrast injection. and a number of patient-related factors (Table 3.13). Published incidence data vary dramatically depending on how adverse reactions are defined. A possible grading is as follows: • Minor reactions: nausea, vomiting, heat sensation. flushing. mild urticaria. sneezing • Intermediate reactions: mild hypotension. generalized urticaria. mild bronchospasm. diffuse erythema. angioedema. vasqvagal reaction • Severe reactions: hypotension (systolic pressure <70mmHg). pulmonary edema. epiglottic edema. severe bronchospasm. cardiac arrhythmia, cardiac arrest.

As a general rule, nonionic contrast media are well tolerated but are still associated with a 2-4% incidence of early adverse reactions and a 4-30% incidence oflate reactions (depending on definition). According to the study by Katayama in 1990, most early reactions are mild to moderate (3.1 %), and only 0.4% are severe (Table 3.10). The incidence of very severe reactions (causing critical illness or death) was 0.1 %. Late reactions are not uncommon but occur more frequently in patients who received interleukin2 therapies (10-30%). They often cause nonspecific symptoms like rash, mild fever, dizziness, chills, or rigors. In most cases no or only supportive treatment is indicated. Cutaneous

Reactions

Patients with cutaneous reactions are usually cooperative. Urticaria are anaphylactoid reactions resulting in raised focal areas of the skin and are most common in the face, neck and chest. They are associated with pruritus (itching) and can be mild or generalized. Patients may also develop diffuse erythema or subcutaneous (angio)edema that may develop into hypotension or alrway edema and therefore should be closely monitored. Respiratory

Reactions

Respiratory reactions usually result in agitation. Airway or laryngeal edema is associated with a feeling of tightness in the throat, a change of

Treatment of Adverse Reactions voice, or even inspiratory stridor. It can be preceded by swelling of lips or tongue, Bronchospasm can be seen in asthmatics and cause dyspnea with wheezing and difficulty exhaling, Pulmonary edema may be seen in patients with underlying congestive heart failure, The symptoms often develop slowly and dyspnea is the most important symptom Anxiety is a diagnosis of exclusion, but it can cause hyperventilation with chest tightness, •

Cardiovascular

Reactions

These are usually the more dramatic reactions in which patients are looking for help but may become unresponsive, Hypotensive patients often complain of light-headedness or weakness, Mild reactions often respond well to fluids and leg elevation, Vasovagal reactions are associated with bradycardia (pulse < 60bpm) and are more frequent in younger males, while tachycardia is suspect for an anaphylactoid reaction, It may be difficult to differentiate these two in patients that use (3-blocking agents, If patients do not improve after initial measures or if they become unresponsive, direct assistance should be sought

Treatment of Adverse Reactions

Oxygen should be administered

at high dosage (l0-1211min) via a face mask, preferably with an oxygen reservoir. This enables administration of 100% concentrations, Nasal cannulae are suboptimal for delivery of such high doses, All patients should receive a high dose regardless of the existence of chronic obstructive pulmonary disease, HI and HZ antihistamines are used primarily to reduce symptoms of skin reactions such as urticaria and diffuse erythema, They can also be used as a second-line drug for respiratory reactions after epinephrine has been given, Corticosteroids do not play an im portan t role in the treatment of the acute reaction, They are effective in prevention of delayed recurrences for as long as 48 hours, especially in respiratory reactions, When administered, a high loading-dose of 500-1000 mg hydrocortisone is advised which can be followed by constant infusion of 500 mg in 250ml saline at a rate of 60mllh, Overdosing in the acute phase is generally not problematic, 500-1000 mg hydro cor-

tisone is therapeutically equivalent to 125250 mg Prednisolone, 100-200 mg methylprednisolone or 20-40 mg dexamethasone, Epinephrine (adrenalin) is the most important medication, The 1:1000 dilution (l mg/ml) is used for subcutaneous or intramuscular injection for milder symptoms and is often available in 0,3 ml preloaded syringes (e.g, , Epipen ®), The 1: 10000 dilution (0.1 mg/ml) is used for intravenous injection in moderate to severe symptoms and may also be available in preloaded syringes, In a recent UK report from the Project team of Resuscitation Council, IV administration by non experienced personnel is strongly discouraged and requires monitoring, Instead, 1M injection of 0,5 ml 1: 1000 adrenalin is recommended, Care should be taken in patients using [3-blocking agents, as the selective a-adrenergic stimulation with vasoconstriction may lead to a hypertensive crisis (use isoproterenol instead), A detailed outline of the most important reactions and suggested treatment is given in Table 3,14,

Table 3.14 Treatment suggestions for adverse reactions to contrast media (protocols have to be checked with local regulations and available medication)

Ht.utancous symptoms Nausea or vomiting • No treatment needed Mild or asymptomatic urticaria: • No treatment needed Generalized

or symptomatic

urticaria:

• 50 mg diphenhydramine po/im/iv ·300 mg cimetidine or 50 mg ranitidine saline slowly iv Angioedema

or diffuse

n 20 ml

erythema:

• 50 mg diphenhydramine iv ·300 mg cimetidine or 50 mg ranitidine in 20 ml saline slowly iv • 0.5 ml epinephrine 1:1000 im-repeat after 5 min-if patient is hypovolemic • Saline 0.9% or Ringer's lactate rapid iv Respiratory

symptoms

Mild laryngeal

edema:

• Oxygen 10 I/min by face mask • 0.5 ml epinephrine 1:1000 im-repeat

after

5min Severe

laryngeal

edema-add:

• 300 mg cimetidine or 50 mg ranitidine in 20 saline slowly iv - Call code-consider tracheal intubation

ml

Continue

A

95

3 Patient Preparation and Contrast Media Application Table

3.14

Contrast Injection

(Continue)

lllliild

bronchospasm: • Oxygen 10 Ifmin by face mask • Albuterol, terbutaline or metaproterenol doseinhaler 2-3 deep inhalations (or nebulizer Smg i1 2 ml saline)

Severe bronchospasm-add: • O.Smlepinephrine 1:1000 • 500-1000 mg hydrocortisone (optional-see text) • Call

im-repeat

code t patient tracheal intubation

emergency

-consider I; Cardiovascular

after 5

rrm

iv or equivalent does

not respond

symptoms

Hypotension

with

bradycardia

(vagal

reaction):

• Elevate legs and release any abdominal pression • Oxygen 10 Ifmin by face mask • Saline 0.9% or Ringer's lactate rapid iv

com-

• 0.5-1.0 mg atropine slowly iv-repeat every 5minupto3mg Mild, isolated hypotension with tachycardia: • Elevate legs and release pression • Oxygen 10 I/min by face • Saline Severe

0.9%

or Ringer's

hypotension

with

any abdominal

com-

mask lactate

rapid

tachycardia

iv (shock)-

add: • 0.5 miepinephrine 1:1000 im-repeat after 5 rrm • 400 mg dopamine in 250 ml glucose 5%-0.2 mil mn infusion by pump (or 2-5 u,g/kg/min) • Call emergency code if patient does not respond Pulmonary edema: • Elevate head or have patient sit up • Oxygen 10 Ifmin by face mask • 40-80 mg furosemide slowly iv • 500-1000 mg hydrocortisone iv or equivalent (see text) • Call emergency

code

if patient

does

Angina: • 0.4 mg nitroglycerin sublingual • Oxygen 10 Ifmin by face mask • Call emergency code for cardiologist Cardiac arrest: • Start CPR • Call emergency Neurological

code

ECG

Anxiety reaction: • Monitor vital signs and reassurance • Paper bag for breathing (when hyperventilating) • If severe -2.5 mg midazolam or 5 mg diazepam slowly iv aspiration slowly

iv

All adverse reactions should be adequately documented in the radiology report-type of CM used, patient symptoms, 2 mg Clemastine dramine

iv

I 2 3 4 5

Contrast volume V (ml) Flow rate F(mlls) Scan delay D (s) Saline flush N (ml) Position X (ofthe reference region for bolus triggering) 6 Concentration C (mg iodine/ml) 7 Osmolarity (osmoll1) 8 Viscosity (kP) Parameters I through 3 are the main ones for contrast injections, while parameters 6 through 8 are specific for the product used and generally are not modified by the examiner. The injected contrast material can be more fully utilized for contrast enhancement ifthe contrast material is pushed forward and the injection vein is flushed by a bolus of isotonic NaCI (normal saline, parameter 4), Ifthe initiation of scanning is determined by the appearance of contrast medium in a reference region X (parameter 5), the scan delay D (parameter 3) denotes the interval from that point until a designated level of enhancement is achieved in that region, The contrast parameters should be documented as a comment on the images, as they may have important implications for image interpretation, We suggest using the notations from Table 3.1,

Intravenous and

immediately

• Oxygen 10 Ifmin by face mask ·2.5 mg midazolam or 5 mg diazepam

The most important parameters for intravascular contrast injection are:

not respond

symptoms

Seizures: ! Tum on side to avoid

Parameters

therapy, outcome and follow-up. iv alternative to 50 mg diphenhy-

Contrast Administration

In the great majority of cases, parenteral contrastmedium is administered by an intravenous route, Such an access route is quite long and leads to a defined sequence of vascular and organ enhancement with various mixing processes along the way to the target organ,

Basic Principles Contrast material has to flow through the injection veins into the vena cava, enter the right atrium, pass the pulmonary circulation and finally arrive in the aorta, Mixing with nonopacified blood occurs along the way to the right atri urn, In the rightventricl e, mixing 0 fop a cifi ed and non-opacified blood should be completed,

Intravenous Contrast Administration Once the contrast material has entered the aorta, it enhances various capillary beds during an arterial phase, enters draining veins that eitherjoin the vena cava or enter the portal venous system. Contrast material in the portal system enhances the liver parenchyma and drains into the liver veins before it reaches the right atrium again. As the contrast material flows back to the right heart from various organs, recirculation effects occur. Typical arrival times for various organ systems are shown in Table 3.15. Aortic enhancement ideally reaches a plateau phase shortly after the contrast material arrives in the aorta. The slope of this initial enhancement is an indicator of the cardiac output of the patient but may be slowed if there are any obstructions along the venous inflow. As more contrast medium is injected, the aortic "plateau" increases further from cumulative effects that culminate as the final amount of contrast material enters the aorta. Thus, the aortic enhancement "plateau" is not a real horizontal plateau but increases as the scan progresses (Fig. 3.1). Peak aortic enhancement is seen at the end of this phase. This peak enhancement should not be regarded mistakenly as the ideal time for imaging the arteries but it is well suited for imaging arterially perfused organs. Since the spontaneous flow in the injected veins is often slower than the injection rate, the inflow of contrast material slows as the injection is completed. This leads to a premature end

Table 3.15 Contrast arrival times after injection into the right cubital vein 6-12s 9-15s 13-20s 15-22S 16-24s 18-27s 22-33 s 22-30 s 22-30 s 24-32 s 120-250 s 30-45 s 35-50 s 50-80 s 120-250 s

Right atrium Main pulmonary artery Left atrium Aorta Carotids Renal arteries Femoral arteries Jugular vein Renal veins Suprarenal IVC Infrarenal IVC Splenic vein Mesenteric veins Liver veins Femoral veins

of the aortic plateau and does not take full advantage of the injected contrast material. A saline flush, injected at identical flow rates immediately after contrast administration, pushes the contrast material forwards and thus prolongs the aortic plateau phase. In theory, the time from contrast arrival to peak enhancement in the aorta should last about as long as the injection time (= VIF), but it often ends earlier in patients with a high cardiac output and lasts longer in patients with a low cardiac output. Most organs have an exclusively arterial blood supply. Only the liver and the lungs have a

300

200

Liver veins

100

Fig. 3.1 Contrast enhancement for the liver: example of a time-density curve for 150ml contrast material injected at 4mils.

o

20

40

60

80

100

120

140

Time (5)

3 Patient Preparation and Contrast Media Application

dual blood supply. In the lungs, there is a minor contribution from the bronchial arteries, which can increase in patients with pulmonary hypertension. In the liver, the main blood supply is via the portal vein (75-90%), and the remainder via the hepatic artery. If there is obstruction to portal flow or ifhypervascular lesions are present, arterial flow will increase. Organs with purely arterial perfusion such as the pancreas, the bowel, the bladder, or the adrenals are best imaged during peak organ enhancement, which occurs some 5-15 s after peak aortic enhancement. The kidneys are an exception because their perfusion characteristics are also determined by contrast excretion, and homogenous enhancement requires scanning in the nephrographic phase (80-100 s post injection). The liver requires scanning in at least the portal venous phase, which is always a phase that combines arterial enhancement with enhancement via the portal veins. Lesion detection and characterization is markedly improved if arterial phase scanning is performed as well. This phase should be timed so that there is optimum enhancement of tumor capillaries but there is still little portal enhancement.

Injection Parameters and Organ Enhancement The initial level of the aortic contrast plateau is important for imaging the arteries (CT angiography). It increases with the amount of iodine injected per second, i.e., with flow rate and contrast material concentration (see Fig. 3.4). If only very small contrast volumes are injected « 50ml in most patients with a normal circulatory state), the contrast enhancement curve (time-density curve) of the aorta shows only a single peak and does not level off (see Fig.3.3b). The maximum enhancement of this curve strongly depends on contrast material volume, a fact that is important to know if a very short scan duration is sufficient, such as with 16-slice CT angiography. For such a purpose, too Iowa volume of contrast material yields insufficient contrast enhancement. The peak aortic enhancement is important for imaging arterially perfused organs and tumors. It, too, increases with flow rate and contrast material concentration, but also, to a lesser degree, with the total volume of injected contrast material (see Fig. 3.3 b).

The duration of the aortic enhancement plateau increases with longer injection times, i.e., it increases with larger volumes and slower flow rates. In practice, there is a trade-off between the level and the duration of aortic enhancement. The faster the scanners (multislice CT), the higher are the flow rates and, thus, the higher the enhancement that can be maintained during the scan. At the same time, however, optimum timing of the scan becomes cruicia!. Portal venous enhancement depends mainly on the total amount of iodine injected. There are reports that suggest that faster flow rates increase early portal venous enhancement. For biphasic or multiphasic liver studies, the flow rate ("4 mlls) determines arterial enhancement, while the iodine load (=" 2 mllkg body weight at 300 mg iodine/ml) determines portal enhancement (see Fig. 3.4). Parenchymal enhancement of arterially perfused organs (e.g., pancreas) depends mainly on peak aortic enhancement and the total amount of iodine injected. The most important patient-related factors are weight, cardiac output and venous (or arterial) inflow obstruction. The higher the cardiac output, the greater the dilution effect and the lower the level of vascular enhancement. In patients with low cardiac output (see Fig. 3.6), arterial enhancement levels can be very high but one has to keep in mind that vascular and parenchyma! enhancement can be substantially delayed (the aortic plateau starting as late as 60s post injection). Thus, proper timing is an important factor (see below). Patient weight has moderate influence on peak aortic enhancement but strongly influences organ enhancement (pancreas and liver).

Contrast Injection For most contrast-enhanced CT studies, a mechanically controlled injection is necessary to achieve optimum results in terms of enhancement levels and contrast utilization. Peak vascular and organ enhancement and the duration of the plateau phase can be controlled by the proper selection of contrast volumes and flow rates. Based on the short scan time in spiral or multislice CT, imaging should be performed during a specific phase of enhancement (Ta-

Intravenous Contrast Administration Suggested start delay for specific contrast Table 3.16 phases (35 s duration of contrast injection)

Delay

Phase

Triggering' SA

Arterial phase (CTA)

205

SA

Portal venous phase

60s

155

Hepatic venous

80s

Hepatic

arterial

phase

phase

Parenchymal (pancreas)

phase

405

20A

Parenchymal

phase

405

20A

255

lOA

(bowel) Corticomedullary Nephrographic

phase

1005

phase

Excretion phase

5-15min

Systemic venous phase

150s

20V

(IVe) 1

Trigger ROI: A

cava

=

=

aorta,

S=

spleen, V

=

inferior vena

IVC

ble3.16) so that optimum results are achieved. In scanning protocols that include two enhancement phases. contrast administration is geared toward the arterial phase of enhancement. A power injector is essential for this type of study. using initial flow rates in the range of 3-5 mlls. For most clinical applications. monophasic contrast injections with a constant flow rate suffice. It has been shown, however, that an individually adjusted biphasic contrast material injection actually produces a more arterial constant plateau. This technique (see p. 103) is still too complex for routine use.

NaCl Bolus (Saline Flush) The injection of a saline bolus (saline flush) is useful only in spiral CT examinations that require arterial phase imaging. If only later phases are of interest, there is usually no need for a saline bolus. All injection techniques that employ small amounts of contrast material should include a saline bolus. Rationale

Saline bolus injection is based on the following considerations: when a contrast bolus is injected into a vein, the flow velocity in the vein usually returns to normal after the injection is completed. These normal velocities are highly

variable but generally are less than the injection velocity of the contrast medium. At the same time, venous pooling of contrast medium occurs in the veins through which the medium is delivered. Injecting a saline bolus immediately after the contrast bolus will flush the pooled contrast from the veins and should propel the contrast bolus ahead of the saline bolus at a constant velocity. This technique has been successfully used for some time in intravenous DSA and MR angiography, and can be suitably adapted for CT scanning. A saline bolus thus can be used to increase the duration of contrast enhancement, and may allow for a reduction in the amount of contrast material or for the use of higher injection rates at identical duration of contrast enhancement. In general, we recommend 40-60 ml of saline. Above that amount, almost no further improvements can be noted. A saline bolus yields the effect of contrast material for the cost of saline. Single-barrel CT Injectors

When using a single-barrel injector, at least a 200 ml syringe should be used for the injection. The contrast medium and NaCI are drawn successively into the syringe, using a technique that will cause the NaCI to form a supernatant layer on the heavier contrast medium. To perform the injection, the pump should be directed vertically downward (this is not possible with all devices and should be considered when the device is acquired). There are alternative techniques used to fill the syringe:

• Tip of injector pointing vertically downward: the saline is drawn first, taking approximately 10ml more than the planned volume of the NaCI bolus (a precaution against air injection). Air is expelled from the syringe, and the selected contrast volume is drawn carefully (slowest setting) into the barrel. Slight mixing will usually occur at the interface between the contrast medium and NaCI, but this will not significantly affect opacification.

Tip of injector pointing vertically upward:

the selected followed by ting) of the inadvertent

contrast volume is drawn first, careful drawing (slowest setNaCI bolus (+IOml to prevent air injection). This creates a

99

Patient Preparation

and Contrast Media Application

smooth meniscus at the interface of the contrast medium and saline. Air is expelled from the syringe, which is then carefully rotated 180 into the injection position, the NaCI bolus remaining above the contrast medium. This method provides a sharp demarcation between the injection solutions but is more laborious than the first technique. 0

Double-barrel

CT Injectors

and Roller

Pumps

Double-barrel syringe-based injection systems for CT are just appearing on the market. Like roller pumps with two or more reservoirs they markedly simplify the saline flush technique. The injector automatically switches to a separate saline reservoir or the second saline-filled barrel after the contrast bolus has been injected. Such injectors can also be used to vary the dilution of the injected contrast material by simultaneously injecting contrast and saline. Allowance must be made for the deadspace volume (5-30 ml) within the tubing system. This means that the volume of the saline bolus must be increased by an amount equal to the dead-space volume. Another potential difficulty is the fact that the tubing system is filled with saline rather than with contrast medium after each injection. This will not create problems if bolus triggering is consistently used. Otherwise the delay time to scan initiation must be increased correspondingly, or the tubing must be flushed with contrast medium.

Contrast Material Concentration Most institutions will use one concentration of contrast material as a standard for the vast majority of their CT examinations. All suggestions for contrast material injection given in this book rely on contrast material with 300 mg iodine/ml (unless stated otherwise). It has to be noted that the contrast enhancement in CT only depends on the local iodine concentration in the vasculature or tissues. In the vessels, this concentration is reached by a mixture of the injected contrast material with (non-opacified) blood. It depends on the injection rate of iodine in mg/sec. It should therefore be irrelevant whether contrast material of a concentration of 400 mg/ml is in-

jected at 3 mlls or whether a concentration of 300 mg/ml is injected at 4 mlls. Lower concentrations of contrast medium have the advantage of a more physiologic osmolarity, resulting in better tolerance and less intracapillary dilution from the back-diffusion of interstitial fluid. They are also less likely to cause high-contrast artifacts in the injected veins (important in chest CT), resulting in a theoretical preference for less concentrated media. However, they produce less vascular and organ enhancement than more concentrated media injected at an equal rate. To achieve the same level of enhancement, the fiow rate must be increased correspondingly. This means that the contrast volume also must be increased to achieve a similar duration of enhancement, but this taxes the limitations of CT injectors and may impose an excessive volume load on the patient. Only a few manufacturers supply reduced concentrations of contrast media in injection bottles of adequate size. Users can make their own dilutions by adding NaCI solution or injectable distilled water. It should be noted that 0.9% NaCI will cause less change in the solution osmolarity than distilled water. Excessive dilution of contrast media with distilled water may produce hypotonic solutions, with an associated

risk of edema

Higher concentrations of contrast medium allow the same amount of iodine to be injected at a lower flow rate than standard concentrations. Higher concentration of contrast material are particularily suited for short scan duration with multislice CT (4- and especially 16-slice scanners) because improved arterial enhancement over lower concentrations has been reported, even if identical amounts of iodine are injected per second. High-concentration contrast materials (>350mg iodine/ml) have a substantially increased viscosity and require pre-warming of the contrast material. Without the use of a saline flush, the duration of contrast enhancement drops early because the (more viscous) contrast material remains in the injection" veins.

Intravenous ContrastAdministration dium by IV bolus injection (preferablyusingmechanical injection at the same rate as the diagnostic scan) and performing several trial scans in a reference region using the lowest possible tube output. The scans may be taken at 1-2 second intervals, depending on the dose and type of scanner. Even for these trial scans, it is advantageous to have an 8-12 second delay from the start of the injection to the initial scan. The delay time depends on the patient's presumed circulatory status but rarely exceeds 20 seconds (except in patients with heart failure or valvular disease). The test is evaluated by identifying the image that shows maximal enhancement in the target region (e.g., the aorta). The time to peak At, measured from the start of the injection to the time index ofthe selected image, equals the optim um scan delay time. Experience shows, however, that the best results are achieved by adding 2 to 5 seconds to the calculated delay time (Table 3.17).

Scan Delay Ind ivid ual d iff erences in circulation tim e are difficult to assess since they are based on heart rate, patient age, and previous illnesses. Because ofthese differences, the scan delay time should be individualized so that imaging can be performed in the optimum phase of enhancement. This is the more important the faster the data acquisition becomes. Modern multislice scanners require just 6 s for a liver scan with 4x 2.5 mm collimation, and the next generation of 16-row scanners requires even less time (2-4 s. depending on collimation). The risk of mis-timing the injection increases with such equipment. The arterial phase of perfusion is the most difficult one to capture. Individual adaptation is mandatory for fast scanners. The parenchymal or portal phase of perfusion is less critical, thus standard delay times can be used for most routine examinations. Even in these cases, better results can be achieved by individualizing the scan delay (Table 3.17). •

Bolus Triggering

A test bolus is not the preferred method for determining the scan delay. A more efficient method is bolus triggering, in which the contrast bolus itself is used to initiate the scan. A growing number of manufacturers are offering this technique, in which "monitor scans" are

Test Bolus

The injection ofa test bolus is a robust method available for determining the individual circulation time and the optimum scan delay. It consists of administering 10-20 ml of contrast meTable

3.17

Tailoring

the scan delay to the patient's

circulation

time

Test bolus Reference

Contrast Scan

region:

parameters:

parameters:

Scan delay 0

Ascending

aorta Start of scanning

(Thoracic

(CTA

region

CTA)

of neck or abdomen)

10/5/8-12 5 mm section collimation, ning sequence: 20-30 s Calculated D = At + 2

minimal

from time to peak

dose,

scans

at 2-4

s

intervals;

duration

of scan-

At.

s (At < 20 s) D=At+5s(At>20s)

Bolus triggering Target

region:

Spleen

(5) or portal

Contrast parameters:

According

Scan

Section

parameters:

(Thorax, eTA)

Ascending aorta (A) Abdominal aorta (A) to protocol,

collimation

Scan delay 0

Depending

Notation:

V/FIDX, aorta

(Arterial phase in liver or pancreas) (Portal venous phase)

veins

no test injection

as in spiral scan,

on target

region:

e.g.: 120 + 60/4/5

A

minimal

dose,

delay starts when =

scan

initiated 5

scans at 1 to 4s intervals

enhancement

s

in

after appearance

ROI

exceeds

of contrast

50

HU

in the

101

3 Patient Preparation and Contrast Media Application

taken at a suitable table position using a greatly and the contrast material is injected rapidly C'mlls). On the other hand, a delay of <4s reduced dose, and a region of interest (trigger ROI) is defined in which the progression of en- does not allow adequate time to give breathing hancement is monitored (Fig. 3.2). Once an ade- instructions. We suggest that patients should quate level of enhancement is achieved (deter- hyperventilate immediately prior to contrast inmined visually or by a threshold CT number, the jection but breathe shallowly and not hold their trigger threshold), the table moves to the breath during the trigger scans. The position of the trigger ROI should be tailored to the clinical starting level, and helical scanning is initiated. For technical reasons, there is a delay of problem and to the time delay that is necessary 3-9 seconds between the last monitor scan and for giving breath-hold commands (Table 3.17). the start of helical scanning. Up to 50 ml of con- Because of a delay of at least 5s in most trast medium may be injected during that time, patients, it may be advisable to go upstream depending on the flow rate. Thus, bolus trigger- with the trigger ROI (e.g., descending aorta for ing for purely arterial imaging (CT angiography) abdominal studies, or ascending aorta for the should not be performed if the delay exceeds 5 s chest), and to use a relatively low trigger threshold of 50 H, which is above the level of most artifacts, and markedly below the enhancement level reached later during the examination. Alternatively, the scan can be started manually as soon as the contrast material is seen in the target region. Problems may arise if the software is cumbersome and makes the procedure time-consuming. Artifacts may occur in trigger ROIs in the aorta at the level of the diaphragm (respiratory artifacts). Thus, this level should be avoided. Instead, we suggest a trigger level either in the descending aorta, well above the diaphragm, or in the abdominal aorta, at the level of the SMA. Positioning the trigger ROI in the cardiac ventricles may be tricky because the scanograms are acquired during inspiration b a while the monitor scans are performed during 90 shallow respiration. A table position about (HU) 80 ,,",Enhancement ,., , ..",' " ,.. """" "'" ..,,'" 4-5 em below the tracheal bifurcation yields 70 _. - . ... \ i goods results in most patients. 60 "", "' .., ""..,'" "'"'' ,"" """",~"".'''".."'" ,. '"''''''..,.." ! Triggering can also be performed for exami50+-------~--------~--~--- nations of parenchymal organs. Choosing a trig40 ger ROI within the organ parenchyma, however, 30 is risky. If desired trigger level is not reached, ... h··············t--' 20 10 ,,,", .,,,,, ,,,,, ..',, the examination fails or a "default delay" has to , . --... ..... -. _. ... be used. In patients with excellent parenchymal o enhancement, the trigger level is reached too 10';'" --" , Time (5) 20+-------~1--------~----~~ early, starting the scan before the optimum en10 15 o 5 hancement could be reached. For this reason, triggering in the aorta is more advisable, but the Fig. 3.2 Bolus triggering. Placement of trigger ROI (a), delay has to be adjusted to the time expected last image of series of low dose trigger scans (80 kVp, for proper organ enhancement. 2mGy) (b). Scans were started 10s after begin of the I .••

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contrast material injection and were repeated every 1.5s. Time-density curve (c). As soon as the trigger threshold of 50 h over baseline is surpassed, the spiral scan is initiated. Note that there is an additional delay of 2-8 s with most scanners before the actual data acquisition starts.

Intravenous ContrastAdministration can calculate how the patient would respond (in terms of enhancement curves) to larger amounts of contrast material or higher flow rates. Such calculations show that the aortic enhancement after injection small volumes of contrast material has a short peak that is substantially less than with larger contrast volumes (see Fig. 3.3b). This behavior suggests that contrast volumes cannot be proportionally reduced as the scan duration decreases with new 8- and 16-slice scanners. For larger volumes there is a bent in the enhancement curve with a more rapid increase in the initial phase and a smaller increase as more contrast material is injected. Maximum enhancement is reached close to the end of this" enhancement plateau".

Individualized Contrast Material Injection

Circulatory parameters such as cardiac output, patient size, and inflow obstructions have a substantial influence on contrast enhancement in individual patients. Two main approaches towards a more individually adapted contrast material injection have been suggested: mathematic modeling and Fourier-deconvolution of a test bolus injection. The test bolus technique (Fourier-deconvolution technique) uses a test bolus injection to determinetheindividual patientresponsefunction (for the aorta) to the injection of contrast material. Assuming a linear behavior of the patient's circulatory system towards contrast material, one

8

16 ml@4 mils

Enhancement (HU)

300 4

2

o

Time (s)

o

8

16

24

100

o

32

8

I~-----~:: 8

16

24

32

40

48

56

64

72

00

40

48

56

64

72

80

-uu

Contrastbolus

Enhancement (HU)

8x16ml@4ml/s

6+-----------------·~------

300

4

zoo

2

100

o

o

8

16

24

32

Fig. 3.3 Individual adaptation of contrast material injection [modified from Fleischmann 1999]. The timedensity curve derived from a test bolus injection (a) can be usedto calculate an individual patient function that allows the effect of varying injection parameters on contrast enhancement to be predicted (b). Note that the aorticenhancement "plateau" grows over time and that the maximum enhancement also grows as larger

o

o

8

16

24

32

volumes of contrast material are injected. By setting a target level for contrast enhancement and the duration of the plateau phase, an ideal contrast injection curve can be calculated (c). Approximating this ideal injection curve with a biphasic injection with a rapid initial and a slower later injection phase agrees closely with an ideal enhancement plateau (d). Fig.3.3c.d

>

104

3 Patient Preparation and Contrast Media Application

10

i Theoretical ,ideal' enhancement

Theoretical ,ideal' bolus

8

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time (5)

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Fig. 3.3(, d

Conversely, it is also possible ttl pre-determine the level of contrast enhancement and the duration of the plateau phase, and then calculate the volume and flow rates required for this individual patient (Fig. 3.3). It could be shown that an individualized biphasic injection with a smaller amount of contrast material injected at higher flow rates followed by a larger amount injected with slower flow rates can produce a more constant enhancement than a monophasic injection. Up to now, however, the goal of reaching a predefined height of the plateau phase cannot be achieved in all cases. Mathematic modeling uses patient-specific data to determine the individual organ enhancement produced by a specific contrast injection protocol. It is also possible to calculate estimates for the volume and flow rates of contrast mate-

rial required to obtain a specific enhancement level in a target organ. This approach could show that an exponentially decreasing flow rate is ideally suited for a constant enhancement plateau in the aorta and various organs (Fig. 3.4). The model predicts that aortic enhancement increases as higher flow rates are used but hepatic enhancement remains almost independent of flow rate (Fig. 3.5). It also predicts that maximum liver enhancement can only be increased by injecting higher amounts of contrast material. With this model, the effect of body weight and cardiac output can also be determined. Liver enhancement increases almost proportional with the injected amount of iodine per kg body mass. Conversely, liver enhancement for a given contrast material volume depends on

Intravenous Contrast Administration Exponential decrease of flow rate

Optimized

enhancement

5 4 ---------------.-----

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20

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30

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Time (s)

Injection duration (s)

Fig. 3.4 Mathematic modeling predicts that an exponentially decreasing flow rate for contrast constant plateau of contrast enhancement [modified from Bae and Heikenj.

Fig. 3.5 Mathematic modeling allows the effect of increased injection speed on aortic and hepatic enhancement to be predicted (a). Note that maximum aortic enhancement increases while maximum hepatic enhancement remains largely independent offlow rate [modified from Bae 1998].

50

injection will yield a

Contrast enhancement (HU)

350 300 250 200 150 100

liver parenchyma'

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-- ..

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o

2

4

6

8

10

Flow rate F (mils)

patient weight and decreases for more obese patients. The effect of weight on aortic enhancement is much less. Conversely, cardiac output strongly effects maximum arterial enhancement (Fig. 3.6 a) but dose not effect maximum hepatic enhancement. However, hepatic enhancement is substantially delayed with low cardiac output (Fig.3.6b).

The system is not yet commercially available and suffers from the fact that cardiac output cannot be precisely determined beforehand.

Perfusion Studies

The administration of 30-80 ml of contrast medium by rapid bolus injection (4-10 mlls) can be used to study the perfusion characteristics in a specific organ region (e.g., for evaluating a focal hepatic lesion). CT scanning in these cases is performed with a stationary table, and contrast dynamics are analyzed through the inflow, peak, and clearance phases of enhancement. For optimal results, administration with a power injector and standardized flow rates is advisable. Two basic concepts for image analysis are available: one is based on the slope of contrast

105

3 Patient Preparation and Contrast Media Application

500

Fig. 3.6 Effect of decreased cardiac output on aortic and hepatic parenchymal enhancement in an experimental porcine model [modified from Bae 1998].

Cardiac output

==>

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enhancement (see Fig. 24.71) while the other depends on Fourier deconvolution techniques. Both techniques are able to calculate various circulatory parameters for each pixel in the image. The simpler, slope-based technique is more robust but requires higher injection rates and does not allow for absolute flow measurements. The other technique could also work with slower injection rates and provides calibrated numbers, but is more susceptible to noise and data inconsistencies. Both techniques, however, require data sets that are free from respiratory motion, a fact that makes them difficult to use for most body applications.

120

140 Time (s) ~

Direct Arterial, Portal, or Venous Contrast Administration

Arterial Contrast

Direct intra-arterial contrast administration through an angiographically placed catheter is necessary only in very selected cases. The commonest application is CT arteriography of the liver for the detection of hypervascular lesions. This procedure has been superseded almost completely by dual-phase scanning with intravenous contrast or MRI. Other applications include determination of vascular territories that are supplied by a particular vessel.

/\

DirectArterial, Portal, or Venous ContrastAdministration Because the injected artery is usually located within the target region, the contrast concentration C should be reduced to 150mg/ml to avoid artifacts, Flow rates should be similar to the ones used for angiographic injection of the same vessel.

Portal Contrast Generally the portal vein is, opacified indirectly by means of splenic or mesenteric portography, This procedure, called CT during arterial portography (or CTAP) , relies on angiographic insertion of a catheter into the superior mesenteric or splenic artery, It is unclear which of the two vessels is better suited for the procedure, Some authors recommend the simultaneous catheterization of both arteries for best results, A minimal dose of contrast medium should be used for angiography to avoid the premature enhancement of hepatic lesions, CT scanning is performed during intra-arterial injection of the contrast medium, A great variety of scanning protocols have been recommended in the literature (see also Chapter 11), The only means of direct portal opacification is through a catheter that has been inserted intraoperatively into the portal vein or superior mesenteric vein, This would be done only at the time of an arterial port insertion for regional infusional chemotherapy of the liver. Given the large tumor size in most of such cases, this procedure is very susceptible to artifacts and can be recommended only in patients with small lesions.

Venous Contrast There are few situations in which the high, uniform contrast enhancement of systemic venous structures can be achieved in CT examinations, Homogeneous venous opacification does not appear until the venous phase of enhancement, i.e, approximately 40-100 seconds after the start of the intravenous contrast injection, depending on the vascular region, Generally only a moderate level of enhancement is observed, Given the differences in the perfusion times of different organs, the mixing of opacified and nonopacified blood at venous confluences can

mimic an intravascular filling defect (pseudothrombus artifact, see Fig, 7.39), If scanning is performed during the direct enhancement, the contrast medium should be diluted by about 1:3 to 1:5 to avoid high-contrast artifacts, Even when this technique is used, artifacts can still occur due to mixing effects, Artifacts can also result from layering of the heavier contrast medium in dependent areas of laminar flow (e.g, within the inferior vena cava), Consequently, the injection should be performed bilaterally if possible (in both arms or legs), and the inferior vena cava should be imaged during an interval of adequate flow to promote mixing of the contrast medium with non opacified blood,

Intrathecal Contrast Administration Intrathecal contrast administration is useful for the optimum CT evaluation of spinal lesions or intervertebral disk disease, CT scanning is preceded by lumbar myelography using a standard technique (nonionic dimeric contrast material), The patient remains supine for about 1 to 2 hours and is then scanned prone, This interval is necessary to reduce the concentration of contrast material and avoid high-contrast artifacts,

Intracavitary Contrast Adm inistration Parenteral (preferably nonionic) contrast media can also be used for the opacification of anatomic and pathologic spaces, This is most commonly done in urinary tract studies (antegrade contrast administration through an indwelling nephrostomy tube or retrograde administration through a bladder catheter), Direct contrast injection can also be used to visualize abscess cavities and fistulous tracts, which is especially useful in the pelvis, Intraperitoneal or pleural contrast administration (dilution 1:10-30) is used only in highly selected cases for the imaging of encapsulated areas, demonstration of continuity between regions (Fig, 3.7), or for imaging small peritoneal metastases,

108

3 Patient Preparation and Contrast Media Application Fig. 3.7 Intraperitoneal contrast material (diluted to < 30 mg iodinel ml) demonstrates communication between abdominal ascites and an intractable left-sided pleural effusion.

a

b

jC;liiAagsgaa;'>SsA'aAiAsAA

High-contrast artitacts can be prevented by adequate dilution of the contrast medium (1:3 to 1:5, i.e., < 100 mg iodine/ml). If necessary they can be somewhat reduced by using the highest possible tube current (usually 140kV).

4 Optimization of Scanning Technique M. Prokop

Scanning

Strategy

.

Scanogram (p. 110) Settnq up the Scan Sequence (p. 110) Scan Parameters

.

General Acquisition Parameters (p. 112) Gantry Angulation (p. 112) Scan Field of View (SFOV) (p. 113) Scan Time and Rotation Period (p. 113) Scan Duration and Phase of Respiration (p. 113) Scan Length (p. 114) Scanning Direction (p. 114) Tube Voltage (kVp) (p. 115) Tube Load (mAs) (p. 115) Tube Output Characteristics (p. 115) Single-slice Spiral Acquisition Parameters (p. 116) Section Collimation (p. 116) Table Feed and Ptch (p. 117) Multislice Acquisition Parameters (p. 118) Section Collimation (p. 119) Table Feed and Pitch (p. 120) Vendor-specific Approaches (Four-slice Scanners) (p. 121) Eight- and 16-row Multislice Scanners (p. 122) Reconstruction and Display Parameters (p. 123) Section Width and Raw Data Interpolation (p. 123) Reconstruction Interval (p. 124) Convolution Kernel (Reconstruction Field of View (p. 127) Window Selection (p. 127) Image Processing (p. 128) 2D Processing (p. 128) 3D Processing (p. 129) Special Scanning Techniques (p. 130) Ouantitative CT (OCT) (p. 130) Dual-Energy CT (p. 130)

Algorithm)

(p. 124)

4 Optimization of Scanning Technique

Scanning Strategy Scanogram

The scanogram, known alternatively as a scan projection radiograph, topogram, scout view, or pilot scan, is used to locate and define the regions to be examined by CT. A scanogram is obtained by keeping the x-ray tube stationary while the patient is moved through the scan plane, producing a projection radiograph that resembles a conventional plain film. In the scanogram, however, the contiguous slit scans and collimated detectors greatly reduce scattered radiation compared with plain films. This has numerous advantages, such as a marked improvement in the delineation of the abdominal organs (important in determining the scan length). Spatial resolution is relatively poor, however, depending on the selected slice thickness (0.5-2 mm). Especially when low-dose scanning is performed, the scanogram may contribute substantially to overall patient dose. For this reason, the lowest possible mAs and kVp settings should be used in such situations (Fig. 4.1). Setting up the Scan Sequence

The following guidelines are helpful in devising a strategy for CT examinations:

additional, secondary target regions. For example, the pancreas is the primary target organ for thoracoabdominal staging in a patient with known pancreatic carcinoma while the thorax, liver, and the rest of the abdomen represent secondary target regions. (Intravenous) contrast administration should be tailored to the primary target organ. If timing is not critical for the primary target region, use the secondary target organ for which contrast timing is most critical. Determine the enhancement phases in which imaging should be performed (see chapters dealing with specific body regions). Next decide how many scans are required and determine the sequence in which the primary and secondary target regions will be scanned. The choice of scan parameters strongly depends on the available technology. With multislice CT scanning, either a volumetric imaging protocol with near-isotropic resolution or a fast spiral scanning mode with rapid data acquisition will be chosen. With single slice spiral CT the length of the individual scanning regions and the available time window determines the choice of scan parameters.

• The clinical problem will dictate the primary target organ for scanning as well as

Fig. 4.1 Scanogram using 120 kVp and 100 mAs (manufacturer's standard) (a) and 80 kVp and 50 mAs (approximately one fifth ofthe dose)(b). a

b

Setting up the Scan Sequence

Increasing attention is being given to radiation safety issues in CT examinations. The level of radiation exposure in CT is multiple times higher than that incurred in conventional radiography. This mandates ajudicious selection of imaging parameters, for even slight changes in the parameters can lead to marked absolute differences in radiation exposure. An effort should be made to reduce radiation exposure in accordance with the patient's size and the clinicalpro blem. The diagnostic accuracy of CT will generally be enhanced with a more specific clinical request and a more limited target region • Non-contrast scanning followed by a contrast -enhanced examination are usually indicated in patients with suspected hemorrhage, renal lesions, or suspected hypervascular hepatic tumors. • For most contrast-enhanced studies, the upper abdominal organs should be scanned first during the contrast injection, because organ perfusion and enhancement in that region will critically influence the quality of the examination. • Thoracic scanning should precede examination of the lower abdomen, because

111

vascular opacification in the pulmonary hilar region is more critical. The neck should be scanned last (interstitial enhancement), preceded if necessary by the injection of additional contrast medium. Repositioning of the arms should be considered when neck and chest have to be examined. Keeping the arms in the scan filed will cause major image degradation (Fig.4.2) The scan parameters should be tailored to the scanned region according to the anticipated lesion size, bearing in mind the limitations imposed by the available scanner technology. For CTA and virtual endoscopy, the smallest possible FOV (for better resolution) and a soft convolution kernel (for less noise and better 3D rendering) should be used. Sample protocols for CT examinations of varying complexity are shown in Table 4.1.

b Fig. 4.2 Raising the arms above the head should be tried whenever possible for scanning of the chest and abdomen: There are substantial streak artifacts with the

arms placed next to the body (a) while raising at least one arm above the head leads to a substantial reduction in artifacts and decreases dose requirements (b).

112

4 Optimization of Scanning Technique Table 4.1 Single-slice

Sample protocols of varying complexity D

for a single-slice scanner (left) and 4-slice scanner (right)

MultisliceD SC/TF/RI

NxSC/TF

V/F/DX

SW/RI'

MPRb

V+N/F/DX

7{12{6

1. Chest, inspiration

1{10 1{30

t t

5{1O{5 2. Tumor + hila:

t

2{4{2

1 i

noncontrast

4x2.5{15

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noncontrast

8{8

HR'

1{1

HR'

1{1

noncontrast

1. Adrenals:

4xl16

j

3{2

90{3{30

2. Chest: tumor region

4x1{6

i

3{3 1.5{1.5

non contrast cor

90 + 50{3{30

cor+saq''

lymph node staging token over 1~hr p'eriod

- Preparation: 7 L oral eM 1. Liver + chest:

7{12{6

i

120{2{40

1. Liver to neck:

2. Rest of abdomen:

7{12 6

t

105 ISO

2. Rest of abdomen:

- Reposition arms next to body 3. Neck:

3{5{3

t

jl t

Suspected retroperitoneal hemorrhage 1. Abdomen

d

50{2{20

4x2.5{15

t

5{4

~=~~~:=====:;:i::;:;i===1:::::50{4{25 lOs ISO

non contrast

CPR

150+50{5{5A'

cor

lOs ISO

105 ISO

lOs ISO

SC = slice collimation (mm), TF = table feed (mm{rotation), RI = recon. increment (mm), i t = scanning direction, SW = effective section width (mm), MPR = multiplanar reformats. axial = axial sections. cor= coronal, sag = sagittal eM = contrast material, Contrast concentration = 300 mg/ml iodine V=volume of CM (rnl). N = volume of saline flush (ml) F=flow rate (ml{s), 0= start delay, X=trigger region a axial section for diagnostic review, an additional reconstruction to create a "secondary raw data set" may be necessary b MPR are reconstructed from a .secondary raw data set: with SWIRl = 1-1.5/0.7 Pancreas: CPR along pancreatic duct; CTA: MIP I VRT for routine display mode, CPR for stenoses C sequential CT d reconstruction from the same raw data set e read: 5 A = start delay 5 s after reaching 50 HU enhancement in the infrarenal aorta

Scan Parameters User-selectable scan parameters are tailored to the specific clinical request so that adequate spatial and contrast resolution will be achieved. Reasonable trade-offs should be made that will balance the requirements of an acceptable radiation dose with acceptable image quality. The selection of scan parameters is significantly influenced by the available scanner technology. For the notation of scan parameters, we recommend the abbreviations listed in Table3.7.

The following section describes the various trade-offs for the choice of data acquisition, reconstruction, and display.

General Acquisition •

Parameters

Gantry Angulation

Most IT examinations do not require gantry angulation, i.e., the scan plane remains perpen-

General Acquisition Parameters dicular to the table plane. In intervertebral disk studies, however, the gantry angle should be adjusted on the lateral-projection scanogram so that the scan plane is parallel to the center of the intervertebral disk space for each level that is examined. Gantry angulation can also be used in neck scanning to minimize dental artifacts and should always be used in head examinations to exclude the eyes from the scan field. With thin-section multislice scanning, gantry angulation is almost never necessary for body imaging because any desired imaging plane can be reconstructed after the data set has been acquired (see Fig. 25.45). In some scanners, gantry angulation will cause distortion in 3D reconstructions or will not be available for spiral scanning. •

Scan Field of View (SFOV)

On some scanners, the size of the scanned field can be reduced to increase the sam pling rate and thus improve spatial resolution. This is feasible mainly for relatively small object diameters (neck, extremities). If the object diameter exceeds the SFOV, "out-of-field artifacts" will appear at the periphery ofthe object (see Fig. 7.45). For this reason the object should be precisely centered within the gantry aperture, and, if in doubt, a trial scan may be advisable. Out-of- field artifacts influence only a relatively narrow band in the periphery of the (round) SFOV (see Fig. 7.45b). This can be used for indications with a focused interest on only a portion of the body cross-section (suchas in multislice cardiac imaging, see Chapter 23). When positioning the structure ofinterest exactly in the center ofthe gantry aperture, and reconstructing a slightly smaller fieldofview than the actually exposed field, the image quality will be high, and radiation exposure to the more peripheral body portions (e.g., skin,breast) will be reduced. Information outside

the SFOV, however, will be lost. This type of scan is often called a "target scan". For multislice scanning, cone beam artifacts are least severe for structures close to the rotational axis. For this reason, the examined body part should be placed as close as possible to the center of the gantry (com pare Fig. 1.22). •

Scan Time and Rotation Period

Shorter scan times per slice are associated with fewer motion artifacts. The scan time can be shortened by limiting the scan arc to an angle equal to 180 plus the fan angle (partial scan) rather than performing a full 360 rotation. Motion artifacts can be reduced by assembling multiple rotations into one image. Short rotation periods (rotation time, RT) in spiral and multislice CT increase the available scan length and shorten the time required to scan a particular volume. A short rotation time will also allow for using thinner sections while covering a given scan volume. In some scanners, very short rotation periods will cause some degradation of spatial resolution because the number of available projections decreases (e.g., for the "flying focal spot" technology used by Siemens). For high-resolution application, such ultra-short rotation periods should be avoided on these scanners (Fig. 4.3). For cardiac scanning, adapting the rotation time to the heart rate may improve temporal resolution. This will be the case if a two- or threesector reconstruction instead of a one-sector reconstruction can be used (see Chapter 23, Heart). 0

0

•

Scan Duration and Phase of Respiration

When selecting the scan duration, the radiologist should make certain that the patient has adequate breath-holding capacity and that imaging can be performed during the desired enhancement phase. When brief hyperventilation precedes the scan, most patients can tolerate a 30 second breath hold in inspiration.

Fig.4.3 Image quality of HRCT is improved with maximum number of projections. Some scanners using flying focal spot technology require a reduced rotation time (0.75s) (a) for optimum spatial resolution. Resolution with 0.5 s rotation time and the same exposure dose is slightly reduced (b).

a

b

4 Optimization of Scanning Technique

Most body spiral CT examinations (singleslice or multislice) are performed in inspiration. Scans during forced expiration are only required for the chest (high-resolution CT) to detect air trapping or emphysematous lung areas. Multislice scanning during expiration may be used to quantify the amount of air trapping in the whole lungs. There are a few cases in which scanning may be performed during shallow respiration, e.g., for patients in respiratory failure or sedated children. Spiral scanning will su bstantially reduce artifacts on the axial scans if only mild movement is present. If a breath-hold is attempted, the amount of artifacts will commonly increase because such a patient will usually have to continue breathing during the scan with deep inspirations to compensate for his dyspnea (Fig. 4.4). Breath-hold scanning should not be attempted in dyspneic patients, unless the scan duration is very short. Breath holding is unnecessary in examinations of the head, axial skeleton, and extremities. •

Scan Length

The scan range can be determined from the scanogram in most instances. For reasons of radiation protection, pre contrast scans to precisely define the scan range should be avoided.

Fig. 4.4 In dyspneic patients, trying to have the patient hold the breath may cause severe artifacts when the patient has to start breathing again. This can be avoided if the patient is instructed to breathe shallowly. Note the lack of artifact in the upper portion of this craniocaudal scan (arrows) and massive artifact in the lungs and bronchi further distally (arrowheads).

The radiation exposure to the patient (dose-length product, DLP) increases with the scan length. Therefore, the scan length should be kept as short as possible without omitting any portions of relevant organs. Proper patient instruction, training of the technicians and the use of standardized scan protocols will help to reduce the "safety margins" for examinations of the chest and abdomen that are commonly used to make sure all organs are covered, even in a patient with breathing problems. Generally the extension of the scan range will make a prolonged breath-hold and a wider section collimation necessary. This trade-off is less relevant with multislice CT but remains important when deciding on the total amount of contrast material or the scan duration that can be tolerated by the patient. •

Scanning Direction

The direction in which scanning proceeds can substantially influence image quality, depending on the contrast protocol and anticipated respiratory artifacts. A caudocranial direction is best for spiral scanning ofthe thorax, because it reduces motion artifacts toward the end of the scan in the lung apex. For optimum results of breath-holding enough time should be calculated for patients to breathe in and hold their breaths before the scan is started. If the scan is initiated a few seconds after the breath-holding instructions the results

a Fig. 4.5 Starting a caudocranial scan too early causes breathing artifacts (a) that are most prominent on MPR

(b).

General Acquisition

will be substantially improved (Fig. 4.5). This can be accomplished by a start delay of 4-5 s. Caudocranial scanning generally reduces contrast -induced streak artifacts in the cranial mediastinum particularly, if the IV contrast injection is followed immediately by a saline flush at the same inj ection speed: the brachiocephalic veins are then captured at a point in time when they already contain saline solution, the highly concentrated contrast material being flushed to therightatrium. When the liver is examined early during the portal contrast perfusion phase, caudocranial scanning will improve visualization of the hepatic veins because they will be covered during the late part of the scan when they are more readily opacifed. This effect is of course less pronounced with multislice CT because scan duration is much shorter. •

Tube Voltage (kVp)

If the tube voltage is reduced'from 140kV to 80 kV, a greater part of the radiation is absorbed in the prefilter and in the outer fatty tissue layers of the patient. As a result, the CTDIvoi for identical mAs settings is reduced by a factor of 3-4. This is accompanied, however, by a strong increase in noise but also an increase in CT attenuation (and therefore an enhanced radiation contrast) of substances containing high atomic numbers, such as bone, iodinated contrast agents, or metals. Hence, the relationship between image quality and kVp settings is complex (see more detailed discussion Chapter 5). Our own results suggest that a high kVP setting (140kVp) is advantageous for obese patients, while a low kVp setting (80-100kVp) is advantageous in children or in low-absorption areas such as the chest, especially when contrast

Fig. 4.6 Low-dose scan of a patient with chronic thromboembolic pulmonary hypertension performed with 80 kVp and a CTDlvoi=2.3 mGy. There are multiple peripheral pulmonary artery stenoses (arrows). Note the excellent contrast (window settings W/L= 1500/300) on the thin-slab MIP (a) and the good quality ofthe volume rendered display (b).

Parameters

material is injected (Fig. 4.6, see also Figs. 5.16, 5.19, 24.18, 24.19 a, 24.20 a, 24.64). •

Tube Load (mAs)

For constant tube voltage and beam filtration, the tube current is proportional to both, the patient dose and the detector dose. The mAs per tube rotation provides a useful measure of image noise (detector dose), while the mAs per tube rotation divided by the pitch (effective mAs = mAs per rotation/pitch) provides a measure of the patient dose. A high mAs setting results in low noise, permits the use of high-resolution kernels, and improves image quality (see Chapter 5) but also increases radiation exposure to the patient. On most newer scanners, the volume CT dose index (CTDIvoi) is displayed on the user interface (as CTDI) and provides a better information about radiation dose than the mAs settings ever could. Radiologists and technicians therefore should get adapted to using CTDIvoi instead of mAs as a measure of exposure settings to compare scanning protocols. •

Tube Output Characteristics

The x-ray tube output, heat capacity, and heat output are the tube performance characteristics that limit the mAs settings available for the duration of the spiral CT scan. This has a direct effect on image quality. The longer the scan duration or the greater the number of successive spiral scans that are acquired at short intervals, the higher the necessary tube output and the lower the available mAs per tube rotation. In older scanners, trade-offs have to be made in terms of image quality or available enhancement phases. With multislice CT, two, four, or more sections can be acquired simultaneously. Although

115

116

4

Optimization of Scanning Technique

this feature can be used to reduce the requirements on the x-ray tube by a similar factor, most users choose thinner sections instead, which increase the scan duration and keep mAs requirements high. In addition, subsecond rotation times (e.g., 0.5 s) and high pitch factors further increase the rnA settings that have to be used for obtaining a good signal-to-noise ratio. With a pitch of 2 (P * =8) and 0.5s rotation time, the rnA settings have to be four times higher than for a conventional incremental CT (1 s rotation time) at identical patient dose and signal-to-noise ratios.

ters are discussed below. Table 4.2 gives an overview of suggested scanning parameters depending on the clinical indication. Table 4.3 describes the trade-offs for the choice of various acquisition parameters. •

Section Collimation

A narrow section collimation (SC) has the advantage of reduced partial volume effects, better spatial resolution along the z-axis, and a con- f: sequent improvement in the image quality of • multiplanar reformations and 3D reconstrucReduced mAs, soft convolution kernels, tions. The resulting increased image noise can shorter scan lengths, and thicker slice collimation be reduced subsequently by increasing the seccan be used in an attempt to minimize image tion width in axial reformations, which will ennoise without overloading the tube. These tech- hance the signal/noise ratio without corn- ji y niques also reduce the radiation dose to the promising z-axis resolution, The main advantage of a wide section collipatient and, therefore, should be considered in mation is that less radiation dose (lower mAs the formulation of scan protocols. Maximum dose per section is required for setting) is required at a constant level of image very obese patients. This can be achieved by noise, whereas at a constant dose, there is less decreasing the rotation time and increasing the noise resulting in a better contrast resolution, section collimation and mAs. Image noise can be The most practical advantage of a wide collimareduced by reconstructing thicker sections (sec- tion is the ability to scan a given volume more , tion thickness SW= 10mm) and by using soft rapidly because thicker sections can also be re- ( convolution kernels. In single-slice CT, a pitch of constructed from thinner ones using MPR func1 and a 360 LI interpolation algorithm may be tions (or in the case of multislice scanning, the & used (twice as dose-efficient as 180 LI). With raw data of the scan). multislice scanning, low pitch factors are reThe choice of a collimation setting depends F on the clinical request and the nature of the required to allow for high mAs settings. gion that is to be examined. Since most anatomic structures are oriented along the longitudinal axis ofthe body, a section collimation of Single-slice Spiral Acquisition Parameters 5-8 mm can often be used without introducing Spiral CT has become the standard technique for detrimental partial volume effects. Large most applications in the body. The most impor- volumes are imaged with a large section thickness in order to reduce the scan duration. The tant trade-offs in the choice of the scan parame0

0

Table 4.2

Suggested scan parameters for single-slice spiral CT ofthe body

Ai; ! 1 "Indication I

Of

------------

Single-slice cT SC/TF/RI"

High-resolution CT of the lungs 9 Skeleton (facial bones, upper cervical spine, hand, foot, elbow, knee) Tracheobronchial system, CTA (renal arteries) Adrenals, skeleton (spine, pelvis, shoulder, elbow, knee), CTA (carotid artery: 2/411) 1 Head, neck, pancreas, kidneys, CTA (aorta: 3/6/2) 1 Thorax, liver, pancreas, kidneys, lower abdomen I Screening (chest + liver), abdomen 9

1

1-2/10-2013 1/2/1 2/3/1 2/4/2 3/5/3 5/8/4 7/12/6

SW 1 1.3 2.3 2.6 3.6 5.9 8.5

For optimum 3 D results (MPR MIP, SSD, VRT), choose RI = SW/2 incremental scanning technique SC = section collimation (rnrn). TF = table feed (mrn). RI = recon. increment (rnrn). SW =section width (mm)

b

Single-slice Spiral Acquisition Table 4.3

Trade-offs

Acquisition

that influence

Parameter

Section collimation

SC

Table feed TF

the choice

of scanning

Parameters

parameters

Trade-off large small

--

large

->

less nose, better low-contrast resolution, more partial volume effects less partial volume effects, better z-axis resolution, more noise faster coverage,

longer scan range per time, less motion

artifacts

on

MPR smalls

FItch P

smaller

large

->

small-> T uberotationtime

RT

resolution

large

-> more projections coverage _"' less motion

small Scan length L

better z-axis

or faster coverage,

increased

patient

+ better in-plane

artifacts,

better z-axis

less patient

resolution

(some scanners),

resolution

or faster

-> long scan range, thinner collimation possible, more contrast material required -> shorter scan range, less artifacts, less contrast better

anatomic

dose

dose

large

-» large coverage,

small

longer scan duration TI required -> less patient dose, thinner collimation

overview,

more motion material

thicker

possible,

less

coverage artifacts,

required

collimation

shorter

SC or

scan time

mAs settings

high low

-> less noise, better contrast resolution, _" less patient dose, more image noise, low-contrast details

kVp

high

-> better penetration,

low

-r

large small

-> less noise, better low-contrast resolution, more partial volume effects -> less partial volume effects, better z-axis resolution, more noise

large small

->

settings

Reconstruction

more patient dose decreased detection

less dose for given signal-to-noise

patients, especially in abdomen higher contrast (especially with iodinated contrast minirrum dose, less dose for given signal-to-noise patients, chest, and neck

Recon. increment

materia), lower for children, slim

RI

less images bu lesions may be missed more images, better MPR/3D representations,

more lesions

Z-filter

wide -> less image noise and dose requirements, lower z-axis narrow _"' increased noise, higher z-axis resolution

Recon. filter kernel

smoothing

less image noise better 3D images

high resolution Field of view (FOV)

large small

-> substantially

-> better overview, -c-» higher maximum

section collimation should be reduced, however, if it is necessary to depict fine anatomic details, With low-contrast lesions such as hepatic metastases, the signal-to-noise ratio, and thus detectability decreases for lesions that are smaller than the section width, Table

In spiral used for improves phase, or

of small

in obese

Parameter

Section width SW

•

SC possible

less spiral artifacts,

small large

Scan duration TI

collimation

Feed and Pitch

scanning, a pitch of 1.5 to 2 should be the vast majority of indications, This scan coverage during a breath-hold it can be used to improve spatial reso-

and dose requirements,

lower spatial

increased

spatial

noise,

higher

detected

resolution resolution

resolution

smoothing filter kernels are less disturbing spatial resolution (requires HR filter kemels)

lution by reducing section collimation (see Fig, US), At the same time, less dose is required than with conventional slice-by-slice scanning (see Fig, 5.12), A pitch in this range provides an optimum z-axis resolution for a given scan length and simultaneously reduces the radiation exposure, If the collimation is very thin (SC < 1 mm): even a pitch as high as 3 can be used without causing serious additional artifacts (Fig, 4.7),

118

,

4 Optimization of Scanning Technique Fig. 4.7 A pitch of 3 may be possible with thin collimation (SCI TF /RI = 1 /3/1) for CTA of the pulmonary vessels (a) and even the lung parenchyma (b).

b

a

MultisliceAcquisition Parameters

For the vast majority of applications there are two essentially different ways to use a multi-

Table 4.4

Comparison

of multislice CT scanning Volumetric

slice CT scanner: as a fast spiral CT scanner or as a volumetric imaging tool (Table 4.4). Fast spiral scanning takes advantage of the increased acquisition speed but still uses the

protocols for fast spiral scanning

and volumetric

imaging

Fast spiral scanning

imaging

16-slice

Protocol

4-slice NXSClTF

16-slice NXSClTF

4-slice NXSCITF

NXSC/TF

GE

4x1.25/7.5

16XO.625/13.75

4X2.5/15

16X1.25/27.5

Philips/Siemens

4X1/6

16XO. 7 5/16-18

4X2.5/15

16X1.5/32-36

Toshiba

4X1/5.5

16XO.5/11.5

4X3/16.5

16X1-2/23-46

Performances

TT TT TT TT

T

Coverage

T TTT TTT

Speed Z-resolution

lrnaqenolse" Pulsation

artifacts

Breathing

artifacts

TT TT

U U

A

1

Applications Standard,

Tumor

Chest

Tumor staging I nterstitial disease

Metastases Mediastinum

Abdorne?

Pancreas Liver, biliary system Bowel Preoperative evaluation

Standard abdomen Liver (noncontrast phase) Kidneys

Carotids Pulmonary vessels Abdomen

Aorta Oysponeic Veins

Small joints Cervical + thoracic spine Small bones

Pelvis Lumbar spine Long bones

Skeleton

3

c

staging

lymph node staging

Neck

relative to 1 s-single-slice 7/12/6 use 16x1-1.5 mm for volumetric

protocol imaging

b

in obese patients

patients

at identical patient dose (CTDlvoi)

Multislice Acquisition Parameters scanner primarily as an axial imaging tool. It has the advantage that a thinner collimation than with single-slice CT can be used, although the acquisition time is substantially reduced in comparison with single-slice spiral CT Images are reconstructed with a section width that is equal to or slightly less than section width in conventional spiral CT These images serve as the primary evaluation tool. Thinner sections are reconstructed retrospectively only if questions arise, and may be used to create good quality multiplanar reformations, This technique has the advantage of a known workflow, high patient throughput, limited numbers of images on a routine basis, and the option for problem solving with thin sections and multiplanar imaging, However, it does not take full advantage of the capabilities of multi slice CT, and fine detail may be missed if thick axial sections are to be used as the primary tool for making a diagnosis, Volumetric imaging takes full advantage of the near isotropic spatial resolution that becomesavailablewithmultisliceCT Ittransforms CT into a true multiplanar imaging tool that is, in this respect, superior to MRL It is the preferred, if not only, mode used with 16-slice scanners, Volumetric imaging requires the reconstruction of thin, overlapping axial images that serve as a secondary raw data set, These images are used as a 3D data volume from which further images are generated for diagnostic purposes, In most instances, these thin images will suffer from increased noise and require the reconstruction of thicker multiplanar reformations with a section width that is tailored to the clinical question, Such MPR are reconstructed as part of the standard data evaluation and can be "pretailored" to the anatomical site (such as coronal MPRofthe tracheobronchial tree, sagittal MPR of pulmonary masses, curved planar reformations of the pancreatic duct) or they may be created interactively at the CT workstation to probe the data volume interactively and obtain problem-adapted sections which is, in the interactive 3D data evaluation, similar to ultrasound, This approach requires very fast workstations and networks with proper software to be timeand cost-efficient Volume rendering may become the primary tool for evaluating such data because it combines the advantages of MPR with additional depth information,

Volumetric imaging takes full advantage of the capabilities of multislice CT scanning, However, there are marked differences in workflow, and the patient throughput is lower due to the longer processing times, Increased data volume has to be reconstructed, processed, analyzed, and stored, which requires a proper network and archiving system as well as fast, intuitive CT workstations and trained personnel. At present, such constraints are major and lead to markedly increased effort with such a technique, Therefore, volumetric imaging is used most often only in selective cases, such as planning of surgery, CT angiography, or complex cases that cannot be resolved with conventional techniques, Section

Collimation

Fast spiral scanning can rely on a 4x2,5 mm (4 x 2-3 mm) detector configuration for most indications, With a volume pitch (P*) around 6, a table feed of 15 mm per rotation can be obtained, which translates into a table speed of 18.75 mmls (0,8s rotation time) to 30 mmls (0,5s rotation time), This allows for covering the chest (24cm) in 8-12,8 s. which is a very comfortable breath hold period for most patients, A 40 em long abdomen can be covered in 13,3-21 s. which also can be met by most patients, Even combined thoracoabdominal scans become possible: a 60cm range can be covered in 20-32 s. Using a larger collimation but a lower pitch will reduce multislice artifacts but a similar if not better result is obtained if such thicker sections are reconstructed from the 4 x 2,5 mm data, Thicker collimation may be useful for very rapid scanning or in very obese patients as the image noise can substantially be reduced, Volumetric imaging relies on the highest possible spatial resolution at a reasonable patient dose, With current four-slice scanners, this can be achieved with a 4x 1 mrn or 4 x 1.25 mm detector configuration and the reconstruction of thicker sections for image analysis from the raw data or from the secondary raw data set that should consist of LCl-L5 mm thick sections reconstructed every 0.5-1 mm With current four-slice scanners, dose efficiency is reduced by some 20-30% with this detector configuration, In obese patients, a 4x2 mm or 4x2,5 mrn acquisition may be used instead, which requires reconstruction of 2.5-3 mm wide sections every 1-1.5 mm

4 Optimization of Scanning Technique

Similar to single-slice spiral CT a high pitch factor allows thinner collimation for a given scan coverage per time to be used. It has been shown that it is better to use thin collimation with a high pitch factor and then reconstruct thicker sections than to use thicker collimation with a low pitch factor primarily. For an identical reconstructed section width, artifacts are less with

thinner sections, and there is still the opportunity to reconstruct thinner sections if multiplanar imaging has to be performed. Thus, a volume pitch P* of 5.5 to 6 with thinner sections should be preferred over a volume pitch of3 for most applications (Fig. 4.8). Exceptions from this are situations in which the thinnest possible sections have to be reconstructed. In such cases (e.g., for the cervical spine or the extremities, Fig. 4.9), the amount of artifact can be reduced by using lower pitch factors (P *=3, P < 1). Another exception are obese patients, in whom image noise is a limiting factor. If too thin a collimation is employed electronic noise may dominate the images (see Fig. 7.33). Image quality then can no longer be recovered by reconstructing thicker sections, For such patients, thicker sections and maximum mA settings should be used with low pitch factors (p*= 3, P< 1). Adjusting the pitch P even higher, P < 1 to a maximum of 2 (P*=8 for 4-slice units-if allowed by the scanner) will result in an increase of artifacts. However, this may be advisable in cases in which faster scanning or longer coverage is required. Such artifacts can be decreased efficiently by either reconstructing thicker sections or reformatting thicker MPRs. In addition, such high pitch factors may be advisable for low-dose scanning because the mA settings at an identical patient dose is higher for a larger

a

c

Ultra-thin collimation (0.5-0.625 mm) is possible with most multislice CT scanners but there is a substantial increase in patient dose with current systems (see Fig.5.5b), and an increased susceptibility towards electronic noise which further decreases the signal/noise ratio. Such protocols are useful only for very selective applications, e.g., for the lung parenchyma or for small bones, where it can provide exquisite spatial detail. With 16-slice scanners, volumetric imaging is used in almost all cases. Thin (SC= 1-1.5mm) to very thin (SC = 0.5-0.75 mm) sections become available. Because of noise characteristics, thin sections will be the standard mode for the abdomen, while very thin sections can be recommended for the neck, chest and extremities. Only in slim patients and specific diagnostic questions (CT of the pancreas and biliary system, CTA of the mesenteric arteries) very thin sections may be used in the abdomen as well. •

Table Feed and Pitch

b

Fig. 4.8 For identical volume coverage per time (identical table feed), using a higher pitch factor and thinner sections may improve the quality both ofMPRaswell as thick axial sections, at identical dose to the patient. Comparison of a 4x2.5 mm collimation with volume pitch P* of 3 (a, c) to a 4 x 1.25 mm collimation with a

d

volume pitch of 6 (b. d). A CTDlyoi= 12 mGy was usedfor both examinations. Axial sections of 3 mm thickness (a, b) yield comparable image quality but coronal sections of 3 mm thickness (c. d) are substantially better with thinner collimation.

Multislice Acquisition Parameters

a

thickness) as multiples of a single section collimation (1.25 mm, 2.5 mm, 3.75 mm, etc., see Table 1.8) and then decide which detector configuration to use. This approach favors the fast spiral scanning technique and is very intuitive and time-efficient. GE provides only two volume pitches, which are named high-quality (HQ.) mode (P" = 3) and high-speed (HS) mode (P" = 6). As discussed above, the high-speed mode should be employed for optimum system performance (see also Table 1.7). For similar image quality the high-quality mode will require about 50% increase in dose. There are only limited combinations of section width and section collimation possible with the software on current four-detector row scanners (see Table 1.8): it is, for example, not possible to choose a 4 x 2.5 mm collimation for a 7.5 mm section width (reconstruction slice thickness), or a 4 x 1.25mm collimation for a 5mm section width, respectively. To take advantage of direct axial image reconstruction from the raw data, users will have to use the next thicker collimation. Note that the minimum section width is some 28% larger that the numbers displayed on the user interface if a high speed mode is chosen (see Table 1.8). To obtain thick axial sections even when thin-section scanning (volumetric imaging) was employed, the data have to be reconstructed with the minimum possible section width and an overlap of some 50% (e.g., with 1.25mm sections, every 0.7 mm for 4 x 1.25mm configura-

b

Fig. 4.9 For the skeletal system, a lower pitch factor yields fewer artifacts. Comparison of coronal MPRofthe cervicalspine obtained with a volume pitch P* = 3 (a) and a volume pitch P* = 6 (b) from a 4 X 1 mm detector configuration.

pitch, which leads to a higher detector signal and thus less contribution of electronic noise (Fig.4.10). •

Vendor-specific Approaches (Four-slice Scanners)

Table 4.5 compares the parameters settings for the volumetric imaging and fast spiral modes that are available with the various vendors. General Electric (GE) lets the user first choose the section width (reconstruction slice

a

b

Fig.4.10 For low-dose applications, noise decreases substantially with larger pitch factors despite an identical patient dose. Comparison of images obtained with a volume pitch P*=6 (a, c), and P*=8 (b. d) from a 4 x 1 mm detector configuration with 80 kVp and

c

d 40 mAs (CTDlyoi = 1.2 mGy) each. Note that there is a reduction of image noise not only in the 1.25mm thick secondary raw data set (a, b) but also on the 5 mm thick sections reconstructed for image review (c. d).

121

4 Optimization of Scanning Technique tion). After transfer to a CT workstation (e.g.. Advantage Windows) thick axial sections are then reformatted using the MPR mode, and additional MPR can be reformatted in any desired plane. Philips provides the real section width (effective slice thickness) and the pitch factor, not the volume pitch, on its user interface. Fast spiral scanning and volumetric imaging can be applied as described above. Good results are found for a pitch of 1.5 (corresponding to P*=6). Thick sections can be reconstructed from the raw data in a similar fashion as with all other vendors. Siemens has implemented an adaptive interpolation that assures that the section width is independent of the chosen pitch factor. The noise is also kept constant due to the increase of mA with increasing pitch (constant mASeff and thus, patient dose). Artifacts are somewhat lower at lower pitches but the advantage of increased coverage or higher spatial resolution should prompt the user to employ a volume pitch of 6-8, unless bone imaging is performed. Axial images of almost arbitrary width can be reconstructed from any detector configuration as long as the section width is larger or equal to the collimation (see Table 1.9). A special z-filter is available that allows for reconstructing sections of 1 mm width from a 4 x 1 mm collimation. This will increase image noise substantially and therefore cannot be recommended on a routine basis. Fast spiral scanning and volumetric imaging can be applied as described above. Toshiba uses a z- filtering algorithm (MUSCOT) that also assures that the chosen section width differs only minimally from the section width provided on the user interface. Axial images of almost arbitrary width can be reconstructed as long as the section width is larger or equal to the collimation. If the reconstructed section width is identical to the single section collimation, noise will be substantially increased (see Table 5.5). lt is therefore advisable to use a slightly (by 0.5mm) wider section width for most indications of volumetric imaging. The Toshiba detector gives the largest choice of detector configurations. For fast spiral scanning the user has the option of choosing between a 4x2 mm and a 4 x 3 mm collimation. In practice, the thicker collimation can be recommended for obese patients and in all cases where faster data

acquisition is required. The Toshiba hybrid detector is the only one that has four rows of 0.5 mm collimation, making ultrahigh resolution scanning more feasible and less dose-intense. Although the pitch can be varied freely, Toshibasuggests preferred pitch factors (see Table 1.10). Again, higher volume pitches of 5.5 or 6 yield the highest performance. Otherwise, fast spiral scanning and volumetric imaging can be applied as described above. Eight- and 16-slice Scanners

With the advent of more detector rows, there is a shift from fast spiral acquisitions to volumetric scanning with thin sections. These scanners have the advantage of better geometric dose efficiency compared to four-slice scanners, and most will also use more sophisticated detector technology that reduces electronic noise. Nevertheless, ultrathin collimation (0.5mm) will cause a substantial increase in image noise and will require reconstruction of thicker axial and multiplanar sections as described for volumetric imaging. This provides excellent image quality for the lungs as well as for the peripheral skeleton and the cervical spine. It is doubtful whether such thin sections can be used in the abdomen, especially in more obese patients. They prove most useful for the chest, neck and peripheral skeleton, where radiation absorption is not too high. The main difference between 8- and 16slice scanner types and four-slice scanners will be the substantially increased scanning speed, which will all but eliminate movement artifacts, Pulsation effects are spread out over longer distances, thus improving the display of the thoracic aorta and the origin of the supraaortic vessels. Optimization of contrast material administration requires special attention because of the short scanning time. Precise separation of scan phases is possible but timing is a crucial issue and frequently requires longer short delays. Only a limited number of applications allow for savings in contrast material (mainly CTA and chest imaging), most other applications (especially in the abdomen) require similar amounts of intravenous contrast material because organ enhancement is more dependent on the volume than the flow rate during contrast injection. I The application that profits most is cardiac imaging because eight-to 16-row scanners

Reconstruction an d Display Parameters

overcome many of the limitations of spatial and temporal resolution with present four-slice scanners, especially because some new scanners allow for 0.42 s or less tube rotation time. In addition. new techniques for dose reduction (ECGgated mAs modulation) and reduced scan field of view substantially reduce dose requirements with cardiac cr. Since the coverage is substantially increased. lower pitch factors can be chosen to keep artifacts to a minimum without compromising scan speed. However. all manufacturers use more elaborate reconstruction schemes that sufficiently compensate for cone beam gecmetry and allow for a maximum pitch Ps 1.5. Apart from these issues, the same general principles described above for volumetric scanning and fast spiral scanning hold true, and acquisition parameters only have to be adapted to the collimation settings avallable with a specific scanner (Table 4.6). GE supplies an eight-detector row scanner with 0.5 s rotation speed. Similar to their fourslice scanner, multiples of 1.25mm can be chosen as section collimation and reconstructed section width. The manufacturer allows for a volume pitch of 5 (UQ, P= 0.625), 7 (UM, P=0.875), 10.8 (UF, P=1.35), and 13.4 (US, P= 1.675). A volume pitch of 10.8 is suggested for most applications because it provides the best compromise between coverage, dose requirements, and image quality. A detector configuration of 8 x 1.25 mm at a table feed of 13.5 mm/rot, can be suggested for volumetric imaging, and a 8x2.5 mm configuration at 27 mm/rot, yields best results for fast spiral imaging. A special one-slice mode is available for HRCT of the chest. GE is now also supplying a 16-slice scanner that gives the choice between 16 x 0.625 mm and 16 x 1.25 mm collimation. The pitch can be varied in four steps. A volume pitch P*=9 (P =0.5625) and P*= 15 (P= 0.9375) yields a minimum SW that is similar to SC, while P*=22 (P= 1.375) and P*=28 (P=1.75) are more doseefficient but provide a 30% wider SW (0.8mm and 1.6mm, respectively). Apart from high-resolution applications, a pitch P*=22 (28 for CTA) provides the best compromise. Philips and Siemens have introduced a 16-detector row scanner that allows for 16 x 0.75 mm and 16 x 1.5 mm collimation. Like the present scanners, standard section width will be some

30% larger than the collimation (SW= 1 mm for SC=0.75mm, and SW=2mm for SC=1.5mm). This ensures improved signal-to-noise ratios. In addition, thin sections (identical to the collimation) can also be reconstructed but at the cost of a higher image noise. Such thin sections are only useful for the lung parenchyma and the peripheral skeleton. Pitch factors can be increased up to 1.5 (volume pitch P* up to 24) for Siemens and up to 2 (P* = 32) for Philips scanners. A standard pitch around 1.5 (P'" = 24) will suffice for most applications. Both vendors allow for continuous selection of pitch factors. For volumetric imaging ofthe chest, as well as for the abdomen in slim patients, a 16x0.75 mm detector configuration is best suited. In obese patients, a 16 x 1.5 mm configuration should be employed in the abdomen. Fast spiral scanning relies on a 16 x 1.5 mm collimation. Toshiba offers an 8-slice as well as a 16-slice scanner. On the latter, Toshiba provides a 16 x 0.5 mm, 16x 1 mm and 16x 2 mm collimation. Preferred pitch values are P*=ll (P=0.6875), P*=15 (P=0.9375), and 23 (p= 1.4375). When reconstructing sections SW that are identical to the collimation SC, image noise will be increased. Thus, this should only be done for high-resolution applications but not for standard techniques in the chest or abdomen. In case of doubt, it is better to use the next smaller section collimation instead and reconstruct (at least 30%) wider sections. Only for the peripheral skeleton, the neck, and the lungs can a 16xO.5 mm ultrahigh resolution protocol be recommended. In most standard situations in the neck, chest and abdomen, a 16 x 1 mm collimation yields less noise and thus a better image quality for volumetric imaging. A 16x2 mm collimation should be used for volumetric imaging in obese patients as well as for fast spiral scanning in standard patients. Fast image reconstruction is possible if the reconstruction increment RU is chosen 0.2, 0.5,1.0 or 2.0times SC.

Reconstruction and Display Parameters •

Section Width and Raw Data Interpolation

In single-slice spiral CT, the chosen collimation, the pitch factor, and the raw data interpolation algorithm determine the section width (see Figs. 1.13, 1.15). The section width SW grows continuously from 100% to 130% of the section collimation SC when the pitch is increased from 1 to 2 at a 180 LI algorithm. 0

123

Optimization of Scanning Technique A 360' LI algorithm is available only on a few scanners and requires the use of a pitch = 1 (SW=1.3xSC). lts main advantage is a better utilization of the radiation dose. In fact this algorithm reduces the noise as much as an increase in mAs by a factor of 2. Because scan coverage is low. this algorithm has been used mainly for dual slice systems in which this disadvantage is compensated by simultaneous acquisition of data from two detector rows. Reconstruction of thicker section width is possible with both single and multislice spiral CT but there are hardly any implementations in single-slice scanners available yet. Thicker sections for single slice CT have been employed for partial volume artifact reduction in the posterior fossa but have seen few if any use outside this application. They may. however. be used in a similar fashion to multislice CT, and then can serve as a tool to reduce image noise when thinsection spiral scanning had been employed. In multislice CT, the collimation, pitch factor, and reconstruction algorithm determine the minimum available section width SW (see Fig. 1.25). SW varies in a complex fashion between 100% and 130% of the section collimation SC if pitch is increased from 1 to 2 and a 180' MLI interpolation is used. A similar behavior is found for 360' MLI up to a pitch of 1. Adaptive interpolation schemes fix the section width at some 130% of the collimation, independent of the pitch (see Fig. 1.29). For fast spiral scanning, thick sections are reconstructed from the raw data set, even if thin collimation was used to obtain a scan. The section width depends on the size of the patient and the clinical indication (Table 4.5, see also Table 2.2). Thin sections and MPR are only reconstructed as a problem-solving technique if thick axial sections alone cannot resolve the clinical question. For volumetric scanning, overlapping thin sections of 0.5-1.5 mm width are reconstructed every 0.4-1 mm. This "secondary raw data set" serves as the basis for arbitrary cut planes using thick MPR. lt can be employed as a standard for indications such as staging of bronchogenic cancer or CT colonography (see organ chapters for more details), or as an adjunct if suspicious lesions are found (see Fig. 7.29). For indications, in which standard axial, coronal, and sagittal sections are desired for image interpretation, optimum results are gained if the axial plane-

as well as all other required planes-are tioned orthogonally relative to the patient and not relative to the scan table. Hence, positioning of the patient can be easily rected by imageprocessing. •

Reconstruction

posiaxes malcor1

Interval

The reconstruction interval Rl (synonyms: reconstruction increment, reconstruction index, or spacing) in spiral or multislice CT can be chosen independently from the collimation or section width. The advantage of a large reconstruction interval is that it reduces the number of images that must be reconstructed and interpreted. The main disadvantage of non-overlapping sections is that they do not provide optimum lesion detection, resulting in a less confident evaluation and a lower detection rate (up to 30% lower in the liver and lung). Overlapping reconstruction results in a greater number of images that have to be interpreted, processed, and stored. The quality of multiplanar and 3D reconstructions will generally improve with a higher degree of image overlap. Theoretical considerations suggest that a reconstruction increment that is some 30% less than half the section width is an optimum balance between practicality, accuracy, and image quality. In thin-section multislice CT, however, it makes little sense to choose a reconstruction interval that is smaller than the pixel size in the scan plane. For practical reasons, a reconstruction increment that is identical to half the section width suffices for most indications. Usually, in fact, conventional spiral CT as well as multislice CT require an overlap of only 30%, which is sufficient for most routine applications, e.g., lesion detection (Tables 4.2 and 4.5). Convolution Algorithm)

Kernel (Reconstruction

The selection of the convolution kernel always involves a trade-off between spatial and contrast resolution (see Fig. 5.11). High-resolution kernels always result in a disproportionate increase in image noise and may even introduce edge-enhancement artifacts. They are suitable only for regions in which image contrast is high, such as lung parenchyma, and bone. Soft kernels are suitable for cases in which high contrast resolution is required or image noise is excessive due to a low detector signal. This applies particularly to abdominal scanning in obese patients

Reconstruction Table 4.5

Suggested

scan parameters

for multislice

CT scanning

and Display Parameters

of the body (4-slice scanners)

Acquisition

Pitch factor"

N-SC/TF

P

P*

SW/Rlb

SWIRl'

2XO.625/1.S75

1.5

3

0.65/0.5

0.65/0.5

Volumetric resolution)

imaging

(ultrahigh

4X1.25/7.5

1.5

6

1.6/0.Sd

5/3"

Volumetric

imaging

(standard)

4X1.25/3.75

0.75

3

1.25/0.5

1.25-2.5/2

Volumetric

imaging

(skeleton)

4x2.5/7.5

0.75

3

2.5/1.3

7.5/5"

Volumetric patients)

imaging

(obese

4X2.5/15

1.5

6

3/1.5d

5/5

Fast sprial scanning

(standard)

4x3.75/22.5

1.5

6

5.0/2.5

7.5/5

Fast sprial scanning patients)

(obese

0.S-1.5

1.6-3

0.5/0.5

0.7/0.5

Volumetric resolution)

(ultrahigh

Image reconstruction

Application

General Electric

Philips. Siemens 2XO.5/0.S-1.5

imaging

4X1/6-S

1.5-2

6-S

1.25/0.7

3-5/3

Volumetric

imaging

(standard)

4X1/3

0.75

3

1.0-1.5/0.5

1-2/2

Volumetric

imaging

(skeleton)

4X1/S

2

S

2/1

6-S/5

Volumetric patients)

imaging

(obese

4x2.5/15

1.5

6

3/1.5

6/5

Fast spiral scanning

(standard)

4x2.5/20

2

S

4/2

S/5

Fast spiral scanning patients)

(obese

4XO.5/1.5-2.75

0.75-1.375

3-5.5

0.5/0.5

0.5/0.5

Volumetric resolution)

imaging

(ultrahigh

4x 1/5.5

1.375

5.5

1.5/0.7

3-5/3

Volumetric

imaging

(standard)

4X1/3

0.75

3

1.0-1.5/0.5

1-2/2

Volumetric

imaging

(skeleton)

4x2/11

1.375-1.75

5.5-7

2.5/1.3

S/5

Volumetric patients)

imaging

(obese

4X2/11

1.375

5.5

2.5/1.3

6/5

Fast spiral scanning

(standard)

4X3/16.5

1.375

5.5

4.0/2

S/5

Fast spiral scanning patients)

(obese

N = number of active detector rows; SC = section (mm), RI = recon. Increment (mm)

collimation

(mm); TF = table feed (mm), SW = section

width

a Pitch factor

P=TF(NxSC); Volume pitch P* =TF/SC parameters for reconstruction of an overlapping 3D volume (secondary raw data set) from which MPR and 3D displays can be generated; not necessary for fast spiral scanning unless MPR or 3D display is required c parameters for image review and reporting (may vary depending on clinical indication), may be reconstructed by thick MPR from 3D volume (secondary raw data set) d user interface displays SW = 1.25 and 2.5, respectively e needs to be reconstructed by thick MPR from 3D volume (secondary raw data set)

b

as well as to situations that require the use of a greatly restricted dose (e.g., pediatrics and pregnancy). When switching to a more smoothing kernel,images at first appear "blurred" but radiologistsquickly get used to the new appearance.

Apart from classic HRCTapplications, most soft tissue imaging will not require the extra spatial resolution provided by a sharper kernel (see Fig. 1.4). Hence, they can be used to decrease image noise at a lower exposure dose. In obese

125

126

4 Optimization of Scanning Technique Table 4.6

Suggested scan parameters for multislice

Acquisition

CT scanning of the body (16-slice scanners)

Image reconstruction

Pitch factor"

Application

P

p.

SW/Rlb

SW/Rlc

16xO.625/5.625

0.5625

9

0.625/0.4

0.625/0.5

Volumetric olution)

imaging (ultrahigh res-

16xO.625/13.75

1.375

22

1.25/0.7

5/4

Volumetric

imaging (standard)

16xO.625/9.375

0.9375

15

1.25/0.5

1-2/2

Volumetric

imaging (skeleton)

16X1.25/27.5

1.375

22

1.25/0.7

7.5/5

Volumetric imaging (abdomen or obese patients)

16X1.25/27.5

1.375

22

1.25/0.7

5/4

Fast spiral scanning (standard)

8x2.5/27

1.35

10.8

2.5/1.25

7.5/5

Fast spiral scanning (obese patients)

16XO.75/7.5

0.625

10

0.8/0.5

1/1

Volumetric olution)

imaging (ultrahigh res-

16xO.75/18

1.5

24

1/0.7

5/4

Volumetric

imaging (standard)

16XO.75/11.25

0.9375

15

1/0.7

1-2/2

Volumetric

imaging (skeleton)

16x 1.5/36

1.5

24

2/1

7/5

Volumetric imaging (abdomen or obese patients)

1.5

24

2/1

5/4

Fast spiral scanning (standard)

1.5

24

3/1.5

7/5

Fast spiral scanning (obese patients)

16XO.5/5.5

0.6875

11

0.5/0.3

1/1

Volumetric imaging (ultrahigh olution, peripheral skeleton)

16XO.5/11.5

1.4375

23

1/0.5

5/4

Volumetric

imaging (standard)

16XO.5/7.5

0.9375

15

0.5/0.5

1/2

Volumetric

imaging (axial skeleton)

16x 1/23

1.4375

23

2/1

7/5

Volumetric imaging (abdomen or obese patients)

16x1/23

1.4375

23

1/1

5/4

Fast spiral scanning (standard)

16X2/46

1.4375

23

3/1.5

7/5

Fast spiral scanning (obese patients)

NxSC/TF General Electric

Philips/Siemensd

16X1.5/36 16X1.5/36

e

Toshiba" res-

N = number of active detector rows; SC = section collimation (mm); TF = Table feed (mm); SW = section width (mm); RI = reconstruction increment (mm) "Pitch P = TF / (N x SC); Volume Pitch P* = TF/SC b parameters for reconstruction of an overlapping 3 D volume (secondary raw data set) from which MPR and 3 D displays can be generated; not necessary for fast spiral scanning unless a second plane of reconstruction of 3 D display is required C parameters for image review and reporting (vary depending on clinical indication, and may be reconstructed by thick MPR from secondary raw data set) d pitch factors may have to be varied according to newest developments by the manufacturers e optional: 8-slice mode (8x3 mm) may be used in very obese patients on a Philips unit

patients with a large body cross-section, the pixel size is large (0.8-1 mm) and limits spatial resolution. In such a situation, the smoothing kernels not only reduce noise, but also do not markedly influence spatial resolution.

Judicious selection of the convolution kernel and mAs will help reduce the radiation dose. A smoothing kernel should be used in obese patients.

Reconstruction

•

Field

of View

The field of view (FOy) selection limits image reconstruction to a particular body region of interest to obtain the best possible spatial resolution (Table 4.7). For example. portions of the subcutaneous fat can usually be omitted from the scanned field. A constant FOV should be used throughout the scanned body region to allow for multiplanar reformatting and 3D reconstructions. For this reason. the FOV should be based on the widest portion of the scanned region. For high-resolution thoracic CT, some authors recommend a separate reconstruction of each lung to obtain the smallest possible FOV. However, the gain is minimal compared with a tightly limited field confined to both lungs. In addition, some modern scanners allow for reconstruction of a 10242 matrix, which will provide a large field of view at optimum spatial resolution.

Table 4.7 a Effect of reconstruction filter kernel spatial resolution (example of single-slice scanner) Kemel

3.2 Lp/cm

5 Lplcm

Smooth

4.0 Lp/cm

6.2 Lp/cm

Standard

4.8 Lp/cm

High resolution

8.5 Lp/cm

7.8 Lplcm 12.6 Lplcm

Very smooth

Exact

numbers

resolution hgher

a small objects b

vary

between

for multi slice

manufacturers,

4J

scanners

that size are displayed

original contrast boundary resdutim

at 4

to two times with 20% of

%MTF

Table 4.7 b Effect of FOV selection attainable spatial resolution

FOV

on the

maximum

Boundary

Pixel size

resolution

S'l Zmm

1mm

5 Lplcm

256mm

0.5 mm

10 Lp/cm

200rrrn

O.39mm

12.0 Lp/cm

150mm ~xel size limits

O.29mm spatial

Choice of reconstruction small FOV

on

Boundary resomon"

Resolution at 20%MTFa

17.6 Lp/cm

resolution filter

for large

kernel

FOV

is limiting

for

and Display Parameters

A limited field (FOV of approximately 25cm) can be useful in studies of the adrenals and pancreas. Reconstructions including the whole cross-sectional body area should additionally be obtained if abnormalities outside this field of view are suspected (e.g., pancreatic cancer). For CT angiography, it is generally unnecessary to reconstruct a 'whole-body cross-section". A FOV of 20-25 em can substantially improve the delineation of small vessels and intraluminal pathology (pixel size: 0.4-0.5 rnrn). Excessive magnification (FOV too small) only amplifies the apparent noise without, improving spatial resolution (FOV < 15 cm 5 is rarely advised). 2. With a large FOV, high-resolution kernels give poor results and are therefore not recommended (Table 4.7). Window

Selection

The window level (or center) should be chosen close to the CT number of the primary structure of interest (e.g., 50-70 HU for most soft tissue imaging). The window should be wide enough to capture all relevant attenuation numbers (e.g., fat should not be completely black and enhanced vessels not completely white) but as narrow as possible if low attenuation differences shall be detected (e.g., liver imaging). In general, no structure of interest should be completely white or completely black, unless more than one window setting is available (e.g., for the chest). The window width should also be increased for imaging high-contrast structures. This particularly applies to the lung, where a window setting that is too narrow (800-1000 HU) can cause pleural lesions to be missed or can exaggerate the apparent size of small lesions and bronchial wall thickness. Narrow windows are useful only in the detection of emphysematous changes. A large window width decreases the visibility of image noise and may be used for low-dose examinations in which local contrast is not critical. In studies that are strictly confined to high-contrast structures (e.g., dedicated skeletal studies), a wide window setting can be used along with a substantially reduced radiation dose.

127

Optimization

of Scanning

Technique

sponse of the human observer. This may improve local contrast for wide window settings but makes it difficult to compare window settings to other scanners.

Some manufacturers (e.g., Philips) use a gradation curve that assigns gray levels nonlinearly to the CT numbers within the chosen CT window to compensate for the gray level re-

Image Processing

cr.

Image processing gives the best results with data sets that were acquired with thin collimation. For spiral and multislice CT, a thin section width and a reconstruction increment with at least 50% overlap should be chosen for a "secondary raw data set" that serves as the basis for optimum quality of reformations and 3D reconstruction with the acquired data set. In most cases, the section width should be chosen some 20-30% wider than the collimation in order to reduce noise and optimize image quality for a given patient exposure (see above). Image processing, apart from thick axial reformations, is required whenever further evaluation of a volumetric data set is intended. Table 4.8 gives an overview of the types of processing that we suggest for various clinical indications.

single-slice spiral The technique of thick axial reformations is the more effective the thinner the primary sections and the higher the overlap. If such axial sections are reformatted, the operator can also correct for positioning errors, e.g., at the skull base, the neck, the shoulders, or the pelvis. In general. axial. coronal or sagittal reformations should be aligned not to the CT table but to the patient anatomy. The workstation should have a simple tool to perform this realignment of data to patient anatomy. The commonest further processing techniques are coronal or sagittal multiplanner reformations. They are routinely indicated for all situations that require a second imaging plane for optimum evaluation. This is the case for many musculoskeletal applications, such as for the cervical, thoracic, and lumbar spine. For joint trauma in the shoulders, elbows, hands, hip, knee, and foot, biplanar reformations are mandatory. In the chest and abdomen, most diagnostic questions can be resolved by axial sections alone. Coronal, and sometimes sagittal reformations are mainly required for planning surgery, e.g., for staging of bronchial cancer, complex liver lesions, pancreatic tumors, kidney

2D Processing Axial images with a larger section width can be reformatted from the secondary raw data set, even if the scanner does not allow for direct reconstruction of such thick axial sections. The technique can be used for multislice CT as well as for

Table 4.8

Suggested

Processing

image

processing

Technique

Thick axial sections Correct

Standard

for positioning

Coronal

or sagittal

techniques

depending

on clinical

indication

Application

errors

reformations

Petrous Tumor

viewing

technique,

bone,

shoulders,

staging,

bronchi,

noise reduction

pelvis,

intervertebral

bile ducts,

stomach,

disks bowel,

urinary

bladder,

kidneys Interactive

reformation

Curved

planar

Shaded

surface

Maximum

displays

intensity

Minimrm Volume

reformations

intensity rendering

projection projections techniques

Complex

pathology,

Vessels,

pancreatic

Complex

anatomy

CT angiography Bronchi,

pathology aorta,

(standard),

rendering

Virtual endoscopy

CT angiography Preoperative Colon,

interfaces

bile duct

skeletal

lung nodules,

structures interstitial

lung disease

biliary system (standard),

skeleton,

planning Perspective

near tissue

duct, common ofthe

planning

rarely bronchi

or arteries

complex

pathology,

preoperative

3D

tumors, and gastrointestinal malignancies, The evaluation of bowel diseases is improved when coronal reformations are available, Interactive reformation is required in more complex cases that cannot be resolved adequately on axial sections alone, Interactive reformations would be an ideal primary diagnostic interpretation mode but hardly any present workstation is fully capable of the (relatively simple) prerequisites for time-efficient work with this viewing modality (Table 4,9), Basic prerequisites include real-time interaction even for a large data set, full-screen display of the reformatted plane, and easy and intuitive tools for interaction, Interactive choice of the width of the reformatted plane is especially desirable with thin-section multislice imaging because image noise becomes the single most important limiting factor for image quality, In addition, a simple way of aligning the three imaging planes to the actual morphology makes image interpretation more easy, For more complex pathology, interactive variation of the imaging plane is helpful but thick sections should be available for the same reasons as described above, With most present workstations interactive scrolling speed is still insufficient when thick (oblique) sections are reformatted, although appropriate PC-based hardware has been available since the late 1990s, Curved planar reformations (CPR) are used mainly for vessel tracking in CT angiography and allow for optimum display of wall changes and eccentric plaques, and are the prerequisite for accurate measurement of curved vessel segments (e,g" prior to interventions), CPR are mandatory to evaluate the vessel lumen in case of vessel calcification, e,g" of peripheral runoff vessels, but semiautomated programs are required to make this procedure time-efficient Coronal CPR can be Table

4.9 CT workstations: prerequisites use of multi planar capacities of multi slice Real-time

irteraction

even for large data sets

Interactive choice ofthe width ofthe matted plane (noise reduction) Full-screen

Easy

display ofthe

and intuitive

Interactive

reformatted

reforplane

tools for interaction

control of scrolling

speed

Tools for aligning the three orthogonal planes to the actual anatomy • Interactive

for optimum

CT

adjustment

(with thick sections)

of the imaging

imaging plane

Processing

used as a standard display tool for longitudinal evaluation of the sternum, the spinal column, and the larynx and trachea, Coronal CPR through the pancreatic or common bile duct are used for CT cholangiography and CT pancreatography, Coronal CPR through the ureters are excellent for demonstrating the position of ureter calculi and for differentiating between calculi and ph leboliths. Such CPR, however, are quite time-consuming and difficult, and are made easier if the curve can be interactively corrected,

3D Processing

Shaded surface displays (SSD) have been all but replaced by high-quality (>5122 matrix) volume rendered displays, They are helpful for complex skeletal fractures (especially for the acetabulum and the shoulders) and can be used to display complex vascular anatomy (especially of the thoracic aorta), Maximum intensity projection (M I P) is an excellent display modality for CT angiography, especially where small vessels are concerned, For larger vessels and the display of three-dimensional relationships, volume rendering is superior. With the advent of (semi)automated bone removal, MIP has become a time-efficient imaging tooL Still, time can be saved if the images are prepared by the CT technologists, Sliding thin-slab MIP, however, is an excellent tool for evaluating complex anatomy of small vessels, MIP holds promise for the evaluation of pulmonary parenchymal disease but is still not routinely used for this application, Minimum intensity projections (mIP or MinIP) have only fringe indications in the chest (display of air-filled tracheobronchial structures and emphysematous bullae in the lungs) but other display techniques such as volume rendering are often superior. Volume rendering techniques (VRT) have replaced many other 3D display modalities with the advent of fast workstations and dedicated accelerator boards, VRT have become the standard technique for rapid display and evaluation of vascular disease in CTA Often, only minimum editing is needed for good results, VRT is replacing SSD for skeletal trauma, VRT displays should become standard for initial evaluation of trauma cases, At some point in the future, VRT will probably replace conventional axial sections and even

129

130

4 Optimization of Scanning Technique multiplanar reformations, especially when thinslab techniques and opacity settings that simulate conventional CTwindowing are used, Perspective rendering may make interactive movies look more realistic but it rarely yields additional information over other rendering tools, especially when such tools are combined with sectional planes that may reveal otherwise hidden structures, Using it depends very much on individual user preferences,

Virtual endoscopyis an endoluminal visualization technique which has the advantage that the evaluated structures are substantially enlarged, thus makingit possible to detect even subtle abnormalities, It has a proven value for the differentiation of colonic polyps and folds while the practical value for other organ systems (such as the larynx or bronchi) is very limited,

Special Scanning Techniques Quantitative CT (QCT) Quantitative CT is used primarily for the assessment of bone mineral density, It is most commonly used to assess the bone mineral content of the first three lumbar vertebrae, but in principle QCT can be used in other spinal segments and in the neck of the femur. It requires special analytical software and a suitable calibration phantom, The technique is described fully in Chapter 25 (Musculoskeletal System),

Dual-Energy CT Dual-energy CT is used for the detection and quantification of substances with a high atomic number, such as calcium, iodine or iron, The technique is based on the principle that the xray attenuation and CT numbers of these substances are strongly dependent on the tube voltage that is used, The method has found clinical application in bone densitometry (DEQCT) and for assessing liver iron in patients with hemosiderosis or hemochromatosis, The technique can reduce measurement errors due to variations in the fat content of the bone marrow or liver. Dual-energy CT is used only sporadically at present The base material decomposition technique, although more accurate, is no longer available with current scanners, The postprocessing dual-energy technique is still available and can be performed on any scanner. It involves taking two successive CT scans at the level of interest using two different tube voltages (e.g. , 80kVp and -140 kVp) , Though less precise than the base-material decomposition technique, the postprocessing technique is useful for assessing liver iron and bone mineral

content, which are estimated from the mean CT numbers in the region of interest (ROl) measured at the two energies, The scans can be performed at a low dose, provided a sufficiently large ROI (> 1 cm) is selected, I The attenuation difference (ACT) between' the two measurements is roughly proportional to the iron or bone mineral content of the tissue, Generally, this technique is satisfactory for the assessment of hepatic iron, Calibration phantoms are used in quantitative CT to improve accuracy and further reduce measurement errors due to fat Phantoms may consist of solid calibration standards composed of calcium hydroxyapatite or aqueous solutions ofk2HP04, To correct for fat-related errors, the j absorption difference in fatty tissue must be ! estimated by direct measurement in the subcutaneous or abdominal fatty tissue or in a standard ethanol solution, The ethanol solution is F less accurate, because its atomic composition is not identical to that of fat The following equation is used to calculate the mineral concentration Cm in the ROI from the concentration c of the calibration material and the attenuation i differences ACT for the ROl, ACTM for the calibration material, and ACTp for the fatty tissue: em

=

c x fACT-ACTFXACTM-ACT

A

This type of analysis requires an accurate determinationofACTp,otherwisethemeasurement errors would negate the advantage of the technique, Generally, the results are slightly less reproducible than conventional (single-energy) CT measurements, because the negative expressions in the equation tend to amplify the effect ofstatistical variations, For this reason, the results of the method are seldom worth the increased effort

5 Radiation Dose and Image Quality M Prokop

Imponant Dose Parameters

_

Volume CT Dose Index (CTDlvoi) (p. 132) Dose-Length Product (DLP) (p. 134) Effective Dose (E) (p. 134) Other Dose Measures (p. 136) Guidelines (p. 136) Scanner

Technology

_

Scanner Geometry (p. 137) Focal Spot Tracking (p. 138) Geometric Efficiency (p. 138) Detector Efficiency (p. 139) Electronic Noise (p. 139) Noise Filtering (p. 140) Tube Current Modulation (p. 141) I-Axis Tube Current Modulation (p. 142) Dose Traps in Multislice CT (p. 142) Dose and Image Quality

_

Image Noise (p. 143) mAs Settings (p. 144) Tube Voltage (p. 144) Reconstruction Filter Kernel (p. 145) z-Filter (p. 146) Section Collimation (p. 146) Section Width and Pitch (p. 147) Body Diameter and Body Region (p. 149) Field-of-View(p.150) Window Setting and Perceptible Image Noise (p. 151) Dose Reduction

Negligible Reduction in Image Quality (p. 152) Trade-offs in Image Quality (p. 152) low-Dose

Scanning

Prerequisites for Low-dose Scanning (p. 153) HRCT(p.154) Screening CT (p. 154) CT Urography and Urolithiasis (p. 155) Children (p. 155)

_

5 Radiation Dose and Image Quality

Radiation Protection

156

Patient Selection (p. 156) Dose n Children (p. 156) Selection of Parameters (p. 157) Radiation Shielding (p. 158) Exposure of the Examiner to Radiation (p. 160)

The absolute values for radiation exposure in CT depend strongly on the scan parameters, scanner characteristics, and the patient. They may be 5 to 100 times higher than with conventional radiographs of the same anatomic region. This emphasizes the importance of radiation dose and tailoring the scan parameters to the individual patient.

Important

Dose

There are numerous parameters that de- ( scribe the radiation dose to the patient but the I following three are most important and have gained international acceptance to describe the local dose (volume CT dose index, CTDlvoi),Ihe total scan dose (dose-length product, DLP) and the radiation risk (effective dose, E).

Parameters

Volume CT Dose Index (CTDlvoi) The volume CT dose index (CTDlvoi) is a tool to indicate the average local dose to a patient within the scan volume. Its unit is the mGy (milli Gray) and there are separate data provided for body applications (measured in a 32cm PMMA phantom) or head scans (l6cm PMMA phantom). The CTDlvoi is a measure of the average local dose delivered by CT (conventional slice-by slice, single-slice or multislice spiral scanning) to a cross-section of such a phantom. The CIDlvoi is the most accessible dose indicator because it can be directly displayed on the user interface of modern CT scanners. It proivides immediate feedback about the dose delivered to the patient. Such a display is required by EU regulations, but outside the EU some manufacturers only provide display of the CTDlvoi when requested by the customer. The CTDlvoi allows for direct comparison of the radiation dose from different scan parameter settings, even for scanners of different manufacturers. It already corrects for the influence of pitch, mAs, kVp, filtering and other scanner-specific parameters. It does not, however, indicate the precise dose for any in-

dividual patient, but is rather an index of dose for a particular scanner and examination. The CTDlvoi is derived from the primary direct measurement of dose in CT, the CT dose index (CTDl) , which is defined as the integral under the dose profile D (z) of a CT section: 1 D(z)dz N.SC CTDl measurements are made at various positions within a body (32-cm diameter) or head (16-cm diameter) PMMA phantom. The phantom provides scattering media similar (though of course not identical) to a patient. The dose profile is markedly wider than the section collimation SC or section profile because of beam divergence and scattered radiation (Fig. 5.1 a). As a result, contributions from adjacent sections increase the local radiation dose during scanningofwhole body regions (Fig. 5.1 b). In theory, CTDl measurements must include all dose contributions from the tails of the dose profile. As this is often not practical, finite measurement limits are used. The subscript 100 in the term CTDhoo denotes that the measurement was made over lOOmm. The term CTDlw (weighted CTDl) represents the average radiation dose across the diameter of the phantom (in body scanning, the periphery and center measureCIDI

rs

133

Volume CT Dose Index (CTDlyoi)

1.0 0.8

2.0 Dose profile

CTDlvo\

Slice profile

1.5 (])

Vl

0.6

0

U (])

>

~rc

0.4

1.0

(])

e:::

0.5

0.2 0.0 -5 a

-4

-3

-2

-1

0

2

em

4

5

0.0 -10

o

-5

em

5

10

b

Fig. 5.1 Dose profile of a single CT section (a). The area under the curve describes the CT dose index (CTDI). Acquisition of multiple contiguous sections increases the local dose due to contributions from adjacent sections

(b). The maximum dose in the center of the scan volume corresponds to the volume CT dose index (CTDlvoi) if the scanned volume is large enough and a relative plateau is reached.

ment can vary by a factor of 2). The CTDIwis factor P [= TF / (NxSC) 1 and is then termed measured using a 100mm ionization chamber volume CTDI (CTDIvoi): that is placed at various positions within a CTDIvoi= CTDIw/ P 32cm (body) phantom (Fig.5.2a). The CTDIw is then calculated from the measurements in the 0::::::::--:: 1 em center and the average of the four measurements in the periphery, 1 cm below the phantom surface: CTDlw =- CTDIc+ -j- CTDlp The CTDIwis a very good measure of the average radiation dose within the scan volume for contiguous CT scans. To extend this to noncontiguous axial scans or spiral scanning with a pitch + 1, the CTDIw has to be corrected by the pitch

32em

o

16 em

o

o o

a

250% 200 % ..-

Fig. 5.2 Measurements of the CT dose index (CTDI) are performed in the center (CTDlc) and 1 cm below the surface (CTDlp) of a cylindrical Plexiglas phantom of 32 cm diameter for the body and 16cm for the head (a). The resulting CTDlw decreases with larger phantom diameters (b), which implies that the real average dose is underestimated for children and slim patients.

_

.

o b

(])

>

~

~

e<::

100% 50%

O%+---------~-------,---------,--------, o 30 10 20 Phantom diameter (ern)

40

Radiation Dose and Image Quality Hence, CTDIvoi represents an average over the scan plane (x and y dimensions) and the length of the scan (z dimension), It must be understood, however, that the actual local dose to the patient differs from the body CTDIvoi because attenuation properties differ from a 32 cm body phantom (Fig.5.2b), In a larger patient more dose is absorbed in the periphery so that the local dose in the center is less than it would be for a slimmer patient (assuming all scan parameters are unchanged), Dose to the periphery also decreases for larger patients, This is because the peripheral dose is accumulated from all x-ray tube positions and, in a larger patient, the x-rays that have traveled through the patient have been more strongly attenuated, As a consequence, the actual average dose within the scan volume (for the same scan parameters) may be lower for very large patients than the CTDIvoi indicates, In smaller patients or in organ regions with less attenuation (neck, chest) the opposite holds true, While the CTDlvoiis a valuable tool for protocol comparison, it underestimates the average dose within the scan volume for children and slim patients, and overestimates it in obese patients, Some manufactures correct the CTDlvoi tor children, depending on the chosen protocol or field-of-view, This makes comparison between manufactures difficult

Dose-Length Product (DI Pl The dose-length product DLP is a measure ofthe cumulative dose (total energy) delivered to a patient Its unit is mGy • ctn. It not only takes into account the average dose within the scan volume (CTDlvoi)but also the scan length L

DLP~CTDlvoiXL The DLP thus bears witness to the intuitive' fact that, for example, the dose must be less for a limited CT examination of the abdomen as compared with a CT examination of the chest, abdomen and pelvis, even if the same CTDlvoiis used, For conventional (non-spiral) scanning the scan length L is the sum of all section collimations

(e,g" 25xl mm for HRCT=25mm), For spiral scanning, one can use the difference in table positions of the first and last section as a good estimate for the scan length L However, spiral and multislice CT oversample data at the beginning and the end of the scan range because this data is needed for raw data interpolation of the first and last sections, There are differences between the various manufacturers, but approximately one half a rotation at the beginning and another half rotation at the end have to be added to the radiation exposure of the patient Thus, the scan length, as provided by the scanner, should be expanded by at least one table feed, • There is a considerable dose increase due to oversampling at the beginning and the end of the scan when short spiral acquisitions are employed, • The dose increase is particularly high in 16-slice scanners and short scan ranges (children),

Effective Dose (E) The radiation risk to the patient can be estimated from the effective dose E (unit: m Sv-rn llll Sievert), The effective dose can be estimated for a standard male or standard female using mathematical modeling, which computes organ conversion factors that vary along the z-axis depending on the anatomic region. Various computer programs are available that perform this task for a wide range of scanner types and organ systems, These programs can calculate dose for individual organs, which, when appropriately weighted using the organ weighting factors from lCRP-60, can be summed to calculate E. The mathematical modeling done to compute the organ conversion factors used to calculate E are performed on standard sized "mathematical" human phantoms, Separate data are available for males and females of various ages (infants through adults), However, in clinical practice, the data for a standard adult (70kg) is most frequently used, Such estimates of E will underestimate the radiation risk for children and slim patients and overestimates it for obese patients,

Effective Dose (E) Table 5.1 The effective dose E can be estimated as a percentage of the CTDlvoi or using a conversion factorfor DLPw for each scan phase. The estimates have been derived using data from a German survey of over 800 institutions (Galanski and Stamm) and from data for standard patients as described in the ED report 16262 EN. The estimates strictly hold true only for standard patients. For variation of the scan length, the DLP yields better estimates for the effective dose, while the CTDlvoi is probably superior for patients of different body height Organ

Neck Chest Liver Pelvis Abdomen + pelvis Chest + abdomen

Scan length

Conversion E/CTDlvoi

20cm 30cm 20cm 20cm 40cm 60cm

20% 42% 32% 41% 73% 96%

factors for estimation EIDLP

of

0.0090 0.015 0.018 0.017 0.017 0.016

L=scan length, E= effective dose (mSv), CTDlvoi = volume CTdose index (mGy), DLP = dose-length product (mGy -crn). EUAdata from EUR 16262; all other data from Stamm, 2001 (personal communication)

In addition to this size dependence, the in-' creased radiation sensitivity of children causes them to have an age-dependent radiation risk that is 2-3 times larger than that of the general population. A less computationally

intensive but still reasonably robust method of estimating E has been proposed that relies on the reported values of DLP. Individual organ doses are not computed, but rather for a given scan region (head, chest, etc.) a conversion factor from DLP to E is used. Since the calculations are based on standardsize patients, the DLP for very tall or very short patients (each having the same values of CTDIvoi) may over or underestimate E. For these cases, it may be helpful to redefine these conversion coefficient in terms of CTDIvoi (Table 5.1). For any method used to calculate a value of E, it is essential to remember that such values are estimates. The many assumptions used in the calculation process are likely not accurate for any given patient. As with CTDIvoi estimates of E do not indicate the precise radiation risk for any individual patient, but are rather an index of risk for a particular scanner and examination. The risk of death from radiation- induced cancer has been calculated from BEIR V and ICRP-60 data, which extrapolate the risk estimates from

accidentally or occupationally exposed groups (e.g. the atomic bomb survivors). Much of this data reflects high doses and high-dose rates and hence risks from diagnostic radiology procedures (typically low doses and low dose rates) must be extrapolated. It is extremely hard to detect an additional risk in the diagnostic dose range because all other environmental and hereditary risk factors appear to have a much larger influence. That is, the radiation-induced risk per mSv is approximately 0.5/10.000 persons, while the risk of a fatal cancer is about 3.000/10.000 persons. Because radiation risk is markedly age-dependent (Table 5.2), in-utero risks associated with the exposure of a pregnant woman should be considered. These are discussed in Chapter 20 (Female Pelvis).

Table 5.2 Calculated radiation-induced risk (ICRP 60) of dying from cancer. However, such risks can not yet be statistically proven for an effective dose below 20 mSv

Age

Risk (death per mSv)

Child (0-10 years)

14 /100000

Adolescent

18

(10-20 years)

/100000

Adult (20-30 years)

7.5 /100000

Adult (30-40 years)

3.5 /100000

Adult (60 years)

2.0 /100000

Adult (80 years)

1.0/100000

Average

5.0)100000

135

5 Radiation Dose and Image Quality

The entrance skin exposure (£S£) is ameasure of the skin dose but is not a good measure of Apart from the three most practically useful patient dose because it is strongly dependent on dose measures described above, a vast variety of scanner geometry and the energy spectrum ofthe other dose measures are available, x-ray beam, Identical ESE may lead to vastly The CTDlpoA was the first well-defined and different CTDlvoi (by factors of two or more), broadly used measure of dose in CT and is still The energy imparted is the most physically commonly used by manufacturers, It is identical correct measure ofthe radiation exposure, It deto the CTDlwo definition described above but is scribes the energy of the radiation that remains based on measurements over a length of 14 conwithin the body or an organ, It is more complex tiguous sections instead of a fixed length of to determine than the CTDl and therefore is not IOOmm The difference between the CTDlpDA in widespread use in clinical practice, and the CTDlioc is negligible for large collimaOther measures like air kerma (K) are important for dosimetry, One has to keep in mind tion (=27 mm): but the CTDlpDAmarkedly underestimates the dose for narrow collimation that correction factors have to be employed to (A 5 mm) such as in thin-section spiral or multitranslate air kerma into absorbed dose to a slice CT If in doubt whether the CTDlpDAor specific material (also in mGy) , although this CTDhoo is provided by a manufacturer, compare may be done automatically by some measurethe num bers for thick and thin collimation: the ment equipment CT01pDAfalls as the collimation decreases, while the CTDlloo remains constant or even increases, The multiple slice average dose (MSAD) is Guidelines the average dose delivered within a scan region of multiple contiguous scans (see FigS lb), Apart from various local regulations, few guidelines have been established yet, although both Conceptually, the CTDlvoi is identical to the MSAD ifthe 100-mm ion chamber collects all of the European Union and the United States work the dose in the tails ofthe dose profile, on such issues. The CJDlair (dose free in air) is the dose The European Guidelines for Quality in measured in the center of the gantry with all abComputed Tomography EUR 16262 suggests sorbers removed, Because no phantom is re- maximum dose levels for CTDlvoi and DLP that should not be exceeded unless there is a clinical quired it is easier to determine than the head or body CTDL It was used for dose estimates and reason (Table 5,3), These dose levels have been some local European regulations, However, it established from a British survey in the late cannot take into account the influence of the 1980s and represent the 75th percentile of the dose levels applied at the participating institubeam quality (energy spectrum of the x-ray tions, In the late 1990s similar studies were perbeam) and scanner geometry (distance from formed in Germany, Switzerland, and Austria tube focus to gantry center) on the attenuation by the body, The relation between the dose in and found markedly lower dose levels based on spiral CT scanners (Table 5,3), There are now efair and the actual body CTDl varies strongly with scanner type and manufacturer. It thereforts to further refine the suggested dose levels fore is not particularly useful for clinical purwithin the EU, and to start similar projects in poses, It is very important, however, as an input the US, parameter to the mathematical modeling softThe suggested dose levels (CT01voi) proware programs that estimate organ doses and E. vided by the authors of this book (see Table 5,7) The organ dose is a measure that describes are the product of vast personal experience but the average dose to an organ (in nrSv). Depending may have to be adapted to local requirements on the size ofthe body cross-section and the loca(for dose and image quality) and to the intion ofan organ relative to the surface, it can be dividual patient The suggested CT01voirefer to a estimated from CTDlc, CTDlp, and CTDlw (after cor170cm170kg "standard patient", who is obrecting for the influence of the pitch factor P), viously quite slim, The CTDl will have to be inComputer programs are available that can calcucreased substantially in obese patients (somelate the dose more accurately for standard times up to a factor of3 or higher) because x-ray patients (male, female, children of various ages), attenuation doubles for every 4-6 cm in additional soft tissue diameter (see below), Other Dose Measures

Scanner Geometry Table 5.3

Average

-a

exposure dose found in surveys CTDlw[mGy]

in various

countries as compared to the EU guidelines

DLP [mGy*cm]

E [mSv]

EU Larynx Chest Chest (HRCT) Liver Kidney Abdomen

38 18 21 21 21

EU 003

33 15 30 16 17 15

30 35

327 327 748

45

G = German survey of 830 institutions

(Galanski,

A = Austrian

(Novotny,

survey of 15 institutions

SSs Swiss survey of 6 institutions

638 326 96 321 383

415

4ffi

300 500

2.4 6.4

650 280

700 277 580

5.9 5.9 12.9

780

2.4

3.4

4.7 1.5 5.9 6.2 8.6

9 13 4.8 10

2001) 2002 personal

communication)

(Aroua, 2000)

EU= EUR 16262 guidelines

Guidelines for (accidental) radiation exposure in pregnant women are discussed in Chapter 20 (Female Pelvis).

Scanner Technology

Scanner technology markedly influences image noise and image quality at a given radiation exposure to the patient. The following paragraphs discuss the most important factors that influence patient dose, as well as techniques that can be used to reduce patient exposure. Scanner Geometry

Scanner geometry strongly influences the patient dose (CTDI) at a given mAs setting. With

more rapid tube rotation, the centrifugal forces on the tube increase with greater distance between the tube and isocenter of rotation. For this reason, most newer scanners have moved the tube closer to the isocenter (Fig. 5.3). At a given mAs setting, this will increase patient dose, especially skin entrance dose, substantially. This phenomenon causes a potential dose trap: if identical mAs settings are used on a new scanner as compared to an old scanner (even from the same manufacturer), this maysubstan-

X-ray tube

/

,

, Fig. 5.3 Scanner geometry. A large distance (a) between the focal spot of the x-ray beam and the isocenter of rotation suffers from increased centrifugal forces on the x-ray tube. A short distance (b) increases patient dose and detector signal at constant mA settings. Surface dose is increased most.

I I

,

/

/

X-ray tube

,

,, ,

/

, \

/ / /

._- .. Scan field-of-view ---

,,

,

, \

137

5 Radiation Dose and Image Quality

tially increase patient dose if the scanner geometry has changed (e.g., a GE LightSpeed compared with a GE HiSpeed scanner). For this reason, manufacturers recommend adjusting the mAs setting downward so that CTDIvoi remains similar. Focal Spot Tracking

With fast rotational speed, the centrifugal forces on the electron beam within the x-ray tube cause small alterations in the position of the focal spot during a spiral scan. This can be solved by slightly widening the pre-patient collimation ("overbeaming"). Such a technique was used in the very first versions of multislice CT scanners and caused a substantial increase in patient exposure, especially for thin collimation, relative to similar single-slice scanners. Tracking the focal spot by slightly adjusting the collimator setting (focal spot tracking) will keep the dose on the detector constant without having to increase patient dose. Such techniques are now standard with multislice CT. Geometric Efficiency

The geometric efficiency of a detector is determined by the amount of radiation that actually hits the detector elements relative to the amount of radiation that leaves the patient. The geometric efficiency on the one hand depends on the width, spatial orientation, and absorption of the septa that separate single detector

elements. Such septa are necessary to avoid cross-talk between neighboring detector elements and to absorb scattered radiation. Thirdgeneration scanners have an advantage over fourth-generation scanners in that their collimators are centered on the tube focus and not on the center ofthe scan. In multislice CT, radio-opaque septa will slightly reduce geometric efficiency when matrix detectors with a wide total collimation are used. Radiolucent septa will keep geometric efficiency high but will be more vulnerable to scattered radiation. Additionally, geometric efficiency depends on the width of the dose profile (in z-direction) relative to the total width of the detector elements. In single and dual slice CT, there usually is no post -patient collimation, which means that almost all radiation that leaves the patient also hits the detector (Fig. 5.4). In multislice CT, the detector is subdivided into four or more active detector channels. Most manufacturers try to make sure that each active acquisition channel (detector row) "sees" the same amount ofradiation. This makes it necessary to widen the dose profile by opening up the pre-patient collimation (Fig. 5.4) and therefore reducing dose efficiency. In fact, the slope of the dose profile due to the penumbra is not utilized. Since the absolute width of the penumbra and thus additional radiation exposure due to the penumbra is almost independent of the collimation, exclusion of the penumbra will have the greatest relative effect if the total width of collimation (N x SC) is small. It will cause a markedly reduced geometric dose efficiency and thus increased CTDI (Fig. 5.5). As can be seen. the effect is greatest for a 2 x 0.5 mm

Focal spot '. The penumbra contributes to the image and to patient dose

Beam collimator

The penumbra contributes to patient dose but not to the image

Beam collimator

20 cm detector width Single slice CT

Multislice CT

Fig. 5.4 The penumbra of the x-ray beam is caused by the finite size of the focal spot of the x-ray tube (a). It is fully utilized for single and dual detector CT scanners (b) but is excluded for multislice CT scanners (c).

Electronic

detector configuration and improves with a 4 x 0.5 mm collimation. With 16-slice scanners, the effect is almost negligible.

Detector Efficiency Scintillation (solid-state) detectors generally have a 20% to 30% higher quantum yield than Xenon gas detectors. This implies that using scintillation detectors, the same image nose can be achieved with considerably lower dose, relative to Xenon gas systems. The total quantum yield also depends on radiation prefiltering, detector design, detector electronics, and scanner geometry, which tends to reduce the differences between different types of detector.

139

Noise

The smaller the detector signal, the more pronounced the role of electronic becomes. The effect is particularly significant for obese patients, low-dose protocols and thin-section imaging. In multi slice CT, where thin collimation is becoming routine, it is very important to

•

Strong effect with thin collimation

Penumbra -

1111111111111111116x1.25mm 20 cm detector width Effect decreases with adaptive array detectors

.-.

I III

Electronic Noise

t"

I

I 4x 1 mm

20 cm detector width

The electronics of the detector system (signal amplifiers) cause a constant noise level that is independent on the incident radiation dose (Fig.5.6, see also Fig. 7.44). Quantum noise, on the other hand, increases with the detector dose. This increase, however, is substantially smaller than the simultaneous increase in detector signal (square root function). With a high detector signal, the electronic noise is negligible relative to the quantum noise but as the detector signal decreases, the electronic noise may assume a similar order of magnitude to the quantum noise and then contribute substantially to the noise levels on the final CT image.

Effect decreases with thicker collimation 11111

I I I

111114x2.5mm 20 cm detector width Effect decreases with more detector rows

1111111111111111116x1.25mm 20cm detector width

240%

a

reI. CTDI vol

220% 4-slice scanner

200% Fig. 5.5 The geometric efficiency of the detector system depends on the amount of radiation excluded (penumbra) relative to the radiation collected by the x-ray detectors (a). The geometric efficiency decreases with thinner sections but increases if more active detector rows are used. As a consequence, the (TDI of 4-slice scanners grows markedly for smaller collimation but less so for 16-slice scanners (b).

180% 160% 140% 120% 100% ~

16-slice scanner

J-~~'8--,-~::===;===~===;:==-'---+ o

2

3

4

7 5 6 8 Section collimation S( (mm) ~ b

140

5 Radiation Dose and Image Quality Fig. 5.6 Electronic noise limits lowdose applications and thin-section imaging. It dominates the image as soon as it is larger than the quantum noise, New detector electronics can substantially reduce electronic noise for 16-row scanners making thinner sections possible even at reduced dose,

100 ~,

'''''',~~

" "'.Signal

10

-u

'"

,,-,

'0,1

0.01 0.001

'<,

Electronic noise (4-slice scanners)

-TI~-R~-d~-~~-;~-~-~~~-~~~~i~~~~~~;~~~--~--~--~-·~~----,

0.0001 --+----,------,--------,-----,-----------, 100.000

10.000

1000

1 00

10

Rei, detector dose

individually adjust exposure settings, especially for obese patients (see Fig. 7.33). Newer detector electronics with 16-slice scanners substantially decrease electronic noise relative to single slice CT or four-slice scanners (Fig. 5.6) and thus allow for thinner sections and lower dose settings than older scanner models. Noise Filtering

Various manufacturers have already included digital filtering options in their scanners that reduce the effect of low signal (and thus high noise levels) on individual detector elements. Such filters work on the raw data and average the signal from neighboring detector elements if the signal in these detector elements becomes too low. Averaging n detector elements has the

same effect as "locally" increasing the detector dose by the factor n. Because there are often only a few detector elements for each projection angle that have too Iowa signal (e.g., the central elements that cover the spinal column), the averaging influences only a small portion of the total projectional data. As a consequence, inplane spatial resolution is affected only minimally. Most recently, three-dimensional filtering of raw data has been suggested that uses a three-dimensional weighting function to locally average the data from adjacent detector elements, from similar projection angles, and along the z-axis if the detector signal for a particular projection becomes too low. Such a technique can substantially reduce noise-related streak artifacts, for example in the shoulder or pelvic regions (Fig. 5.7).

Fig, 5.7 Noise filters selectively average raw data from projections with low detector signal and, thus, high image noise, Compared to an image without noise filter (a), the image with noise filter displays less noise and less noise-related streak artifacts (b).

a

b

Tube Current Modulation

both noise reduction and improvement of image quality. By increasing the dose selectively for Tube current modulation takes advantage of the those projections in which there is a substantial fact that there is often a substantial difference in increase in attenuation (usually lateral projecradiation attenuation between the lateral and tions) but at the same time decreasing the dose anteroposterior (AP) diameter of a body cross- for projections with little noise (most other prosection. Because attenuation follows an ex- jections), the overall image noise may be ponential function, small changes in diameter decreased while radiation exposure to the will result in major differences in attenuation patient also is reduced. and, thus, related quantum noise at the detector. Various systems are presently on the For an oval body, the exponential nature of at- market that use either sinusoidal dose modulatenuation will yield only a few projection angles tion (for example SmartScan, GE) or on-line with markedly increased attenuation and a adaptive dose modulation (for example C.A.R.E. larger range of angles with a relatively low at- dose, Siemens). Other manufacturers offer simitenuation (Fig. 5.8). lar products and the user should ask the manuWhen looking at how noise propagates facturer to provide a specific description of the from single projections to the final image, one rnA modulation algorithm. The sinusoidal dose modulation technique finds that the projections with maximum noise have a disproportionate contribution to the re- modulates the rnA in a sinusoidal fashion acsulting image noise. This holds the potential for cording to the maximum and minimum patient Tube Current Modulation

Fig. 5.8

Adaptive dose modulation

relies on mA reduction for those projections with less x-ray attenuation (usually ap views) (a). With adaptive dose modulation (right side of image) there is substantially less noise and fewer noise-related streak artifacts (b) even if no noise filtering is employed. As a result, the quality of coronal reformations is improved (right side of image, c).

Absorption, mAs

40 35 30 25 50%

20 15 10 5 0 +-----~~~~~--~~==~----~----+O% 0.4 0 0.2 0.6 0.8 1

1.2

Tube rotations

b

c

141

142

5 Radiation Dose and Image Quality size (within the scan range) as determined by an anteroposterior and lateral scanogram. Some manufacturer's algorithms use only one localization image. Since the minimum and maximum diameters vary along the z-axis, the amplitude of the sinusoidal variation (both the minimum and maximum values) must be adjusted to account for variations in patient size (along the z axis). It is important to determine if a specific algorithm implemented on one's scanner incorporates such z-axis modulation. Finally, because the actual attenuation varies much more than the diameter (Fig. 5.8), a sinusoidal modulation may not fully exploit the dose saving potential of mA modulation. The on-line adaptive dose modulation technique uses the attenuation information from every 1800 rotation during a spiral scan to modulate the mA setting in the following rotation to compensate for the measured differences in detector signal (Fig. 5.8). This technique takes full advantage of the potential of dose modulation. However, the early implementation of this technique keeps the maximum mA per rotation constant.ln the region of the neck and shoulders, for example, this will lead to an increased exposure to the neck if the modulation is to work properly for the shoulders. For multislice CT with wide total collimation (N x SC), large table feeds are used. This will lead to problems in regions with rapid changes in patient size along the z-axis (e.g., in the shoulder region). Adaptive dose modulation allows for dose reduction by some 10-30% without sacrificing image quality, depending on the shape of the examined body area. Noise-related streak artifacts can be reduced substantially. For low-dose applications, however, electronic noise may render the techniques less useful than the noise filtering methods described above. More advanced algorithms can be expected soon.

Z-Axis Tube Current Modulation As noted above, longitudinal dose modulation along the z-axis adapts the dose to the local requirements in the body (Fig. 5.9). Such a technique (e.g., Smart mA by GE) lowers the mA settings for the neck and chest, and increases it for the shoulders or the abdomen. It is a further advance over adaptive modulation and uses the attenuation information of either an AP or lateral

rnA Longitudinal modulation ConstantmA legs z-axis Image noise

, ~Ongitudinal

DV

modulation ~onstantmA

z-axis ~

Too much dose

c=::J

Too much noise

Fig. 5.9 Longitudinal dose modulation relies on mA reduction for those positions along the z-axis that have less x-ray attenuation (neck, lungs) and rnA increase for highly attenuating regions (shoulders, pelvis). Local dose varies while image quality remains more constant.

scanogram, the APjPA or lateral projection during a spiral scan or the continuous information from each spiral rotation to decide upon the overall mA requirements for a particular rotation. It is an important step towards "phototirning" for CT. However, there is little experience yet in how to correctly choose exposure settings using this technique.

Dose Traps in Multislice

CT

Multislice CT suffers from a number of "dose traps" that can substantially increase patient dose if mA settings are used identical to those for conventional spiral CT, even if a scanner from the same manufacturer is employed. Changes in scanner geometry (closer distance between tube and isocenter) will cause an increase in CTDI at identical mAs settings (e.g., GE LightSpeed versus GE HiSpeed scanners). Using narrow collimation (e.g. 4 x 11.25 mm) decreases geometric efficiency with four-slice scanners and leads to an increase in CTDI (see Fig.5.5b). This increase is approxi-

Image Noise

rnately 30-60% for 4xl mm or 4x1.25mm collimation (30% is still acceptable compared to the typical dose variations between sites), but may be as high as 150% with 2xO.5 mm or 2 x 0.625 mm collimation. For this reason. a collimation of 2xO.5-0.625 mm should be avoided for most body applications. This increase is no longer relevant for 16-slice scanners (see Fig. 5.5). The term effective mAs, which is used on Siemens scanners, refers to the values of the true mAs divided by the pitch factor. Some users find that this term makes it easier to choose an given level of noise, as pitch is already taken into account. However, confusion of the two terms (effective mAs and mAs) may lead to a

substantial increase in dose. This can occur if a user chooses to employ the same mAs settings that he previously used on a single-slice scanner of the same manufacturer. For example, a 200 mAs setting at a pitch of 2 (single slice) will correspond to IOOmAseff (multislice). Choosing 200 "mAs" on the user interface of the multislice scanner actually means choosing 200 mAseff, which will cause a twofold increase in patient dose compared to the single-slice system (all other determinants of dose, such as gantry dimensions, being equal). As a consequence, users should not look primarily at the rnA settings but choose the rnA in a way that ensures appropriate patient dose as indicated by the CTDlvoi (see Table 5.3).

SiBahdT mageQttft The minimum dose requirement in a CT examination is determined by the acceptable amount of image noise at the required spatial resolution. Acceptable image noise depends in turn on the necessary contrast resolution, the window width that will be used, the attenuation characteristics of the scanned region, and the diameter of the patient.

Image Noise Image noise (known also as pixel noise) is a measure of the statistical fluctuation ofCTnumbers and is measured as the standard deviation in a homogeneous region of interest. Image noise a is theoretically related to the section collimation SC (in mm): tube current 1 (in rnA), and section acquisition time t (in s) by the equation:

The acquisition time t per slice may equal several rotation periods RT in conventional CT, but in spiral scanning it is always equal to RT. Cp is a constant that depends on the convolution kernel, and k is a normalizing factor. The factor Cz(P) depends on the raw-data interpolation algorithm in spiral CT or on the z- filtering in multis lice CT. In spiral CT, it is usually a constant Cz, independent of the pitch. In multislice CT,

Cz(P) is strongly pitch-dependent (compare Fig. 1.28 b). From this it can be seen that, • The dose must be increased four times to reduce noise to one-half of the original value. • Using 2 mm collimation instead of 8 mm collimation leads to a doubling of image noise. • The convolution kernel has a major effect , on noise (Table 5.4).

Table 5.4 Influence of filter kernel on image noise and the dose required for constant image noise (measured for a Siemens Somatom Plus 4 scanner, Prokop, 1998)

Filter kernel

Relative

image noise

(%)

Relative dose require-

Spatial I resolution (Ip/em)-

ment(%)3 ABIO AB20 AB30 AB40 ABSO AB60 AB70 AB82 AB91

56 71 78

85 100 117 136 281 280

32 50 60 72 100 137 184 789 786

4.1 4.7 5.0 5.2 5.8 6.2 6.6 7.6 7.7

a measured in center of a 30 em water phantom b spatial resolution at 20% MTF (better reflects

clinical reality than cut-off frequency at 4% MTF)]

143

144

5 Radiation Dose and Image Quality The following additional quantities theoretically affect image noise by influencing the normalizing factor k: k = C.~ where M is the x-ray attenuation in the object and w is the sampling aperture. The factor C is an equipment-dependent measure of dose effectiveness (a small C denotes high effectiveness), which is most strongly influenced by the scanner geometry and the detective quantum efficiency of the detector system. The sampling aperture w is determined by the width of one detector element in the x-y plane and by the number of data samples per rotation. It is an important factor in the above equation, indicating that a smaller aperture and thus potentially higher spatial resolution leads to markedly increased noise.

mAs Settings Radiation exposure of the patient as well as the detector dose are proportional to the mAs settings. For a given rnA setting and rotation speed, the detector "sees" the same dose per rotation, independent of the pitch. Only with multislice CT interpolation schemes does the image noise vary with pitch at a constant mAs setting. The patient dose, however, is always proportional to the effective mAs (mAs per tube rotation divided by the pitch). Any measure to reduce patient dose will usually be coupled to a reduction in effective mAs.

As mAs settings per rotation are reduced, image noise grows. This has a major effect on contrast resolution but no effect on spatial resolution as long as the contrast of a small structure exceeds the noise levels by a factor of 2 to 3. Excessive reduction of mAs settings, especially in very obese patients and in combination with thin collimation, may reduce detector dose so much that electronic noise will dominate image quality. Reconstructing thicker sections or using smoothing filter kernels then can no longer recover reductions in image quality (see Fig. 7.33). Such an excessive mAs reduction, therefore, should be avoided by all means.

Tube Voltage For a constant mAs setting, the radiation output of the x-ray tube increases with higher x-ray energy. When the tube voltage is increased from 120 kVp to 140 kVp, the CTD! increases approximately by a factor of 1.4, while decreasing the tube voltage to 80 kVp decreases the CTD! by a factor of approximately 2.2 (Fig. 5.10). This effect may be useful for scanning the newborn and infants where the lowest possible tube output may be desired (the use of 80 kVp allows for further dose reduction below the minimum mAs settings available at 120 kVp). Conversely, the x-ray attenuation in the patient decreases with higher x-ray energy. This effect exceeds the increase in tube output with higher kVp, and results in a better dose utilization at higher tube voltages. This effect is especially important in the obese patient and in

160% 140% 120%

15 Iu Q)

100%

> ;::; ro

80%

c.:

60%

Qj

80

100

120

Tube voltage (kVp)

140

Fig.S.l0 At constant mA settings, the CTDI varies substantially with kVp. There are marked variations between scanners, depending on prefiltering of the x-ray beam.

Reconstruction Filter Kernel regions with high x-ray attenuation (such as the shoulders, abdomen, and pelvis). In such a situation, the highest possible tube voltage should be chosen (e.g., 140kVp). Finally, the relative difference in x-ray attenuation (contrast) between the various structures in the body increases with lower x-ray energy. At lower kVp settings, the CT numbers of metal, iodine, and calcium increase substantially, while the CT number offat decreases. The effect is especially pronounced for any contrast -enhanced (iodine-containing) structure: the enhancement at a given concentration of iodine is about twice as large at 80 kVp as compared to 140kVp (see Table 7.2). The effect can be used to optimize contrast-to-noise ratios at a given radiation exposure, or to decrease the radiation exposure at a given contrast -to-noise ratio. However, the positive effect of contrast is counterbalanced by the increased attenuation described above. When changing the tube voltage from the typical value, it is essential to increase mAs settings to compensate for the reduced tube output at lower kVp (which would increase image noise).

For contrast -enhanced examinations W the neck, chest, and in very slim patients, low kVp settings with properly increased mAs settings may be used (e.g., 80kVp, 150 mAs instead of 120 kVp, 100 mAs). This decreases radiation exposure (CTDI) and improves contrast enhancement. . For obese patients, especially for examination of the shoulders, abdomen, and pelvis, high kVp settings (e.g., 140 kVp) should be employed, Al For most other applications, a standard setting of 120 kVp suffices.

Reconstruction Filter Kernel

The filter kernel used for image reconstruction from the raw data has a substantial influence on image noise (Table 5.4). A slightly smoothing filter kernel will cause only minor reductions in spatial resolution but will require substantially less dose for constant image noise. On the contrary, increasing the spatial resolution by using a sharper filter kernel will substantially increase noise, often to a degree that can no longer be compensated for by raising patient dose (Fig. 5.11).

Although there are no firmly established rules yet, the following suggestions can be given: • For the newborn and infants, low kVp and mAs settings should be used.

300%-]' ReI. Dose Requirements 250%

200%'·······

150%

Fig. 5.11 Image noise depends strongly on the chosen filter kernel for image reconstruction from the CT raw data set. With growing spatial resolution there is a substantial increase in image noise, while a moderate reduction of spatial resolution (smoother filter kernels) allows for substantial dose savings.

50%

.

-

.

--'

O%+-----~------~----~------~----~ 60%

80%

100%

120%

140%

160%

ReI.spatial resolution

5 Radiation Dose and Image Quality

• In-plane spatial in for noise.

resolution

can be traded

z-Filter In spiral CT, the use of a 360' LI algorithm reduces image noise by 18% compared with conventional scanning, while a 180' LI algorithm increases image noise by 15% (Table 5,5), In multislice CT, image noise is strongly pitch-dependent (compare Fig,L28b), At present, only Siemens has chosen an implementation where image noise becomes independent of pitch if radiation exposure to the patient remains constant (see Fig, L29a), In this implementation (as with other vendors for a pitch of 6), the noise is similar to the noise seen with a 360' LI algorithm in conventional spiral CT Similarly it comes with an increase in section width of some 30%, Since multislice CT usually employs thinner sections, such a broadening of the section profile can be accepted easily because it leads to substantially lower dose requirements for identical image noise,

Table 5.5 Relative dose requiremerts raw data interpolation and z-filtering Z-filter

Pitch

algorithm

Conventional

Rel.section width (SW)

CT

depending

ReI. dose requirement

100%

1000/0

100% 66% 128% 110% 110%

133% 128% 66% 117% 93%

100% 128% 100% 128%

133% 66% 133% 66%

SpiralCT 180-LI 360" LI SmartHelical"

Multislice

CT

z-filtering HQ mode" HS mode" a

GE4-slice scanners

0-2 0-2 0.75 1.5

Use of a 180' LI algorithm increases noise while 360' LI decreases noise. A 360' LI algorithm may be used in very obese patients if mAs setting cannot be further increased. A 360' LI algorithm with thinner sections can be used at P = 1 as a standard in dualslice scanners for applications with high image noise (e.g., abdomen and pelvis). Use of an adaptive z-filter interpolation decreases noise.

Section

Collimation

Image noise increases as a thinner section collimation is chosen. This behavior parallels that of the mAs settings: reducing the collimation by a factor of 4 could be compensated for only by an increase in mAs by the same factor. Image noise increases by a factor of 2 if no such compensation is performed. The effect of section collimation SC on radiation exposure (CTDI) varies greatly between manufacturers. For most single-slice scanners, the CTDI remains constant for SC 5: 3 mm, and increases only slightly for smaller collimation (see Fig. 5.5). When a single detector row is used, not only the radiation from the ideal section profile but also the penumbra (see Fig. 5.4) of scattered and extrafocal radiation can be utilized. A dose increase may be seen when manufacturers use a post-patient collimation to improve the section sensitivity profile and open up the pre-patient collimation to compensate for the loss in detector dose. In some single-slice scanners employing post-patient collimation, the CTDI voi doubles when going from a 2-mm scan to a 1 -mm scan. With multislice CT, most manufacturers try to make sure that each active acquisition channel (detector row) "sees" the same amount of radiation. This makes it necessary to widen the dose profile by opening up the pre-patient collimation (see Fig. 5.4) and therefore reducing dose efficiency. In fact, the slope of the dose profile due to the penumbra is not utilized. Since the absolute width of the penumbra, and thus additional radiation exposure due to the penumbra, is fairly independent of the collimation, exclusion of the penumbra will have the greatest relative effect if the total width of colli-

Section Width and Pitch

mation (NxSC) is small. It will cause a markedly reduced geometric dose efficiency and thus increased CTDl (Figs. 5.4 and 5.5). As can be seen. the effect is greatest for a 2 x 0.5 mm detector configuration and improves with a 4xO.5 mm collimation. With l6-slice scanners, the effect is almost negligible. • Z-axis resolution can be traded in for noise. • Image noise doubles when 2 mm collimation is used instead of 8 mm collimation. • Reducing the collimation by a factor of 4. could be compensated by raising the mAsI settings by the same factor, but should be avoided in clinical practice. • With single-slice spiral CT, patient dose is nearly independent of the section collima1 tion • With thin-section multislice CT (four detector rows), patient dose increases by 30% for 4 x 1 mm collimation, up to 60% for 4 x 1.25mm collimation and by up to 250% for 2 x 0.5 mm collimation. • With thin-section multislice CT on l6-row is

Section Width and Pitch The section width determines image noise in a similar way as section collimation. In fact, section width is directly dependent on section collimation (and pitch) in single-slice spiral CT, but it becomes an independent parameter in multislice CTscanning. Section width in single-slice spiral CT is identical to section collimation for pitch of 1 (180 LI) and gradually increases by 28% up to a pitch of2 (180 LI). Such a widening by 28% also occurs with 360 LI at pitch 1. With increasing pitch factor P, the spiral is stretched and patient dose falls proportionally. This leads to the definition of effective mAs (=mAs/P) and CTDIvoi (=CTDlw/P), both of which compensate for the influence of pitch on dose. Consequently, a moderate widening of the section width by choosing a large pitch factor will cause a marked decrease in patient exposure. A pitch of 1 with a 180 LI interpolation is highly dose-inefficient and should be avoided wherever possible. It is the main "dose trap" for single-slice spiral CT 0

0

0

0

scanning. A pitch close to 2 is preferable (Fig. 5.12). Note that Philips is so far the only manufacturer that indicates the section width, not the section collimation, on the user interface. When scanning parameters are chosen to optimize z-resolution (i.e., constant table feed but reduced collimation), image noise will rise due to the smaller collimation. Choosing higher mAs setting can compensate for this. For example, patient dose and image noise with scrrF = 5110 at 200 mAs are identical to the patient dose and noise with scrrF = 10/10 and 100 As, but section width is 6.4 mm instead of 10 mm. • A pitch factor of 1 should be avoided with spiral scanning and 180 LI because of increased patient exposure at a given noise level and section width (Fig. 5.12). • Patient exposure remains constant if mAs are increased proportional to the pitch. • Z-Axis resolution can be improved at identical patient exposure (CTDlvoi) by reducing the section collimation and keeping the effective mAs constant. Image noise will remain similar. • Choosing dose settings according to the CTDlvoi, not according to the mAs, helps avoid dose traps. 0

Section width in multislice CT follows a more complex behavior with varying pitch, but it also ranges between 100% and 128% of the section collimation. Image noise shows a similarly complex behavior. At identical patient exposure, it is markedly higher if the section width is identical to the collimation, and is substantially reduced if the section width is increased by 28%. On GEmultislice CTscanners, dose requirements at identical image noise are about 50% higher for the HQ. mode than for the HS mode. While the HQ. mode allows for a section width that is identical to the collimation, the HS mode has a 30% wider section width. Note that the section width indicated on the user interface is only a rough estimate of the real values (see Table 1.8). With 8- and l6-row scanners the rule remains that higher pitch factors require less dose at a given image noise. Siemens uses z-filtering (four-slice scanners) and cone beam reconstruction algorithms (16-slice scanners) in such a way to keep noise

148

5 Radiation Dose and Image Quality

1000%

100%

sw=S(

10% 0

5

10

15

Section width SW(mm) .~ 1000% 0

c Q)

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E

.•.....

c

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oc

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5

10

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Section width SW (mm)

A.

1000%

100%

5mm Doselowerthan conventional (T

10mm

10%~-------------'-------------T-------------'

o

5

10

15

Section width SW (mm)

Fig. 5.12 Relative dose requirements for constant image noise on single-slice spiral CT images acquired with varying section thickness. Noise on a 5 mm conventional CT section is normalized to 100% (a). Dose requirements in conventional CT grow substantially with thinner section width. Note that section width and section collimation is identical in this case (b). Dose requirements in single-slice spiral CT are halved as the pitch increases from 1 to 2, but that the resulting section width increases by 30%. Note that for a given noise and section width, dose requirements with spiral CT are actually higherthan with conventional CT if low pitch factors are employed (c). For any given section width, thinner collimation and high pitch factors require the lowest patient dose. Note that the dose requirements for a section width of 10 mm are higher for SC =10 mm and P=1 than for conventional CT (SC= 1Omm) or spiral CT withSC= 7 mm and P=1.7.

Body Diameter and Body Region constant independent of the pitch. The default setting uses an increased section width (e.g .. SW= 1.25mm for 4x 1 mm collimation. and 3 mm for 4 x 2.5 mm collimation) and therefore has a reduced noise (as compared to standard CT sections) independent of the pitch. For the 4x 1 mm collimation. a setting is also available that reduces the section width to 1 nun, independent of the pitch. Because of a markedly increased noise level. this setting should be reserved only for few high-contrast applications. in which maximum z-resolution is required. Toshiba has a similar approach. As the user chooses thicker sections for a given collimation. the z-filters vary and the image noise decreases. The minimum section width is identical to the collimation. but suffers from a substantially increased noise as compared to the next wider section width (e.g .. 1.5mm instead of 1 mm). Philips uses the 360 MLI forlowpitch factors P ss 1. and 180 MLI for P > 1 on their four-slice scanners and an advanced cone beam algorithms (COBRA) for their 16-slice units. With the latter algorithm arbitrary pitches A 2 are available that have a comparatively low noise. 0

0

!. On GE four- and 8-slice scanners. the HS t and UF modes yield less noise for a conH stant patient exposure (CTDlvoi). 1 • The HQ. and UQ. modes should only be 1 used in applications in which artifact bel havior is crucial (e.g.. cervical spine, hands). • On Siemens and Toshiba scanners, reconI structing a section width that is identical 11 to the collimation should be avoided for H standard applications because of a substantial increase in image noise. • With four-slice scanners, high pitch factors can be chosen (p* = 5.5 for Toshiba, 6-8 for ( A Siemens and Philips) for standard applications, and low (P*=3) for applications, inwhich artifact behavior is crucial (e.g., cervical spine, hands). • Choosing dose settings according to the CTDlvoi, not according to the mAs, helps avoid dose traps. Reconstructing thicker section width from thinsection multislice CT raw data reduces image noise substantially, similar to using thicker collimation to begin with. The exact extent of the re-

duction in noise is greatly dependent on the implementation of z- filtering for this purpose. However, because of the reduced geometric efficiency at thinner collimations and the larger amount of electronic noise on four-slice scanners (see Figs.5.5b and 5.6), the image noise on thick sections reconstructed from 1 -2 mm multislice scans may be higher than in images of the same thickness reconstructed from thicker collimations. This holds true especially for obese patients. If a thick section is reconstructed from a secondary raw data set by multiplanar reformations, the noise reduction depends on the degree of overlap of these sections and the algorithm used for creating thick MPR. The effect of reducing noise by reconstructing thicker sections is more efficient for MPR perpendicular to the scan plane. This is because image noise on consecutive axial sections is correlated (depending on the reconstructed section width of the axial images that are the basis of the 3D data set used for MPR) while it is nearly independent for adjacent pixels within the scan plane (depending_on the reconstruction filter kernel and the FOV). Because of the increased attenuation in lateral projections, the noise on sagittal MPR is higher than on coronal MPR, especially for highly eccentric body regions such as the shoulders or pelvis (Fig. 5.13). Recommendations for MPR widths are given in Table 4.3. • Multislice CT gives a unique opportunity to maintain z-axis resolution and trading through-plane resolution (MPR thickness) for noise. • Reconstructing thick sections from a thinsection secondary raw data set is most effective if there is sufficient overlap (at least 50% of SW) of the axial images comprising this secondary raw data set. • Image noise is reduced most effectively on coronal reformations. Such MPR require less thickness than axial or sagittal MPR.

Body Diameter and Body Region Noise doubles in the center of a cylindrical water phantoms with every 4-6 cm increase in diameter d. depending on kVp settings, x-ray filtration, beam geometry and software noise fil-

149

150

5 Radiation Dose and Image Quality

a

c

b

Fig. 5.13 Comparison of image noise on 3 m m wide axial (a), coronal (b) and sagittal images (c), reconstructed from a volumetric data acquisition with 4 x 1 mm collimation. This patient could not elevate the

arms above the head. coronal image, and is sagittal image that are the posterior areas of

ters. In humans, the body cross-section is more elliptical. The minimum diameter (di) and maximum diameter (d2) of this cross-section determine an effective diameter d', which should result in identical noise as in a cylindrical body of this diameter. Ideally, d' should be equal to /\ di x ds' but in many scanners, noise filtering will reduce the influence of the larger (lateral) diameter, thus making a linear approximation de Cl xdi + C2xdz more appropriate. Noise ina human body, therefore, will increase less than in a water phantom, especially when noise filtering is employed by the scanner software. In addition, the subjective assessment of image noise does show that noise is less disturbing in obese patients because of a larger amount of body fat that serves as an "intrinsic" contrast material, and because of the larger FOV (see below) that must be employed with such patients.

There is less attenuation in the chest due to the high proportion of air in the lungs. Therefore, compared to the abdomen, noise increases less quickly in the chest as the patient diameter increases. Consequently, dose requirements for the chest are substantially lower than for other regions in the trunk.

X-ray attenuation in the patient rises exponentially with body diameter, so obesity leads to a disproportionate increase in noise. A useful rule of thumb is that the noise level doubles with every 4-8 cm increase in effective body diameter (except for the chest), depending on kVp settings and noise filtering.

Note that noise is least for the highest for those regions on the at the level of the shoulders and the chest (arrowheads).

Field of View

The FOV not only changes the pixel size but also the character of the image noise. As the FOV becomes smaller, the noise appears coarser and becomes more troublesome. Depending on the convolutionkernelandothernoise-determining factors, there is a minimum size for the FOV (approximately 25cm for the trunk, otherwise 15cm) below which the image will only have a noisier, grainier appearance with no improvement in spatial resolution (see also Table 4.4). • A large FOV creates a more finely speckled noise pattern but also reduces spatial resolution. • A small FOV in children makes noise appear coarser and also increases the effect of image blur, especially when smoothing filter kernels are used.

Window Setting and Perceived Image Noise

a

b

Fig. 5.14 The perceptible image noise greatly depends on the window setting. An unenhanced liver scan requires a narrow window setting (a, 200 HU) to optimize contrast between the liver parenchyma and lesion. This also doubles the perceptible image noise compared

Window Setting and Perceived Image

Noise

The perceived image noise 0* denotes the noise level that is perceived visually on the film or monitor. It depends critically on the window width setting W: o*=fxo/W,

where f is a suitable normalizing factor that also depends on the gradation of the display medium and the sensitivity ofthe eye. Thus, the window width has a much greater impact on the visual perception of noise than the dose or collimation: changing from a window width of 400 (standard soft-tissue window) to a width of 200 (used for unenhanced liver studies) causes a two-fold increase in perceptible noise (Fig. 5.14). For this reason, highresolution convolution kernels should be used only with a window width> WOO (e.g., in the lung or skeleton). Conversely, the use of a soft kernel should be considered in cases where a narrow window width is required (e.g., in the liver); otherwise noise can be reduced only by increasing the patient dose.

c

d

with a normal window width (b. 400 HU). The use of a high-resolution (MR) kernel provides excellent definition of the lung parenchyma (c. window width of 1500 HU), but the mediastinum suffers from increased noise (d. 400 HU).

Increasing the window width setting can greatly reduce perceived image noise. Doubling the window width reduces perceived noise by 50%. Doubling the window width also reduces image contrast by 50%. Consequently, structures with a high contrast require a significantly lower radiation dose (doubling the window width reduces the dose requirement by 75%).

151

5 Radiation Dose and Image Quality Dose Reduction

All dose reduction relies on the reduction of CTDIvoi or scan length. Negligible Reduction in Image Quality

On-line adaptive dose modulation will yield excellent results for most body regions where there is a marked difference in attenuation between AP and lateral diameters. This holds true especially for the shoulders but also for the chest, abdomen, and pelvis in slim patients. Dose reduction by some 10-30% will become possible, depending on the shape of the examined body area. Image quality is retained and even improved for the shoulders or the pelvis. Current implementations, however, suffer from a number of problems (see above). Longitudinal dose modulation adapts the dose to the local requirements in the body (e.g., less for the neck and chest, higher for the abdomen). This will become available soon (SmartrnA, GE; DoseRight, Philips; AEC, Siemens; Real EC, Toshiba) and will be an important step towards more constant image quality ("phototiming") in CT. New three-dimensional multislice CT interpolation schemes (noise filtering) substantially reduce noise without causing much smoothing of the data. This technique is not yet commercially available.

a

b

Fig. 5.15 The dose recommended by the manufacturer (a, 300 mAs, standard kernel) can be significantly reduced by the use of a soft kernel (b. 100 mAs). Note that the noise level is identical but the noise appears coarser.

Trade-ofts in Image Quality

The most well known trade-off is the increase in image noise if mAs settings are reduced and all other parameters are kept constant. Image noise can also be reduced by using more smoothing filter kernels for image reconstruction ("soft" instead of "standard", or "standard" instead of "high resolution" or "bone"). Thus there is a trade-off between dose and inplane spatial resolution if a smoothing kernel is employed together with appropriately reduced mAs (Fig. 5.15). Choosing a smoothing kernel for the soft tissues and a higher resolution kernel for the lungs or bones yields excellent results and allows for substantial dose savings (Fig. 5.16).

a

b

Fig. 5.16 The radiation dose in thoracic examinations can be significantly reduced (here: I = 40 mA, RT = 1.0s, pitch =1.6 => 25mASeff; CTDlvoi = 2.3 mGy) by using different kernels (HR and soft) to display the lung parenchyma (a) and mediastinum (b).

Image noise can also be reduced by choosing a thicker collimation or by reconstructing thicker sections (effective section width, SW) from thin axial data. This will reduce z-axis resolution but may be employed to reduce mAs settings. The multiplanar capabilities of multislice CT are best utilized by thin-collimation scan-

Low-Dose Scanning ning. However, patient dose (CTDIvoi) increases with such thinner sections, and image noise also grows substantially. Still, low-dose scanning is feasible with the following approach. An overlapping data set of thin sections (secondary raw data) is reconstructed first. This data set consists of noisy images (see Figs. 2.5, 2.14, and 5.18-19) but has high resolution in all three spatial directions. Thick multiplanar reformations are then performed from these data in any desired direction (axial, sagittal, coronal, oblique). These MPR will have good in-plane resolution as well as sufficient signal-to-noise (see also Fig. 5.13). The only drawback is the reduced through-plane resolution (resolution in the direction perpendicular to the MPR plane) due to the increase in MPRthickness. This is very similar to multiplanar imaging in MRI, where in-plane resolution is high, and a section thickness of 3-8 mm is employed. Image quality with this approach is excellent but-with four-slice scanners-the patient dose will be somewhat increased as compared to scanning with thicker sections because of the higher CTD] at thinner collimation (Fig.5.5a). With 16-slice scanners, geometric efficiency is substantially improved, and this negative effect is almost completely eliminated.

a

b

Fig. 5.17 Low-dose examination for colon cancer screening. Acquisition parameters: 4x 118 at 80kVp and CTDlvoi = 1.88 mGy, resulting in an effective dose of 1.3 mSv. Image noise is excessive with a soft tissue window setting (a) but is acceptable with a wide window

Low-Dose Scanning Prerequisites for Low-dose Scanning

Under the following four conditions, low-dose scanning becomes feasible: •

Reduced image quality requirements

Image quality is a critical feature in radiographic examinations. Reductions in quality are possible if there is pre-existing information about the disease (e.g., follow-up scans), if there is only one specific finding that is of importance in a certain clinical setting (e.g., presence of a polypoid lesion in the colon in colonic cancer screening) , or if only one organ system with high contrast has to be evaluated (e.g., lungs in lung cancer screening). Thick sections have been used as a standard in the past. If thick sections suffice (such as for the search for abdominal abscesses or hematoma) , dose may be reduced compared to protocols with a thinner section and similar signalto-noise ratio (e.g., by a factor of 2 if 10 mm instead of 5 mm sections may be employed). In multislice CT such thick sections may be reconstructed retrospectively as a means to improve in-plane image quality. Thin sections reduce partial volume effects and therefore increase the contrast of small structures. For this reason, an increase in image noise is acceptable on thin sections used for diagnostic re-

c setting ofW/L= 200010 (b) and is sufficient for virtual colonoscopy (c). In more obese patients, higher kVp settings should be used, and dose has to be increased to maintain image quality.

5 Radiation Dose and Image Quality view. The dose therefore needs not be proportionally increased as compared to thicker sections. •

Discontinuous

evaluation

of target organ

If the structure of interest can be evaluated in a discontinuous fashion. such as in HRCT for diffuse lung disease, effective radiation dose is reduced by a factor that is roughly identical to the relation between the width of the section and the table feed (e.g., factor 10 for 1 mm collimation and l Omm table feed in HRCT with a single-section technique). In multislice CT four or more sections are acquired simultaneously. Thus, the advantage of discontinuous scanning is reduced by that factor if no single-slice mode is available on such scanners. As a result, exposure dose for HRCT using such multislice scanners is still lower than with continuous CT but substantially higher than with single-slice CT. In addition, thin sections usually increase the CTDl somewhat, and will cause an increase in image noise that has to be compensated for by an increase in rnA settings. •

High contrast between the structures interest and their surroundings

of

High contrast makes it possible to employ wide window settings. Such wide window settings efficiently suppress noise (which is immediately evident when comparing lung and soft tissue window settings from HRCT of the lung, see Figs.5.14c, d). Thus, major dose reductions become feasible only if a wide window setting is required. This holds true for the lungs and the skeletal system. Improved contrast in CTA (by appropriate contrast injection protocols or scanning protocols) may be used to reduce patient dose while maintaining a reasonable signal-tonoise ratio between vessels and surroundings. •

Examined body part has little x-ray attenuation

Slim patients or children have lower x-ray attenuation than obese patients. This translates into lower dose requirements. The same holds true for organ regions with intrinsically low attenuation such as the lungs, the neck, and the extremities. For roughly every additional 4cm of soft tissue diameter, the required radiation dose for identical image noise doubles in phantom experiments. In clinical practice this increase is probably less (first results suggest some 8 cm for

doubling the dose) because obese patients contain more fat as an intrinsic contrast agent and thus are less vulnerable to noise, while children require somewhat sharper kernels for image reconstruction and thus require more dose than such a calculation would suggest. In practice, however, every 4-8 cm more soft tissue diameter requires double the dose, while for slim patients, the dose can be halved if the diameter is smaller by 4-8 em.

HRCT

High resolution CT (HRCT) of the chest is performed in a discontinuous fashion: 1-2 mm thick sections are acquired every 10-20 mm. This substantially reduces radiation exposure compared to spiral scanning. In standard size patients, the DLP should remain below 50mGyxcm, and the effective dose E should remain well below 1 mSv. Follow-up examinations or expiratory scans may be performed with further reduced dose.

Screening CT CT is excellent for screening of bronchogenic cancer patients since primary thin-

Multislice

section scanning allows for reconstruction of suspicious areas with a high spatial resolution and thus avoids additional scans in patients in whom a nodule was detected. In addition, it becomes much simpler to follow up patients with multiple nodules, especially when automated detection algorithms are used (Computer Aided Diagnosis or CAD). The mAs settings have to be adjusted to the size of the patient so that the CTDIvoi varies between 0.6 and 2mGy. Note, however, that image quality may deteriorate substantially if the patient is too obese and too Iowan exposure setting has been used (see Fig. 7.32, electronic noise). With single-slice spiral CT, a 5 mm collimation with a pitch of 2 is employed for lung cancer screening. Because of less noise on such thick sections than with thin-section multislice CT. an even lower CTDlvoi can be chosen, in the range of 0.3-1 mGy. However, additional thinsection scans are required if a nodule is detected.

Children Fig.5.18 CT Urographywith reduced dose. Acquisition parameters: 4x-1/8at120kVpand CTDlvoi= 1.88 mGy, resulting in an effective dose of 1.6 mSv. I mage noise on the thin-section images from the secondary raw data set (SWIRl = 1.25/0.7) is excessive (a) but can be substantially reduced when 20 mm thick curved planar reformations a re used to demonstrate the course of the ureter and the renal pelvis (b). Note the decrease contrast excretion by the tumor -bearing left kidney. a

In colon cancer screening there is a high contrast between the air-filled lumen and the bowel. Dependent on patient size, a CTDIvoi between 2 and 5 mGy can be used (Fig. 5.16). Smoothing reconstruction is mandatory and a section with of some 2mm is sufficient (and suffers from less noise) even for the secondary raw data set of overlapping axial images. CT Urography and Urolithiasis

CT urography after injection of IV contrast has an intrinsic high contrast. Thus, low-dose scans may be sufficient but mAs settings must be adapted to patient size (CTDIvoi =2-5 mGy). Thin-slab MIP or thick curved planar reformations (10-20 mm) parallel to the course of the ureters are used to evaluate the images (Fig. 5.1S) For the detection of urolithiasis, low-dose scanning is feasible. We found that even with CTDlvoi =2 mGy all therapeutically relevant stones > 2.5 mm could be seen in a 20x30 cm water phantom when thin-slab MIP were employed. However, noise increases quickly with more obese patients. A substantial increase in dose (5 mGy) may be necessary for this patient group.

b

Children

For the newborn and small infants it is advisable to reduce the tube voltage to SO-100 kVp because the CTDI falls with lower kVp. This allows for further dose reduction below the minimum mAs settings available with a standard of 120 kVp. Often as little as 0.3-1.5 mGy at SOkVp is sufficient (Fig. 5.19). The chest requires less dose than the abdomen. Contrast administration improves tissue differentiation and signal-to-noise, and therefore should be considered for examinations of the neck, mediastinum, and abdomen. For children between 4 and IOyears of age, 120 kVp with CTDIvoi= 2-5 mGy can be recommended (see Table 5.6). For all indications, a thin secondary raw data set should be obtained from which thicker sections can be reconstructed in arbitrary planes. The width of the MPR should remain as narrow as possible but as wide as necessary for a good signal-to-noise ratio (chest: 2-5 mm, abdomen 3-S mm).

155

156

5 Radiation Dose and Image Quality

a Fig. 5.19 Low dose examination in a 5-year old boy with a right-sided thoracic mass. Acquisition parameters: 4 x 1 18 at 80 kVp and CTDlvoi = 0.88 mGy, resulting in an effective dose of 0.35 mSv. Excessive noise on a 1 pixel thick coronal reformation from a secondary raw

data set that was reconstructed with a high-resolution kernel (a). The 5-mm thick coronal section from the same data set yields excellent image quality and demonstrates a right-sided interlobar effusion (b).

Radiation Protection

CT accounts for a large percentage (>30%, depending on the country) of the collective effective radiation from medical sources to which the general population is exposed. Patient selection, proper choice of scanning parameters, and radiation shielding can contribute to lowering overall radiation exposure. Patient Selection

The most important step is patient selection based on rigorous criteria. The question that is to be addressed by CT should be defined as clearly as possible, and the imaging modality itself must be capable of answering the question. The radiologist should also consider whether alternative modalities such as MRI or ultrasound could provide comparable information with less invasiveness. MRI should be considered as a primary imaging study mainly in young patients or in patients with non-neoplastic diseases that require frequent follow-up. Dose in Children

Radiation-induced risk in children can be substantially higher than the risk in the adult population. For this reason, the radiologist should

double-check the indication for CT, and should adapt the radiation dose to the body cross-section. Longitudinal and adaptive dose modulation would be ideal techniques, but presently only rough suggestions based on the body weight are available (Table 5.6). One has to keep in mind, however, that the body weight may underestimate the dose requirements in obese children because their body cross-section can be larger than that of a taller child of identical weight. Table 5.6 Suggested exposure settings depending patient size (CTDlvoiderived from data by Donnely, 2001 for aGE CT Ii). The pitch factor should always be 1.5 or larger (120 kVp) Weight(kg)

Chest

Abdomen/pelvis

4.5-8.9 9.0-17.9 18-26.9 27 -35.9 36-44.9 45-70 >70

1.7 mGy 2.1mGy 2.5 mGy

2.5 mGy 2.9 mGy 3.3mGy 4.1mGy 5.0mGy 5.8-6.2 mGy /\7.0mGy

2.9 mGy

3.3mGy 4.1-5mGy > 5.8mGy

Selection of Scanning Parameters Selection of Scanning Parameters

Table 5.7 gives recommendations for selecting the convolution kernel, slice collimation, kVp, and CTDIvoi for various clinical indications. The numbers indicated are based on a "standard patient" of 170cm and 70kg body weight (20x30 cm body cross-section), and have to be increased for obese patients. The following considerations are important in the selection of imaging parameters.

For objects with high intrinsic contrast (lung parenchyma and bone), spatial resolution is almost independent of image noise. Thus, the wide window setting used for skeletal imaging and HRCT of the lung permits the use of narrow collimation, a high-resolution kernel, and low mAs values (Fig. 5.16). Contrasts are lower in CT angiography, so a soft kernel must be used to compensate for the narrow collimation that is required (Table 5.7).

Table 5.7 Suggested exposure parameters for various clinical indications for standard patients (70 kg/170 ern). These exposure levels should be adjusted to individual requirements because dose efficiency and filter kernels vary between scannertypes and manufacturers

w- _ ,1'

_1

I

Iflndication

r:-t'-!!r--,.....,,~

Standard patients (70 kg/170 em) Neck (benign disease) Neck (tumor staging) I Chest (HRCT) Chest (benign disease) Chest (pulmonary embolism) Chest (tumor staging) Liver (pre-contrast) Liver (post contrast) Pancreas (tumor staging) Urolithiasis Abdomen (benign disease)

I

CTAchest CTAabdomen Obese patients (100kg/170cm) Neck (benign disease) I Neck (tumor staging) I Chest (HRCT) jfefChest (benign disease) Chest (pulmonary embolism) Chest (tumor staging) Liver (pre-contrast) Liver (post contrast) Pancreas (tumor staging) Urolithiasis Abdomen (benign disease) CTAchest CTAabdomen a b

C

smoothing kernel for the mediastinum, high resolution kernel (or display filter) for the lungs 5 mm sections for the mediastinum and lungs; additional 1.5 mm sections for peripheral tumors (multislice CT) thinner sections and higher dose are chosen for preoperative evaluation of tumor patients

157

Radiation Dose and Image Quality

For structures with low intrinsic contrast, detection of small objects (as described by a contrast-detail curve) depends on image noise, and small structures may be completely obscured by noise. As a general rule, one should use the convolution kernel that provides the best trade-off between low noise (smoothing effect) and adequate spatial resolution (acceptable unsharpness). While this trade-off is always based on a subjective appraisal, in most cases it can substantially reduce the dose requirement below manufacturers' recommendations. Contrast resolution in liver studies is critical, because there is often very little attenuation difference between a hepatic lesion and the surrounding parenchyma. Lesions smaller than 3 mm can almost never be detected. A narrow window setting is required for noncontrast examinations, and therefore soft kernels should be used to avoid an excessive increase in dose (Fig. 5.15). Intravenous contrast medium generally increases the attenuation difference between tumor and liver, so the dose can be reduced when a soft kernel is used (Table 5.7). In abdominal examinations, similar principles to contrast-enhanced liver studies apply. In CT examinations of the lungs and neck, relatively little attenuation occurs within the tissues. Since many neck examinations require a detailed structural analysis, a somewhat higherresolution kernel should be used than for mediastinal studies. The dose in thoracic examinations can be reduced by using different display kernels for the lung parenchyma and the mediastinum (Fig. 5.16). Intervertebral disk studies have the highest dose requirement: the attenuation difference between the disk and its surroundings is small, thin collimation is used, and abdominal attenuation is frequently high. Again, the dose requirement can be reduced by using a soft convolutionkerne1. The following parameters are critical: • Number of scans: are precontrast scans, biphasic scans, or delayed scans really necessary? • Scan length (preferably limited to the region of interest). • CTDlvoi (choose mAs judiciously, using a lower setting for thin patients and children) .

• Pitch (should be "1.5 or more whenever possible). Slice width (thicker sections, whether axial or multiplanar. reduce noise and allow for CTDl reduction). Convolution kernel (soft kernels reduce noise and allow for CTDl reduction).

Radiation Shielding Outside the scan volume lead shielding can be used to reduce scattered radiation. It has to be noted, however, that most of the scattered radiation arises from within the patient and therefore cannot be shielded. Lead shields must not be included in the scan range because they cause substantial streak artifacts that warrant repeat scanning of the affected region. Shielding of direct radiation is only in its infancy. Bismuth-containing shields have been suggested that are radio-transparent, but still reduce surface exposure substantially. Such shields are especially helpful for reducing dose to superficial organs such as the breasts, the testes, and the eye lenses. The more superficial the organs, the higher the radiation doses to which they are exposed. This particularly applies to organs that are directly exposed in the scanning field, but it also pertains to those receiving scattered radiation. Testes that are unprotected in the scanning field may receive organ doses higher than 20m Gy per examination. In women, the breasts are exposed to particularly high dose levels (up to 25mSv) if they are within the scanning field. The dose is several times that received in grid mammography, so particularly stringent criteria should be used in selecting young women for CT examinations. It should be noted that in women with large breasts, CT scanning of the liver will incur some radiation exposure to the mammary gland. This can increase the organ dose to more than 50mGy, especially in biphasic examinations. Consequently, the breasts should be positioned to minimize their exposure in liver examinations.

Radiation Shielding Table 5.8

159

Oose and image quality

Examination parameters

t i

mAs-settings Tube voltage

Remarks

radiation exposure radiation exposure

i

Section collimation Pitch facto r

Effect

i

radiation

image noise image noise

1800-lnterpolation

image noise

z-Filtering: SW = SC

image noise

z-Filtering: SW> SC

image noise

t

image noise image noise

High resolution

filter kernel

image noise

Window width

t

noise perception

t

Body cross-section

image noise

t

t = increase; t

i tt

image noise

it;

(t)

t

preferentially

t

between 1.5 and 2

adapt to indication

(see Table 2.2)

for P = 1 for P > 1 increased dose requirements decreased dose requirements

=

standard with multislice

CT

for mediastinum/abdomen

xy-resolution t t t ; contrast t

t ; dose t

as low as possible couple with mAs

only in obese patients

t ; z-resolution t t ; z-resolution t t ; z-resolution t t ; z-resolution = t ; z-resolution ( t ) t ; in-plane-resolution t t ; xy-resolution t

Smoothing filter kernel

Size of lungs

noise

t ; z-resolution t exposure t ; z-resolution

i

MPR-thickness

noise

image noise

3600 -Interpolation

Section width

t ; image i ;image

for lungs/bone for lungs/bone/CTA

requirements

t

reduce mAs chest only; predictable gram

from scano-

= decrease; = = constant

Table 5.9 Relation of mAs settings (at 120 kV) to CTOlvol (single slice data calculated from data from Nagel 2000. multislice data from www.impacscan.org). The multislice data hold true only for 5 mm section collimation. For smaller sections. the CTOI increases (see Fig. 5.11)

Single slice scanner GE9800/HiLight/HiSpeed

series

Philips SR series/AV series/Secura PhilipS/Picker PQ series Philips/Elscint/Picker

Twin Flash

Siemens AR series/Emotion/Balance Siemens Plus 4 series Toshiba Xpeed/Xpress/XVision

CTOlvol at 100 mAs

mAs at CTOlvol = 10 mGy

Pitch = 1

Pitch=2

Pitch = 1

Pitch=2

5.4

2.7

185

370

8.0

4.0

125

250

11.2 4.3

5.6

89

178

233

466

14.4 8.3 8.8

2.15 7.2 4.15 4.4

69

138

120

240

114

228

89

178

Shimadzu 7000 T

11.2

5.6

Multislice scanner

Pitch = 1

Pitch =2

GE LightSpeed series (SC = 5 mm)

10.1

5.1

99

198

7.0 7.3

7.0a 7.3a

143

143a

137

137a

11.2

5.6

89

Philips/Picker MX 8000 Siemens VolumeZoom Toshiba Aquilion

Multi

Pitch = 1

178

Numbers for 120 kV. For 140 kV the CTOI is larger by a factor of approximately 1.36. The mAs settings have to be reduced by 26% to keep the CTOI constant. a Siemens and Philips multislice scanners display the effective mAs-settings. Thus. there is no pitchdependence.

•

160

5 Radiation Dose and Image Quality Exposure of the Examiner to Radiation Only individuals who remain in the scanning room are subject to radiation exposure (interventional procedures, ICU patients, persons accompanying the patient) as structural shielding can easily be designed and installed that essentially eliminates exposure outside of the scan roo 111. Every scanner has its own isodose map with contour lines indicating the spatial distribution of scattered radiation (Fig. 5.20). As always, scattered radiation depends on the applied dose (mAs), tube voltage, and patient cross section. Dose to persons within the scan room can be minimized using the radiation protection principles of shielding (use of a lead apron), time (limit time within the scan room), and distance (move as far away from the scan plane and patient as is possible). Since the radiation emanates from the gantry aperture, a bystander will receive a minimum dose by standing at an angle next to the gantry or close to the narrow side of the gantry. Persons involved in interventional procedures should not leave their hands in the scan plane during imaging (especially during CT fluoroscopy). If necessary, this can be avoided by using suitable holding devices for the interventional instruments.

4

3 2

o -1

-2-

-2

-1

o

2

3

m--

Fig. 5.20 Typical isodose map showing radiation emissions from a modern CT scanner. The dose figures are stated per 100 mAs for 120 kV and a 10 mm slice thickness (CTD1w~ 8 mGy).

6 CT-Guided Interventional Procedures M. Prokop, A.J. van der Molen

CT-Guided Biopsies

....

General Principles (p. 162) Biopsies in the Chest (p. 167) Biopsies in the Abdomen (p. 169) Musculoskeletal Biopsies (p. 171) CT-Guided

Drainage Procedures

General Principles (p. 174) Specific Techniques in the Chest and Abdomen Specialized Interventions

<, (p. 177)

.............................•...

Liver Tumor Ablation (p. 178) Treatment of Osteoid Osteoma (p. 181) Spinal Interventions (p. 183) I Injection Therapy for Vertebrogenic Pain (p. 183) 6 Nerve Blocks (Neurolysis) for Pain Management in Cancer (p. 184) P Percutaneous Laser Disk Decompression (p. 185) ~ Percutaneous Vertebroplasty (p. 185)

j;

~-~

162

6 CT-Cuided Interventions! Procedures CT -guided interventions are common procedures. They can be diagnostic, such as biopsies or nerve root injections, or minimally invasive therapeutic procedures for drainage of abscesses, ablation ofliver lesions, nerve blocks for pain management, or as therapy of bone lesions.

CT-Guided

Biopsies

CT is an established guidance technique for collecting samples for cytological, histological, or bacteriological evaluation. It is particularly suited for sites that would be difficult to access with ultrasoundor fluoroscopy-guided procedures. The diagnostic accuracy and complication rate of CT-guided biopsies depend on the size and location of the lesion and the gauge of the percutaneous needle. Negative results must be viewed with caution and should be interpreted within the context ofCTmorphology.

General

Principles

Indications

Biopsies and abscess drainage still make up the majority of these procedures. CT serves as an important modality for guidance, but increasingly CT is complemented by ultrasound and fluoroscopy, sometimes in dedicated interventional suites.

and Contraindications

An important consideration in patient selection is whether other guidance modalities might be simpler or more effective for tissue sampling. CT offers advantages in cases that involve difficult routes of approach, small focal lesions, and bony or aerated structures. There are no absolute contraindications for CT-guided biopsies. Biopsy is relatively contraindicated in patients with deranged coagulation states (e.g., due to coagulation disorders or drug therapy) unsafe route of approach, uncertainity about the vascula nature of the focus, or other factors specified further on pp. 167. In general. adequately informed patient consent, preparation, post- interventional surveillance and aftercare are hallmarks of good practice and are as important for a successful outcome and a representative biopsy result as patient compliance and cooperation.

Risks

CT-guided biopsies generally have a low complication rate. Biopsy risks include nerve or vessel injuries with bleeding or paralysis, infection, and the dissemination of tumor cells (Table 6.1). The risk of bleeding is substantially increased Kn patients with abnormal blood coagulation (i.e., low platelet counts, anticoagulation), as well as for a transabdominal approach in patients with ascites. Other risks relating to the specific location of the biopsy site must be considered on an individual basis. The risk of tumor cell dissemination is low «0.5%). It depends on the puncture technique and appears to increase in biopsies of hypervascular tumors (e.g., hepatocellular and renal cell carcinomas (Fig. 6.1).

Table

6.1

CT-guided

needle

procedures

Risks Hemorrhage (up to 2%) Hemoptysis (2-5% after lung biopsy) Pneumothorax (10-60%, 10-30% of which require treatment) Pancreatitis (up to 2% after pancreatic biopsy) Abscess, empyema, peritonitis Renal injuries (very rare) Tumor cell dissemination (very Air embolism (rare) Risks

are dependent

(very

rare)

rare)

on:

Route of approach Number of biopsies Needle diameter Lesion vascularity

and puncture

Coagulation status Patient cooperation

(especially

technique platelet

counts)

General Principles

•

Position

A supine, prone, lateral, or oblique position may be necessary, depending on the approach. In all cases the patient should be placed in a stable, comfortable position with at least one arm extended above the head to avoid streak artifacts. Ample padding should be used. The table height should be set as low as possible to provide adequate space for the needle insertion and other interventional maneuvers within the gantry aperture. • Fig. 6.1 Metastasis caused by tumor seeding along the track of a fine-needle aspiration biopsy of hepatocellular carcinoma (HCC) in the left lobe of the liver. There is a coexisting contralateral adrenal metastasis.

•

Preparation

At the time a biopsy is scheduled, all possible approaches have to be considered on the basis of preliminary studies to assess the risks and costs of the procedure and determine the requirement of additional imaging. An up-to-date coagulation status, including platelet counts, must be available before the procedure. Reference values for safe biopsy are: platelets

> 70000/mm3,PPT <50s,Quick PT>50%

These values should be checked against the reference values of the reader's own laboratory. If possible, there should be timely termination of anticoagulation therapy before biopsy to allow a safety window for the procedure. Warfarin is terminated and IV heparin instituted instead. Patients with deranged clotting parameters may require substitution of their blood coagulation factors (e.g., fresh frozen plasma). Patients who are under direct threat of pulmonary embolism from deep venous thrombosis may require implantation of a temporary inferior vena cava filter before anticoagulation can be interrupted. If coagulopathy is suspected, extended clotting tests are obtained before planning the biopsy and special clearance by the hematology department may be required in particular cases.

Planning the Approach

Planning of the biopsy requires review of relevant previous cross-sectional and other imaging. A recent (contrast-enhanced) study of the biopsy region should be available. Alternatively, a diagnostic examination can precede the biopsy. However, in many cases, a limited non-enhanced study directly before the biopsy will suffice for safe evaluation of the access path. Additionally, this study serves as a test to assess if the position is comfortable for the patient. For abdominal lesions it is advantageous to have oral or rectal contrast medium on board to avoid bowel puncture. In doubtful cases an additional quick reference can be obtained with abdominal ultrasound to complement the assessment of bowel peristalsis. When selecting a representative area for tissue sampling, the operator should consider that the central part of a lesion often contains necrotic areas devoid of active tumor cells. In the case of pulmonary lesions, perifocal inflammatory reactions may occur at the periphery of the lesion (near the pleura), so the sample should be obtained as close to the hilum as possible. The route of approach should be as safe as possible (and the patient forewarned of any potential risks when consent is sought). Hence, it is essential to avoid nerves and vessels, the pleura and uninvolved organs if at all possible. In some cases the approach can be substantially facilitated by changing the depth of respiration, moving the patient's arm, or repositioning the patient. Approaches through the liver or lung parenchyma, or small bowel are acceptable, provided a sufficiently thin needle (18 G or smaller) is used and there is no significant amount of pleural or intraperitoneal fluid.

163

164

6 CT-Guided Interventlonal Procedures

b

a

c

Fig. 6.2 Needle biopsy of a small lesion in a lung transplantat patient. Planning (a), local anesthesia (b), fine core biopsy with a 20 G needle (c).

It is a sound general principle to take the simplest route of approach, bearing in mind that changing from a vertical or horizontal path to an oblique path, or even angling the gantry to place the needle path within the scan plane, increases the complexity of the approach. Only experienced examiners should select an approach that cannot be imaged in one plane of section. Multiplanar reformations of spiral or multislice CT data are useful in planning this type of approach. By viewing a suitable reference image, the operator can monitor the depth of infiltration anesthesia, the minimum distance between the needle insertion site and the lesion, and the maximum safe depth of needle penetration on the CT console. The cutaneous puncture site is marked on the z-axis with the laser-positioning device of the scanner (table position of the reference image). The location within the scan plane can be determined from anatomic landmarks (e.g., a vertebral spinous process), or from radiopaque markers attached to the skin (Fig. 6.2). •

Single scans will usually suffice for needle localization, but short spirals may be required for more complex approaches. With multislice CT, multiple sections are Simultaneously available, and the various manufacturers offer different solutions to take advantage of these sections. A good approach is to fuse the innermost sections to obtain a thicker target plane. The sections above and below this target plane serve as a control that easily demonstrates whether the needle deviates in a caudal or cranial direction and requires repositioning (Fig. 6.3).

Needle Localization Scans

For needle localization, it is generally sufficient to use the lowest scanning dose available (the smallest mAs and, if desired, the lowest kV setting). A foot switch and a display monitor in the examination room can vastly speed up the procedure, because the needle position can be quickly checked by the radiologist without having to leave the examination table.

Fig. 6.3 Multislice CT provides simultaneous sections. The inner ones can be used to define the target plane. Note that the needle is seen in the cranial image, indicating in which direction the needle deviates.

General Principles Guiding Systems

Various guiding systems are available to choose the correct needle path in a quicker and more reproducible way. Since less control scans are required. such guiding systems may also allow for a substantial reduction in radiation dose applied to the patient. The less experienced a user. and the more complex the access route. the more will the procedure profit from a guiding system. Simple guidtng devices help to define the correct angulation by providing an angular measure that can be directly attached to the needle or to the patient. Laser guides may either be part of a CT unit or may be provided by a different manufacturer and can be added to an existing unit. After planning the approach the correct angulation is transferred to the laser guiding system that indicates it as a laser beam either outside the gantry or inside the gantry opening. For systems that remain outside the gantry opening. no direct control of the needle position is possible as the needle tip is advanced. This has to be kept in mind when organs that move with respiration are biopsied. Even more sophisticated guiding systems are based on electromagnetic tracking of the tip and angulation of the needle. This information may then be overlaid on a reference CT data set that was acquired prior to the procedure. ifnecessary using contrast-enhancement. Such systems allow for complex angulation and reduced radiation exposure and promise not to compromise biopsy accuracy. Vital organ structures can be avoided more easily. especially since such systems (e.g .. the UltraGuide system) are aiso designed to compensate for respiratory movements. CT Fluoroscopy

CT fluoroscopy is a procedure that allows for online tracking of the needle as it passes through the tissue (see Fig. 1.33). The technique was developed for complex approaches or for regions with substantial respiratory motion (e.g .. lung bases). in which guided biopsies by conventional CT may be cumbersome. CT fluoroscopy should use low-dose settings because radiation dose can quickly become excessive with longer fluoroscopy times. However. such low-dose scans may suffer from

reduced image quality. which is particularly disturbing in difficult needle approaches. For practical purposes. fiuoroscopy should only be used intermittently during critical phases of the procedure. Having the patient change the depth of inspiration during fiuoroscopy may help bring the target lesion into the scanning plane. The needle is then advanced under fiuoroscopic control. Fluoroscopy with multislice scanners can demonstrate in which direction the needle leaves the target plane and thus make repositioning simpler. It has to be noted. however. that there is a potential dose increase if four sections are exposed instead of one. Speciai needle holders are available that make it possible to direct the needle without getting direct radiation exposure to the hand of the radiologist. Biopsy Needles

Percutaneous biopsies may be performed with fine needles (20-22 G). small core needles (1618 G). or large-core needles (14G and up). depending on the type of sample required. the risk of the approach. the anticipated pathology. and the experience of the evaluating pathologist. The smaller the lesion and the riskier the approach. the thinner the needle should be. Generally the presence of a trained cytologist and on-site analysis of the salvaged material help to significantly reduce the number of biopsy passes and to decrease the gauge of the required aspiration and guidtng needles. In many cases the biopsy needle can be substituted by aspiration cytology and unnecessary risk of hemorrhage and pneumothorax is avoided. This is especially important as the choice of the needle type (Fig. 6.4) or biopsy mechanism seems to have less effect on the diagnostic yield than the experience of examiner and cytologist. Aspiration needles can be of several types: Chiba needles (flat bevel tip). Franseen needles (serrated tip) or Westcott needles (with specimen side notch). Core biopsy needles generaily employ the Tru-Cut principle. There is a current trend toward semiautomated or automated high-speed biopsy instruments in which the trocar and core needle are individually or successively fired with a spring-loaded mechanism (Blopty, Temno, Gallini, etc.). There are severai coaxial biopsy systems in which multiple samples can be retrieved

165

6 CT-Cuided Interventional Procedures

Chiba

Greene

Westcott

Rotex

~

~

22,23

G

20,22

G

•

((E

18,20,22

18,19,5,21

~

G

G

22G

~

Fig. 6.4 Various types core biopsy needles.

G

22 G

~

Tru-Cut

G

16,18,20,22

~

Franseen

Otto

22,23

~

Turner

Madayag

making certain that the pleura, peritoneum, or periosteum are adequately infiltrated. For lung biopsies, the pleura should be anesthetized without piercing the pleural tissue with the needle. Striking a nerve will elicit a sharp, shooting pain, and the needle should be repositioned. If desired, localization of the inserted anesthesia needle can be performed to recheck the direction of the approach. Injections in children may be easier if a topical anesthetic cream (e.g., EMLA) is applied to the skin \hour before the procedure.

Size

Type

: ~

and sizes

14-22G.

of fine needles

and

through an indwelling outer cannula. Needles with graduated markings simplify the biopsy procedure, as do sleeve-type depth indicators that are mounted on the needle shaft. Some questions cannot be adequately resolved by fine-needle aspiration biopsy and cytology, and core biopsy is needed to furnish material for histological evaluation (Table 6.2). •

Local Anesthesia

The usual technique is to inject 5 to 20ml of local anesthetic (e.g., lidocaine, mepivacaine), Table 6.2 Techniques opsy indications

recommended

for common

Fine-needle aspiration adequate for: Pulmonary lesions: bronchial carcinoma, metastasis Hepatic lesions: HCC, metastasis Suspected pancreatic carcinoma Suspected lymph node metastases Core biopsy necessary for: Hepatic lesions: exclusion of benign disease Mediastinal tumors Retroperitoneal lesions Pleural lesions

bi-

Medication

for Sedation

and Analgesia

For some biopsies as well as for more complex interventions, conscious sedation may be needed for optimal results. In these cases adequate monitoring of pulse oximetry, ECG, heart rate and blood pressure is needed. A number of medications are frequently used, often in combination. Benzodiazepines give anxiolysis and sedation but do not provide analgesia. In older patients, and patients with cardiovascular disease, hypotonia and depression of breathing may be seen. Patients often like the short -term amnesia that may be associated with these drugs. They are contraindicated in patients with allergy, muscular diseases, and in pregnancy. Diazepam is the best known with good anxiolytic properties, but has a long elimination-time. For this reason, midazolam is more often used. Benzodiazepines can be antagonized byflumazenil. Propofoi in a selected dose can be used effectively for sedation in patients who breathe spontaneously. It is short acting and elimination is fast, but it has no analgesic effects. Dosing should be done carefully as overdosing may induce depression of breathing and there is no antidote available. Also, patients should be adequately monitored. In children, ketamine can be used for sedation in short procedures. It has both sedative and anesthetic effects. It is usually combined with atropine sulfate to counteract hypersalivation that might compromise the airway. Emergent reactions preclude the use of this agent in adults, but in children these are rare. For longer procedures it can be combined with benzodiazepines like midazolam. Opiates are mainly used for analgesia, but they may have some sedative effects in higher doses. Side effects include nausea, vomitting,

Biopsies in the Chest

breathing depression and hypotension. Frequently used examples are the strong opiates piritramid and the short-acting alfentanil and remifentanil. These opiates can be combined with benzodiazepines or propofol (dose adaptations'). Naloxone can be used as an antagonist for opiates. Non-steroid anti-inflammatory drugs can also be used for analgesia and also have antipyretic and anti-inflammatory profiles. They can be used to supplement opiates for pain reduction. They should be used carefully in patients with ulcer disease. asthma, and reduced renal function. Parenteral administration is possible with diclofenac or metamizole, but the latter can have serious side effects. Needless to say, adequate training should be sought before these drugs are administered by radiologists and their nursing staff. Hence, conscious sedation is often supervised by the referring clinician or anesthesiologist. Puncture Technique

A small stab incision is made in the skin, and the biopsy needle is introduced and advanced along the pre-selected path. It is easier to control the needle alignment when an assistant stands at the head or foot of the table. The laser guide of the scanner or external biopsy aids can be used to improve orientation. Depending on the risk of the approach, the needle may be inserted directly into the lesion or it may be advanced in stages while each stage is checked with a localization scan. The needle tip appears as a hypodense streak (hardening effect) in continuity with the axis of the needle shaft. In structures that move with respirations, the needle should not be fixed but should be allowed to move with the excursions, as otherwise it might lacerate the liver, lung, or other organs during respiration. The safest technique is to advance the needle while respiration is suspended. Sudden shooting pains signal that a nerve has been struck, and the needle should be repositioned. For a fine-needle aspiration cytology (FNAC), the needle is advanced to the surface of the lesion orjust inside it. The stylet is removed, a syringe is attached (20ml Luer-Lock), and the needle is moved in a fan-shaped pattern ("needling"). hence piercing the lesion several times, while suction is applied. The suction is released

before the needle is withdrawn to avoid seeding potentially malignant cells into the needle tract. When a core biopsy is performed with a core biopsy needle (or its automated or semiautomated variants), the biopsy needle is again advanced up to orjust inside the lesion. It should be noted that the core biopsy needle is surmounted by about a 5 mm-long trocar point that does not collect tissue. Advancing the trocar into the lesion loads tissue into the 5-20 mm-long sample notch of the needle, then the sharp-edged outer sleeve is rapidly advanced over the trocar to collect the tissue core. A spring-loaded mechanism may be used to accomplish one or both of these steps, usually providing a less hemorrhagic sample that is easier to evaluate. With larger core biopsy specimens simultaneous cytology "roll" smears can be made, or when coaxial technique is performed, the core biopsy can be preceded by fine needle aspiration cytology (FNAC). Final Check

After the sample has been obtained, a control CT scan should be taken to exclude a pneumothorax or acute hemorrhage. If a small pneumothorax is detected with CT, a conventional chest radiograph should be taken at 1 -4 hours after the biopsy and the clinicians informed. Depending on the needle size and route of approach, several hours' bed rest may be advised to reduce the risk of post biopsy hemorrhage.

Biopsies in the Chest Lung The location of the lesion is an important factor in deciding whether to perform a trans bronchial biopsy (central bronchial lesion, "positive bronchus sign" of a bronchus directly entering the lesion on CT) or a percutaneous transthoracic fine needle aspiration cytology (FNAC) or small core needle biopsy (peripheral lesion not related to the bronchi). Transthoracic biopsy is contraindicated in patients with a contralateral pneumonectomy, pulmonary hypertension, or pulmonary arteriovenous malformation and severe chronic obstructive pulmonary disease (COPD). FNAC of the lung is most often performed for the diagnosis of carcinoma, while core biopsies are more important for benign lesions,

167

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CT-Guided Interventional Procedures

sclerotic lesions, lymphomas, and other noncarcinomatous malignancies, For diagnosing malignancy, transthoracic biopsy has reported sensitivities of 70-100% and high specificities, For benign processes, the yield is 85-95% but the specificity is much lower and varies between 15% and 65%, Prerequisites for a higher yield are repeated sampling from different parts of the lesion and expert on-site cytopathological advice with histological confirmation ff needed. Pulmonary lesions should be biopsied with the patient prone (if access is short enough) in shallow breathing and suspended respiration during needle movements, It is sound practice, especially with small lesions, to advance the needle into the lesion before the stiletto is withdrawn, In biopsies of extensive lesions, material should be sampled from perihilar portions of the lesion and not from its necrotic center. The risk of pneumothorax in lung biopsies may be as high as 40%, and patients must have adequate pulmonary function (Oz partial pressure > 60 mm Hg) and should not have serious emphysema or advanced pulmonary hypertension, The risk of pneumothorax is increased in patients with obstructive airway disease, emphysema, multiple needle passes, and in deep-seated or small lesions, Between 10 and 30% of all pneumothoraces have been reported to need chest drainage, The number of air leaks may be reduced by sealing the needle tract with Gelfoam or throm bin at the end of the procedure, Another method that has been successful for avoiding pneumothorax is a coaxial technique using a guiding canula with a sharp and blunt trocar. The sharp trocar is only used to advance the needle close to the pleura, which is then penetrated slowly using the blunt trocar. A sharp biopsy needle is only inserted close to the target lesion, This technique is less prone to causing pleural tears as the needle is advanced into the lung, The risk of a delayed pneumothorax can be reduced by positioning the patient with the biopsied side down, Small intrapulmonary hemorrhages along the needle track (see Fig,9,30a) are common, However, hemoptysis occurs only in 2% to 5% of cases, and more often at biopsy of vascularized

lesions, It is generally self-limiting, Rare complications like air embolism or needle tract seeding occur in 0.1 % and fatal complications have been reported in 0,2% of cases, Mediastinum

Percutaneous biopsy is appropriate for lesions that are inaccessible by the transbronchial route or by mediastinoscopy, It is also used in cases where a previous transbronchial biopsy has been attempted without success, Hilar lesions should be biopsied at a central site to avoid false-negative results from areas of posts ten otic pneumonia or atelectatic lung, The safe biopsy of mediastinal lesions requires that the location of vascular structures be known precisely, and contrast medium must be administered for vascular opacification, The risk of pneumothorax can be reduced By performing the biopsy at full expiration (increases contact area on anterior chest wall) and by injecting 20-50 ml of physiologic saline solution to displace the lungs laterally (Fig, 6,5), The chosen approach will depend on the location of the lesion, and is either anterior parasternal or posterior paravertebral. A direct mediastinal route is preferred to a trans pulmonary one to avoid the risk of pneumothorax, However, if a trans pulmonary approach cannot be avoided, FNAC should be preferred, Another option is the transsternal approach (Fig, 6,6), in which a hollow needle (e.g. a 2mm Ostycut needle (Bard) for biopsy with an 18 gauge cutting needle) is passed just through the sternum to establish access for the aspiration cytology or core biopsy, While FNAC has a high sensitivity and specificity for metastatic carcinoma, core biopsies are needed for non carcinomatous malignancies, lymphoma, and in benign lesions, For the diagnosis of thymoma, a surgical technique is generally suggested and may directly precede tumor resection, The success rate of mediastinal biopsies is Significantly higher with core biopsies (approximately 90%) than with FNAC, Pneumothorax is reported in some 10% of cases and hemorrhage in 3%,

Biopsies in the Abdomen

Fig.6.5 The route of approach can be expanded by the injection of physiologic salinesolution.

• Pleura CT is a good imaging modality in the identification of and guidance for aspiration and tap of loculated collections of the pleura, and biopsy of associated pleural thickening. A supra costal approach is mandatory to avoid damage to the intercostal neurovascular bundle. In most cases, the biopsy path is tangential to the thoracic wall. 18-20 G core biopsies give the best results in pleural masses, because the lesions are often inhomogeneous and immunohistochemical methods are needed for differentiation. For malignant disease, a sensitivity of >80% and specificity near 100% has been reported. In benign disease, the main role is identification of tuberculous pleuritis if CT morphology is equivocal. As with most biopsies, multiple passes are required for optimal accuracy. In experienced hands, major complications occur only in less than 1 % and include pneumothorax or hematothorax. There is a risk of tumor seeding, especially in mesothelioma. For mesothelioma and lymphoma, however, thoracoscopic (VATS) biopsy remain the reference standard. Biopsies in the Abdomen •

a

b

Fig. 6.6 Transsternai approach using a 2 mm Ostycut needle (a) followed by 18 gauge core biopsy (b) of a suspected recurrence ofT -cell non-Hodg kin's lymphoma.

Liver

In selecting patients for a liver biopsy, it should be decided whether the diagnosis could be adequately established by imaging procedures (typical findings in two separate procedures) rather than percutaneous biopsy. If hepatocellular carcinoma (HCC) is suspected, the risk of peritoneal tumor spillage would contraindicate liver biopsy in patients who require surgical treatment regardless of the biopsy result. An ultrasound-guided biopsy is preferred in many cases, because the biopsy can be monitored in real time and can often be completed during one inspiration. Care should be taken in patients with free intraperitoneal fluid as any liver biopsy can induce severe hemorrhage in these patients. Therefore, in patients with diffuse liver disease transjugular biopsy via a liver vein should be preferred, and in patients with focal liver pathology pre- interventional ascites tap may be helpful to reduce the risk of hemorrhage.

169

CT-Guided

imervenuone;

Procedures

A liver core biopsy is preferred over FNAC because of its greater accuracy, especially in the differentiation of benign hepatic lesions, The accuracy rate of core biopsy is greater than 90%, and its complication rate (bleeding, pneumothorax, puncture of other organs) is low, The individual route of approach to a hepatic lesion depends on its location and size, The shortest transhepatic route is generally preferable, Hemangiomas and other hypervascular tumors, should be biopsied through a longer, self-tamponading approach through normal liver tissue to reduce the risk of hemorrhage, If a subphrenic lesion cannot be biopsied in full inspiration via the extrapleural route, the biopsy should be performed in expiration or in the right lateral decubitus position to reduce the volume of lung tissue in the pulmonary recess, Biopsy employing gantry angulation can be beneficial and even trans pleural approaches have been used successfully with low complication rates in selected cases, Gallbladder,

Spleen

Percutaneous biopsy of the gallbladder is rarely indicated, because generally there is no difficulty in diagnosing a lesion that has expanded past the gallbladder to involve other tissues, Splenic masses are usually associated with systemic lymphomatous diseases that are more easily biopsied at other sites, Another reason to avoid gallbladder and splenic biopsies is their high rate of complications (biliary peritonitis and hemorrhage), FNAC should be performed only in highly selected cases, Pancreas

CT (fluoroscopy) and ultrasound are prerequisites for guidance of pancreatic biopsies, as they allow for real-time monitoring of the procedure, Using routine CT (non-fluoroscopy) guidance it is difficult to obtain a representative sample from small lesions, and negative cytology is generally unreliable, The sensitivity of FNAC is variable (70-100%) but is comparable to the accuracy of an intraoperative biopsy, The complication rate of pancreatic biopsy is low when a small-gauge needle is used, Many pancreatic lesions are difficult to detect without contrast medium, hence the planning scan should routinely include contrast CT Pancreatic biopsies can be performed through an anterior or posterior approach, A transgastric

or trans intestinal approach requires 6 hours' fasting to reduce the risk of contamination and infection, For safety reasons FNAC is generally preferred, The biopsy should be performed with a breath-hold in an intermediate position because respiratory excursions cause movement of the liver and other intraperitoneal organs relative to the retroperitoneal pancreas and deep abdominal respiration could withdraw an unfixed anterior needle from the pancreas, Kidney

Renal biopsy is generally not indicated in patients with renal cell carcinoma (RCC), because the biopsy sample cannot often differentiate the lesion from adenoma, the rich vascular supply increases the risk of hemorrhage, and tumors tend to disseminate into the needle tract or the lung, In doubtful cases the diagnosis should be established by surgical enucleation, which can be combined with intraoperative frozen section analysis, Renal biopsy for focal lesions is indicated only in patients with a suspected isolated renal lymphoma or metastasis, Exceptions are patients with tuberous sclerosis and equivocal findings in cross-sectional imaging, in whom repeated laparotomies cannot be performed, More frequent indications are core biopsies of native kidneys for diffuse disease or transplant kidneys for rejection, For best results, "14G core biopsies are taken from the lower-pole cortex, If ultrasound guidance is not preferred, the biopsy can be performed under CT-guidance by a posterior approach, and FNAC will generally furnish an adequate sample, Complications in the form of retroperitoneal or intraperitoneal hemorrhage or gross hematuria are rare, Adrenal Gland

Adrenal biopsy is appropriate in cases where a suspected metastatic lesion or indeterminate adrenal mass cannot be adequately diagnosed with CT MRI or other imaging procedures, In patients with a known primary tumor, FNAC is satisfactory for the detection of adrenal metastasis, In patients with a suspected primary adrenal tumor, an effort should be made to establish a histological diagnosis, The sensitivity and specificity are close to 100% with core biopsy and 90-100% with FNAC, If a pheochromocytoma cannot be excluded, appropriate pharmacological prepara-

Musculoskeletal

tion with' 0,1 and 0.2 blockade is indicated because of the risk of a hypertensive crisis. For this purpose the expertise of the local endocrinology department should be enlisted. Adrenal biopsy is best performed through a posterior approach with the patient positioned prone or with the affected side down. The adrenals can also be biopsied through a lateral approach or through an anterior approach (transhepatic, if necessary). Transrenal and transsplenic approaches should be avoided. Saline injection can expand the posterior paravertebral space and facilitate a posterior extrapleural approach (see Fig. 6.5). Retroperitoneal

and Mesenteric

Masses

A posterior paravertebral approach is most suitable for the biopsy of retroperitoneal lymph nodes and indeterminate retroperitoneal masses. Core biopsies are preferred, although injury to the lumbar arteries can produce extensive retroperitoneal hemorrhage. For lesions located between the vena cava and aorta, FNAC should be preferred using an anterior (trans intestinal) approach avoiding the colon. Nodular mesenteric masses in patients with an unknown primary tumor should be investigated by core biopsy from an anterior approach. Pelvic Region

The difficulty of differentiating scar tissue from recurrent tumor on cross-sectional irnagingespecially in rectal carcinoma-is perhaps the most typical indication for biopsy in this region. FNAC is usually adequate, especially since core biopsies can be very painful and, when scarring is present, often do not yield a diagnostic specimen. The complication rate of FNAC is extremely low, and its accuracy rate is higher than 90%. Structures cranial to the linea terminalis are biopsied via a trans peritoneal approach. Structures caudal to the linea terminalis are accessible through the abdominal wall or sciatic foramen. In selected cases specialized routes may be needed which can give good results (see also section on CT-guided drainage).

Biopsies

Musculoskeletal Biopsies Biopsy of Bone Lesions

In recent years, percutaneous biopsy of bone lesions has become a routine procedure in centers specializing in their treatment. For guidance, CT as well as fluoroscopy or ultrasound can be used. CT-guidance is especially valuable in deep-seated lesions and lesions that lie adjacent to vital structures. Biopsy is planned in close communication with the referring specialist (especially for malignant lesions). Bone scintigraphy is used to select the most accessible site for biopsy. Osteolytic regions are preferable for biopsy and thinslice CT is indispensable before biopsy to guide the appropriate access, as it can better differentiate osteolytic from sclerotic regions within the lesion. It is also used for selecting the proper needle type. Additional MRI may have a supplementary advantage in differentiating vital tumor regions from cysts or necrosis as well as gutding the needle pass in highly vascular lesions. All bone biopsies must be performed under strict aseptic conditions due to the high risk of infection. The periosteum should be thoroughly anesthetized. Sedation and general anesthesia should not be used for biopsies in proximity to neural structures. Bone biopsy can be performed as aspiration or trephine (core) biopsy. In general, trephine biopsy is more efficient than the alternative, both for neoplasms and infections, especially tuberculosis. However, aspiration can be useful in lytic lesions. A multitude of special needles are available for bone biopsy. Short trephine needles (e.g., Yamshidi, Tanzer) have large calibers of 8-11 G and are useful for superficial lesions or the penetration of cortical shells. They have the disadvantage that reinsertion for multiple biopsies is required. Longer trocar systems (e.g., Ackerman, Craig, Laredo-Bard) are placed coaxially and allow for multiple passes in deeper-seated lesions. Another coaxial system is the Bonopty needle which has an eccentric drill and is useful for penetrating intact cortex in peripheral bones. It can be used in combination with softtissue needles for lytic lesions. Pneumatic motor-drill systems (e.g., Ostycut) may be useful in locations where the cortex is thick or in sclerotic lesions. Recommended needle types are summarized in Table 6.3.

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6 CT-Cuided Interventional Procedures

a

b

Fig. 6.7

c

Sample approaches for vertebral biopsies in the cervical spine (a), thoracic spine (b), and lumbar spine (c).

Table 6.3

Recommended

needle types (after Laredo,

1999) Bone tumors

Superficial bone lessions

Yamshidi, Tanzer

Deep bone lesions

Ackerman, Craig, Laredo-Bard

Sclerotic

Ostycut. Bonopty

Lytic

Tru-Cut core biopsy needle

iytic

+

bony shell

Yamshidi

+

Tru-Cut

Lesions in the thoracolumbar spine are approached via a posterolateral route with the patient in a lateral or prone position (Fig. 6.7). Lesions of the posterolateral body or pedicles are best reached via a transpedicular route, while lesions in the more anterior part require a steeper lateromedial angle (40°-60°) directly to the body. Needle tilt can be important to avoid damage to exiting nerve structures, and may require tilting the gantry. In the cervical spine an anterolateral or lateral approach should be chosen for lesions in the vertebral bodies. Neural arch lesions can be reached via posterior routes with tangential approaches. In peripheral bones the approach may be variable but especially in primary tumors should be chosen in close consultation with the referring surgeon, because the needle path should be resected at surgery. The path should be perpendicular to the cortex (Fig. 6.8) and aspiration of the bone marrow can be performed simultaneously. Penetration of the marrow may be painful, requiring general anesthesia. For some malignant lesions, such as osteosarcomas, open surgical biopsy may be

a

b

Fig. 6.8 Percutaneous biopsy of a lytic lesion in the talus that proved histologically to be a giant cell tumor.

preferable for better control of bleeding complications. In flat bones a tangential approach is better for sampling the maximum amount of pathologic bone and to avoid injuries to underlying structures. It is advisable to sample different sites of the lesion. Both the sampled bone tissue and any blood aspirated from the bone should be submitted to the pathologist. Close communication with the pathologist is needed for proper fixation, e.g., when electron microscopy is contemplated. If an inflammatory process is suspected, the bulk of the biopsy material should also be prepared for microbiological testing. In experienced hands complications are few and can be less than 1 %. Complications may adversely affect surgical treatment and outcome, especially in malignant bone tumors, in a significant number of patients. Therefore, proper

Musculoskeletal technique is important and these biopsies are best performed in specialized centers. In general, percutaneous biopsies are less invasive and associated with a lower morbidity and cost than surgical biopsies. The diagnostic accuracy depends on many factors, like the nature of the lesion, quality of preoperative work-up, selection of biopsy site, biopsy material, and technique and experience of the pathologist. The accuracy cited in the literature ranges from 80% to 95% for bone metastases and round cell tumors (Ewing' sarcoma, lymphoma) and 60-80% for primary bone tumors. In infection, success rates of 90% for skeletal tuberculosis and 60% for pyogenic infections have been reported. •

Biopsy of Malignant Soft Tissue Tumors

For many years, biopsy of malignant soft-tissue tumors has been the terrain of the oncologic surgeon because of the risk of compartmental contamination as well as the histopathological heterogeneity of most soft-tissue tumors. Recently, percutaneous biopsies have been more frequently used for the primary diagnosis with good results. Close interaction with the surgical oncologist is necessary and therefore these biopsies are best done in centers specializing in surgery of the tumors. CT is most commonly used for deep-seated lesions, while ultrasound is a strong competitor for large and more superficial lesions. CT has the added benefit that it can document the needle track accurately. Before biopsy, close communication with the operating surgeon is important as the needle track should be resected during subsequent surgery. General pre-biopsy principles should be followed as outlined above, and generally a preoperative gadolinium-enhanced MRI should be available to select the viable tumor regions. Biopsy can be performed as needle-core biopsy or fine needle aspiration biopsy (FNAC).Core biopsy is usually preferred in larger tumors and has the advantage of allowing specific subtyping of sarcomas. FNACis more difficult in mesenchymal tumors and is often reserved for small lesions.The preference of the surgeon or radiologist as well as the experience of the cytologist and pathologist will also influence the choice of the type of biopsy in the individual patient. Core biopsy is usually performed with 14-18 G side-cutting (e.g., Tru-Cut, Quick-Core, Gallini) or tip-cutting (Surecut, IBI) needles. To

Biopsies

prevent seeding of the biopsy track a coaxial approach may be taken through an outer sheath or Yamshidi needle, which is placed into the periphery of the tumor. Often the side-cutting needles are connected to a biopsy gun for ease of the procedure. Fine-needle biopsy employs 20-23 G side-cutting (Westcott, Burney) or tipcutting (Chiba) needles connected to a syringe. It is best to have a cytopathologic technician onsite for direct evaluation of the biopsy material. A suggested algorithm from the literature is presented in Table 6.4. Complications from percutaneous biopsies are similar to complications for other CT-guided biopsies. They occur in less than 1-4% of biopsies, and are less frequent than with open surgical biopsy. This is important since any complication may alter the timing and the type of subsequent surgical procedures, and thus may considerably influence patient outcome. It has been demonstrated that the number of clinically relevant complications in specialist centers is significantly smaller. Few studies have evaluated the accuracy of this technique. Core biopsies have an accuracy of 84-90%, while the accuracy of fine needle aspiration lies in the 90-96% range. Major reasons for inaccurate results are failures to obtain diagnostic samples and to sample parts of the tumor that are not representative for the whole lesion. The latter is more important in lowgrade malignancies and malignancies with necrotic and cystic areas.

Table 6.4 al.,1996)

Recommended

biopsy algorithm

(Logan et

Soft tissue tumors < 3 cm

20-22 G FNAC

;;. 3 cm

18 G core biopsy

Bone tumors Lytic + soft tissue mass <3cm

20-22 G FNAC

Lytic + soft tissue mass ;;.3cm

18 G core biopsy

Lytic + bony shell

Combination

Sclerotic

14 G Bone biopsy

technique

173

CT-Guided

Drainage

Procedures

CT-guided drainage procedures are indicated for pathologic fluid collections that require drainage (abscess, biloma, hematoma, etc.) and that are not amenable to or do not respond to ultrasound-guided aspiration, If a sufficient access to the lesion can be established, percutaneous drainage is preferable to the open surgical alternative owing to the significantly lower complication rate and shorter hospital stay, The puncture technique of CT-guided drainage is similar to that of CT-guided biopsy (see above),

General Principles Preparation

A brief period of hospitalization is generally required, If there is a high index of suspicion for abscess, prophylactic systemic antibiotics should be administered just before the procedure to prevent septic complications, Approach

The safest and most effective route of approach should be selected, As many non- involved organ structures as possible should be avoided, In the upper abdomen, a transhepatic approach is safe enough in most patients, or a trans peritoneal or transgastric approach may be considered, especially in patients with pancreatitis, The pleural recess should be avoided due to the risk of inciting pleurisy or pleural empyema, Injury to the spleen and intestinal structures must be scrupulously avoided, For pelvic abscesses, a transgluteal approach through the greater sciatic notch is generally employed, To avoid the sacral plexus and vessels the tract should be close to the sacrum, preferably above the sacrospinous ligament Specialized

Approaches

There are a number of new developments and approaches in CT-guided abscess drainage, Contrary to classical convention, subphrenic collections can be drained via a transpleural route if no safe transabdominal route is available, However, higher complication rates are associated with this technique, which is usually performed via a trocar puncture,

Deep pelvic collections may be difficult to approach via transgluteal or transabdominal access, Studies have demonstrated that in these cases transvaginal, transrectal, or transperineal routes can be safe alternatives for catheter placement Transrectal approaches can be especially useful in presacral lesions, Transperineal access can be obtained under CT-guidance, while transvaginal and transrectal access is guided more commonly by ultrasound, In many cases abscesses can be managed with a combination of systemic antibiotic therapyand (repeated) image-guided aspiration and lavage only, Even without placement of any indwelling catheters, similar results for pelvic abscesses, compared to hepatic ones, can be achieved, Diagnostic Aspiration

The site should be infiltrated with copious local anesthetic (10-20 ml): because often a large drainage catheter is used and its insertion can be quite painful, It is a sound general principle to precede drain insertion with a diagnostic fine-needle aspiration, as this allows sampling of fluid for gross inspection, and for bacteriological and cytological work-up, Purulent or turbid fluid is suggestive of an infection, greenish-brown fluid usually indicates biloma, and clear yellowish fluid is found in ascites or a seroma Catheters

Percutaneous drainage is performed with a pigtail or J-catheter whose gauge (French size) is matched to the viscosity of the fluid, While a 6-8 F catheter is usually sufficient for draining serous fluids (bilorna, seroma. low-viscosity abscesses), highly viscous (abscess) fluid will require a 9-12 F catheter. A suction- irrigation system (e.g. sump) may even require catheter diameters up to 16 F. Catheters for hematoma drainage are usually larger than 9 F. Necrotizing or suppurative pancreatitis often requires the use of multiple catheters and generally requires drainage by very large systems up to 28 F. The catheter should have side holes of adequate size, It is unsafe to leave side holes outside the abscess, because they facilitate dissemination of abscess, biliary and other hazardous

General Principles material to other sites in the body (especially with a trans peritoneal approach). •

Direct Puncture (Trocar Technique)

This technique employs catheter systems mounted on a long gUiding cannula with a rigid trocar. The system is introduced through a stab incision of the skin and passed directly into the collection. The trocar is then replaced by a stiff wire (e.g., Amplatz Super Stiff) and the catheter is then advanced further from the guiding cannula, which is held in a stable position to maximally enhance the guiding effect of the stiff wire. Alternatively the procedure can be performed without the additional help of a guidewire; the catheter is then directly inserted from cannula and trocar. Once the drainage catheter is in a safe position, the cannula is removed and material aspirated. Subsequently the catheter is locked and secured in place (Fig. 6.9). If no aspirate can be obtained, the system must be repositioned. The trocar technique is suitable for lesions that are easy to reach. It is fast and holds less risk of disseminating (infectious) material because the advancing catheter immediately seals the puncture hole in the fluid collection. It is

therefore ideally suited for drainage of pleural fluid or ascites and abscesses when spillage of material along the puncture tract should be avoided. •

Coaxial Technique (Seldinger Technique)

The coaxial technique is used in more difficult anatomic situations with a higher risk of puncture-related complications. The procedure is performed with a diagnostic aspiration with a fine needle, which serves as a guide to advance a sufficiently stiff guide wire into the lesion. This is often followed by insertion of a trocar/dilator combination (e.g., Acustix) to allow for guide wire exchange to standard size (0.035 inch) and the tract is subsequently dilated to increasing diameter before the final drainage catheter can be introduced (Fig. 6.10). If several wire and catheter changes are necessary, care must be taken not to dislodge the catheters, because this would make a new puncture necessary. •

Abscess Drainage

When draining an abscess, care should be taken to avoid spillage of material along the drainage tract that could contaminate neighboring organs

b Fig. 6.9 Abscess drainage by direct puncture, (a) The puncture set (drainage catheter containing a hollow needle with trocar) is inserted into the abscess, (b) The

puncture needle is withdrawn while the catheter is advanced into the abscess, (c) When the catheter has coiled into its definitive shape, the abscess is aspirated.

175

6 CT-Cuided Interventional Procedures

b Fig. 6.10 Abscess drainage by the coaxial technique. (a) The abscess is punctured with a hollow needle, (b) A guidewire is passed into the abscess and the puncture needle is removed, (c) A drainage catheter is threaded into the abscess over the guidewire. Depending on the

caliber of the puncture needle and the gauge of the drainage catheter, an intermediate step may be required in which a dilator is passed over a thin guidewire to enlarge the needle tract before inserting the thicker guidewire and drainage catheter.

or body cavities. After the drainage catheter has • Drainage Catheter Aftercare been successfully placed, the operator should attempt good evacuation of the abscess cavity. Catheters should be fixed to the skin with a Irrigation of an abscess cavity may induce tolerance slag to compensate for breathing sepsis and should always be performed under an- movements in transabdominal or transpleural tibiotic cover. Irrigation effectively liquefies positions. They should be directly fixed to the viscous material and is an important aid in re- skin if the catheter is not affected by significant moving cellular debris and other solid com- movement. Large bore drainage catheters ponents to prevent or clear catheter blockage. An should be fixed by suture. Several techniques irreversibly occluded catheter should be ex- can be performed such as multifilar Mersilene changed for a larger size drainage system or re- suture through an adhesive tape strip that is moved. Continuous irrigation systems such as the wrapped around the catheter. To avoid clogging van Sonnenberg sump catheter can work well and to facilitate drainage of viscous fluids, the using only a single access tract; however, double catheter should be flushed 3-4 times per day catheter irrigation is often more effective. with sterile normal saline. Control CT after inVarious compounds such as N-acetylcys- jection of water-soluble contrast media may be teine (ACC) and urokinase can be added to irri- beneficial to assess the drainage success. gating solutions to enhance the liquefaction efThe drainage should be maintained until fect. The dosage of urokinase can follow a the retrieved fluid is less than 10 ml/day. Ususimple, recently published protocol (Haaga ally, the catheter can be removed after 2000): abscess size <3cm: 12500IU, 3-5 cm: 7-10 days. In pancreatitis and abscesses com25000IU, 5-10 cm: 50000IU and >10cm municating with small or large bowel longer du100 000 IU followed by 10ml saline and ration (2-6 weeks) is generally required. Catheters with special suture-locking mechanisms are 15 minutes contact time, three times per day. removed under imaging guidance.

Success Rates

The results of percutaneous drainages depend on the location and nature of the lesions and on the techniques employed. Success rates may vary between less than 30% in central pancreatic abscess and more than 90% in the chest.

Specific Techniques in the Chest and Abdomen •

Drainage of Complicated Pleural Fluid Collections and Empyemas

CT-guidance for placement of thoracostomy tubes in empyemas. superinfected hematomas and other complicated fluid collections is usually reserved for those cases in which US-guided methods are not successful. This is often the case in fibrinopurulent and organizing loculated collections. Multiple loculations will need more than one drainage access and CT is optimal for planning the most appropriate approach of each one. Once again. the choice of catheter will depend on the viscosity of the fluid. Usually 8-16 F catheters are placed using a supracostal Seldinger technique or direct puncture. The entry should be in the most dependent part of the collection. If possible. posterior approaches or entry sites medial of the scapula should be avoided because of the risk of catheter displacement. Lung atelectasis should be re-expanded gradually to avoid pulmonary edema. Limited CT is the follow-up modality of choice. For catheter repositioning and exchange. however. CT alone can provide only suboptimal guidance and should be complemented by thoracic ultrasound. Guided drainage systems generally are smaller caliber than the non-guided alternative. This will in many cases result in a longer drainage duration as well as a higher susceptibility for catheter occlusion in most cases. However. the technical success rate of nonguided drainage will be improved from 50% to approximately 65-75% if catheter placement is aided by imaging guidance. The combination of drainage with intracavitary fibrinolysis (ICFT) using urokinase can further improve results (6595%). ICFT (technique see p. 176) should be instituted as early as possible in the fibrinopurulent phase and is not beneficial in effusions that are more than six weeks old.

Drainage of Malignant Pleural Effusions

Over 75% of malignant effusions are caused by carcinomas of breast. lung. and ovaries. and by lymphomas. Image-guided drainage insertion using small-bore catheters and sclerotherapy is equally successful as a nonguided large-bore access. Ultrasound is the first choice for guidance, whereas CT is usually reserved for difficult cases. Catheters (14-16 F) are placed in the sixth or seventh intercostal space and up to 1000 ml is slowly aspirated. The drainage catheters are put on 20-30 mm H20 suction and are irrigated with saline twice daily. Due to ex vacua phenomena postprocedure chest films show pneumothoraces in 30% of cases that should resolve spontaneously. After complete drainage palliative sclerotherapy is performed using talcum, antibiotics, or chemotherapeutics. Loculation may be treated first by ICFT (e.g., 250000 IU urokinase in 100ml saline). With this technique ambulatory palliation becomes possible. Drainage in Pancreatitis

Severe forms of pancreatitis are associated with life-threatening complications such as pancreatic abscesses and pancreatic necrosis with or without superinfection. In particular, infected necrosing pancreatitis is associated with a very high morbidity and mortality (up to 80%). Parenchymal necrosis can occur at any time in the course of pancreatitis and is detected on CT as areas of reduced or absent enhancement. Central organ necrosis is likely to affect the pancreatic duct integrity with the complication of fluid collections or pseudocysts refractory to fine needle aspiration. Pancreatic abscesses occur in less than 5% of acute pancreatitis cases and usually require 4-6 weeks to form. The infection is usually introduced by bacterial translocation from the colon. Unfortunately, CT is not very reliable in the differentiation of infected necrosis from abscess. Abscesses can appear as more or less well circumscribed collections in the vicinity of the pancreatic bed, the mesentery or even the psoas muscle, with evidence of gas inclusion in only 30-40% Abscess drainage requires large bore catheters sizes (12-28 F) for optimal results and multiple catheter changes are often indicated because average drainage times range between four and eight weeks. Infected necrosis is usually

178

6 CT-Cuided Interventionaf Procedures treated surgically, but in selected non-surgical candidates percutaneous drainage can be attempted using very large catheters over a long time period that are exchanged in timely fashion. Close cooperation between the interventional radiologist, surgeon, and lCU specialist is a prerequisite for the interventional management of patients with these complex conditions, The catheter placement follows the general principles outlined above, Multiple catheters

ASpecialized

may have to be placed within one collection. Post-drainage protocols should be strictly obeyed, With these techniques the success rate of abscesses and infected necrosis for drainage is 35-50%, Abscesses located centrally within the pancreatic parenchyma are associated with significantly poorer results and are better treated surgically,

Interventions

An increasing number of organ-specific interventions are now performed with the help of CT guidance, These include interventional techniques in the liver, spine, and the peripheral musculoskeletal system, as well as imageguided injection techniques and blockades for pain management This section provides a brief overview of selected procedures, For more information on this topic, please refer to the literature on interventional radiology listed in Suggested Reading atthe end of this book.

Liver Tumor Ablation Only few patients with primary or secondary hepatic neoplasms are good candidates for liver resection (10-15%), A number of minimally invasive therapies have been introduced recently that may are expected to improve and complement the otherwise poor outcome of conventional nonsurgical treatment such as radiation and chemotherapy, Radiofrequency

(RF) Ablation

RF ablation is currently one of the fastest progressing nonvascular interventional treatments in hepatic malignancy and is being investigated by many groups in Europe and the US, lt can be used exclusively or in combination with other ablative and transcatheter therapies and with hepatic resection, Patients should have less than five lesions, none measuring more than Scm, and be unfit for resection or have nonresectable disease, For primary hepatic malignancy such as HCC, the Child- Pugh classification and other functional data are also taken into account

Electric currents in the RF range (460480kHz) are used to heat tumor cells to 90-100°C via shielded needle electrodes and produce coagulative necrosis, For adequate (complete) tissue ablation, a perilesional rim of liver tissue should be included and subjected to cytotoxic temperatures (> 50°C), Vessels around the lesion may cause rapid cooling of tissue resulting in less optimum coagulation (heat sink effect), It has been demonstrated that overlapping treatment areas can increase the efficacy of thermo ablation in larger lesions, Ideally 6-14 overlapping ablations of 3cm in a lesion of 2-3 cm diameter should be used but for reasons of clinical practicality the number of overlapping areas is usually reduced to below six, Ablation of a cuff of 5-10 mm of normal liver tissue should always be attempted, Ablation needles with a variety of features are available, These include retractable prongs and internal cooling of the tip, Newer devices optimize the amount of tissue that is coagulated in a Single session, Simultaneous intratumoral saline or doxorubicin injections and an occlusion of the local (parenchymal) blood flow (Pringle maneuver) have been used to enhance the effect oftherrnoablation for lesions larger than 3-5 ern. The procedure can be performed intraoperatively as well as percutaneously under guidance ofCT or ultrasound or both, Before the procedure is scheduled, recent CT and MRI scans should be available or additional imaging may have to be obtained prior to intervention. Large-bore venous access is made available and supportive therapy with IV fluids and peri-interventional antibiosis is initiated, One or two ground pads are positioned on the

Liver Tumor Ablation patient's back or thigh and the patient is anesthetized (general anesthesia or local anesthesia including the liver capsule under conscious sedation). The vital parameters are closely monitored at all stages of the procedure. The needle is then advanced into the target area and the electrode is connected to a 100200 W RF generator. After deployment of the electrode prongs, the lesion is ablated with continuous or pulsed protocols at the desired temperature for 8-25 minutes, depending on the equipment and individual lesion size. Temperature sensors in the electrode tip control the temperature delivered to the tissue. Depending on its design, the electrode generates a sphere of tissue coagulation 3-7 cm in diameter (Fig. 6.11). If multiple overlapping ablations are required, the needle can be repositioned and the procedure repeated. Individualized strategies are employed for complex situations with subcapsular tumors and central lesions in close proximity to large vessels. Multiple ablations with only partial deployment of the prongs and the use of different designs of needle may be required to safely enhance efficacy. During and after the procedure, most patients experience pain and nausea that subsides within a week. Therefore good practice should include the best available management by the local pain team including the use of

a

b

Fig. 6.11 Radiofrequency (RF) ablation of a liver metastasis from colorectal cancer. The hypovascular lesion (a) is treated via an anterior approach using a needle with extendable prongs (b). After ablation (c),

epidural on-demand analgesia with morphine analogs. Major complications can include capsular necrosis, intraperitoneal hemorrhage, pleural (hemorrhagic) effusion, perforation of bowel or bile ducts, and cholangitis. A baseline CT to document therapy success is performed within a week after the procedure. Multiphasic technique is employed, preferably with portal venous and early delayed phases for hypovascular (metastases) and arterial and portal phases for hypervascular lesions (HCC). Follow-up with CT, MRI, or contrast -enhanced ultrasound every 3-4 months is recommended, ideally within a standardized surveillance program. As thermal ablation is a new, rapidly changing therapy, no long-term studies or studies with surgical correlation are yet available. Initial studies in HCC reported complete necrosis rates of 48-95% and a tumor -free interval of 12-24 months in 64-71% of patients. For small tumors, survival rates of approximately 60% after 3 years and about one-third after 5 years can be expected. For metastases, complete necrosis was achieved in 52-93% of lesions and tumor-free rate was 11-52% at 9-18 months follow-up. Long-term data indicate a 46% survival after 3 years. Local tumor control appears significantly better for small metastases with patient survival rates of78% for lesions below 2.5 cm diameter and 32% for lesions larger than 4.0cm.

c there is coagulation necrosis with lack of contrast uptake but a small remnant of tumor tissue in the periphery (arrow).

179

6 CT-Cuided

Interventionaf

Procedures

Laser Ablation

A second type of energy that can induce tissue coagulation in the liver is the laser. Laser ablation is commonly referred to as interstitial laser photocoagulation (ILP) or laser-induced thernnotherapy (UTI), and is most frequently used for the treatment of metastasis, Thermal ablation with laser uses neodymium: yttrium-aluminum-garnet (Nd: YAG) or solid-state laser types delivering their energy through 400 urn fibers, Different fiber geometries result in different areas of necrosis, Bare tip fibers will create spherical lesions and cooled-tip diffuser fibers will result in elliptical lesions <2cm, Multiple fibers are needed for creating larger areas of confluent necrosis, Currently coagulation lesions with a diameter of up to 6-7 cm can be created, The procedure is done percutaneously under general anesthesia or conscious sedation, CT or ultrasound are most commonly used as guidance modalities, although MRI is also well suited (LITT) because it can use thermometry sequences to monitor the coagulation process, Multiple (2-8) 18-19 G needles are placed deep in the tumor and exchanged for sterile laser fibers, The lesion is heated at 2W for 50 seconds, and the fibers are gradually withdrawn more proximally for additional ablation cycles, Total procedure time is in the order of 6O-90minutes, The procedure is generally well tolerated and no major complications have been reported, Minor complications include pain, pleural effusions, and hematoma formation, Ultrasound can be used for follow-up, but usually multiphasic contrast-enhanced CT or MRI are preferred, In metastases, complete necrosis has been reported in approximately 50% of lesions, Reported survival rates are some 75% after 2 years, and some 25%-30% after 5 years, Microwave

Ablation

Microwave energy is an alternative means to generate tissue coagulation for ablation of liver tissue, It is used most often in patients with unresectable HCC. Most published experience comes from centers in Iapan. Microwaves generate an elliptical zone of thermal coagulation around the tip of the electrode needle (maximum diameter of 2 em). The

technique is effective against fibrous tumors and metastases that are resistant to chemo-emboltzation. Candidates for this method are inoperable patients who are not candidates for chemo-embolizatlon. Patients should have four or less HCC lesions, each preferably smaller than 3 em, The procedure is usually performed percutaneously under local anesthesia or conscious sedation, Under CT-guidance, a 14 G access needle is placed at the periphery of the tumor. Subsequently a 18 G microwave needle is advanced through the guiding needle into the tumor and connected to a 2.450 MHz microwave generator. Single ablations are performed for 60-120 seconds at 60 W, which can be repeated in multiple (up to 12) sessions until total tumor ablation has been achieved, Complications are only minor and consist of pain, fever, pleural effusion, or hemorrhage, Success of therapy is evaluated with multiphasic CT or MRI follow-up, Complete tumor necrosis has been reported in over 70% of cases with survival rates of approximately 70-80% after 2 years, Percutaneous

Ethanol Injection

(PEl)

Ablation by percutaneous ethanol injection (PEl) is one of the most frequently used techniques for the therapy of primary liver malignancies in cirrhotic patients, Administration of 95% ethanol results in cell dehydration and protein denaturation and induces coagulation necrosis followed by fibrosis (Fig, 6.12), The amount of necrosis induced depends on the amount of alcohol injected, Necrotic lesions as large as 8 cm can be created with multiple sessions, PEl is used in patients with liver cirrhosis and HCC with a tumor load below 30% liver volume, It is most effective in lesions smaller than 3cm. The procedure can be performed either on an outpatient basis under local anesthesia with multiple (4-12) sessions, or on an inpatient basis under general anesthesia with a 'singleshot" approach, In each case 20-22 G needles are positioned in the tumors under ultrasound or CT guidance, A few ml of ethanol are injected in small aliquots during each session and necrosis is monitored with ultrasound or CT Usually the needle is advanced through the tumor and ethanol is injected as the needle is withdrawn

Treatment ofOsteoid Osteoma

a

c

b

Fig. 6.12 Percutaneous ethanol injection (PEl) of an inhomogenously hypervascular hepatocellular carcinoma (a). The needle is advanced after injecting small aliquots

of ethanol (b). Tumor necrosis corresponds to the hypoattenuating region after therapy (c).

under imaging guidance. Coagulation necrosis will cause a bright echogenic blush on ultrasound that makes the evaluation of regions distal to the injection site impossible. The patient will experience pain as soon as the alcohol flows back through the needle tract and reaches the liver capsule and peritoneum. This pain will limit the amount of ethanol that can be injected in a single session in a non-anesthetized patient. As a technical variant that is possible only with CT-guidance, the needle tip can be placed in the proximal portion of the tumor and then be advanced while small ethanol aliquots are injected. With this technique larger volumes of ethanol can be used because the proximal needle tract is sealed by the coagulation necrosis. This will delay the back-flow of ethanol to the liver capsule and consequently the onset of symptoms. PEl is relatively safe in experienced hands. However, patients usually present symptoms including pain and fever during and after the procedure. Major complications like abscess, pleural effusions, ascites, or hemorrhage are more frequently observed with "single-shot" than with multisession treatment. All patients require follow-up by contrast-enhanced CT or MRI. Complete tumor ablation can be achieved in 70-75% of cases. Depending on patient characteristics (Child-Pugh classification), and size and multiplicity of the lesions, the survival rates in HCC that can be expected are 60-80%

after 3 years and 30-60% after 5 years. Results for metastases are generally poorer with complete necrosis only in 50-55% of lesions and 3-year survival in the order of35-40%. Treatment of Osteoid Osteoma

Osteoid osteoma is a benign osteoblastic bone tumor with no risk of malignant transformation. It usually occurs in the weight -bearing bones before the age of 25 and is more frequent in men. Osteoid osteoma induces local pain that worsens at night and responds well to aspirin. Conventional treatment is surgical excision, which is a major intervention. Recently a number of minimal invasive techniques have been used to treat osteoid osteomas with very promising results. For successful treatment it is necessary to destroy the central nidus of the tumor. Various ablative techniques have been used, most commonly radiofrequency (RF) ablation (thermocoagulation), interstitial laser photocoagulation (ILP) , or-mechanical-percutaneous drilling. •

RF Ablation

All patients with osteoid osteoma are amenable to treatment by RF ablation. Pre-procedural imaging includes plain films, CT, and MRI for precise localization. The intervention is generally performed under general or epidural anesthesia because the

181

6 CT-Cuided Interventionai Procedures penetration or drilling of the osteoma is extremely painful. The lesion is localized with CT and the approach carefully planned to avoid all vital structures. First, a biopsy of the nidus is taken using an 18-20 G bone trephine needle with or without drill (e.g., Ackerman, Bonopty), and a hole with a diameter of 1 mm or more created for placement of the RF electrode. This is always performed under CT guidance (Fig. 6.13). In selected cases, it can be advantageous to enter the bone from the opposite cortex. The electrode is connected to the RF generator and the grounding pad is placed close to the entry site of the needle. Within 1-2 minutes the electrode is heated to a temperature of 90°C , which is maintained thereafter for 4-6 minutes, depending on lesion size. This gives tissue coagulation 5 mm around the needle tip. For lesions > "10mm, multiple electrodes may have to be placed for optimal results. Post -procedural pain should be alleviated with injection of a long-acting local anesthetic in the operation area before anesthesia is terminated. The procedure is safe and there usually are no major complications if performed under strict asepsis. Successful RF ablation of osteoid osteoma has been reported in 90-95% of cases (longterm follow-up). Rapid pain relief is seen within 24-72 hours. Recurrences can be treated with the same procedure, and late recurrences after successful therapy are rare. •

Interstitial Laser Photocoagulation (lLP)

As with RF ablation, all patients with osteoid osteoma are amenable to treatment by ILP. With the patient anesthetized, the lesion is localized with CT and the approach is planned.

Subperiosteal lesions can be targeted with a 18 G spinal needle, while cortical lesions may require 14 G bone biopsy needles with or without drilling (e.g., Bonopty, Osticut). When the tip is in the center of the nidus, a 400 u,m laser fiber is inserted. ILP is performed by using a diode laser operating at 805 nm. In continuous wave mode, a power of 2 W is applied for 200-500 seconds, depending on the maximal size of the nidus. The procedure is safe, and complications are rare. They may be associated with the biopsy procedure, but reflex sympathetic dystrophy following the procedure has also been reported. Results from studies with 2 year follow-up show success rates over 95% with rapid pain relief within 24-72 hours. Recurrences can be treated by the same procedure. •

Percutaneous Drilling

Instead of ablation by heat, percutaneous drilling can also be employed for treatment. As the motor-driven equipment is a standard ORtool for the orthopedic surgeon, the procedure can also be organized within a multidisciplinary team. The procedure is usually performed under general or epidural anesthesia and with CT guidance. After positioning, plain CT is performed to evaluate the lesion and choose the optimum approach and materials. Drilling is performed with motor-driven Kohler trephine or Auger drill sets. The latter are available with different drill sizes, but usually 3-7 mm is adequate. However, many alternative approaches have also been described. When using a Kohler system, the introducing trocar is anchored in the cortex and a K-wire

Fig.6.13 Radiofrequency (RF) ablation of an osteoid osteoma in the femur. After localization of the subperiosteal nidus (a), the needle is advanced into the lesion under CT guidance low-dose fluoroscopy and RF ablation is performed (b). Note the reduced quality of the low-dose image obtained during the intervention.

a

b

Spino'interventions is drilled through this trocar towards the center of the nidus. Subsequently, a serrated sheath is hammered in the cortex and the first trocar is removed. The bone is then drilled to the edge of the lesion and the lesion is subsequently removed with a 7 mm trephine needle. With the auger set, the procedure is simpler: after anchoring the trocar in the cortex, the entire path and nidus of the lesion is drilled out with the hollow auger drill. If smaller drills are used and the nidus is incompletely removed, the drilling path may subsequently be sclerosed with 96% ethanol. All removed tissue is submitted to pathologic analysis to demonstrate the nidus, which may be an advantage over the ablative therapies as outlined above. Patients infrequently complain of persisting pain, which can be treated by analgesics. When long paths are drilled through weight-bearing bones, the risk of fracture may be increased. Thus, RF or laser ablation are the preferred technique in these regions. Other major complications are few, but osteomyelitis is well recognized. After the procedure patients should be mobilized non-weight-bearing for six weeks and should refrain from sports for three months. Results from small studies with "1-4year follow-up show success rates of 84-100% with rapid pain relief within 24-72 hours. Treatment of recurrence can be attempted with a similar subsequent procedure. Comparable CT-guiding techniques can also be used to facilitate complex percutaneous screw fixations of pelvic fractures.

Spinal Interventions In recent years there has been a marked increase in the number of image-guided diagnostic and therapeutic interventions in the spine. Although C-arm fluoroscopy was initially used as the main modality for guidance, CT is used increasingly for more complex situations and a number of institutions have even installed combined CT-fluoroscopy interventional suites. It should be noted that performing most of these procedures require special expertise and are best done by interventional, musculoskeletal, or neuroradiologists after subspecialized training.

Injection

Therapy

for Vertebrogenic

Pain

Local spinal pain and radiculopathy are frequent conditions affecting many individuals during their lives. Percutaneous injection therapies for pain management have been used for a long time, mainly by specialized anesthesiologists without imaging guidance. Recently, imagingguidance with CT or fluoroscopy has increased in popularity and can increase precision. Based on clinical symptoms, patients may be selected for a variety of injection procedures. Indications are localized pain syndromes not responding to conservative therapy. Indications include: facet joint syndromes, radicular syndromes associated with disk herniation or spinal stenosis, inflammatory sacroiliitis, and spondylosis. The procedure is performed on an outpatient basis. Relative contraindications are hemorrhagic diathesis and skin infections. Neurological disorders causing pain may be masked by the procedure. In practice, the technique for facet joint injection is comparable with techniques for selective nerve root, epidural, sacroiliac joint, and spondylolysis injections. The procedure can be performed under local anesthesia or with conscious sedation. Following local anesthesia 20-22 G spinal needles are placed under imaging guidance. The type of guidance is operator-dependent, but usually fluoroscopy, CT, or CT-fluoroscopy are employed. The location of the needle will depend on the procedure and can be inside a facet or another joint, adjacent to the dorsal root ganglion, or along the medial branch of the dorsal root of the spinal nerve (Fig. 6.14). When the needle is placed correctly, a combination of long-acting local anesthetics (e.g., bupivacaine) and/orlong-actingsteroids (e.g., triamcinolone) are injected in small aliquots. After the procedure the patient is observed for another 2 hours in a short-stay unit or recovery room. The aim of most therapeutic injections is to provide longer-term (i.e., three months) pain relief. The published success rates of the procedures vary and strongly depend on patient populations, techniques and outcome definitions. There remains a considerable controversy about the (long-term) usefulness of interventional spinal pain relief procedures.

183

6 CT-Cuided Interventional Procedures Table 6.5 Sympatheticchain blocks and indications in oncology (Gangi, 1996)

Fig. 6.14 For spinal pain treatment, CT allows precise positioning of the needle tip either inside or close to a facet joint, adjacent to the dorsal root ganglion or along the medial branch of the dorsal root of the spinal nerve.

•

Nerve Blocks (Neurolysis)

for Pain

Management in Cancer

Tumor infiltration or compression of nerve roots or plexus may be a cause of excruciating pain in cancer patients. In a significant number of patients symptoms of causalgia (burning pain, hyperalgesia, hyperesthesia, hyperpathia) develop that can be associated with vasomotor changes. These patients often benefit from sympathetic chain blocks, the most important of which are summarized in Table 6.5. For reasons of brevity, we will only discuss the technique of celiac plexus blockade, which is technically similar to nerve blockade in other regions. Contrast -enhanced CT is preformed before the intervention to locate the celiac trunk and mesenteric artery. The celiac plexus is located

Block type

Indications

Cervicothoracic sympathetic (stellate ganglia)

tumors of the lung apex

Thoracic sympathetic

tumors of the posterior mediastinum, esophagus

Lumbar/sacral sympathetic

tumors of the uterine body and cervix

Celiac plexus

tumors of the pancreas, adrenals, stomach, liver and gallbladder

Splanchnic nerves

tumors of the lower mediastinum and upper abdomen

(Pancoast)

immediately lateral to the celiac trunk in front of the anterior wall of the aorta. Intervention may be impossible if the celiac trunk is entirely engulfed by tumor. Celiac blockade can be performed via anterior or posterior approaches. For an anterior approach the patient is in supine position and a long 22 G needle is directed to the plexus via a transhepatic route. For a posterior approach the patient is in prone position and a paravertebral route is selected (Fig. 6.15). For this access, bilateral injections are usually necessary and the gantry may have to be angled to avoid the posterior pleural sinus. Once the needle is in the correct position, a few ml of diluted iodinated contrast are injected to confirm the extravascular location of the needle tip and to anticipate the diffusion of ethanol to be injected. Then, 10ml local anes-

Fig. 6.15 Sympathetic nerve blockade. After positioning the needle tip in the region of the sympathetic plexus, contrast material is injected to check the position of the affected region. Finally ethanol! contrast mixture is injected for nerve ablation. The procedure can be done in the thoracic (a) or lumbar spine (b). a

b

Sp/'nol inrervenrions

thetic (e.g., lidocaine) is commonly injected followed by a mixture of 20-25 ml of 96% ethanol and 5ml contrast medium for neurolysis. The added contrast medium allows for CT documentation of the ethanol deposit. The procedure is usually very well tolerated and major complications are few. These can include orthostatic hypotension, intravascular injection, and pneumothorax. With this technique notable immediate pain relief can be achieved in a high percentage of patients. Reported studies show variable success rates of 40-90%, with long-term pain relief in 15-70% of patients. •

Percutaneous Laser Disk Decompression

Suboptimum results of open surgery for degenerative disk disease have led to a number of minimally invasive alternatives such as chemonucleolysis, percutaneous diskectomy, and percutaneous laser disk decompression (PLDD). In PLDD the aim is to reduce intradiskal pressure by evaporation of disk material using Nd:YAG or Ho:YAG laser energy. Patient selection is crucial and indications for laser disk decompression include: • contained disk herniation as demonstrated by CT or MRI, • disk protrusions and extrusions with positive neurological findings, and • failure of 6 weeks of conservative therapy. Contraindications include hemorrhagic diathesis, previous surgery at involved segment, spondylolisthesis, and spinal stenosis. PLDD is performed on an outpatient basis and is guided by a combination of CT and (lateral) fluoroscopy. The patient is placed prone on the table with elevation support of the abdo-

men to open up the disk spaces. The exact pathway is determined with CT to avoid nerve roots (Fig. 6.16). Under imaging guidance the skin, subcutaneous tissues, and articular process are infiltrated with a local anesthetic. It is important not to anesthetize the exiting nerve root. From a posterolateral approach an 18 G curved tip needle is subsequently introduced into the disk under (CT) fluoroscopy and its position is checked with diskography. This check can also be used to evaluate the patient's complaints. If the needle is in a correct position, the stiletto is removed and the optical laser fiber is inserted into the disk 5 mm beyond the needle tip. The laser procedure entails 15 W pulses of 0.5-1.0 s at intervals of 4-10 s. For the lumbar levels an energy of 1200-1500J is usually administered, except for L4-L5 where 1500 -2000J is more appropriate. CT is performed every 200250J to visualize and control the extent of the evaporated area. At all times patients should be able to communicate and respond to pain during the procedure. Complications of the procedure are rare. The most common is septic (spondylo)diskitis. Good to excellent results in the range of 60-76% have been published in the literature. The average follow-up was 2-3 years. However, larger prospective comparative trials are still lacking. •

Percutaneous Vertebroplasty

Osteoporosis is an increasingly important disease with more than 1.5 million fractures occurring annually in the US, especially in white women. The most frequently involved levels in the spine are T8, T12, LI and L4. The technique of vertebral vertebroplasty (PVP) was pioneered in France in the mid 1980s by Galibert and Dera-

Fig. 6.16 Percutaneous laser disk decompression (PLOD). Placement of the needle (a). Control scan during the evaporation procedure

(b). a

(PVP)

b

185

6 CT-Guided Interventional Procedures mond and was popularized throughout Europe and North America during the second half of the 1990s. Persistent. painful. or unstable vertebral collapse not responding to conservative therapies form the main indication. and therapy is geared toward pain relief. improvement of stability. and improved patient mobility. Apart from osteoporotic fractures. the technique can also be used for destructive vertebral hemangiomas. hematologic malignancies like multiple myeloma and for osteolytic metastatic disease (limited experience). Relative contraindications include significant compromise of the spinal canal. advanced posterior cortex destruction. collapse of more than 90% of the vertebral body (vertebra plana) and long-standing pain for more than one year. Patients are selected after close consultation with the referring physician. including selected screening examinations and review of all old examinations available. The procedure is nowadays most often performed on an outpatient basis. It can be performed under general anesthesia or conscious sedation with continuous monitoring of vital signs in cooperation with an anesthesiologist andlor an anesthesiology nurse. With the patient prone. CT or (biplane) fluoroscopy is used to guide needle placement in the vertebral body via a lateromedial transpedicular route. Although fluoroscopy is more frequently used. CT with or without CT-fluoroscopy may offer more control for ideal needle placement. Preferably the needle tip should be positioned near the midline in the anterior part of the vertebral body. When using CT guidance. needle placement can be performed under local anesthesia before moving the patient to the interventional suite. A variety of needles are available for this technique: generally bone biopsy needles of 11-14G are used. but more specialized needles like the Osteosite or Accuthread needles are currently available. When the needle is in the desired position, vertebrography with a small

amount of diluted iodinated contrast medium is performed to avoid needle placement in the basivertebral venous plexus. The actual vertebroplasty is performed using polymethylmethacrylate (PMMA) bone cement (e.g., Codman cranioplastic, Simplex P, Osteobond): which is mixed with manganese or tantalum for opacification. Some 6-7 ml of this paste is injected under fluoroscopic control in O.Sml aliquots using either small syringes or commercially available high pressure cement delivery systems. During impaction of the injected cement, the needle is pulled back and the injection is terminated when the cement reaches the posterior one-fourth of the vertebral body. If the cement remains unilateral, needle placement from the contralateral side is usually necessary. After the procedure the patient is closely monitored for 2 hours in recumbent and sitting positions and is re-evaluated clinically before discharge. Complication rates are low and vary from '1-10%. However, they include nerve root and spinal cord injury, epidural abscess, and pulmonary embolization ofPMMA particles via the epidural venous plexus. With rapid diffusion of the method, these rates can be higher than reported during the learning curve of the procedure. Based on the largest series in the literature, the results of PVP were good to excellent with prolonged analgesia in 90% of patients. PVP for vertebral hemangiomas has a success rate of over 90%. This rate can be further enhanced by concomitant alcohol or Ethibloc embolization of the remainder of the lesion. In metastatic lesions, experience is limited. PVP has a stabilizing and analgesic effect and should precede radiotherapy to which it is complementary. Improvement in quality of life and analgesia can be achieved in over 80% of patients. Standard vertebroplasty differs from the newer technique of balloon kyphoplasty in which a balloon is inflated in the vertebral body for restoration of height and reduction of kyphosis.

Image Analysis M. Prokop

Basic Rules Image Analysis and Scanning Strategy (p. 188) Interpretation Strategy (p. 189) Anatomy CT Numbers Measurement of CT Numbers (p. 191) Air and Gas Collections (p. 193) Fat(p.194) Mixed Tissues (p. 194) FtJKJs (p. 194) Blood (p. 195) Hemorrhage (p. 195) Abscess(p.196) Calcifications (p. 197) Hemosiderin (p. 197) Foreign Bodies (p. 197) Contrast Media (p. 198) CT Morphology

.

Size Determination (p. 199) Boundary Surfaces (p. 200) Lesion Localization (p. 201) Tubular versus Round Structure (p. 202) Displacement versus Infiltration (p. 202) Inflammation versus Tumor (p. 203) Infiltration versus Retraction (p. 203) Enhancement Characteristics Normal Enhancement Characteristics Focal Lesions (p. 206) Artifacts

. (p. 204)

and Pitfalls

Anatomy (p. 209) Previous Surgery (p. 209) Partial Volume Effects (p. 210) Window Setting (p. 211) Beam-Hardening Artifacts (p. 212) Image Noise (p. 213) Motion Artifacts (p. 214) Artifacts Due to Contrast Media (p. 217) Scanner-Related Artifacts (p. 219) Spiral and Multislice Artifacts (p. 220)

.

7 I mage Ana lysis The goal of image analysis in CT is to detect and then evaluate pathologic structures. CT morphology is rarely unequivocal, but patient history and clinical parameters usually allow for narrowing down the differential diagnosis or

making a definite diagnosis. When using CT to evaluate the extent of a disease process, it is important to know the pitfalls and artifacts that may simulate or hide real disease.

Basic Rules

The basic rule of CT image analysis is to focus istration can facilitate the differential diagnosis not on just a single section but also to view ad- of certain hepatic lesions and can differentiate jacent sections to trace all structures until they cysts from solid hypovascular lesions. They are helpful for evaluating the contrast-filled renal are identified. The recognition of suspicious findings requires a precise knowledge of collecting system and lower urinary tract. sectional anatomy. All features that cannot be However, they are only essential in patients positively identified as anatomic structures re- with a suspected urinoma or with trauma to the renal pelvis or ureter (Fig. 7.2). Often they are quire further investigation. First the CT number (attenuation value) of the suspicious area the only means of positively distinguishing a should be analyzed. Attention is then turned to poorly opacified bowel loop from a soft-tissue the morphology of the suspicious finding, and, mass. (Additional oral contrast may have to be where applicable, its enhancement characteris- administered in these cases to obtain sufficient bowel opacification.) tics are assessed. Artifacts and common pitfalls An additional scan with narrow section should be considered during each step of the width reduces partial volume effects, improves analytical process (Fig. 7.1). resolution along the z-axis, and makes it easier to appreciate fine image details. However, many lesions or anatomic structures can best be Image Analysis and Scanning Strategy appreciated during a short time period, so that Uncertainties of image interpretation may ne- delayed scans are no longer useful. In such cessitate additional scans. Ideally, a gross image cases, reconstructing images from the original analysis should be performed immediately after data set with a greater degree of overlap may the examination so that action can be taken if help to resolve uncertainties. the patient is still available for additional scans. With the increased number of examinations that is possible with modern equipment, however, this may become impossible. In fact, only in a limited number of cases benefit from additional late scans because most CT examinations use contrast enhancement, which has an optimal time window for most applications. Such late scans after contrast admin-

Morphology

Fig. 7.1

Enhancement characteristics

The foundations of CT image analysis.

a

b

Fig. 7.2 Renal trauma. Scans obtained during the parenchyma! phase of enhancement demonstrate a perirenal mass (a), which can be identified as a urinoma only on delayed scans (b). Note the nonenhancing parapelvic cysts.

Anatomy Consequently, it is more important to decide beforehand which examination technique to use, If in doubt, multislice CT offers the option to reconstruct thicker sections as a standard for primary image interpretation from a thinsection data set If questions arise, they can be resolved by reconstructing thin overlapping sections (a "secondary raw data set") and creating problem-adapted multiplanar reformations, With respect to delayed scans, one could either perform them routinely in appropriate patients or try to save radiation exposure by selectively reviewing the image data in such patients and deciding whether late scans are useful in a particular case or not Interpretation Strategy Image analysis starts with the precontrast examination, if available, Then the scans acquired after intravascular contrast administration are reviewed and compared with the precontrast images, The radiologist should give attention to "incidental findings" that could have life-threatening implications but may not be causing symptoms at the time of the examination, Prime examples are thrombi in the femoral or pelvic veins and signs of pulmonary embolism, In immunocompromised patients, the radiologist

should be alert for potential signs of infection, Asymptomatic hemorrhages are rare, Most other incidental findings do not have immediate implications, If suspicious findings are noted during image analysis, a search should be made for concomitant findings that might permit an etiologic classification, At the same time, close scrutiny should be given to sections that are apparently uninvolved, since focusing on the principal finding may cause other findings to be missed, With spiral and especially multislice CT, the axial sections can be reviewed interactively on a monitor using the cine mode or interactive multiplanar reformations, This makes it easier to identify boundary surfaces, curved structures that cross the scan plane multiple times, and small structures that are difficult to trace in single slices, An interactive image analysis is even recommended as a routine measure for certain indications (search for pulmonary metastases, hepatic studies, bowel studies, CTA) , Further image manipulation with 3D surface rendering or MIP views is reserved for the analysis of complex findings, such as in orthopedics, traumatology, and CT angiography, With the advent of multi slice CT and faster processing workstations, interactive multiplanar evaluations using volume rendering techniques may become the primary evaluation tool (see Fig, 2,27),

Anatomy CT scans of the various organ systems demonstrate typical anatomic patterns, which are described in the chapters dealing with specific body regions, An extensive review of crosssectional anatomy would exceed our scope, but anatomic atlases may be consulted for a detailed account CT scans in some body regions permit a side-to-side comparison (neck, trunk muscles, pelvic muscles, extremities) and can disclose asymmetries that, especially in the neck region, may be an important indicator of pathology, Partial volume effects appear in CT sections that cut the cranial or caudal portions of an anatomic structure or simultaneously cut structures with different CT densities (Fig, 7,3), The effects are most pronounced in the region of the thoracic

inlet, the diaphragm and adjacent structures, the upper and lower poles of the kidney, and the boweL Partial volume effects are also caused by anatomic structures that are small in relation to the selected section thickness, This can occur in bronchi, pulmonary vessels, the adrenals, and many abdominal vessels, causing a spurious change in attenuation values (Fig, 7.4), Tubular structures may appear round, oval, or elongated depending on their course relative to the plane of the CT section, Whenever an indeterminate structure is found that cannot readily be identified or assigned to a specific organ, the structure should be traced through adjacent sections until a positive anatomic identification can be made, Features that are not identified as anatomic should

189

7 Image Analysis Fig. 7.3 Partial volume effects. Changing from a thick slice collimation to a thinner setting can reduce the effect of surrounding structures on the CT number of a lesion.

00 Boundary surface

Lesion

Tubular structure

be considered pathologic, though one should bear in mind the possibility of anatomic variants, artifacts, and other potential sources of error (see Artifacts and Pitfalls, p. 209).

Fig. 7.4 Effect of slice thickness on the appearance of small lesions. With an 8mm section thickness (a), the CT numbers of nodules decrease from the center to the periphery. In addition, the maximum CT number of small nodules is reduced. Only larger nodules retain soft-tissue attenuation in their center. With a 2 mm slice thickness (b), almost all the lesions show similar attenuation values and well-defined margins.

CTNumbers

CT numbers (CT attenuation) are of limited use for tissue discrimination. Air, fat, protein-poor fluids, and fresh hemorrhage are the only substances in the body that display characteristics attenuation values (Fig. 7.5). Calcified structures are clearly distinguishable from noncalcified soft tissues, but areas that enhance intensely after iodinated contrast administration display similar attenuation values. Metallic foreign bodies or implants have extremely high CT numbers that in some cases may greatly exceed the maximum values that can be displayed by most scanners (approximately 3000 HU). Lower CT numbers may be seen with surgical clips and other small structures that are subject to partial volume effects.

The relationship between the CT number of a structure and its physical density p is defined by the equation: p=CT + 1000 (p in rug/em', CT in HU) This equation holds true as long as the effective atomic number of the substance (i.e., its atomic composition) in the displayed voxel is similar to that of water. This applies with reasonable accuracy to fluids, soft tissues, and less to fat but not to iodinated contrast media or calcified structures. For this reason, the CT number of a soft-tissue structure is often used interchangeably with "density." A lesion may be described as hyperattenuating (hyperdense), isoattenuating (isodense), or hypoattenuating (hypodense), de-

Measurement of CT Numbers

3000

100

Hemorha

Ie----+--

Liver ----+---

,

Soft tissue --t-~ Blood ----f-Exudate-----+Transsudate -+--Water ---t---~~ Mixed tissues

50

o

-+--------lic~

-50

Fat -------t-100

Air Fig. 7.5 Scale of CT attenuation numbers. The only physiologic media with fairly unique CT numbers are air, fat, protein-poor fluids, fresh hemorrhages, and calcifications.

Measurement of CT Numbers

The quantitative measurement of CT attenuation values is necessary only in doubtful cases. Air, fat, hemorrhage, and calcifications can usually be identified visually by their characteristic attenuation. This visual interpretation can be aided by comparing the feature of interest with "internal standards" such as extracorporeal air, subcutaneous fat, or bone (Fig. 7.6). The gallbladder, urinary bladder, and CSF provide good internal standards for identifying fluids, although errors can result from sludge or contrast medium in the gallbladder, contrast medium in the urinary bladder, or spinal cord tissue in the vertebral canal. In interpreting a low-density hemorrhage or a suspected liquid lesion, it is often necessary to measure the CT numbers directly on the monitor. When CT attenuation is measured in a suspicious structure, consideration should be given to local variations caused by image noise. Thus, measurements should not be limited to one pixel but should cover a sufficiently large re~ion of interest (ROI) , preferably larger than 5 mm . If doubt exists, the measurement should be repeated in adjacent sections or in other parts of the suspicious region. The ROI should not be located in an area that is subject to motion artifacts, respiratory misregistration, or beam-hardening artifacts. When CT numbers on pre- and postcontrast scans are to be compared, identical kVp settings, reconstruction filter kernels, and section width

pending on whether its CT number is higher than, equivalent to, or lower than its surroundings. Thus, a hyperattenuating lesion appears "brighter" than the surrounding tissues, an isoattenuating lesion is not delineated, and a hypoattenuating lesion appears "darker." Note that these terms are always used in reference to surrounding structures. For example, a lesion with a CT number of 30 HU may be hyperattenuating to the fluid- filled renal pelvis but hypo attenuating to the renal parenchyma. Fig. 7.6 Hypoattenuating lesion of the right kidney. Comparison with the subcutaneous fat shows a similar CT number, indicating that the mass is an angiomyolipoma and not a cyst.

191

7 Image Analysis

a

c

b

d

Fig. 7.7 Comparison of CT numbers on various phases of contrast enhancement: arterial phase image (a) and delayed phase image (b) with standard widow setting (300/60) reveals no apparent difference in lesion en-

hancement (arrow). With a binary window setting (0/85) adjusted to the CT number of the lesion in the arterial phase (c), there is clear washout with more "black" pixels on the delayed phase scan (d).

should be chosen. If there is marked enhancement of the surrounding tissue (such as the renal parenchyma), a smaller section width may be preferred to reduce pseudoenhancement due to partial volume effects (see Figs. 7.24 and 18.16). A simple technique to compare contrast enhancement on CT scans in various phases is to reduce the window width as much as possible to obtain a binary (black and white) image, and then to chose the window level in such a way that on the first phase there are approximately the same number of black and white pixels within the lesion. If the number of black pixels increases in the corresponding section on another phase, enhancement has decreased while a larger number of white pixels indicates contrast uptake (Fig. 7.7).

relative volume within the voxel. Thus, when a ROI is selected, it must be considered that spurious densities will be measured in the periphery of a suspicious region, especially the caudal and cranial portions (Fig. 7.8). Spiral and multislice CTwith reconstruction of overlapping sections provides seamless coverage of a structure of interest. But because of the widened slice profile, the CT numbers measured in spiral

•

Determination Structures

of CT Numbers

in Small

For small structures less than 10 mm in diameter, the smoothing or edge-enhancing effect of the convolution kernel will alter the maximum CT density that can be measured in the structure. In particular, the use of high-resolution kernels can mimic the presence of calcifications in small lesions. It is best, therefore, to use a "standard kernel" for measuring CT attenuation values in small lesions. Partial volume effects can distort attenuation measurements because the CT number in a pixel is always affected by the CT numbers of all structures contained within the associated voxel. These structures contribute proportionately to the CT number according to their

Fig. 7.8 Partial volume effects can distort the attenuation values measured in the cranial or caudal portions of a lesion and in lesions smaller than the selected slice thickness.

Air and Gas Collections 193 CT are theoretically correct only if the structure is larger than the sum of the table feed (TF) and slice collimation (SC). A good rule of thumb is that the structure of interest should be one and a half times to twice as large as the slice width to ensure that accurate density measurements can be made even at eccentric sites within the section. Air and Gas Collections

Air and gas collections are best appreciated with a lung window setting. In doubtful cases this setting should also be used in extrathoracic regions (e.g., to exclude free intra-abdominal air) while the image is viewed interactively on the monitor. Window settings in which fat appears too dark are not suitable for the detection of gas (Fig. 7.9). Particular care should be taken when a liver or soft-tissue window setting is used, since gas collections may be missed or areas of very low attenuation (fat with scant connective tissue) may be misinterpreted as air. Extrapulmonary air or gas is usually easy to identify despite partial volume effects. CT numbers that are significantly below the attenuation of fat mean that air is present in the corresponding voxel. An area containing less than 10% air by volume may show fat -equivalent attenuation, however. A CT density <-150HU in soft tissue is pathognomonic for the presence of gas. Pathologic gas collections should be sought in trauma patients (cutaneous emphysema, pulmonary laceration), in cases of suspected gastrointestinal perforation or infarction, and in patients with inflammatory processes (gas-

forming bacteria). Because free air rises within a closed cavity system (pleural cavity, peritoneal cavity), air collections in the supine patient (pneumothorax, free intra-abdominal air) are most likely to be found in the anterior portions of body cavities. Air within a fluid -filled cavity (small bowel, pleural cavity, aspirated cysts, etc.) forms a horizontal air- fluid level on CT images (Fig. 7.10). Air collections near the center of a presumed liquid mass suggest the presence of internal septations. This finding is highly suspicious for an abscess when corroborative symptoms are present (Fig. 7.10). The colon contents have a characteristic CT morphology due to gas-forming bacteria in the stool, enabling the colon to be distinguished from small bowel. Diarrhea or enemas can alter the appearance of the colon by creating fluidfilled bowel loops with fluid levels.

000

00

0

0

a

b

0

0

0

0

Q

c

Fig.7.10 Free air within a liquid mass (e.g., postoperative seroma, small bowel contents) rises toward the anterior side of the mass, forming an air-fluid level (a). If the contents of the mass are viscous or have solid elements (e.g., abscess, colon contents), intralesional gas inclusions are seen (b). If the mass is loculated or has internal septa (e.g., abscess, fibrinous seropneumothorax), multiple air-fluid levels are observed (c).

Fig. 7.9 A narrow window setting may prevent the detection offree air (a), which requires the use of a wider window (b).

a

0

b

7 Image Analysis Fat Fat is another tissue that can be identified by its characteristic CT density of approximately -IOOHU (±20HU, depending on kVp settings). Body fat provides a natural negative contrast medium that delineates different organ systems from one another. As a result, the neck in most patients cannot be adequately evaluated without intravenous contrast administration. Even in the abdomen, noncontrast examination can be quite difficult in thin patients who have a paucity of body fat. Fat can generally be identified even when partial volume effects are present. Whenever CT numbers measure in the range from 0 to -!OOHU, it may be assumed that fat is present within the measured region. When fat occurs in adipose tissue cells, there will usually be at least some small areas with CT densities less than -50 HU (given sufficiently thin sections). But when other cells (liver, pancreas, muscle) acquire fat as a result of fatty infiltration, it is rare to find densities lower than 0 HLJ. A decrease in the attenuation of such an organ by 10 HU or more is considered evidence that fatty infiltration has occurred. The attenuation decrease is proportional to the fat content of the tissue: in the liver, which has a normal attenuation of about 60 HL], CT density decreases at a rate of approximately 16 HU per 10"10 fat content ([CTliver-CTfat[ X 10%). The detection of fat in a mass is an important differentiating sign. If the entire mass consists of fatty tissue, this is diagnostic of a lipoma. But a predominantly fatty lesion that also contains streak-like or solid components of softtissue density is most likely a liposarcoma. Predominantly solid tumors with fatty elements can have numerous etiologies and are discussed in the chapters dealing with specific body regions. Notable examples are intrathoracic and intra-abdominal teratomas, adrenal myelolipoma, and renal angiomyolipoma.

Mixed Tissues Mixed tissues occur ubiquitously in the human body. Their CT densities are intermediate between the densities of their individual components, and this can occasionally lead to misinterpretations.

CT attenuation ofOHU almost always signifies fluid (water) but also may be caused by mixed tissue in which fat is combined with other soft tissues (e.g., adrenal adenoma). Another typical mixed tissue is cancellous bone. which is composed of calcium salts, bony matrix, connective tissue, hematopoietic marrow, and fatty marrow. For a constant calcium salt content, the measured CT attenuation varies with the amount of fat contained in the trabecular bone. This is one of the basic problems of bone densitometry using quantitative CT This technique assumes that trabecular bone is composed of a standard soft-tissue component (with a water-equivalent CT number of OHU) and a variable amount of bone mineral. Thus, variations in the fat content of the vertebrae can cause distortion of the CT measurements. But because the calcium salt content of the bone declines with progressive osteoporosis while the fat content increases, these effects tend to be mutually reinforcing, so that it is possible to detect smaller changes in bone composition. Another mixed tissue is the lung parenchyma, which consists of aerated alveoli, capillary vessels, and interstitial connective tissue. An increase in lung attenuation is always due to a reduction in aerated lung parenchyma. This may result from deficient alveolar expansion, or a material of soft-tissue density (fluid, protein, cellular material) may have infiltrated the alveoli or interstitium. Thus, both alveolar and interstitial changes can lead to a "ground glass" pattern of increased lung attenuation on CT scans. Fluids Water, by definition, has a CT attenuation of o HLJ. In theory, the density of fluids can be determined from CT scans based on the close relationship between the CT number and physical density of a fluid. In particular, it should be possible to differentiate exudative fluids (> 10l8mgiml) from transudative fluids (
Hemorrhage

numbers higher than 25 HU suggest an exudative or hemorrhagic collection. In the range from 10 to 25 HU, no differentiation can be made. Very protein-rich fluids and hemorrhagic fluids may have a CT density higher than 35 HU and may even appear hyperattenuating to parenchymal organs (chocolate cysts in the kidney or ovary, fresh hemorrhage). When an indeterminate structure is found that is less attenuating than ordinary soft tissues, the likelihood that the structure is a liquid area increases as its CT number approaches OHU. While a CT number of approximately 0 is highly suspicious for a protein-poor fluid, this is far from conclusive a similar values may be found in soft -tissue structures that have undergone fatty infiltration. For example, adrenal adenomas have a variable fat content and typically have a CT density of approximately 0 HU. Densities in the range of about 20-40 HU are nonspecific and may be seen in protein-rich or hemorrhagic fluids, areas of fatty infiltration, or solid tumors. In some cases a cystic mass can be reliably differentiated from a solid lesion (e.g., in the kidneys or adrenals) only after intravenous contrast administration. Cysts should show no apparent alteration in CT attenuation by more than 10HU after contrast administration (see also Pseudoenhancement, Figs. 7.24 and 18.16). Pleural exudate may show enhancement during the interstitial phase because contrast material can diffuse through the damaged pleura and enter the effusion.

20

40

60

80

Blood

The CT number of blood plasma is approximately 28 HU. The CT attenuation of blood increases with the relative amount of corpuscular elements and hemoglobin (iron) present. The CT number of blood is proportional to the hematocrit (Fig. 7.11a). When the hematocrit is normal, blood has approximately the same attenuation as muscle and vessel walls. A simple sign of anemia is hypoattenuation of the cardiac ventricles relative to the cardiac septum (Fig. 7.11b). If very severe anemia is present, the walls of the aorta and great vessels may show higher attenuation than their lumina. In a liver that is free of fatty infiltration, severe anemia will increase the hypoattenuation of the hepatic vessels (and hemangiomas) relative to the parenchyma and can mimic the appearance of a hyperattenuating liver (as in the setting of hemochromatosis) . Hemorrhage

The CT attenuation of extravasated blood depends greatly on its age, location, and the presence of clots (Fig. 7.12). Fresh hemorrhage is typically associated with clot formation and the rapid reabsorption of serum elements. Both processes increase the hematocrit of the collection, leading to a corresponding increase in CT numbers (up to 80 HU). Fresh hemorrhage may have a high overall density or may contain hy-

100Het(%)

100

HE

80 60 Soft tissue

40 20 8

16

24

32 Hb (g%)

b Fig. 7.11 The CT density of blood is proportional to the hematocrit but is also influenced by kilovoltage (a). When anemia is present, the cardiac ventricles become

hypoattenuating to the cardiac septum (b). Note the density of the hemorrhagic pericardial effusion.

195

7 Image Analysis

a

b

d

c

Fig. 7.12 CT features of hemorrhage. Fresh hemorrhage with typical hyperattenuating clots (a). Hyperattenuating perisplenicnen-iorrhage after the reabsorption of serum elements (b). Sedimentation effect in a

fresh hemorrhage due to active bleeding, showing typical linear contrast accumulation at the interface (c). Acute hemorrhage with fresh extravasation of isoattenuating blood (d).

perattenuating, coagulated elements that are pathognomonic for hemorrhage. The age of an intracranial hemorrhage can be estimated with reasonable accuracy, but extracranial hemorrhages are more difficult to assess. As a hematoma ages, it becomes less attenuating due to the breakdown of blood and the absorption of hemoglobin. The hemorrhage initially appears iso- or hyperattenuating to the soft tissues and later becomes hypoattenuating. The rate at which the blood is broken down depends on numerous factors, including the site of the hemorrhage: with intrahepatic bleeding, hyperattenuating areas may persist in the liver for several weeks. An old liquefied hemorrhage is indistinguishable from a cyst.

A very fresh hemorrhage in which serum reabsorption has not yet occurred is isoattenuating to intravascular blood (Fig. 7.12d). Hematomas do not enhance after intravenous contrast administration unless active bleeding is present (Fig. 7.12c). Abscess

Abscesses have nonspecific CT densities that can range from OHU (infected cyst, seroma, or biloma) to 80 HU (infected hematoma). Pure pus has a CT density of approximately 30 HU. Abscesses may mimic old hematomas (Fig.7.13a, compare with Fig.l6.20b, c), both with respect to CT attenuation as well as enhancement in the membrane around the fluid collection (abscess

Fig. 7.13 (a) Localized, encapsulated pleural empyema with rim enhancement and a hazy increase in CT attenuation in the center of the collection (arrow) similar to an old hematoma. Some 500 ml of pus were removed by CT-guided drainage, (b) Conversely, the air inclusions in this patient with recurrent bleeding after surgical removal of a psoas hematoma were due to fibrin sponges placed at surgery. a

b

Foreign Bodies

membrane, granulation tissue due to resorption of the hematoma). Air inclusions are suggestive of gas-producing bacteria but may also be a consequence of surgery (Fig. 7.13b). Calcifications

A rule ofthumb for detecting calcifications m small round lesions is that noncalcified foci should have approximately the same CT density as horizontal vascular segments of the same diameter. Local beam hardening and other phenomena cause characteristic hypo attenuating areas near small foci of calcification, which can be detected using lung-window settings (Fig. 7.14).

A CT density higher than 100 HU in soft tissue signifies a calcium-containing structure, a metallic object, or contrast medium. Pure calcific plaques often have CT densities higher than 1000 HU when imaged in a sufficiently thin section. Lower densities may be caused by partial volume effects or diffuse calcification. Calcifications may form as a consequence of hematoma, abscess, fat necrosis, in granulomas (tuberculosis, histoplasmosis), in hydatid cysts, and less commonly in noninfectious cysts. Calcifications may also develop after tumor therapy (e.g., metastases from thyroid carcinoma after radioiodine therapy, malignant lymphomas). Calcifications have been described in a variety of tumors and can provide a useful differential diagnostic criterion. The detection of calcium in pulmonary nodules is generally a sign ofabenigne lesion. In larger lesions (» Smm), the morphology of the calcification must also be considered (see Chapter 9). The attenuation values in small lesions depend on numerous factors, most notably the slice thickness and convolution kernel. A high-resolution kernel tends to overestimate the density of small lesions due to its edge-enhancing effect.

The CT attenuation of foreign bodies depends on their physical density and effective atomic number (Table 7.1). The attenuation of substances with a high atomic number (leaded glass,

a

c

b

Fig. 7.14 Calcified granulomas. Subtle local beamhardening artifacts are typically visible with a lung window setting (a) and are even seen in association with

Hemosiderin

The presence of iron in the liver causes a proportionate increase in CT attenuation. MRI is better for the detection of iron storage, however, because concomitant fatty infiltration of the liver creates a mixed attenuation pattern in which hepatic density may be normal, or even decreased, despite an iron overload. Dual-energy CT can be an\ effective technique for reducing the effect'Toffat and increasing densitometric accuracy (see Chapter 4, p.a). With tube voltages of 80 and 140 kVp, the attenuation difference in the liver should not exceed SH. An attenuation difference greater than 10 HU suggests increased iron storage. Foreign Bodies

d

very small lesions (c). With a soft-tissue window, calcium can be detected only in larger lesions (b) while small foci may display soft-tissue attenuation (d).

197

7 Image Analysis Table 7.1

CT characteristics of various foreign bodies

Safety glass

Hyperattenuating

Ordinary glass

Very slight hyperattenuation

Polyurethane

Soft-tissue

Teflon

Very

Polyethylene

Near-fat density

Metal

Extremely hyperattenuating, streak artifacts

Wood

Air inclusions

Table 7.2

F.

Contrast enhancement

Tube voltage

density

hyperattenuating

metals) is strongly influenced by the energy of the x-ray beam (kilovoltage, filtering). Metals have CT numbers outside the range that can be displayed with a 12-bit encoding ofCT numbers (i.e., > 3000 HLJ). They also cause significant beam hardening and may cause almost complete absorption, which will lead to artifacts due to photon starvation. This consistently leads to problems of image reconstruction due to hyperdense and hypodense streak artifacts. Some scanners have 16-bit encoding available (maximum CT numbers> 65,000 HLJ), or expand the CT number scale by a factor of 10, thus permitting the direct evaluation of metallic structures (such as a defective implants). New techniques can correct for beam-hardening artifacts as well. Contrast Media

The following rule of thumb applies to the effects of iodinated contrast media: at 120 kVp, the CT attenuation in a voxel increases by approximately 25 HU for each 1 mg/ml of iodine that is administered. The precise attenuation is determined by the local x-ray energy, which depends numerous factors that include tube voltage, prefiltering, and beam hardening in the

J-

140kVp 120 kVp IOOkVp SOkVp

per rnq/crn"

iodine

; Enhancement* 20 HU 25 HU 32 HU 40 HU

numbers vary between scanners

patient (Table 7.2). As the kilovoltage and iodine concentration are increased, less contrast enhancement occurs within the scanned region. Low kVP scanning can be used to increase the CT numbers of contrast-enhanced structures substantially, thus improving signal-to-noise and creating a potential for dose savings (Fig. 7.15). There are just a few situations in which the attenuation values after contrast administration are useful for identifying the underlying pathology. During the arterial phase of enhancement, for example, when the attenuation in the aorta exceeds the attenuation in the veins, areas that are isoattenuating to the contrast-filled arteries are most likely aneurysms, pseudoaneurysms, or active hemorrhage (e.g., Figs. 11.33and 14.17). In another example, the late phase after intravenous contrast administration is marked by urinary excretion of the contrast material. If highly concentrated contrast medium is found within the abdomen and outside the urinary tract, there must be a perforation or rupture of the renal pelvis, ureter, or urinary bladder (see Fig. 7.2). Cavernous hemangioma (e.g., in the liver) should appear isoattenuating to the vessels in the late phase of enhancement due to the pooling of blood within the lesion ("blood pool effect", see Figs. 11.9and 11.15).

Fig.7.15 Lower kVp settings improve contrast enhancement. Despite identical scan and contrast injection parameters average CT numbers are some 90 HU higher at 100 kVp (a) than at 140 kVp (b). Effective patient dose with 100 kVp could be lowered substantially (E < 2.5 mSv). Emboli (arrows) are harder to distinguish from partial volume effects (arrowhead) on 140kVp scans.

a

b

Size Determination CT Morphology

Morphologic criteria are hel pful in differentiating and localizing pathologic processes and evaluating their spread. Frequently, however, they are based on relatively imprecise subjective assessments, and in some cases a confident morphologic evaluation cannot be made. Size

Determination

Many factors influence the accuracy of dimensional measurements in CT. Accuracy in the image plane is determined by image sharpness, i.e., it depends on the pixel size (image matrix and field of view) and the convolution kernel and ranges from 0.6 to 1.5mm. The accuracy of measurements along the z-axis is influenced by the scan parameters, most notably the slice thickness. Spiral CT can provide an accuracy range of 2-3 mm along the z-axis, and submillimeter accuracy can be achieved when thin slices are used with spiral or multislice CT. For volume measurements, a constant breath-holding depth is essential in order not to over- or underestimate the size of a lesion. With regard to image display, care should be taken to select a window setting of sufficient width. The structure to be measured and its immediate surroundings should be displayed in gray levels and should not contain any black or

white areas. The caliper of the digital measuring tool should not be placed at the outmost margin of a lesion but in the center of the slope at the lesion border (Fig. 7.16). The goal is to measure the "full width at half maximum" of this lesion, which accurately represents the true diameter as long as the lesion is large enough. Alternatively, it has been shown that a relatively narrow window width, combined with a window level that is adapted to the contrast of the structure of interest, can yield very accurate measurements. For small lesions, the size of the lesion, and its orientation relative to the scan plane has also to be taken into account. Thus, such an approach has little practical value. Digital measurements on the monitor are necessary for optimum results, but for most practical applications it is sufficient to perform measurements on the CT film, using the reference scale on the film to calibrate the measurements. The size of very small structures (e.g., small pulmonary nodules) can only be estimated unless the region of interest is magnified by selecting a smaller FOV. Focal lesions, especially tumors, are measured by drawing two perpendicular lines that mark the greatest diameters of the lesion (Fig. 7.16).The longitudinal extent ofa large lesion can be estimated by determining the table positions for the first and last sections on which the

Line profile

o FWHM 8.0 mIT' -500

-1000

F?;;:::O""'''T"''I'''......,........-r-T

o

10

20

.•.•....••...••••.•

30

mm

a

b

Fig. 7.16 The size of a focal lesion can be estimated by drawing a line representing the major axis of the lesion and drawing a second, perpendicular line representing its largest minor axis (a). The size of smaller lesions can

be most accurately determined by the "full width at half maximum" of a line profile through the major and minor axis of this lesion (b).

199

7 Image Analysis

lesion can still be seen; the difference will give the approximate longitudinal extent of the lesion. Greater accuracy can be achieved by performing the measurement on mul tiplanar reformatted images. The volume V of an ellipsoid mass can be estimated from the three greatest mutually perpendicular diameters a, b, and c as follows: V=(axbxc)/2

In parenchyma! organs, the contrast of the lesion with its surroundings is a limiting factor. This is particularly a problem in the liver and pancreas, where unfavorable contrast parameters can result in significant underestimation of lesion size. A hypervascular margin may appear isoattenuating to normal liver parenchyma on portal phase scans and thus will lead to underestimation of the true lesion size. For this reason, measurements on precontrast or arterial phase scans may be more accurate. CT during arterial portography (CTAP) will generally define a hepatic lesion in its entirety. The occlusion of small portal venous branches, however, may produce a peripheral wedge-shaped perfusion defect leading to the overestimation of lesion size with CTAP. There is an increasing demand for more accurate volume measurements, e.g., for determining the volume of a tumor or a whole organ or part of an organ prior to and after therapy or transplantation. When performing such measurements manually, the organ or lesion has to be isolated form the data volume by proper editing techniques (see Chapter 2, p. 45). At present, numerous programs are being developed that aid in volume measurements and may even perform those measurements automatically (Fig. 7.17). Automated volume measurements are critical to distinguishing be-

tween benign and malignant disease, especially for the detection and follow-up of lung nodules. Boundary Surfaces

Most relevant tissue planes and boundary surfaces in the neck and trunk are oriented parallel to the longitudinal body axis. As a result, the boundary surfaces between the principal organs are well depicted by CT even when a relatively large slice thickness (7-10mm) is used. Problems with axial scanning arise at boundary surfaces that are parallel to the scan plane, such as blood vessels, bronchi, diaphragm, and the superior and inferior borders of the liver, kidneys, spleen, stomach, and bowel. Partial volume effects at these interfaces blur the boundaries of the structures, causing a gradual rather than a sharp transition in CT attenuation from the structure to its surroundings. Most uncertainties can be resolved by using a thinner sections and generating reformatted images perpendicular to the boundary surface. Multislice CT with thin sections offers substantial advantages over spiral CT in this regard because it can cover whole body regions with such a high spatial resolution (Fig. 7.18). If the contrast between the structure and its surroundings is low (e.g., a right -sided renal tumor abutting the liver parenchyma), often even thin sections do not show the distinction between a well-defined boundary (no tumor infiltration) and an indistinct boundary (invasion by tumor). Fine bony fissures, thin fascial planes, and interlobar fissures in the lung require thin sections and multiplanar reformations to be reliably detected.

Fig. 7.17 Automated size measurements are available for the vasculature (a) and for the determination of the volume of pulmonary nodules (b).

a

b

Organ of Origin

a

b

Fig. 7.18 Transfissural infiltration of this bronchogenic carcinoma is hard to evaluate on a 5 mm axial section (a) but is easy to detect on sagittal reformations from a multislice CT data set (b).

Organ of Origin

Attri bu ting a mass lesion to a specific organ is often the key to diagnosis. If the lesion is sur-

rounded by fat and has broad-based contact with just one organ, this question is easy to resolve. Difficulties arise when a lesion is found to be in contact with at least two organs. This is illustrated by the problem of differentiating empyema from a subpleural lung abscess, a subcapsular hepatic lesion from a peritoneal lesion, or a large renal tumor from a large adrenal tumor. An obtuse angle or bulge between the lesion and the organ suggests that the lesion arises from that organ, whereas an acute angle or indentation at the lesion periphery suggests that the lesion is extrinsic to the organ (Fig. 7.19). Typical routes of spread are usually oriented along pre-existing fascia-defined spaces. With a tumor that has invaded an adjacent organ, intravenous contrast administration may be necessary to heighten the attenuation difference between the lesion and adjacent organs and thus distinguish indentation from invasion. In all but a few cases, this differentiation can be successfully accomplished (Fig. 7.20).

Fig. 7.19 An acute angle between a lesion and contiguous organ suggests impression on the organ by an extrinsic lesion (a, adrenal tumor), while an obtuse angle suggests that the lesion arises from the organ itself (b, liver metastasis).

a

b

a

b

Fig. 7.20 Peritoneal metastasis from ovarian cancer that infiltrates the liver and simulates a liver lesion (a). Note, however, the obtuse angle between the mass and the liver surface (arrows) that could only be appreciated on the postcontrast scan. Bronchogenic carcinoma with retraction and thickening of the pleura (arrow) (b). The contrast-enhanced scan demonstrates that there is no invasion of the chest wall yet.

7 Image Analysis Tubular versus Round Structure

Displacement versus Infiltration

The differentiation of a round or oval structure (lesion, lymph node) from a tubular structure (vessel, bronchus, esophagus, bowel) generally requires an analysis of contiguous CT slices. Tubular structures can be traced through numerous sections, whereas round or oval structures are depicted in only a few sections. The cine mode is very useful in these cases, as it enables contiguous or overlapping sections to be viewed in a movie-like sequence. Another useful criterion is the attenuation in the periphery of a lesion, which is particularly helpful for identification of lymph nodes and pulmonary nodules. Tubular structures that are cut at right angles by the sectional plane show a sharp decline in contrast at the periphery of the structure. In the case of true round structures, this occurs only with a relatively large lesion whose central portion has been cut by the scan. Sections near the cranial or caudal end of the structure show a peripheral fading of contrast due to partial volume effects. This also applies to lesions that are smaller than the section width. Thus, for example, a pulmonary nodule is more easily distinguished from blood vessels on thick sections than on thin sections that pass through its center (Fig. 7.21).

Expansile lesions tend to displace or compress adjacent anatomic structures (blood vessels, parenchymal structures, bronchi, bowel). Typically this is manifested by an acute angle between the edge of the mass lesion and the border of the displaced organ. The organ capsule is intact, so the abutting mass usually forms a lens-shaped indentation. The boundary surfaces should be clearly depicted on perpendicular sections (axial or MPR). With an infiltmtive lesion, the contours of the lesion and infiltrated organ are blurred due to partial volume effects occurring along the infiltrative margin of the lesion. Again, an adequate evaluation requires images perpendicular to the boundary surface and obtaining sufficient contrast between the lesion and its surroundings. Unenhanced CT can resolve the question of fatty infiltration with an organ-transgressing tumor, but scans after contrast administration are generally required to evaluate the infiltration of other adjacent soft-tissues. Most organs are separated from one another by fa t planes. Expansile lesions cause thinning of these planes but generally do not disrupt them. Obliteration of the intervening fat plane suggests infiltration by an organ-transgressing tumor or inflammatory process. This sign is highly sensitive but only moderately specific. MRI with out -of-phase imaging is often more accurate for detecting remnants of a fat plane. Vascular invasion is an important prognostic criterion for many tumors. Because of their high intraluminal pressure, arteries present a circular cross section on imaging (unless this is prevented by atheromatous plaque). The only definitive signs of vascular invasion are a contour abnormality or the tumor-induced stenosis or occlusion of an artery. Contact with a tumor in itself is not a sufficient sign of arterial invasion, although the likelihood of invasion increases with the area of contact. If tumor covers more than 25% (one-fourth) of the circumference of an artery, it is likely that the tumor has invaded the vessel. With a more than 50% wide contact area, tumor invasion is almost always present. Venous cross sections depend on the fullness of the vein, its size, and its contact with surrounding structures. Because a venous wall is

Fig. 7.21 Decreasing attenuation due to partial volume effects in the periphery of a small nodule can help distinguish it from a tubular anatomic structure. The small nodule is difficult to detect with a slice 2 mm thick (a), but in one 8 mm thick it is clearly demarcated from surrounding vessels (b).

Infiltration versus Retraction thinner than an arterial wall, there is a greater likelihood of invasion when tumor contact is present. In particular, hepatic veins showing even a small area of tumor contact have almost certainly been invaded, or cannot be surgically separated from the tumor. Stenoses and occlusions are reliable indicators of venous infiltration. While it is rare for tumors to invade arterial vessels (this usually occurs with highly malignant tumors such as sarcomas and angiosarcomas), venous invasion by tumors is far more common. This is often associated with the formation of appositional thrombi, which can be distinguished from the actual tumor thrombus by their lack of contrast enhancement (no blood flow). The presence of a renal cell carcinoma, hepatocellular carcinoma, or sarcoma should elicit a search for venous invasion. If venous vessels are found to pass undisturbed through a suspicious mass with no displacement or constriction, it may be assumed that the mass represents a diffuse, nontumorous process. A typical example is focal fatty infiltration of the liver, which does not distort the architecture of the organ. Inflammation versus Tumor

Inflammatory processes and tumors may be differentiated in borderline cases only by taking into account clinical parameters of infection (bearing in mind that inflammatory signs may be less in immunosuppressed patients). Problems arise in the differentiation of:

• • • • •

focal lesion rounded shape mass effect lymph node> 2 mm in diameter absence of clinical signs of infection active protein =CRP, leukocytosis) • lack of response to antibiotics

(C-re-

Areas of liquefaction may occur in tumors as well as infectious processes. Generally tumors are distinguished by a larger soft-tissue component, a thicker wall, and less regular margins. In doubtful cases, an effort should be made to establish a tissue diagnosis by fine-needle aspiration or core biopsy.

Infiltration versus Retraction

The margins of a lesion can provide clues to its etiology. Smooth margins are typical of expansile lesions and most metastases. Spiculated margins may indicate an infiltrative lesion or may signify retraction effects due to scarring (Fig. 7.22). Infiltration may have a tumorous or nontumorous etiology. Tumorous infiltration is most commonly seen with primary organ tumors (bronchogenic carcinoma, hepatocellular carcinoma, pancreatic carcinoma) but is occasionally caused by heavily vascularized metastases (Kaposi sarcoma, angiosarcoma). N ontumorous infiltration may be caused by infectious processes with a cellulitic component.

• alveolar cell carcinoma from pneumonia (compare Fig. 9.31 to 9.39 and 9.40) • recurrent tumor from chronic infection following surgery (see Fig. 15.19) • chronic pancreatitis from pancreatitis in pancreatic-head carcinoma • inflammatory bowel disease from intestinal lymphoma • gastric cancer (Tl) or early lymphoma from gastritis • diverticulitis versus colon cancer (compare Fig.15. 21 toI5.15c). While there are no generally valid rules, the following clinical and radiologic features are suggestive of a tumor:

Fig. 7.22 Spicules radiating from a lesion may be the result of an infiltrative process or scarring: bronchial carcinoma (a), scar tissue in silicosis (b).

203

Image

Analysis

Retraction effects are typically caused by scarring as a result of trauma, surgery, radiotherapy, infection, or successful tumor therapy, lt can be difficult to differentiate the various causes of spiculated margins in any given lesion, but the following guidelines should be helpful, Radiating strands of scar tissue generally have well-defined margins and are less tightly clustered than the spicules of an infiltrative lesion, Perifocal edema is rare, Scars may distort the parenchymal architecture, which seldom occurs with infiltrative processes,

Within soft tissues (neck, mediastinum, abdomen), ill-defined spiculated margins on CT scans suggest an infiltrative process (inflammation or high-grade malignancy, especially nonHodgkin lymphoma), Again, well-defined spicules are more consistent with retractile processes due to scarring, Perifocal edema can obscure fat planes and is often indistinguishable from infiltration; its presence signifies an active process (tumor, infection, recent surgery, lymph stasis, venous stasis),

Enhancement Characteristics Normal Enhancement Characteristics Pulmonary Vessels

Opacified blood and nonopacified blood are generally well mixed within the right ventricle, As a result, the maximum enhancement of the pulmonary vessels is identical to peak aortic enhancement Maximum opacification depends strongly on the cardiac output (dilution effect) and the injection parameters (contrast concentration, flow rate), Enhancement of the pulmonary arteries is some 5 s earlier than the enhancement of the pulmonary veins, If there is a temporary enhancement of the aorta before contrast arrives in the pulmonary vessels, a persistent oval foramen can be suspected, Such a foramen opens temporarily as there is a rise in right atrial pressure due to contrast material injection and allows blood to enter the systemic circulation first Systemic Vessels

The aoJta and systemic etteties are defined with exquisite clarity during the arterial phase of enhancement The enhancement of the aorta (and its branching arteries) reaches a relative plateau rapidly after contrast material arrives at a particular level in the aorta, As contrast material is injected the level of this "plateau" increases further (see Fig, 24.3) until shortly after the injection (contrast material and saline flush) stops, The coverage of a large imaging volume with high spatial resolution during this phase requires the use of multi slice scanning, About 20 to 30 seconds after the onset of arterial enhancement, contrast material is

delivered to the portal system, with inflow of contrast occurring first through the splenic vein and later through the mesenteric veins, This delay can lead to artifacts in the portal vein, where non opacified blood can mimic intraluminal thrombosis (see Figs, H.53 and 15,34), The time at which systemic veins enhance after intravenous contrast injection depends on their location, The renal veins enhance very rapidly because of the high renal blood flow, accompanied by enhancement of the intrahepatic segment of the inferior vena cava, A similar pattern is seen in the jugular veins, Venous drainage of the lower extremity and azygos system proceeds more slowly, but even in these regions nonopacified blood at confluence sites can mimic thrombosis, If the contrast material is injected at a very slow rate, the arterial and venous vessels will show similar patterns of enhancement determined solely by the total volume of contrast medium and the patient's circulating blood volume, Various enhancement phases are best separated if the contrast material is injected as a short bolus (limited volume, high flow rates), Arterial Phase

The onset of aortic enhancement after contrast injection depends largely on the circulation time and can range from less than 10 seconds to more than 60 seconds, It correlates with the heart rate only in patients who have a healthy circulatory system. For this reason, tailoring the examination to the individual patient by bolus triggering or test bolus injection is superior to a standardized examination technique,

Normal

The arterial phase denotes the period in which the enhancement of an organ is caused predominantly by contrast medium delivered to the organ by arterial perfusion. This phase continues as long as a high concentration of contrast material is delivered through the aorta. But the purely arterial phase lasts only a short time (15 to 30 seconds) before other effects. such as contrast diffusion into the interstitium. contrast delivery through the portal vessels. and renal parenchymal enhancement. are superimposed. Typical patterns of arterial-phase enhancement are observed in the spleen (serpentine hypo- and hyperattenuating regions due to the differential perfusion of the red and white pulp). the kidney ("corticomedullary phase" with intense cortical enhancement and faint medullary enhancement). the liver (minimal enhancement of normal parenchyma. since 75% of the hepatic blood supply is from the portal vein). the pancreas (strongest enhancement toward the end of the arterial phase). and the thyroid gland (intense enhancement of the entire gland). Mucosal enhancement in the bowel is strongest at the end of the arterial phase and beginning of the parenchymal phase.

Enhancement

Characteristics

portal phase approximately 60 to 90 seconds p.i. In the kidney, the renal cortex shows intense initial enhancement, and shortly thereafter contrast is delivered to the collecting ducts in the renal medulla. Thus the renal cortex shows earlier and more intense peak enhancement than the medulla. The true parenchymal phase starts when the cortex and medulla show approximately equal levels of attenuation (at about 80 to WO seconds p.i.). The normal intestinal mucosa shows the strongest enhancement at the beginning of the parenchymal phase and then loses enhancement over time. The muscles do not show significant enhancement in either the arterial or parenchymal phase. Interstitial

Phase

The interstitial phase is determined by the diffusion of contrast medium into and out of the interstitium; perfusion effects are of minor importance. Little organ enhancement persists during this phase, and the vessels in most organs appear isodense to the parenchyma. Contrast

Excretion

Contrast material is normally excreted almost entirely by the kidneys. At 3 to 5 minutes after Parenchymal Phase the injection, highly concentrated contrast meThe parenchymal phase denotes the period in dium appears in the excretory system of the uriwhich the parenchyma of an organ shows its nary tract. Because contrast medium is denser than urine, it forms a dependent layer in the urihighest level of enhancement. The time required to reach the parenchymal phase depends on the nary bladder. It is very rare for intravenously inorgan (Table 7.3). and this phase shows varying jected contrast to fill the bladder completely; degrees of overlap with the arterial phase or in- generally this requires retrograde contrast administration. terstitial phase of enhancement. The liver excretes approximately 2% of the In the pancreas. for example. maximum parenchymal enhancement is produced by arte- contrast medium into the bile. This percentage rial perfusion and occurs only about 40 to may be substantially higher in patients with im60 seconds after the start of the injection (p.t.). paired renal function. Opacification of the gallIn the liver. however. enhancement results pre- bladder occurs approximately 6 to 24 hours dominantly from the portal delivery of contrast after intravenous contrast administration. medium, so peak attenuation occurs during the

Table 7.3 , Organ

Phases

of enhancement Arterial

phase

Liver

Minimal

Spleen

Tiger-striping

Homogenous

enhancement

Homogenous

Pancreas

Slowly increasing

enhancement

Maximum

Kidneys

Corticomedullary

differentiation

Homogenous

Bowel

Increasing

mucosal

enhancement

Decreasing

after after

50-

120s

after musocal

p.i.

60 s p.i.

enhancement 80-100

at 40-60 s p.i.

enhancement

s p.

7 Image Analysis

Focal Lesions

•

Peripherally Enhancing Tumors

Focal lesions in parenchymal organs have approximately the same CT attenuation as their surroundings, but their vascular supply (perfusion) can differ markedly from the surrounding parenchyma. As a result, contrast administration can improve lesion detection and narrow the differential diagnosis. The best phase of organ perfusion for lesion detection is the phase that provides a maximum attenuation difference between the lesion and its surroundings. The pattern of contrast delivery and clearance relative to the surrounding parenchyma can be used for lesion characterization.

Tumors with a rich peripheral vascular supply are most common in the liver. Most such tumors are metastases (usually from adenocarcinoma), but cholangiocarcinoma (Fig. 7.23 b) may also occur. Rim enhancement is seen during the arterial phase, and may persist through later phases. In some lesions, however, scans during the portal phase may show only the central hypovascular portion of the tumor, resulting in underestimation of tumor size. A similar enhancement pattern may be seen outside the liver (e.g., in neck tumors) but does not have particular diagnostic significance.

•

U

Hypervascular Tumors

Hypervascular tumors show greater enhancement than the surrounding parenchyma during the arterial phase, generally appearing as a focus with transient hyperattenuation. If the scan is performed too late, many hypervascular tumors are no longer detectable. As a result, small lesions can often be detected only with (biphasic) spiral or multislice CT scanning. Examples of hypervascular tumors are renal cell carcinoma (Fig.7.23a), hepatocellular carcinoma, sarcomas, and most malignant tumors with endocrine activity. Benign hypervascular tumors include renal adenomas, hepatic adenomas, focal nodular hyperplasia, pheochromocytoma, and benign endocrine pancreatic tumors.

a

Hypovascular tumors are less perfused than the surrounding parenchyma, so they are most conspicuous in the phase of maximum organ enhancement. Some hypervascular tumors are also hypo attenuating to their surroundings in this phase (e.g., all renal tumors), provided the attenuation produced by arterial perfusion is below the level of parenchymal enhancement. Almost any tumor histology may present as a hypovascular lesion. A large percentage of all hepatic metastases are hypovascular, and hypovascular lesions are typical of pancreatic carcinoma (Fig. 7.23c). The maximum enhancement of an organ during the parenchymal phase is strongly dependent on the contrast injection parameters.

c

b

Fig. 7.23 Lesions are optimally displayed times, depending on the vascularity of the organ: pancreatic metastasis from renal cell in the arterial phase (a), cholangiocarcinoma

Hypovascular Tumors

at different tumor and carcinoma in the early

portal phase (b), pancreatic carcinoma in the parenchymal phase (c). Note the primary tumor and the periarterial invasion (arrows) as well as the atrophic tail of the pancreas with a dilated pancreatic duct (arrowheads).

Focal Lesions

Maximum attenuation in the pancreas occurs about 20 to 30 seconds earlier than in the liver, while maximum attenuation in the renal nephrogram occurs 20 to 30 seconds later than maximum liver enhancement. As a result, the timing of optimum lesion detection varies from organ to organ, and contrast administration always involves a trade-off with regard to imaging goals. •

Tumors Outside Parenchymal Organs

Tumors outside parenchymal organs, such as softtissue tumors and head and neck tumors, must be delineated from muscle tissue. Except for sarcomas, they are frequently hypovascular or slightly hypervascular. Tumor enhancement is based on arterial perfusion and increased contrast diffusion into the interstitium due to altered vascular permeability. Depending on the vascularization, tumors can best be appreciated towards the end of the arterial phase (arterial vascularization) or later, up until the interstitial phase (diffusion effects). Optimum detection of these lesions requires scanning with a long delay (60 s or more) and a narrow window setting. •

Cysts

Cysts do not enhance after contrast administration. A diagnostic criterion for cysts is the absence of a significant attenuation increase (> 10 HU) during any perfusion phase. Attenuation changes less than 10 HU are still within normal accuracy limits or may result from random fluctuations. Scans during a late hepatic phase or during the excretion phase in the kidneys often improve differentiation between tumors and cysts because even small tumors tend to

8-mm Sections

pre-contrast

Fig. 7.24

Pseudoenhancement

post contrast

of small cysts.

have taken up contrast during these phases and become hard to visualize while cysts remain distinctly hypo attenuating. Pseudoenhancement is a problem in areas where there is marked contrast uptake in normal parenchyma outside a cyst. It is most common in the kidneys and is the more pronounced the smaller the lesion. Pseudo enhancement is a consequence of partial volume effects, which change in magnitude as the extracystic parenchyma enhances (Fig. 7.24). For small cysts, a less stringent criterion of > 15 HU enhancement may be used, but it is better to reduce the section width for such an examination and even perform a late scan with thin sections to keep partial volume effects low. Accurate measurements can be expected if the cyst is larger than twice the section width. There are no enhancing (solid) elements found in the periphery of a true cyst. The presence of such elements, even when very small, should raise suspicion of a cystic tumor, or, when membrane-like, of infection. •

Inflammation and Abscesses

Inflammatory diseases of the mucosae (e.g., in the gastrointestinal tract) are characterized by intense mucosal enhancement during the arterial phase and early parenchymal phase. The inflammatory involvement of deeper wall layers leads to enhancement of the entire wall, in some cases obscuring the adjacent fatty tissue. A focal inflammation often produces no detectable changes within a parenchymal organ, but unenhanced scans may occasionally show a hypo dense area (edema), and a slightly hyperdense area may be seen after contrast adininis-

2-mm Sections

pre-contrast

post contrast

207

7 Image Analysis

(ration (arterial hyperperfusion or impaired interstitial back-diffusion). The predominant finding in fatty tissue is a diffuse or streak-like attenuation increase caused by edema and an inflammatory reaction. Abscesses may develop from a region of focal inflammation or from the infection of an existing fluid collection or hematoma. Liquefaction and abscess formation are marked by a central, nonenhancing cystic region with an attenuation of approximately 30HL] (pus) surrounded by an enhancing rim of relatively uniform thickness (Fig.7 .25a) . Usually this granulomatous rim is best appreciated during the interstitial phase of enhancement. Hepatic abscesses may be rimless or display a hypodense rim due to the intense enhancement of the surrounding parenchyma (Fig.7.25b). Because of this often invisible ring of granulation tissue, hepatic abscesses usually appear to have indistinct margins. Superiniected fluids (seroma, biloma, hematoma) have nonspecific CT densities that can range from 0 to 80 H. They do not show central enhancement, and it takes several days for a typical granulomatous rim to develop at the periphery (Fig.7 .25c). Generally this rim is narrow and is most conspicuous during the interstitial phase. A superinfected fluid collection is indistinguishable from a noninfected collection in its early stage. Diagnostic needle aspiration should be considered if laboratory findings are consistent with infection. An enhancing rim may also form in association with resorptive processes such as hem a-

a

b

toma breakdown, so this sign is highly suggestive of, though not specific for, an infectious process. •

Hemangiomas

Cavernous hemangiomas of the liver are loculated, blood-filled cavities with a delayed, centripetal pel-fusion pattern and very thin walls. This accounts for their enhancement characteristics on CT scans: On noncontrast images, hemangiomas are isodense to blood (even in anemic patients). Following contrast administration, the enhancing areas in the lesion show somewhat delayed but approximately equal attenuation to the aorta. Contrast -filled lacunae appear first at the periphery of the lesion and subsequently spread toward its center. By the portal phase at the latest, all the lacunae are isoattenuating to the blood vessels (blood pool effect). The time required for the complete opacification of a hemangioma can range from seconds to more than 30 minutes, the lesion displaying a characteristics pattern of peripheral-to-central ("centripetal") enhancement. If the blood and liver parenchyma are isoattenuating on delayed scans, the hemangioma can no longer be discerned. Note that there are a few atypical hemangiomas that may appear as a homogeneous hypervascular tumor (small lesions up to 2cm), as a heterogeneous tumor (large lesions greater than 3cm), or as a hypovascular lesion (thrombosed hemangiomas).

c

Fig. 7.25 Loculated hepatic abscess with an enhancing rim (a), hepatic abscess with a hypodense rim and air inclusion (b), superinfected biloma (c).

Artifacts and Pitfalls Anatomy

Previous Surgery

Anatomic variants or normal structures scanned on an unusual plane should not be misinterpreted as mass lesions (Fig. 7.26). Differentiation is accomplished by comparing the adjacent slices and by taking enhancement characteristics into consideration (Fig. 7.27). Table 7.4 lists some of the anatomic structures that are most commonly misidentified on CT scans.

Scars can be quite difficult to distinguish from tumors if the scar tissue forms a mass lesion. This is particularly true after proctectomy. Pulmonary metastases after therapy may resemble bronchial carcinoma. Dynamic contrast studies can sometimes differentiate tumors from scar tissue, since tumors enhance during the arterial or parenchymal phase of perfusion, while scar

b

a

Fig. 7.26 An accessory left superior vena cava mimics a mediastinal lymph node (a). The cisterna chyli mimics a retrocrural node (b). Bowel simulates lymph nodes (c),

a

b

c

d

and the fatty infiltration of a lymph femoral vein thrombosis (d).

c

Fig. 7.27 Cervical veins simulate lymph nodes (a, b). Compression (c). Perisplenic varices simulate small bowel loops (d).

node

mimics

d by a synovial cyst mimics femoral vein thrombus

7 Image Analysis Table 7.4 some anatomic identified on CT scans

structures

that may be mis-

Structure

May be mistaken for

Geniohyoid muscle

Lymph node

Scalene muscles

Lymph nodes

Sartorius muscle

Lymph node

Supra-aortic

Lymph nodes

arteries

Left superior vena cava

Lymph node

Aberrant vein

Lymph node

pulmonary

Pericardiac recess

Lymph node

Right atrial appendage

Tumor, lymph node

Atypical renal veins

Lymph nodes

Paravertebral veins

Lymph nodes

Pelvic venous plexus

Infiltrative growth

Fatty infiltration lymph node

Venous thrombosis

of

tumor

Cisterna chyli

Azygos vein, lymph node

Crus of diaphragm

Lymph node, esophagus

Accessory spleen

Lymph node, renal or adrenal tumor, peritoneal tumor

Bowel

Intra-abdominal tumor, lymph node, varices

Splanchnic varices

bowel loops

tissue does not enhance until the interstitial phase. Bowel loops after reconstructive surgery of the biliary system often collapse and may easily

be mistaken for a local mass. Oral cholegraphic contrast agent may be used to opacify such loops and allow for reliable differentiation from a real tumor (see Chapters 12 and 15). M ul tislice scanning may improve anatomic delineation as long as there is sufficient local abdominal fat between various soft tissue structures. Bladder wall edema following a transurethral resection (TUR) can mimic a residual tumor (see Fig. 19.7). "Cysts" can be seen followingTUR of the prostate. Partial Volume Effects

The influence of partial volume effects depends on the size of the structure relative to the section width and the position of the structure in relation to the scan plane. Thus, structures that are approximately parallel to the scan plane are far more subject to partial volume effects than structures perpendicular to the scan plane, i.e., structures oriented along the z-axis. In studies using relatively thick sections and nonoverlapping reconstructions, partial volume effects can reduce the contrast of small pulmonary or hepatic lesions located in an unfavorable position (between two adjacent slices), causing them to be missed. Scans that cut a portion of an adjacent structure may simulate lesions where none exist. For example, a section traversing an ossified, posteriorly protuberant attachment of the first rib to the sternum can mimic a solitary pulmonary nodule (Fig.7.28a). Similarly, a sec-

c Fig. 7.28 The sternal attachment of the first rib mimics an intrapulmonary hepatic lesion (b).

d nodule (a). The air-filled

colon mimics a

Window Setting Window Setting

a

b

Fig. 7.29 Suspected low attenuation mass in the left adrenal gland on a 5 mm section (a) that can be attributed to partial volume effects due an almost horizontal course of a normal size gland (b) if coronal MPR are reconstructed from an overlapping secondary raw data set in multislice CT (4 x 1/6).

tion cutting the upper pole of the right kidney, the gallbladder, or adjacent air-filled bowel segments can mimic an intrahepatic lesion (Fig.7.28b). The lower part of the caudate lobe may be mistaken for a lymph node at the porta hepatis. The portal venous confluence can simulate a lesion of the pancreas. Thick sections through an obliquely positioned adrenal gland may mimic a mass (Fig. 7.29). Unsharp tumor interfaces in oblique scans may prompt a spurious diagnosis of infiltration.

a

b

Fig. 7.30 Dependence of CT measurements on window width. When a narrow window is used (a), the bronchial wall thickness is overestimated compared with a wide window setting (b). The degree of calcified

CT window settings affect image contrast and noise. If the window is set too wide, image contrast is reduced, with the result that low-contrast lesions in the liver or pancreas may be missed or the enhancement of an abscess membrane, for example, may go undetected. Conversely, too narrow a window setting can significantly increase image noise and abolish grayscale differentiation in fatty tissue areas. This can hamper the evaluation of fine structural details and cause pathologic changes in fatty tissues (edema, infiltration) or free air to escape detection (see Fig. 7.9). The accurate estimation of lengths and diameters on CT scans requires a sufficiently large window width. Both the structure of interest and its surroundings should still contain gray levels and should not appear totally black or white. A typical example is the evaluation of bronchial wall thickness. This requires a window width > 1OOOHU (preferably 1500 HU). Other window settings lead to the overestimation of bronchial wall thickness (Fig.7.30a, b). CTA of calcified stenoses or stents requires wide window settings to correctly assess the degree of narrowing (Fig. 7.30c, d).

c

d

stenoses on CTA of the carotids varies with window settings. While a setting ofW/L=500/150 overestimates the stenosis (c), a setting of 1500/300 displays the lumen correctly (d).

211

7 Image Analysis

Beam-Hardening Artifacts

The radiation emitted from the x-ray tube of a CT scanner represents a spectrum of high and low energies. As the thickness of the scanned object increases, the low-energy spectral components are absorbed more than the higherenergy components. This phenomenon is called beam hardening. In defining CT numbers, a certain "intermediate" energy must be assumed for the absorption of x-rays by water. Since absorption is reduced at higher energies, increased beam hardening would cause a decline in CT numbers. The technical solution to this problem is to estimate the thickness of the object based on the x-ray absorption for each individual projection and mathematically correct for beam hardening based on the estimated thickness. One assumption in this computer correction is that the entire object is composed of just one substance (usually water). If the local atomic composition of the object differs markedly from that of water (bone, metal, concentrated iodinated contrast medium), beam-hardening artifact will continue to occur. Newer techniques are available for further reducing beam-hardening artifacts. This can be done either by using complex computer algorithms to determine beam hardening from the density distribution in the CT image (e.g., RASP, Toshiba) or by acquiring thinner slices that are then assembled to make a larger slice thickness (this can reduce beam hardening by structures that pass obliquely through the slice).

a

b

•

Ribs

Low-attenuation beam-hardening artifacts that occur behind ribs can mimic intrahepatic lesions, but they can usually be correctly identified by their characteristic location (Fig. 1.31 a). •

Extremities

If both arms are left in the scanning field in an adducted position at the patient's sides disturbing horizontal streak artifacts will appear between the bony structures due to increased xray absorption and beam hardening. For this reason, at least one arm should be removed from the scanning field and extended above the patient's head. Similar beam hardening effects can be seen between the shoulders (Fig. 7.31 b) and between the femurs. The denser the skeletal structures, the more pronounced the artifacts. •

Metal Implants

Metallic objects cause significant beam hardening ranging and may also cause almost complete x-ray absorption (photon starvation at the detector), resulting in the appearance of hyperdense and hypodense streak artifacts (Fig. 7.31c). These artifacts generally do not occur with small objects (staples, vascular clips) because of partial volume averaging. Titanium implants absorb less radiation than other metals, so they cause fewer artifacts. Beamhardening artifacts caused by bilateral total hip implants cause decreased attenuation in the lesser pelvis (bladder, obturator compart-

c

d

Fig. 7.31 Beam-hardening artifacts may occur behind ribs (a) and may be caused by the shoulder girdle (b), metal implants (c), and highly concentrated contrast medium in the urinary bladder (d).

Image Noise

merit), making it difficult to evaluate this region. CT is not useful for evaluating metal objects (e.g., checking for implant fracture) unless the range of measurable CT densities is expanded upward (> 3,000 HLJ). This is possible if the scanner already employs 16bits per pixel (maximum CT number> 65,000) or if special software is available that extends the CT number scale. Algorithms for metal artifact suppression are available from some manufacturers. Such algorithms often use iterative correction. In a first step they determine the location of dense objects based on a reconstructed CT image. They then use this information for predicting beam hardening effects and photon starvation effects, and correcting for them. This procedure can be repeated in an iterative manner, thus further improving image quality. Such algorithms, however, require substantially longer for reconstructing the final image. •

Contrast Medium in the Bladder

A contrast-filled urinary bladder typically causes areas of very low attenuation to appear between the lateral bladder wall and the bony pelvis (Fig. 7.31d). •

Gastrointestinal Contrast Media

Barium residues in the gastrointestinal tract can give rise to very intense local streak artifacts. Undiluted iodinated contrast material can produce a similar effect. For this reason, CT examination of the abdomen should be postponed if gastrointestinal contrast residues can be seen on the scanogram. Contrast residues in diverticula may persist for days or weeks, but usually are responsible only for minor artifacts.

Image noise is particularly disturbing shaded surface displays because it will "flying pixels" and irregularities of the contour (Fig. 7.32). Similar effects can countered on volume rendered images. •

on 3D cause object be en-

Electronic Noise and Photon Starvation

If radiation dose to the detector is reduced substantially, then the electronic noise of the detector system (amplifier noise) becomes a major issue. Electronic noise adds random fluctuations of the detector signal and is negligible for standard radiation exposure. The absolute amount of noise is independent of the incident radiation. As the detector signal decreases, the relative effect of electronic noise becomes more apparent. Such a decrease in detector signal can be due to high absorption in the patient (e.g., obese patients, metallic objects, large cross-section such as in the shoulders) or due to low-dose scanning. As there is a continuing trend towards lower exposure dose, electronic noise will become more important. If the number of photons that hit the detector becomes too low (photon starvation), electronic noise becomes the predominant feature (Fig. 7.33). Reconstructing thicker sections or using smoothing kernels to reconstruct the raw data set can no longer compensate for this sudden increase in image noise. For this reason, a photon starvation effect should be avoided by all means because there is no way of compensating for the reduced image quality.

Image Noise

Image noise is a normal effect on CT sections that were not obtained with an excessive amount of radiation exposure. It represents random fluctuations of the measured CT number and is mainly caused by quantum noise (see also Chapter 5). Image noise may reduce the conspicuity of low-contrast detail but can be reduced by increasing the section thickness (axial images or MPR) or using a smoothing filter kernel (see Fig. 2.5).

Fig. 7.32 Image noise will lead to flying pixels on 3D shaded surface displays (a). Using a smoothing kernel for image reconstruction can reduce this effect (b).

213

214

7 Image Analysis

100 Signal

~

,-

'" i

.~

Quantum

noise

.~ ~

001

Electronic

norse

00001+-----,----.----.----.----, 10,000 1000 10n 10 Detector dose

Fig. 7.33 As exposure dose is decreased, image noise exceeds the numbers predicted from quantum noise alone. The reason is the growing contribution of electronic noise. which becomes the predominant feature as the detector signal drops below a certain threshold

and photon starvation occurs (a). Reconstructing thicker sections from multislice CT data sets can no longer sufficiently compensate for the reduction in image quality (b, c).

Motion Artifacts

•

Motion in the scanned section during one rotation of the x-ray tube will result in non-consistent projection data because of different configurations of the scanned objects in the various x-ray projections. As a result, varying degrees of motion artifact will appear throughout the reconstructed image, being most pronounced in the region of the moving structure.

Pulsation of the heart, aorta, and pulmonary vessels may cause double or multiple contours to appear on vascular and organ boundaries. When this effect occurs in the ascending aorta, it can simulate an intimal nap (see Fig.23.13). Pulsation of the heart may cause streak artifacts in adjacent organs and may simulate dissection in the descending aorta (Fig.7.34a). Vascular pulsation may lead to displacement of the vessel as such and may cause double contours

Fig.7.34 Pulsation of the heart may cause streak artifacts in the descending aorta simulating dissection (a). Pulsation results in displacement of the renal arteries

leading to serrated contours in this patient after renal transplantation (b). Pulsation artifacts can produce kymographic effects on MPRs (c).

Pulsation

Motion Artifacts

(Fig.7.34b) or serration of the vessel wall (see Fig. 23.13). Pulsating structures in multiplanar reformations cause serration artifacts to appealalong the z-axis (kymographic effect, Fig. 7.34c). Most pulsation effects decrease with faster scanning (multislice CT, e.g., with 4x2. 5 mm detector configuration) because one heart cycle spreads out over a larger range along the z-axis. Pulsation of the heart and great vessels causes the trachea to appear serrated on 3D or multiplanar displays (Fig.7.35a). Pulmonary vascular pulsations or transmitted lung pulsations most commonly occur behind the heart in the left lower lobe, causing herringbone-like distortion of vascular bifurcations in the lung periphery. Hypodense areas appear near the bifurcation sites and can mimic focal emphysema (Fig.7.35b). Pulsation may cause local areas of increased CT density that present as "pseudocalcifications" in the lung hilum, the heart, or the liver (Fig. 7.35 c). Fast ECG gating or prospective triggering can almost completely suppress pulsation effects in the aorta or pulmonary vessels. However, pulsation is non-uniform because pulsation waves travel from the heart to the periphery. Thus, there are different points in time that are optimum for the aorta and the pulmonary vessels. •

Respiratory Artifacts

Insufficient breath holding can cause double contours to appear on organ boundaries that

move with respiration. These artifacts can obscure pathology and are rarely mistaken for true pathology, although double contours within the lung can mimic bronchiectasis or pleural disease (Fig. 7.36a). These effects are caused by vigorous respiratory motion, such as a sudden deep inhalation or a coughing fit (Fig. 1.36 b). At the upper or lower border of large ovalshaped structures such as the kidneys, spiral artifacts may occur if the patient breathes during the scan (Fig.7.36c). Such artifacts do not occur if the patient holds his or her breath properly. With spiral and multislice scanning, acceptable results can be achieved even during shallow respiration. This applies only to axial scans, however. Multiplanar reformations are highly sensitive to any kind of patient movement, and even small respiratory excursions will cause undulating surface contours (Fig. 7.37a). Organs, vessels, or tumors may appear elongated or compressed, depending whether the patient breathes in or out during the scan (Fig. 7.37b). Deeper breathing leads to discontinuities in the moving structures (Fig. 7.37 c). Patient instruction is therefore critical if any type of 3D processing or 3D measurement is to be performed, such as in CT angiography or CT volumetry. •

Other Motion-Related

Effects

Motion artifacts also result from gross movements such as those that occur in patients with acute fractures or unrest due to pain (Fig. 7.38 a). While spiral CT has greatly reduced the motion problem on axial images, conspicuous motion

a Fig. 7.35 Pulsation causes the trachea to appear serrated on multiplanar reformations (a). Pulsation artifacts produce emphysema-like hypodensities at the bi-

furcations of pulmonary calcifications (c).

vessels

(b) and can mimic hilar

215

216

7 Image Analysis Fig. 7.36 Gross respiratory excursions during the scan (e.g., due to coughing) can mimic areas of bronchiectasis (a) or produce a double margin on organ boundaries (b). At the upper or lower border of the kidney, breathing may cause spiral artifacts.

a

b

a

b

Fig. 7.37 On multi planar reformations or volume-rendered displays motion artifacts can be appreciated as undulating contours of the body surface (a) or elonga-

tion or shortening of an organ or lesion (b). Marked breathing may cause discontinuities in vascular structures (c).

Fig. 7.38 Gross movements (here due to swallowing) lead to structural discontinuities that even can simulate fractures (a). A sagittal reformation of the dens in Parkinson's disease shows pronounced motion artifacts (b) that were not visible in the axial sections.

a

b

Artifacts Due to Contrast Media

artifacts can still appear in m ul tiplanar reformations and shaded surface displays (Fig. 7.38 b).

sequent blending of opacified and nonopacified blood from the two vessels can create a spurious intraluminal filling defect, especially when there is laminar flow. Pseudothrombi can occur at numerous sites of venous confluence (Table 7.5). Generally this artifact is easy to recognize, but occasionally it can be mistaken for throm bosis (Fig. 7.39, compare also Figs. H.53c and 15.34c). In the case of a pseudothrombus, scans acquired 30 to 60 seconds later will show complete, uniform opacification of the region in question.

Artifacts Due to Contrast Media

The short scan duration with spiral and multislice CT can increase artifacts caused by differential enhancement of various vascular territories. Venous structures are most commonly affected. •

Pseudothrombus

Artifact

A pseudothrombus artifact occurs most commonly at the confluence of veins that carry blood from anatomic regions with different circulation times, such as the infrarenal vena cava (slow return from pelvic and leg veins) and the renal veins (fast return though the kidneys). The artifact occurs if scanning is performed during a phase that is too early for complete opacification of both venous systems. The con-

Table 7.5

Sites of venous confluence

Affected vein

pseudothrombus

Inferior vena cava

Renal veins

Portal vein

Superior vein

Pseudothrombi

in Pulmonary

Vessels

During early scanning (particularly with multislice CT), there may be differential enhancement between various portions of the pulmonary arteries that rarely may simulate thrombus (Fig. 7.40 a). The differential enhancement of pulmonary arteries and veins during the early phase of

Causative vein,

Superior mesenteric

artifacts

may occur (listed in order of frequency) Mechanism Inflow of opacified

mesenteric

vein

blood

Inflow of nonopacified Reflux of opacified

Splenic vein

blood

blood

Mesenteric vein

Mesenteric

Inflow of nonopacified

blood

Superior vena cava

Azygos vein

Inflow of nonopacified

blood

Common

Internal iliac vein

Inflow of nonopacified

blood

Profunda femoris vein

Inflow of opacified

Brachiocephalic

Inflow of nonopacified

iliac vein

Common femoral

vein

Superior vena cava

a

where

•

veins

b

Fig. 7.39 Pseudothrombus artifacts due to differential portal vein (c), common femoral vein (d).

vein

c venous opacification:

blood blood

d azygos vein (a), inferior vena cava (b),

217

218

7 Image Analysis

a

Fig. 7.40 Pseudothrombus in the pulmonary arteries is a rare finding and is due to differential opacification of various pulmonary artery branches (a). Non-opacified pulmonary veins may simulate thrombi but are easily

scanning is more common, which may lead to misinterpretation of a non-opacified vein as an arterial em bol us (Fig. 7.40b). Because of the shorter scan duration, this effect is seen more frequently with multislice CT. Differentiation of arteries and veins is simple using a cine display or by checking for the bronchus that is adjacent to pulmonary arteries, not veins. •

c

b

Pseudolesions due to Hepatic Veins

Nonopacified hepatic veins appear hypoattenuating to the enhanced liver parenchyma and can appear as pseudolesions in some sections (Fig.7 .40c). This is most apparent during the early portal phase (about 50-60 seconds p.i.), when the portal vein is brightly enhanced but the hepatic veins are not yet opacified. It is better to delay scanning of the liver or to choose a caudocranial scanning direction during this phase, as this will delay imaging of the major hepatic veins until they are more adequately opacified. Pseudolesions can be correctly identified by following a suspected lesion through adjacent sections and comparing it to pre contrast or later contrast -enhanced scans. An identical effect can occur in arterial-portal CT (CTAP), in which contrast medium is injected into the superior mesenteric artery and the liver parenchyma is enhanced purely by contrast that is delivered to it through the portal vein. The hepatic veins are not opacified during the early phase of CTAP (about 20-40 seconds p.i.) and may appear as pseudolesions.

distinguished based on anatomical considerations (b). Non-opacified liver veins may simulate hypovascular lesions, especially during the early portal venous phase (c).

•

High-Contrast

Artifacts

If the concentration of contrast material is too high, the signal at the detector may be so weak (photon starvation) that typical streak artifacts are produced. This effect is accompanied by beam hardening, which also produces low-attenuation artifacts. After the administration of intravenous contrast, artifacts may appear in areas where the venous inflow carries a high concentration of contrast medium, most commonly the brachiocephalic vein or superior vena cava. These artifacts can completely obscure underlying signals, mimicking the CT appearance of an aortic intimal flap or pneumothorax (Fig. 7.41; see also Fig. 24.5). This problem can be reduced by lowering the contrast concentration, or by scanning in a caudo-cranial direction

Fig. 7.41 High-contrast artifact caused by contrast medium in the subclavian vein mimics a pneumothorax.

Scanner-Related

Artifacts

while using a saline flush to wash out the injection veins (see Chapter 3, p. 99). Scanner-Related Artifacts

CT scanners can cause a variety of equipmentrelated artifacts, most of which are due to errors of application, adjustment problems, or scanner defects. •

a

Inaccurate CT Numbers

Improper calibration of the scanner can lead to significant CT number errors (Fig. 7.42a). On multislice CT, improper calibration of the detectors will cause hypo attenuating rings that rotate around the patient and cause a pattern of undulating densities (zebra stripe artifact) on MPR (Fig. 7.42b). The scanner should be calibrated at least once a day, depending on the manufacturer's recommendations. If inaccuracies are noted, the scanner will require recalibration or service. In some scanners, a low radiation dose or certain convolution kernels can lead to inaccuracies in the measured CT numbers. In particular, high-resolution kernels should not be used for CT number measurements (Fig. 7.43). The effect of kilovoltage on CT numbers should also be considered. The more the effective atomic number of the scanned region differs from that of water, the greater the kilovoltage effect. Lowering the kVp will increase the CT numbers of structures with high atomic number (e.g., calcium- or iodine-containing compounds, see also Fig. 7.15), while it will reduce the CT number of fat.

b

Fig. 7.43 A high-resolution kernel for image reconstruction will simulate the presence of calcifications in small nodules (a). Correct assessment requires the use of a standard kernel (b).

•

Tube Malfunction

Breakdown of the x-ray beam during a spiral acquisition yields specific artifacts (Fig. 7.44) that are due to a complete lack of data from some projection angles. For these angles, only electronic noise contributes to the image. In some scanners such raw data cannot be reconstructed at all, others allow for image reconstruction and thus may avoid re-exposing the patient. •

Detector Malfunction

If a single detector malfunctions during one single projection, filtered back-projection along that ray will cause a single black line to appear on the CT image (Fig. 7.45a). The malfunction of several detectors in one projection will be represented as multiple lines centered on the corresponding tube positions. These artifacts

Fig. 7.42 Incorrect scanner calibration: the calibration processes was interrupted and led to a circular band of a low attenuation that simulates perihepatic fluid (a). On multislice CT, alternating bands of low and normal attenuation may occur that present as "zebra stripe artifacts" on MPR (b).

a

b

220

7

Image Analysis

Fig.7.44 Tube malfunction with complete interruption of the x-ray beam leads to purely electronic noise at the detector. Resulting artifacts with partial detector signal (a) and no detector signal (electronic noise only) (b).

b

a

Fig.7.45 Failure ofa single detector during one projection (a). Ring artifacts caused by faulty calibration of the detectors relative to one another (b). Out-of-field arti-

occur sporadically and reflect problems with the detector electronics. If a single detector malfunctions for all the projections in one tube rotation, all the CT numbers will be underestimated, and the image will show a hypodense ring the position of which correlates with the angular position of the faulty detector. This type of artifact may occur sporadically or may signify a permanent detector defect. •

c

b

a

Ring Artifacts

Multiple ring artifacts appear when several detectors malfunction in one tube rotation or, more commonly, when detector sensitivities are poorly calibrated relative to one another (Fig. 7.45 b; see also Fig. 7.41 b). The latter problem can usually be corrected by recalibrating the scanner.

facts in an obese patient, caused by tissues located outside the scanned field of view (c).

•

Out-of-Field

Artifacts

Out -of- field artifacts can occur in very obese patients or if the selected scanned field of view is too small. Areas of the body cross section located outside the scanned field appear hyperattenuating (Fig. 7.45 c, see also 25.14c). Spiral and Multislice Artifacts •

Spiral Artifacts

Spiral artifacts may occur in spiral or multislice CT at interfaces that are slightly angulated relative to the scan plane. They are due to data inconsistencies during one rotation that cannot be corrected by interpolation. They increase with pitch factor and present as an object distortion (Fig. 7.46) or spiral indentation of the object sur-

Splrol and Multislice Artifacts

Fig. 7.46 Spiral artifacts in the dome of the acetabulum from a multislice CT scan (4 x 2.5/15) with 3 mm section width.

face on 3D renderings. They are exaggerated if additional motion occurs (see Fig. 7.35c). •

Cone Beam Artifacts

Cone beam artifacts are due to the geometry of the x-ray beam and are more pronounced the more detector rows present. Only objects in the rotational axis (center of the gantry) are "seen" by the same detector row. Objects localized in the periphery of the scan field will be detected by different detector rows as the tube rotates around the patient (see Fig. 1.22). This effect leads to marked undulations and serrations on MPR (see Fig. 2.9) because the apparent position of the object cannot be precisely localized. New multislice cone-beam interpolation techniques promise to reduce this effect.

The artifacts can be seen in almost all objects with high contrast, such as bone, vessels, and intraluminal air in the bowel. Artifacts are more pronounced with a higher pitch factor and may even cause slight streak artifacts at object interfaces (Fig. 7.47). When comparing the image quality of thin sections with a high pitch factor to thick sections with a low pitch factor, such cone beam artifacts are less with the lowpitch images. However, if images of similar thickness (section width) are reconstructed from a thin-collimation data set, artifacts are further reduced. Thus, scanning with thin collimation yields the best image quality as long as image noise does not become a limiting factor. •

Noise and Spatial Resolution

Noise and spatial resolution are not uniformly distributed throughout a set of spiral CT images. Each table position along the z-axis corresponds to a particular angular position of the x-ray tube. At this position, little raw data interpolation is required, which will yield a locally improved spatial resolution. At an opposite position in the image, maximum interpolation is necessary, which yields less image noise but a somewhat lower spatial resolution (Fig. 7.48). Image noise is increased in thin-section multislice CT images because of the narrower collimation and the reduced detector dose. This makes such a technique more vulnerable to electronic noise as well.

Fig. 7.47 Cone-beam artifacts at high-contrast interfaces in the cervical spine (arrows) are hard to appreciate on axial sections (a) but are obvious on coronal reformations (b) from a data set acquired with N x SCITF=4 x 115 and reconstructed with SWIRl =1.25/0.7.

a

b

221

222

7 Image Analysis Fig. 7.48 Image noise and spatial resolution rotate in conjunction with the angular position of the x-ray tube. Close to the position of the tube, image noise is highest and spatial resolution is best (arrow) while opposite, spatial resolution as well as noise are reduced (arrow heads) (a). Image from a 5/10/5 data set. (b) Image 5 mm further distally. Note that now the areas with high and low noise are shifted by 180°.

a

b

o

IVt-LK A. J. van der Molen, M. Prokop

Anatomy

...........•.

225

Oral Cavity (p. 225) Pharynx (p. 225) Larynx (p. 226) Lymph Nodes (p. 227) Lymphoid Tissue (p. 227) Salivary Glands (p. 230) Thyroid and Parathyroid Glands (p. 230) Spatial Approach to Anatomy and Pathology

232

Fasciae (p. 232) Approach to Radiographic Evaluation of Masses of the Deep Face and Neck (p. 232) Spaces of the Suprahyoid Neck (p. 232) Spaces of the Infrahyoid Neck (p. 240) Masses Involving Multiple Cervical Spaces (p. 241) Examination Technique Cystic Lesions Thyroglossal

242 .

245

Duct Cyst (p. 245)

Branchial Cleft Cysts (p. 245) Thymic Cyst (p. 246) Cystic Hygroma/Lymphangioma (p. 246) Dermoid and Epidermoid Cyst (p. 247) Laryngocele (p. 247) Tornwaldt's Cyst (p. 247) Benign Tumors

248

Pseudotumors and Tumor-Like Lesions (p. 248) Neurogenic Tumors (p. 248) Mesenchymal Tumors (p. 249) Juvenile Angiofibroma (p. 250) Benign Tumors of the Larynx and Hypopharynx (p. 250) Malignant Tumors

.

251

Lymph Node Staging (p. 251) Nasopharyngeal Carcinomas (p. 252) Oropharyngeal Carcinomas (p. 253) Oral Cavity Carcinomas (p. 254) Hypopharyngeal Carcinomas (p. 256) Laryngeal Carcinomas (p. 258) Continue>

Lymphoma (p. 261) Post-Transplantation

Lymphoproliferative Disorder (PTLD) (p. 262)

Malignant Mesenchymal

and IMeurogenic Tumors

Sarcoma (p. 262) Salivary Gland Pathology Cysts (p. 263) Benign Tumors (p. 264) Malignant Tumors (p. 265) Inflammatory and Autoimmune Lesions (p. 266) Granulomatous Diseases (p. 267) Thyroid and Parathyroid Gland Pathology Goiter (p. 268) Thyroid Cyst (p. 268) Benign Thyroid Tumors (p. 268) Malignant Thyroid Tumors (p. 269) Parathyroid Tumors (p. 270) Inflammatory Thyroid Lesions (p. 271) Infectious

Diseases

Cellulitis and Abscess (p. 271) Tuberculous Adenitis (Scrofula) (p. 272) Necrotizing Fasciitis (p. 272) Miscellaneous Diseases with Adenopathy (p. 272) Fungal Infections (p. 273) Vascular lesions

.

Jugular Venous Thrombosis (p. 273) Arterial Thrombosis and Dissection (p. 273) Trauma Edema and Hematoma (p. 274) Vascular Injuries (p. 274) Laryngotracheal Injuries (p. 274) Post-Treatment

Neck

.. ,

Larynx Surgery (p. 275) Neck Dissections (p. 277) Radiation Therapy (p. 277)

Pharynx CT is an established imaging modality for examination of the head and neck (Table 8.1). but is increasingly being superseded by magnetic resonance imaging, especially in cooperative patients. A simple rule of thumb is that CT is preferable for inflammatory pathology. while MRI is preferred for tumor imaging. For tumor imaging, MRI is currently more important in the higher part of the neck while helical cr is still the preferred modality in the lower part, where more moving structures are present. This rule is, of course, prone to change with new developments in technology, cost and availability, and radiologist experience. The development of short bore magnets, fast imaging sequences, phased array neck coils, and dedicated protocols have led to an increased use of MRI, while with the advent of multislice helical CT some of the indications may shift back to cr. CT has advantages and should be used as the primary modality in several circumstances: • intensive care patients: shorter examination time, better monitoring; • severely debilitated patients: shorter examination time, less motion artifact; • patients with suspected destructive bone lesions or bone erosion.

Table 8.1 neck

Major indications for CT examination of the Detection, characterization, TNM-staging, bone erosion

I \lnfeGtion

Extent of infectious processes, abscess formation, differentiation of infection from tumor

{Ongenita""diseases

Evaluation of extent, characterization

lraUJT1a

Laryngotracheal injury, vascular injury

All in all, in many patients CT and MRI are complementary imaging modalities rather than competitive ones, with MRI the primary modality for tumor imaging in the upper neck. Modern ultrasonography also has a major role in imaging the neck. This is a primary modality for evaluating the superficial parotid and tile thyroid glands, Combined with ultrasoundguided fine needle aspiration it is probably the most reliable technique for evaluation of lymph node staging, although the retropharyngeal nodes are still difficult to assess with this technique.

Anatomy Most cervical structures are arranged symmetrically with respect to the midline. This facilitates side-to-side comparison, and soft-tissue asymmetry may provide the only radiological clue to the presence of a pathologic neck mass.

The anterior tongue consists of three intrinsic muscles (genioglossus. styloglossus. hyoglossus) that are supported by the floor of the mouth. The floor of the mouth is U-shaped and lined with mucosa and is formed by the mylohyoid and the anterior belly of the digastric muscles.

Oral Cavity The oral cavity is the anterior part of the oral aerodigestive tract and is separated from the oropharynx by a ring formed by the junction of the hard and soft palates, circumvallate papillae, and anterior tonsillar pillars. It contains the anterior two-thirds of the tongue, lips, hard palate, buccal mucosa, gingiva, mandible, maxilla, and the floor of the mouth. The major types of soft tissue found here are muscle, fat, and lymphoid tissue.

225

Pharynx The air-containing structures of the pharynx are traditionally divided into three levels, which are still routinely used in the description of squamous cell carcinomas and other tumors (Fig.8.1).

•

8 Neck

middle constrictor muscles. The oropharynx contains the base of the tongue and lingual tonsils in the anterior part and the faucial tonsils laterally, which are part of the Waldeyer's ring (see Fig. 8.3 b,c; Fig.8.4a,b). Hypopharynx

Fig. 8.1 Sagittal anatomy of the pharynx. The neck is commonly subdivided in suprahyoid (SHN) and infrahyoid parts (IHN). NC= nasal cavity; NP= nasopharynx; OC=oral cavity; OP=oropharynx; H = hyoid bone; HP = hypopharynx; L= larynx.

The hypopharynx stretches from the valleculae superiorly to the lower edge of the cricopharyngeal muscle inferiorly. There are three important subsites of the hypopharynx. Piriform sinus. This is formed by invaginations between the aryepiglottic folds medially and the thyroid cartilage laterally and anteriorly (see Fig. 8.3d). Its inferior tip reaches to the level of the cricoarytenoidjoints (true vocal cords). Postcricoid area or pharyngoesophageal junction. This is the anterior wall of the hypopharynx at the level of the cricoid. This site is difficul t to evaluate with cross-sectional imaging techniques. Posterior hypopharyngeal wall. This is composed of mucosa and the constrictor muscle.

Nasopharynx

The nasopharynx is the upper part of the pharynx. It is bordered superiorly by the sphenoid sinus, inferiorly by a horizontal line through the soft palate (see Fig.8.4c), anteriorly by the nasal choanae and posteriorly by the posterior pharyngeal wall. A landmark on the lateral wall is the cartilaginous end of the eustachian tube (torus tubarius). The pharyngeal recess (fossa of Rosenmuller) is located posterior and superior to the torus, which is difficult to evaluate clinically and may harbor malignancy (see Fig.8.3a). The adenoids are situated on the posterosuperior wall (roof) and are the most superior part of the Wakleyer's ring of lymphoid tissue. Oropharynx

The oropharynx stretches from the soft palate superiorly to the valleculae inferiorly. Anteriorly, it is separated from the oral cavity by a ring formed of the junction of the hard and soft palate, circumvallate papillae and anterior tonsillar pillars, and posteriorly by the superior and

Larynx

The soft tissue structures of the larynx are arranged around a cartilaginous framework of the cricoid, thyroid, and arytenoids. The epiglottic cartilage serves as "lid" for the laryngeal "box" (Fig.8.2b) . The thyroid cartilage has protective laminae that meet at an angle anteriorly (Fig. 8.3 e). From the posterior part of the laminae, inferior horns extend downward to articulate with the cricoid and superior horns stretch toward the hyoid bone providing attachment for the thyrohyoid ligaments. The cricoid cartilage is a complete ring. Its inferior border represents the junction of the larynx with the trachea. The greatest part or lamina is located posteriorly and its superior part lies at the level of the vocal cords. The paired, pyramidal arytenoid cartilages sit on top of the cricoid lamina (Fig.8.3e) and serve to move the vocal cords during phonation. All are made of hyaline or elastic cartilage and start to OSSifyaround the age of 20. The true vocal cords span the airway from anterior to posterior and are the inferior part of the vocal cord-ventricle complex. Anteriorly,

Lymphoid Tissue

,----

Epiglottis +---

Fig. 8.2 Corona] (a) and sagittal (b) anatomy of the larynx.

Hyoid bone

Supraglottis

.-+-f,Z-If---

Cricoid cartilage

Subglottis

a

o

Vocal cords and ventricle

Trachea

--tlt-----

b

they converge into the anterior commissure. The false cords are slightly more superior and are the inferior continuation of the aryepiglottic folds. The ventricles are clefts that also run anteroposterior and that curve upwards to a level above the false vocal cords (Fig. 8.2, Fig. 8.4b). The leaf-shaped epiglottis is the flexible anterosuperior part of the larynx and is separated from the hypo pharyngeal piriform sinuses by the aryepiglottic folds (Fig.8.3d). It is made of fibrocartilage and seldom shows any significant calcification. Its function is to protect the airway during swallowing. It is connected to the oropharynx by the anterosuperior pharyngoepiglottic folds and is also attached to the tongue by the glossoepiglottic fold in the midline. Anterior to this "lid" is the preepiglottic fat behind the anterior thyrohyoid membrane. The larynx is routinely subdivided into three major subsites. • The supraglottis extends from the tip of the epiglottis to the laryngeal ventricles. It contains the epiglottis, pre-epiglottic fat, aryepiglottic folds, false cords, arytenoids, and ventricles. • The glottis contains only the true vocal cords and anterior-posterior commissure. • The subglottis extends from the undersurface of the true vocal cords to the inferior edge of the cricoid. It contains only the mucosa lining the cricoid ring. Lymph Nodes

Approximately 300 of the 800 lymph nodes of the body are located in the neck. Although there

are many classifications, the most widely used are the ones of the Union Internationale Contre Ie Cancer/American Joint Committee on Cancer (LJICC/ AJCC) and the American Academy of Otolaryngology, Head and Neck Surgery. Very recently, an imaging-based variant based on these has been introduced-see Chapter 18. In the present chapter, the original regional classification of the American Academy of Otolaryngology, Head Neck Surgery (1991) is used. In this classification nodal groups, like the facial, OCCipital, retropharyngeal, and parotid nodes, are not included and should always be mentioned separately. The major nodal groups are classified into six levels: Level I: Level Level Level Level Level

Submental and submandibular groups II: Upper jugular group-skull base to hyoid II I: Middlej ugular group-hyoid to omohyoid muscle IV: Lower jugular group-omohyoid muscle to clavicle V: Posterior triangle group VI: Parapharyngeal and paratracheal groups

Lymphoid Tissue

Lymphoid tissue is primarily located in Waldeyer's ring, which is composed of the palatine and lingual tonsils, faucial tonsils, and the adenoid. The adenoid lies high in the nasopharynx and may also extend into the lateral fossae (Rosenmuller). The adenoid normally involutes

227

8 Neck

Maxilla Maxillary sinus -----Pharynqealorifice eustachian' tube

~---

_

[Medial [pterygoid process

/ ~-zygomaticbone

of ~

/"

~Temporali.slmuscle

Massetenrnuscle ~

<,

/

Eustachian tube ~

~

~ »<:>:

Lateral [pterygoid [process

/

Lateral pterygoid imuscle ~ Pharyngealrrecess lnternal.carotid.artery

~

------

[Parotid gland

------------

Styloid process

------""--+~'--7/~

Glossopharynqeajnerve

h~~~(

r-------

Mastoid process --------( Rectus capitis anteriorunuscle Sigmoid sinus----------./ Erector spinae muscles

Iinternal jugular vein (bulb)

'---------

~

Hypoqlossal nerve

'------------.------

---------=~~~~~~7r'

~===--

Maxilla --------------------..

Clivus

'----------Splenius

Perioralmuscles

~-----------

[Hard palate

~---------

Medial pterygoid muscle ------,

[Parotid duct

Iinferior alveolar nerve. lingual nerve

Masseter I muscle . ---------~

---------

Cranial nerves IX and X -----~

-----..---------

_

artery I

(posterior 'belly)

----\:-1'-+-.11-

[Mandible

Styloid process and styloid muscles

---______.

Internal jugular vein -----

Digastric imuscle

'capitis muscle

-----r-----------

Palatine tonsil, superior -------, pharyngeal, constrictor imuscle

Vertebral

Mandibularcondyle

--------

------_J

Vagus nerve -.--------~-/

Retromandibularvein

lLongus capitis unuscle

--

[Parotid gland

0f~--'2I-~-+f-----

[Internal carotid artery and cranialnerve Xll

'-----

Lonqus.capitis muscle

Longissimus, capitis muscle -.------\,.,\ Obliguus capitis i inferior .and :superior unuscle

Sternocleidomastoid

Deep cervica Ivein ----_-_ -_-_-_ -_-_-_-_ -_-_-_-•••-.,. -§-~-§, -~ -§.\~~\~~~~~~~~ Erector spinae muscles

Depressor anguli 'Orisimuscle -------

_

'---------

Atias(lateral

'--------

..---------------------------..-------

muscle mass)

[Dens of axis

[Mandible (body) Genioqlossusrmuscle [Mylohyoid muscle [Hyoglossus muscle

Masseter muscle -------------.... Submandibular gland -- ••••• ---------...,. Digastric Imuscle I (posterior [belly) ----I, Auriculotemporal nerve (branch) --~ and retromandibularvein Internal carotid artery --------/·---'!1L--F

4---'-."""'-+-1------

Internal jugularvein Levator scapulaemuscle

Trapezius I muscle

•••••

----------\'\\\'

r---f----

Spleniuscervicis

Sternocleidomastoid

muscle muscle

'------------

Vertebral artery

'------------

Axis (body)

7------------

Fig. 8.3 Axial cross-sectional anatomy of the neck. (a) nasopharynx; (b) oropharynx; popharynx-sinus piriformis; (e) larynx-vocal cords; (f) trachea-thyroid gland.

Spinous process

(c) floor of the mouth; (d) hy-

Lymphoid Tissue

~------

Epiglottis ----------______.

Thyrohyoid I membrane and preepiglottic space

~-----~nfrahyoid

muscles (sternohyoid and sternothyroid)

Platysma ----------______. ~------------

Piriform sinus

Sympathetic trunk ----------.... Carotid artery (bifurcation) Internal jugular vein

-:>.c-~-",y-::::::,,~

------~f----t,::/

..A-~~-\--------

Vagus nerve ----------f---7',\.,..__ Sternocleidomastoid

Inferior pharyngeal constrictor muscle Longuscolli muscle

----~.J

muscle

Anterior scalene muscle

l------

------""'''-\-""\t\

------------

Levator scapulae muscle ------\----t-',----tr-

Vertebral artery

--~;;;;;;:;;;;~~~~;t~~~~~~Z& -----------

Erector spinae muscles

facet joint

C4/C5

'--------

Body of C4 vertebra

d Infrahoid muscles (sternohyoid, -------... omohyoid, sternothyroid)

?~-------r-~-------~----~

Vocalis muscle ----------------.,L~~L/-!-'-I-IJ Cricothyroid joint ----------~~~+:.!..I---1 Common carotid artery --------,.(§ jugularvein -------------(--/--..... Inferior pharyngeall constrictor muscle Sternocleidomastoid

Anteriorlaryngeallcommissure Thyroid cartilage

X'--';p~,------'",,------

~(g'=-----""'~"""..---------Thyroid

gland

~~~~~~~~~~~~~-------~-

--t-t-----t--r--rn..-?

Esophagus --n;i-----~

Longus colli muscle

\'--1-----

Anterior scalene muscle

_

"'--------~

muscle

Vertebral artery

"----C5/C6 facet joint

Levatorscapulae muscle

----...d.~~~~~~~~s5f~~~:z "------

Erector spinae muscles

Cricoidlcartilage(lamina)

~~~-:::;:?!7/

L_--------

Body of C5 vertebra Nuchal ligament

Trapezius muscle

Sternohyoicd muscle ---------

_

SternothyroidJ muscle ----------_...... Cricoid cartilage

---------------;;~»<

Platysma .-------------

Esophagus

'x--------

Cornrnoru carotid artery

Thyroid cartilage, inferior cornu ---~~~...:, Thyroid glan~ Intern alj ugu Iarv ein -------f/;(-f---r:;;;-w-: Vagus nerve ------------I'!ffl--+--"7'---1 Vertebral artery

Sternocleidomastoid

~~~~~~;-T~~~~

Inferior pharyngeall constrictor muscle ~-----

----------r-'-r--T-:-'"ff

Longuscolli muscle

Brachial plexus Anterior scalene muscle

------/---:;?'-=-.J'-f11

Facetjoint ----------<--r----:r--Irw C7 vertebrae -------+--'~'<::_----T'"'II\

'-------

/~--

Fig.8.3d-f

muscle

Medial and posterior scalene muscles Levatorscapulae muscle

Splenius cervicis and capitis muscles

229

8 Neck

with age and in most adults no tissue is found by the age of 30. Incomplete involution may lead to subtle asymmetries that can mimic superficial tumors. The tonsils are usually symmetric soft tissue masses that may be prominent in children and young adults. Tonsils also involute with age. Asymmetry. especially when associated with lymphadenopathy, is suggestive for pathology. After infection dystrophic calcifications can be seen, especially in the faucial tonsils. After contrast injection, lymphoid tissue enhances more strongly than muscle but is hard to differentiate from other soft tissue structures. Gadolinium-enhanced MRI with fat suppression is the imaging technique of choice to image lymphoid tissue and to differentiate lymphoid hyperplasia from tumor.

deep tongue and ends in the sublingual papilla, at the level of the fatty sublingual septum. Usually this duct cannot be seen on CT. The sublingual gland is the smallest gland and lies on top of the mylohyoid muscle against the surface of the mandible in the sublingual space (SLS). Sometimes intraglandular ducts fuse into the sublingual (Bartholin's) duct that joins the submandibular duct just before the sublingual papilla. More than 750 minor salivary glands are situated within the mucosa of the oral cavity and upper aerodigestive tract. They are especially numerous in the buccal, palatal and lingual regions and have the same structure as the major salivary glands. These minor glands can host the same pathologic processes as their major counterparts. On plain or contrast-enhanced CT these glands cannot be distinguished from other surrounding soft tissues.

Salivary Glands The parotid is the largest salivary gland (Fig.8.3a, b). Its superficial lobe makes up 80% of the gland and overlies the ramus of the mandible and masseter muscle. The remaining 20% of the deep lobe extends through the stylomandibular tunnel (see Fig.8.7) adjacent to the parapharyngeal space (PPS). The gland has an equal amount of fat and acinar tissue, resulting in a hypodense aspect at non-contrast CT. With aging, the gland will undergo fatty change. The posterior facial (retromandibular) vein is the CT landmark that defines the course of the facial nerve which runs laterally to the vein. It marks the division of the deep and superficial lobes of the parotid. The parotid (Stensen's) duct (Fig.8.3b) emerges from the anterior part of the gland as a thin, soft-tissue dense band, courses over the masseter and buccal fat pad and pierces the buccinator muscle at the level of the second upper molar. The gland has 20-30 intraparotid lymph nodes which drain into the internaljugular chain of nodes. The submandibular gland is located in the floor of the mouth and curves around the free posterior edge of the mylohyoid muscle (Fig. 8.3 c, 8.4 b). In comparison to the parotid, the submandibular gland has less fat and a higher density on non-contrast CT. The submandibular (Wharton's) duct exits from the deep part of the gland, courses obliquely through the

Thyroid and Parathyroid Glands The thyroid gland is horseshoe shaped and located anterolateral to the larynx and trachea (Fig. 8.3 f). Its size varies, but on average each lobe measures approximately 6x4x2 em. A minority of patients have a central thyroid isthmus. From it the midline pyramidal lobe may arise that ascends anteriorly to the thyroid cartilage. Because of its high iodine content, the thyroid is relatively hyperdense (60-80 HU) to the surrounding soft tissues on non-contrast CT. The parathyroid glands are located posteriorly to the upper and lower poles of the thyroid lobes. Usually there are four glands, but a minority of patients has more. They are frequently not visible with CT, but when visible appear relatively hypodense to the thyroid. The superior and inferior thyroid vessels run posterior to the glands and may be used as markers on the axial CT slices when the glands are not visible.

Thyroid and Parathyroid Glands

Superior orbital fissure (with optic, trochlear" oculomotor" ophthalmic and abducent nerves)

Temporalis muscle

-f-j--------

Pterygopalatine fossa ------_ Lateral pterygoid muscle -----_ Med ial pterygoid muscle ------Parotid gland ----------\ Soft pa Iate ----------~ Tongue

-----------.---l~-rr,~~

Lingual nerve ------------,--m~~\ Mylohyoidl muscle ---------~

~~LC:~Cl3f7L---------Submandibular (S

Platysma ---------------'

Hyoid bone

Geniohyoid muscle ------------~ '--------------

Thyroid cartilage

~---~---

Medial pterygoid muscie -~Mandible(ramus)

gland

Digastric. muscle (tendon)

Y----------

Zygomatic arch Lateral pter-ygoid muscle

-------

--------

Masseter muscle

'?~~*_~:____h;H~/----------

Mylohyoid muscle --------

Soft palate

--~~-~H+-+~-------------Tongue Submandibular gland ------Omohyoid muscle ------Thyrohyoid muscle --------

~LL/----------

Hyoglossus muscle

gL-----------

Hyoid bone

~~-I-~---------Laryngeal ventricle -------------\--+''--1+Sternohyoid muscle

Sternocleidomastoid

Vestibular ligament Vocal ligament

~1_t_-I-------------

-------------/--+

muscle ---------/+

b

Temporalis muscle

--------1-~

'111iI---f-,.-.,-\------------

Lateral pterygoid muscle ------\---',,--,,~\Medial pterygoid! muscle Mandible (ramus)

Mandible (condyle)

-Ih-+--I--------

-------\-f------I---fl-H--/

----------\-\--\-\1

Uvula ------~-~~-----------_____\-~= Submandibular gland --------~-\

Hyoid bone (greater cornu) Vestibularfold

++!-rt-fofN---/-------

7Jrl11-.r~+-I--------

------------f

--------------------Common carotid artery

Soft palate

.----,'---j-----------------F---1-------

Epiglottis---===------------\--"--+1\ Laryngeal vestibule

Sphenoid sinus

-----------<Je

Vocal cord Hyoid cartilage

Omohyoid, sternohyoid and thyrohyoid muscles

\\"-----------

Fig.8.4

Coronal cross-sectional

anatomy of the neck (a) mid-tongue;

(b) tongue base; (c) pharynx.

Thyroid gland

231

8 Neck

Spatial Approach

to Anatomy

and Pathology

With the increasing use of cross-sectional imaging, the spatial approach to anatomy of the suprahyoid and infra hyoid neck became popular in the late 1980s and early 1990s and is commonly used today in the description of the extent and differentiation of neck masses,

Fasciae

For the description of the fascial spaces, a basic knowledge of the cervical fasciae is needed, There are two main cervical fasciae, the superficial and deep cervical fasciae, The superficial cervical fascia (SCF) is really a fat-filled layer of connective tissue that surrounds the neck and which also contains the platysma, superficial lymph nodes, nerves, and vessels, The deep cetvtcsl Iescis (DCF) consists of three parts: the superficial, middle, and deep layers (Fig, 8.5), The superficial layer of the deep cervical fascia (SLDCF) or the investing fascia also completely encircles the neck and runs from the skull base superiorly to sternum, clavicles, and scapulae inferiorly, It encloses the trapezius and sternocleidomastoid muscles, encircles the parotid space, forms a fascial sling for the inferior belly of the omohyoid muscle and contributes anterolaterally to the carotid sheath, The middle or buccopharyngeal (visceral) layer ofthe deep cervical fascia (MLDCF) is more variable, It also runs from skull base and is inferiorly continuous with the pericardium, The muscular layer envelops the infrahyoid strap muscles and the visceral part encloses the thyroid and parathyroid glands, larynx, and the trachea and so forms the visceral space, It is the anterior boundary of the retropharyngeal space and is also part of the carotid sheath laterally, The deep or prevertebral layer of the deep cervical fascia (DLDCF) runs from the base of the skull and merges inferiorly with the MLDCF and then continues into the mediastinum, It encircles the prevertebral and paraspinous muscles as well as the trunks of the brachial plexus, phrenic nerve, and vertebral vessels, It attaches to the transverse processes of the vertebrae and forms the anterior boundary of the prevertebral space, Laterally it contributes

to the margins of the retropharyngeal space and carotid sheath, Based on these fascial boundaries, a number of spaces can be distinguished, divided into spaces of the suprahyoid neck and infra hyoid neck, Four spaces are common to both the supra- and infra hyoid region, these are the carotid space (sheath) and the retropharyngeal, prevertebral, and posterior cervical spaces (Fig, 8.5),

Approach to Radiographic Evaluation of Masses of the Deep Face and Neck After identification ofa mass in the neck, a stepwise approach is taken: L Is the lesion a real mass or a pseudomass? 2, What is the space of origin of the mass: • Where is the epicenter of the mass? • In what directions are the surrounding structures or spaces displaced? 3. Evaluate the radiographic features of the mass, including: • Extension of the mass; • Is there erosion or invasion of the surrounding bony and cartilaginous structures? • Is there perineural extension of the mass? 4, Combine clinical and imaging features with space-specific differential diagnosis; 5, Provide the clinician with the most likely differentials with reasons (maximum three),

Spaces of the Suprahyoid Neck Parapharyngeal Space (PPS) The PPS has an inverted pyramidal form that extends from the base of the skull to the hyoid bone and lies anterior to the styloid process, Anteroinferiorly it is continuous with the submandibular space, Primarily composed of fat, it is the key space in the suprahyoid neck because PPS displacement can be used in assigning a lesion to a space of origin,

Spaces ofthe Suprahyoid Neck Fig. 8.5 Layers of the deep cervical fascia and spatial anatomy of the neck. (a) axial: suprahyoid neckoropharynx; (b) axial: infrahyoid neck-thyroid gland; (c) coronal: mid-tongue.

ACS = anterior cervical space BS= buccalspace CS= carotid space MS = masticator space PS= parotid space PPS = parapharyngeal space PMS = pharyangeal mucosal space PCS = posterior cervical space PVS= prevertal space AC= anterior compartment PC = posterior compartment RPS = retropharyngeal space SZMS = suprazygomatic partof masticator space

Sl.Ss sublinqual space SMS = submandibular space VS= visceral space

BS PMS MS RPS

BS MS ACPVS PS PPS CS

PPS PS

PCS

CS PCS

PC PVS *

PC PVS

:+:

:+:

a

ACS

VS ACS

CS

CS ACPVS PCS

PCS

PC PVS

b

I----SZMS

1=-1-----

MS-----\

--='-¥I:-----;l+--IHI------

MS PMS

I-----PPS SLS-----\~~~~ SMS-----~-~

I------SLS ,/-------

-

c

'I~ -

"*"

SMS

Superficial Layer Deep Cervical Fascia Middle Layer Fascia Deep Cervical Deep Layer Deep Cervical Fascia Dangel- Space

----_.

--

233

The contents of the PPS are:

The PPS is sandwiched between many other spaces: medially it is separated from the pharyngeal mucosal space, laterally it is adjacent to the masticator and parotid spaces and posteriorly it is bounded by the carotid sheath. Intrinsic masses from the PPS like neurinomas of the submandibular nerve (Vs) or minor salivary gland carcinomas are rare, but this space can act as an elevator shaft for tumors or infections either to the submandibular space or the skull base. Most common lesions result from a secondary extension of pathology originating in neighboring spaces (Figs. 8.6, 8.7; Table 8.2).

fat

ascending pharyngeal artery (from the external carotid artery) branches of the submandibular nerve (Vs) pharyngeal (pterygoid) venous plexus internal maxillary artery (from the external carotid artery) minor salivary gland rests salpingopharyngeus muscle.

PMS-----I-T~~ MS-----...

~---MS I-I~---RPS -I-----Ps

PS----\

PPS CS

I--J----~---I-----

CS ------./

\.!.......-+-------I----

AC

rvs

PC

rvs

.'::.L-I---.I------

Fig. 8.6 Patterns of spread of masses in suprahyoid neck spaces. Left: masticator space mass. Right: retropharyngeal space mass. Legends see Fig. 8.5

PPS--~ ~---MS

MS----------

-i-----PS PS----I

cs-----, RPS -----\--'\---(

PPS

1-1-----

~-I-----cs ~/--'/----.. ~L_/_-----

rvs PC pvs

AC

Fig. 8.7 Patterns of spread of masses in suprahyoid neck spaces. Left: parotid space mass. Right: pharyngeal mucosal space mass. Legends see Fig. 8.5

Spaces

Table 8.2 Differential space lesions

diagnosis

Pseudotumor:

Asymmetric plexus

Cystic lesions:

Atypical

Inflammatory lesions:

Malignant tumors:

parapharyngeal

pterygoid

second

venous

branchal

cleft cyst

Abscess spread from adjacent deep cervical spaces: adenoidsltonsils (PMS), odontogenic

Benign tumors:

of

(MS),

parotid

Pseudotumor:

Cystic lesions: Inflammatory lesions:

from

Mucoepidermoid and adenoid cystic carcinoma of salivary gland rests Direct spread of tumor from adjacent spaces: squamous cell carcinoma, nonHodgkin's lymphoma (PMS) or adenoid (PMS, PS)

diagnosis

of

Neck

pharyngeal

mu-

Asymmetric fossa of Rosenmuller Infectious or radiation-induced pharyngitis T omwaldt's Adenoidal abscess

cyst or faucial

Postinflammatory

(PS)

Lipoma Pleomorphic adenoma savilary gland rests

sarcoma (MS) mucoepidermoid cystic carcinoma

Table 8.3 Differential cosal space lesions

of the Supra hyoid

tonsillitis retention

or cyst

Benign tumors:

Pleomorphic adenoma salivary glands

Malignant tumors:

Squamous cell carcinoma (SCCa) of the pharyngeal mucosa Non-Hodgkin's Mucoepidermoid

from minor

lymphoma or adenoid

cystic

The most important intrinsic masses in the PMS are the squamous cell carcinomas arising from the mucosal surfaces of the nasopharynx, oropharynx, and hypopharynx (Fig, 8,7: Table8,3),

Pharyngeal Mucosal Space (PMS) The PMS includes the mucosal surfaces and submucosa of the nasopharynx, oral cavity, oropharynx and hypopharynx, This mucosa is lined with pseudostratified columnar or stratified squamous epithelium which can be the origin of squamous head and neck cancers, The PMS is surrounded posterolaterally by the middle layer of the DCF, which superiorly also encloses the posterior aspect of the pharyngobasilar fascia (PBF), This fascia attaches the pharynx and superior constrictor muscle to the skull base, The levator veli palatini muscle and the eustachian tube pass through the PBF superolaterally via the sinus of MorgagnL This provides a route of spread for nasopharyngeal tumors to the skull base, The contents of the PMS are: pharyngeal mucosa lymphoid tissue: tonsils, adenoids minor salivary glands torus tubarius superior and middle constrictor muscles palatoglossus muscle palatopharyngeus muscle levator veli palatini muscle salpingopharyngeus muscle pharyngobasilar fascia

Parotid Space (PS) The PS is located posterior to the masseter muscle and mandible and is lateral to the PPS, It is enclosed completely by the split layers of the superficial layer of the DCF, The posterior belly of the digastric muscle can indent the parotid from a posteromedial direction and can help in differentiating superficial lobe lesions from deep lobe lesions of the parotid, The contents of the PS are: • parotid gland • facial nerve (lateral to the retromandibular vein) retromandibular vein • intra parotid lymph nodes (20-30) • external carotid artery (medial) • parotid duct (Stensen's duct), Parotid gland masses are most important here and are termed intra parotid masses if> 50% of the circumference of the lesion is surrounded by normal parotid gland tissue, These lesions often extend through the stylomandibular tunnel, which can be widened by the mass (Fig, 8,7; Table 8.4),

8 Neck Table 8.4 sions

Differential

diagnosis

of paratoid

space

le-

Table 8.5 Differential diagnosis sions (suprahyoid and infrahyoid) Pseudotumors:

Pseudotumors:

Masseter hypertrophy Accessory parotid gland

Cystic lesions:

First branchial Benign

Lymphangioma

(children)

Parotitis

lesions:

Reactive adenopathy Sjogren's syndrome

Benign tumors:

or parotid

gland

Malignant

Hemangioma (children) Pleomorphic adenoma Warthin's tumor (cystadenoma

Carotid artery thrombosis, dissection, (pseudo)aneurysm IJV thrombosis or thrombophle-

node metastases from skin, breast, or lung

cancers Perineural tumor facial nerve

spread

along

Benign

Iymphoepithelial

cleft

bitis Cellulitis

Reactive or suppurative nopathy Tuberculous adenitis

or abscess

Benm

Paraganglioma: carotid body, glomus vagale or glomus jugulare Cranial or sympathetic nerve schwannoma, neurofibroma Neuroblastoma Meningeoma (via jugular

Malignant tumors:

ade-

foramen)

Encasement from direct infiltration by SCCa Lymph node metastases from SCCa, thyroid, melanoma Hodgkin's and non-Hodgkin's lymphoma Lymph node metatases and abdominal

lesions

Cysticlesions:

branchial

Inflammatory

cystic carcinoma

lymphoma-primary and metastatic Lymph SCCa,

second

lesions:

tumors:

Adenocarcinoma Non-Hodgkin's

Iftnultifocal

Atypical

Mucoepidermoidcarcinoma Adenoid

Asymmetric carotid bulb or ectatic carotid artery Asymmetricjugular vein

Cystic lesions:

abscess

Iymphomatosurn) Facial nerve neuroma

tumors:

le-

cyst cyst

(AIDS) Inflammatory

space

Vascular lesions:

cleft cyst

Iymphoepithelial

of carotid

from lung

carcinoma

cyst

(AIDS) Inflammatory lesions:

Sjogren'ssyndrome

Benign solid

Warthin's

lesions: Malignant lesions:

tumor (cystadenoma

Iymphomatosum) solid

Non-Hodgkin's

lymphoma

-

metastatic Lymph node metastases from SCCa, skin, breast, or lung cancers

Carotid Space (CS) The two carotid spaces run from the jugular foramen at the skull base to the aortic arch and are thus both suprahyoid and infrahyoid neck spaces. They are located posteromedial to the PPS and lateral to the retropharyngeal space (RPS). All three layers of the DCF condense into the thick carotid sheath. which prevents transgression of disease. The contents of the CS are: • common/internal • internaljugular

carotid artery vein (IJV)

• cranial nerve IX, XI, XII (upper part) • cranial nerve X (upper and lower parts) • sympathetic plexus • deep cervical lymph nodes. Lesions in the CS typically abut the carotid artery or jugular vein and are tubular or fusiform in shape. The most important lesions are the lymph node metastases of the deep cervical chain (levels II-IV) in adults and neurogenic tumors in children (Table 8.5). The lymph nodes of the neck are more extensively addressed in Chapter 22.

Masticator Space (MS) The masticator space is anterior to the PS, anterolateral to the PPS, and posterior to the buccal space (8S). It is most extensive in the craniocaudad direction and reaches from the superior border of the temporalis muscle to the inferior edge of the mandible. It can be thought of as

Spaces of the Supra hyoid Neck

two continuous parts, the suprazygomatic MS and the naso-oropharyngeal MS, This is important for imaging and axial slices should always continue above the zygomatic arch when delineating this space (Fig, 8,9), The MS is surrounded by two slips of the superficial layer of the DCF, which ends superiorly at the skull base medial to the foramen ovale. Through this elevator shaft, MS lesions can communicate with the intracranial structures (cavernous sinus), There is also a possibility of communicating with the pterygopalatine fossa through the pterygomaxillary fissure superomedially. The contents of the MS are: ramus and posterior body of mandible masseter muscle medial and lateral pterygoid muscles temporalis muscle inferior alveolar artery and vein masticator nerve branches (Vs) inferior alveolar nerve branches (Vs).

Differential

of masticator space

diagnosis

Pseudotumors:

Accessory parotid gland Hypertrophic masseter muscle Atrophy of mastication muscles

Inflammatory lesions:

Odontogenic abscess

Benign tumors:

Hemangioma Lymphangioma Leiomyoma Myositis ossificans Schwannoma, neurofibroma

from

Malignant tumors:

V3 de

nervation

Osteomyelitis

Malignant

of the mandible

fibrous

rhabdomyosarcoma,

histiocytoma, leiomyosar-

Chondrosarcoma, osteosarcoma Malignant schwan noma Perineural tumor spread along VB nerve Non-Hodgkin's lymphoma Squamous cell carcinoma of oropharynx

(retromolartrigone)

Differential

Pseudotumors:

diagnosis

Accessory

Cystic lesions: Inflammatory lesions:

Sebaceous

of buccal

parotid

space

lesions

gland

cyst

Spread of infection from adjacent spaces (MS) Reactive lymphadenopathy

Benign tumors:

Hemangioma Lipoma Fibromatosis

Malignant tumors:

SCCa-direct invasion or lymph mode metastases Non-Hodgkin's lymphoma Mucoepidermoid or adenoid cystic carcinoma from minor glands Soft tissue sarcomas

Buccal Space(BS)

Sarcomas (soft tissue and chondroand osteosarcomas) are the most dangerous lesions here and they can spread perineurally to the skull base via the masticator and inferior alveolar nerves (Table 8,6),

Table 8.6

Table 8.7

le-

The BS is a horizontal elongated space along the anterior part of the parotid duct, and is not a true fascially defined space, It lies between the buccinator muscle medially and the superficial muscles of facial expression laterally (Fig,8,5a), This space is almost entirely filled with fat that has some finger-like projections: laterally along the parotid gland, medially along the medial side of the mandible and superiorly along the both sides of the temporalis muscle, The contents of the BS are: buccal minor parotid facial buccal nerves buccal

fat pad salivary glands duct (Stensen's duct) artery and vein branches of facial and mandibular lymph

nodes,

The commonest lesions in the space are squamous cell carcinomas in buccal lymph nodes or that have spread from other spaces such as the MS and submandibular space (SMS) (Table 8,7), Minor salivary gland tumors occur infrequently,

Retropharyngeal

Space

(RPS)

The RPS is a midline space posterior to the airway that runs from the base of the skull to the upper mediastinum, It is sandwiched between

237

8 Neck

the pharyngeal mucosal space and the prevertebral space and is bounded by the middle layer of the DCF anteriorly and the deep layer of the DCF posteriorly. Its lateral walls are made of slips of the DCF called the alar fasciae. The anterior extension of the alar fascia divides this space in two parallel subspaces. the posterior of which has been referred to as the "danger space" (Fig.8.4c). This danger space continues even further downward. to the diaphragm. Both these spaces form natural conduits for spread of infection or tumors from the neck region into the (upper) mediastinum or skull base. The contents oftheRPSare: • lymph nodes (only suprahyoid) e fat.

The retropharyngeal nodes are important in staging SCCa. The lateral chain (nasopharynx and high oropharynx) can be visualized on imaging and contains the nodes of Rouviere, while the medial chain (nasopharynx to hyoid bone) is normally not visible on imaging studies. Besides lymph node metastases, frequent lesions are edema following radiation therapy or trauma, and direct invasion of the RPS from posterior extension of nasopharyngeal SCCa (Fig. 8.6; TableS.S).

Table space

Differential diagnosis of retropharyngeal lesions (suprahyoid and infra hyoid)

Prevertebral Space (PVS)

The PVS is also common to the suprahyoid and infrahyoid neck. It is a midline space from skull base to coccyx that lies posterior to the RPS. It is bounded anteriorly and posteriorly by the DLDCF which is attached to the transverse and spinous processes of the cervical vertebrae forming two compartments: the anterior compartment (AC) or PVS proper and the posterior compartment (PC) or paraspinal PVS. The corpus and pedicles of the vertebrae as well as the intervertebral disks are part of the anterior compartment, while the spinous process and laminae are part of the posterior one. The contents ofthe PVS are: prevertebral muscles vertebral artery and vein scalene muscles proximal brachial plexus cervical vertebrae and disks (posterior) paraspinous muscles (posterior) phrenic nerve (posterior) muscle motor nerve branches (posterior).

Table 8.9 Differential diagnosis of prevertebral lesions (suprahyoid and infra hyoid) Pseudotumors:

Degenerative lesions of the vertebrae and facet joints Cervical disk herniation

Inflammatory lesions:

Vertebral body osteomyelitis diskitis (pyogenic, TB)

8.8

Pseudotumors:

Ectatic carotid artery Edema secondary to radiation or (internal jugular) venous obstruc-

Inflammatory lesions:

Cellulitis or abscess and adenoids)

tion

Reactive pathy Benign tumors: Malignant tumors:

extension

Benign tumors:

of SCCa

Post-traumatic

Air

lesions:

Edema,

Malignant

Malignant

vical spine Vertebral body or epidural

bone tumors

of the cer-

metastasis: Lung, breast, prostate, thyroid and renal cell primaries Direct extension of SCCa into the

from

PVS Non-Hodgkin's

lymphoma-sec-

ondary Rhabdomyosarcoma

leukemia

hematoma

Chordoma Osteoblastoma, osteochondroma, ABC, giant cell tumor

tumors:

neighbouring spaces (PMS) Lymph node metastases from (nasopharyngeal) SCCa Lymph node metastases from melanoma, thyroid carcinoma Lymphoma,

or

colli tendinitis

Brachial plexus schwannoma/neurofibroma

adeno-

Hernangioma Lipoma Direct

Longus

(from tonsils

or suppurative

space

Post-traumatic lesions:

Edema,

hematoma

(paediatric)

Spaces of the Suprahyoid Neck

In the anterior compartment, lesions typically displace the pre vertebral muscles anteriorly, In the posterior compartment the fat of the posterior cervical space is displaced posterolaterally (Table 8,9), Bony lesions of this space are clinically the most important.

Sublingual Space (SLS) The teacup-shaped SLS is located deep in the oral tongue, superomedial to the mylohyoid muscle (the floor of the mouth) and lateral to the genioglossus-geniohyoid muscles, The space is not bounded by fascial structures and is posteroinferiorly continuous with the posterosuperior part of the submandibular space, For this reason lesions can easily spread out to the submandibular region, The contents of the SLS are: anterior hyoglossus muscle lingual artery and vein cranialnerves IX, XII lingual nerve(Vs) deep lobe of submandibular gland submandibular gland duct (Wharton's su blingual glands and ducts siiblinguallymph nodes,

tongue base from posterior to anterior ble 8.10), Spreading odontogenic infections commonly involve this space,

Submandibular

(Taalso

Space (SMS)

The horseshoe-shaped SMS lies above the hyoid bone and inferolateral to the mylohyoid muscle, The anterior part is bounded by fascial slips from the SLDCF. In the posterior part are no fascial boundaries, and here the SMS is continuous with the SLS and more superiorly with the inferior PPS,This creates a potential route from the submandibular region to the skull base, At the level of the hyoid bone the SMS is continuous with the anterior cervical spaces, The contents of the SMS are: anterior belly of the digastricus muscle facial artery and vein inferior loop of hypoglossal nerve superficial lobe of the submandibular gland submandibular and submental lymph nodes fat

duct)

The space is important in SCCa of the tongue, The tumors of the anterior tongue invade the SLS from superior to inferior and those of the

Congenital cystic lesions, submandibular gland lesions, and nodal metastases represent the commonest lesions in this area, depending on the age of the patient (Table 8,11), It is rare for lesions in the SMS to extend up into the SLS,

Table 8.11 Differential diagnosis of submandibular space lesions Table 8.10 lesions

Differential

diagnosis

of sublingual

space

Pseudotumors:

Pseudo-

Tongue

tumors:

poglossal nerve injury Lingual thyroid tissue

muscle

atrophy

from

hy-

Cystic lesions: Lymphangioma- cystic hygroma Ranula or diving ranula Dermoid-epidermoid Inflammatory lesions:

Cellulitis or abscess genie focus

from

odonto-

from calculi

Hemangioma

tumors:

Pleomorphic

adenoma

lesions:

Cellulitis

or abscess

Reactive adenopathy

sec

Invading a from anterior tongue or tongue base

Mucoepidermoid or adenoid cystic carcinoma (sublingual gland)

gland

inflamma-

tion duct from calculi Benign

Hemangioma

tumors:

Lipoma Pleomorphic adenoma (submandibular gland, parotid tail)

Malignant tumors:

Lymph node metastasis of face or oral cavity

(sublingual

gland) Malignant tumors:

Inflammatory

Second branchial cleft cyst Suprahyoid thyroglossal duct cyst Cystic hygroma-lymphangioma Diving ranula Epidermoid-dermoid

Submandibular

Ludwig's angina Dilated submandibular gland duct Benign

Cystic lesions:

Atrophy of digastric and mylohyoid muscle (V3 motor injury)

from SCCa

Lymphoma Mucoepidermoid or adenoid cystic carcinoma (submandibular)

239

8 Neck Table 8.12

Spaces of the Infrahyoid Neck Visceral Space (VS)

The miclline VS is the only space that is unique to the infrahyoid neck and is completely enclosed by parts of the MLDCF or the visceral fascia. It lies anterior to the RPS and PVS proper and superomedial to both carotid spaces. For practical purposes, the space can be further subdivided in four areas: laryngeal, thyroid, parathyroid, and esophageal areas. The contents of the VS are: thyroid and parathyroid glands paratracheal lymph nodes recurrent laryngeal nerve hypopharynx larynx cervical esophagus trachea. The visceral space is clinically most important thyroid and parathyroid lesions and squamous cell carcinomas of the larynx and popharynx, and lymph nodes (Fig. 8.8; ble8.12).

for for hyTa-

Anterior Cervical Space (ACS)

These small paired spaces lie in the anterolateral part of the infrahyoid neck. They are located lateral to the central visceral space, medial to the lateral strap and sternocleidomastoid muscles, and anterior to the carotid spaces. Their fascial boundaries are complex and all three layers of the DCF contribute to these. Su-

Differential

diagnosis

of visceral

space le-

Pseudotumors:

Pyramidal lobe of thyroid gland Prominent thyroid isthmus Patulous esophagus in tracheoesophagealgroove

Inflammatory lesions:

Cellulitis or abscess Reactive adenopathy

Laryngeal lesions:

Laryngocele Squamous cell carcioma Chondrosarcoma

Thyroid lesions:

Infrahyoid thyroglossal duct cyst Colloid cyst Goiter Acute/chronic thyroiditis Thyroid adenoma Thyroid carcinoma: papillary, follicular, medullary etc. Extranodal non-Hodgkin's lymphoma-primary or metastatic

Parathyroid lesions:

Parathyroid Adenoma

Esophageal lesions:

Zenker's diverticulum Esophageal carcinoma

Other malignant lesions:

Nodal metatasis from SCCa, thyroid Nodal lymphoma

cyst

periorly they are continuous with the SMS and, as no fascia separates the two, SMS lesions can spill into the fat-filled ACS as well. The contents of the ACS are: - fat.

The most important lesions in the ACS are transspatial diseases from the neighboring spaces, such as tumors or infections (Table 8.13). -------VS -------ACS

VS------------~-~

-------

ACS-------r~~"

~~

~b~~\"--==::s:.:-

CS RPS

Fig. 8.8 Pattern of spread of visceral space masses in the infrahyoid neck. (Legends see p. 233)

Spaces

Table 8.13 Differential space lesions

diagnosis

of anterior

cervical

Table 8.14 Differential space lesions

of the Infrahyoid

diagnosis

of posterior

Pseudomass:

Hypertrophied muscle

Second branchial cleft cyst Cystic hygroma-lymphangioma

Cystic lesions:

Cystic hygroma-lymphangioma Third branchial cleft cyst

Inflammatory lesions:

Cellulitis or abscess

Inflammatory lesions:

Benign tumors:

Lipoma

Malignant tumors:

Spread of thyroid carcinoma, phoma from adjacent spaces

Pseudotumors:

Enlarged

Cystic lesions:

anterior jugular

vein

Iym-

Benign tumors:

The contents

of the

pes

are:

fat preaxillary brachial plexus spinal accessory nerve (XI) dorsal scapular nerve spinal accessory lymph nodes. Lesions specific to the PCS should be centered in the fat and have a thin fat stripe that separate them from the CS. This can be difficult in the suprahyoid region where the PCS is only very small. Most important in this area are lymphoma or metastases from SCCa and other tumors in the spinal accessory lymph nodes (Table8.14).

Masses Involving Multiple Cervical Spaces There are few diseases that do not follow the spatial anatomic confinement or involve multiple spaces, either contiguously or non-contiguously (Table 8.15).

Cellulitis or abscess Reactive or suppurative nopathy Tuberculous adenitis

phoma Liposarcoma

Table 8.15 multispatial

ade-

neurofibroma

Nodal metastasis from SCCa (nasopharynx) Hodgkin's or non-Hodgkin's lym-

Posterior Cervical Space (PCS) The pes are also paired and lie in the posterolateral part of the suprahyoid and infrahyoid neck and completely encircle the paraspinal muscles in the paraspinal PVS. They have a posteriorly tilted. triangular shape, corresponding well with the occipital and subclavian triangles behind the sternocleidomastoid muscle. The fascial boundaries of the pes are complex. The DLDCF separates it from the perivertebral space and superficially the SLDCF forms a boundary to the subcutis and sternocleidomastoid muscles.

cervical

levator scapulae

Lipoma Hemangioma Schwannoma,

Malignant tumors:

Neck

(rare)

Differential diagnosis diseases of the neck

Transspatial Cystic lesions:

Inflammatory lesions:

diseases

of transpatial

(contiguous

and

spaces)

~

Cystic hygroma-lymphangioma Branchial cleft cyst Thyroglossal duct cyst Transspatial cellulitis Diving ranula

or abscess

Benign tumors:

Congenital hemangioma Lipoma Juvenile angiofibroma Schwannoma, neurofibroma

Malignant tumors:

SCCa-primary or metastatic Non-Hodgkin's lymphoma Rhabdomyosarcoma, thyroid carcinoma, minor salivary gland malignancies,

Multispatial Inflammatory nodal:

lesions

melanoma

(noncontiguous

spaces)

Reactive adenopathy from respiratory tract infection Idem, from mononucleosis, scratch fever, TBC Sarcoidosis

Malignant nodal:

Squamous

Non-nodal

Neurofibromatosis Hematogeneous metastases

disease:

cell carcinoma

Non-Hodgkin's lymphoma Other metastatic tumors

-;-'!ssSs~s~

upper cat-

241

Examination Technique

The CT examination relies on contrast-enhanced spiral or multislice scanning with thin sections. Care must be taken to choose the proper contrast injection protocol because scanning too early may yield suboptimum enhancement of malignant lesions. Scan Preparation and Patient Instruction

Patients are examined in the supine position with the neck slightly hyperextended and the shoulders lowered as much as possible. A proper alignment of the head in the cephalocaudal axis to prevent any asymmetry is important. Patients are instructed to breathe shallowly (to keep the vocal cords open) and to avoid coughing. Scanning during suspended inspiration is possible, but this may lead to artifactual narrowing of the airway. Suppression of swallowing is important because swallowing can cause major artifacts and may render an examination useless. There are two approaches: not to mention the word "swallowing" at all and to instruct the patient to breathe normally but keep their mouths open, or to ask the patients to swallow multiple times immediately prior to the examination to "exhaust" the swallowing reflex. A digital lateral scout radiograph is made to adequately plan the examination. Proper gantry angulation is important. In general the gantry is angulated parallel to the vocal cords (laryngeal ventricle) for the neck and larynx region and parallel to the hard palate for the face. If the vocal cords are not distinguishable on the scout view, angulation parallel to the hyoid bone or midcervical disk spaces are good approximations. The presence of large amounts of dental amalgam may require different neck positions and angulations to avoid metallic artifacts. Often this can be accomplished by using separate, overlapping acquisitions with different angulations for the face and neck (Fig. 8.9). Another source of artifacts is the shoulder region. In this region spiral scanning may be replaced by incremental technique using higher mAs and l
Fig. 8.9 Separate, overlapping acquisitions with different angulations forthe face and neck avoid metallic artifacts in patients with large amounts of dental fillings.

which provides high mA in lateral projections and markedly reduced mA in AP projections. For optimum results, an increase of the maximum mA to about 2-3 times that of a standard neck examination should be chosen. Despite this, the local patient exposure (effective mAs) is often less than with conventional scanning because of the marked dose reduction in the non-lateral projections. Radiologists are advised to check with the manufacturer whether the particular dose modulation program is able to reduce the mA settings to normal as the scan progresses to the neck. If not, there is a su bstantial increase of the patient's radiation exposure. Acquisition

Parameters

Spiral CT is performed with a collimation of 2-3 mm, a pitch of r. 1.5, and image reconstruction every 1-3 mm (Table 8.16). To optimize spatial resolution a field of view of 14-18cm and a 512 x 512 matrix are employed. For lesions of the nasopharynx, tongue, palate, and skull base, the axial images may be complemented by direct coronal scans with the patient positioned prone. This technique, however, is becoming obsolete with thin-section spiral and multislice scanning because high-quality coronal and sagittal sections can be reformatted from the axial data set. This requires overlapping image reconstruction for optimum quality.

Acquisition Parameters Table 8.16

Protocols for

CT

243

scanning of the neck

General Oral Contrast

None

Preparation

Remove dentures,

Patient position

Axial sections:

let patient swallow repeatedly

Coronal sections: Scan range Respiratory

prior to scan

Supine, axially symmetric with arms at the sides and shoulders low (may require strap) Prone, head reclined

lateral scanogram: skull base to jugular fossa parathyroid tumor, abscess, etc. may require scanning to the tracheal bifurcation phase

Quiet, shallow respiration; additional scans may be acquired during E phonation Noncontrast CT: Contrast-enhanced

Seen parameters

1-slice SC/TF/RI

a-suceSC

16-slicea SC

axial SW/RI

Standard (e.g. lymph nodes)

3/5/3 "

2-3 "

1-1.5 "

3/2

Tumor staging

24/2 "

1-l.25"

0.75-1.25

larynx (focused exam)

1/2/1!

0.5-1"

Contrast injection

t-suce V/F/D

4-slice V+N/F/D

Standard

70/2/20

70+ 50/2/20

Tumor staging Biphasic injection Biphasic scanning

100/2/60 70/2+50/1/80' 150 + 50/4/30 + 120d

100 + 50/2/60 70/3 + 60/1/80' 150 + 50/5/30 + 120

CT:

•

or a Valsalva maneuver

Windowing

W/L = 300/40 W/l=300/60

0.5-0.75

" "

MPRb SW/RI

3/2

2/2 cor

2/2

2/1 cor

50 + 30/2/30

d

100 + 50/2/70 70/3+50/1/80' 150 + 50/5/40+

120

d

SC= slice collimation (mm), TF = table feed (rnmjrotatlon}, RI'" recon. increment (mm), i! '" scanning direction. SW = effective section width (mm), MPR::: multiplanar reformats, axial= axial sections, cor= coronal, seq= sagittal eM = contrast material, Contrast concentration = 300 mg/ml iodine V=volume of CM (ml), N =volume of saline flush (ml), F= flow rate (mils), 0 =start delay, X '" trigger region a Pitch P=TF/(NxSC) should be chosen close to 1.5 with a-slice a scanners, 1.3-1.5 with 16-slice scanners b MPRare reconstructed from a "secondary raw data set" with SWIRl'" 1-1.510.6 or 0.5-0.810.4 ( read: 70 ml CM at 2-3 mils flow. followed by 50-60 ml CM at 1 mils, overall start delay of 80 s dread: 150 ml CM followed by 50 ml of normal saline, both at4-5 mils flow, start delay of 30-40s for l ", 120s for 2nd scan

The larynx should be scanned with thin (05-2 rnrn) sections and an imaging plane that is parallel to the vocal cords. Optimum alignment is possible using axial MPR. Focused spiral scans can also be performed separately, thus combining a survey examination of the neck with high resolution larynx imaging. E-phonation, modified Valsalva maneuvers, or puffed cheeks maneuvers can be employed to improve the evaluation of the vocal cords, the piriform sinuses. and the oral cavity, respectively. For a high success rate in these

functional scans, good patient instruction prior to the examination is needed. Multislice IT can either be performed with a spatial resolution similar to that of spiral CT (e.g., 4 x 2.5 mm

collimation)

or it can

be op-

timized to obtain near isotropic resolution (0.75 x 1.25 mm section collimation with 4- to 16-s1ice

scanners)

which

allows

for high-quality

reformations in any required imaging plane. The higher resolution scanning protocol is essential for optimum tumor staging: the lower resolu-

8 Neck

tion should be reserved for less critical indications such as lymph node staging. For evaluation of the larynx. 0.5-0.75 mm collimation may be used to further improve spatial resolution. For clinical evaluation. 2-3 mm thick axial sections should be reconstructed from a multislice CT data set. This can be done primarily from the raw data or by reformatting a thin-section overlapping "secondary raw data set". From these secondary raw data. 2-3 mm axial sections that may even be tilted retrospectively to match the anatomy (e.g .. parallel to the vocal cords) and 1.5-2.5 mm coronal or 2-3 mm sagittal sections can be reformatted. Different protocols for specific clinical questions should be devised. with adjustments to the parameters mentioned above. as needed. Complex protocols have been proposed for screening the entire neck using multiple spiral scans with optimized angulation and technique in the face and upper neck. lower neck. and shoulder regions. With multislice scanning and dose modulation many of these protocols will become obsolete. Scans should be reconstructed with a standard algorithm and displayed using soft tissue and bone window settings. For the soft tissues it is advantageous to keep the window fairly narrow (250-350 HU). With pathology in bone or cartilage. the raw data should be reconstructed with a higher resolution bone algorithm as well.

Contrast Administration The injection of intravenous contrast is mandatory for all but a few indications (e.g. dislocation of the arytenoid cartilages). If contrast material injection is contraindicated. other imaging techniques (MRl) should be preferred. Nonionic contrast medium is injected with a power injector using a monophasic or biphasic injection protocol. For good vascular opacification and delineation of lymph nodes. scanning can start after 20-25 seconds. and 80-100 ml of contrast material suffice. With multislice CT, the amount of contrast material can be further reduced for this specific purpose. Good enhancement of tumors requires a longer waiting period of more than 60 seconds because it relies on tumor hypervascularization as well as increased leakage of contrast material

into the interstitium. Most institutions will use a monophasic injection of 100-150 ml of contrast material with a flow rate of 2-3 mlls and a long enough scan delay of 60-70 s to obtain good tumor and vascular opacification. More sophisticated injection schemes use a biphasic injection that allows for continuing contrast injection well into the scan to ensure good opacification of the whole neck despite a relatively late scan phase. Such biphasic protocols rely on the injection of 50-70 ml at 2-3 ml/s, followed by 50-70 ml at 1 mlls. They allow for an increase in scan delay to 80-100 s. Another alternative is to inject a high volume of contrast material (e.g .. 150ml. followed by a 50ml saline flush) at a high flow rate of 4-5 mlls and perform biphasic scanning with a scan delay of 30s and 120s. This will allow for separation of the effects of tumor hypervascularization and interstitial enhancement.

Image Processing For complex clinical questions and for tumor imaging. near isotropic data acquisition should be performed. A "secondary raw data set" with 0.5-1.5 mm section width and 0.4-0.6 mm reconstruction increment is then used for further image processing. Standard processing of such data includes optimally angulated axial sections (e.g .. MPR parallel to the vocal cords or the hard palate). A symmetric display ofthe neck can be gained by appropriately tilting the reconstructed plane and correcting for a suboptimum alignment of the patient. Sagittal and coronal planes are excellent for displaying the full extent of the tumor and its relation to neighboring structures. Coronal planes may be tilted to be parallel to the anatomic structure of interest, e.g., the larynx and trachea. To keep the signal-to-noise levels acceptable and the partial volume effects low, the width of the reconstructed axial and multiplanar sections should be in the range of 1.5-2 mm. Thin-slab maximum intensity projections of 5-10 mm thickness are helpful for evaluating vascular structures, in particular carotid abnormalities and vascular involvement in tumor patients. Casts of the air-containing structures of the neck may provide an overview of asymmetries and gross deformation (see Fig. 2. 26a). Tfssuetransition projections provide a better under-

Contrast Administration standing of wall irregularities (see Fig.2.26b). Care has to be taken not to mistake a collapsed portion of the hypopharynx with tumor infiltration. In cases of doubt. additional scans using a modified Valsalva maneuver should be performed.

Virtual laryngoscopy can demonstrate subtle irregularities and asymmetries of the larynx and hypopharynx and can provide a roadmap to sites that are difficult to assess by conventional laryngoscopy (see Figs. 2.41 d and 8.21 b).

Cystic Lesions There is a large variety of cystic lesions in the neck that are best characterized by their location in a particular cervical space (Table 8.17).

Thyroglossal

Duct Cyst

The thyroid descends from the tongue base to its normal position during the first 7 weeks of gestation. During migration it is connected to the tongue via the thyroglossal duct. This duct normally involutes during the 8- Wthweek. If parts persist. they can give rise to midline (75%) or paramedian (
diagnosis are given

of cystic lesions in brackets)

Parotid space

Cystic hygroma (Warthin tumor) (Mucoepidermal

Mandibular

Branchial

(Neurogenic angle

regan

carcinoma)

tumors)

cleft cyst

CT Morphology The cyst is an unilocular, well-defined mass with fluid attenuation along the course of the thyroglossal duct in the anterior neck. It has a thin wall and septations can be seen infrequently. Normally it has a fluid-like attenuation and thin peripheral rim-enhancement may be seen on contrast-enhanced CT. After infection, the attenuation can increase, septations can occur and the enhancing rim becomes thicker and irregular. Sinus tracts/fistulae can be seen in the subcutaneous tissues as irregular tube-like fluid collections. Modularity or a softtissue mass within the cyst is suggestive for malignancy.

Cystic hygroma Dermoid cyst External

laryngocele

Branchial Cleft Cysts

Ranula Abscess (Malignant Sublingual

space

Epidermoid

lymphoma) cyst

Renna Mucocele

of submandibular

gland Median reqcn

of

anterior to the strap muscles infrahyoidally (65%). lt is the commonest congenital mass and the second commonest benign mass, usually presenting before the age of 20 and 4-5 times more frequently in females than males. The mass is 15-30 mm in size and can move upward with tongue protrusion. Very rarely, thyroglossal duct abnormalities can be associated with thyroid carcinoma, usually of the papillary type.

cervical

Thyroglossal duct cyst Submental dermoid cyst Lymphangioma Necrotic metastases

Lymphoma

Carotid space

(Lymphadenitis

and liquefac-

tion) (Cyst

thyroid

in ectopic

Aneurysm (Malignant

tissue)

of carotid artery lymphoma)

These rests sinus lateral adults. ratory

cysts probably arise from epithelial cell and can manifest themselves as cysts, tracts, or fistulae. They are usually uniand occur in older children or young The cyst can distend during upper respitract infections. First branchial cleft cysts: These manifest as recurrent abscesses around the ear or mandibular angle. A recurrent parotid abscess not responsive to antibiotics is typical. The cyst may be superficial, deep or located in the parotid gland. Sinus tracts can extend to the ear or skin. Second branchial cleft cysts: These are by far the commonest branchial cleft anomaly and

245

8 Neck

several subtypes exist. They are most frequently found lateral to the carotid sheath and posterior to the submandibular gland (type II) or extending medially in the carotid bifurcation (type III). The mass is painless, slow-growing and measures 1-10 cm. It can become painful when infected. Atypical forms can lie in the parapharyngeal space and project upward from the faucial tonsil to the skull base. Third and fourth branchial cleft cysts: These are rare. Third cleft sinus tracts or cysts arise from the apex of the piriform sinus and extend posterior to the sternocleidomastoid muscle and carotid space into the posterior cervical space. Anomalies of the fourth cleft are usually sinus tracts extending from the piriform sinus down as far the aortic arch or right subclavian artery. Most tracts are short and sometimes there is a cutaneous fistula at the base of the neck. •

CT Morphology

These cysts are well-circumscribed and homogeneously hypoattenuating. The cyst wall is thin, but may thicken after infection, and enhances after administration of contrast (Fig. 8.10). If large, they can displace the surrounding structures from their characteristic locations. Thymic Cyst

These uncommon cysts probably arise from thymopharyngeal duct remnants. They are found immediately adjacent to the carotid sheath and

can occur anywhere from the mandibular angle to the mediastinum. In 50% of cases there are connections with mediastinal thymic tissue. Patients usually present with a painless, slowly growing mass in the lateral neck near the upper mediastinum and in close relation with the sternocleidomastoid muscle. In 65% of patients they present during the first decade of life, are more common in males, and occur most frequently on the left side. When cysts are large, patients can complain of hoarseness, dysphagia, or stridor. •

CT shows an unilocular or multilocular, well-defined cystic mass with fluid attenuation. The cyst size can vary between 1 cm and 20cm. The mass is often elongated and located adjacent to the carotid space and may extend into the mediastinum. Thymic cysts may enlarge during Valsalva's maneuvers. Cystic Hygroma (Cystic Lymphangioma)

This is the commonest form of lymphangioma in infancy and childhood and is usually detected when the patient is 2 years old. It involves the lower face and neck and is most commonly found in the posterior cervical space or oral cavity. The lesion does not respect fascial boundaries and may extend inferiorly into the axilla and mediastinum or anteriorly in the floor of the mouth and tongue. Most are asymptomatic. Cystic hygromas manifest as painless soft masses of variable size. They grow slowly, but may suddenly increase in size after trauma or infection. Resection is difficult and many recurrences are seen after surgery. Other types of lymphangioma include the smaller cavernous (tongue, salivary glands) and capillary (skin) forms that are significantly smaller. •

Fig. 8.10 Superinfected lateral neck cyst (second branchial cleft cyst). The cyst contents have perforated into the surrounding soft tissues (arrow).

CT Morphology

CT Morphology

Cystic hygromas show multiple, poorly circumscribed cystic spaces with a homogenous fluid attenuation. Infected lesions show an increased attenuation. The spaces in the hygroma can vary in size from a few mm to more than 8 cm. They are commonly centered in the posterior cervical or submandibular spaces and show transspatial growth.

Contrast Ad ministration

Dermoid and Epidermoid Cyst

These cysts are manifestations of benign teratomas and are covered with squamous epithelium. Unlike dermoids, epidermoids do not contain skin appendages. Epidermoids present during infancy, while dermoids manifest during the second or third decade. In the neck, dermoids are more frequent than epidermoids. Both cysts are mobile, soft masses in the suprahyoidal midline that grow slowly and are of variable size. The most frequent location in the head and neck is the oral cavity. •

CT Morphology

These cysts are thin-walled, unilocular masses in the submandibular or sublingual space that have a fluid attenuation. The dermoid cyst may show "marbling": presence of fat nodules in the fluid matrix. The location of the mass with respect to the mylohyoid muscle is crucial for therapy planning, which requires coronal images or reformations. Laryngocele

A laryngocele is a dilation of a small pouch in the anterior roof of the laryngeal ventricle, the laryngeal saccule. Internal laryngoceles (40%) are limited to the larynx, while external ones (25%) extend through the thyrohyoid mem-

a

b

Fig. 8.11 (a) Mixed laryngocele with intratralaryngeal components (scan at the level of bone). There is a mass effect with displacement rowing of the hypopharyngeal lumen, (b)

brane and only have an external dilatation. Mixed laryngoceles (35%) have a dilatation both in the larynx and outside. A laryngocele develops due to a combination of a congenital long saccule and longstanding increased supraglottic pressure (shouting, coughing, glass blowing, playing wind instruments). Rarely they are congenital but are usually acquired and manifest in adult life with dysphagia, hoarseness, and stridor. In 1 % of cases they are associated with laryngeal cancer. •

CT Morphology

Laryngoceles appear as smooth, well-defined masses in the paralaryngeal space. They may cross the thyrohyoid membrane at the level of the superior laryngeal neurovascular bundle. Depending on the contents (air, fluid, mucoid secretions) the attenuation will vary and airfluid interfaces can occur (Fig. 8.11). T ornwaldt's

Cyst

This cyst is derived from embryological rests of the notochord and is found in 4% of the normal population. It measures 1-5 mm and is characteristically located in the midline of the nasopharyngeal roof between the longus capitis muscles. Infection may lead to postnasal discharge, prevertebral muscle spasm, or even abscess formation.

c and exthe hyoid and narAir-filled

right-sided laryngocele in a typical supraglottic location. (c) Volume-rendered coronal cut through the larynx in the same patient.

247

8 Neck •

CT Morphology

The cyst is usually smooth and well defined. It may have a fluid attenuation but is more often

isoattenuating to muscle due to a high protein content of the fluid.

Benign Tumors Pseudotumors and Tumor-Like Lesions Lingual Thyroid

This is the commonest form of ectopic thyroid tissue and is seen more frequently in women. It usually present during puberty. It represents an arrest of migration of thyroid tissue within the tongue, usually between the circumvallate papillae and epiglottis. This arrest may be incomplete and in 70-80% of patients there is no thyroid tissue in the lower neck. •

CT Morphology

This is seen as a high-density soft tissue mass in the posterior tongue with vivid enhancement after contrast administration (Fig. 8.12). Frequently, no thyroid tissue is seen in the lower neck. Vascular Pseudomasses

Normal variants can resemble masses of vascular origin. Often there is a marked asymmetry of

the internal jugular veins, with the right one bigger than the left. In elderly patients the carotid arteries can have a tortuous, more medial course. The pterygoid plexus can also be asymmetric, appearing as an enhancing area along the medial border of the lateral pterygoid. Neurogenic Tumors Schwan noma and Neurofibroma Schwannomas are small, encapsulated

tumors of the nerve sheath arising from Schwann cells in which the tumor grows eccentrically from the nerve. Patients are between 20-50 years and present with a painless, slow-growing mass in the anterolateral neck. The cervical nerve roots, vagus nerve, or sympathetic plexus are most frequently involved. They are usually solitary, but can be multiple in patients with neurofibromatosis 2 (5 % of cases). Rarely, schwannomas can show malignant degeneration (malignant peripheral nerve sheath tumor =MPNST, or neurosarcoma) . Neurofibromas are nonencapsulated and grow concentrically, giving an enlargement of the nerve of origin. They occur in patients between 20-40 years and in 90% are of the localized variety. Infiltrative growth can occur with plexiform subtypes. They can be multiple and are in 10% part of neurofibromatosis syndromes. •

Fig. 8.12 Lingual thyroid (noncontrast scan). The dystopic thyroid tissue shows nonhomogeneous CT densities. Note the resemblance to the CT morphology of a carcinoma of the tongue base. Two regressive punctate calcifications (arrows) suggest the possible etiology of the mass.

CT Morphology

Fusiform, well defined tumors with low density, frequently in the carotid space. Paraspinal tumors can have a characteristic dumb-bell shape. The density of schwannomas is related to lipid-rich Schwann cells and fat. Neurofi bromas tend to be more fatty, while schwannomas can have cystic or necrotic components (in 20% of cases). After contrast administration, there is a moderate, heterogeneous enhancement.

Mesenchymal Tumors quires angiography for exact delineation tumor vessels and feeding arteries. •

Fig. 8.13 Glomus tumor (paraganglioma) in the right carotid bifurcation. CT shows an intensely enhancing mass with splaying of the internal carotid artery (ICA) and external carotid artery (ECA).

of

CT Morphology

The lesions are usually well defined and show intense enhancement in the carotid bifurcation, along the vagus nerve or in the jugular bulb (Fig. 8.13). This enhancement is homogenous but shows a rapid washout. The tumors can distort the normal course of adjacent vessels. In large tumors hypodense, necrotic regions are visible. Lesions adjacent to the skull base can show permeative bone erosion (glomus jugulare). A CT angiography technique with thin sections, preferably using multislice CT, yields the best results. Mesenchymal Tumors

Paragangliomas

Paragangliomas (chemodectomas or glomus tumors) are rare, slow growing, hypervascular tumors arising from neural crest cells and are classified by location and origin. The commonest types are: glomus caroticum in the carotid bifurcation, glomus jugulare from the nerves in the bulbus jugularis, glomus vagale from the ganglion nodosum of the vagal nerve, located between carotid bifurcation andjugular bulb, and glomus tympanicum from the tympanic plexus in the cochlear promontory. Symptoms depend on location of the tumor: glomus caroticum and vagale present as slow-growing masses with no other symptoms, while glomus jugulare and tympanicum may have associated pulsatile tinnitus, hearing loss and cranial nerve palsy. There is a familial history in 30% of patients. There is an increased incidence in females and in patients living at high altitudes. The lesions are multiple in 30% of patients with familial paragangliomas but less than 5% in non- familial types. Paragangliomas can occur in association with other paragangliomas or tumors in other body parts, but these combinations are rare. Head and neck paragangliomas are hormonally active in only "170. The lesions can be malignant in 10% of cases. Therapy of carotid and solitary vagal body tumors is surgical, whereas forjugular and tympanic glomus tumors a wait-and-see policy is advocated. Further preoperative work-up re-

Lipoma

Lipomas are frequent tumors of the neck, usually presenting at advanced age. They are soft, mobile, and painless masses. They can arise in any neck space, but are most frequent in the posterior cervical space. The retropharyngeal ones can become large before clinically apparent. Hibernomas are lipoma variants that contain brown fat. •

CT Morphology

Well-defined masses with a homogenous fat density and thin septa. There is no clearly identifiable capsule or only a very thin capsule that does not enhance after contrast administration. Hemangioma

These are the commonest cervical tumors in children. Girls are more often affected than boys. The two most frequent varieties are capillary and cavernous hemangiomas. Capillary hemaugiomas usually present directly after birth and show proliferative growth during the first 12 months. They may be located in nearly all spaces, but have a preference for the masticator or parotid spaces. These lesions are more frequently superficial and may show a bluish discoloration of the skin. They show fatty replacement and involution in adolescence.

249

Neck

Cavernous hemangiomas and other vascular malformations occur in adolescence and do not involute at later ages. Often they are deeply located and can grow large involving multiple neck spaces. Phleboliths may arise in these hemangiomas. Large AVMs with high flow can show hypertrophic feeding vessels. •

CT Morphology

Hemangiomas may be well defined or grow in an infiltrative pattern. Most lesions are isoattenuating with muscle and show intense enhancement after contrast administration. Phleboliths may be present in cavernous hemangiomas. Rhabdomyoma

These are very rare lesions of striated muscle cells with a predilection for the head and neck. They usually affect the oropharynx. submandibular region. or larynx. On CT the lesions are circumscribed. smoothly marginated and have muscle density. After contrast. enhancement equal to that of muscles is seen. Aggressive Fibromatosis (Desmoid Tumor)

This is a well-differentiated fibrous lesion with locally infiltrative growth. originating from the musculoaponeurotic junctions. The lesions occur most often in the face or supraclavicular region of young adult patients and are usually painless and slow-growing. The CT findings are nonspecific and may resemble malignancy. Due to the high fibrous content. there is persistent interstitial enhancement of the tumor. Benign Fibrous Histiocytoma

Rare benign tumors consisting of fibroblastic cells. They are common in the subcutaneous tissues and rarely found deep to the superficial cervical fascia. Young adults are most frequently involved. The CT findings are nonspecific and may resemble MFH.

Juvenile Angiofibroma

Angiofibromas occur most often in teenage boys and originate from the superior wall of the nasal cavity or nasopharynx. The lesion is highly vascular and. although benign. is locally infiltrative. Extension can be to the pterygopalatine fossa laterally. sphenoid sinus or orbit superiorly. and even into the middle cranial fossa. It is often accompanied by local bone destruction. CT Morphology

Juvenile angiofibroma presents as a soft tissue mass with infiltrative growth pattern and often bone destruction. The tumor enhances strongly after contrast administration. Benign Tumors of the Larynx and Hypopharynx

Benign laryngeal tumors account for approximately 10% of all laryngeal masses. Benign tumors of the larynx and hypopharynx include fibromas, angiofibromas, and fibromyxomas (sessile or pedunculated "laryngeal polyps"), which most commonly involve the anterior part of the vocal folds. Adenomas are rare tumors that may involve the aryepiglottic fold, ventricular fold, or subglottic area. Myogenic tumors, lipomas, hemangiomas, chondromas (most arise from the cricoid cartilage), and fibroneuromas are also very rare. They are diagnosed by visual inspection and are not an indication for CT, although they are occasionally noted as incidental findings. Papillomavirus-induced laryngeal papillomas occur predominantly in young males and most commonly affect the anterior vocal cords. The multiple superficial growths of papillomatosis are common in younger patients « 10years), whereas most lesions in older patients are solitary. Malignant transformation may occur. CT is indicated only if there is suspicion of malignancy (stage > TI). CT Morphology

Benign tumors generally have smooth margins. Laryngeal or hypopharyngeal polyps may present as small nodular protrusions at the anterior commissure of the vocal cords, or as a large, smoothly marginated mass within the larynx or hypopharynx (Fig. 8.14). Pedunculation is in-

Lymph Node Staging Fig. 8.14 Small calcified fibroma of the posterior wall of the hypopharynx (a). The relationship to the surrounding structures is best appreciated on a volume-rendered coronal view (b).

a

b

frequently detected at CT. Absence of tissue invasion may suggest the diagnosis. Papillomas may present a cauliflower-like surface when extensive. Chondromas have a lower attenuation than most soft tissue tumors, typically in the 20-30 HU range (Fig. 8.15). They frequently lead to displacement of the smaller cartilages. From CT morphology alone there is no secure differentiation between benign and malignant disease, although of course, infiltrative growth suggest malignancies. Fig. 8.15 Laryngeal chondroma causing subtotal obstruction of the larynx. The noncontrast scan shows a markedly hypodense mass with splaying of the arytenoid cartilages (arrows).

Malignant Tumors Lymph Node Staging

The presence oflymph node metastasis is an important prognostic factorin head and neck cancer. Asinglenodalmetastasisreducestheprognosis by 50%, and the presence ofacontralateral metastasis byanother 50 %. Ifextranodalspreadis detected the prognosis is further reduced by an additional 50%. Evenin palpable normal necks the riskofoccult metastases can be as high as 20-50%, depending on the type of the primary tumor. In evaluating the lymph nodes of the neck it is important to discriminate between two clinical scenarios, patients with a palpable NO neck and patients with a palpable positive neck (N411).

NO patients are often electively treated by neck dissection if the expected risk of occult metastasis exceeds 20%, based on characteristics of the primary tumor. If the risk is lower, a wait -andsee policy under strict clinical and imaging control is advocated. Negative imaging studies may thus reduce the number of dissections while positive studies (upstaging) can guide appropriate therapeutic procedures. In palpable N + necks, finding additional positive nodes at imaging may guide the type of neck dissection and adjuvant therapy, depending on the lymph node levels involved. Lymph node invasion can be evaluated with CT and is usually staged according to the UICC (TNM) cri-

251

8 Nec!<

Table 8.18 Lymph node staging of cervical tumors (excluding the thyroid gland and proximal esophagus) NO

No regional lymph node metastases

Nl

Metastases in a single ipsilateral node, 3 cm or less in greatest dimension

N2a

Metastases in single ipsilateral node, greater than 3 cm but no more than 6 cm in greatest dimension

N2b

Metastases in multiple ipsilateral nodes, none more than 6 cm in greatest dimension

N2c

Metastases in bilateral or contralateral nodes, none more than 6cm

N3

Metastases in a node more than 6 cm in greatest dimension

teria for head and neck tumors (Table 8.18). Criteria used for differentiation are size, shape, grouping, necrosis, and extracapsular spread. The criteria frequently used are: minimal axial diameter of 11 mm for level II, minimal axial diameter of 10mm for other levels, grouping of three or more borderline (8-10 mm) nodes and irregular enhancement. These criteria are based on neck dissection studies and probably lack sufficient sensitivity for patients with clinically NO necks. Because CT is relatively inaccurate in preoperative diagnosis, ultrasound combined with fine needle aspiration biopsy is most often used as a first line modality. Other methods in current development like (CT)-PET, thallium-SPECT or USPIO-enhanced MRI may improve the accuracy of preoperative lymph node imaging in the near future. A more detailed overview of lymph node levels and criteria for malignancy can be found in Chapter 22.

The cervical nodes can also harbor metastases from primaries other than squamous cell carcinomas of the neck (Fig. 8.16). Thyroid cancers, salivary gland cancers, renal cell carcinoma, breast carcinoma, lung carcinoma, and melanoma are all known to metastasize to this region. Nasopharyngeal Carcinomas

Up to 80% of nasopharyngeal malignancies are squamous cell carcinomas. A number of risk factors have been established including EpsteinBarr virus, genetic predisposition in ethnic Chinese, poor living conditions, and chronic sinonasal infection. Males are 2.5 times more often affected than females. S

CT Morphology

The main tumor subsites are the posterosuperior, anteroinferior, and lateral walls. The tumor is based most often in the fossa of Rosenm uller (lateral wall). It may spread submucosally in all directions along fascial planes, with early infiltration of palatal muscles (Fig. 8.17). The pharyngobasilar fascia represents a relative boundary so that posterior extension is infrequent. Deep extension often involves the PPS and MS. Advanced lesions may erode bone, spread perineurally, and reach the skull base or the cavernous sinus via the foramen lacerum (intracranial in 30%). Nodal metastases are the rule: 80-90% of patients develop metastases in lateral retropharyngeal, level II, III, or V nodes. In 35-50% these nodes are bilateral (depending on stage of primary tumor). There is also a high

Fig. 8.16

(a) Lymph node metastases with central necrosis (arrow) from squamous cell carcinoma of the oropharynx. (b) The coronal MPRin a patient with NHL demonstrates the rounded shape (ratio of longitudinal: transverse diameter < 2) even of small pathologic nodes.

a

b

Oropharyngeal Carcinomas Oropharyngeal Carcinomas

Fig. 8.17 Nasopharyngeal carcinoma of the torus tubarius on the right side. Note the asymmetry and the polypoid protrusions. In this scan after 100 s delay, there is only minimum contrast uptake by the tumor.

frequency (5-40%) of distal metastases in bone, lung, and liver. The T-stages are given in Table 8.19.

In the oropharynx, 90-95% of malignant tumors are squamous cell carcinomas. Tumors in the oropharynx tend to be more aggressive and more poorly differentiated than in the oral cavity. The commonest risk factors are a long history of tobacco and alcohol use, but increased risk is also associated with Plummer-Vinson syndrome and syphilis. The tumors are seen twice as frequently in men as in women. Because of their aggressive nature and mild symptoms, these tumors are often advanced at presentation. Symptoms include sore throat, foul breath, dysphagia, or ear pain. The tumors tend to extend submucosally and along fascial planes. Four separate tumor subsites are of interest: tongue base, tonsillar region, posterior wall, and soft palate. Tonsillar and tongue base carcinomas are most frequent. Each tumor subsite has a specific pattern of spread and lymphatic drainage. •

Important imaging features for evaluation of nasopharyngeal carcinomas: • assessment of tumor spread in surrounding deep spaces: PPS, MS, CS • skull base erosion (requires coronal thin section CT) • perineural tumor spread via the mandibular nerve • intracranial (cavernous sinus) extension.

Table 8.19 T-staging (UICC, 1997)

of

nasopharyngeal

carcinoma

Tis

Carcinoma in situ

T1

Tumor confined to the nasopharynx

T2

Tumor extends to soft tissues of oropharynx and/or nasal fossa

T2a

Without para pharyngeal

T2b

With para pharyngeal

T3 T4

extension

CT Morphology

The T-stages of Oropharyngeal carcinomas are given in Table 8.20. Tongue base: the tumor remains unilateral until it is quite extensive. It can spread anteriorly into the oral tongue, inferiorly in the valleculae or pre epiglottic fat and superiorly to the tonsillar fossae (Fig. 8."18).Frequently the tumors initially present with cervical node metastases. Adenopathy is present in 50-80% of patients, and is bilateral in 20-30%. Usually, this involves levels II, III, and 1.Surgical treatment options depend on the involvement of the neurovascular bundle, submucosal extension in adjacent areas and crossing of the midline. Table 8.20 T-staging (UICC, 1997)

of

Oropharyngeal

carcinomas

Tis

Carcinoma

in situ

T1

Tumor 2 cm or less in greatest dimension

T2

Tumor invades bony structures and/or paranasal sinuses

Tumor between 2 and 4 cm in greatest dimension

T3

Tumor with intracranial extension and/or involvement of infratemporal fossa

Tumor greater than 4 cm in greatest dimension

T4

Invasion of adjacent structures: pterygoid muscles, mandible, hard palate, deep tongue muscles, larynx

extension

253

8 Neck

a

b

c

Fig. 8.18 Tongue base carcinoma seen on axial (a) and coronal sections (b) as well as sagittal volume-rendered ages (c) from a thin-section multislice CT data set.

Important imaging features carcinoma:

in tongue base

• extension to floor of the mouth and surrounding structures • relationship to ipsilateral lingual neurovascular bundle • extension across midline and relation to contralateral neurovascular bundle. Tonsillar region: the tumor may spread inferiorly into the tongue base or infiltrate the surrounding structures like the tongue or pterygoid muscles. Because of the asymmetry of the normal tonsils, small tumors are difficult to detect. Tonsillar fossa tumors show early lymphatic metastasis, which is seen in 60-75% of patients at the time of presentation. In tumors of the anterior tonsillar pillar this figure is somewhat lower at 40-55%. Bilateral adenopathy is seen frequently and lymph node levels II, I or less commonly 111 orIVareaffected. Soft palate: these tumors are often well differentiated and have the best prognosis. They usually affect the oropharyngeal side of the palate and extend in all directions, but most often to the hard palate, tonsillar pillars, and PPS. Lymphatic metastasis is present in 40-60% of patients at time of diagnosis, which is frequently bilateral and involves usually level I, II,III,orretropharyngealnodes. Posterior wall: tumors lead to a thickening of the pharyngeal wall and can extend along the constrictor muscles superiorly to the skull base

im-

or inferiorly to the hypopharynx. Deeper invasion can be seen in the tongue base, retropharyngeal space or prevertebral space. Cervical adenopathy is common and seen in 50-70% of patients at diagnosis. The retropharyngeal nodes are first affected, followed by nodes in levels II, I II, or V. Important imaging features oropharyngeal carcinomas'.

in

other

• submucosal extension into surrounding soft tissues of the neck (PPS, CS, nasopharynx) • invasion of the tongue base • bone erosion (for skull base thin section coronal CT needed) • encasement of the carotid artery • prevertebral muscle invasion.

Oral Cavity Carcinomas

As in the oropharynx, the great majority of malignancies are squamous cell carcinomas. There is a histological gradient with the more aggressive tumors located posteriorly and most tumors arise in the dependent positions, such as the floor of the mouth. The most important risk factors are (chewing) tobacco and alcohol use. Pipe smoking and exposure to sun are specifically associated with carcinomas of the lip. Superficial lesions are normally asymptomatic, but larger lesions can be associated with

Oral Cavity Carcinomas

ear pain, foul breath or ill fitting dentures. ~ive separate tumor subsites are of interest: Ilps, floor of the mouth, oral tongue, buccal mucosa, and gingiva and hard palate. The lip and the floor of the mouth are most freq uently affected. Each tumor location has a specific pattern of spread and lymphatic drainage. •

CT Morphology

The T-stages of oral cavity carcinomas are given in Table 8.21. Lips: Imaging is only required in extensive lesions. Most often, lip tumors arise on the vermilion border of the lower lip and can extend in the orbicularis oris muscle, skin, and mandibula. Perineural extension along the mental nerve i? rare. Lymphatic spread is seen in 10-20% of patients (depending on stage prima~y tumo~) and usually involves levels I and II. ThIS tumor IS frequently seen in younger women.

Table 8.21 1997)

T-staging

of oral cavity carcinomas

(UICC,

Tis

Carcinoma

in situ

T1

Tumor 2 cm or less in greatest dimension

T2

Tumor between dimension

T3

Tumor greater than 4 cm in greatest sion

T4 lip

Invasion of adjacent structures: skin of the face, inferior alveolar nerve, floor of the mouth, cortical bone

T4 rest

Invasion of adjacent structures: extrinsic muscles of the tongue, cortical bone, maxillary sinus, skin

2 and 4 cm in greatest dimen-

Important imaging features in lip carcinomas: • soft tissue invasion • bone erosion (buccal surface of mandible). Floor of the mouth: Most tumors

arise within 2cm of the anterior midline and can extend in the extrinsic tongue muscles, sublingual or submandibular spaces. Encasement of the ipsilateral neurovascular bundle is seen frequently and may lead to denervation atrophy. Crossing of the midline with infiltration of the contralateral neurovascular structures is important therapeutically as this requires a total 910ssectomy. Although infiltration of the ma~dIbular periosteum occurs early, frank destru~tlOn of the mandible is a late phenomenon (FIg. 8.19). At the time of diagnosis, 30-50% of patients show nodal metastases (depending on stage primary tumor), usually in level I and I I nodes. Important

imaging

in floor of the

• deep invasion along mylohyoid and hyoglossus muscles • relation to ipsilateral neurovascular bundle • extension across midline and in contralateral lingual neurovascular bundle • tongue base invasion • invasion of soft tissues of the neck • extent of bone erosion. Oral tongue: Most tumors arise from the under-

surface and lateral borders. They infiltrate

Fig. 8.19 Small squamous cell carci noma of the floor of the mouth on the right side (a). The air marks the oral cavity, (b) Large floor of the mouth carcinoma with encasement of the mandible. There is central tumor necrosis. Similar enhancement is seen in an ipsilateral nodal metastasis anterior to the sternocleidomastoid muscle (arrow).

a

features

mouth carcinomas:

b

the

255

8 Neck

deep intrinsic and extrinsic muscles and can extend submucosally to the floor of the mouth, tonsils, and mandible, Crossing of the midline has important therapeutic implications, Bone erosion of the mandible is only seen in very advanced lesions, Tumors metastasize early to level I. 11, and III lymph nodes and 35-65% of patients (depending on stage primary tumor) present with (bilateral) adenopathy, Important

cinomas:

imaging features

in oral tongue car-

• relation to ipsilateral neurovascular bundle • extension across midline and in contralaterallingual neurovascular bundle invasion of floor of the mouth and associated bone erosion.

Buccal mucosa: These tumors commonly arise in the lateral walls, They spread laterally along the buccinator muscle to the masticator space or the pterygomandibular raphe and can erode underlying bone from the maxilla or mandible, There are often aggressive with early lymphatic spread to level I. 11, and parotid nodes, Important

nomas:

imaging

features

in buccal carci-

• submucosal extension • bone erosion.

Gingiva and hard palate: The extent of these tumors is often underestimated by crosssectional imaging, Primary hard palate SCCa is rare and usually an extension from an upper gingival tumor. Via the hard palate the tumor can break into the nasal cavity, Lower gingival tumors can extend into the floor of the mouth or erode the mandible, The amount of bone erosion will determine the type of surgical therapy, Up to 50% of patients present with lymph node spread to the level I (gingiva) or retropharyngeal and level II (palate) nodes, Retromolar tligone carcinomas are a special kind of gingival tumor with a complex spread pattern. They can extend anteriorly into the buccal region, posteriorly in the tonsillar fossa, superiorly along the pterygomandibular raphe into the nasopharynx and skull base, and inferiorly into

the floor of the mouth, Bone erosion can occur early and can only be evaluated accurately by thin section imaging, It is typically located at the ventral side of the ramus mandtbulae. Important

imaging

features

hard-palate carcinomas:

in gingiva and

• bone erosion • submucosal extension • perineural extension (e.g. incisive and greater/lesser palatine foramen),

canal

Hypopharyngeal Carcinomas The vast majority of hypo pharyngeal tumors are squamous cell carcinomas, Risk factors for hypopharyngeal carcinomas are alcohol and tobacco use and for postcricoid lesions there is an association with Plummer-Vinson syndrome, H ypopharyngeal tumors are clinically silent and often present in advanced (T3-T4) stages, When symptomatic, patients will complain of sore throat, (referred) ear pain, or dysphagia, All tumors tend to grow submucosally, so endoscopic diagnosis frequently underestimates the extent of the tumor. Tumor volume evaluation is important as low-volume lesions «6,5cm3) without apex involvement can be managed effectively by radiotherapy, Early forms of hypopharyngeal carcinoma can be difficult to diagnose endoscopically because of their site of occurrence or submucosal growth characteristics, A typical situation is a cervical lymph node metastasis in which a negative endoscopy leads to classification as a carcinoma of unknown primary (C UP), Given an adequate examination technique with expanded piriform sinuses, early tumor stages can be detected with CT CT Morphology The T-stages of hypopharynx carcinomas are given in Table 8,22, Suspected early lesions should be interpreted with reference to endoscopic findings or oral contrast examination, A careful side-to-side comparison for asymmetries as an early sign of tumor is mandatory, Collapsed, asymmetric sinuses or unilateral mucus retention, however, can mimic tumors, When in doubt, another scan

HypopharyngeaJ Carcinomas Table 8.22 T-staging (UICC, 1997)

of

hypopharynx

carcinomas

Tis

Carcinoma in situ

T1

Tumor is limited to one subsite and 2 cm or less in greatest dimension

T2

Tumor involves more than one subsite or measures between 2 and 4cm in greatest dimension, without fixation of hemilarynx (vocal cord)

T3

Tumor greater than 4 cm in greatest dimension or with fixation of hemilarynx

T4

Tumor invades adjacent structures: thyroidl cricoid cartilage, carotid artery, soft tissues of neck, prevertebral muscles, thyroid gland, esophagus

with Valsalva maneuver should be performed to yield optimum expansion of the piriform sinuses. More advanced hypopharyngeal carcinomas show an asymmetric intraluminal growth pattern with associated mass effects

(displacement, compression, or deformation of adjacent structures) and possible signs of cartilage destruction or fatty infiltration. Hypopharyngeal tumors may compress the internal jugular vein, and tumors invading the vessel wall can lead to thrombosis. Invasion of the carotid artery is manifested by asymmetric luminal narrowing or occlusion. Invasion is also indicated by tumor encasing more than a 90 segment of the vessel wall and by circumferentialluminal narrowing. There are three important subsites of the hypopharynx: piriform sinus, postcricoid area, and posterior wall. Each subsite has its specific pattern of spread and lymphatic drainage. Piriform sinus: small tumors often involve one wall and can easily be missed at endoscopy. More often these tumors will infiltrate all walls of the sinus (Figs. 8.20, 8.21). Extension can be endolaryngeal in the paraglottic fat, via the posterior wall or postcricoid area to the esophageal junction, or into the apex of the sinus. Cartilage 0

Fig. 8.20 Hypopharyngeal carcinoma. Postcontrast CT demonstrates a collapsed piriform sinus with an enhancing, thickened wall (a). Virtual endoscopy (b) only demonstrates an asymmetry of the piriform sinuses. P= Piriform sinus, V = valieculae, E = epiglottis.

a

b

a

b

Fig. 8.21 Piriform sinus carcinoma. Postcontrast multislice CT shows a polypoid, moderately enhancing mass projecting far into the pharyngeal lumen (a). Note the multiple hyperenhancing bilateral lymph node metastases (arrows), some of which display central necrosis. Virtual laryngoscopy (b) demonstrates a mass in the piriform sinus. Anatomic orientation on such endoluminal views may be difficult.

257

8 Neck

invasion (thyroid, arytenoid) is seen frequently and can lead to extralaryngeal spread, Lymph node metastases occur in 55-75% of patients at presentation and involve mainly level 11, III and (less frequently) V nodes, Posterior wall: tumors here commonly extend into the oropharynx as well. The tumor may extend upward into the piriform sinus, caudal to the esophagus, or in unusual cases dorsally in the prevertebral muscles, Cartilage invasion

is uncommon

Supraglottic Tumors

Oncologically, the supraglottis is divided in five subsites: suprahyoid epiglottis. aryepiglottic folds (laryngeal aspect), arytenoids, infrahyoid epiglortts and false vocal cords (UICC),

for tumors in this loca-

tion, Lymph node invasion is seen early with retropharyngeal, level 11and III nodes affected, Postcricoid area: primary tumors in this location are rare, They will grow circumferentially in the submucosa, thereby narrowing the hypopharynx, The tumors can extend in the esophagus or in the posterior larynx with associated invasion of the cricoid cartilage, Lymph node metastases occur primarily in level III and (less frequently) 11or IV nodes, Important imaging features in hypopharyngeal tumors: • tumor volume

midline extension involvement of the apex of the piriform sinus

cartilage invasion • extension into esophageal verge or paraglotttc space • extralaryngeal spread • prevertebral

(contra-)indications are addressed in the section on the post-treatment neck,

muscle invasion.

CT Morphology

The T-stages of supraglottic larynx carcinomas are given in Table 8,23. Ventral tumors usually extend anteriorly in the preepiglottic fat Low lying and lateral tumors ofthe false cords, ventricles, or aryepiglottic folds primarily extend into the fat of the paraglottic space, Because of their submucosal growth, they are usually diagnosed in advanced stages, Posterior tumors of the (inter)arytenoid region infiltrate posteriorly in the postcricoid hypopharynx, Inferior extension is most important to decide whether supraglotttc laryngectomy is feasible and high quality coronal images with thin sections are important Tumor volume is important for nonsurgical management and low-volume « 6,Ocm3) tumors can be managed effectively by radiotherapy, Cartilage invasion, although infrequent, leads to an upgrade of the tumor and increases the risk of therapy failure, Lymph node spread is commonest in lesions involving the aryepiglottic folds and epiglottis. Spread is to the level II-IV nodes and nodal metastasis is seen frequently, in 30-50% of patients at the

Laryngeal Carcinomas

Almost all tumors are squamous cell carcinomas that are etiologically related to alcohol and tobacco use, Endoscopy and imaging are complementary in evaluating the extent of the tumors as they often extend submucosally Because these tumors may be surrounded by a peri tumoral inflammation zone, imaging with CT or MRI may lead to overestimation of tumor volume, The tumors are traditionally subdivided into supraglottic, glottic and subglotttc tumors, each with their own TNM-staging, pattern of spread and lymph node involvement The types of conservational surgery of the larynx and their

Table 8.23 (UICC,1997)

T-staging

of supraglottic

larynx carcinomas

Tis

Carcinoma

T1

Tumor confined to one subsite vocal cord mobility

T2

T umcr invades more than one adjacent subsite or supraglottis, glottis, or region outside supraglottis (mucosa of tongue base, vallecula, medial piriform sinus)

T3

Tumor

limited to

tion and/or

pre epiglottic T4

in situ

larynx with

with normal

vocal

invasion of postcricoid space tissues

cord fixaarea

or

Tumor extends through thyroid cartilage and/or extends into the soft tissues ofthe neck, thyroid and/or esophagus

Laryngeal Carcinomas time of presentation the primary).

(depending on the stage of

Important imaging features larynx tumors:

Table 8.24

T-staging of glottic larynx carcinomas

(UICC,1997)

in supraglottic

• tumor volume • distance of caudal edge of tumor to commissura anterior • trans glottic extension • invasion of piriform sinus or pre-epiglottic fat • exolaryngeal spread • cartilage invasion.

Tis

Carcinoma in situ

T1

Tumor limited to vocal cords (and commissures) with normal mobility

T1 a

Tumor limited to one vocal cord

T1 b

Tumor limited to both vocal cords

T2

Tumor extends to supraglottis and/or subglottis and/or with impaired cord mobility

T3

Tumor limited to larynx with vocal cord fixation

T4

Tumor invades through thyroid cartilage and/or to other tissue beyond the larynx: trachea, soft tissues of neck, thyroid, pharynx

Glottic Tumors

The subsites of the glottis are the true vocal cords and the anterior and posterior commissures. •

CT Morphology

The T-stages of glottic larynx carcinomas are given in Table 8.24. Vocal cord paralysis can be detected by CT scanning during shallow respiration. The affected vocal cord is not abducted and causes partial luminal obstruction. Glottic tumors typically arise in anterior part of the vocal cords and extend in the anterior commissure (Figs. 8.22, 8.23). From there they may spread into the supraglottic, subglottic or prelaryngeal regions. Lateral extension in the thyroarytenoid muscle may lead to vocal cord fixation, destruction of cartilage, and perilaryngeal spread (Fig. 8.24). Inferior spread to the

subglottis is also common and difficult to detect endoscopically. These extensions determine the type of larynx surgery to be performed. Tumor volume and cartilage status are important for nonsurgical therapy. Low volume tumors «3.5cm3) without cartilage sclerosis can be managed effectively by radiotherapy as well. Lymph node metastases are rare as long as the tumor is confined to the larynx, but become more frequent with extralaryngeal extension. Important imaging features in glottic larynx tumors: • tumor volume • craniocaudal extension (transglottic or subglottic) • involvement of anterior commissure • invasion of the deep paraglottic fat

Fig. 8.22 Small glottic carcinoma (T1) with nodular enhancement of the tumor on the vocal cord. Axial section (a) and coronal volumerendered image (b). a

b

8 Neck

Fig.8.23 Epiglottic carcinoma. The tumor has crossed the midline, has invaded the body and stem of the epiglottis, and is occupying most of the ipsilateral vallecula (arrow).

involvement of cricoarytenoid joints extension to the posterior commissure cartilage invasion.

Subglottic Tumors

True subglottic tumors are infrequent, and most often these represent inferior extension from a glottic lesion. •

Fig. 8.24 Advanced transglottic laryngeal carcinoma causing high-grade laryngeal obstruction and extensive destruction of the thyroid cartilage on the right side.

vanced stages. Cartilage invasion is less frequent than in other laryngeal tumors and may involve the cricoid or thyroid. Lymph node metastases are quite common and involve the level VI (Del phi an) , IV and upper mediastinal nodes. Important imaging larynx tumors '.

1

features

in subglottic

• inferior extent • cartilage invasion.

CT Morphology

The T-stages of subglottic larynx carcinomas are given in Table 8.25. True tumors extend inferiorly to the trachea and thyroid gland or posteriorly to the cervical esophagus. Direct extension through the cricothyroid membrane is seen only in more ad-

Table 8.25 T-staging of subglottic larynx carcinomas (UICC, 1997) Tis

Carcinoma in situ

T1

Tumor limited to the subglottis

T2

Tumor extends to vocal cord with normal or impaired mobility

T3

Tumor limited to larynx with vocal cord fixation

T4

Tumor invades through cricoid/thyroid cartilage and/or extends to other tissues beyond the larynx: trachea, soft tissues of neck, thyroid, pharynx

Cartilage Invasion

Invasion of the cartilage tends to occur at those sites where the perichondrium is interrupted by attachment of collagen fibers. These are located, for instance, at the anterior commissure, cricoarytenoid joint, cricothyroid membrane attachment, and along the thyroid laminae. Nonossified cartilage is relatively resistant to tumor infiltration compared to ossified cartilage. Ossification may precede actual tumor involvement through erosion via osteoclast activity. The diagnosis of tumor invasion with CT is difficult as the ossification pattern of laryngeal cartilage is very variable and tumor tissue has an attenuation similar to nonossified cartilage. Studies by Becker (1998)

Lymphoma have revealed four diagnostic signs of neoplastic invasion using CT (Fig. 8.25): • • • •

extralaryngeal tumor spread sclerosis of cartilage erosion of cartilage lysis of cartilage.

Sclerosis is a sensitive sign for tumor involvement but has limited specificity, especially in the thyroid cartilage. Erosion and lysis of cartilage adjacent to a tumor have a specificity of over 90% but a low sensitivity. This also holds true for extralaryngeal tumor, which is only seen in advanced stages. By combining these signs, a sensitivity and specificity of 80% can be reached with proper technique. State-of-the-art MRI however, may be the preferred modality due to a higher specificity and negative predictive value than CT. Lymphoma

Lymphomas can be classified as either Hodgkin's disease (HD) or non- Hodgkin's lymphoma (NHL) and constitute the second most common malignancy in the neck. In children HD is the most frequent, but overall 75% of cases are associated with NHL. Hodgkin's disease usually involves the lymph nodes, whereas non- Hodgkin's lymphoma frequently involves extranodal sites like the lymphatic tissue of Waldeyer's ring, salivary glands or thyroid.

Nodal Lymphoma

Most patients present with enlarged nodes at the level III -V nodal stations, but superficial nodes or higher level nodes can also be involved. The disease can be either unilateral or bilateral. HD shows contiguous spread between nodal stations, whereas in NHL the spread is often non-contiguous. Necrosis and extracapsular extension in primary disease is infrequent, but can be seen following therapy. It is more frequent in NHL. The nodes rarely calcify unless patients had previous radiation or chemotherapy and are most prominent with sclerosing forms of Hodgkin's disease. Nodal lymphoma with associated nasopharyngeal lymphoid hyperplasia or lymphoepithelial cysts in the parotid is suggestive of HIV infection. In these cases extranodal NHL is often present. •

CT Morphology

Unilateral or bilateral enlarged lymph nodes with a homogenous density are seen. The nodes are often round and clustered. After contrast administration slow and homogeneous enhancement is present. Necrosis or calcification can be seen occasionally in patients with a history of radiation or chemotherapy. Differentiation from nodal metastasis is often impossible. Extranodal Lymphatic Lymphoma

In the neck this form is the most common presentation of extranodal NHL. It may be present in the lymphatic tissue of the adenoids and tonsils as an asymmetrical bulky mass with no infiltration of the surrounding structures or bone ero-

I a Extralaryngeal tumor spread Fig. 8.25 Criteria for CT diagnosis of neoplastic cartilage invasion (Becker, 1997) (a) extra laryngeal tumor spread; (b) sclerosis; (c) lysis (top) and erosion.

b Sclerosis

lll = tumor

o

=

nonossified cartilage

c Lysis and Erosion

0 •

= =

ossified cartilage sclerosis

261

8 Neck

sion. Often. concomitant nodal disease is present. Contrast enhancement is moderate and necrosis is absent. Differentiation of the mass from squamous cell carcinoma is usually not possible. but affected nodes in uncommon locations for SCCa may suggest the diagnosis. In young patients it is also indistinguishable from infectious mononucleosis. In the nasopharynx. NHL masses lack septa that are present in lymphoid hyperplasia {best seen on fat-suppressed Cd-enhanced MRI).

Extranodal Extralymphatic Lymphoma

Parotid gland lymphoma is rare. The gland is involved with either Hodgkin's disease or nonHodgkin lymphoma. There are associations with Sjogren's syndrome, HIV and Waldenstrom's disease. At CT, the disease is most often confined to homogeneous intraparotid nodes that enhance moderately. True extranodal disease is exceedingly rare and can show an ill-defined infiltration of (part of) the gland. Concomitant nodal disease is frequent. Thyroid lymphoma is seen in up to 10% of patients, usually middle-aged or elderly women. Over 80% of patients have a history of Hashimoto thyroiditis, but the two conditions cannot be separated by imaging. Regional adenopathy is seen frequently. Other locations for NHL include the nasal cavity, paranasal sinus, orbit and skin. The presence of multiple masses in extralymphatic sites is highly suggestive ofNHL.

Malignant

Mesenchymal

and Neurogenic

Sarcoma

All sarcomas are rare in the head and neck and require surgical biopsy for definitive diagnosis.

Post-transplantation Disorder (PTLD)

Lymphoproliferative

Post-transplantation lymphoproliferative disorder is a serious complication in patients after solid organ transplantation. It encompasses a spectrum of disorders that range from lymphoid hyperplasia to (non-Hodgkin's) lymphoma. It represents an unregulated B-cell proliferation and the development is associated with EpsteinBarr virus infection. The disease occurs in up to \0% of transplant recipients and depends on the organ transplanted and the immunosuppressive regimens employed. Most often it involves the lung and gastrointestinal system. The neck is an uncommon site for PTLD. It can manifest as focal masses in Waldeyer's Ring or as lymphadenopathy. cr Morphology In the lymphoid tissue of the nasopharynx and oropharynx focal soft tissue masses can be seen that can extend in the parapharyngeal space submucosally. Central necrosis is often present, and there is a variable amount of contrast enhancement. Adenopathy can manifest as a large soft tissue mass or as excessive numbers of normal-sized nodes that appear in clusters. CT is valuable in diagnosis and for monitoring response to treatment.

Tumors On CT the density of the fat is often higher than subcutaneous fat and enhancement may be seen in the soft tissue parts.

Fibrosarcoma

Liposarcoma The neck is a rare location for liposarcomas, but they may arise in the anterior and posterior cervical spaces in patients 40-60 years old. They are often infiltrative and inhomogeneous with fat and varying amounts of soft tissue elements.

The neck is a common location for fibrosarcoma. It presents in adults with a slow-growing painless mass that may reach a considerable size before showing symptoms. Metastases are very common (>60%). The infantile form is much more benign and has a low incidence of

metastasis. CTshows a homogeneous soft-tissue mass that may erode bone. Calcifications. necrosis. and hemorrhage can be present.

Chondrosarcoma In the neck. chondrosarcoma most often originates in the chondroid larynx skeleton. The thyroid and cricoid are most commonly involved. Other locations are the masticator space and skull base. It is usually well defined with submucosal growth. but shows irregular. ring-like calcifications on CT. The airway may be compromised in laryngeal tumors.

Rhabdomyosarcoma This is the commonest sarcoma in children. usually occurring before the 10th year. The nasopharynx. parapharyngeal space. or infratemporal fossa are involved in 30% of cases. The tumor frequently extends through the skull base into the cavernous sinus. Lymph node. lung. and bone metastases are seen in up to 50% of patients. At CT. the tumors present as homogeneous. infiltrative bulky masses with soft tissue density and variable enhancement. There is often bony erosion or remodeling of the skull base or maxillary sinus.

Synovial Sarcoma

in the retropharyngeal space and patients present with hoarseness and dysphagia. The tumors show slow, infiltrative growth and frequently metastasize to the lung. CT shows homogeneous or heterogeneous lesions that often have welldefined margins. Fluid - fluid levels, hemorrhage, and calcifications are common. Malignant Peripheral Nerve Sheath Tumor (MPNST) MPNST is the unified name for malignant neurogenic tumors and describes a highly malignant sarcoma. These sarcomas may arise de novo or from malignant transformation of a neurofibroma or schwannoma, usually a plexiform neurofibroma. Some 65% show distant metastases. They are commonest in patients with neurofibromatosis I and following radiation (latency period 10-20years), but are uncommon in the head and neck. CT is nonspecific and shows a large, fusiform, invasive tumor with necrosis and hemorrhage. Enhancement is heterogeneous.

Malignant Fibrous Histiocytoma The neck is a rare site for this tumor. CT findings are a circumscribed mass of homogeneous softtissue density, often within muscle or along fasciae. Small foci of calcification, necrosis, or hemorrhage may be present.

The neck is the most usual site of this tumor outside the extremities. Lesions are commonest

Salivary Gland

Pathology

Cysts

Retention cysts develop secondarily to an incomplete or intermittent obstruction caused by inflammatory stricture, calculus, surgery, or trauma to the salivary ducts. They have an epithelial lining and most often occur in the submandibular gland. A ranula is a mucous retention cyst of the sublingual gland. The simple form is a true cyst limited to the sublingual space. The plunging or

diving ranula develops from a rupture of a simple ranula and is in fact a mucocele or pseudocyst without an epithelial lining. It extends over the posterior margin of the mylohyoid muscle into the submandibular space. Lymphoepithelial cysts are associated with AIDS and often multicentric and bilateral. They are usually located in the parotid glands and associated with cervical adenopathy.

8 Neck

and have and lack more illtail into

rhage, or calcification, When left untreated, 25% of the tumors will develop malignant transformation (usually adenocarcinoma), When incompletely resected, the tumor can recur in up to 50% of cases within lOyears after surgery, MRI is the preferred modality in these cases,

Between 70% and 80% of parotid, 40-60% of submandibular, 15-30% of sublingual and 20-40% of minor gland tumors are benign, Imaging is unreliable in giving a definite diagnosis but should be used to select therapy and serve as a roadmap for surgery, Table 8,26 gives an overview of solid lesions in the parotid gland,

Small tumors are homogeneously hyperattenuating and well defined on non enhanced studies, With increasing size the masses are more inhomogeneous with hyperattenuating areas of hemorrhage or calcification andlor more hypodense regions of necrosis and cystic degeneration, Enhancement after contrast injection increases (Fig, 8,26) and becomes more homogeneous over time, and is best evaluated on delayed scans,

CT Morphology

These cysts are smooth, well defined fluid attenuation, The walls are thin calcifications, A diving ranula can be defined and often has a characteristic the sublingual space,

CT Morphology

Benign Tumors

Pleomorphic Adenoma Pleomorphic adenoma or benign mixed tumor is the commonest and represents 70-80% of all benign solid masses in the salivary glands, The majority occurs in the parotid, usually lateral to the plane of the facial nerve (superficial lobe), They are usually solitary, well-defined and ovoidshaped, Larger tumors may have a lobulated contour and may show areas of necrosis, hemor-

Table 8.26

Differential diagnosis of parotid gland

tumors Benign

tumors

Pleomorphic adenoma Warth in's tumor"

Oncocytoma Lipoma Facial nerve Malignant tumors

schwannoma

Mucoepidermoid

lymphoma"

Acinar cell carcinoma Malignant mixed tumor Squamous cell carcinoma Metastases

Melanoma' Squamous

Inflammatory

cell carcinoma

Abscess Lymphoepithelial

"9 Sarcoidosis' .• May

CT Morphology

These are small, ovoid, well-defined masses often arising in the tail of the superficial lobe, Small tumors are homogeneous, well-delineated masses that can be multifocal or bilateral. In larger tumors, cyst formation is seen frequently, These cysts may have focal tumor nodules in their walls, Contrast enhancement is inhomogeneous, The differential diagnosis for noncystic lesions includes lymphoma, adenopathy, and granulomatous disease, and, for cystic lesions, cavitating metastases, and lymphoepithelial cysts,

carcinoma

Adenoid cystic carcinoma Non-Hodgkin

Warth in's Tumor Warthin's tumor or cystadenoma lymphomatosum is a monomorphic adenoma of the parotid gland accounting for 2-10% of benign tumors, It is the commonest lesion to occur as a multifocal or bilateral (10%) disease, The lesion is rare in non-whites,

be multifocal

cysts

of skin' (In AIDS)';

Oncocytomas

n1g~~del'n

Oncocyto~~~gl:FedU~~~~ymln~ patients over 55 years old, They occur preferentially in the parotid gland, are slow growing, and are well circumscribed,

Malignant Tumors

a

c

b

Fig. 8.26 Pleomorphic adenoma of the deep lobe of the parotid gland ("iceberg tumor") (a). Hypervascularized mass medial to the right mandible displacing the origin of the pterygoid muscles and the soft palate. Bilateral fatty infiltration of the parotid glands (arrows) is

a common finding. A pleomorphic adenoma at the lower pole of the right parotid gland (b, c) simulates a lymph node. There was a typical echo-free appearance on ultrasound with posterior echo enhancement. Note the dental artifacts on the coronal view (c, arrowheads).

•

Table 8.27 1997)

CT Morphology

The imaging characteristics are nonspecific and mimic both pleomorphic adenomas and Warthin's tumor. Malignant Tumors

Malignant tumors are more frequent in smaller salivary glands and occur more often in Inuit Eskimos and in children. Pain, nodal metastases, and facial nerve paralysis are associated with poor prognosis. Staging is based upon the TNMclassification (TableS.27). Mucoepidermoid Carcinomas

Mucoepidermoid carcinomas are the commonest salivary malignancies in adults and children. They represent 30% of parotid malignancies and occur most often in the parotid or minor salivary glands. They can be oflow, intermediate, or high grade. •

CT Morphology

Low-grade tumors are often well described while the high-grade tumors are diffusely infiltrative. Cyst formation is frequent in lower grade tumors, and hemorrhage or necrosis may be present. At imaging the morphology varies with the histological grade and as such may mimic pleomorphic adenoma or squamous cell carcinoma.

T-staging of salivary

gland tumors

(UICC,

T1

Tumor 2cm or less (greatest dimension) withoutextraparenchymal extension

T2

Tumor greater than 2 cm but less than 4 cm without extra parenchymal extension

T3

Tumor greater than 4 cm but less than 6 cm ortumorwith extra parenchymal extension without facial nerve involvement

T4

Tumor greater than 6 cm or tumor invades facial nerve or skull base

Adenoid Cystic Carcinomas

This tumor occurs most often in the minor salivary glands of the palate, in the parotid or submandibular gland of patients in the fifth or sixth decade. It is the commonest malignancy in the submandibular gland. These tumors grow slowly and have a tendency to show perineural invasion that can lead to pain as well as spread along the facial or mandibular nerve. Cd-enhanced MRI is preferable to CT for evaluating cell ularity and perineural extension. •

CT Morphology

Tumors show a more aggressive growth in the minor salivary glands than in other glands where their aspect is more benign, depending on the subtype and cellularity. Tumors in the minor salivary glands arise in the mucosa and

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therefore cannot be morphologically differentiated from pharyngeal carcinomas. The imaging characteristics are compatible with their histological grade.

Carcinoma ex Pleomorphic Adenoma Carcinoma ex pleomorphic adenoma is a malignant transformation of a (resected) pleomorphic adenoma.

cr

Morphology

Small tumors resemble pleomorphic adenoma with benign characteristics. but larger tumors are more inhomogeneous and infiltrative with areas of necrosis. The CT morphology directly reflects this variation in macroscopy.

Acinic Cell Carcinoma Acinic cell carcinoma occurs almost exclusively in the parotid gland and can be bilateral in 5% of cases. They usually occur after the 40th year but also are the second commonest salivary malignancy in children. CT Morphology

The tumors can either be solid or cystic. The imaging characteristics are nonspecific. The tumors generally have a benign appearance.

Metastases Metastases occur almost exclusively in (the intraglandular nodes of) the parotid gland. The commonest primary is melanoma of the temporal skin or other parts of the neck. Less frequent are head and neck. renal, lung, breast, and Gl carcinomas.

cr

Morphology

Unilateral or bilateral, enlarged intraglandular lymph nodes with a homogeneous density. After contrast administration homogeneous enhancement is seen. Necrosis can be seen occasionally, and is suggestive for malignancy. Differentiation from nodal lymphoma is often impossible.

Inflammatory and Autoimmune Lesions Sial adenitis Acute bacterial and viral sialadenitis commonest pathologies of the salivary Bacterial infections usually ascend from cavity when saliva flow is decreased and pathogens like Staphylococcus aureus,

are the glands. the oral involve

Streptococcus pyogenes. Streptococcus pneumoniae and Haemophilus influenzae. Viral infections can be

caused by mumps, Coxsackie or (Para)influenza viruses and preferentially involve the parotid glands. Intra- and periglandular lymph nodes may be involved in the process. The main role for imaging is to differentiate adenitis from abscess formation. Abscesses are often formed when acute adenitis is undiagnosed or incompletely treated. These abscesses can spread quickly in the para pharyngeal space or other parts of the neck. CT Morphology

The involved gland is dense and somewhat enlarged. It enhances slightly. Abscesses are demarcated regions of low attenuation.

Sialolithiasis Stones can be located in the salivary gland ducts or the extraglandular ductal system. From 80-90% occur in the submandibular gland, W-20% in the parotid and <5% in the sublingual glands. The stones are usually solitary and most often located in the extraglandular ducts. Complete obstruction will lead to gland atrophy while incomplete obstructions are often associated with recurrent infections and swelling. The main role of CT lies in differentiating obstructive from nonobstructive diseases. This directly affects the choice of management. CT Morphology

Sialoliths are readily identified at CT, and even noncalcified stones can be detected. Some 80% of submandibular stones and 60% of parotid stones are calcified. The gland is often enlarged and more ill defined. After contrast there is increased enhancement of the gland on the affected side. In case of obstruction, hypoattenuating dilated ducts can be visualized (Fig. 8.27).

Granulomatous Diseases Fig. 8.27 Inflammation leads to hypervascularization of the affected left submandibular gland (arrows). Note the dilated duct (arrowhead) on the coronal volume-rendered image (a). A curved planar reformation is able to display the whole length of the dilated duct (b). Note that there is no stone visible, which suggests a stricture, e.g., after spontaneous passage of a sialolith.

a

Sialadenosis

This is a noninflammatory and nontender chronic enlargement of the parotid glands that can lead to xerostomia. It seems to be caused by a degeneration of the autonomic nervous system and is associated with a variety of endocrine and nutritional diseases, the most important being diabetes mellitus. It can also be associated with medications, in which case also the submandibular glands can be involved. •

CT Morphology

The CT appearance is nonspecific. The glands are enlarged and appear either dense or infiltrated by fat. Sjogren's syndrome

This systemic autoimmune disorder can occur alone or in association with connective tissue diseases like rheumatoid arthritis. Classically, symptoms are keratoconjunctivitis sicca, xerostomia and connective tissue disease. The adult form is much commoner than the childhood form and often proceeds to more advanced stages. It is mainly seen in women 40-60 years old and is associated with parotid enlargement. The peripheral ductuli and acini are involved first and superinfection may lead to gland destruction and multiple abscesses. There is a 40-fold enhanced risk of subsequent development of lymphoma in the gland.

b

•

CT Morphology

In early stages of the disease the gland appears normal. With progression of disease, the gland enlarges and becomes denser. Later, a honeycomb glandular pattern may develop, which can also be seen in granulomatous diseases or chroni csialadenitis. Contrast can be retained in small punctate or globular collections. Rarely, a macrocystic change can occur resembling HIV -associated lymphoepithelial cysts. Granulomatous Diseases

These systemic diseases can also affect the intraparotid or periglandular lymph nodes. CT usually shows multiple benign appearing masses throughout the gland that may mimic lymphoma or other granulomatous diseases. Diffuse adenopathy is frequent. Sarcoidosis affects the parotids in 10-30% of patients leading to a chronic bilateral nontender multinodular enlargement. Often there is associated adenopathy. When combined with uveitis and facial nerve paralysis this condition is known as Heetiottit's syndrome. Tuberculosis or atypical mycobacterial infections are rare and usually arise from a focus in the tonsils. The parotid and submandibular glands may be affected. The presentation can be acute or more insidious and both clinical and CT findings are nonspecific and can mimic infectious sialadenitis or tumor. Other causes include cat-scratch fever, toxoplasmosis and actinomycosis. In the last two

261

S Neck

forms the disease can be associated with more diffuse lymphadenopathy. In actinomyces infec-

tions the soft tissues (masticator space) are also infected and sinus tracts are formed.

Thyroid and Parathyroid Gland Pathology

Primary diagnostic methods in the thyroid are serology, scintigraphy, and ultrasound. However, on CT examinations for other indications one may come across coincidental findings in the thyroid. CT is also indicated in the evaluation oflarge thyroid tumors, especially when malignancy is suspected. Iodinated contrast material should only be given after TSH-values have been assessed, as hyperthyroid patients may develop long-standing iodine-induced hyperthyroidism (IIHT). The use ofiodinated contrast agents is also contraindicated when patients are scheduled for ablative therapy with radioactive iodine (see also Chapter 3). In these cases, MRI is the modality of choice. Goiter

Goiter is a benign enlargement of the thyroid that is often asymmetric. It can be simple, associated with iodine deficiency, or multinodular, associated with autoimmune disorders or dietary factors. The nodules in multinodular goiter represent either colloid nodules or true follicular adenomas of variable sizes. Patients can be hypothyreotic, euthyreotic, or hyperthyreotic. Multinodular forms can become bulky, extending in the anterior mediastinum. The frequency of carcinoma developing in m ul tinodular goiter is 4-7%. •

Fig. 8.28 Unilateral goiter causing enlargement of the left lobe of the thyroid gland and displacing the trachea to the right side. Hypodense areas within the enlarged left lobe and a small calcification are present, signifying regressive changes.

H

CT Morphology

These cysts are smooth and well defined. Their attenuation depends on the cyst content and varies from hypodense to isodense. The walls are thin and lack calcifications. Large cysts may be hard to identify as arising from the thyroid but hyperattenuating (compressed) thyroid tissue in the wall of these large cysts is typical (Fig. 8.29).

CT Morphology

Mixed solid and cystic zones are present in a nodular and enlarged gland. Hemorrhages and calcifications may be present (Fig. 8.28). Enhancement is patchy and inhomogeneous. The trachea, esophagus or neck vessels can be displaced or compressed. Thyroid Cyst

These are the result of degeneration of adenomas and are usually well-defined lesions, often with fluid attenuation. With elevated protein content, hemorrhage, or infection, these lesions have a higher attenuation.

Benign Thyroid Tumors

Thyroid nodules are very common and occur in 4-7% of the adult population. Colloid {adenomatous} nodules are composed of hyperplastic epithelium and are not true neoplasms. They are usually non- functioning and may be cystic (colloid cyst). Follicular adenomas present as slow growing well-defined encapsulated nodules that can be functioning or non-functioning. They are usually solitary and measure up to 3 cm. They may undergo cystic degeneration, calcification, or hemorrhage. Although benign adenomas do not undergo malignant transformation, "10-'15% of all single (non-functioning)

Malignant

b

a

Thyroid Tumors

c

Fig. 8.29 Large thyroid cyst that leads to displacement and extrinsic compression of the larynx (compare to Fig. 8.11 a). It can be identified due to the hyperenhancing rim that is isoattenuating to thyroid tissue and is

contiguous with the thyroid gland. Axial coronal (b) and sagittal volume-rendered from a thin-section multislice CT data set.

adenomas are malignant. The Hiuthle cell subtype is more variable in size and shape and has less well-defined borders.

Papillcny carcinoma is the commonest thyroid malignancy accounting for 55-75% of cases, especially in young or middle-aged adult females. They may be multifocal or bilateral and can have cystic, hemorrhagic, or calcified components. Although these tumors grow slowly, 50% of patients have metastases in the 10coregional lymph nodes at diagnosis (Table 8.28). Follicular carcinoma is most frequently seen in young female adults and may be either well encapsulated or diffusely infiltrative. It frequently invades vessels. Cystic degeneration is rare. Spread to nodes is infrequent « 10%) but hematogenous dissemination to lung or bones occurs early. Anaplastic carcinoma occurs more frequently in the elderly, especially in people with longstanding goiter. These tumors are often densely calcified and show necrosis. The

•

CT Morphology

Follicular adenomas are usually well defined, relatively hypodense solid masses on noncontrast scans. Cyst formation and calcification can often be seen. Some may show a marked enhancement with iodinated contrast. It is virtually impossible to differentiate benign tumorous nodules from a nodular goiter. Malignant Thyroid Tumors

The commoner malignancies of the thyroid are usually well differentiated. Risk factors include exposure to ionizing radiation during childhood and prolonged increased levels ofTSH. Table 8.28

image (a), images (c)

Thyroid carcinomas

PTC

FTC

ATC

MTe

Incidence

55-75%

15-20%

8-15%

2-8%

Iodine-storing

+ +

Metastases

Regional LN Lungs

Lungs Bone

Regional

Regional LN Lungs Liver Bone

PTC = papillary thyroid carcinoma, FTC = follicular thyroid carcinoma, ATC MTC = medullary thyroid carcinoma, LN = lymph nodes

= anaplastic

thyroid carcinoma,

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8 Neck Table 8.29 1997)

TN-staging

of thyroid

carcinoma

(UICC, •

CT Morphology

T1

Tumor 1 cm or less in greatest dimension, limited to thyroid

T2

Tumor between 1 cm and 4 cm in greatest dimension, limited to thyroid

T3

Tumor more than 4 cm in greatest dimension, limited to thyroid

T4

Tumor of any size extending beyond the capsule

NO

No regional lymph node metastasis

N1a

Regional metastasis in ipsilateral cervical lymph nodes

The T-stages of thyroid carcinomas are given in Table 8.29. The pattern of spread of the various tumor types is provided in Table 8.28. Imaging findings are nonspecific. There is much overlap between benign and malignant tumors. Thyroid carcinomas are well- to ill-defined lesions, often with inhomogeneous contrast enhancement. Areas of hemorrhage, necrosis, calcification, and cystic degeneration are often present. Aggressive carcinomas show extrathyroid spread or vascular invasion.

N1 b

Regional metastasis in bilateral, midline or contralateral cervical or mediastinal lymph nodes

Parathyroid Tumors

tumor is aggressive and invades neighboring structures commonly. It metastasizes early, with 70-80% of patients having affected nodes, which are often necrotic. Medullary carcinoma is rare. It has a familial incidence and can be part of MEN -syndromes: MEN IIa associated with pheochromocytoma and parathyroid adenoma and MEN lib associated with mucosal neuroma and marfanoid facies. About 50% of patients have locoregional metastasis in the lymph nodes and distant metastases are common. Metastases to the thyroid are rare and often clinically occult. The commonest primaries are bronchogeniC and renal cell carcinomas. The lesions are usually multiple and sometimes hemorrhagic.

Adenomas of the parathyroid can be found along the tracheoesophageal groove anywhere from the upper neck to the mediastinum, but frequently lie posterior to the thyroid poles. In most cases they are associated with hyperparathyroidism. Multiple adenomas can be associated with MEN I and MEN IIa syndromes. Carcinomas are very rare and usually cause hyperparathyroidism. They are slowly growing tumors that may mimic adenomas but grow locally invasive. They spread via lymphatics and hematogenously in 30% of cases. •

CT Morphology

An adenoma is a well-defined soft tissue lesion that is hypodense relative to the thyroid (Fig. 8.30). Some 25% of adenomas enhance. Sensitivity ofCT is however only 60-70%. The diagnosis using ""Tc-sestamibi scintigraphy is much more sensitive. Carcinomas can only be differentiated if local invasion or metastases are visible.

Fig. 8.30 Parathyroid adenomas may appear as a hypodense expansile mass on the posterolateral aspect of the thyroid gland (a), showing a marked attenuation difference to the thyroid tissue (after IV contrast administration). Ectopic parathyroid adenoma lateral to the aortic arch (b).

a

b

Cellulitis

Inflammatory

Thyroid

Lesions

Acute infective (suppurative) thyroiditis is a rare cause for swelling with pain, fever and dysphagia. Commonest pathogens are Streptococcus hemolyticus, Staphylococcus aureus and Streptococcus pneumoniae. It can be associated with a fourth branchial cleft anomaly and may be the result of piriform sinus or thyroglossal duct fistulae. Hashimoto's [lymphocytic) thyroiditis is a chronic autoimmune disorder with inflammation that leads to a symmetric nodular enlargement of the gland. It occurs in middle-aged women with hypothyroidism and predisposes to non- Hodgkin's lymphoma. It is associated with Graves' disease and other autoimmune disorders.

and Abscess

Riedel's thyroiditis is a variant chronic flammation giving extensive fibrosis leading hypodense, enlarged gland at CT. It may press the esophagus and is associated with diastinal and retroperitoneal fibrosis. Slight trast enhancement may be seen.

into a commecon-

De Quervain :s {granuloma taus} thyroiditis is a

subacute disease of middle-aged women. It is associated with viral infections ofthe upper respiratory tract. Patients may progress from hyperthyroidism through hypothyroidism to euthyroidism in 1-2 months. At CT the gland is symmetrically enlarged or may show a dominant mass.

Infectious Diseases Cellulitis

and Abscess

Most localized infections are treated by antibiotics. CT is only required in nonresponsive cases, and can differentiate cellulitis from suppurative adenitis with edema and abscess formation. Dental infections: Infections from dental origin are common and may spread to the sublingual/submandibular, parapharyngeal, or masticator spaces. Abscess formation is seen frequently and there may be cortical erosion of adjacent bone. Miicosal lesions: Infections of the nasopharynx, oropharynx, tonsils, and middle ear can extend into the retropharyngeal space via spread to the retropharyngeallymph nodes. Cellulitis or suppurative adenitis may occur and associated swelling may cause bowing of the pharyngeal wall. Abscesses can occur locally ([perij tonsillar) or as a complication of an abscessed adenitis (retropharyngeal). In this age of broad-spectrum antibiotics, retropharyngeal abscess formation is more often caused by penetrating trauma, diskitis, or surgery. In the larynx, acute supraglottitis can spread locally to involve the glottic or subglottic larynx and hypopharynx and may cause life-threatening swelling of these structures. Ludwig's angina is an extensive, acute cellulitis of the floor of the mouth that can be life threatening. It is generally seen in middle-aged men with infected molars. Streptococci or

staphylococci are the causative agents and the infection involves the sublingual and submandibular spaces bilaterally. Abscess formation is infrequent. If not controlled, rapid spread to the suprahyoid soft tissues can ensue. Cellulitis can easily spread to adjacent facial spaces or extend along fascial planes, upward or downward. It may extend into the mediastinum via the visceral and posterior cervical spaces. Abscesses conform to fascial spaces and can occur anywhere in the neck. Localized abscess collections may give a clue to their origin by their location. Extensive infections may be complicated by venous thrombosis, osteomyelitis, and airway narrowing from mass effect. CT Morphology

Cellulitis may present

with a soft tissue mass showing marked contrast enhancement. There is infiltration and edema with thickening of the skin, obliteration of normal fat planes and irregular thickening offasciae (Fig. 8.31 a). Gas inclusions are indicative of infection with gas-producing bacteria (Fig. 8.31 b). Myositis may cause enlargement of muscles. Infective (suppurative) adenitis is characterized by enlarged lymph nodes with a low-attenuation center, suggestive of liquefaction necrosis. Surrounding edema leads to a smooth expansion of the structures within the space with displacement of the pharyngeal wall anteriorly. Abscesses are masses with low attenua

271

8 Neck

a

c

b

Fig. 8.31 (a) Cervical cellulitis appears as an inflammatory mass that obscures the sternocleidomastoid muscle and the intrinsic neck muscles, (b) Gas-forming cellulitic inflammation of the cervical soft tissues on the right side, with significant soft-tissue swelling and mycotic aneurysm of the common carotid artery due to salmonella sepsis. The inflammatory process shows pe-

ripheral enhancement (arrow), (c) Incipient parapharyngeal abscess (arrow) contains a very small area of liquefaction partially surrounded by an enhancing wall. There is significant concomitant soft-tissue swelling with slight displacement and narrowing of the pharyngeal lumen.

tion that show enhancing peripheral capsules (Fig. 8.31 c). Abscesses may contain air and often show surrounding edema. Severe complications of neck infections include septic jugular vein thrombosis with or without distant embolization and mediastinitis (see Fig. 8.32). Arterial rupture is a very severe complication of disseminated neck infection. It is preceded by small hemorrhages with hematoma formation and has a protracted course. It usually affects the internal carotid artery.

changes. Infections with atypical mycobacteria are more often seen in children and usually involve a more localized, unilateral lymphadenopathy, commonly in the upper cervical nodes.

Tuberculous

Adenitis (Scrofula)

This accounts for 5% of cases of cervical lymphadenopathy. The disease is caused by Mycobacterium tuberculosis or atypical mycobacteria, is usually painless and seen especially in young adults. In tuberculosis, it mayor may not be a manifestation of a systemic disease. In AIDS-patients there may be co-existence of tuberculous adenitis and Kaposi's sarcoma. •

CT Morphology

Nodes can be homogeneous or show central necrosis with rim enhancement or calcification. A multilocular complex of nodes is a specific sign. The posterior triangle or jugular nodes are involved without surrounding inflammatory

Necrotizing Fasciitis

Necrotizing fasciitis causes a rapidly progressive infection of subcutaneous tissue and deep fascia. It can be caused by anaerobes or streptococci and leads to widespread necrosis, especially in immunocompromised patients. •

CT Morphology

Gas may be present, but otherwise distinction from cellulitis is difficult. Miscellaneous Diseases with Adenopathy

Infectious mononucleosis is primarily caused by the Epstein-Barr virus. It is associated with lymphadenopathy, and most often the posterior cervical nodes are involved. Hyperplasia of the lymphatic tissue ofWaldeyer's ring is common. Castleman's disease (angiofollicular lymphnode hyperplasia) originates in the mediastinum but can be multicentric in W% of cases. Most often this affects male patients in their

Arterial Thrombosis and Dissection 50 s or 60 s. The nodes show an early dense uniform enhancement following contrast administration. Sarcoidosis gives rise to diffuse adenopathy. The nodes are homogenous and do not show necrosis. There is often involvement of Waldcyer's ring and the salivary glands. Cat-scratch disease is a granulomatous infection in young patients caused by Bartonella benselae. It can be associated with painful cervical adenopathy. Usually one node group is involved, but multiple sites can be found in disseminated disease (in 5-'10%). Kimura and Klkucbi disease are rare causes for cervical adenopathy in oriental patients. Kimura occurs in young males and shows enhancing nodes and salivary gland infiltration.

Kikuchi is more frequent in young females, showing a necrotizing lymphadenopathy. Fungal Infections

Histoplasmosis is ubiquitous in the central US. Rarely, it may show granulomatous masses in the larynx, oropharynx, or trachea. Cryptococcosis is usually seen in immunocomprornised patients that in rare cases may show a neck mass originating from suppurative adenitis. In South-America, acute or chronic paracoccidioidomycosis (South American blastomycosis) can be encountered which shows mucosal infection of the oral cavity or lymphadenopathy.

Vascular Lesions Jugular Venous Thrombosis

Thrombosis of the internal jugular vein can be the result of infection, obstruction, advanced tumors, IV drug abuse, or sinus thrombosis, postoperative or after insertion of central venous catheters. •

CT Morphology

A fresh thrombus is hyperattenuating. The acute thrombophlebitic phase shows an enlarged thrombus- filled vein with loss of adjacent soft tissue planes and enhancing vasa vasorum (Fig. 8.32). This is often associated with fluid (venous transudate) in the retropharyngeal space. In the thrombosed phase perivascular changes have resolved, but enlarged collaterals are shown.

•

CT Morphology

Thrombosis: The imaging features are comparable to venous thrombosis Dissections: CTA can show an endoluminal membrane separating the false (often bigger) and true lumen (often smaller). If one lumen is thrombosed, the only sign of dissection may be a long narrowing of the artery (Fig. 8.33).

Arterial Thrombosis and Dissection

Thrombosis is often associated with pseudoaneurysm formation and leads to ischemic symptoms or Homer's syndrome. Dissections of the large arteries of the neck can be post-traumatic, part of an aortic dissection, or spontaneous. They are an important cause of ischemic cerebral lesions in young patients.

Fig. 8.32 Fresh thrombosis of the internal jugular vein (arrow). Note the inflammation in the anterior parapharyngealspace.

273

274

8 Neck Fig. 8.33 Traumatic dissection of the carotid artery in a young man following a motor vehicle accident. The false channel is completely thrombosed (a) making a diagnosis from axial sections almost impossible. There is, however, occlusion of the medial cerebral artery on the right side, seen on coronal thin-slab MIP from a multislice CT data set (b). The distal portion of the internal carotid artery (arrow) and the A 1 segment of the anterior cerebral artery are stenosed. Note the excellent collateral supply of the right-sided cortex.

b

a

Trauma

Spiral CT has become the primary investigation for important soft tissue trauma and can be used for soft tissue, vascular, pharyngeal, and laryngeal lesions.

aneurysms. Due to a higher incidence of spinal lesions, vertebral artery lesions are more frequent than carotid lesions, with most lesions occurring in the C1-C2 and C6-C7 segments. These patients are often asymptomatic.

Edema and Hematoma

•

In traumatic lesions of the cervical spine or associated ligaments, prevertebral space edema or hematomas may develop that can extend up or downward. Retropharyngeal hematomas are much less frequent, but can result in significant airway compromise. Air can dissect retropharyngeally after laryngeal trauma or barotrauma from assisted ventilation after intubation. Vascular Injuries

Vascular injuries are most frequently caused by penetrating trauma such as stab or shot wounds and are preferentially diagnosed by CT angiography. The source images can also visualize the trajectory of the penetrating injury in the soft tissues. Penetrating injuries lead most frequently to carotid occlusion (35%), pseudoaneurysm formation (35%) and much less frequently to arterial dissections. Vertebral artery in] ury is usually associated with spinal fractures. Blunt trauma leads to stretching of vessels and is more often associated with occlusions (35%) and dissections (35%) than pseudo-

CT Morphology

Direct findings of vessel injury include irregular margins, contrast extravasation, caliber changes, or lack of enhancement. Indirect signs like carotid space hematoma or bone or metal fragments close to vessels can be evaluated at the same time. In blunt trauma, there are frequently signs of vertebral fractures. Laryngotracheal

Injuries

These injuries are frequently a complication of penetrating trauma and seen only seldom after blunt trauma. The mortality rate of these injuries, especially major dislocations, is high. CT provides the best evaluation of the extent of fractures, dislocations and hematomas. •

CT Morphology

Fractures: cricoid ring fractures are often m ul tipIe with the anterior fragment collapsed posteriorly. There may be associated tears at the petiole of the epiglottis due to shearing forces. Exposed fragments in the airway are prone to infection and must be removed. Thyroid fractures are either horizontal (more often in adults), vertical (more in children), or com-

Larynx Surgery pound. It can sometimes be difficult to detect horizontal ones by CT. There is associated soft tissue swelling and hematoma formation, all leading to airway compromise. Dislocations: cricoarytenoid dislocations can be caused by relative minor trauma such as intubation. There is usually a ventral dislocation. Vocal cord paralysis and aryepiglottic fold hematoma can serve as indicators of such dislocations. Cricothyroid dislocations require much

greater forces and are associated with cricoid or thyroid fractures in most cases. The distance between the cricoid and thyroid is enlarged and the cricoid can be rotated relative to the thyroid. There is a large risk of recurrent laryngeal nerve injury in these dislocations. Hematomas: hemorrhage in the soft tissues is associated with fractures, but can also occur independently. The paraglottic fat and subglottic region are the most likely regions involved.

Post-Treatment Neck

Because of the changed morphology of the treated neck, a thorough knowledge of the therapy undertaken and its expected changes is very important in the evaluation of patients after surgical or radiation therapy. Even so, posttreatment imaging remains a significant challenge and is often associated with a limited sensitivity for diagnosing local recurrence. LarynxSurgery

The goal of oncologic surgery in the larynxpharynx is complete resection of tumor with preservation of speech and deglutition. In the larynx, the most frequent conservational procedures and their resulting morphology are reviewed below (Fig. 8.34; Table 8.30).

Vertical Hemjiaryngectomy

In this procedure the ipsilateral vocal cord, paraglottic space and part of the thyroid cartilage are resected. Variations include resection of one arytenoid for posterior spreading lesions or the anterior aspect of the opposite true cord in anterior extensions. These procedures are indicated for limited glottic carcinomas with normal cord mobility and no supra/subglottic extension. Contraindications are extension to the false cord or in the subglottic as well as paraglottic or cartilage invasion. •

CT Morphology

The laryngeal axis is tilted and the thyroid on the side of resection is shortened, often with a paramedian plane of section. The adjacent car-

b VH Fig. 8.34 Lines of resection of procedures used in conservation surgery of the larynx, (a) horizontal supraglottic laryngectomy (HSL), (b) vertical hemilaryngectomy

(VH), (c) supracricoid laryngectomy with cricohyoidopexy (SL + CHP), and (d) supracricoid laryngectomy with cricohyoidoepiglottopexy (SL + CHEP).

275

276

8 Neck Table 8.30

CT landmarks

after conservative Hyoid

Horizontal supraglottic laryngectomy

bone

Present

laryngeal

surgery

Present

Supracricoid larynqectorny- CUP

Present

1997)

Thyroid

Cricoid

Arythenoid'

Arytenol

Resected

Upper third resected

Not modified

Present

Present

Ipsilateral

Not modified

(resected)

Vertical hemilaryngectomy

(Maroldi,

Epiglottis

Present

ala resected

Resected

Resected

(resected) Resected

Present

Shifted

Resected

Present

closer to

(present)

hyoid bone Supracricoid

qectomy-Cf-lf

laryn-

l>

Present

Presert

Resected

Suprahyoid

Shifted

Resected

closer to

(present)

Present

hyoid bone a

Ipsilateral,

"Contralateral,

Cf-H'ecricohyoidopexy,

CHEP=cricohyoidoepiglottopexy

tilage often becomes irregular and sclerotic. Glottic symmetry is usually distorted from scar tissue between the thyroid and arytenoid and often a laryngocele is present.

Supracricoid Laryngectomy Supracricoid laryngectomy with cricohyoidoepiglottopexy entails resection of the thyroid cartilage (except the cornu inferior). one arytenoid. paraglottic tissue. both true and false vocal cords and the lower portion of the epiglottis ineluding the pre epiglottic space. The epiglottic tip and hyoid are sutured to the anterior arch of the cricoid. This is the procedure chosen for more advanced glottic carcinomas. Contraindications are arytenoid fixation. invasion of preepiglottic fat. or massive extralaryngeal growth. Supracricoid laryngectomy with cricohyoidopexy differs in the fact that the entire epiglottis and its preepiglottic fat are resected. followed by pexis of the hyoid to the anterior cricoid. lt is performed for advanced supraglottic carcinomas extending into the ventricle as well as for transglottic carcinomas with limited cartilage invasion. Arytenoid fixation. invasion of cricoid or extension beyond the larynx (tongue base or valleculae) are contraindications.

CT Morphology

The thyroid and (part of) the epiglottis are absent while the hyoid and cricoid are adjacent in the midline. The redundant mucosa around the apex of the preserved arytenoid is regular, but a soft tissue pseudocord is seen extending downwards into the subglottis. The axis of the asymmetric larynx is often oriented in the transverse plane and pharyngeal pouches with air or fluid can be observed lateral to the preserved arytenoid.

Horizontal Supraglottic Laryngectomy In this procedure. the false cords. epiglottis and pre epiglottic space and upper part of the thyroid are resected. although variations exist in which part of the tongue base. piriform sinus or one arytenoid are also resected. lt is performed for selected supraglottic carcinomas with normal cord mobility and without extension to the ventricle. Contraindications are lesions that involve both arytenoids. postcricoid area. tongue base. cartilage or glottic region (crossing of vent riel e). CT Morphology

The anterior glottis is elevated just below the tongue base with hyoid and thyroid remnant close together and often irregular. The residual mucosa around the preserved arytenoids used for reconstruction is often thickened and redundant resulting in asymmetry of this neovestibule.

Radiation

Total

Laryngectomy

Total laryngectomy has to be performed if lesions cannot be treated conservatively as in subglottic extension or extensive cartilage invasion by the tumor. This is also the procedure for local recurrences or if radiotherapy has failed or produced osteoradionecrosis. All laryngeal structures. the hyoid bone. the piriform sinus. and strap muscles are resected and the defect is reconstructed in a tube-like neopharynx connecting the base of the tongue with esophagus. Contraindications are synchronous tumors or hematogenous distant metastases. •

CT Morphology

The resected structures

are absent resulting in a collapsed. irregular aspect of the mucosa. The neopharynx is a tubular structure with a regular wall thickness and smooth external margins. Also, the walls of the tracheostoma are thin and regular.

Neck Dissections

Surgery to the head and neck is often associated with some form of neck dissection for resection of (possible) positive lymph nodes. The terminology has recently been standardized and several forms are practiced. Radical neck dissection: In the classical radical dissection, all lymph nodes at levels I-V are resected, and the spinal accessory nerve, the sternocleidomastoid muscle and the internal jugular vein removed. Sometimes the superior belly of the omohyoid muscle is also removed. Modified radical neck dissection: In this procedure all lymph nodes of the radical dissection are removed but one or more major non-lymphatic structures are preserved. Selective neck dissections: Beside preservation of major non-lymphatic structures, selected lymph node levels are also preserved. There are four subtypes:

• supraomohyoid:

resection of all lymph nodes at levels I - III located above the superior belly of the omohyoid and anterior to the posterior margin of the sternocleidomastoid; • posterolateral: resection of all lymph nodes at levels II-Vas well as suboccipital and

Therapy

retroauricular lymph nodes (especially used for melanoma of the scalp); • lateral: removal of all lymph nodes at levels II-IV along the internaljugularvein; anterior compartment: resection of all lymph nodes at level VI from the hyoid bone level down to the suprasternal notch level.

Extended radical neck dissection: In this procedure one or more additional lymph node groups andlor nonlymphatic structures are resected compared to the radical neck dissection. Examples can include level VI, suboccipital or periparotid nodes and nonlymphatic structures like the carotid artery, vagus or hypoglossal nerve, or paraspinous muscles. Because the post-surgical neck is usually evaluated by a combination of clinical and endoscopic means, the role of CT in diagnosing recurrence is not clear. It is probably best reserved for patients with a high risk of recurrence based on pretreatment findings and clinical follow-up and for evaluation of late complications. It can evaluate deep extension or guide biopsy. For these high-risk patients, an early post-treatment baseline study may be beneficial. Tumor recurrence is suspected by the presence of an irregular, enhancing soft tissue mass > 1 cm, thickened commissure or lysis of residual cartilage. CT Morphology

With radical dissections the neck is markedly asymmetric with absence of lymph nodes and characteristic nonlymphatic structures. The trapezius muscle is atrophied with concomitant hypertrophy of the levator scapulae muscle. In selective dissections only minor differences may be present, especially with regard to lymph nodes and surrounding fat.

Radiation

Therapy

Primary radiation therapy is generally possible for Tl and T2 tumors as well as selected T3 tumors of the neck. It is the primary modality for nasopharyngeal and small glottic tumors but for other tumors there is an institutional variability based on local experience. In general. large volume tumors or tumors with massive invasion of the pre-epiglottic fat are a contraindication

8 Neck

for primary radiation therapy. Secondary radiotherapy is often performed in T3 and T4 tumors as an adjunct to conservation or radical surgery to get a better local control. Successful radiation therapy will lead to considerable shrinkage of the tumor within months after its institution. Failure of radiation therapy is suspicious when at 4 months after therapy 50% or more of the tumor mass is still visible. However. the high doses (>60Gy) used in radiation therapy lead to diffuse changes in the tissues in the neck. mainly as a result of inflammatory reaction with edema. fat infiltration. and fibrosis. The high doses can also be associated with complications like soft-tissue necrosis with ulceration or fistulae and osteochondronecrosis. Radiation treatment can cause a progressive inflammatory mucositis with edema. Severity varies with the type of treatment. The process is most severe 2-4 weeks after treatment and often involves the pharynx or larynx. There may be associated swelling of the retropharyngeal structures and airway narrowing. In closes of 45 Cy and larger. the salivary gland is affected too. resulting in a chronic sialadenitis with hypoplastic glands and xerostomia. Postradiation surveillance by palpation or endoscopy is often difficult. Therefore. it would be preferable to rely on imaging. Cost-effectiveness issues limit routine follow-up with imaging studies in high-risk patients. Pretreatment fac-

tors like tumor volume. cTNM-staging and cartilage invasion can identify patients at high risk of treatment failure and in these patients a baseline study at 4 months is advisable. Patients at low risk or those where there is clinical suspicion of complications can be imaged as needed. Early results of FDC-PET and especially 'Thallium-SPECT are promising. resulting in an improved accuracy for distinguishing between recurrence and radiation changes. •

CT Morphology

Expected changes after radiation therapy can be manifold and depend on the dose given. type of therapy and irradiated portals. The changes are often symmetric. start within weeks. and may resolve within 12-24 months or persist indefinitely. Usually, the skin and platysma are thickened with reticulation of subcutaneous and deep fat structures. The mucosal structures and pharyngeal wall may thicken also, with surrounding edema and increased enhancement. Salivary glands show a chronic adenitis with increased enhancement and atrophy and normal lymph nodes will atrophy. In the larynx, specific changes include infiltration of pre epiglottic and paraglottic fat, thickening of vocal cords and epiglottis, and mucosal thickening at the glottic commissures and subglottic region. Cartilage is usually unaffected by the radiation.

Acknowledgments We are grateful for the advice of Jonas A. Castelijns. MD PhD and Suresh K. Mukherji. MD who have helped to organize and write this

chapter and who have given incorporate their material.

us permission

to

9 Lungs and Tracheobronchial System c. Schaefer-Prokop,

M. Prokop

Anatomy Examination Congenital

282 Technique Disorders

286 ........................................................

292

Tracheal Bronchus, Cardiac Bronchus (p. 292) Bronchial Atresia (p. 292) Tracheobronchomegaly (p. 292) Bronchogenic Cyst (p. 293) Pulmonary Sequestration (p. 293) Venolobar Syndrome, Scimitar Syndrome, Congenital Hypogenetic Lung Syndrome (p.294) Anomalous Pulmonary Venous Drainage (p. 294) Acquired Disorders of the Tracheobronchial System Nonneoplastic Tracheal Stenosis (p. 295) Bronchiectasis (p. 296) Bronchiolitis(p.298) Allergic BronchopulmonaryAspergiliosis (ABPA)

295

(p. 302)

Tracheobronchial Tumors

303

Benign Tumors (p. 303) Malignant Tracheal Tumors (p. 304) Carcinoid Tumor (p. 304) Bronchial Carcinoma (p. 305) Secondary Tracheobronchial Neoplasms (p. 313) Pulmonary

Nodules

314

Metastases(p.315) Granuloma (p. 317) Pulmonary Pseudotumor (p. 317) Hamartoma (p. 317) Carcinoid Tumor (p. 317) Septic Emboli (p. 318) Peripheral Bronchial Carcinoma (p. 318) Predominantly Focal Pulmonary Opacities Atelectasis(p.321 ) Rounded Atelectasis (p. 321) Bronchiolitis Obliterans with Organizing Hemorrhage, Hemosiderosis (p. 323)

Pneumonia

.

319

(BOOP) (p. 321)

Continue

/\

Alveolar Cell Carcinoma (p. 324) Lymphoma (p. 324) Kaposi Sarcoma (p. 327) Pulmonary Blastema (p. 328) Pulmonary Infarction (p. 328) Wegener's Disease (p. 328) Churg-Strauss Syndrome (p. 329) Vascular Malformation (AVM) (p.330) Infectious

Diseases

330

Bacterial Pneumonia in the Normal Host (p. 332) Atypical (Nonviral) Pneumonia in the Normal Host (p. 333) Viral Pneumonia in the Normal Host (p. 334) Tuberculosis and Atypical Mycobacterioses (p. 335) Bacterial Pneumonia in Immunocompromised Patients (p. 336) Viral and Pneumocystis carinii Infections in the Immunocompromised Opportunistic Fungal Infections (p. 338) Lung Abscess, Cavitation and Pneumatocele (p. 340) Differentiation of Infectious from Noninfectious Disease (p. 341)

Patient (p. 337)

Diffuse Interstitial Reticular or linear lung Changes Pulmonary Edema (p. 341) Idiopathic Interstitial Pneumonia Asbestosis (p. 347) Diffuse Nodular lung

341

(liP), Collagen Diseases (p. 344)

Changes

. . . . . .. . . . . . . . .. . . . . . . . . ..

348

Pulmonary Lymphangitis Carcinomatosa (PLC) (p. 348) Sarcoidosis (p. 350) Silicosis and Coal Worker's Pneumoconiosis (CWP) (p. 352) Diffuse lung Changes with Increased Parenchymal Density.. Extrinsic Allergic Alveolitis, Chronic Allergic Pneumonitis Pulmonary Alveolar Proteinosis (PAP) (p. 356) Lipoid Pneumonia (p.357) Eosinophilic Pneumonia (p. 358) Drug-Induced Lung Disease (p. 358)

354

(p. 355)

359

Parenchymal Diseaseswith Decreased Density Emphysema (p. 359) Congenital Lobar Emphysema (p. 361) Swyer-James Syndrome (p. 362) Pulmonary Diseaseswith Cystic Changes Langerhans Cell Histiocytosis (p. 362) Pulmonary Lymphangioleiomyomatosis

.

(LAM) and Tuberous Sclerosis

Acquired Disorders of Pulmonary Vasculature Chronic thromboembolic pulmonary hypertension Pulmonary Veno-occlusive Disease (p. 366) Hepatopulmonary Syndrome (p. 366)

. (CTEPH)

362

(p. 364)

365

(p. 365)

Continue

>

Trauma

and

Postoperative

Chest

Trauma

Adult

Respiratory

Pneumonectomy lung

Chronic Airway

Distress and

Syndrome

Lobectomy

Transplantation

Reimplantation Acute

367

Changes

(p.367)

Rejection

(p. 368)

(p. 370)

371

.

Response

(p. 371)

(p. 371)

Rejection Complications

(p. 371) (p. 371)

Plain chest radiographs are an indispensable part of the diagnostic work-up of suspected thoracic disease. However. projection effects and limited density resolution limit the value of plain films as a diagnostic tool. Computed tomography. with its excellent contrast resolution and nonsuperimposed views. is the most sensitive modality for evaluating the lung parenchyma. CT is the method of choice for tumor staging. the diagnosis and follow-up of pulmonary and mediastinal lesions. and the detection of occult thoracic lesions that cannot be diagnosed with conventional x-ray films (Table 9.1). High-resolution thin-slice CT scanning (HRCT) improves our ability to define and evaluate fine structural details in the lung parenchyma. HRCT is used for the localization. characterization. and quantification offocal and diffuse parenchyma! lung diseases. The pathomorphologic response pattern of the lung parenchyma is fairly uniform for different etiologies. however. so clinical information is essential for the interpretation offindings. CT is an important adjunct to bronchoscopy for evaluating the airways. By defining the precise location of an abnormality, CT can accurately direct bronchoalveolar lavage (BAL) or bronchoscopic biopsy. CT can visualize surrounding structures as well as bronchial segments that are inaccessible to bronchoscopy.

Table 9.1 Spiral

Indications and Multislice

for CT examination CT

Tumor diagnosis • Pulmonary and mediastinal • Metastases • Tumor characterization • Tumor Detection • (Occult)

tumors

(solitary

nodule)

staging and quantification of infectious processes

• Cavitation Asbestosis, silicosis • Emphysema • Bronchiectasis Localization prior to • Biopsy, bronchoscopy, bronchoalveolar lavage • Differentiation of pulmonary from pleural lesions 1

High-resolution

CT(HRCT)

Diffuse lung diseases • Detection of subtle parenchymal • Morphologic characterization • Ouanification • Localization choalveolar

changes

of parenchymal changes for open lung biopsy or bronlavage

Anatomy •

Trachea

The trachea starts at the inferior border of the cricoid cartilage and is 9 to 12 cm long. In 50% of patients the membranous posterior wall forms a convex bulge projecting into the tracheal lumen. The average transverse diameter of the trachea is 15mm in women and 18mm in men. The upper limits for coronal and sagittal diameters are 26mm in men and 22mm in women; the lower limit of normal is 13 mm in men and 10 mm in women. The extrathoracic tracheal diameter varies markedly during forced inspiration and expiration (by up to 35%), and this should not be misinterpreted as wall instability. The intrathoracic part of the trachea remains fairly constant during the normal respiratory cycle. •

Central Bronchial System

The right main bronchus is shorter than main bronchus and appears adjacent upper lobe bronchus in longitudinal (Figs. 9.1, 9.2). The bronchus intermedius to 4cm distally and presents an elliptical

the left to the section runs 3 section

on the CT scan. The apical segmental bronchus of the lower lobe (B6) arises at approximately the same level as the middle lobe bronchus. The latter then bifurcates into a lateral and medial segment. The left upper lobe bronchus divides into only two segmental bronchi (apicoposterior, B1I2, and anterior, B3). Further distally, the lingular bronchus branches anteriorly from the inferior aspect of the upper lobe bronchus. Its division into a superior (B4) and inferior segmental bronchus (B5) is poorly visualized on axial CT scans because of its oblique course. It originates 1 to 2 cm cephalad to the right middle lobe bronchus. Past the origin of the B6 segmental bronchus, the right lower lobe bronchus divides into four segmental bronchi while the left lower lobe bronchus usually divides into three segmental bronchi (there is no B7). The branching pattern is subject to a considerable range of normal variation.

Fig. 9.1 Anterior view of the central tracheobronchial system. Upper Lobe 1 apical (R) 112 apicoposterior 2 posterior (R) 3 anterior (R/L)

(L)

Middle Lobe 4 lateral (R) 5 medial (R)

,-.:1'----1/2 1Mi!t----3

----5

4

---6 8 ~--9 -T-"'r~~-1

0

Lingula 4 superior (L) 5 inferior (L) Lower Lobe 6 superior (R/L) 7 medial basal (R) 8 anterior basal (R) 8 anteromedial basal 9 lateral basal (R/L) 10 posterior basal (R/L)

Anatomy

Fig. 9.2 Axial anatomy of the pulmonary segments and central tracheobronchial system. The major and minor fissures can be clearly identified (as fine lines or

avascular zones). The intralobar segmental boundaries (broken lines) can only be estimated from the course of veins and segmental bronchi.

283

9 Lungs and Tracheobronchial System

Thickening of the posterior wall of the intermediate bronchus that borders directly on the lung parenchyma is a sensitive sign of a mass lesion (except in the approximately 2 % of individuals with an aberrant superior pulmonary vein). Pulmonary

Vessels

The main pulmonary trunk bifurcates into a short left and longer right pulmonary artery. The right pulmonary artery runs between the superior vena cava and the right main stem bronchus and divides into an upper branch (truncus anterior) and the descending interlobar branch. The left pulmonary artery bridges over the left main bronchus. In some cases. the left upper lobe artery directly originates from the main artery. More frequently. the main pulmonary artery continues into a vertical interlobar or descending pulmonary artery running posteriorly to the lower lobe bronchus. from which the segmental arteries to the upper and lower lobe originate. The upper limit for the vascular diameter is 29mm for the pulmonary trunk. 28 mm for the left pulmonary artery and 24 mm for the right pulmonary artery. The right inferior pulmonary artery presents a circular cross section in the angle between the middle lobe bronchus and the origin of the 86 segmental bronchus (Fig.9.2d). Increased soft-tissue structures or scalloped borders in this area are a sign ofhilar lymphadenopathy. On the right, there are three pulmonary veins: the descending superior pulmonary vein and the middle lobe vein, which both run into the superior venous confluence at the posterior aspect of the left atrium, and the inferior pulmonary vein running more horizontally into the inferior pulmonary venous confluence. On the left there are two veins: the superior pulmonary vein draining the upper lobe and the lingula and the inferior pulmonary vein draining the lower lobe. They run into the superior and inferior venous confluences, respectively. Pulmonary

of each lung is easily traced on axial CT scans in its

posterosuperior-to-anteroinferior

Table 9.2 Azygos

Accessory fissures fissure

caused by downward invagination of azygos vein

Inferior accessory fissure

between medial segment (7) and the remaining lower lobe

Superior fissure

between upper segment and the basal segments t..pper lobe

accessory

Segments

There are ten pulmonary segments in the right lung and eight in the left lung. The major fissure

course

(Fig. 9.2). The minor fissure between the upper lobe and middle lobe of the right lung runs approximately parallel to the scan plane and is identifiable as a nonvascular zone on one to three axial slices. The anatomic relationship between the segmental bronchus and the concomitant artery is helpful to differentiate the middle from the upper lobe: in the upper lobe the artery runs medially to the segmental bronchus while in the middle lobe the artery is located laterally to the segmental bronchus. Thin slices and multiplanar reformations based on multislice CT data sets better demonstrate the course of the lobar fissures than axial slices alone. Additional fissures can be identified more frequently than with any other technique. The commonest accessory fissures include the azygos fissure, the inferior and superior accessory fissure, and the left minor fissure (Table 9.2). Incomplete fissures are frequent findings that allow for collateral air drift and spread of disease. The bilateral inferior pulmonary ligaments represent parietal pleural reflections extending from the inferior aspect of the hilum to the posterior diaphragm. On axial CT images they appear as broad bands between the posterior mediastinum (region of esophagus) to the diaphragm. CT cannot define the segmental boundaries in normally aerated lung, but the segments can be identified indirectly by tracing the central segmental bronchi and the interlobar veins. Substantial variations may be encountered in segmental pulmonary anatomy. A slice collimation of 5 mm or less is necessary for the positive identification of segmental bronchi.

Left minor fissue

between lirwla upper lobe

and left

(6) ofthe

Anatomy •

Peripheral Bronchi

The airways undergo approximately 23 divisions between the trachea and the respiratory bronchioles (Fig. 9.3). Standard CT can define fourth-order bronchi (3 mrn), while HRCT can resolve eighth-order branches (1-2 mrn). The CT detection of normal bronchi relies on visualization of the bronchial wall. The thickness of the bronchial wall is approximately one-tenth the diameter of the bronchus. Normal intralobular bronchi are not visible, as their wall thickness is less than 0.15mm. The presence of visible bronchial structures in the lung periphery (within 2 to 3cm subpleural space) signifies pathologic bronchial wall thickening or ectasia of the small airways. The small airways of the lung « 2m m in diameter) have a total cross-sectional area greatly exceeding that of the larger airways but account for only about 25% of the total airway

resistance. The small airways must undergo severe destructive changes before pulmonary function is measurably affected. •

Lung Parenchyma

The smallest structural unit visible on CT lung scans is the acinus, which is about 6-8 mm in diameter and consists of alveolar ducts, alveolar sacs and approximately 4000 alveoli distal to the terminal bronchiole (Figs. 9.3, 9.4). The secondary lobule is the smallest discrete portion of the lung that is surrounded by connective tissue. It is the smallest anatomical unit that can be clearly identified on HRCT, and assessment of the distribution of abnormalities within it can be helpful in the differential diagnosis of parenchymal disease. It consists of a maximum of 12 acini and has a characteristic polygonal shape with an edge length of 1-2.5 ern, a septal boundary, and a central core consisting of a bronchiolus and arteriole (Fig. 9.4).

Fig. 9.3 Schematic representation of the bronchial divisions, showing the limits of spatial resolution of CT and HRCT.

Sekundary lobe (1-2,5 ~ cm) Bronchi (2-4)

Acinus (6-8

Q

mm)

Primary lobe Bronchioles

Terminal :Respiratory' bronchiole:,bronchiole: , :(17-19) :

, , ,

Alveolar ducts (20-22)

Alveolar saccules (23) 23 generations

I

Visible with CT

Visiblewith HRCTwhen norma

j

Visible with HRCTwhen diseased ,

Bronchiectasis:

Bronchiolectasis

I Emphysema

I

285

286

9 Lungs and Tracheobronchial System

_-----------

Interstiti u m: a) central b) septal c) peripheral

Viszeral pleura

c~ _""f-'--

b

\

)7

~

----

Secondary

Acinus lobule

*

Centrilobular artery -

Interlobular

septum

Terminal ::;', ~\'T-----

bronchiole

ILobular artery

Terminal bronchiole

Respiratory I bronchiole· Central acinar artery *----''

*

Lymph vessels" Pulmonary vein

* Visible only when diseased

• Never visible

Fig. 9.4 Schematic representation ing veins, and interstitium.

of two adjacent

secondary

The arterioles lie 5-10 mm deep to the pleura. The veins take an interlobular course and are visible 1-2 cm below the pleura. Normally the bronchioles cannot be seen on CT. This means that normal Iung contains an immediate subpleural zone 5-10 mm wide that is devoid of visible vascular and bronchial structures. The interstitium is the connective-tissue framework of the lung. Its fiber tracts support the bronchi, the pulmonary vessels, and the membranes for alveolar gas exchange. The interstiti urn consists of the central or axial interstitium, which surrounds the central airways and vascular structures, the septal interstitium, which supports the alveolar walls, and the peripheral interstitium, which forms the basic

lobules with afferent arteries

and bronchioles,

drain-

framework of the lobar and lobular septa. The interlobular septa normally are not visible on CT or may appear only as faint linear structures in the basal lung zones. In the presence of thickened lymphatic vessels or interstitial disease, however, they may become very prominent and define the architecture of the lung parenchyma. The CT density of the lung parenchyma normally shows an anteroposterior and an inspiratory-expiratory gradient. The anteroposterior density gradient is 20 ± IOHU in inspiration and 150±20HU in expiration. The average pulmonary density in inspiration ranges from -900 to -850 HLJ.

Examination Technique

The choice of examination technique for the Iung depends on whether the whole lung is evaluated (usually in cases offocallung disease) or whether a disseminated process can be examined by discontinuous scans with high spatial resol utian (HRCT) . Only multislice scanning allows for a combination of these two different requirements.

Oral contrast is usually not indicated for CT of the lungs unless tumor involvement of the esophagus is suspected (see Chapter 15, p. 548). Intravenous contrast material should always be used for the investigation of mediastinal and pleural abnormalities. Contrast administration is not only mandatory for the evaluation of focal intrapulmonary lesions (tumor, rounded atelec-

Routine Chest CT 287 tasis). Enhancement characteristics are not important for the characterization of diffuse parenchymal changes. Contrast administration is essential for all vascular investigations (pulmonary embolism. aortic studies) and for staging examinations.

Table 9.3

Suggested

protocols

for CT scanning

Routine Chest CT Spiral CT is the standard for examinations of the lung (Table 9.3). Most patients will be scanned in inspiration. Patients with marked dyspnea who are not be able to hold their breath during the scan should breathe shallowly instead.

of the thorax

General Preparation

not necessary

Patient

Supine with arms raised (standard) Prone position with elevated arms: • Differentiating hypostasis from subpleural • Differentiating edema from infiltration

position

Scan range

Respiratory

From thoracic From thoracic phase

inlet to posterior pleural recess (standard) inlet to below the renal veins (suspected bronchogenic

carcinoma)

End-inspiratory scans (standard) End-espiratory scans: • Air trapping: early detection of peripheral airway obstruction • Differentiation of air trapping from ground-glass opacity • Compliance testing (fibrosis versus edema) • Bronchogenic carcinoma: chest wall fixation

Windowing

Lung (standard): Emphysema: Mediastinum (noncontrast): Mediastinum (post contrast):

Scan parameters

l-slice SC/TF/RI

4-slice' SC

t

2-3.75

Standard

5/10/5

Tumor staging, CT-bronchography

3/5/2 t or 2/4/2 i

Screening

fibrosis

5/10/5 i

t'

W/L ~ W/L~ W/L~ W/L~

1500/- 650 800/-800 400/40 400/60

16-slice' SC

1-1.25

t t

1-1.25

i

axial SW/RI

1-1.5 i

t'

MPRb SW/RI

5/4

0.75-1.25

r

0.75-1.25

t

3/3

3/3 cor' 4/4 sag'

5/4

t'

HRCT(discontinuous) (spiral scan)

1-1.5/10-20

Contrast

l-slice V/F/D

4-slice V+N/F/D

16-slice V+N/F/DX

Standard

70/2/20

70+50/2/20

50+50/2/30

Tumor staging

90/3/30

90+50/3/30

90+50/3/25A

Pulmonary vessels e

140/3/20

120+60/3/20

90+50/3/5pd

injection

1-1.25/10-20 1-1.25i

0.5-1/10-20 0.5-0.75i

1-2/10-20 1-1.5/10

1.5/10 cor 1.5/10 sag

d

SC= slice collimation (mm), TF = table feed (mm/rotation), RI = recon. increment (mm), i t = scanning direction, SW = effective section width (mm), MPR = multi planar reformats, axial = axial sections. cor= coronal, sag = sagittal CM= contrast material, Contrast concentration = 300 mg/ml iodine V = volume of CM (ml), N = volume of saline flush (ml), F = flow rate (rnlts), D = start delay. X = trigger region a Pitch P = TF/(NxSC) should be chosen close to 1.5 with 4-slice CT scanners, 1.3-1.5 with 16-slice scanners b MPRare reconstructed from a "secondary raw data set" with SW/RI = 1-1.5/0.7 or 0.5-0.8/0.6 c read: single section mode HRCTwith 0.5-1.5 mm collimation and 10-20 mm increment between sections dread: 25A (5P) = start delay 25 s (5 s) after reaching 50 HU enhancement in the desc. aorta (pulmonary artery) e choose lower dose and kVp setting (CTDI", = 3-6 mGy; 80-100 kVp, depending on patient size and CT scanner)

II

9 Lungs and Tracheobronchial

System

When such a patient is scanned during inspiration, marked artifacts will occur as soon as the patient begins breathing again, Breathing artifacts decrease with caudocranial scanning, The technician, however, has to take care that there is a sufficiently long waiting period (2-4 seconds suffice) between the inspiration command and the beginning of the scan, Otherwise, the lung bases will suffer from marked artifacts due to the final inspiration of the patient. Patient compliance can be improved if they are instructed prior to the scan and asked to hyperventilate (2-5 deep breaths) immediately prior to the spiral examination, With spiral CT, a 5 mm collimation and a l Ornm table feed (pitch = 2) will give a good compromise between spatial resolution along the z-axts, scan duration (usually some 30 seconds), and patient exposure, Reducing the pitch to 1.5-1.8 will slightly improve image quality at the base of the lung because lower pitches are somewhat less susceptible to pulsation artifacts, For multislice CT with 4-slice scanners in a routine setting, a 4 x 2.5 mm (4 x 2-4 x 3 mm) collimation usually suffices unless there is a high likelihood that coronal or sagittal reformations will have to be reconstructed from the data set Under such conditions, volumetric imaging using 4xl-l.25 mm is advocated, With 8- and 16-slice scanners, 1-1.5 mm section collimation can be used on a routine basis, Reconstructing axial sections of 5-7.5 mm section width is sufficient for rou tine interpretation, Reconstruction of an overlapping secondary raw data set with SWIRl = 3/1.5 (for 4 x 2.5 mm collimation) or SWIRl =l.25-2/0,7 (for 1-1.5 mm collimation on 4- to 16-slice scanners) are required only if multiplanar reformations were initially planned or become necessary to solve a particular question on the axial sections, For optimum image quality, the images of the mediastinum should be reconstructed using a soft kernel, while images of the lung require a high resolution kernel. Instead of directly reconstructing two sets of images from the raw data set (larger amount of images to be processed and archived), display filters may be used to smooth images from a single data set reconstructed with a high resolution kernel. For opacification of the mediastinal vessels, relatively small amounts of contrast material are required for spiral CT or multislice CT acquisitions: the injection of some 50-70ml of contrast

material followed by 50ml of a saline flush (2 mlls) is usually sufficient. Because of the substantially reduced absorption in the lungs the radiation exposure (mAs settings) for the chest can be approximately halved as compared to abdominal scanning, Usually a CTDlvol = 5 mCy (more for obese patients, less for slim tndtviduals) yields good results, One has to keep in mind, however, that image noise in the cranial portions of the liver as well as in the shoulders may then be substantially increased, This usually does not affect diagnostic accuracy because liver imaging using a chest protocol is far from optimum (and should be optimized separately anyway), and the spectrum of neck pathology in chest examinations is very limited (lymph nodes, masses extending into the neck), Image quality in these regions may be improved even at reduced exposure settings with the use of longitudinal dose modula-

tion techniques.

Tumor Staging The highest possible image quality should be employed for tumor staging during the initial evaluation of primary tumors or prior to surgery. Since high space resolution is possible only for a short scan range with spiral CT, a composite approach with thin and thick sections has to be taken. We suggest focusing primarily on the pulmonary lesion including the pulmonary hila first, and then scanning the rest of the lung or repeating a scan of the whole lung using the protocol described above. For the initial high resolution scan, a (SWI TFIRI =) 214/2 protocol can be recommended if the scan range can be covered within one breath-hold phase. It is advisable to test the breath-holding capacity prior to the scan because breathing artifacts substantially impair the quality of reformatted images. For larger ranges, a 3/5/2 protocol can be recommended. When only the primary lesion (peripheral nodule) has to be scanned (for in-detail analysis of morphologic features or for nodule volumetry), a 11211 protocol can be employed. With a 11311 protocol, a 50% longer scan range can be achieved at only slightly reduced image quality (if the scanner supports a pitch of 3). With 1 mm sections, the mAs settings have to be increased to ensure sufficient image quality.

High resolution CT (HRCT)

From this thin-section scan multiplanar reformation should be reconstructed as described below for multislice CTscans. For multislice CT, volumetric imaging with a 0.5-1.25 mm collimation (P ':1.5) yields excellent results for 4- to 16-slice scanners. Only in very dyspneic or obese patients should thicker sections be used. The acquired data are reconstructed as a "secondary raw data set" with SW IRl= 0.7 -1.5/0.7. From this secondary raw data set, axial sections of 4-5 mm width are reconstructed, followed by coronal and sagittal thin sections across an intrapulmonary tumor (SW = 1. 5mm). If lymph node, mediastinal, or chest wall involvement is suspected, additional 3-4 mm coronal and 4-5 mm sagittal reformations are reconstructed across the central portions of the mediastinum (including the hila), or across the affected portions of the chest wall. Additional expiratory scans can determine whether a tumor adheres to the parietal pleura (no movement on expiration). Scans after inducing a pneumothorax have been suggested for the same purpose but are substantially more invasive and have the same problems with differentiation between mere adhesion and chest wall infiltration. For best results, a high resolution kernel should be used for the lungs and a soft kernel for the mediastinum and chest wall. However, this will create up to WOO images per patient (two secondary raw data sets). As a compromise, a Single secondary raw data set can be reconstructed with a high resolution kernel. and only the multiplanar images that are reconstructed from this data set are subsequently subjected to softening display filters. For tumor staging, a volume of 70-100 ml of contrast material should be injected at 2-3mlls, followed by a saline flush of 50ml using the same injection rate. A scan delay of 30 seconds yields good results in most patients. Larger volumes and higher flow rates improve contrast enhancement but add little to the diagnostic yield of the scan. Radiation exposure is no limiting factor for tumor staging. Instead, exposure setting should be chosen so that image quality is optimized. If thick axial (and coronal or sagittal) sections are sufftctent, a CT01voi of 5-10 mGy is recommended, but thin-section imaging requires a CTDlYoiof 10-25 mCy, depending on the size of the patient.

High Resolution CT (HRCT) High resolution CT is indicated for diffuse lung disease. If focal lesions are suspected, a sole examination by HRCT is not indicated. In such cases, an (additional) spiral CT should be performed. When volumetric imaging using multislice CT is performed, thick conventional as well as thin HRCT sections can be reconstructed form the same data set. HRCT examination of the lung parenchyma is usually performed with a slice thickness of 1 -2 mm, a high-resolution kernel and discontinuous acquisition with a 10 to 20mm gap between sections. Scanning is generally performed at end inspiration with the patient in a supine position. Expiratory scans are necessary to detect air trapping and thus early (peripheral) airwayobstruction, and to differentiate air trapping from ground-glass opacities. They have also been suggested for testing lung compliance (fibrosis versus edema) or bronchial collapse. It is possible to use a wider gap of > 20 mm for this purpose. Scans in prone position help differentiate hypostatic edema from subpleural fibrosis, and edema from infiltration. Only the areas that were suspicious on the supine scans need to be examined again (usually the lung bases). Scanning primarily in a prone position may be useful for patient with asbestos exposure (HRCT screening examination) because the initial findings with this disease predominantly affect the posterior portions of the lung. For screening purposes, the number of sections may be reduced to keep radiation exposure low. The mAs settings for HRCT of the lungs should be about 50-100% higher than for routine chest CT because of the higher noise on thin sections. Despite the fact that the CTDlw (dose per section, displayed on the user interface for incremental CT) may be quite high (1020 mGy), the dose to the patient remains low because of the large interslice gap (CTDlvol =1-4 mGy) Multislice CT scanners should be used in a Single slice mode. If available, only a Single section should be acquired at each table position, because a set of adjacent sections rarely adds to the diagnostic information but substantially increases patient dose. Acquiring Single sections is not yet possible with most 4-slice scanners but

9 Lungs and Tracheobronchial System can be clone with most 8- and 16-slice scanners. If no single-section mode is available, dedicated HRCT examinations should preferably be performed on single-slice systems. When 4-slice scanners have to be used, discontinuous sections of 4 x 1 mm to 4 x 1.25 mm width (or 4xO.5 mm width on Toshiba units) should be acquired every 10-15 mm, preferably every 20mm. In order to take full advantage of the increased number of sections per scan, one should not only view the axial sections but create thinslab MIP and mIP across the (four) simultaneously acquired sections. This may improve the detection of initial disease in some patients. Multislice HRCT of the whole lung is based on volumetric imaging with 1-"1.25mm section collimation for 4-to 8-slice scanners and 0.5-0.75 mm collimation for 16-slice scanners. In general. a pitch P s 1.5 (P" '6 for 4-slice scanners) can be recommended. A sufficiently high dose (CTOIvoi 'IOmCy) is required for good results. Image noise is reduced when 1.5 mm thick axial as well as coronal or sagittal reformations are reconstructed from a secondary raw data set. Multislice HRCT of the whole lung may add findings unsuspected from conventional discontinuous HRCT. Because very few of these findings appear to hold therapeutic consequences, HRCT scanning of the whole lung cannot yet be recommended on a routine basis (radiation exposure is increased by a factor of 5-10). Because scanning of the whole lung improves side-by-side comparison of findings, it may be indicated for patients in whom even minor changes during follow-up warrant a change in therapy. In such a follow-up situation, scans with reduced dose may be feasible.

Low-dose

CT

Because of the low absorption of the lungs, radiation exposure can be substantially reduced ifC T scanning can focus on the lungs only. With spiral CT. a pitch of 2 should be employed to get the best image at a given (low) exposure dose (see Fig. 5.11). For multislice CT, the same holds true: high pitches yield better quality (in terms of noise) at a given low exposure dose, especially when thin sections are employed (less effect of electronic noise, see pp.120, 139, and 213: also compare Fig. 4.10). The CTDIvoi can be reduced to 2 mGy for most patients.

U sing a less edge-enhancing standard filter substantially improves image quality, especially for ultra-low dose applications, such as bronchogenic cancer screening (see below). For HRCT, a reduced dose (CTDIw= 2-6 mGy) is feasible for follow-up examinations (depending on the therapeutic implications and the subtlety of disease). Increasing the gap between sections to 2cm or higher also reduces patient dose but is only feasible for relatively evenly distributed disease processes. If in doubt, additional sections may be added in the gap between the original sections. With multislice CT and thin-section lowdose scanning, the noise on the original axial sections may be unacceptably high, especially in the mediastinum. Reconstructing thicker sections either primarily from the raw data or from a secondary raw data set will substantially improve image quality (see Fig. 5.18). Since low-dose scanning is at the lower edge of acceptable image quality, any further decrease in quality may reduce diagnostic yield. Obese patients require more dose, otherwise image quality may deteriorate substantially, especially in the lung apex (shoulder regions). Also, a higher kVp setting (usually 140 kVp) should be employed to improve radiation penetration. If in doubt, a single section test scan (which is often performed anyway because of the selection of the reconstruction field-ofview) with the same mAs and kVp settings (not CTDIvoi) as compared to the planned spiral or multislice scan can be used to judge if image quality is sufficient. For this test scan, the CTOIvoi provided on the user interface will be 1.5-2 times (= pitch) higher than the CTDIvoi of the actual scan because (per definition) a single section has a pitch of 1. If available, adaptive noise filtering (see Fig.5.7) substantially improves image quality in critical regions (lung apices). For children, the dose should be adapted to their size (see Table 5.6). In small children, 80 kVp and minimum mAs settings may be employed. The test scan used for determining the reconstruction field-of-view can be used to decide whether image quality is too high (lower dose possible) or too low (higher dose or thicker sections necessary). Low kVp settings can also be recommended for imaging of the pulmonary vessels (see also p.837)

kernel for image reconstruction

vtnuet Low-Dose

Screening

of Bronchogenic

Virtual

Bronchoscopy

Bronchoscopy

and CT Bronchography

and

CT

Cancer

Bronchography

For low-dose screening of bronchogenic cancer, even lower dose may be employed, With conventional spiral CT, a 5110/5 protocol at 120 kVp with a CTDlvoi < 0,8 mGy may be feasible in normal size patients (depending on the minimum mAs setting of the scanner), This corresponds to an effective dose of approximately OAmSv for females and 0,3 mSv for males, It has to be compared to conventional chest radiography, which yields some 0,05-0.1 mSv for a single PA projection, and 0.12-0,2 mSv for PA and lateral radiographs, Again, the dose has to be increased in obese patients, A standard filter kernel for image reconstruction substantially improves image quality, If a suspicious nodule is detected on a screening scan, the patient is brought back and a thin-section CT with moderately increased dose (CIDlYol= 2-4 mGy) is performed with a 112/0,5 protocol and a field-of-view (FOV) that is targeted on the nodule of interest This scan is used for in-detail morphologic analysis (presence of calcifications) and for nodule volumetry. Depending on the size of the nodule (usually < 1 cm) and the screening algorithm, the scan is repeated after 6 weeks to 6 months, Changes in nodule volume by 20%-100%, depending on its size, indicate Significant growth and thus malignantpotentiaL With multislice CT, scanning can be performed immediately with thin sections (0,751,25mm collimation for 4- to 16-slice scanners), Thick sections of some 5 mm width are usually used for image review unless an automated detection program is available, If a nodule is detected, thin sections can be reconstructed around this nodule using a small FOV, and morphologtc and volumetric analysis can be performed, Automated programs substantially simplify nodule analysis because local multiplanar reformations centered around the nodule, volume rendered displays, and volumetric analysis become almost instantly available (see Fig, 9.18), No second scan is required, and the detection of nodule growth may become feasible at an earlier point in time (6-12 weeks follow-up),

Virtual bronchoscopy is a three-dimensional (3D) visualization technique that requires perspective rendering and various interactive tools to obtain the visual impression that the observer is moving within the data volume, The technique can be used to assess anomalies of the central tracheobronchial system noninvasively but cannot substitute for fiberoptic bronchoscopy in patients in whom biopsy has to be performed, lis most useful indications are for the planning of complex transbronchial biopsies (peripheral bronchi or central masses next to major vessels), lt is potentially more sensitive for detecting small irregularities of the tracheobronchial wall but such irregularities are often caused by mucous retention, especially in smokers, Other rendering techniques (compare Fig, 2.16, 2.18, and 2,36) are often superior to virtual bronchoscopy for the display of the tracheobronchial system (CT bronchography). Such techniques present an "intraluminal cast" (using SSD or VRT, see Fig.2.36a) or display the tracheobronchial anatomy as three-dimensional structure with semitransparent walls (tissue transition projection, see Fig.2.36b). Virtual bronchoscopy and CT bronchography require a highly resolved data set. Thus a protocol should be used as described above for tumor staging. M ultislice CT has the advantage that there are fewer pulsation or breathing artifacts because of the more rapid scanning procedure. In addition, it not only provides good results for the central tracheobronchial system but also for more distal bronchial segments. Due to the perspective distortion, measurements should not be performed on the endoscopic views but on the axial or reformatted images.

9 Lungs and Tracheobronchial System Congenital Disorders

Congenital disorders of the lungs and airways are very rare and may cause recurrent infections. CT is indicated if chest radiographs and bronchoscopic findings are equivocal. Tracheal Bronchus, Cardiac Bronchus

Tracheal bronchus refers to a bronchus (upper lobar, apical segmental, or middle lobar) arising from the trachea at an atypical site or a supernumerary bronchus arising directly from the trachea. In most cases the anomalous airway arises from the right lateral wall of the distal third of the trachea. Cardiac bronchus is a supernumerary bronchus, usually rudimentary, that most commonly arises from the medial wall of the intermediate bronchus and passes downward and medially toward the heart (Fig. 9.5). These anomalies are clinically significant in that they predispose to recurrent infections and bronchiectasis, frequently isolated to the bronchopulmonary territory of this bronchus. After endobronchial intubation, the balloon may inadvertently obstruct an atypical bronchus causing right upper lobe atelectasis. Bronchial Atresia

The apicoposterior segment of the left upper lobe is most commonly involved. The lesion may be associated with a bronchogenic cyst.

•

CT Morphology

While there is atresia of the lobar or segmental part of the bronchus, the more distal structures are well preserved. Mucus secreted within the airways distal to the obliterated lumen cannot pass the bronchial stenosis and subsequently causes a mucous plug and a mucocele. Collateral air drifts keep the involved segment inflated. It becomes hyperinflated due to expiratory air trapping. CT reveals the mucocele as perihilar branching structure and hyperinflation of the involved segment. Tracheobronchomegaly

Tracheomegaly can have various causes (Table 9.4), the commonest being destruction of the cricoid cartilage. •

CT Morphology

Tracheobronchomegaly is present when the tracheal diameter exceeds 2.5 cm and the diameter of the main bronchus exceeds 2.0 and 2.3 ern, respectively. Absence or hypotrophy of elastic fibers and hypotrophy of the muscles result in pathologic airway dynamics with dilatation in inspiration and collapse in expiration. Intrapulmonary bronchiectasis is a frequent associated finding.

Fig.9.5 VRT(a)and mIP(b)of the central tracheobronchial system in a newborn: two bronchial buds arise from the intermediate bronchus. Images were derived from a reduced dose CT scan (4 x 1/6, CTDlvoi = 1.8 mGy). Note the small size of the bronchial system with a tracheal diameter of 4mm.

a

b

Pulmonary Sequestration Table

9.4

Etiology of tracheobronchomegaly

Traumatic • Long-term

irtubation

Congenital syndromes • Ehlers-Danlos (cartilage defect) • Mounier-Kuhn (tracheobronchial Chronic inflammation • Recurrent childhood • Immune defect • Cystic fibrosis

diverticulosis)

Pulmonary

Bronchogenic

upper lobar or diffuse pulmonary sarcoidosis, idiopathic pulmonary

Cyst

Bronchogenic cysts are the commonest bronchopulmonary anomaly and result from abnormal bronchial arborization during embryologic development. They are classified by location as intrapulmonary (70%) or mediastinal (30%). •

Sequestration

infections

Traction • Predominantly fibrosis (e.g., fibrosis)

area. They communicate only very rarely with the tracheobronchial system. Infection is extremely rare. The mediastinal bronchogenic cyst is usually larger as compared to the pulmonary site (c- 20 em).

CT Morphology

The intrapulmonary type occurs most frequently in the lower lobe. The cyst may reach a diameter of several centimeters (central type) or may be a multilocular mass composed of multiplesmallcysts (peripheral type). Cysts with very proteinaceous contents may have a CT density as high as 50 HU. causing the "cystic" mass to exhibit soft-tissue density. The uninfected lesion does not enhance after intravenous contrast administration. Characteristically the cysts do not communicate with the tracheobronchial tree until they become infected. which occurs in about 75% of cases. When communication is established. the cysts contain air or show an air-fluid level. With infection of the cyst. the usually sharp definition of the cyst border becomes obscured by consolidation in the adjacent lung parenchyma. the cyst wall becomes thickened and contrast-enhancing, in that way simulating the appearance of a lung abscess. Differentiation between a bronchogenic cyst and an acquired cyst (e.g., after adult respiratory distress syndrome (ARDS) or a healed abscess) may be difficult. Mediastinal cysts are most commonly seen in the right subcarinal area but can also be located paratracheally or in the paraesophageal

A pulmonary sequestration is a nonfunctioning mass of aberrant lung tissue that lacks a normal communication with the bronchial system and has a systemic arterial blood supply. Sequestrations are found in the posterior left lower lobe segment in approximately 65% of cases and in the right lower lobe segment in the remaining cases. Two forms are distinguished according to their relationship to the pleura: • intralobar sequestration (75-85%) which is located within the normal pleural covering of the lung and drains into the pulmonary veins, it typically manifes ts in ad ults; • the extralobar form (15-25%) which has its own separate pleural covering and drains into the vena cava or azygos vein, and is diagnosed in the newborn or early infant. The diagnosis should be considered in any patient with persistent or recurrent lower lobe pneumonia. CT Morphology

An intralobar sequestration may be aerated (by collateral air drift through the pores of Kohn) and frequently shows areas of hyperinflation due to air trapping phenomena. In this case it can be identified as a sequestration only by its atypical arterial supply from the descending aorta. Sites of mucoid impaction in the rudimentary bronchi are a typical finding. If the sequestration has become infected or has perforated into the bronchial tree, CT may show uni- or multilocular cysts containing variable amounts of air and fluid. Extralobar sequestrations generally do not communicate with the bronchi and are of softtissue density throughout (Fig. 9.6). Homogeneous enhancement of the mass coinciding with aortic enhancement suggests the correct diagnosis on postcontrast scans. Some patients show emphysematous changes in the adjacent lung parenchyma and cystic areas within the sequestration.

293

294

9 Lungs and Tracheobronchial

System

Fig. 9.6 Extralobar sequestration in a newborn child. Left lower lobe consolidation (a) with an systemic arterial blood supplyfrom the infradiaphragmatic descending aorta (arrows) seen on a 3 mm thick MIP (b). The arrowheads on (b) indicate a pneumothorax treated by a pleural drain (arrowhead in a). The black arrowheads point to a gastric tube. Ultralow dose scan with 0.6 mGy CTDlvoi after intravenous injection of 2 ml contrast material (4 x 1/6).

a

b

The anomaly is frequently associated with other congenital abnormalities. In rare cases the sequestered segment may fistulate to the esophagus or stomach. Venolobar Syndrome, Scimitar Syndrome, Congenital Hypogenetic Lung Syndrome

Venolobar syndrome is a collective term for various congenital anomalies of the thorax, which frequently coexist in the same patient (Table 9.5). •

CT Morphology

CT demonstrates a small, hypoplastic lung with anomalies in the course and branching of the airways. Bronchial diverticula and bronchiectatic areas are common associated findings. The examiner should be alert to anomalous pulmonary venous return to the vena cava, right atrium, or atrial appendage. Most commonly there is a single vein draining the entire right lung, that runs inferiorly parallel to the right Table 9.5

Venolobar

cardiac border into the inferior vena cava below the diaphragm. (On a frontal radiographs this vein has the appearance of a Turkish sword or scimitar.) Atypical pulmonary veins may be found even in the absence of detectable anomalies of the lung parenchyma. The lung usually receives a systemic blood supply from the descending or abdominal aorta, and sequestrations may be present. Anomalous

Pulmonary Venous Drainage

Anomalous venous drainage is characterized by venous drainage directly into the right heart or the systemic veins (Table 9.6). It may be partial or complete, involving some or all veins, and in any case producing an extracardiac left -to-right shunt. The anatomy is highly variable with approximately 30 different patterns. It is frequently associated with other cardiovascular abnormalities (e.g., atrial septal defect). One diagnostic clue is the identical oxygen saturation in all four heart chambers because pulmonary venous blood mixes with systemic venous blood. The long-term effect of the increased

syndrome

Hypoplastic right lung, lobar agenesis, aplasia or hypoplasia

69%

Partial or complete anomalous veins

pulmonary

31 %

of the right puimo-

14%

Absence of hypoplasia nary artery

Table 9.6

Pulmonary sequestration

24%

Partial or complete systemic arterial supply to the lung without sequestration

10%

Absence or disruption of the inferior vena cava Duplication of the diaphragm right hemithorax

separating

7% the

Patterns of anomalous venous drainage

Supracardiac (50%) • Venous return into superior vena cava, persistent left vena cava or the left innominate vein Cardiac (30%) • Venous return into right atrium or right coronary sinus Infradiaphragmatic (15%) • Venous return into the portal vein or its branches

7% Mixed (5%)

Nonneoplastic Tracheal Stenosis

a

b

c

Fig. 9.7 Partially anomalous pulmonary venous return (PAPVR). Axial sections demonstrate a large superior vena cava (SVC) and a symmetrical vein on the left side (a). There is direct drainage of the right upper lobe vein into the SVC (b). The volume-rendered image demon-

strates that the left-sided mediastinal vein drains the almost the whole left lung into the left braehioeephalie vein (c). The right upper lobe vein drains into the superior vena cava (4 x 1 16).

blood flow in the involved lung is pulmonary hypertension.

left superior vena cava lateral of the aortic arch (Fig. 9.7), and a lack of veins that drain into the left atrium. The anomalies, however, are best described and detected on volume-rendered displays of the pulmonary vasculature because such a display gives the best overview over the -often complex-anatomy. In the presence of arterial septal defects there may be a jet of highly concentrated (undiluted) contrast material entering the left atrium which even can be seen on volume-rendered images (see Fig.23.19b).

•

CT Morphology

CT angiography using spiral or multislice CT allows for complete description of such anomalies, including hypoplastic native pulmonary veins. Without prior clinical suspicion, the anomalies can be missed easily on axial sections. Typical features involve a pulmonary vein that directly enters into the right atri urn or the superior vena cava (Fig.9.7a), an aberrant left pulmonary vein in the position of an accessory

Acquired Disorders of the Tracheobronchial Nonneoplastic

Tracheal Stenosis

Tracheal narrowing may affect the entire trachea or portions of it and may result from pulmonary disease, systemic disease, or a process confined to the trachea itself. CT can be used to distinguish an intrinsic lesion from an extrinsic process (compression) and exclude malignant disease. A "saber-sheath" trachea occurs in patients with COPD and is related to an abnormally high intrathoracic transmural pressure.

•

System

CT Morphology

Tracheal stenosis mayor may not be associated with wall thickening (Table 9.7). CT morphology is often nonspecific, so histological evaluation is frequently necessary when wall thickening is present. CT is performed to document the affected tracheal segment, define the degree of stenosis, and visualize involved bronchial segments. A "saber-sheath" trachea is present when the tracheal area is reduced by 70% and the transverse (coronal) tracheal diameter is reduced to less than one-half of its sagittal diame-

295

9 Lungs and Tracheobronchial Table 9.7 Nonneoplastic Tracheal Intrinsic

stenosis

tracheal

System stenosis:

etiology and CT features

without wall thickening

Rare, 2-4 cm in length, smooth or irregular

Idiopathic Saber-sheath

Intrathoracic trachea, long segment involved, elliptical to triangular cross-section, coronal diameter < half of sagittal diameter

trachea

After tracheostomy

Region of stoma or lower edge of tube, hourglass configuration

After intubation

At cuff site or tube tip, hourglass

Post-ischemic

after lung transplantation

Smooth,

Post-traumatic

configuration

Region of anastomosis hourglass

configuration

Extrinsic Vascular

Detection of vessel (aberrant right subclavain duplicated aortic arch, pulmonary sling)

Goiter

Relation to thyroid gland, intense contrast ment, cysts, calcium

Tracheal

stenosis

Osteochondroplastic

with wall thickening t rac he ob ron ch op ath ia

Calcified tracheal

Polychondritis

Submucosal

Scleroderma

Diffuse thickening,

Wegener's

granulomatosis

cartilage,

irregular tracheal

nodularity of anterolateral

central and peripheral stenoses

Rare, smooth or irregular wall thickening, calcified cartilage, focal or diffuse wall thickening, laryngeal involvement Submucosal

Sarcoidosis

Mural granulomas, smooth or irregular, laryngeal involvement (tuberculosis,

fungal, scleroma)

nodules, diffuse>

local

Rare, smooth or irregular, tuberculous

cavities

Smooth or irregular

Postirradiation

ter. Typically ber beginning ing extending

contour

walls

Amyloidosis

Postinfectious

artery,

enhance-

there is abrupt change of the caliat the thoracic inlet, the narrowfor the entire intrathoracic length.

Bronchiectasis Bronchiectasis is a localized dilatation of the bronchial tree. The changes may be very subtle and may be regional or widespread. Today, bronchiectasis is commonest in patients with cystic fibrosis, mucociliary dysfunction, or an immunodeficiency disease (Table 9.S). Reversible bronchiectasis may occur in the setting of pneumonia and may resolve completely in four to six months. HRCT is superior to standard CT

in the detection of bronchiectasis, providing sensitivity and specificity greater than 90%.

a

CT Morphology

One criterion for bronchiectasis is an internal bronchial diameter that is enlarged in relation to the corresponding artery (normally the artery has a slightly larger caliber than the bronchus). But since the ratio of bronchial to arterial diameter shows a relatively large range of physiologic variation (values found in a normal population range from 0.5 to 1.4), two additional CT criteria for bronchiectasis have been introduced:

Bronchiectasis

Table 9.8

Etiology of bronchiectasis

Congenital diseases • Structural wall defect: Kartagener's syndrome • Mucociliary dysfunction: Williams-Campbell syndrome • Cystic fibrosis • Alpha-l-antitrypsin deficiency • Congenital or acquired immune deficiency

Infections • • • • • •

Allergic bronchopulmonary aspergillosis Chronic granulomatous infection (tuberculosis) Measles Whooping cough Swyer james syndrome Chronic aspiration or inhalation

Bronchial obstruction • Tumor • Foreign body • Retained secretions • Lymphadenopathy

or compression

(mucoid

impaction)

Pulmonary fibrosis (traction bronchiectasis)

• failure of the bronchi to taper toward the periphery (a bronchus has the same diameter as its parent branch for a distance of more than 2 cm); and • visualization of bronchi within "1 cm of the costal pleura and visualization of bronchi abutting the mediastinal pleura. A bronchoarterial diameter ratio can be considered a reliable sign of bronchiectasis by itself only when it is at least 1.5. When it is between 1 and 1.5, it must be present in several airways or be associated with other findings such as bronchial wall thickening and lack oftapering, or both. Care should be taken to assess the smallest crosssectional airway diameter, to avoid comparisons near bifurcations of bronchi and vessels and not to overcall bronchiectasis in areas with decreased vessel size as a result oflocal vasoconstriction. On 10 mm collimated scans normal bronchi are seen only in the inner third of the lung, on HRCTimages Table 9.9

normal bronchi can be seen in the inner twothirds of the lung. Optimal HRCT assessment is obtained using a window level of -700 HU and a width of /\lOOOHU; smaller window width leads to artificial bronchial wall thickening. An extended window width (1500 HU) may be used routinely. However, it reduces the contrast between lung parenchymal structures and makes slight attenuation differences less conspicuous. Vessels may have double contours due to pulsations or respiratory motion, and this can mImIC bronchiectasis (see Fig.7.36a). Bronchiectasis may be missed in areas of dense consolidation or atelectasis. Three morphologic types of bronchiectasis are identified in the Reid classification: cylindrical (tubular, fusiform), varicose, and cystic (saccular) (Table 9.9). Depending on the relation of the bronchi to the plane of section, CT scans of cylindrical bronchiectasis may show signet -ring configurations in cross sections or "tram lines" when the bronchus is scanned in-plane. Tram lines often can be traced far into the periphery (Fig.9.8a, d). The rare varicose form is recognized on CT by marked variations in the calibers of airways that are imaged in longitudinal section (Fig. 9.8 b). Areas of cystic bronchiectasis often show retained secretions with fl ui d levels (Fig.9.8c). Air-filled cystic lesions in atelectatic lung areas have a cluster-of-grapes appearance. Dilated bronchi that are filled with secretions appear as Y- or V-shaped structures of soft-tissue density [mucoid impaction, Fig.9.8c). Thickening of the bronchial walls indicates a bronchitic component or mucosal edema (Fig.9.8b). Mucoid impaction in bronchiectasis is particularly common in patients with cystic fibrosis and allergic bronchopulmonary aspergillosis (ABPA).

Bronchiectasis

Cylindrical

Varicose

Cystic

Mildest form Tram lines (horizontal) Signet ring sign (vertical)

Rare (Swyer -james) String-of-beads contours

Most severe form Transverse diameter> 1 em Frequent wall instability Clustered cysts Air-fluid levels

297

298

9 Lungs and Tracheobronchial System

b

a

c

d

Fig. 9.8 Bronchiectasis. (a) Cylindrical bronchiectases (signet ring sign, arrow), (b) Extensive mucous plugging in right lower lobe bronchiectases (arrowheads). The bronchi are larger than the accompanying arteries. (c) Varicose and cystic bronchiectasis with fluid levels. (d) Semicoronal reformat from a low dose multislice CT data set (4x1/6, 1.2mGy) demonstrates cylindrical

and varicose bronchiectasis in a patient with cystic fibrosis. There is bronchial wall thickening (sign of chronic inflammation) and retained secretions (mucoid impaction). The secretion-filled bronchi produce typical Y- or V-shaped structures. The subpleural "tree in bud" pattern is also evident.

Bronchiolectasis is detectable only if the dilated bronchioles are filled with mucus. CT shows thin, branching structures of soft-tissue density in the subpleural lung zone (5-10 mm) that have small bulbous terminations ("tree in bud" pattern, Fig. 9.8d).

Table 9.10

The isolated circumscribed dilatation of a mucus-filled bronchus may be an indirect sign of a small intraluminal tumor (e.g., carcinoid, bronchialcarcinoma). Differentiation of mucoid impaction from arteriovenous malformations is easily achieved after intravenous administration of contrast material.

Bronchiolitis

A variety of pulmonary diseases are characterized predominantly by inflammation of membranous and respiratory bronchioles. Various classifications schemes are found in the literature that differ with respect to whether they are based on etiologic, histologic, clinical or radiologic features (Table 9.10). From a clinical point of view it is most useful to differentiate infectious bronchiolitis from non-infectious, the latter being related to immunologic reactions.

Different forms of bronchiolitis

Etiology

Morphology

Acute infectious

Tree-in-bud, nodules

Chronic inflammatory (asthma, chronic bronchitis)

Bronchial wall thickening, tree-in-bud

Panbronchiolitis

Marked tree in bud, air trapping, bronchial wall thickening

Respiratory

Centrilobular nodules (predominantly in lower lobes)

bronchiolitis

Respiratory bronchiolitis with interstitial disease

(RBIO)

acinar

Centrilobular nodules, emphysema, bronchial wall thickening

Follicular bronchiolitis

1-3 mm (occasionally 1-2 ern) subpleural illdefined nodules

Obliterative bronchiolitis (bronchiolitis obliterans)

Air trapping, pattern

mosaic

Acute Infectious Bronchiolitis

Acute infectious bronchiolitis is most commonly the result of infection by viruses (respiratory syncytial virus, adenovirus), Mycoplasma pneumoniae and Chlamydia species. Immunocompromised patients develop acute Aspergillus bronchiolitis or endobronchial spread of tuber-

Bronchiolitis

culosis. In principle, each bacterial infection can cause acute bronchiolitis if confined to the small airways. •

CT Morphology

The hallmarks of infection-related bronchiolitis in HRCT are small centrilobular nodules with branching lines (tree- in-bud) that represent thickening of the bronchiolar wall or filling of dilated bronchioles with granulation tissue, mucus, or pus (Fig. 9.9 a, b). The tree-in-bud sign is most strikingly appreciated in the peripheral one-third of the lung parenchyma. In some cases it is accompanied by scattered areas of ground-glass attenuation or consolidation. Acute infectious bronchiolitis has to be differentiated from chronic inflammatory conditions (asthma, chronic bronchitis), diffuse panbronchiolitis and respiratory bronchiolitis, which present with centrilobular small nodules (see Fig. 9.9 c): The centrilobular nodules in infectious bronchiolitis are usually well-defined and have sharp margins whereas those seen in patients with respiratory bronchiolitis typically have poorly defined margins. In infectious bronchiolitis nodular opacities are usually combined with branching linear opacities (tree- in-bud). In infectious bronchiolitis centrilobular nodules may enlarge then resembling acinar nodules that can come together to form rosettes.

a

b

Fig.9.9 Bronchiolitis. (a) Infectious bronchiolitis with tree-in-bud-pattern and air-trapping (expiration) indicating disease of the small airways, (b) Infectious bronchiolitis with small nodules and tree-in-bud. (c) Respira-

Respiratory Bronchiolitis and Related Disease

Respiratory bronchiolitis and respiratory bronchiolitis associated interstitial lung disease (RBILD) have recently been grouped together with desquamative interstitial pneumonia (D IP) as part of a single spectrum of diffuse interstitial infiltrative pulmonary disease (see Table 9.34) since they have clinical similarities and significant overlapping of histological and HRCT features. Both diseases are strongly associated with cigarette smoking, underlining the concept that all three diseases represent different degrees of severity of small airway and parenchyma! reaction to cigarette smoking. Radiological as well as histological features of respiratory bronchiolitis and extrinsic allergic alveolitis are also in a continuum, since bronchiolitis is a prominent component of extrinsic allergic alveolitis. Inhalational injury caused by S02, oxides of nitrogen, or a variety of other gases or fumes is characterized by identical CT morphology. •

CT Morphology

Respiratory bronchiolitis is characterized by illdefined centrilobular nodular opacities (Fig.9.9c) producing an image that may be virtually indistinguishable from acute extrinsic alveolitis (see Fig.9 .56a). In most patients, the upper lobes are involved predominantly or even exclusively (as opposed to extrinsic allergic alveolitis) .

c

d

tory bronchiolitis with centrilobular nodules and bronchial wall thickening, (d) Follicular bronchiolitis with multiple ill-defined centrilobular nodules, predominantly in subpleural location.

299

9 Lungs and Tracheobronchial System In addition to small centrilobular nodules, patients with RB-ILD show more prominent bilateral areas of ground-glass attenuation mostly in a patchy distribution and subtle findings offibros is (honeycombing, intralobular lines) predominantly in the lower lung zones, An upper lobe centrilobular emphysema is frequently present; bronchial wall thickening may be an associated finding, In contrast to DIP, lung volumes are usually normal, Similar findings as seen in RB and RB-ILD are also seen in asymptomatic smokers, but the findings are usually less extensive, Clinical and radiological findings improve promptly when the patient stops smoking and is treated with corticosteroids,

Diffuse Panbronchiolitis (DPB) Diffuse pan bronchiolitis is a disease ofunknown etiology, It is characterized by luminal obliteration through accumulation of macro phages, The disease has been recognized almost exclusively in East Asia, The patients present with a chronic productive cough and progressive dyspnea, The long term prognosis is poOL CT Morphology

The hallmarks are extensive diffuse centrilobular nodules with branching lines (tree-in-bud), marked bronchial wall thickening, bronchiectasis, and localized areas of decreased attenuation (air trapping) and perfusion,

gates (Fig,9,9d), The nodules are usually small (1-3 mm). and only occasionally larger (1-2 em). Tree-in-bud, bronchial wall thickening, and patchy areas of low attenuation are associated findings,

Bronchiolitis Obliterans (Constrictive Bronchiolitis) Bronchiolitis obliterans (BO) -also termed constrictive bronchiolitis-is characterized by proliferation of submucosal and peri bronchiolar fibrous tissue resulting in a more or less concentric narrowing of the airway lumen (without intraluminal granulation) through to complete obliteration in extreme cases, The idiopathic form is rare, and the condition can have various causes (Table 9.11), Patients present clinically with progressive obstructive lung disease that frequently shows no abnormalities on plain chest radiographs, Bronchiolitis obliterans is a separate entity from bronchiolitis obliterans with organizing pneumonia (BOOP, see P 321), Table

9.11

Etiology

of obliterative

Inhalation of toxic gases • (e.g., nitrogen dioxide, chorine) Infectious

bronchiolitis

nitric oxide,

ammonia,

agents

• Mycoplasma (in children) • Viruses (in adults) Drug reactions • Penicillamine • Lomustine

Collagen diseases

Follicular Bronchiolitis Follicular bronchiolitis is characterized by the presence of abundant lymphoid tissue within the walls of bronchioles and to some extent of bronchi. The finding is not specific and can be found in association with connective tissue disease (Sjogren's disease and rheumatoid arthritis), imm unodeficiency syndrome, and systemic hypersensitivity reactions, CT Morphology

CT shows nodular opacities mainly in a peribronchovascular or subpleural distribution, consistent with the presence of lymphoid aggre-

• Rheumatoid • Scleroderma

arthritis

-SLE Transplantation

(lung, heart,

• Graft-versus-host • Chronic rejection In setting

of other

lung diseases

• BOOP • Chronic

bronchitis • Cystic fibrosis • Bronchiectasis • Extrinsic allergic alveolitis Ulcerative Idiopathic

colitis

bone marrow)

reaction

•

CT Morphology

The hallmark of bronchiolitis obliterans is air ~rapping: the density of the affected lung areas IS normal or reduced on inspiratory scans, and on expiratory scans the normal parenchyma shows increased density while affected areas show little or no increase in attenuation (due to check-valve obstruction, Fig.9.10a, b). Regions of air trapping may show decreased vascularity with redistribution and consequent vasodilatation in other lung areas, giving rise to a "mosaic" pattern (Fig. 9.10c). . Peribro?chiol.ar fibrosis is rarely detected directly but ISmamfested by centrilobular punctate or branching structures that are most conspicuous in the peripheral third of the lung. Bronchiectasis (mainly central), bronchial wall thickening, and signs of interstitial fibrosis are common associated findings but are not primaryfeatures ofbronchiolitis obliterans. It can be difficult to distinguish the mosaic pattern of bronchiolitis obliterans from a geographic pattern of ground-glass opacities. Ground-glass areas of increased density are pathologic and are seen in the setting of acute alveolar diseases and interstitial inflammatory proc~s:es leading to fibrosis. Ground-glass opacities show a very slight, homogeneous density increase that does not obscure pulmonary vessels or parenchyma! structures.

Mosaic pattern is seen in bronchiolitis obliterans (1), an acute inflammatory process (2) or in chronic thromboembolic disease (3), which can be differentiated from each other by expiratory CT scanning (Table 9.12): 1. In the case of air trapping, the areas of increased attenuation are normal while the are~s of decreased attenuation are patholOgIC. They do not show an increased attenuation on expiratory scans but become even more pronounced in stead. 2. In infiltrative disease, ground-glass opacities represent areas of pathologically increased density. In expiratory views, they show an attenuation increase like that observed in normal lung parenchyma. 3. Pulmonary vessels are decreased in size in the low attenuation areas when air trapping or vascular disease are present, but are normal in acute infiltrative disease. Expiratory scans help to exclude air trapping in hypo attenuating areas.

r

a

b

Fig. 9.10. Bronchiolitis obliterans is characterized by air trapping due to check-valve obstruction, (a) Inspiratory CT shows sharply demarcated areas of different lung density with vascu lar rarification in the hyperlucent areas (mosaic pattern), (b) Expiratory CT shows marked increase of lung density in the normal parenchyma

c while lung areas with air trapping show no change in volume or density, (c) Obliterative bronchiolitis secondary to a drug reaction. CT reveals a patchwork of sharply marginated areas of increased lucency (air trapping) adjacent to areas of normal or apparently decreased lucency with dilated blood vessels ("mosaic pattern").

9

Lungs and Tracheobronchial System

Table 9.12

Differential

diagnosis

of mosaic pattern

Bronchiolitisobliterans

Infiltrativedisease

Chronicpulmonary embolism

Small air ways disease

Ground-glass

Vascular

Airtrapping

Density of normal and opacified areas increases with expiration

No air trapping

Normal vessel

Vessel diameter is decreased areas of hypoattenuation

in expiration

Vessel diameter is (slightly) decreased in areas of hypoattenuation

opacification

diameters

obliteration

in

--

Allergic Bronchopulmonary Aspergillosis (ABPA)

Allergic bronchopulmonary aspergillosis is caused by a pathologic immune response to colonization of the bronchi by Aspergillus species (not to be confused with invasive aspergillosis). Patients with asthma or cystic fibrosis are predisposed. Other rarer forms of hypersensitivity reaction to Aspergillus are extrinsic allergic alveolitis and a Loffler-like syndrome. 1

•

CT Morphology

The initial finding is that of migrating alveolar infil trates in a setting of pneumonitis. The infiltrates are usually bilateral and show a predilection for the upper lobes. The hallmarks of ABPA are bronchiectasis and mucoid impaction involving predominantly the segmental and subsegmental airways of the upper lobes (Fig. 9.11). Retained secretions in segmental bronchi lead to V- or Y-shaped opacities that may persist for months. Focal areas of mucoid impaction and bronchiectasis may mimic a nodule or mass lesion on CT. CT typically shows branching tubular opacities that emanate from the hilum (finger-in-glove appearance). Bronchiectasis may also be widespread and involve more peripheral bronchi; involvement of main and lobar bronchi, however, is very uncommon. The combination of bronchiectasis on CT and blood eosinophilia is typical of allergic bronchopulmonary aspergillosis. Segmental atelectasis and peripheral subsegmental atelectasis give rise to patchy areas of consolidation. Postobstructive cavitation may occur. Late changes consist of hyperinflation and fibrosis. Involvement of small bronchioles leads to a "tree- in-bud" pattern and air trapping on expiratory scans.

255 137 1

o

a

b

Fig. 9.11 Allergic bronchopulmonary aspergillosis (ABPA) with upper lobe tubular bronchiectasis.

Bronchiectasis is seen more frequently in asthmatic patients with ABPA than in asthmatic patients without ABPA. However, bronchiectasis may also occur in uncomplicated asthma, and may not be present in all patients with ABPA.

Benign Tumors Tracheobronchial Tumors Benign Tumors

Benign neoplasms of the trachea and bronchi are very rare and are always intraluminal. Commonest are mesenchymal tumors (hemangioma, hamartoma), but occasional paragangliomas, neurofibromas, and papillomas can occur. The detection of extraluminal tumor growth on CT excludes a benign lesion. Tracheobronchial papillomatosis is caused by a viral infection (papilloma virus). It is usually confined to the larynx, rarely spread of papillomas to the trachea, bronchi and lung occurs. Papillomas may be solitary or multiple. Histologically they consist of central fibrovascular bundles covered by layers of well-differentiated squamous cells. Transformation oflesions into invasive squamous cell carcinoma is known. •

a

Fig. 9.12 Tracheal lesions, (a) Shaded surface display of a benign intraluminal tracheal papilloma (2/4/2). (b) Coronal reformatted image of a malignant esophageal tumor invading the trachea (3/5/2). Note the intraluminal and extraluminal components of the tumor.

CT Morphology

They have a pathognomonic CT morphology with nodules that frequently cavitate (Fig. 9.13).

Benign neoplasms usually have smooth, welldefined margins and are less than 2cm in diameter. They do not display specific morphologic CT features, however, and biopsy is necessary to establish a diagnosis (Fig. 9.12). Polypoid and sessile masses protrude into the lumen and may cause subtotal obstruction. Tracheobroncbial papillomatosis may present as polypoid lesions on the surface of the tracheobronchial system, but frequently intrapulmonary papillomas are the only findings on CT.

a

b

b

Fig.9.13 Intrapulmonary papillomatosis. (a) Intrapulmonary cavitating, irregular nodules (HRCT). (b. c) Lowdose multislice CT (4x1/6, CTDlvoi=1.8mGy) of a

Particularly in elderly patients with an obtunded cough reflex, secretions may accumulate in the central airways and create a pseudomass appearance. Usually these can be identified by their teardrop-shaped configuration, and coughing tends to displace the collection or cause it to disappear.

c

~ d

young girl with multiple small cavitating nodules due to papillomatosis. (d) Compare to cavitating intrapulmonary metastases.

303

9 Lungs and Tracheobronchial

System

Malignant Tracheal Tumors

Carcinoid Tumor

Primary malignant tumors of the trachea are extremely rare (180 times rarer than bronchial tumors). Clinical symptoms are a late finding, so generally these tumors are in an advanced stage when diagnosed (significant luminal narrowing in more than 75%, mediastinal invasion in 40%). The three commonest tracheal malignancies are squamous cell carcinoma, adenoid cystic carcinoma, and mucoepidermoid carcinoma. Secondary malignant tumors involving the trachea include the thyroid and the esophagus.

Carcinoids are classified as neuroendocrine tumors. They account for less than 4% of all bronchial neoplasms. They are locally invasive and metastasize to regional lymph nodes, bone, and liver. Approximately 90% are endobronchial tumors that involve the central airways and may become obstructive. Only 10% involve the lung parenchyma. According to their histology and their biological behavior, typical, carcinoids are differentiated from the more aggressive atypical carcinoid (metastasizes into the lymph nodes) and the small-cell carcinomas (mostly associated with bulky mediastinal disease). Radion uclidecoupled somatostatin analogs (octreotide scan) are used to visualize somatostatin-receptorbearing tumors.

•

CT Morphology

Primary tracheal tumors grow exophytically and by infiltration. They may spread over a distance of several centimeters and sometimes up to 10cm. The full extent of intramural (submucosal) spread tends to be underestimated with CT. While squamous cell carcinoma causes irregular sometimes nodular wall thickening, adenoid cystic carcinoma leads to smooth and long-standing wall thickening and consecutive lumen narrowing. For surgical planning, it is crucial to assess the involvement of the infraglottic larynx (indication for laryngectomy) and of the principal bronchi.

a

b

Fig. 9.14 Axial section (a) and shaded surface display (b) of an intraluminal carcinoid at the level of the carina (3/5/2). It is typical to find the smooth-bordered mass located in proximity to bronchial divisions. A carcinoid

•

CT Morphology

Endobronchial carcinoids appear on CT as wellcircumscribed nodular masses that occur predominantly at bronchial divisions (Fig. 9.14). They frequently cause airway obstruction and may be associated with air trapping, bronchiectasis, mucoid impaction, or pneumonia. Calcifications are seen in 30% and due to their good vascularization tumors show a marked enhancement after application of contrast.

c

d tumor (arrowheads) in another patient (c) has caused atelectasis of the anterior segment of the right upper lobe (d). Carcinoid tumors are hypervascular and thus may be hard to distinguish from adjacent atelectasis.

Bronchial Carcinoma 305 sis, idiopathic pulmonary fibrosis, scleroderma), The therapeutic approach depends on the local tumor extent, the invasion of mediastinal and hilar lymph nodes, and the presence or absence of distant metastases, Bronchial carcinoma is characterized by early lymphogenous and hematogenous spread, Lymph node metastases are commonest with small-cell carcinomas, The highest rates of metastasis are to the liver (up to 40%), adrenals (up to 40%), brain (up to 43%), bone (up to 33%), kidneys (up to 25%), and abdominal lymph nodes (up to 30%), The incidence of metastases depends on histology and primary tumor stage,

Bronchial Carcinoma Bronchial carcinoma is the most frequent tumor in the statistics of neoplasm-related death rates and the commonest tumor of the central tracheobronchial system. The five-year survival rate is only 8 to 13% lower as compared to that of other neoplasms (colorectal carcinoma=62%, breast neoplasm=85%), Active cigarette smoking increases the risk for developing a bronchial carcinoma by factor 10, passive smoking by factor 2, The histological classification was revised in 1999 and differentiates the small-cell carcinoma (SCLC=20%), the adenocarcinoma (30%), the squamous cell carcinoma (25%) and the large cell carcinoma (10%) as most frequent tumor types (Table 9.13), Certain occupational agents (asbestos, coal, arsenic, chromium, chloromethyl ether, mustard gas) are known to lead to increased risk of lung cancer, the most important of these being asbestos, A combination of asbestos exposure and Cigarette smoking is multiplicative, Other diseases that have an increased prevalence of lung cancer are characterized by diffuse or focal pulmonary fibrosis (e,g" tuberculosis, asbesto-

Table 9.13

Staging The staging of bronchial carcinoma (Tables 9.149.16) is useful in selecting patients for surgical resection, planning radiotherapy, and monitoring the response to chemotherapy, Stages 1 and 11 are amenable to resection, Stage III was divided into two classes: lllA includes tumors with limited mediastinal or chest wall invasion and are considered potentially re-

Bronchiogenic carcinoma Frequency

Histology

Etiology and

Region of

Squamous

cell carci-

30-35%

noma (subtype:superiorsulcus tumor)

Special features

predilection

Prognosis

Best prognosis; local

2/3 central

Central: atelectasis

metastases nodes

endobronchial 1/3 peripheral

strucnve pneumonitis Peripheral: thick walled

to lymph

or postobcavi-

tation (30%), solitary nodule Small-cell

carcinoma

20-25%

(sci c)

Strong association with smoking; most aggres-

Central (85%)

sive with early lymph node Large-cell

(undifferen-

<5%

tiated) carcinoma Large-cell neuroendocrine carcinoma

<5%

Grows along pre-existing tissue spaces; hilar or perihilar mass (massive nopathy)

metastasis

Strong association with smoking, poor progno-

Peripheral

Early lymph node

Central

(70%)

Large tumors

lymphade-

(>

4cm)

Extended intratumoral (30-7fP/O)

necro-

sis

metastases

(LCNEC) Adenocarcinoma

33-50%

Association

with fibroearly

sis; metastasize

Peripheral (55%)

Slowly growing, solitary nodule or mass; often large

and spiculated Bronchioalveolar cell carcinoma (special from of adenocarcinoma)

2-5%

Good small

prognosis tumors

for

Parenchyma

May imitate

inflammatory

infiltrate, hazy ill-defined ground gram,

glass;

air broncho-

consolidation, nodule (solitary or multiple)

306

9 Lungs and Tracheobronchial Table 9.14 1997)

System

TNM staging of bronchial carcinoma (UICC,

TO

No direct evidence of a primary tumor

T1

Intrapulmonary tumor :s 3 ern, surrounded by lung parenchyma or visceral pleura, no infiltration of the intermediate or main bronchi (except for tumors confined to bronchial wall)

T2

T3

T4

Tumor> 3 cm, or tumor involving the intermediate or main bronchus but at least 2 cm distal to the carina, or tumor involving the visceral pleura; associated atelectasis or postobstructlve pneumonia must involve less than an entire lung Tumor of any size involving the main bronchus within 2 cm of the carina, or tumor directly invading the parietal or mediastinal pleura (including superior sulcus tumors) diaphragm, chest wall, or pericardium, associated atelectasis or postobstructive pneumonitis involving the entire lung Tumor of any size invading the heart, great vessels, trachea, carina, esophagus, or spinal column, or the presence of malignant pleural effusion, satellite lesions within the some lobe as primary tumor

NO

No metastasis to regional lymph nodes

N1

Metastasis to ipsilateral bronchial or hilar lymph nodes

N2

Metastasis to lpsilateral mediastinal and subcarinallymph nodes

N3

Metastasis to scalene or supraclavicular lymph nodes, or metastasis to contralateral mediastinal or hilar lymph nodes

M1

Distant metastases, satellite lesions in different lobe as primary tumor (ipsi- or contralateral)

sectable, while IIIB is considered non-resectable. T4 means infiltration of vital mediastinal structures (e.g .. heart, trachea, esophagus, great vessels). N1 alters the prognosis but not the surgical approach. N2 diseases are considered resectable but usually patients undergo neoadjuvant chemotherapy sometimes combined with radiation therapy before operation. N3 disease precludes resection. It is therapeutically important to differentiate between stage II and iliA tumors (lobectomy versus pneumonectomy) and between stage lilA andlllB disease (resectable versus unresectable) (Fig. 9.15 b). Transfissural tumor spread, pulmonary vascular invasion, the invasion of a main bronchus, or the involvement

Table 9.15 Size criteria for mediastinal lymph nodes (Glazer 1985 adopted to AJCC/UICC classification 1996) Station

Description

Normal size

2R

Right upper para tracheal nodes

:s7mm

2L

Left upper paratracheal nodes

:s7mm

4 R

Right lower paratracheal nodes

:s 10 mm

4 L

Left lower paratracheal nodes

:s 10 mm

5

Subaortic aortopulmonary nodes

:59mm

6

Para-aortic (anterior mediastinal) nodes

::=;8mm

7

Subcarinal nodes

8R

Right para-esophageal nodes

:s 11 mm :s 10 mm

8L

Left para-esophageal nodes

:s;7mm

10 R

Right hilar nodes

:s 10mm

10 L

Left hilar nodes

:s7mm

Table 9.16 Stages of bronchial from Mountain, 1998)

T1 T2 T3 T4 M1

carcinoma

(adapted

NO

N1

N2

N3

M1

IA IB liB IIIB IV

IIA liB lilA 1116 IV

IliA lilA IliA IIiB IV

IIIB IIiB 1116 IIIB IV

IV IV IV IV IV

of upper and lower lobe bronchi, preclude a lobectomy and necessitate pneumonectomy (iliA). Invasion of a main bronchus less than 2 ern from the carina signifies a T3 tumor (Fig.9.15c). Involvement of the carina or involvement of contralateral lymph nodes indicate advancement beyond stage iliA and preclude surgical intervention. The non-small-cell neoplasms are staged according to the TNM classification (last modified in 1997). A small-cell carcinoma is classified as "limited disease" (stages I through iliA). Stage IIIB, denoting "local extensive disease," is still considered to be localized from a radiotherapeutic standpoint. Stage IV denotes "extensive disease" with detectable distant (ex-

Bronchial Carcinoma

a

b

Fig. 9.15 Central bronchial carcinoma, (a) Tumor has encased the intermediate bronchus (stage I) without invading the pulmonary artery. Small lymph nodes, not considered significant, are visible on the contralateral hilum (arrow), (b) Stage IliA bronchial carcinoma has obliterated the right lower lobe bronchus (arrow) and invaded the pulmonary artery, necessitating a pneumonectomy rather than lobectomy. (c) Tumor has en-

trathoracic) metastases (cervical or axillary lymph nodes or other distant metastases). •

CT Morphology-

T Staging

CT has an accuracy of only 50% to 70% in evaluating the local invasiveness (T staging) of bronchial carcinoma. CT and MRI are equally limited in their ability to assess involvement of a main bronchus, the carina, or the trachea, due to a lack of precision in the delineation of submucosal tumor spread. Intrabronchial structures of soft-tissue density, bronchial wall thickening, and an irregular reduction in bronchial caliber are indicators of bronchial tumor involvement (Fig.9.15). Contrast-enhanced CT is essential, though not always successful, in differentiating bronchogeniC tumor from areas of poststenotic atelectasis and pneumonia. Atelectasis shows more intense and more uniform enhancement than tumor-involved lung tissue on postcontrast scans. Pneumonia shows similar characteristics, though to a considerably lesser degree. The following findings are more suggestive of an occult central tumor with postobstructive consolidation rather than of simple pulmonary consolidation due to pneumonia: • S-shaped distortion of the lung fissure due to tumor-induced lobar atelectasis;

c

d

cased a long segment of the anterior upper lobe bronchus and the right main bronchus to within 2 cm of the carina (stage IliA disease), (d) Inoperable tumor (stage IV) has invaded the mediastinum and encased the aorta. It has also invaded the pulmonary artery and partially obstructed the left main bronchus. The atelectasis shows greater postcontrast enhancement than the tumor itself (arrowheads).

• Loss of volume due to lobar atelectasis, or the presence of an air bronchogram (in patients over age 35); • 'Irregular stenosis in the central bronchial tree; • Hilar lymphadenopathy; • Insufficient regression of pneumonia in response to antibiotics. Reconstruction of thin axial sections and MPR are particularly worthwhile in patients with focal lung lesions that are in the vicinity of pleural interfaces, because they frequently provide additional diagnostic information with respect to segment localization, pleural dissemination, and transfissural tumor growth (Fig. 9.16). It was demonstrated that the sensitivity of CT scanning for the presence of extension through the major fissure was 57% for thick 10mm sections, but could be increased to 87% with thin 2 mm sections. Additional multiplanar reformations (MPR) could further improve sensitivity to 100%. For the minor fissure, 6/51 cases were inconclusive on axial section but only one remained so on MPRs. Three-dimensional images based on shaded surface displays were found to facilitate the differentiation of simple pleural tags (i.e., bands extending from the lesion to the visceral pleura; T-stage =s T1) from pleural puckering associated

308

9 Lungs and Tracheobronchial System

a

b

c

d

Fig. 9.16 Multiplanar reformations improve staging of bronchial carcinoma (4x1/6, 7.2 mGy). (a) Transfissural tumor growth and broad tumor adherence along the superior vena cava and the right atrium, (b) Incomplete lobar fissures support translobar spread of carcinomatous lymphangitis, (c) Large tumor infiltrating the pulmonary artery (arrow) with pathologic sub-

carinal lymph nodes (T4N2, stage IIIB disease) and a contralateral acute pulmonary embolus (arrowhead). (d) Bilateral metastatic lymph nodes in the right perihilar region, pretracheal, and in the aortopulmonary window. These nodes are round but smallerthan 10 mm in short axis diameter.

with an indrawn locally thickened pleura seen with visceral pleural invasion (> T2). While the sensitivity of pleural puckering was high, specificity was only 76% because it was not only seen with tumor invasion but also with reactive fibrotic changes.

sidered a reliable indicator for intrapericardial tumor extension, while infiltration of the inferior pulmonary vein does not necessarily mean that the pericardium is infiltrated. Although chest wall and limited mediastinal invasion of lung cancer are no longer contraindications to surgical treatment, the mortality rate associated with en bloc resection of the tumor and the contiguous structures is still substantial. Therefore, information about mediastinal and chest wall invasion is still one of the important factors in the clinical decision to perform surgery. Gross extrapulmonary chest wall invasion does not impose problems. However, accurate distinction between tumor contiguity with adjacent extrapulmonary structures and actual invasion is often difficult on static images, especially when there is no obvious soft tissue mass in the mediastinum or chest wall. Contiguity of the tumor with parietal pleura was found not to be equivalent to definite invasion of the chest wall, even when associated with a pleural thickening contiguous to the tumor. Apart from gross invasion, focal rib destruction is the only sign that is "100% sensitive for chest wall invasion. All other findings such as pleural thickening, an obliterated epipleural fat plane, an obtuse angle between the tumor and chest wall, more than 3 cm of contact between the tumor and chest wall

Presence of a pleural effusion requires the exclusion of pleural involvement (pleural tap, if negative PET). Pleural involvement corresponds to stage IV and makes the tumor irresectable. Mediastinal invasion represents stage T4 and in almost all cases makes the tumor irresectable. It should be diagnosed if CT shows tumor encasement or constriction of vital mediastinal structures, e.g., of the large mediastinal vessels, the heart, the esophagus, spine, or the trachea, or if the tumor is transgressing the midline (Fig. 9.15d). Local infiltration of the mediastinal fat after transgression of the mediastinal pleura is not considered a criterion for irresectability. MRI is superior to CT in differentiating tumor from fat and will reveal even minimal changes, but it is no better than CT for distinguishing tumor infiltration from an inflammatory reaction of the epipleural or mediastinal fat. Infiltration of the superior pulmonary vein is con-

Bronchial Carcinoma and a ratio between tumor-pleura contact and tumor diameter of > 0.9 showed sensitivities between approximately 60% and more than 90%. Obliteration ofthe epipleural fat plane may be due to tumor invasion or inflammatory reaction and is equally unreliably differentiated with MRI and CT (sensitivity/specificity of 85%/87%withCTand90%/86%withMRI) . In general, there is a superiority of \ mm slices as compared to 10mm slices for the delineation of anatomic chest wall structures and thus for evaluating the presence of chest wall involvement. For the delineation of the extrapleural fat plane and the assessment of soft tissue structures a smoothing standard reconstruction algorithm was superior to the high resolution algorithm due to its lower noise level. Both, CT and MRI have been employed to use dynamic imaging during different phases of respiration to assess the respiratory shift that is defined as a change in the relative location between the peripheral lung tumor and the chest wall with deep inspiration and expiration. Expiratory dynamic CT was found to provide accurate information but only for tumors located in the middle and lower lobes. It has also to be noted that only absence of tumor invasion can be confidentially diagnosed in those patients in which free movement of the tumor directly proves lack of attachment between the visceral and parietal pleura. Benign fibrous adhesion may cause false positive results by simulating tumor infiltration. Such limitation holds true for

a

both CT and MR imaging as well as alternative approaches using ultrasound or pneumothorax CT. The latter is based on the idea that tumor infiltration can be differentiated from the parietal pleura on the basis of air in the absence of tumor infiltration. However, this technique is invasive, it may cause symptomatic pneumothorax (described in 4/43 patients) and therefore has never found broad application. Owing to its superior contrast resolution, MRI used to be the method of choice for imaging of superior sulcus tumors. The use of surface coils provides high resolution images and chest wall invasion is best depicted as a disruption of the extrapleural fat line (bright signal of fat in Tl-weighted images). MRI was also thought to be more accurate than CT in depicting chest wall involvement from superior sulcus tumors with respect to involvement of the subclavian vessels, the brachial plexus and the vertebra. However, high quality spiral CT (thin sections, bolus injection of contrast agents, and sagittal and coronal reformats) provide the same anatomic information of the anatomical environment of the plexus, and MRI may be reserved for patients with extension into the neural foramina and the epidural space (Fig. 9.17). •

Staging

The evaluation of mediastinal lymph nodes is an important aspect of staging in patients with non-smaIl-cell lung cancer. The nodal status is an integral part of the TNM staging system and

b

Fig. 9.17 Superior sulcus (Pancoast) tumor (4 x 1/6). On 5 mm axial sections (a) there is no clear demonstration of chest wall invasion, which is clearly demonstrated on 3 mm coronal reformations from the same

CT Morphology-N

c multislice CT data set (b). Coronal MPR in another patient with small cell lung cancer extending into the cervical soft tissues and invading the superior vena cava (c).

309

9 Lungs and Tracheobronchial

System

provides critical information for planning treatment and determining prognosis. Traditionally imaging evaluation of mediastinal lymph node involvement has relied on anatomic features of the nodes. most notably nodal size. CT has been shown to be the best predictor of nodule volume. With regard to the determination of the nodal status. however. its limitation with relatively low sensitivity (4167%) and low specificity (79-86%) are well documented. based on the fact that enlarged lymph nodes may be hyperplastic rather than neoplastic. especially in patients with post-obstructive pneumonia. and normal-sized nodes may contain neoplastic cells. Microscopic tumor involvement was seen in S-64% of normal-sized lymph nodes. while IS-30% of enlarged lymph nodes (IO-IS mm) were free of tumor. Patients with COPD. recurrent cardiac failure. or previous granulomatous disease also tend to have enlarged lymph nodes. Various approaches have been taken to refine the size criterion by modulating the threshold size for various nodal locations within the mediastinum. or taking into account not only the smallest diameter as measured on the axial scan but also the shape of the lymph node (round versus oval). An increased diagnostic accuracy was described for morphologic evaluation of thin section imaging ofhilar lymph nodes. In 9S% oflocations with normal lymph nodes there was a straight or concave pleural interface between hilar structures and lung parenchyma, while in 9S% of locations with malignant lymph nodes there was a convex interface. Application of this criterion led to an improvement in sensitivity from SO% for the 10 mm size criterion to 87% for the morphologic criterion, and of specificity from 80% to 83 %. The approach of considering size and shape of lymph nodes takes advantage of the availability of thin and high quality multiplanar images provided by multislice CT. No quantitative data of diagnostic accuracy are available up to now. However, even though it is likely that refinement of morphological criteria in evaluating mediastinallymph nodes will improve the diagnostic accuracy, it will most likely never be able to achieve the same diagnostic accuracy as functional imaging using PET or (still investigational) lymph node-specific MR contrast agents (USPlO). Lymph node metastases may arise through a continuous mode of spread (intrapulmonary

=> bronchopulmonary or hilar => ipsilateral mediastinal => contralateral mediastinal). or a group of nodes may be skipped (in up to 30% of cases). Tumors of the right lung tend to metastasize to ipsilateral nodes, with lower lobe tumors usually causing more extensive metastasis. Contralateral nodal metastasis is more common with tumors of the left lung {> 30%). CT serves not only as basis for selecting those patients who have to undergo further invasive procedures (mediastinoscopy, mediastinotomy, thoracoscopy, trans bronchial biopsy) for lymph node staging but also to select the appropriate procedure depending on the location of enlarged lymph nodes to be sampled. Though the inability of CT to detect microscopic metastases within normal sized lymph nodes is known, it is generally agreed that preclusion of enlarged lymph nodes is sufficient to obviate further evaluation and patients directly undergo thoracotomy. However, to avoid over staging it is common understanding that enlarged lymph nodes seen in CT must be sampled by biopsy. The paratracheal, subcarinal, and proximal bronchopulmonary lymph nodes are accessible to mediastinoscopic biopsy, but not the commonly affected aortopulmonary nodes.

Therapeutically significant lymph nodes that appear enlarged on CT (Tables 9.1S, 9.16) should be biopsied due to the limited specificity ofCT. A mediastinoscopy is recommended in all patients with T3 tumors, with centrally located adenocarcinoma because of their high incidence of lymph node metastases, and in patients with chest wall infiltration or with Pancoast tumors, because of their poor prognosis with advanced lymph node staging.

Screening with Low Dose CT Previous studies using chest radiography and sputum cytology had not been able to statistically prove reduction of mortality rates, most likely due to the too low sensitivity of these tests for early tumor stages. Spiral CT is acknowledged as the most sensitive diagnostic method for the detection of small intrapulmonary nodules. The option for Significant reduction of acquisition dose

Bronchial Carcinoma

without comprormsmg the conspicuity of intrapulmonary nodules makes this technique feasible for screening (Fig. 9.18). Such CT examinations have a sensitivity >90% for nodules larger than 5 mm in diameter and an acquisition dose that is only about 2-3 time larger than chest radiographs in two projections. If a nodule is detected, the patient has to be rescanned with thinner sections unless thin-section multislice CT has been employed (see p.291). Multislice CT, in addition, facilitates automated detection, morphologic, and volumetric analysis of nodules. Depending on the geographic area and the frequency of histoplasmosis or tuberculosis, there may be a high rate of nodules with benign histology (granuloma, hamartoma, focal infection, lymph nodes). Thus, the screening protocols may vary locally. Biopsy has been suggested for all nodules> A cm that are not definitively benign (e.g., because of benign calcification, see p.314, or a known long-term course). Nodules between 5 and W mm in size are either biopsied or followed, nodules below 5 mm are followed with a larger time interval for rescanning. •

CT Morphology

Nodule detection is performed on axial sections with a lung window setting and a section width of some 5 mm. Overlapping reconstruction is mandatory, and viewing in a cine display speed up the evaluation and improves the detection rate. Automated detection algorithms work best on thin-section multislice CT data sets. It is not yet clear whether these algorithms are good

a

b

Fig. 9.18 Lung cancer screening Low-dose scans (CTDlvoi=2.3 mGy) demonstrate a small solid nodule (adenocarcinoma, a) and a ground-glass nodule (alveolar cell carcinoma, b). The volumetric assessment of

enough to be used exclusively or whether visual assessment of the whole data set is still necessary. Detection failures in a Japanese trial were due to the misinterpretation of lesions in close vicinity to pulmonary vessels or associated with pre-existing disease due to old tuberculosis. Three types of nodules have been identified: ground-glass nodules (focal areas of ground-glass opacities), solid nodules (no ground-glass associated), and mixed nodules (both features present). For nodules below "I em, patients with ground-glass and mixed nodules may be put on a course of antibiotics and are rescanned after 3-6 weeks. Solid nodules are biopsied or removed if thin-section scanning reveals morphologic criteria that are suggestive of malignancy (see Table 9.18). All other nodules are followed for 6 weeks to 6 months, depending on the local protocols and the available evaluation software. Computer-based evaluation algorithms were found to be able to reliably assess volumetric changes already after 4 weeks. Morphologic analysis is improved if volume rendered displays of the nodule surface are available and if multiplanar reformations centered on the nodules can be performed. •

Whom to Screen and When?

All studies published so far have in common that CT was found to be highly superior to radiography in terms of sensitivity. Bronchial carcinoma was detected in 1.1-2.7% of screened patients. Most (62-93%) tumors were determined to be stage 1. There is no general consen-

c

d

another nodule at the lung base (c) demonstrates no increase in tumor volume in follow-up scans after up to 6 months (d).

311

9 Lungs and Tracheobronchial

System

sus yet, however, as to whether screening can be generally recommended, and which risk groups are eligible. There are numerous potential sources of bias in the present trials (e.g., ELCAP, various Japanese and European studies). A lead time bias mainly affects the initial (prevalence) screening and is caused by tumors that are detected earlier but without an effect on overall life expectancy. A length time bias is caused by tumors, such as adenocarcinomas, that are slow growing and are thus more likely to be detected during follow-up (incidence screening) while more aggressive tumor types, such as small-cell cancers, may be diagnosed based on clinical symptoms in the time intervals between two screening procedures. Peripheral tumors are more likely to be detected early than central tumors {location bias}. In addition, there is the danger of overdiagnosis in those individuals who normally would die from other causes: small tumors detected in patients with large other risk factors (such as coronary artery disease, previous heart attacks, severe emphysema, or other smokingrelated diseases) may induce unnecessary treatment. Most present evidence, however, suggest that lung cancer screening programs in a selected risk population may be even more effective than mammography screening. There are no general rules yet but patients should be over 50 or 60 years old and have a major history of smoking (20-40 pack years). Obviously, the older the patient and the longer the smoking history, the higher is the likelihood that screening would be effective in detecting tumors and reducing overall mortality. Control after Treatment/Tumor Recurrence

CT remains the procedure of choice for routine follow-up or in suspected tumor recurrence in the post-pneumonectomy patient. The superiority of MRI for the detection of soft tissue recurrence is shown by the combination of mass effect and altered MR signal intensity. In fact any inhomogeneous signal in the pneumonectomy space should be suspicious for tumor recurrence (Fig. 9.19). A problematic topic is the reevaluation of the residual tumor and tumor activity after

a

b

Fig. 9.19 Tumor recurrence after lobectomy (4 x 1/6). Axial section (a) and coronal reformation (b) demonstrate a large recurrent tumor that invades the chest wall, the pulmonary arterial trunk, and thejugular vein and superior vena cava, leading to extensive venous collaterals (c).

neoadjuvant chemotherapy to be followed by surgery. CT criteria tend to underestimate the therapeutic effect demonstrated by pathologic examination. PET is likely to be the superior diagnostic method for evaluation of residual tumor or tumor recurrence. •

CT Morphology

There is a considerable variation in the appearance of the post-pneumonectomy space ranging from complete obliteration (in 27%) to a persistent fluid collection (in 73%). Filling in of previously patent ectatic bronchi is suggestive for recurrent disease. A 50% size reduction, a change in tumor morphology (round versus irregular), and the disappearance of density enhancement after application of intravenous contrast have been suggested as criteria for complete tumor response. Virtual Bronchoscopy and CT Bronchography

Virtual bronchoscopy yields excellent displays of the central tracheobronchial system and has various advantages over fiberoptic bronchoscopy. It is less invasive and therefore well tolerated by the patients. It can display the bronchial system distal to a narrow stenosis. Inversion of the viewing direction of the virtual

Secondary Tracheobronchial

bronchoscope is possible and one is able to look towards the proximal portions of the tracheobronchial system. Most important, however, is that all the additional information from the surrounding structures is available from the scans. Thus, there is information of the transmural extent of a tumor, the presence and location of lymph nodes, and the location of suspicious areas wi thin the 1ung parenchyma. In a clinical trial it was found that virtual bronchography improves the recognition of mild changes in airway caliber that otherwise are difficult to appreciate. It also helps in the understanding of complex tracheobronchial anomalies. The technique can be used to noninvasively assess diseases of the central tracheobronchial system (Fig. 9.20) but cannot substitute for fiberoptic bronchoscopy in patients in whom biopsy has to be performed or in whom visual assessment of the mucosal surface (e.g., color changes) is necessary. For pretherapeutic staging, virtual bronchoscopy serves as adjunct to bronchoscopy. Virtual bronchoscopy with transparent walls or colored extramural pathology may be used for guiding and planning trans bronchial biopsy procedures. In post-therapeutic imaging, non- invasive virtual bronchoscopy may be used alternately with fiberoptic bronchoscopy, e.g., in patients after bronchoplastic surgery with complex anatomic situations and a susceptibility for developing bronchial strictures. However, other visualization techniques based on MPR or volume rendering often provide equivalent or superior information (Fig.9.20).

Secondary Tracheobronchial Neoplasms

Neoplasms of the larynx, thyroid gland, or esophagus can involve the trachea and central bronchi by direct invasion or distant metastasis. Breast cancers, renal carcinomas, colorectal carcinomas, and melanoma have a propensity for endobronchial metastasis, which may be palliable by laser ablation and stent insertion, depending on the extent of the extrabronchial lesion and the severity of the obstruction. •

CT Morphology

CT has proven more sensitive than bronchoscopy for evaluation of the subglottic larynx. The subglottic spread of a laryngeal tumor into the trachea or, conversely, the spread of a trachea! tumor to the undersurface of the vocal cords will dictate the preferred therapeutic approach (laryngectomy versus partial tracheotomy). When a thyroid tumor is present, the ill-defined boundary that is often found between the thyroid gland and tracheal wall should not be misinterpreted as a sign of tumor invasion. Tracheoesophageal fistulas are most likely to develop in patients with advanced esophageal tumors, and CT can often locate the fistula more easily than endoscopy. Endobronchial metastases display nonspecific CT features and should be biopsied if doubt exists. Hodgkin's disease can lead to the formation of obstructive endobronchial lesions with atelectasis. In very rare cases, non-Hodgkin's lymphoma (NHL) may infiltrate the central airways and cause diffuse submucosal nodules.

Fig. 9.20 Virtual bronchoscopy (a) and CT bronchography (b) in a patient with a stenosis following sleeve resection of bronchogenic cancer(2/4/1 ).

a

Neoplasms

b

313

9 Lungs and Tracheobronchial System Pulmonary

Nodules

Pulmonary nodules are among the commonest focal pulmonary lesions detected at imaging (Table 9.17). Vascular processes or atelectatic lung areas can have features that resemble pulmonary nodules. For many diseases. however. the presence of a nodule is simply one variant in a range of possible radiologic presentations of the disease.

Table dules

9.17

Differential

Benign

neoplasms

• Hamartoma • Chondroma

Approximately 50% of surgically removed nodules turn out to be benign and noninvasive. Evaluation of the solitary pulmonary nodule remains a substantial challenge in modern medicine. Table9.18 reviews morphologic findings that may be helpful in differentiating benign from malignant lesions. In selected cases. ancillary findings such as pleural thickening. vascular relationships. cavitation. or enhancement characteristics may also aid in making the correctdiagnosis. Clusters of small intrapulmonary nodules with an internodular distance of less than Wmm and without associated infiltrates most frequently represent postinfectious granulomaFeatures

that help to differentiate

benign

Pseudotumor

Malignant neoplasms • Peripheral bronchial carcinoma • Alveolar cell carcinoma •

Metastases

• Carcinoid • Kaposi sarcoma •

Lymphoma

Septic

emboli

Intrapulmonary

lymph

nodes

AV malformation Round

atelectasis

and malignant

Benign Margins

pulmonary

• Tuberculosis • Histoplasmosis • Sarcoidosis

• Pulmonary

9.18

of

Granulomas

Differentiation of Pulmonary Nodules

Table

diagnosis

solitary

pulmonary

nodules

Malignant

Smooth

Stellate,

spiculated

Rigler notch sign Converging

vessels

Pleura! tail <Scm

Size Change

in size

Associated

>3cm

Size unchanged

blood vessels

for 2 or more years

No feeding artery, septic emboli

except

for

Feeding Draining

artery pulmonary

Calcifications

Central Dumbbell-shaped Peripheral lamination Calcification >10% of volume

Eccentric Stellate Disseminated Calcification

CTdensity

>150HU

<100HU

Enhancement

characteristics

Gradual

enhancement

<15HU Cavitation

Associated

Concentric,

findings

Rapid

vein

<1cr/oof

volume

enhancement

>15HU thin wall

Eccentric (squamous thick wall Lymphadenopathy

cell carcinoma),

no-

Metastases

tous disease (Aspergillus, mycobacteria, histoplasmosis), Intrapulmonary lymph nodes are typically less than 15 mm in size and locate within 15 mm distance from the pleura or interlobar fissures, They have a sharp contour, are round, oval or lobulated, and show contrast uptake of 35-85 HU, While preliminary work in vitro and in vivo had indicated that dual-kilovolt peak computed tomography may be useful in the identification of benign lesions this could not be confirmed in a recent multicenter trial. There was no significant increase of CT numbers with 80kVp compared to 140 kVp images based on a high calciumcontent. The value of enhancement after bolus injection of contrast medium was also assessed in a multicenter trial. Absence of significant nodule enhancement (A 15 HU) at CT was found to be strongly predictive of benignity, With a threshold of 15 HU the sensitivity was 98%, the specificity was 58% and the accuracy was 77%, The size of the nodules ranged between 14mm and 55 mm. A 15 mm spiral CT cluster of 3 mm slices was obtained L 2, 3, and 4 minutes after the onset of injection of 420 mg iodine/kg body weight with an injection rate of 2 ml/sec (the concentration was 300 mg iodine/ml). Data analysis was based on peak nodule enhancement and time attenuation curves, Malignant lesions enhanced (median of 38 HU) significantly more than benign lesions (median of 10 HU), This technique may also be used for follow-up of doubtful lesions, While malignant (42 HU) and acute inflammatory benign lesions (44 HU) both showed significantly higher peak enhancement levels than noninflammatory benign nodules (13 HU), in follow-up CTs after treatment (3 to 4 months later), the enhancement rates in nodules with active inflammatory reactions significantly decreased, making discrimination possible, Other discriminating features between malignant and inflammatory lesions are a retarded wash-out resulting in a slower decline of the density curves in malignant nodules, a lower attenuation on plain scans, and pronounced peripheral enhancement in inflamma tory n od ule s.

The low false positive rate of the results of the CT nodule enhancement technique is potentially valuable in the treatment of patients, A nonenhancing lung nodule can be managed with radiological surveillance alone even in a supportive clinical situation, An enhancing nodule either will be appropriately followed, if clinical suspicion for malignancy is low, or will undergo biopsy, Intratumoral necrosis and low cardiac output may contribute for low peak enhancement and thus false negative evaluation, It is important to note that PET achieves a comparable sensitivity for the assessment of malignancy in nodules as compared to the reported values for enhancement characteristics (94-100% versus 98%) but superior specificity values of over 90% (versus 60 to 75%), The high specificity ofFDG PET for the diagnosis of benign lesions has important clinical utility, Lesions with low FDG uptake may considered benign, However, they should be followed radiographically since false negative results were reported for carcinoid tumors and bronchioalveolar carcinomas if lesions were smaller than 1 ern. Observation of nodule growth is another valuable tool for monitoring pulmonary nodules, especially if they are too small for the assessment of enhancement characteristics and occur in patients at risk, Computer programs have been developed that allow for nodule segmentation and volume rendering and proved to be more precise and sensitive towards small volume changes than visual assessment on 2D axial follow-up scans or manual assessment oflesion diameter.

Metastases

Pulmonary metastases most commonly originate from carcinomas of the breast, kidneys, colon, stomach, and pancreas, They can also develop from seminomas and sarcomas, CT is the most sensitive modality for detecting metastases in the staging of tumors, Spiral CT can detect pulmonary nodules as small as 2-3 mm in diameter. Conventional CT has similar capabilities, but up to 30% of smaller lesions (A 10 mm in diameter) may be missed on

315

316

9 Lungs and Tracheobronchial System

sequential tion. •

monary lymph nodes, focal scars) are also being detected with increasing frequency. This leads to problems of differential diagnosis in preoperative tumor staging. Since a definitive histological diagnosis cannot be established for all of these findings, other differentiating strategies must be considered, such as scheduling followup examinations at 3 to 6 months or defining a minimum size of 3-5 mm for focal lesions that require further evaluation. At present, however, there are no adequate data to substantiate this approach. Also, some tumors such as thyroid carcinoma (Fig. 9.21 b) often produce miliary metastases that measure only a few millimeters in size. Calcifications are rare and generally suggest benignancy, although calcified metastases are known to arise from osteosarcoma and less commonly from chondrosarcoma. These lesions cannot be reliably distinguished from granulomas. Rarely, calcified metastases develop from the spread of mucous- forming adenocarcinomas (breast, colon, ovary), thyroid carcinomas, soft-tissue sarcomas, and after chemotherapy. Cavitation is typical of septic emboli but rarely occurs also in metastases (Fig. 9.22 a), e.g., in squamous cell carcinoma. Fatwithin a pulmonary nodule usually suggests the presence of a harmatoma, but may also be due to a metastasis from liposarcoma (Fig.9.22b) .

scans due to respiratory misregistra-

CT Morphology

All focal lesions in the lung that are not attributable to vessels are suspicious for pulmonary nodules. Linear densities without a central mass are not considered to be nodules and generally represent small areas of scarring (e.g., post infection). Small nodules that are imaged in thick CT slices show fading peripheral density due to partial volume effects (see chapter 7 on Image Analysis) . Metastases typically have smooth margins. A mass with ill-defined margins should raise suspicion of a peripheral bronchial carcinoma. Metastases may acquire ill-defined margins as a result of local lymphedema, intralesional hemorrhage (angiosarcoma, choriocarcinoma), or postchemotherapeutic scarring. Solitary metastases are rare, accounting for just 5% of all solitary nodules. The likelihood that pulmonary nodules are metastatic increases with the numbers of nodules that are seen. Metastases are commonest in the outer portion of the lung (90%) and near the pleura. They predominantly involve the lower lobes (66%). Approximately 40% of metastases are found to have associated blood vessels (feeding vessel sign, Fig. 9.21 a). Spiral CT can detect pulmonary nodules with such high sensitivity (range of 2-3 mm) that small benign lesions (granulomas, intrapul-

a

b

Fig. 9.21 Pulmonary nodules, (a) Predominantly subpleural hematogenous metastases with feeding vessel sign. (b) Multiple miliary metastases from thyroid carci-

c

d

noma, predominantly in the lower zones, (c) Tuberculoma with central calcification, (d) Pulmonary hamartoma with lobulated margins and fat deposits.

Carcinoid Tumor ganizing focal pneumonia in patients with subclinical infection, and by others to be a precursor of pulmonary lymphoma (see also p.324). They show a variety of histologic entities including xanthoma, histiocytoma, and mast-cell or plasma-cell granuloma. •

CT Morphology

They present as well-defined solitary nodules, calcifications are seen in about 20%. Airway involvement is extremely rare.

a

b

Fig. 9.22 Atypical presentation of pulmonary metastases: Cavitating pulmonary nodule on 1.5mm thick sagittal reformation (4 x 1/6) mimicking a pneumatocele (a). Nodule containing fatty tissue (-55 HU) in a patient with liposarcoma (b).

Hamartoma

Hamartomas constitute more than 5% of all solitary pulmonary nodules, and the great majority (> 95%) occur after 40 years of age. •

Granuloma

Tiiberculomas are focal lesions in which a balance is established between the activity of the infectious organism and the host defenses. Histoplasmosis is prevalent only in certain regions such as the southern and eastern United States; it is rare in Europe. Lung infection gives rise to calcifying granulomas. •

CT Morphology

A tuberculoma is a sharply circumscribed nodule 0.5-4 cm in diameter. Approximately 25% of tuberculomas have lobulated margins. A pathognomonic feature is calcification (Fig. 9.21 c), which usually develops at the center of the nodule or may fill it completely. The upper lobes are a preferred site. Cavitation is rare and, when present, is usually small and eccentric. The nodules are multiple in 80% of cases. Histoplasmosis is marked by the development of granulomatous nodules that often show central calcification like tuberculomas. Cavitation is very rare, and there is a predilection for the lower lobes. Pulmonary Pseudotumor

The etiology is unknown, though they are thought by some to represent sequelae of Of-

CT Morphology

Hamartomatous nodules are usually smaller than 4cm and are more often lobulated than round. They may be slow growing. Approximately 90% of hamartomas occur in the peripheral lung (within 2cm of the pleura). Approximately one-third show calcifications (popcornlike, stippled, central), and an equal percentage contain fatty areas, which are considered pathognomonic (Fig. 9.21 d). Identification of intrapulmonary fat deposits requires density measurements in thin (1-2 mm) sections to avoid misinterpretation of partial volume effects as negative density units. Rarely, a fat-containing nodule may be a metastasis from liposarcoma (Fig.9.22b).

Carcinoid Tumor

The lung parenchyma is an uncommon site of occurrence for carcinoid tumors (10% of all intrathoracic carcinoids). Carcinoids belong to the group of neuroendocrine tumors, and may be associated with ectopic hormone productions (e.g., ACTH). Peripheral carcinoids tend to be type 2 tumors, with metastases in up to 50% of patients. Typical carcinoids (type 1) rarely metastasize.

317

9 Lungs and Tracheobronchial

•

System

CT Morphology

Intrapulmonary carcinoids appear as well-circumscribed nodules. Isolated bronchial dilatation with mucoid impaction may provide the only indirect sign of an occluding intraluminal carcinoid. These lesions rarely show calcification or cavitation, and most enhance intensely with intravenous contrast. Dependent on the histologic subtype they are slow growing; up to 30% of carcinoids contain calcifications. Septic Emboli

Thromboembolic material can arise in septic conditions and cause the occlusion of small pulmonary arteries. Septic emboli may originate from an infected catheter, abscess, endocarditis, or pulmonary or urinary infection. •

CT Morphology

CT usually shows multiple round or wedgeshaped densities in various stages of cavitation (abscess formation in approximately 50%) (Fig. 9.23). The detection of a feeding vessel (feeding vessel sign) confirms the hematogenous origin of the lesion. The typical appearance is that of disseminated bilateral lesions that are mainly located in the lower lobes. There may be an accompanying pleural or pericardial effusion.

Fig. 9.23 Septic emboli. (a) Unsharply demarcated nodules, (b) From a patient with history of intravenous drug abuse showing multiple round opacities with cavitations and signs of bronchogenic spread of disease as indicated by the centrilobular densities.

Septic foci have less distinct margins than metastases and often appear as wedgeshaped opacities based on the pleura, some of which are cavitating.

Peripheral Bronchial Carcinoma

Approximately 40% of all bronchial carcinomas develop distal to the segmental bronchi. Peripheral bronchial carcinoma should always be included in the differential diagnosis of a solitary pulmonary nodule. Many bronchial carcinomas can be confidently identified by their CT morphology alone. •

CT Morphology

The CT appearance ranges from that of a solitary, well-circumscribed pulmonary nodule to lesions that show definite morphologic signs of malignancy (Table 9.14). Malignancy is suggested by signs of infiltrative growth with retraction of surrounding structures. The signs include ill-defined margins, radiating spicules (corona radiata), radiating strands with pleural retraction (pleural tail), and the distortion of small vessels by the nodular lesion. Other signs of bronchial carcinoma are visible intraluminal tumor growth, a notched indentation in the tumor border caused by the entry of a pulmonary arterial branch (the Rigler notch sign), and the detection of a pulmonary vein leaving the tumor (Fig. 9.24). Small air inclusions often represent opened bronchi [pseudoca vitation) or foci of intratumoral necrosis and are a common feature of alveolar cell carcinoma. When CT demonstrates a bronchus entering the tumor ("positive bronchus sign," Fig.9.24b), the accuracy rate of bronchoscopic histologic or cytologic sampling increases from less than \5% to 60%. This finding is best appreciated on thin sections and is helpful in diagnostic planning (bronchoscopy vs. transthoracic needle biopsy vs. surgery). Multiplanar imaging is advantageous for imaging morphologic detail, the relationship to pleural interfaces and the localization of lesions in close vicinity to anatomic interfaces (chest wall/interlobium) (Fig. 9.25).

Atelectasis

A

c

b Fig. 9.24 Peripheral bronchial carcinoma, (a) Axial scan demonstrates ill-defined, spiculated margins and intraluminal tumor growth, (b) Semicoronal reformatted image shows a bronchus passing into the spicu-

lated mass (positive bronchus sign), (c) Shaded surface display shows a pulmonary vein (arrow) draining the tumor.

-4 Fig. 9.25

Peripheral bronchial carcinoma (4x 1/6). The coronal MPR (b) better demonstrates the transfissural tumor growth of this mixed type nodule (adenocarcinoma) than the axial section (a).

·/'\../'\.Af'... __

»<:

Predominantly Focal Pulmonary Opacities

Atelectasis Lobar or segmental atelectasis generally presents no problems of differential diagnosis. It is defined as a volume loss of a lung or portions of

Upper lobes

Fig. 9.26

Lobar atelectasis.

a lung. Types of atelectasis are differentiated according to their etiology (Table 9.19). The CT findings correspond to those in conventional radiography (Fig. 9.26).

Middel lobe orlingula

Schematic

representation

of CT morphology.

Lower lobes

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9

Lungs and Tracheobronchial System

Table 9.19

Types of atelectasis

Resorption atelectasis (endobronchial) • Tumor • Lymphoma • Mucoid impaction • Foreign body • Stricture (tuberculosis, trauma, postoperative) • Postinflammatory middle lobe syndrome Resorption atelectasis • Mediastinal mass • Lymphadenopathy • Enlarged left atrium

If there is a central tumor causing the lobar atelectasis, the lateral contours of the lobes may run convexly rather than concavely (similar to Golden's reverse S sign in posteroanterior radiographs in patients with upper lobe atelectasis due to a central tumor).

(extrabronchial)

Passive (compression) • Pleura! effusion • Pneumothorax • Rounded atelectasis • Adjacent space occupying disease (bulla, mass, ascites) • Decreased diaphragmatic excursions Cicatrization • Granulomatous disease • Pneumoconiosis • Interstitial fibrosis Adhesive • ARDS • Postoperative

•

dially. The lower lobes collapse posteromedially against the posterior mediastinum and spine.

Poststenotic (resorptive) atelectasis generally does not contain an air bronchogram, and mucus-filled bronchi appear as hyperdense tubular or ring-like structures. With compression atelectasis (e.g., secondary to basal effusion), a positive air bronchogram is usually observed. Cicatrization atelectasis occurs in areas of pulmonary fibrosis and is almost invariably associated with bronchiectasis. Adhesive nonobstructive atelectasis or microatelectasis is related to a decreased amount of surfactant in patients after surgery or ARDS and most commonly presents with quite extensive consolidations. After contrast administration, the atelectatic lung tissue shows stronger and more homogeneous enhancement than lung parenchyma involved by neoplastic or inflammatory infiltration (Fig. 9.27).

CT Morphology

Depending on the extent of the atelectasis, CT may show a homogeneous area of increased densitywith a typical location and configuration. Important indirect signs that help distinguish atelectasis from infiltration are the bowing (concave displacement) of the interlobar fissures and the crowding of vascular structures as a result of the reduced lung volume. There may be compensatory overinflation of the remaining lung with the vascular structures being spread apart. In addition to the location and configuration of the opacity itself, the displacement ofthe hilar structures is most helpful in identifying the atelectatic part of the lung. While the right upper lobe collapses against the mediastinum, the left upper lobe retains more contact with the anterior and left lateral chest wall (Fig. 9.26). The middle lobe is particularly susceptible to chronic postinflammatory /\collapse; CT is used to exclude a central endobronchial tumor in these patients and well demonstrates the patent origin of the middle lobe bronchus. The medial margin of the middle lobe abuts the right heart border and the posterior margin ofthe middle lobe is displaced anterome-

a

b

Fig. 9.27 Atelectasis. (a) Comparison of atelectasis (lower lobe) and pneumonia (lingula) after contrast administration. The atelectasis shows marked, homogeneous enhancement due to the rich vascular supply of the collapsed lung parenchyma. Pneumonia, like other types of consolidation, shows considerably less enhancement, (b) Left upper lobe atelectasis due to a central bronchial carcinoma.

Bronchiolitis Obliterans with Organizing Pneumonia (BOOP)

or cryptogenic organizing pneumonia, histologically characterized by:

COP) is

• granulation tissue polyps within the lumina of bronchioles and alveolar ducts; and • patchy areas of cellular infiltrates (mainly mononuclear cells and macro phages) in the surrounding air spaces.

b Fig. 9.28 Rounded atelectasis. Pulmonary window settings show a typical comet-tail configuration of the bronchovascular structures (a), with a broad area of pleural contact and pleural thickening. Soft tissue window settings in another patient (4 x 1/6, coronal MPR) demonstrate the homogeneous enhancement after contrast administration (b).

Rounded Atelectasis

Rounded atelectasis is a persistent compression atelectasis secondary to effusion. It may present a tumor-like morphology, but biopsy is rarely necessary owing to the characteristic CT features of the lesion. Rounded atelectasis can be seen with any cause of pleural thickening but is commonest in asbestos-related disease. •

CT Morphology

CT typically shows a solitary rounded or wedgeshaped mass located near the pleura, most commonly in the lower lobe, and showing homogeneous contrast enhancement. The affected lobe shows signs of volume loss. Diagnosis is aided by the comet tail sign caused by the distortion and displacement of bronchi and blood vessels in a whorled pattern leading into the atelectatic mass (Fig. 9.28). Pleural thickening is always an associated sign. Air bronchograms or focal air collections (pseudocavitation) may be present. Bronchiolitis Obliterans with Organizing Pneumonia (BOOP)

Bronchiolitis obliterans organizing pneumonia (BOOP) (also named organizing pneumonia (OP)

Most cases are idiopathic. Clinical manifestations include an unproductive cough, subfebrile temperatures, several months' history, and restriction of pulmonary function. The symptoms regress with steroid therapy. A form with identical pathomorphologic features, called the BOOP reaction, may occur as a sequel to viral, bacterial, or fungal pneumonia, chronic eosinophilic pneumonia, collagen diseases, radiotherapy, and after lung or bone marrow transplantation, Wegener's granulomatosis and after toxic inhalation (Table 9.20). A recent international consensus on standardizing classification of idiopathic interstitial pneumonias included idiopathic BOOP in the group of idiopathic interstitial pneumonias (see p. 344). This view has been the subject of debate since the lesion is caused by airspace organization and is not primarily interstitial in a histological sense. Clinical history may be helpful for differential diagnosis. BOOP usually appears as a subacute illness with duration of symptoms for about 2 to 6 months before diagnosis. In many patients, the clinical and radiographic signs of disease remit completely after systemic corticosteroid therapy. Only a subset of patients show a rapid progression and have a worse prognosis. In these patients BOOP is frequently associated with connective tissue disease or drug therapy. While infectious diseases may be excluded on the basis of clinical symptoms, lung biopsy is usually performed because of suspicion of a carcinoma.

Table 9.20

Causes of BOOP

Idiopathic Following infection Chronic eosinophilic pneumonia Collagen vascular diseases Radiotherapy Bone marrow or lung transplantation Wegener's granulomatosis Following toxic inhalation

9

•

Lungs and Tracheobronchial System

CT Morphology

The morphologic features are variable and nonspecific (Fig. 9.29). The commonest pattern consists of patchy (non-segmental) air space consolidations in a subpleural (cortical) or central peribronchiolar distribution in more than 50% of the patients. The patchy consolidations may be migratory, they may show pleural tags or spiculae (in approximately 30 to 40%). While consolidations may be the only finding in about 50% of the patients, ground-glass opacifications are mostly part of a mixed pattern. The size of the individual opacities ranges from 3 cm to nearly complete lobar consolidations. The margins of the consolidations are indistinct; they may contain air bronchograms. Bronchial wall thickening and dilatation are seen in most patients and are usually restricted to areas of consolidation or ground-glass opacifications. It is not yet clarified how well these bronchial abnormalities are reversible. In 15% to 50% of patients in the various study populations multiple peri bronchiolar (centrilobular) nodules were seen that mostly measured between 1 and 10mm. These tend to have mostly irregular margins (88%) and may also show air bronchograms (45%). Nodules are the only finding in about one-third of the

a

b

Fig. 9.29 Various morphologic appearances of BOOP. (a) Peripheral pleural-based consolidations, (b) Peribronchial consolidation with air-filled bronchi mimicking pneumonia, (c) Multiple peribronchial nodules.

patients showing nodules. Rarely bilateral areas of ground-glass opacities are seen with overlying thickened intralobular septal lines. Nodules and ground-glass attenuation are seen more frequently in the immunocompromised than in the immunocompetent patient. Additional findings are pleural thickening (33%), small pleural effusions (30%), and parenchyma! bands (25%). Dependent on the range of distinctive patterns, the differential diagnosis for BOOP is rather extensive (Table 9.21). In cases with solitary mass-like focal consolidations that have morphologic criteria for malignancy, such as spiculae, pleural tagging or regional pleural thickening, a bronchial carcinoma cannot be differentiated from BOOR Multiple patchy air space opacities may also be associated with alveolar cell carcinoma, vasculitis, or pulmonary hemorrhage. Numerous bilateral small nodules in BOOP have to be differentiated from sarcoidosis or acute infectious bronchiolitis (atypical and typical mycobacteria). Chronic eosinophilic- pneumonia has to be considered for differential diagnosis if the pattern is dominantly peripheral.

c

d

(d) Patchy, well-circumscribed consolidation with ectatic air-filled bronchi and combined with signs of architectural distortion and air trapping of adjacent lung.

Hemorrhage, Table 9.21

Differential diagnosis

of BOOP

Focal mass-like lesion

bronchial

carcinoma,

Multiple

vasculitis,

lymphoma

patchy air space opacifications

Numerous small nodules

sarcoid,

infectious

Subpleural

chronic

eosinophilic

patchy consolidation

Hemorrhage, Hemosiderosis

Table 9.22

b Fig. 9.30 Pulmonary hemorrhage, (a) Fresh pulmonary hemorrhage with ill-defined, confluent acinar opacities following transpulmonary needle biopsy of a central tumor, (b) Fresh hemorrhage in the posterior apical segmental bronchus of the upper lobe, manifested by ground-glass opacity of the affected sec-

carcinoma

bronchiolitis pneumonia

syndrome

Collagen diseases and vasculitis • Lupus erythematosus • Wegener's disease • Polyarteritis nodosa • Rheumatoid arthritis Hemorrhagic necrotizing pneumonia (e.g., leptospirosis, acute viral pneumonia)

CT Morphology

The CT appearance ranges from small, indistinct (acinar) focal densities (Fig. 9.30) to more diffuse ground-glass opacities and widespread homogeneous opacification with an air bronchogram. The opacities clear in two to three days. Some irregular patchy or linear opacities may persist for a longer time period. With chronic recurrent hemorrhage (increasing amount of hemosiderin deposits within the interstitium), irreversible interstitial changes are superimposed. The severity of

bronchoalveolar

Causes of pulmonary hemorrhage

Goodpasture's

Regardless of its etiology (Table 9.22), pulmonary hemorrhage is characterized by increased alveolar density in the acute stage. Idiopathic pulmonary hemosiderosis (IPH) is characterized by recurrent episodes of pulmonary hemorrhage without associated glomerulonephritis or serologic abnormalities. •

Hemosiderosis

Idiopathic

pulmonary

hemosiderosis

Coagulation disorders • Medications • Myelodysplastic diseases Trauma, biopsy

parenchymal changes is dependent on the time sequence and the number of hemorrhages that have occurred.

c

d

ondary lobules, (c) Intrapulmonary hemorrhage demonstrating confluent acinar opacities as well as dense consolidation in a patient with Goodpasture's syndrome. (d) Ground-glass opacities associated with thickened interlobular septa indicating resorptive processes in hemosiderosis.

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9 Lungs and Tracheobronchial System

Alveolar Cell Carcinoma

Alveolar cell carcinoma (bronchiolar adenocarcinoma, bronchioloalveolar carcinoma) accounts for no more than about 5% of all lung cancers. Bronchogenic spread (e.g., by coughing) is common and leads to multiple lesions and even bilateral involvement in more than 40% of cases. The multifocal or diffuse form has a much poorer prognosis than the local form. •

CT Morphology

The local form of alveolar cell carcinoma presents as a solitary pulmonary lesion that is often peripheral or subpleural (Fig. 9.31 a). It may appear as a well-circumscribed nodule or may show the characteristic features of bronchial carcinoma including a "pleural tail" (streak-like densities extending to the pleura, >50%) and spiculated margins (> 70%). Most lesions exhibit a mixed density pattern. It is common to find intact air-filled bronchi within the tumor (pseudocavitation), and the embedded bronchi may appear stretched. Atelectasis generally does not occur. The multifocal or djffuse form resembles air space consolidation due to pneumonia. The spectrum ranges from patchy ground-glass opacities to areas of consolidation (Fig. 9.31 b, c). Involvement of both lungs may occur.

b Fig. 9.31 Variable morphology of alveolar cell carcinoma. (a) Solitary ill-defined lesion (ground glass nodule), (b) Pneumonia-like subpleural opacities with pseudocavitation (air-filled bronchi), (c) Disseminated confluent acinar opacities, (d) Positive "CT angiogram"

Whenever CT demonstrates multiple lesions that show the morphologic features of bronchial carcinoma or focal pneumonia, alveolar cell carcinoma should be considered in the differential diagnosis. Pleural effusion is seen in approximately onethird of cases, and mediastinal lymphadenopathy is seen in one-fifth. Cavitation is rare. The "CT angiogram" sign appears when pulmonary vessels coursing through the tumor show greater enhancement on postcontrast scans than the surrounding tumor. While a "CT angiogram" is commonly seen with alveolar cell carcinoma, it is not specific and may also occur in consolidation due to lymphoma, pneumococcal infection, or tuberculous infection (Fig. 9.31 d). Lymphoma

Pulmonary involvement by malignant lymphoma occurs in up to 40% of cases and is commoner in Hodgkin's lymphoma than in the nonHodgkin's type (6%) (Table 9.23). In Hodgkin's disease, more frequently, the lung is secondarily involved by widespread mediastinal disease, primarily involvement of the lung parenchyma is extremely uncommon. Recurrent Hodgkin's disease, however, is often

c

d

caused by greater vascular enhancement relative to the surrounding tumor. This sign is common but not specific for alveolar cell carcinoma. Note also the calcified pleural plaque secondary to asbestos exposure.

Lymphoma Table 9.23 Hodgkin's

Types of Lymphoma and lymphoid lymphoma

Lymphoma (WHO classification, listing only the most frequent subtypes)

Non-Hodgkin's B-Cell

Neoplasm

• Immature (precursor cell) 8-cell neoplasm Lymphoblastic leukemia/lymphoma • Mature (peripheral) 8-cell neoplasm Chronic lymphatic leukemia/smail cell B-cell lymphoma myeloma extranodal marginal cell B-cell lymphoma (MALT) follicular lymphoma mantiecell lymphoma diffuse large cell lymphoma (DLBCL) Burkitt lymphoma T-Cell

Neoplasm

• Immature (precursor cell) T-cell neoplasm Lymphoblastic leukemlallymphoma • Mature (peripheral) T-cell neoplasm Peripheral T-eelllymphoma (PTL) Anqioimmunoblestic T-cell lymphoma (see below) Anaplastic MycosIs funfIOJd,es/,;ezB'ry Angiocentric

immunoproliferative

lesions

• Grade 1: benign lymphocytic angiitis • Grade 2: lymphoid granulomatosis • Grade 3: lymphoma Lymphoid • Focal:

hyperplasia

nodular lymphoid hyperplasia, pseudolymphoma

• Diffuse

Lymphoid interstitial pneumonia (LIP) Follicular bronchiolitis

within the thorax along the radiation treatment port margins. Less than 50% of nonHodgkin's lymphoma initially develop a thoracic manifestation. The most frequent manifestation of thoracic involvement in non-Hodgkin's lymphoma (NHL) is mediastinal or hilar lymph node enlargement. If lymphoma is limited to the lungs (with or without mediastinal lymphadenopathy) it is classified as primary pulmonary lymphoma (no extrathoracic involvement for at least three months), while cases with previous or concomitant extrathoracic lymphoma are classified as secondary pulmonary lymphoma. The most recent working formulation subdivides B-cell non- Hodgkin's lymphoma into low, intermediate, and high grade, and predominantly T-cell angioimmunoproliferative lesions (lymphoid granulomatosis) .

Mucosa-associated lymphoid tissue (MALT) represent one type of intrapulmonary low grade B-cell lymphoma in the lung, also described as bronchus-associated lymplzoid tissue (BALT). Also most high grade NHL are B-cell lymphomas. Patients may suffer from underlying disease such as AIDS, or have undergone organ transplantation. In contrast to patients with low grade lymphoma, they suffer from respiratory or systemic symptoms. High grade non-Hodgkin lymphomas tend to recur at the initial sites of disease. Angiocentric immunoproliferative lesions represent a histologic spectrum and are therefore subdivided into three grades, dependent on the severity of cytologic atypia and polymorphism. While grade 1 describes a benign lymphocytic angiitis, grade 2 is defined as lymphoid granulomatosis, and grade 3 represents true lymphoma. From the literature it appears that there is also some overlap between pulmonary lymphoid hyperplasia and low grade lymphoma, and it remains unclear whether the first represents a truly benign (possibly reactive) process, a premalignant form, or a malignancy difficult to diagnose. Focal

lymphoid hyperplasia

(synonyms:

nodular lymphoid hyperplasia, pseudolymphoma, well-differentiated lymphocytic proliferation, monoclonal B-cell proliferation) can be differentiated from diffuse pulmonary lymphoid hyperplasia. For the latter, two varieties have been described: lymphoid interstitial pneumonia (LIP), predominantly affecting the parenchymal interstitium, and follicular bronchiolitis, predominantly affecting the peribronchiolar interstitium (see also p. 300). Secondary pleuropulmonary involvement with lymphoma in patients with extrathoracic disease is much commoner than the primary condition. Spread occurs either by direct involvement from mediastinal or hilar lymph nodes or by hematogenous dissemination. Post-transplant lymplioproliferative disorder (PTLD) is seen in about 3% of organ transplant recipients and is related to immunosuppression and the Epstein- Barr virus. CT

Morphology

Lymphomas have a broad, nonspecific spectrum of CT findings that range from miliary nodulation and discrete nodules to ground-glass opacities and confluent, pneumonia-like patches of

325

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9

Lungs and Tracheobronchial System

c Fig. 9.32 CT appearances of pulmonary lymphoma. (a) Well-defined nodule, (b) Patchy nodules with nodular pleural thickening and pleural effusion, (c) Consolidation with perifocal ground-glass opacification in

consolidation with or without an air bronchogram (Fig. 9.32). The pattern of intrapulmonary Hodgkin and non-Hodgkin's manifestations can be identical. In Hodgkin's disease the commonest findings are ill-defined nodules or masses spreading out from the hila along the central bronchovascular bundle. The appearance may be similar to that of sarcoidosis or Kaposi sarcoma. Air bronchogram and cavitations are frequent findings. Recurrent disease also demonstrates intrapulmonary or perihilar nodules and mass lesions, sometimes along the radiation treatment port margins. Care should be paid to the diaphragmatic and pericardia! lymph nodes that are not included in the mantle radiation treatment. In non-Hodgkin's disease chest wall involvement is rather common and results from contiguous spread from the mediastinum. The pleura may be involved by intraparenchymal disease as well as by chest wall involvement. While plaque-like pleural thickening represents lymphomatous spread, pleural effusions are mostly due to lymphatic obstruction and do not represent direct pleural involvement. The lung parenchyma is a common site of involvement by extranodal NHL. Though there is morphologic overlap, some characteristic features or the different grades of NHL can be described: CT findings in low grade NHL (MALT or BALT) comprise solitary nodules or ill-defined focal opacities between 2 and Scm in size with air bronchograms in about 50% (Fig. 9.33). Bronchi

d

acute myeloic leukemia, resembling invasive aspergillosis. (d) Ground-glass opacifications sparing the subpleural space.

Fig. 9.33 MALT lymphoma. (a) III-defined focal opacity. (b) Diffuse micronodular opacities with tree-in-budpattern and bronchial wall thickening simulating acute endobronchial spread of infection.

within affected lung parenchyma may appear stretched and slightly narrowed. Less commonly there are multiple nodules or infiltrates. They slowly grow over months or years. Lymphadenopathy is primarily seen in less than 5%. CT findings of high grade NHL are nonspecific and range from focal to diffuse consolidations with or without air bronchogram and occasionally with a diffuse reticular pattern. Patients may show rapid progression of consolidations with respiratory failure. The commonest manifestations of englecentric immunoproliferative lesions are multiple

Kaposi Sarcoma

nodules or masses between 0.5 and 8 cm in size. They may start as ill-defined opacities, and develop to confluent nodules or masses. In subpleural location they mimic pulmonary infarcts. Cavitation is seen in up to 40%; air bronchograms are extremely rare. Since medium to small-sized vessels are involved it mimics the appearance ofWegener's disease in the lung but never involves the kidneys (see also p. 328). A focal lymphoid hyperplasia presents with a solitary nodule or a focal area of consolidation usually limited to one lobe, 2 to 5 cm in diameter and rarely exceeding to masses and infiltrates with W cm diameter. Virtually all lesions contain air bronchograms. Typically there is no associated lymphadenopathy; if there is, lymphoma should be suspected. UP is frequently associated with diseases with abnormal immune reaction (Table 9.24, see Fig. 9.65 b). Follicular bronchiolitis has been seen in up to 20% of patients with rheumatoid arthritis. The most frequent abnormalities consist of bilateral areas of ground- glass opacification, small nodules, and mostly centrally located cysts. In follicular bronchiolitis CT shows small nodular opacities in a peribronchiolar and centrilobular location (1-3 mm in diameter, rarely up to 10 mm). In contrast to primary pulmonary lymphoma, secondary involvement tends to affect the central airways resulting in atelectasis and poststenotic pneumonitis. Lymphatic congestion presents with thickened interlobular septa. The commonest intrapulmonary manifestations are solitary or multiple ill-defined nodules or masses, most frequently in the lower lobes, with linear stranding extending into the surrounding parenchyma. Cavitation is rare. Post-transplant lymplioproliferative disease presents with solitary or multiple nodules and occasionally hilar adenopathy. The lesions usually regress with decrease ofimmunosuppression. Table 9.24

Diseases associated with LIP

Sjogren's syndrome

AIDS Chronic hepatitis Renal tubular acidosis Myasthenia gravis Autoimmune thyroiditis Systemic lupus erythematosus (SLE) Primary biliary cirrhosis Allogenic bone marrow transplantation

Kaposi Sarcoma

Kaposi sarcoma in HIV -infected patients may involve the lung in addition to the skin and gastrointestinal tract. The overall incidence is on the decline (from 60% to the current 15-20%). •

CT Morphology

CT typically demonstrates cloudy nodular masses or patchy opacities that show a predominantly perihilar distribution (bilateral, often asymmetrical) and are oriented along the bronchovascular structures. This may be accompanied by nodular thickening of the interlobular septa, creating a pattern that resembles carcinomatous Iym phangitis. The lesions have ill-defined margins due to infiltration of the surrounding interstitium (local lymphangiosis), leading to spiculated, lanceolate, or flameshaped densities. Similar findings are seen with angiosarcoma (Fig. 9.34). Endobronchial tumor spread can lead to atelectasis. Being highly vascular, Kaposi sarcoma enhances intensely after intravenous contrast administration. Extensive pleural effusions and mediastinal lymphadenopathy are commonly found.

a

b

Fig. 9.34 Kaposi sarcoma (a) appearing as multiple perivascular focal opacities surrounded by halos caused by lesional hemorrhage. Perilesional hemorrhage is also seen with pulmonary angiosarcoma (b).

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9 Lungs and Tracheobronchial System

Pulmonary Blastoma

Pulmonary blastoma is a rare primary lung tumor, histologically consisting of immature epithelial and mesenchymal components. There is a biphasic age distribution with peaks in the 15t and 7th decade. The prognosis is poor and appears to be dependent on the size of the primary tumor. A histologic variant resembling pulmonary blastoma without its sarcomatous stroma is named "well differentiated fetal adenocarcinoma" or "pulmonary endodermal tumor resembling fetal lung" . •

CT Morphology

CT demonstrates a large well-defined intrapulmonary nodule or mass with diameters ranging from 2.5 to 25cm. Occasionally metastases and pleural invasion are seen. Rarely tumors are cavitated, calcified or multiple. Well differentiated fetal adenocarcinoma tend to be smaller than the classic blastoma. Radiologic findings are nonspecific consisting of a peripheral nodule or mass. Pulmonary Infarction

The thromboembolic occlusion of a pulmonary artery restricts blood flow to the more distal lung segments, leading in 10-15% of cases to pulmonary infarction. The hypoperfused tissue is prone to secondary infection (postinfarction pneumonia) or abscess formation (postinfarction abscess) and may cavitate when a connection with the bronchial system is established (postinfarction cavitation). •

CT Morphology

Pulmonary infarction typically appears as a wedge-shaped density that is broadly based on the pleura and points to the hilum (Fig. 9.35). The lower lobes are more commonly affected than the upper lobes. Contrast-enhanced scans show a central hypodense area (necrosis) and peripheral rim enhancement (inflammatory reaction). An accompanying pleural or pericardial effusion may be found.

a Fig. 9.35 Pulmonary infarction presents as a pleuralbased opacification with reduced contrast enhancement that is typically wedge-shaped (a), but may be more rounded (b).

Wegener's Disease

Wegener's disease represents a systemic disease with necrotizing granulomatous vasculitis involving several organs. Approximately 90% of patients develop pulmonary manifestations (renal changes occur in 85%, joint changes in 50%), thus the lung has to be examined obligatorily once the diagnosis of Wegener's disease has been established. Approximately 90% of patients have an increased level of C-ANCA antibodies, which is also seen in patients with microscopic polyangiitis, progressive glomerulonephritis, Churg-Strauss, and some infectious diseases (aspergillosis, amebiasis, bacterial endocarditis, chromomycosis). Wegener's disease is characterized by a necrotizing granulomatous vasculitis of the upper respiratory tract, a disseminated vasculitis (small and medium-sized vessels) and a focal necrotizing glomerulonephritis. •

CT Morphology

Typical CTfindings are multiple intrapulmonary nodules or focal densities (0.5 to 10cm) predominantly in the lower zones and frequently subpleurally located. The nodules may show spiculae or feeding vessels that are also described for metastases or infarcts. Cavitation is seen in up to 50% of cases, resulting in thickwalled cavities with irregular inner margins (see Fig. 9.36 b) that are susceptible to secondary infection (e.g., with Aspergillus], an air broncho-

Churg-Strauss Syndrome

, a

b

c

d

Fig.9.36 Wegener's Disease, (a) Diffuse confluent acinar opacities due to pulmonary hemorrhage, (b) Chronic changes during resorption of pulmonary

hemorrhage, (c) Thin-walled nodule with cavitation an air-fluid level, (d) Thick-walled cavity.

gram in 25% and pleural effusion also in about 25%. Women frequently show a smooth subglottic tracheal stenosis typically over 3 to 4cm length. CT shows inflammatory reactions around the thickened tracheal ring. Strictures can also be located in the main or lobar bronchi. A pneumothorax or pneumoserothorax are the result of subpleural cavitations and the development of bronchopulmonary fistulae. Diffuse bilateral ground-glass opacifications or consolidations are caused by alveolar hemorrhage (Fig.9.36a). Recurrent hemorrhage results in fibrotic changes. Nodules and hemorrhage are seen in active disease, while fibrotic parenchymal bands and septal thickening indicate an inactive phase. Follow-up studies may show marked resolution of parenchymal changes. The lymphomatoid granulomatosis shows identical CT features to Wegener's disease (it does not affect the kidneys) but is considered a neoplasticlymphoproliferativedisease.

phase characterized by pulmonary or gastrointestinal eosinophilic infiltrates and peripheral eosinophilia, and the vasculitis phase with hemoptysis, purpura erythema urticare, glomerulonephritis and gastrointestinal ulcers, depending on the affected organs.

Churg-Strauss

•

and

CT Morphology

The most frequent HRCT findings are patchy ground-glass opacifications and consolidations predominantly in subpleural locations. Rarely, there are bronchiectases, bronchial wall thickening, centrilobular nodules, and septal thickening, the latter caused by cardiac insufficiency and interstitial edema (Fig. 9.37).

Syndrome

Churg-Strauss syndrome (allergic angiitis or allergic granulomatosis) is defined as a combination of systemic necrotizing vasculitis affecting the small and medium sized vessels in at least two organs (e.g., heart, lung, skin, nervous system, or kidneys), blood eosinophilia, and lung infiltrates. There are three clinically distinctive phases: the prodromal phase characterized by asthma and allergic rhinitis, the eosinophilic

a

b

Fig. 9.37 Churg-Strauss syndrome demonstrates areas of consolidation and ground glass opacification indicative of pulmonary hemorrhage. Thickening of interlobular (a) or intralobular septa (b) may be present.

329

9 Lungs and Tracheobronchial

System

The following diseases are associated with a peripheral eosinophilia: idiopathic pulmonary fibrosis, allergic bronchopulmonary aspergillosis, chronic eosinophilic pneumonia and Churg-Strauss syndrome.

Vascular

Malformation (AVM)

More often congenital than acquired (post-traumatic) , pulmonary vascular malformations are characterized by the abnormal shunting of blood between the pulmonary arteries and veins or, less commonly, between the bronchial and pulmonary arteries or between the bronchial arteries and pulmonary veins. Because there is no capillary network between the arterial and venous vessel, blood is shunted to the left atri urn without being oxygenated. From 40 to 60% of patients with pulmonary arteriovenous malformation (pA VM) suffer from Osler-Weber-Rendu disease with cutaneous and mucosal teleangiectasias and AVM also in other organs (e.g., liver). •

CT Morphology

Arteriovenous malformations appear as rounded opacities that measure several millimeters to several centimeters in diameter and typically have a feeding arterial vessel and a draining venous vessel (Fig. 9.38). Multiple feeding and draining vessels are seen in complex pAVMs. Most pAVMs are solitary (60-70%) and

a

b

Fig. 9.38 Arteriovenous malformation appearing as a group of peripheral nodules (a). The feeding and draining vessel in another AVM (4 x 1 /6) is demonstrated on a MIP display (b).

are located in the central part of the lung. Calcifications have been described. With spiral CT, an pAVM can be identified on unenhanced scans by its vascular anatomy, which is most clearly demonstrated by shaded surface displays and MIP images (Fig. 9.38). Delineation of the angioarchitecture is important for planning of interventional therapy (embolization). A sign, pathognomonic for pAVM, is its intense contrast enhancement synchronous with the arterial vascular opacification. Only rarely does throm bosis lead to a lack of enhancement.

Infectious Diseases

As a rule, pulmonary infections in themselves are not an indication for CT scanning. CT is used mainly to detect complications (cavitation, abscess formation); detect or exclude predisposing diseases (bronchiectasis, tumor); differentiate among infiltration, atelectasis, and effusion; monitor progression and therapeutic response; and localize lesions prior to bronchoscopy or lavage. Clinical features are important for differentiating various infectious agents. Communityacquired pneumonias occurring in previously healthy individuals have another spectrum of infectious agents as compared to nosocomial

pneumonias acquired in the hospital by patients who were already ill (Table 9.25). Immunocompromised patients are susceptible to a greater diversity of infections that also include atypical forms of pneumonia [Pneumocystis ceruiu, cytomegalovirus, mycobacterial infections, and fungal infections; Table 9.26). Experience has shown that chest radiographs are negative in approximately 10 of patients with pulmonary infection. This percentage is substantially higher in immunocompromised patients. Today, the clinical suspicion of pulmonary infection in a patient with a nega%

Infectious

Table 9.25

Common

Community-acqu

infections

in the normal

Diseases

host

ired pneumonia

Bacterial • Streptococcus

lower lobes,

• Staphylococcus

postviral,

• Haemophilus

lower lobes,

consolidation,

children>

rapid spread,

adults,

cavitation,

bronchopneumonia

round

(children)

or sublobar,

effusion

and cavitation

rarely lobar

empyema

(COPD),

rarely

Atypical diffuse

• Mycoplasma

tions,

centrilobular mosaic

nodules,

pattern

tree in bud, patchy

with air trapping,

• Legionella

upper lobes, segmental

• Chlamydia

similar to Mycoplasma

consolidation,

(lobular)

lymphadenopathy rapid spread

ground

glass

or consolida-

in up to 40%

to diffuse

disease

Vims diffuse acinar shadows, pathyor diffuse ground solidations due to bacterial superinfection

• Influenza

in small

• RSV' • Adenovirus • Varicella

zoster

Nosocomial

children,

bronchiolitis

consolidation

like in bacterial

diffuse acinar

opacities,

glass opacification

frequently

con-

with airtrapping infection

ground

glass,

nodular

calcifications

postinfectious

pneumonia

Bacterial • • • •

see above, septic emboli (catheters) lower lobes, consolidation or multiple nodules, upper lobes, expanding consolidation, necrosis lower lobes, consolidation, multiple cavitations

Staphylococcus Pseudomonas Klebsiella Escherichia coli Respiratory

Table

9.26

syncytial

Common

virus

infections

in immunocompromised

patchy

Bacterial Viral infections • Cytomegalovirus • Herpes

(CMV)

simplex

Pneumocystiscarinii

cavitation and cavitation

patients

or lobar consolidations

patchy or diffuse ground glass, consolidation post transplantation (first 4 months), under chemotherapy, HIV«20 c ells/mm") associ ated with mucocutaneous ground glass

consolidation

disease ->

fibrosis.

cysts

HIV Atypical Fungal

mycobacterial infections

acinar

or nodular

patchy opacities,

opacities,

consolidation

consolidations,

halo sign

• Candida • Aspergillus

HIV, under chemotherapy,

post transplantation

• Cryptococcus

tive chest radiograph is considered an indication for CT or HRCT. Given the nonspecific response pattern of the lung. it is only possible to reduce the number of causative organisms based on morphologic criteria. Clinical features (e.g .. CD4

cell count) can help to further reduce the differential diagnosis of etiologic agents (Tables 9.279.29).

331

9

Lungs

Table

and Tracheobronchial

9.27

(with respect After

Common

System

infections

after

bone

marrow

< 30 days

Bacterial Pneumonia

transplantation

Aspiration Gram-negative Aspergillus

Pneumocystis Cytomegalovirus (CMV) Idiopathic pneumonitis Graft-versus-host reaction

> 4 months

Streptococcus, Staphylococcus Varicella Graft-versus-host reaction

solid organ

transplantation

< 30 days

Aspiration Gram-negative bacteria Catheter infections

< 4 months

Aspergillus Nocardia,

Mycobacteria

Pneumocystis Viral infections > 4 months

A lobar pneumonia is characterized by diffuse alveolar wall injury accompanied by hemorrhagic edema. Bronchopneumonia (lobular pneumonia) results from infectious organisms in the epithelium of distal airways causing peribronchiolar exudates. Chronic (carnifying) pneumonia is considered to be present if the infiltrated alveoli do not clear within 15 to 30 days. This condition involves a proliferative fibroblastic inflammation with scarring and fibrous change. Depending on their etiology. bacterial infections are divided into community-acquired and nosocomial infections. The radiographic pattern is quite variable depending on the organisms. One organism may produce different patterns depending on the progression of disease. the patient's immune status, or preexisting or concomitant disease.

bacteria

< 4 months

After

Infectious Diseases in the Normal Host

transplantation

to time of occurrence)

(e.g., CMV)

Pneumocystis Cryptococcus

Table

9.28

Predominant

pattern

of opportunistic

infections

Consolidations

Nodular/patchy

Bacteria Legionelia Mycobacteria Nocardia

Aspergillus Cryptococcus Nocardia Bacterial abscess Septic emboli

Cryptococcus

Table 9.29

Incidence

of HIV-associated

pulmonary

opacities

infections

Diffuse

ground-glass

opacities

Pneumocystis Virus

related

to immune

status

(CD4 cell count)

Bacterial pneumonia Nocardia Typical tuberculosis

any cells/cm3 < 200 cells/cm3 any cells/em'

MAC

< 50 cells/cm3

Histoplasmosis Cryptococcus Aspergillus

< 100 cells/cm3 < 100 cells/cm3 < 50 cells/em'

Viral

Cytomegalovirus

<

Protozoan

Toxoplasmosis

<

100 cells/em'

<

200 cells/em'

Bacterial

Fungal

Pneumocystis

20 cells/em'

Infectious Diseases in the Normal Host

•

-Delayed resolution beyond two months requires exclusion of an underlying systemic disease and evaluation of the immune status it should also initiate the search for complications of infections such as atelectasis, empyema, or cavitation, or for a predisposing local factor, such as an obstructing neoplasm, or bronchiectasis.

CT Morphology

The typical pattern in bacterial pneumonia is focal or patchy areas of consolidation with a positive air bronchogram (Fig. 9.39). Broncho pneumonia features multiple peribronchial air space opacities that frequently involve several lobes. They may become confluent and produce. segmental or lobar air space consolidations. The\ opacities show decreased contrast enhance ment that is very nonhomogeneous in some cases. Central cavitation and lung abscesses may Atypical (Nonviral) Pneumonia develop, and fluid levels-may-be.seen in-cavities that communicate with airways (see Fig.9.46b). Atypical pneumonias refer to infections that do not respond to antimicrobial therapy or which A hypo dense border or "halo" is caused by perifocal hemorrhage (compare Fig. 9.41 d). have different clinical features compared to bacterial infection (fever without chills, nonproA lobar pneumonia (e.g., Streptococcus ductive cough, extrathoracic symptoms such as pneumoniae, Klebsiella, Legionella pneumophila right upper quadrant pain headache, myalgias and Mycoplasma pneumoniae) is characterized by a lobar infiltration that is sharply marginated or diarrhea). Atypical non-viral pneumonias are by the interlobar fissure (Fig.9.39b). A lobar caused by Mycoplasma pneumoniae, Legionella pneumonia may result in expansion of the lobe pneumophila and Chlamydia. due to extensive edema (most typical of Kleb• CT Morphology siella pneumoniae). Chronic (carnifying) pneuMycoplasma pneumonia is characterized by a monia is characterized by scarring and fibrotic change that may involve an entire lobe, loss of bronchiolitis progressing to a bronchopneuvolume, and bronchiectasis (Fig.9.39d). Most monia. Accordingly the dominant finding of pneumonias resolve radiologically within a acute Mycoplasma pneumoniae infections are month, often within 10 to 21 days. centrilobular nodules (tree-in-bud), signs of airway obstruction with air trapping, and mosaic perfusion in addition to ground-glass opacifications and patchy consolidations in a lobular dis-

a

b

Fig. 9.39 Bacterial pneumonia, (a) Incipient patchy bronchopneumonic infiltrates, (b) Upper lobe pneumonia with air bronchogram and opacifications of varying density. Note the volume loss under treatment (arrowheads). (c) Pneumococcal pneumonia (4x1/6,

c

d

2.4mGy, sagittal MPR) with complete consolidation of segment 5 of the middle lobe with a positive CT angiogram sign (arrow), (d) Chronic carnifying pneumonia with a sharply circumscribed area of consolidation, traction bronchiectasis and signs of volume loss.

333

9 Lungs and Tracheobronchial System •

a

b

Fig. 9.40 Atypical pneumonia, (a) Multiple centrilobular nodules with tree-in-bud pattern due to mycoplasma-induced bronchiolitis. (b) Segmental consolidation with positive air bronchogram in a patient with Legionella infection.

tribution (Fig. 9.40). In children, HRCT may show signs of airway obstruction after an interval of 1 to 2 years in those areas that had been infiltrated. In legionnaires' disease the typical pattern consists of segmental consolidation, predominantly in the upper lobes, with rapid spread to the entire lobe and the adjacent lobes. The parenchymal changes are extensive. A pleural effusion is rare. The radiologic findings of Chlamydia pneumonia may be identical to those of mycoplasma pneumonia with a predominance for patchy areas of consolidation in the middle or lower lobes.

CT Morphology

Viral respiratory infections progress in a predictable sequential fashion. After inhalation of the agent, the inflammatory process initially starts in the airways. CT shows patchy areas of heterogeneous parenchymal attenuation (Fig. 9.41 a, b) and a diffuse tree-in-bud sign indicating bronchiolar impaction. Scans in the subacute phase show the accentuation of interlobular septa by lymph vessels thickened due to the absorption of intra-alveolar exudate in addition to the thickened central bronchovascular interstitium and the bronchiolar abnormalities. With further progression, infection also involves the alveolar walls inciting intra-alveolar edema and hemorrhagic inflammation. The results of filling in of peribronchiolar air spaces are patchy consolidations resembling a bronchopneumonia pattern. Peri bronchiolar consolidation may also cause an acinar pattern. Perifocal hemorrhage produces a hypodense border, or halo (Fig. 9.41 c). Influenza pneumonia is most frequently complicated by bacterial pneumonia and may therefore develop more extensive consolidations (Fig. 9.41 c). Radiologic features of adenovirus pneumonia are very similar to bacterial infection (pneumococcal pneumonia). RSV occurs most frequently in children, predominantly involving the bronchioles. Varicella can cause severe pneumonia in adults. It presents with diffuse ill-defined acinar nodules (4 to 6mm). The findings persist over several weeks and may heal with discrete nodular calcifications. Hilar adenopathy is seen in up to 40% of cases.

Viral Pneumonia

Primary respiratory viruses are the parainfluenza and influenza group (most frequent agent in adults), the respiratory syncytial virus (RSV, most frequent in children), the adenovirus, and the picornavirus. The infection begins in the central large airways and only produces parenchymal changes when disease involves the peribronchial or peribronchiolar alveoli. Especially in children it predominantly affects the small airways leading to airway obstruction.

Anthrax (Bacillus anthracis)

Bacillus anthracis is a gram-positive spore-forming organism. After intra-alveolar inhalation the spores are phagocytosed and transported via lymphatics to hilar lymph nodes where they germinate into the vegetative form. The latter pass via efferent lymphatics into the systematic circulation. In addition to cutaneous and gastrointestinal manifestations, the thoracic manifestation represents the most dramatic form and is lethal in the vast majority of untreated patients. Pathologic organ manifesta-

Tuberculosis and Atypical Mycobacterioses

••••

a

b

~ d

c

Fig. 9.41 Viral pneumonia may show various appearances ranging from focal to diffuse ground-glass opacification or dense consolidation, (a) and (b) CMV pneumonia with patchy ill-defined peribronchial (a) or focal

(b) ground-glass opacification. (c) Diffuse ground-glass opacities in CMV infection, (d) Influenza pneumonia with patchy, confluent opacities, air bronchogram, and perifocal halos due to hemorrhage.

tions are thought to be caused by toxin overload duringsepti cemia.

ment of cavitation. As the immune status decreases further, consolidations are more widespread and cavitation is less common. Patients may develop large bihilar lymphadenopathy. Atypical mycobacterial infections occur in late stage AIDS «50 cells/em"). Patients may show the same features as in typical mycobacteriosis, lymphadenopathy with nodular consolidations, or even lymphadenopathy alone.

•

CT Morphology

Hemorrhagic and thus hyperdense enlargement of mediastinal and hilar lymph nodes is the most typical finding. Peribronchial thickening may be due to inflammation or lymphatic obstruction. Bilateral hemorrhagic pleural effusions are frequent. Intrapulmonary ground-glass opacifications or consolidations are due to intraalveolar edema and hemorrhage. Tuberculosis and Atypical Mycobacterioses

CT is more sensitive than conventional radiography for detecting and following cavitation, cavity superinfections (often with Aspergillus'), bronchopleural fistulas, pleural empyema, and mediastinal lymphadenopathy. Infection with HIV or any other condition of immunosuppression carries a risk of development of tuberculosis. While patients with HIV may suffer from tuberculosis at any CD 4 count, appearance of tuberculosis changes with the immune status. In patients with existent immune reactions (CD4>200 cells/em") tuberculosis has the typical reactivation pattern, with areas of consolidation in the apical and posterior segment of the upper lobes or the apical segment of the lower lobes, with the develop-

•

CT Morphology

CT morphology varies according to the host immune status and the course of the disease. 111defined acinar shadows are caused by endobronchial spread of the infection and always signify active disease. The shadows are 2-10 mm in size, show a peribronchial (centrilobular) distribution, and tend to coalesce to form rosettes or patchy opacities (Fig.9.42a). Miliary lesions result from hematogenous dissemination of the disease. They appear as discrete nodules 1-2 mm in diameter typically distributed throughout the lung parenchyma and showing a predominantly perivascular and paraseptal distribution. Patchy areas of consolidation with a positive air bronchogram signify a "caseous pneumonia" in which the bronchi often show irregular (varicose) dilatation. Differentiation from other forms of lobar pneumonia is aided by noting the small satellite lesions that are usually seen in tuberculosis.

335

336

9 Lungs and Tracheobronch'ial System

a

b

c

d

Fig. 9.42 Various appearances of mycobacterial tuberculosis. (a) Miliary disease, (b) Bronchogenic tuberculosis with ill-defined, confluent acinar nodules in a centrilobular (peri bronchiolar) distribution, (c) Peribron-

chovascular infiltrates, multiple acinar nodules bronchogenic spread.

(d) Lobar consolidation with in the other lobes indicating

Tuberculous cavities may be thick- or thinwalled. While there is no correlation between CT morphology and disease activity, the thickness of the cavity walls decreases in response to therapy and in cases that become chronic (Fig.9.4 2a). Enlarged mediastinal lymph nodes typically show central necrosis and intense rim enhancement. Atypical mycobacterioses [Mycobacterium aviumcomplex=MAC, Mycobacteriumfortiiitum, Mycobacterium kansasii, and others) have imaging features that resemble classic tuberculosis, including the presence of acinar, nodular, or miliary opacities and accompanying lymphadenopathy. There is no upper lobe predominance, however, and cavitation is uncommon. Bronchiectasis is also seen. When miliary opacities are found, the differential diagnosis should include pneumoconioses, fungal infections, and hematogenous metastases. Ill-defined acinar shadows are consistent with alveolar cell carcinoma, extrinsic allergic alveolitis, or viral infection (Table 9.22).

Infectious Diseases in Immunocompromised Patients

Bacterial Pneumonia

The most commonly encountered types of infection in the immunocompromised patient are bacterial in origin. Many of the bacteria that infect the immunocompromised patient are also capable of producing illness in the healthy person. In the immunocompromised patient, however, bacterial infections have the propensity to progress more rapidly, the pneumonia is more severe, and the recovery prolonged. Patients with a decreased humoral immunity (multiple myeloma, chronic lymphatic leukemia, lymphoma, graft -versus-host disease, HIV) are at increased risk of developing a pneumococcal pneumonia, an infection resulting from the encapsulated (more virulent) form of Haemophilus influenzae or Mycoplasma pneumoniae. Chronic obstructive lung disease is a major risk factor for developing H. influenzae pneumonia because many of these patients are colonized with these bacteria. Hospitalization results in an increased risk of developing pneumonia with gram-negative organisms [Pseudomonas, Klebsiella, Acinetobacter, Escherichia coli, Serratia, Enterobacter}. Staphylococcus aureus is second to gram-negative bacteria as a cause of nosocomial pneu-

Infectious Diseases in Immunocompromised Patients monta, especially in patients with granulocytopenia and chronic granulomatous disease, Immunosuppression is a predisposing factor in the development of nocardiosis, Solid organ transplantation, especially heart transplantation, is associated with an increased incidence of Nocardia infections, Patients with leukemia, or lymphoma are also at increased risk, In patients with HIV infection, nocardiosis often occurs simultaneously with other infections such as mycobacterial tuberculosis, or My-

cobacterium evium.

Immunocompromised patients are more susceptible to developing an infection with Legionella; other risk factors include chronic pulmonary disease, diabetes, alcohol abuse, smoking, and old age, Infected central venous catheters account for most nosocomial bacteremias [Staphylococ-

cus aureiis, Staphylococcus epidermidis), •

CT Morphology

The typical pattern in bacterial pneumonia is focal or patchy areas of consolidation with a positive air bronchogram, The pneumonia begins as an area of poorly defined opacity, The opacity increases in size to involve a segment or alobe. In infection with Haemophilus. pleural effusion is common (>50%), cavitation is rare and usually occurs in the lobar form, Cram-negative bacteria develop a patchy bronchopneumonic pattern, abscess formation may develop in any of the gram-negative pneumonias but is most frequently encountered with

Pseudomonas. The commonest manifestation of nocardiosis is air space opacities that may involve large areas of the lung. Mass-like densities are also seen. Small diffusely distributed nodules are a less frequent pattern. More than 60% of patients have pleural effusions. Cavitations occur in consolidations and nodules.

Viral

and

Pneumocystis

carinii

Infections

Viral and Pneumocystis caIinii pneumonia (PCP) are common pneumonias in patients with impaired cell-mediated immunity. Because of overlapping imaging features they are often considered simultaneously in the differential diagnosis.

Pneumocystis carinii is the commonest causative agent of pneumonia in HlV-infected patients (60-75% of all patients have at least one episode of PCP). The infection rate has been declining since the introduction of inhalational chemoprophylaxis. Cytomegalovirus (CMY) infections are common in patients during chemotherapy or in patients following organ transplantation within the first four post-transplant months (>50%, especially after renal or liver transplantation). Reactivation CMV infection (in seropositive patients) is more common. Since introduction of prophylactic anti-CMV therapy, the incidence of CMV pneumonitis in bone marrow transplant (BMT) recipients substantially decreased (usually 50th to 60th day after transplantation, more frequent after allogenic than after autologous BMT). If it occurs it is still associated with high mortality. CMV is less common in the AIDS population. CMV pneumonia in AIDS should be considered only in patients with severe immune compromise (<100 cells/em'). especially when extrathoracic CMV infections have been documented. Herpes simplex virus (HSY) pneumonia is rare and usually occurs after clinically evident mucocutaneous disease. Squamous metaplasia in the tracheobronchial tree (after radiation or chemotherapy) predisposes patients to lower respiratory HSV infections. CT Morphology

CT findings in CMV pneumonia include signs of airway disease including bronchial wall thickening, tree-in-bud, and bronchiectasis. In the stage of air space involvement, hazy granular opacities or-less commonly-dense consolidation are noted. Nodules or masses up to several cm in diameter are also reported (in about 60%). In fact, the combination of air space consolidation and interstitial pattern appears to be the commonest finding. Abnormalities tend to be bilateral and symmetric (Fig. 9.41 a). The initial appearance ofa PCP pneumonitis is that of a predominantly alveolar process with patchy ground-glass opacities showing a diffuse bilateral distribution with perihilar accentuation. In patients receiving prophylactic aerosol therapy, the changes show an upper zone predominance (Fig. 9.43). With further progression of the infection, immunocompromised patients develop extensive consolidation with an air

338

9 Lungs and Tracheobronchial System

a

b

Fig. 9.43 Pneumocystis carinii pneumonia, (a) Acute form of Pneumocysfo corini'i pneumonia with diffuse patchy ground-glass opacities, (b) Acute form of Pneumocyshs carinii pneumonia with diffuse ground-

c

d

glass (note dark bronchus sign) and thickened interlobular septa, (c) Fibrosing form of Pneumocystis carinii pneumonia with signs of parenchymal distortion. (d) Cystic changes.

bronchogram while immunocompetent patients Candida and Aspergillus species. The clinical and (receiving treatment) show a granulomatous re- radiologic features are highly variable and often action with interstitial fibrosis and nodular nonspecific. Because many of these organisms changes (Fig. 9.43). Pleural effusion and medi- can colonize the upper airways, sputum cultures astinallymphadenopathy (rarely with calcificaare considered diagnostically unreliable. Defitions) are somewhat unusual. The dynamics of nite diagnosis requires culture of the fungus the ground-glass opacities provide a criterion from infected tissue or demonstration of the orfor monitoring the patient's response to phar- ganism in microscopic examination. macologic therapy. Noninvasive (obstructing) endobronchial AIDS patients receiving prophylactic treat- aspergillosis is caused by massive intraluminal ment with aerosolized pentamidine are the sub- overgrowth of fungus without airway wall invagroup most likely to develop destructive cystic sion'and is commonest in patients with AIDS. lung changes (pneumatoceles or thick-walled Secondary colonlzatjonofa preexisting cavity cysts), which predominantly involve the upper by Aspergilus is also referred to as noninvasive lobes and the apical segments of the lower I aspergillosis.!nvasive aspergillosis denotes peri lobes. Initially, small cysts appear within con- bronchial infection of the parenchyma by transsoli dated areas and later coalesce to form larger bronchial spread of fungal mycelia from invacysts. Pneumothorax is a potential complication sion of the airway walls. It is most commonly' of subpleural cysts. The changes may resolve al- reportedlh patients with AIDS and in allogeneic most completely in response to therapy. Cysts bone marrow and lung transplant recipients, require differentiation from cavitating lesions IChronic necrotizing or semi-invasive aspergillosis caused by septic emboli or fungal infection. typically occurs in patients with chronic obstructive lung disease, sarcoidosis, or underlying malignancy, or in patients treated with corticosteroids. Opportunistic Fungal Infections Pulmonary candidiasis is uncommon and Opportunistic fungal infection is a common occurs only in severely immunosuppressed cause of serious morbidity and mortality in im- patients. It may result from aspiration of ormunocompromised patients (e.g., chemotherganisms of the upper airway, from hematogenous spread from the gastrointestinal tract, or apy- induced neutropenia, AIDS, immunosuppression after organ or bone marrow transplanfrom infected central venous catheters. Diffuse tation). The most important opportunistic fun- endobronchial dissemination of the fungi can gal pathogens include Cryptococcus neoiormans, precipitate the rare "miliary" form of candidiasis.

Infectious Diseases in Immunocompromised

Cryptococcus

neoformans

minimally pathogenic in immunocompetent patients but can cause severe, often disseminated infection in immunocompromised hosts. •

is

CT Morphology

Fungal infections with a propensity for vascular invasion (invasive aspergillosis, mucormycosis) are characterized by nodular opacities and features that resemble pulmonary infarction. The early signs of invasive aspergillosis consist of solitary or multiple pulmonary masses ranging from a few millimeters to several centimeters in size and showing a perifocal halo of ground glass density (Fig. 9.44 a). Although this "halo" sign is not specific (it is also seen in tuberculosis, CMVinfection,herpes, candidiasis, Legionella pneumonia, etc.) ,its presence in immunosuppressed patients should raise suspicion of a-fungal infection If sighs' Of-infection (fever, CRP) persist despite broad-spectrum antibiotic therapy. Mycotic vascular infiltration re'suIting in vascular occlusion leads to wedgeshaped infarcts (Fig. 9.44 c) that show peripheral rim enhancement after contrast administration , (granulation wall) but otherwise remain hy/\dehse7 It Is common to find a nondilated bronchus entering the infiltrated area and terminating abruptly within the infiltrate. Progression of the infection is_.initially characterized by small patchy opacities that resemble bronchopneumonia. Later the opacities coalesce and may form extensive areas of con-

a

b

Fig. 9.44 Pulmonary aspergillosis. (a) Angioinvasive Aspergillus: nodular opacity with a ground glass halo. (b) Airway-invasive Aspergillus: tree-in-bud sign with patchy peribronchiolar opacities, (c) The air crescent

Patients

solidation. Cavitation is a late finding, observed .after two to three weeks. The cavities typically" show an "air crescent sign that indicates a necrotizing process and Heralds an upturn in the patient's imm une status (Fig. 9.44 b.l.These findi ngs require differentiation from other types of superinfected cavities (tuberculosis, Wegener's disease, noninvasive aspergillosis). Lymphadenopathy, effusion, and the infiltration of soft-tissue structures are very rare. Intrapulmonary mycotic aneurysms pose an acutely life-threatening complication due to their potential for rupture after the pneumonia has resolved (Fig. 9.44(1). In obstructing tracheobronchial aspergillosis, CT demonstrates large branching mucoid impactions, usually in the lower lobes. Distal atelectasis or consolidation is common. Chronic necrotizing aspergillosis manifests with .slowly progressive cavitary consolidations_ that usually affect the upper lobes. Intracavitary mycetomas are seen in more than 50% of cases. Pulmonary mu'cormycoses like that ofAspergillus have a propensity for invasion of arteries. It mimics the pathologic and radiologic features of invasive aspergillosis showing consolidations that frequently cavitate; the air crescent sign is seen less commonly. Candidiasis is characterized by scattered patchy or nodular infiltrates (septic fungal emboli) predominantly in the lower lobes that also may have halos of ground-glass attenuation. Pleural effusion occur in 25%, adenopathy and

c

d

sign is virtually pajJToc)rio"rnpnjc;JQrasc)ergill.o.sJs-and signal"ITrcreasincfriost resistance, (d) Wedge-shaped subpleural infiltrate caused by a mycotically induced infarction.

339

9 Lungs and Tracheobronchial System

cavitation are rare. Most patients with pulmonary candidiasis suffer from superimposed infections. Radiologic manifestations of cryptococcosis are solitary or multiple nodules or masses. Diffuse small nodular opacities, cavitation, adenopathy and effusion are more frequent in the immunocompromised patient.

Table 9.30 cavitation

Pulmonary masses that are associated

Abscess

Lung Abscess, Cavitation and Pneumatocele

A lung abscess is a circumscribed intrapulmonary infection that is associated with parenchymal necrosis. If the abscess erodes into the Jxonchial system, the necrotic tissue is replaced .byair, and a cavity develops. The differential diagnosis of pulmonary cavities includes infec-tifius, inflammatory granulomatpus, neoplastic. and post-traumatic etiologies (Table 9.30; Fig. 9.45). A pneumatocele represents solitary or multiple focal air collections most commonly in a subpleural location resulting from alveolar rupture (e.g., Staphylococcus aureus).

cavity

Tumor

Bronchial carcinoma Metastases Hodgkin's disease Kaposi sarcoma

Granuloma

Wegener's disease Rheumatoid nodules Progressive massive fibrosis (silicosis) Sarcoidosis Histiocytosis

Trauma

Contusion or hematoma (secondarily infected) Traumatic pneumatocele

CT Morphology

A lung abscess appears as a mass of central low density within a pneumonic infiltrate or rounded opacity. Contrast administration more .clearly demarcates the hypodense-center from _the enhancing rim. Multiple small cavities or microabscesses are seen in necrotizing pneu-

a

Staphylococcus aureus Klebsiella pneumoniae Tuberculosis Fungal infections (invasive aspergillosis) Aspiration (gram-negative bacteria, anaerobes) Others: nocardiosis, actinomycosis, Pneumocystis carinii

Septic emboli Postinfarction

•

with

c

b

Fig. 9.45 Differential diagnosis of cavitating sions. (a) Infectious: thick-walled tuberculous the upper lobe. (b) Parasitic: air-fluid level and ed membrane in a ruptured hydatid cyst. (c)

monia (e.g., gram-negative bacilli, anaerobic bacteria, Staphylococcus aureus, Streptococci). If the abscess communicates with the bronchial system, the resulting air collection forms a cavitati on (Fig.9 .46a, see also Fig.lO.18c). Extensive necrosis results in vasculitis and thrombosis

lung lecavity in collapsNeopla-

d

stic: liquefying metastases from a teratocarcinoma. (d) Large cystic bronchiectasis mimicking a lung abscess. Note also the small peripheral foci of bronchiectasis, some of which are secretion-filled.

Pulmonary Edema

a

b

c

d

Fig. 9.46 Complications of pneumonia, (a) Sharply demarcated abscess within dense consolidation. (Note the inhomogeneous enhancement of the consolidation and the space occupying effect of the abscess), (b ) Cavitating pneumonia with abscess formation (arrowheads)

and air inclusions (arrows) due to connections between abscess and bronchi, (c) Cavitating pneumonia with air inclusions and pleural effusion, (d) Pneumatocele with airfluid level and signs of endobronchial spread of infection.

causing ischemic necrosis, pulmonary gangrene, and sloughed lung portions (e.g., Klebsiella pneumoniae).

Differentiation of Infectious from Noninfectious Disease

An abscess may have a similar appearance to a necrotic tumor (e.g., squamous cell carcinoma). A cavitation requires differentiation from air-filled (dilafed) bronchi, preexisting emphysema, and a pneumatocele. The cavitation wall is thicker than the pneumatocele wall and has an irregular inner margin.

Diffuse Interstitial

It can be quite difficult in immunocompromised patients to distinguish the pathologies listed in Table 9.18 from infectious pulmonary infiltrates on the basis of CT morphology.

Reticular or Linear Lung Changes

Interstitial reticular and linear lung changes (Tables 9.31, 9.32; Fig. 9.47) produce varying pathomorphologic patterns on CT scans. Foci of interstitial thickening may be uniform or irregular (beaded) depending on whether they are caused by fluid accumulation (edema), cellular infiltration (tumor, inflammation), or connective tissue proliferation (fibrosis). The goal of CT imaging is to detect subtle early changes and to locate, characterize, quantify, and follow diffuse diseases of the lung parenchyma. Individual morphologic findings are frequently nonspecific. It is necessary to analyze distribution patterns and consider mor-

phologic and clinical findings in conjunction in order to draw meaningful differential diagnostic concl usi ons. Pulmonary Edema

Pulmonary edema is not a primary indication for CT but is by far the commonest cause of increased interstitial markings and should always be considered in the differential diagnosis. Pulmonary edema may be due to either of two mechanisms: an elevation of the pulmonary venous pressure or increased permeability of

341

342

Lungs and Tracheobronchial System

9

Table 9.31

Differentia]

diagnosis

of noninfectious

parenchymal

densities

in immunocompromised

patients

Edema • Most common cause of interstitial changes in immunocompromised • Uniformly thickened interlobular septa, ground-glass opacities

patients

Drug reaction during chemotherapy • Bleomycin, • Mosaic-like Rejection

busulphan, methotrexate pattern of reticular and linear densities,

response,

• Patchy or diffuse Hemorrhage

• • • •

opacities,

and patchy consolidations

reaction after transplantation ground-glass

opacities,

opacities,

consolidations,

ground-glass

pleural effusion,

bronchial wall thickening

opacities

lymphomas

2% of all transplantation patients Weeks to years after transplantation Ground-glass opacities or solid, tumor-like infiltrates Often reversible with reduction of immunosuppressive

Table 9.32

opacities,

in myelodysplasticdiseases

• III-defined alveolar Secondary

graft-versus-host

alveolar

ground-glass

Linear and reticular changes

therapy

(see Fig. 9.47)

1 Peribronchovascular thickening Affects axial interstitium (bronchi) a Thickened bronchial walls (cuffing) b Irregular interfaces with lung parenchyma face sign)

(inter-

2 Prominent intralobular interstitium Affects septal interstitium (acinus, alveoli) • Fine reticular

pattern

3 Thickened Affects lobule)

interlobular septa peripheral interstitium

(secondary

• Lines 1-2 cm long, running perpendicular pleura ->, Peripheral arcades -> Coarse reticular pattern -» Impaired lymphatic drainage

£ 4 Parenchyma!

to the

bands

• Fibrotic parenchymal scars • Chain of thickened interlobular

septa

5 Subpleural lines • Lines running parallel to the pleura -> Reversibleatelectasis ->, Irreversible fibrosis -> Impaired lymphatic drainage 6

Fig.9.47 Schematic representation of interstitial reticular and linear lung changes (compare Table 9.32 for numbering) .

Honeycombing

• Multiple cysts (mm to ern) with thickened -> End stage ofi rreversi blefi brosis

walls

Pulmonary Edema

the alveolar capillary membrane. As the edema progresses, the tissue fluid tends to spill from the interstitium into the alveolar airspaces. The changes may be localized as a result of regional venous inflow stasis or the obstruction of lymphatic drainage (e.g., by a perihilar tumor). •

Table 9.33 densities

Peribronchovascular thickening

CT Morphology

An edematous interstitium is manifested by smoothly thickened interlobular septa, thickened bronchial walls ("cuffing"), and thickened bronchovascular interfaces (Fig. 9.48). A fine reticular pattern also may be present at the intralobular level. Subpleural lines result from obstructed lymphatic clearance due to interstitial edema. Mild cases show hazy ill-defined centrilobular opacities caused by increased prominence of the centrilobular artery and thickened perivascular interstitium. Alveolar edema is manifested by focal, patchy or diffuse areas of increased parenchyma! density that can range from ground-glass opacities to air space shadows. Frequently the density of the lung parenchyma is diffusely increased (dark bronchus sign). The subpleural space may be less involved. The vascular diameter is increased and vascular structures may be followed down to the subpleural space. All possible transitions between the two stages exist with various combinations of thickened septal lines and patchy ground-glass. Pleural effusion may be also present.

a

b

Fig. 9.48 Pulmonary edema, (a) Interstitial edema with smoothly thickened interlobular septa and an anteroposterior parenchymal density gradient. (b) Smoothly thickened interlobular septa with alveolar filling-in of exudates (4 x 1 /6, coronal MPR). (c) Alveolar

Differential diagnosis of linear and reticular

Smooth

Interstitial pulmonary edema Carcinomatous lymphangitis Pulmonary fibrosis

Irregular, nodular

Carcinomatous lymphangitis Sarcoidosis Silicosis Pulmonary fibrosis Chronic allergic pneumonitis

Prominent intralobular

interstitium

Common

Pulmonary fibrosis Asbestosis Chronic allergic pneumonitis Alveolar proteinosis Lipoid pneumonia

Rare

Carcinomatous lymphangitis Interstitial pulmonary edema Sarcoidosis, silicosis

Thickened interlobutar septa Smooth

Interstitial pulmonary edema Carcinomatous lymphangitis Alveolar proteinosis Lipoid pneumonia Lymphangioleiomyomatosis

Nodular

Sarcoidosis (active) Carcinomatous lymphangitis Silicosis (early stage) Continue

c

r-.

d

edema with patchy centrilobular opacities and sparing of the peripheral zone. (d) Atypical presentation in concomitant emphysema causing a reticular pattern with ground glass opacification.

343

9

Lungs and Tracheobronchial System

Table 'I I

9.33

ified the term BOOP and recognized two other forms; the acute interstitial pneumonia (AlP) most closely corresponding to the entity described in 1944 by Hamman and Rich and the nonspecific interstitial pneumonia and fibrosis (NSIP or NIPF, also called non-classifiable interstitial pneumonia). While the original classification implied that UlP and DIP have different etiologies and pathogenesis, other investigators have suggested thatthe patterns ofD IP and UIP represent the cellular and fibrotic spectrum of a single disease. This interpretation is supported by reports of patients who showed both patterns simultaneously in a single lung specimen and by reports of patients who showed UlP in a follow-up biopsy specimen obtained at a site originally showing DIP (Table 9.35). According to the ATS/ERS International Consensus Classification from 2000, the diagnosis of idiopathic pulmonary fibrosis (IPF) is limited to patients who have histologic findings ofUIP. According to this classification, idiopathic interstitial pneumonia with fibrosis (UlPIIPF) is separated from all other IlP such as DIP, AlP, LIP, OP (BOOP), and NSIP.

(Continue)

Thickened

interlobular

Irregular

Sarcoidosis (end stage) Silicosis, pneumoconiosis Pulmonary fibrosis Asbestosis Chronic

Parenchymal

septa

allergic

pneumonitis

bands Asbestosis Sarcoidosis Pulmonary fibrosis Silicosis, pneumoconiosis Tuberculosis

Subpleural

lines Asbestosis Pulmonary Pulmonary

fibrosis edema

Honeycombing Common

Pulmonary fibrosis Asbestosis Chronic allergic pneumonitis

Rare

Histiocytosis,

Sarcoidosis Silicosis,

pneumoconiosis

Usual Interstitial Pneumonia (DIP) Idiopathic Interstitial Pneumonia (liP), Collagen Diseases The idiopathic interstitial pneumonias (IlP) are a heterogeneous group of inflammatory and interstitial fibrosing lesions. The original classification by Liebow (1968) described five subtypes (Table 9.34); it was modified by Katzenstein and Myers (1992), who dropped LIP and GIP, modTable 9.34

Classification

Classification Usual

interstitial

Desquamative

of interstitial

by Liebow

interstitial

pneumonias Classification

1958

pneumonia pneumonia

UlP is the commonest type of idiopathic interstitial pneumonia. The heterogeneity of findings is the characteristic histological finding representing the evolution of fibrosis at early and late stages with a combination of old and active lesions, distributed in a patchy predominantly subpleural distribution with normal parenchymainterspersed.

UIP DIP

Idiopathic

Desquamative Respiratory disease Acute

Lymphocytic Giant

interstitial

cell interstitial

Bronchiolitis pneumonia

pneumonia

pneumonia

obliterans

interstitial

by the ARS/ERS

pulmonary

interstitial bronchiolitis

interstitial

Meeting

fibrosis

IPF

pneumonia interstitial

DIP lung

pneumonia

RB-ILD AI P

LIP

Lymphocytic

GIP

Hard metal

BIP

Idiopathic bronchiolitis obliterans organizing pneumonia = Cryptogenic (organizing) pneumonia

BOOP

Nonclassifiable

NSIP

pneumonia

interstitial

2000

pneumonia

lung disease

(nonspecific)

UP excluded

interstitial

COP orOP

Idiopath'ic Interstitial Pneumonia

Table 9.35

Features of idiopathic

interstitial

pneumonia

UIP/1PF

.p1P111:\

Age

40-70

Therapylprognosis Mean survival

(modified

(liP), Collagen Diseases

from Webb, Muller, Naidich

2001)

RB-ILD

AlP

NS1P

>40

>30

>60

around 50

poor

good

good

poor

good

5 years

n.a.

n.a.

1-2 months

n.a.

HRCT Ground-glass

+++

centrilobular

Consolidation

+

+++

++

Reticular lines

+++

++ late

++ late

Honeycombing

+++

+ (late)

+

n.a.

•

+++

diffuse

=

not available

CT Morphology

Typical HRCT features 0 fU I P are fine or irregular intralobular linear opacities (reticular pattern) often associated with traction bronchiectasis and bronchiolectasis, irregular interfaces and honeycombing consisting of thick walled cystic air spaces (2 to 20mm in diameter) predominantly in the basal and subpleural areas. Honeycomb cysts usually enlarge slowly over time. The hallmark of alveolitis is ground-glass opacities, which may be the dominant morphologic finding in patients with an acute exacerbation of the disease (Fig.9.49a). Ground-glass attenuation is common but usually less extensive than the reticular abnormality. Fibrotic change

a

is manifested by thickening of the intralobular interstitium (fine reticular structure), of the interlobular septa (coarse polygonal lines), and of the peribronchovascular interstitium (irregular boundaries, bronchial wall thickening) (Fig.9.49b). Honeycombing, characterized by numerous thick-walled cysts 5-20 mm in diameter and foci of traction bronchiectasis, is a late change that signifies advanced irreversible parenchymal destruction. Typical sites of occurrence of IPF are the peripheral subpleural lung and basal lung zones. Architectural distortion is present reflecting lung fibrosis with lobar volume loss. A patchy distribution is apparent in most cases with areas

b

Fig. 9.49 Various degrees of pulmonary fibrosis. (a) Acute alveolitis (here: AlP) indicated by patchy groundglass opacification even in prone position, (b) Groundglass opacification associated with traction bronchiectasis is indicative of fibrosis rather than active alveolitis.

(c) Initial fibrosis (UIP) with a subpleural band can be differentiated from hydrostatic changes by scanning in supine position, (d) Advanced fibrosis with honeycombing and traction brochiectases.

9

Lungs and Tracheobronchial System

of reticular pattern intermingled with areas of normal lung parenchyma. The predominantly subpleural distribution of the reticular pattern and the honeycombing is evident in 80-95% of patients and represents the most characteristic feature of IPF on HRCT; lack of this feature should suggest an alternative diagnosis. Mediastinal lymph node enlargement is evident in 70-90% of patients (10-15 mm in the short axis diameter, mostly right paratracheal nodes) and less severe in patients under steroid therapy. The significance of ground-glass opacification in patients with IPF has been the focus of considerable interest and controversy. Groundglass opacities, if a predominant feature (occurring in a minority of patients at about 10%) may be completely or partially reversible under steroid therapy while ground-glass opacities as part of a mixed pattern with reticular opacifications were found to precede and predict development of lung fibrosis (Fig.9.49c). Ground-glass attenuation in patients with histologically proven DIP was likely to improve under corticosteroids, while patients with DIP and ground-glass opacities (and those patients usually show a mixed pattern) showed a progress under steroids. The morphologic changes described for idiopathic pulmonary fibrosis are also seen in rheumatoid pulmonary fibrosis, systemic lupus erythematosus, scleroderma (Fig. 9.49 d) and other collagen diseases, and in histiocytosis (see Fig. 9.65 d). They are morphologically indistinguishable on CT scans. Although the rate of progression is slower in patients with collagen vascular disease.

Desquamative Interstitial Pneumonia (DIP)

Desquamative interstitial pneumonia (DIP) and respiratory bronchiolitis-associated interstitial disease (RB-ILD, see p. 299) have recently been grouped together as part of a Single disease spectrum that is strongly associated with cigarette smoking. RB-ILD is thought to represent an exaggerated respiratory bronchiolitis response.

•

CT Morphology

The dominant HRCT features of DIP are bilateral areas of ground-glass attenuation with predominance in the peripheral and lower lung zones. Irregular linear opacities (reticular pattern), which are seen in 50% of patients, are much more limited in extent compared to those in patients with LJIP. Only very mild honeycombing is seen in about 30% of patients. The differential diagnosis from RB-ILD and DIP includes hypersensitivity pneumonitis, sarcoidosis, NSf? and infections such as ?neumocystis calinii pneumonia. Centrilobular nodules represent the key for differential diagnosis since they are very uncommon in DIP and hypersensitivity pneumonitis.

Nonspecific Interstitial Pneumonia (NSIP) Nonspecific interstitial pneumonia (NSIP) resembles IPF but appears to be associated with a Significantly different course and outcome. While IPF is a recurrent and progressing process resulting in a mixture of signs of active inflammation and long-standing chronic fibrous changes, NSIP shows a more uniform histological picture. The parenchyma! changes appear to have occurred over a Single relatively narrow time span. Originally NSIP was not differentiated IPF (approximately 5-15% of patients with IPF turn out to have NSIP). However, clinical symptoms in NSIP are less severe and disease progression is considerably slower and results in a better prognosis than IPF. NSIP is divided into three histological subtypes with varying clinical outcome. Type 1 is predominantely inflammatory, type II is mixed inflammatory and fibrotic, while type III is predominantely fibrotic. CT Morphology

Typical HRCT features of NSIP are patchy areas of ground-glass opacification with intervening areas of unaffected lung, seen in about 80% of patients (Fig. 9.50). They involve mainly the middle and lower lung zones with a predominance in the subpleural areas. They are the sole abnormality in one-third of cases. A reticular pattern superimposed on the areas of ground-

Idiopathic Interstitial Pneumonia (liP), Collagen Diseases

Fig. 9.50 Various degrees of pulmonary fibrosis in NSIP. (a) Subtle subpleural ground glass opacification with cystic changes (arrow) and traction bronchiectases (arrowhead) in scleroderma. (b) Mixed pattern of ground glass, traction bronchiectases, and a faint reticular pattern.

glass opacification is seen in about 50%. Consolidations, seen in about one-third of patients, are generally bilateral, symmetric and subpleural. The differential diagnosis of NSIP depends on the CT patterns it exhibits. The broad variety of disease entities that have to be enconsidered such as UIP, hypersensitivity pneumonitis, or BOOP mirror the variable CT pattern of NSIP.

attenuation correlates with disease duration. Consolidations are also seen but less frequently than ground-glass attenuations. Ground-glass opacification is seen in all phases of AlP and its meaning depends on the stage: areas of increased attenuation without traction bronchiectasis are associated with the exudative or early proliferative phase ofAlP (reflecting the presence of alveolar edema and hyaline membranes), while areas of opacity with traction bronchiectasis are associated with the chronic fibrotic phase. The combination of ground-glass attenuation, air space consolidation, traction bronchiectasis, and architectural distortion is seen in the majority of patients with AlP. The differential diagnosis of AlP depends on the stage and includes hydrostatic edema, hemorrhage, alveolar proteinosis, bronchoalveolar carcinoma, DIP, and diffuse infectious infiltrations. Patients with an accelerated form of IFF also show multifocal areas of peripheral air space consolidations (mimicking BOOP, DIP, or eosinophilic pneumonia) .

Asbestosis

Acute interstitial pneumonia is a fulminant condition of unknown cause that occurs in previously healthy persons and produces histologic findings of organizing diffuse alveolar damage (DAD). The radiologic, clinical, and also histologic features of AlP are similar to ARDS (suggesting the term idiopathic ARDS). The prognosis is very poor.

Asbestos exposure can induce fibrotic changes in the lung. A causal relationship has been established between asbestosis and bronchial carcinoma. Smoking is a highly significant cofactor, as asbestos-exposed smokers have a 50 times higher risk of developing lung cancer than the normal population, whereas asbestosis or smoking alone is associated with a 5- to lO-fold increase in risk. Asbestos-related parenchymal changes can often be appreciated on high-resolution CT scans (HRCT) even when chest radiographs show no apparent parenchymal changes (an important consideration in workers' compensation evaluations). Scanning should be performed in the supine and prone positions.

•

•

Acute Interstitial Pneumonia (AlP)

CT Morphology

Typical HRCT features of AlP are extensive bilateral ground-glass attenuation in diffuse or patchy distribution with focal areas of spared almost normal lung parenchyma resulting in a geographic pattern. The extent of ground-glass

CT Morphology

Asbestos-related parenchymal changes predominantly involve the posterior portions of the lower lobes. Early changes consist of subpleural lines, punctate centrilobular densities located near the pleura (peri bronchiolar fibrosis), and thickening

347

9 Lungs and Tracheobronchial System

of the interlobular septa (Fig.9.49d). More advanced cases present with honeycombing, parenchyma! bands, an irregular interface between the peribronchovascular structures and the lung parenchyma (interface sign), and areas of traction bronchiectasis or round atelectasis. Pleural plaques in asbestosis are described in Chapter 10. It should be noted that the pulmonary findings are nonspecific in themselves and are referable to asbestos exposure only when interpreted in conjunction with pleural plaque formation and a known history of asbestos exposure. Bronchial carcinoma in asbestosis does not show a particular histology or site of predilection. A tumor is considered to be asbestos-related only when typical accompanying pleural changes are identified.

pecially their pattern of distri bution (Table 9.36; Fig. 9.51). The main diseases to be considered in the differential diagnosis of nodular lung changes are miliary tuberculosis, fungal infections, disseminated metastasis, carcinomatous lymphangitis, sarcoidosis, histiocytosis, and silicosis (Table 9.37). Concomitant fibrotic destruction of pulmonary architecture is seen in sarcoidosis, histiocytosis, and silicosis. The nodular lesions of histiocytosis become less prominent than fibrotic and cystic changes as the disease progresses. Pulmonary Lymphangitis Cardnomatosa (PLC)

Carcinomatous lymphangitis (pulmonary lymphangitis carcinomatosa, PLC) results from the permeation of pulmonary lymph vessels by neo-

Diffuse Nodular Lung Changes

Nodular densities in the lung can be characterized by their size, density, margination, and es-

Table 9.36 Distribution pattern of small nodular densities (see Fig. 9.51) 1 Prominence of centrilobular core structures!A!

la--------------~

~

,------------

..1.. .. t..

~1

•

Affects axial interstitium (bronchioles) a Centrilobuar: punctate, linear, or branched -> Prominence of arteriole -> Visualization of bronchiole b Branched pattern with small bulbous terminations ("tree-in-bud" pattern) -> Dilated mucus-filled bronchioles or peri bronchiolar fibrosis 12 Interstitial (perilymphatic)

nodules

• 1-2 mm in diameter • Usually well-circumscribed • Perilymphatic distribution: peribronchovascular, periseptal, and subpleural; also interlobar, interlobular, and centrilobular 3 Disseminated (perivascular) nodules

2

Fig. 9.51 Schematic representation of nodular and reticulonodular lung changes (see Tables 9.36, 9.37).

• Several mm in diameter • Usually well-circumscribed • Disseminated distribution with no sites of predilection (miliary pattern; perivascular, periseptal, and subpleural but not predominantly interstitial)

• • • •

Several mm to 1 cm in diameter Margins usually ill-defined Ground-glass opacity to soft-tissue density Disseminated distribution, but never subpleural or peri pleural

D i If use Nodular Lung Changes Table 9.37

Differential

diagnosis

Prominent centrilobular Prominent interstitium • • • •

of nodular densities

core structures

Carcinomatous lymphangitis Pulmonary edema Pulmonary fibrosis Asbestosis (early form)

Tree-in-bud pattern

I

• Bronchiolitis, smoker's bronchiolitis • Endobronchial tuberculosis, atypical rioses • Cystic fibrosis, bronchiectasis • Bronchopneumonia

mycobacte-

Centrilobular (acinar) nodules Histiocytosis Endobronchial rioses

tuberculosis,

atypical mycobacte-

BOOP Extrinsic allergic alveolitis Smoker's bronchiolitis Asbestosis Pulmonary edema Alveolar cell carcinoma Vasculitis Bronchopneumonia Viral infection, Pneumocystis Bronchiolitis obliterans (rare) Interstitial (perilymphatic) • Lymphangitis (mm) • Silicosis, pneumoconiosis • Sarcoidosis (2-10 mm)

infection

nodules

•

CT Morphology

HRCT features of PLC are smooth or beaded thickening of the central peribronchovascular interstitium and of the interlobular septa producing a characteristic reticular pattern peripheral arcades) and thickened septa outlining distinct pulmonary lobules (polygonal arcades). Thickening of the intralobular axial interstitium results in prominent vascular and bronchiolar structures (centrilobular core structures). Polygonal septal thickening in combination with prominent centrilobular core structures is one of the most distinctive features of PLC (Fig. 9.52). The centrilobular interstitial thickening predominates in only few patients. In approximately 50% of patients the abnormalities appear focal, unilateral, or asymmetric rather than diffuse. Axial, peripheral subpleural and central perihilar bronchovascular thickening may occur together, or one of these features predominates or occurs alone. Associated findings are intrapulmonary nodules, hilar or mediastinallymphadenopathy (38-54%), or pleural effusion. A preservation of normal lung architecture at the lobular level despite the presence of reticulonodular and linear opacities represents an important feature for differential diagnosis.

(1-5 mm)

Diffuse (perivascular) nodules Hematogenous metastases Miliary tuberculosis Fungal infection Silicosis, pneumoconiosis Histiocytosis (rare)

plastic cells. Tumors that spread in this way include carcinomas of the breast, lung, stomach, pancreas, cervix, prostate, thyroid, and metastases of adenocarcinomas of unknown primary site. PLC usually results from metastatic spread to the lung with subsequent interstitial and lymphatic invasion, but can also occur because of direct lymphatic spread of the tumor or secondary to distension of the vascular and lymphatic channels distal to tumor emboli or tumor obstruction.

Fig. 9.52 Carcinomatous lymphangitis, (a) Irregular beaded thickening of the interlobular interstitium (arrowhead) with accentuation of centrilobular core structures (arrow). (b) Thickened interlobular septa with marked thickening of the bronchovascular bundles (arrow) and pleural effusion.

350

9 Lungs and Tracheobronchial System clues for differentiation between PLC and interstitial edema, in that both are

Diagnostic

characterized by perihilar, peribronchovascular, and reticular septal thickening, are the following: • In PLC, the thickened interstitium is sharply marginated from the adjacent aerated lung, there is no filling-in of the alveoli, which remain well aerated. Interlobular thickening due to PLC is more nonuniform as compared to interstitial edema: different degrees of thickening may occur within one septum as well as alterations between smooth or beaded contours. PLC may occur in a focal and asymmetric distribution. The pulmonary arterial branches adjacent to the bronchi also appear larger than normal, that way maintaining the size relationship of the thick-walled bronchi and adjacent vessels. Interstitial edema is most frequently bilateral and symmetric. The interstitial edema is dependently distributed, various degrees of alveolar opacification represent air space edema. The left ventricle and atrium may be enlarged, and pleural effusions are frequently seen. Administration of diuretics will result in a decrease of interstitial edema within hours. clues for differentiation of PLCfrom sarcoid, pneumoconiosis and pulmonary fibrosis, which may all show nodular or beaded

Diagnostic

peribronchovascular

thickening,

are:

• Septal thickening is less extensive in sarcoidosis and pneumoconiosis than in patients with PLC, and reticular opacities are not the predominant feature. In sarcoidosis, pneumoconiosis, and pulmonary fibrosis cicatricial distortion of the lung architecture and secondary lobule anatomy is common, especially when septal thickening is present. In PLC however, lung architecture remains normal and the lobules preserve their size and shape.

Sarcoidosis Sarcoidosis is a systemic, non caseating epithelioid-cell granulomatosis that can affect all organs and is most commonly manifested in the

lung (90%). The granulomas typically show a perilymphatic distribution in the peribronchovascular (perihilar and centrilobular), interlobular, and subpleural interstitium. The pulmonary changes are accompanied in early (stages 1 and 11) by bilateral hilar and mediastinallymphadenopathy. CT Morphology

The CT appearance of pulmonary sarcoidosis varies greatly and is known to masquerade many other diffuse infiltrative lung diseases. The commonest pattern consists of small (2-10 mm): well-defined nodules in a characteristic "perilymphatic distribution" in relation to the subpleural surface, adjacent to the major fissures, along thickened interlobular septa and adjacent to vessels in the lobular core. As a result, pulmonary vessels may be irregularly enlarged. The nodules may be evenly distributed throughout both lungs though predominantly in the upper and middle lung zones; however, in many cases (50%) they are clustered in the perihilar and peribronchovascular region with relative sparing of the lung periphery, or they may be grouped in small areas uni- or bilaterally. A typical finding is irregular polynodular thickening of the bronchovascular bundle and interlobular septa (Fig. 9.53). Pseudoplaques formed by the coalescence of multiple subpleural granulomas are a characteristic feature. Nodular densities measuring between 1 and 4cm in diameter are seen in 15-25% of patients (so called nodular or nummular sarcoidosis). Confluence of granulomas results in large, mostly ill-defined, opacities or consolidations that are often located centrally around bronchovascular structures. There are patchy areas of ground-glass opacities, which may be superimposed over interstitial nodules or signs of fibrosis. They are rarely seen on the radiograph (0.6%) but are commonly present on HRCT (20-60%) mostly in association with small nodules. Pathological studies have shown that they represent interstitial granulomatous inflammation and occasionally microscopic foci of parenchyma! fibrosis. Approximately 20% of patients develop a pulmonary fibrosis with septal thickening, traction bronchiectasis and honeycombing (Fig. 9.53). Irreversible fibrosis is commonest in stage 3 (diffuse pulmonary disease un associated

Diffuse Nodular Lung Changes

a

b

c

d

Fig. 9.53 Sarcoidosis. (a) Very small diffuse nodular densities that partially coalesce to ground-glass opacification. (b) Grouped nodular opacities partially coalescent to patchy consolidations (arrow) with tiny subpleural nodules ("pseudoplaques", arrowheads), (c) Irregular beaded thickening of the central bronchovascular bundle with nodules in interstitial distribution. (d) Perihilar fibrotic mass with parenchymal distortion and interstitial nodules, (e) Coronal 5mm wide MIP demonstrating the apical predominance of nodular densities (4 x 1/6).

with lymph node enlargement). Conglomerate masses mostly in a perihilar location represent areas of fibrosis causing characteristic traction bronchiectasis. The posterior displacement of the upper lobe bronchi (and later also of the main bronchi) is considered to be an early sign oflung distortion in sarcoidosis, indicating loss of volume in the posterior segments of the upperlobes. Differential diagnosis: conditions that most closely mimic the HRCT appearance of sarcoidosis are pulmonary lymphangitis carcinomatosa (PLC), silicosis. and coal worker's pneumoconiosis (CWP). All of these show small perilymphatic nodules. However, differences of the predominant distribution and the combination with signs of fibrosis represent the diagnostic key factors.

In sarcoidosis, the nodules are predominantly located along the central bronchovascular bundle and in the subpleural area; in PLC nodules are mostly located septally and along the central bronchovascular bundle. Septal thickening in sarcoidosis is a much less dominant feature as compared to PLC and if present, it is usually combined with findings offibrosis and lung distortion, precluding the diagnosis of PLC. There are, however, descriptions of cases in which the parenchymal involvement of sarcoidosis and PLC are quite similar and indistinguishable. In silicosis and CWP, nodules are more evenly distributed throughout the whole lung with a predominantly centrilobular and subpleural location, a finding that is much less typical for sarcoidosis. Differen-

351

Lungs and Tracheobronchial System tiation of conglomerate masses of fibrosis in sarcoiclosis from those in stltcosis can be made by the more common presence of air bronchograms in sarcoidosis. As fibrosis develops over time, sarcoidosis and IPF increasingly share a number of findings. Both show irregular or nodular septal thickening, irregular interfaces and traction bronchiectasis. In sarcoidosis, honeycombing is a less frequent feature of irreversible fibrosis as compared to IPF. However, there appear to be two types of fibrotic progression in sarcoidosis. Most patients develop fibrotic changes with loss of volume, fibrotic bands and cysts predominately in the perihilar region and the upper lobes, that way suggesting the diagnosis of sarcoidosis. There are, however, descriptions of a few cases that developed a diffuse fibrosis with predominant basal and subpleural honeycombing very similar in appearance to UlP. HRCT is the method of choice for showing early it is important to know that HRCT cannot be used to rule out involvement of the lung parenchyma. Several authors report that intrapulmonary granulomas could be histologically shown in patients with normal HRCT appearance of the lung parenchyma. A lot has been written about the meaning of ground-glass opacities in patients with sarcoid. One study indicated that areas of ground-glass opacities are associated with disease activity as assessed by 'Ca-scintigraphy. Correlation with the pathologic specimen, however, could prove that ground-glass opacities reflect the presence of confluence of extensive interstitial sarcoid granulomas rather than active alveolitis. Ground-glass, nodular and irregular linear opacities and interlobular septal thickening were described as representing potentially reversible disease, while cystic air spaces and architectural distortion are irreversible findings. The extent of consolidations and nodular densities showed higher correlations with other indicators of disease activity (serum-angiotensin converting enzyme assay, Ga-scanning and bronchoalveolar lavage) than reticular opacities and ground-glass attenuation. Signs of fibrosis (such as lung distortion and reticular densities),

fibrosis in sarcoidosis. However

however, correlate better with impairment of lung function (obstruction) than extent and distribution of nodules and consolidations. Silicosis and Coal Worker's Pneumoconiosis (CWP) The WHO defines pneumoconioses as irreversible lung changes caused by the inhalation of mineral dust and its deposition in the lungs. Silicosis and CWP are pathologically distinct entities with differing histology, resulting from the inhalation of different inorganic dusts (dust containing crystallized silicon dioxide and coal dust, respectively). However, the radiographic and HRCT appearances are similar and cannot be reliably distinguished. The primary imaging modality is plain chest radiography, but HRCT is more sensitive in the detection of subtle changes. Pneumoconiosis may take an uncomplicated nodular course or a complicated course that is associated with progressive massive fibrosis (PMF). The complicated forms present clinically with severe impairment of pulmonary function and frequently with cor pulmonale. Silicoproteinosis occurs after brief, intense exposure to fine silica dust and is characterized by fulminant respiratory impairment. Silicosis can also predispose to the reactivation of tuberculosis, which may take an accelerated, fulminating course (silicotu berculosis). CT Morphology The nodular form is characterized by multiple 1 to 5 mm nodules, usually with sharp margins, that are scattered throughout the lungs (Fig. 9.54) but which show a predilection for the upper and middle zones especially posteriorly. Small nodules may coalesce to form macronodules (8-20 mm in diameter, with subpleural lesions forming pseudoplaques). In contrast to sarcoidosis, the nodules are typically uniformly distributed in centrilobular (peribronchiolar) and subpleural locations and rarely clustered or distributed along central bronchovascular structures. Calcification may occur within the nodules, and hilar lymph nodes may develop typical eggshell-like calcifications. Sometimes the centrilobular densities show tiny short branches instead of being round, corresponding to irregular fibrosis around the respiratory

Diffuse Nodular Lung Changes

haled stimulating an alveolar and subsequently extensive fibrous tissue (silicoproteinosis). olar ground-glass opacifications crete silica nodules, resembling alveolar proteinosis.

a

b

Fig. 9.54 Silicosis. (a) Diffuse rnicronodular densities (type q' in ILO classification), (b) Conglomerate mass (PMF) with parenchymal distortion, perifocal emphysema, flbrosls. reactive pleural thickening, and diffusely scattered nodules.

bronchioles surrounded by a small zone of focal emphysema. As simple silicosis and CWP progress, the number of nodules increase and increasingly coalesce resulting in distortion of the adjacent lung structure. Coalescence of small nodules to ~arger nodules (> 1 em) and eventually to large, Irregularly defined conglomerate masses refl,e~ts t?e transition from simple to complicated silicosis/pneumoconiosis (Fig. 9.54). The masses of progressive massive fibrosis (PMF) tend to develop in the mid portion or periphery of the upper lung zones and migrate towards the hila. They may cavitate due to ischemic necrosis although this seems to be more common with CWP. Calcifications may befound. Regardless of age and smoking habits, most pati,ent,s with silicosis or coal worker's pneumocomOSIS develop focal areas of centrilobular emphysema. Unlike classic centrilobular emphysema, these areas contain central micronodules and their extent correlates with the impairment of pulmonary function. The extent of emphysema correlates more closely with functional imp,airment than does the degree of nodular profusion. Functional impairment and p:~gn~sis are usually poorer for patients with sIlIcoSIS than for CWP. Accompanying paracicatri~ial emp?ys~ma reflect the process of progressive massive fibrosis, An acute form of silicosis has been described in which a large dose of silica was in-

lipoproteinosis production of CT shows alveinstead of disthe image of

The differential diagnosis of silicosis/CWP includes all diseases that may develop numerous small well-defined nodular opacities such as sarc~idosis, pulmonaly lymphangitic carcinomatosis, Langerhans cell histiocytosis but also infectious diseases such as miliary tuberculosis, fungus infection, and liematogenous metastases. • Dependent on the profusion of the nodules in a seemingly random distributi~n, it may be impossible to distinguish miliary tuberculosis (perivascular nodules) from silicosis (peri bronchiolar nodules). Also the differentiation between sarcoidosis and silicosis can be difficult when the sarcoid nodules are numerous a~d ~o n,ot show the typical perilymphatic distribution but a diffuse centrilobular location. The differentiation between silicosis and Langerhans cell histiocytosis is difficult if the latter is characterized solely by t~e presence of nodules while cystic leSIOns are completely missing. • Si,licosis/CWP and PLC can usually be differentiated quite easily by the different distribution of nodules. In silicosis and CW\, the nodules appear bilaterally, symmetnc and more uniformly distributed. Beaded septa or reticular densities are usually absent. • Perihilar mass-like fibrosis or upper lobe consolidations in combination with p~~en~hymal distortion may occur in both slhcoSlS/CWP and end-stage sarcoidosis. Both entities are characterized by an upp~~ lo~e predominance and by potential calcifications of the nodules. Differentiation of conglomerate masses of fibrosis in sarcoidosis from those in silicosis can be ~ade b~ the, presence of air bronchograms In sarcoidosis and by associated findings such as the distribution of nodules. • Conglomerate masses in PMF are occasionally indistinguishable from tuberculous cavities or bronchial carcinoma.

354

9

Lungs and Tracheobronchial

System

Diffuse Lung Changes with Increased Parenchymal Density

Parenchyma! changes that cause increased radiographic density are evaluated according to the degree of the attenuation increase and the shape and extent of the area of increased attenuation (Tables 9.38, 9.39; Fig. 9.55). The areas may be described as nodular, patchy, geographic, or lobular.

Table 9.38 Differential diagnosis enchymal density in the lung Ground-glass

of

increased

3

par-

opacity

Pulmonary edema Pulmonary fibrosis (acute) Sarcoidosis Extrinsic allergic alveolitis Alveolar proteinosis Lipoid pneumonia Pneumocystis or viral pneumonia Eosinophilic pneumonia

BOOP Smoker's bronchiolitis Hemorrhage Vasculitis Radiation pneumonitis Alveolarcell carcinoma Lymphoma Alveolar opacities (airspace shadowing) Infection (bacterial, atypical, fungal) Alveolar cell carcinoma Sarcoidosis Wegener's disease Eosinophilic pneumonia Lymphoma Pulmonary edema ARDS Alveolar proteinosis Radiation pneumonitis

Fig. 9.55 Patterns of increased (see Tables 9.38, 9.39).

Table 9.39

1 Ground-glass

Hemorrhage Contusion masses

Silicosis, pneumoconiosis Sarcoidosis (end stage)

BOOP

Carnifying pneumonia Wegener's disease

density

opacity

2 Alveolar opacities (PMF)

parenchymal

density

Patchy increase in parenchymal density Vascular structures not obscured Air bronchogram may be present (dark bronchus sign) Thickened alveolar walls, intraalveolar material Occurs with interstitial and alveolarprocesses-> active, treatable, potentially reversible process

BOOP

Conglomerate

Increased

parenchymal

• • • •

(airspace shadowing)

Profuse focal or patchy opacity Obliteration of vascular structures Positive air bronchogram Alveolar airspaces filled with cells or fluid

3 Conglomerate

masses

• Bunched, confluent strands • Traction bronchiectasis • Volume loss

of fibrosis

Extrinsic Allergic Alveolitis,

Extrinsic Allergic Alveolitis, Chronic Allergic Pneumonitis

Extrinsic allergic alveolitis (EAA) or hypersensitivity pneumonitis is an allergic response to inhaled organic antigens (farmer's lung, birdbreeder's lung). Known causal antigens include microorganisms (fungi, bacteria, actinomycetes) and animal and plant proteins. The radiographic and pathologic abnormalities are quite similar, independent of the causing antigen The clinical and CT features ofEAA vary with the duration of the exposure. The changes are classified as acute, subacute, or chronic, and combinations of these forms can be found in individuals who are repeatedly exposed. Mostly HRCT is performed in the subacute and chronic stage. Chronic changes are also referred to as chronic allergic pneumonitis. A careful clinical history, typical HRCT findings and concordant serologic findings confirm the diagnosis of EAA thus precluding the need for lung biopsy. In patients with discrepant or atypical HRCT findings diagnosis is mostly based on trans bronchial biopsy and abnormal Tlymphocytes in the bronchoalveolar lavage. •

CT Morphology

The acute form is generally diagnosed clinically, so little information is available on CT morphology.

Chronic Allergic Pneumonitis

In the subacute stage, HRCT typically (in 40-70%) shows small (2-3 mm, maximum 5 mm) centrilo bular acinar densities, usuall y ill- defined and showing low attenuation (Fig.9.56a). The acinar densities may coalesce to form diffuse ground-glass opacities (Fig. 9.56b). All lung zones are affected, but there is a slight middle and lower zone predominance that often spares the subpleural space (unlike the alveolitis in idiopathic pulmonary fibrosis). The changes tend to regress in 7-10 days after exposure to the antigen is discontinued. Bronchial walls may be slightly prominent. Recurrent EAA episodes and the chronic stage of allergic pneumonitis are characterized by signs of irreversible interstitial fibrosis and parenchyma! distortion (irregular reticular opacities, intralobular interstitial and interlobular septal thickening, visible intralobular bronchioles, traction bronchiectasis, and honeycombing) (Fig.9.56c). Frequently there is a mixture of chronic (fibrotic) and subacute (inflammatory) changes in patchy distribution and combined with signs of air trapping. The distribution of the fibrosis may be variable, sometimes predominantly subpleural, in others patchy or peribronchovascular. Honeycombing is usually localized subpleurally. Divergent findings are reported about the lobar predominance of chronic fibrotic changes. While one study reported a lower lobe predomi-

r

Fig.9.56 Extrinsic allergic alveolitis. (a) Diffuse, iii-defined acinar (alveolar) opacities in a centrilobular distribution. (b) The alveolar opacities may coalesce to form ground-glass opacities. Differentiation is required from viral pneumonia, acute Pneumocystis pneumonia, and acute exacerbation of an early form of interstitial fi-

brosis or sarcoidosis. (c) Chronic allergic pneumonitis with an mixed pattern of acute alveolitis (ground-glass opacities) and chronic fibrosis (thickened septa, thickened interstitium, bronchiectasis, subpleural opacities) associated with regional air trapping indicating a process involving the airways.

356

9

Lungs and Tracheobronchial System

nance in about one-third of patients, others described a mid zone predominance or even distribution for the majority of patients. In contrast to idiopathic pulmonary fibrosis, however, there is no subpleural or lower zone predominance. In rare cases chronic EAA may be indistinguishable from a late form of sarcoidosis. • In patients with bilateral nodular, patchy or diffuse ground-glass opacities, the differential diagnosis includes desquamative interstitial pneumonia (DIP) and pulmonary alveolar proteinosis (PAP). DIP, however, is very rare and usually shows a subpleural predominance of the groundglass opacification and is not associated with centrilobular nodules. • Faint acinar nodules, which may coalesce to a diffuse ground-glass opacity, are seen in various disorders including allergic alveolitis, viral infections, and vasculitis. Classic alveolar proteinosis shows a crazy paving appearance and is readily distinguished by bronchoalveolar lavage. The latter is also suited to rule out infectious diseases such as CMV or Pneumocystis cetinii as etiologies for diffuse groundglass opacities. The differential between an alveolar sarcoidosis and subacute EAA requires trans bronchial biopsy. • Patients with chronic EAA and UIP may show identical HRCT findings, and differential diagnosis may be possible only by the clinical history and laboratory findings. Only a predominant location of fibrotic changes in the upper and/or middle lung zones allow for a distinction between these entities.

tings (in children: lymphopenia, thymic aplasia, immunoglobulin deficiency; in adults: lymphoma and leukemia) and infectious diseases (CMV, mycobacteria, Pneumocystis, Histoplasma, Candida species, etc). Pulmonary proteinosis is very rare. In addition to the suggestive HRCT pattern, diagnosis is usually based on characteristic features of BAL fluid. •

CT Morphology

The classic CT finding is a reticulated parenchymal pattern formed by the juxtaposition of normal areas and pathologic areas of increased density (crazy paving). Opacifications range from ground-glass to consolidation, and may be patchy or geographic with sharp demarcation from the surrounding normal parenchyma. While some borders follow anatomic structures of lobar or lobular septa, others are independent of anatomic boundaries. The smoothly thickened inter- and intralobular septa often create a very fine polygonal pattern that is usually seen only in areas with groundglass opacification (Fig. 9.57). Resolution of air space consolidation can be monitored radiographically or by HRCT. The overall extent of disease and the degree of Opacifications correlate well with the impairment of pulmonary function and severity of hypoxemia.

Pulmonary Alveolar Proteinosis (PAP)

Pulmonary alveolar proteinosis is characterized pathologically by filling of the alveoli with a PAS-positive proteinaceous material rich in lipid. It is caused by a dysfunction of pneumocytes that desquamate in the alveolar spaces secondary to a defect of lipid metabolism at the intracellular level. An abnormality in surfactant production, metabolism, or clearance also has been strongly implicated, as well as an association with a variety of immunocompromised set-

a

b

Fig. 9.57 Pulmonary alveolar proteinosis. (a) Acute: ground glass opacification superimposed by smooth intralobular thickening (crazy paving) in a geographic sharply demarcated distribution, (b) Chronic: after treatment, irregularly thickened intralobular septa may persist.

Lipoid Pneumonia

Although the crazy paving appearance is highly suggestive for alveolar proteinosis with a 100% prevalence of this pattern, it is not specific. A recent publication listed 14 airspace and interstitial lung diseases that showed crazy paving with varying but mostly much lower prevalence of crazy paving. The main differential diagnoses include diffuse alveolar damage superimposed on UIP (prevalence of crazy paving in 67%), acute interstitial pneumonia (31 %) and ARDS (21 %). Other diseases with lower prevalence of crazy paving were drug induced pneumonitis (12%), pneumonias (bacterial 6%, tuberculosis 1%, mycoplasma 6%, Pneumocystis cetinii 7%), BOOP (8%), chronic eosinophilic pneumonia (8 %) , radiation pneumonitis ( 4 %) , and cardiogenic pulmonary edema (14 %) . The wide differential diagnosis is understandable considering that any kind of fluid or cellular filling of the air spaces and the interstitial spaces causes crazy paving. In alveolar proteinosis, the crazy paving appearance was found to be due to an accumulation of PAS-positive material in the air spaces adjacent to the interlobular septa rather than to thickening of the septa themselves. Most disease entities with similar HRCT features can be distinguished from alveolar proteinosis by clinical symptoms and associated HRCT findings. It has to be noted that superimposed infection- frequently by Nocardia asteroides-is a common complication of alveolar proteinosis,

and CT is rather limited in differentiating air space consolidation due to infection from underlying disease. Lipoid Pneumonia

Lipoid pneumonia is caused by the aspiration of oily substances (e.g., foods, laxatives, nasal drops) and is commonest in elderly patients with neuromuscular disorders or swallowing difficulties. A large amount of oily material must be aspirated before symptoms develop. It differs from alveolar proteinosis in that the alveoli are filled with a lipid-containing material rather than proteinaceous material. •

CT Morphology

Lipoid pneumonia presents with a homogeneous low attenuation (-35 to -75 HU) consolidation of the affected area (predominately in the middle or lower zones). It may cause alveolar opacities resembling those in alveolar proteinosis. There is also ground-glass opacity with a superimposed reticular interstitial pattern caused by thickened interlobular septa (Fig. 9.58). Fibrosis and inflammation accompany the presence of intra-alveolar lipid material (HU value lower than chest wall musculature but higher than subcutaneous fat). Sometimes cavitation and necrosis are seen. In rare cases, fat -equivalent CT densities are found in the affected bronchi.

Fig.9.58 Lipoid pneumonia. (a) Diffuse changes with dense ground glass opacification and accentuated intralobular septa, (b. c) Focal changes with a tumor-like presentation (b) but fat-attenuation on soft tissue windows (c) in an actor that uses oily nose drops.

a

b

c

357

358

9 Lungs and Tracheobronch'ial System

Eosinophilic Pneumonia

Drug-Induced Lung Disease

Eosinophilic pneumonia is typically associated with blood eosinophilia and responds promptly to steroid therapy (regression within days with complete recovery within months). An acute type (Loffler) is differentiated from the chronic type (disease duration longer than one month). Some 50% of patients have an atopic disposition, 40% have a history of asthma. It may be idiopathic or represent secondary immunologic reaction to drugs, parasites, or other agents.

Pulmonary drug toxicity is increasingly being diagnosed as a cause of acute or chronic lung disease and has to be considered in the differential diagnosis, especially in selected patient groups (e.g., those under chemotherapy or cardiogenic antiarrhythmic therapy). Affected patients typically present with progressive dyspnea, dry cough, and, occasionally, fever. The radiologic findings consist of a broad spectrum of morphology reflecting the underlying histopathologic process. Pulmonary pathology may start to decrease after cessation of drug therapy (e.g., BOOP, eosinophilic pneu-

•

CT Morphology

Dominating features in the acute eosinophilic lung disease (Loffler's syndrome) are groundglass opacities (" 100%) and consolidation (in 92%) in diffuse distribution without a preferred zone (Fig. 9.59). Infiltrates are transient and "migratory". Ill-defined nodules (in 54%), septal thickening (in 70%), and a thickening of the central bronchovascular bundle (in 60%) are other frequent findings. Bronchiectasis and lymph node enlargement are rare. Two-thirds have a pleural effusion. Dominating features in the chronic type are also ground-glass opacities (88%) and consolidations (100%) but now in a predominantly subpleural distribution in the cranial and middle lung areas. Nodules (38%), septal thickening (18 %), thickening of the bronchovascular bundle (38%), and pleural effusions (10%) are rarer than in the acute form.

Table 9.40

Drug-induced lung disease

DAD

• Early: scattered or diffuse ground-glass opacities • Later: fibrosis • Common: bleomycin*, busulphan, carmustine*, cyclophosphamide, melphalan, mitomycin, gold salts Nonspecific interstitial pneumonia (NSIP) • Scattered or diffuse ground-glass opacities, later: basal fibrosis • Common: amiodarone*, chlorambucil, gold salts, methotrexate, nitrofurantoin (chronic) • Less common: carmustine* (Bicnu), cyclophosphamide, bleomycin* Bronchiolitis obliterans with organizing pneumonia (BOOP) • Patchy opacities, tree-in-bud, bronchiectasis • Common: amiodarone, chlorambucil, nitrofurantoin, penicillamine, sulfalazine • Less common: amiodarone*, bleomycin*, cyclophosphamide, gold salts, methotrexate Eosinophitic pneumonia • Reverse edema pattern • Common: anti-inflammatory drugs, nitrofurantoin (acute), para-aminosalicylic acid, penicillamine, sulfalazine Pulmonary hemorrhage • Patchy opacities with air-space nodules, consolidation • Common: amphotericin B, anticoagulants, cyclophosphamide, cytarabine, penicillamine

a

b

Fig. 9.59 Eosinophilic pneumonia with subpleural filtrates of varying density.

in-

• relationship between lung injury and dose and duration of therapy

Emphysema monia). Some patients require the administration of corticosteroids. •

CT Morphology

Pulmonary drug reactions include findings caused by diffuse alveolar damage (DAD), nonspecific interstitial pneumonia (NSZP), obliterative bronchiolitis, bronchiolitis obliterans with organizing pneumonia (BOOP), eosinophilic lung disease, pulmonary hemorrhage, edema, hypertension or veno-occlusive disease (Table 9.40). DAD is divided into an acute exudative phase (first week after injury) and a late reparative or proliferative phase (second week), which is characterized by a varying degree of interstitial fibrosis, architectural distortion, and honeycombing. NSf? in its early phase shows scattered or diffuse ground-glass opacities, followed by findings of fibrosis (traction bronchiectasis, honeycombing) predominantly in the basal lung areas (Fig. 9.60). Patients with BOOP show nodular or patchy consolidations, diffuse tree- in- bud opacities and bronchiectasis.

Fig. 9.60 Drug-induced lung disease, (a) Nonspecific interstitial pneumonitis (NSIP) due to Glivec therapy. (b) Bronchiolitis with organizing pneumonia (BOOP) as a consequence ofAmiodarone therapy.

Eosinophilic pneumonia presents with peripherally located subsegmental consolidations (reverse pulmonary edema pattern), while pulmonary hemorrhage leads to bilateral patchy or diffuse ground-glass opacities or consolidations.

Parenchymal Diseases with Decreased Density

A regional or diffuse decrease in the attenuation of the lung parenchyma can have numerous

potential causes, several of which are distinguished by their typical morphologic features on HRCT (Tables 9.41, 9.42; Fig. 9.61).

Table 9.41 Differential parenchymal density

Emphysema

diagnosis

of

decreased

Emphysema • Chronic obstructive lung disease (COLD), smoker's emphysema • Alpha-1-antitrypsin deficiency • Check-valve mechanism (foreign body, tumor, stenosis) • Scarring or atelectasis -> hyperinflation • Postinfectious emphysema in early childhood (Swyer-James) Airtrapping • Bronchiolitis obliterans (causes see Table 9.8) • Focal: check-valve mechanism Mosaic perfusion • Chronic

pulmonary

embolism

Emphysema is an irreversible dilatation of the peripheral airways distal to the terminal bronchioles accompanied by destruction of the alveolar walls. Emphysema is classified into three types-centrilobular, panlobular, or paraseptal-according to the arrangement and distribution of the emphysematous areas (Fig. 9.62). Clinical impairment of pulmonary function is related to the extent of parenchymal destruction rather than to the type. The pathogenic mechanisms of peripheral airway enlargement may be based on hypoplasia, atrophy, hyperinflation, or destruction. Hypoplasia of the peripheral airways can occur as a sequel to bronchopulmonary diseases in in-

359

9 Lungs and Tracheobronchial System Table 9.42

Decreased parenchyma! density

1 Emphysema a Centrilobular, b panlobular, c paraseptal Air trapping - Hyperlucent areas with air trapping alternating with areas of normal density • Well-defined margins • On expiratory scans, little or no density increase in hyperlucent areas (air trapping) with a normal density increase in the rest of the lung parenchyma

- Hyperlucent areas with narrowed vessels alternating with areas of slightly increased density with expanded vessels • Margins frequently ill-defined • On expiratory scans, uniform density increase in all areas

5-Table 9.43 Cystic structures

in the lung (see Fig. 9.61)

3 Pulmonary cysts -<1 em • <3 mm thick with smooth (epithelialized) • Surrounded by normal lung tissue

4----"

Fig. 9.61 Patterns of decreased parenchymal density and cystic parenchyma! changes (numbering see Tables 9.41-9.43).

fants and children. Alveolar atrophy involves the secondary loss of alveoli in an initially normally developed lung (e.g., in patients with age-related emphysema). Hyperinflation may result from a check-valve mechanism, atelectasis, or scar traction. Emphysema due to destruction of the alveolar walls can occur in the end stage of inflammatory fibrous diseases (sarcoidosis, histiocytosis, etc.) or in patients with congenital alpha-I-antitrypsin deficiency.

4 Pneumatocele • Post-traumatic • Delicate wall

or postinfectious cavity

5 Emphysematous bullae • >1 ern, up to 20cm • Thin border «1 mm), no wall • Always associated with emphysema 6 Honeycombing • Multiple cysts (mm to cm) with thickened walls • End stage of irreversible parenchymal destruction 7 Cavities • Cavity formation within an opacity • Thick wall (mm to cm)

Terminal bronchiole

k---

-

Normal secondary lobule

wall

Respiratory bronchiole

Centrilobular emphysema

Panlobular emphysema

Fig. 9.62 Schematic representation of Centrilobular and panlobular emphy-

Congenital Lobar Emphysema •

A bulla is defined as an emphysematous space larger than 1 cm in diameter. It results from the expansion or confluence of several secondary lobules due to progressive septal destruction. Unlike a pulmonary cyst, the wall of the bulla is not lined with bronchial epitheli um. Paraseptal or subpleural emphysema involves alveoli adjacent to pleural structures and bronchovascular bundles (Fig.9.63a). It is clinically silent, causing no restriction of pulmonary function. Preferred sites are the costophrenic angle, apical lung, and bronchovascular branches. The CT appearance of emphysema can be graded based on a scoring system that determines the area of emphysema relative to the total area of the axial section. HRCT can also be used to detect and classify very early emphysematous changes that are still clinically occult (normal pulmonary function).

CT Morphology

The CT diagnosis of emphysema is based on the detection of well-defined, unwalled areas of decreased density or increased lucency. There is an accompanying reduction in the number and caliber of pulmonary vessels and their branches. With centrilobular (centriacinar) emphysema, the respiratory bronchioles are affected while the lobular periphery remains intact. Small, circumscribed areas of hyperinflation are found scattered within normal parenchymal (Fig. 9.63 a). The changes show an upper zone predilection and are associated with a history of smoking. Decreased vascularity is seen only in advanced stages. With panlobular (panacinar) emphysema, there is additional involvement of the alveolar ducts and alveoli. The emphysematous areas coalesce, normal parenchyma is sparse, and there is a more pronounced decrease in vascularity (Fig. 9.63 b). The previous distribution pattern is reversed, with the lower zones affected more than the apex. A special form of panlobular emphysema, also with basal predominance, occurs in patients with alpha-I-antitrypsin deficiency. In advanced stages of emphysema, the centrilobular and panlobular forms become indistinguishable.

a

b

Fig. 9.63 Forms of emphysema, (a) Centrilobular emphysema with small, circumscribed areas of hyperinflation within an intact lobular periphery (common in smokers). There is accompanying paraseptal emphysema with subpleural bullae. (b) Pan lobular emphysema

Congenital Lobar Emphysema

Lobar emphysema results from the congenital or postinflammatory stenosis of a lobar bronchus. The most commonly affected structures are, in descending order of frequency, the left upper lobe bronchus (50%), the middle lobe bronchus (25%), and the right upper lobe bronchus (20%).

c with destruction of all intralobular alveoli and decreased vascularity (alpha-1-antitrypsin deficiency), (c) Severe emphysematous parenchymal destruction with progressive loss of tissue (4 x 1 /6).

361

9 Lungs and Tracheobronchial

System

CT Morphology

CT Morphology

CT demonstrates creased lucency and mediastinal

a hyperinflated with diminished displacement.

area of invascularity

Swyer-James Syndrome Swyer-James syndrome (MacLeod's syndrome) is not a congenital disorder but results from injury of the immature lung (hypoplastic lung) caused by a persistent bronchiolitis obliterans in children who have recurrent viral infections during the first 8 years of life.

Pulmonary

Diseases with Cystic Changes

Pulmonary diseases that are manifested predominantly by cystic changes include Langerhans cell histiocytosis (eosinophilic granuloma) and lymphangioleiomyomatosis. They require differentiation from cystic changes with a bronchial etiology and from parenchymal diseases leading to solitary or multiple, nondiffuse cystic lung changes (Tables 9.43, 9.44; see Fig.9.G!).

Langerhans Cell Histiocytosis Langerhans cell histiocytosis (also described as eosinophilic granuloma of the lung, Langerhans cell granulomatosis of the lung, or histiocytosis X) accounts for approximately 4 % of all diffuse Table 9.44 Differential structures in the lung Pulmonary

The typical CT finding is a hyperlucent lung with markedly decreased vascularity (= unilaterallucent lung). A bronchial check-valve mechanism and collateral air drift lead to very pronounced unilateral air trapping. It is important to exclude (by C1) an acute central airway obstruction as the cause. Differentiation is required from pulmonary arterial aplasia, chronic pulmonary embolism, lobar emphysema, central obstruction by a tumor or foreign body, toxic inhalational injury, and bronchiolitis obliterans secondary to a graft-versus-host reaction following bone marrow transplantation.

diagnosis

of thin-walled

cystic

cysts

CT Morphology

• Lymphangioieiomyomatosis • Histiocytosis X • Honeycomb pattern (see Table

9.17)

Pneumatocele • Postinfectious after Pneumocystis

pulmonary diseases. It may occur in children as part of a multisystem disorder (Abt-LettererSiwe disease), and the disease in adults shows a peak incidence between 20 and 30 years of age. In its early stages it is characterized by a bronchiolocentric granulomatous reaction with proliferation and infiltration of the bronchiolar wall and the adjacent blood vessels by Langerhans histiocytes and eosinophils. Bronchiolar obliteration causes progressive alveolar wall fibrosis and cyst formation, eventually resulting in increasing fibrosis and parenchymal distortion. The HRCT findings closely mirror this stepwise histological progression. The etiology of histiocytosis is unknown. However, there is a striking relationship to smoking (>90% of patients with histiocytosis are active smokers). Histologic confirmation of the diagnosis is usually required. Thirty percent of patients are found to have coexisting granulomas in the bone andlor liver.

or Staphylococcus

monia • Post-traumatic (laceration) • After successful treatment

pneu-

of metastases

(rare)

CT reveals nodular and cystic lung changes. The nodular lesions are 1-5 mm in diameter (rarely several centimeters), usually have well-defined margins, and show a peribronchiolar centrilobular distribution (Fig.9.G4a). Larger nodules {> 1 Omm) also occur but are less common. Nodules can be few or numerous depending on disease activity. The nodules have irregular contours particularly when they are surrounded by

Langerhans Cell Histiocytosis

a

b

c

d

Fig. 9.64 CT manifestations of Langerhans cell histiocytosis. (a) Diffuse centrilobular nodules and microcystic spaces, (b) Multiple, predominantly small cysts, some confluent, accompanied by isolated subpleural nodules. The intervening parenchyma shows ground-

glass opacification. (c) Coronal MPR (4 x 1/6) demonstrates the upper lobe predominance of large confluent cysts with bizarre shapes, (d) Advanced parenchymal destruction leads to fibrotic changes.

cystic or reticular disease. They may be solid or show small lucent centers, these "cavitations" are thought to represent dilated bronchioles surrounded by granulomas and thickened interstitium. Development of cavitated nodules into cysts has been observed. The cysts are usually smaller than 1Dmm in diameter with walls that range from barely perceptible to being several millimeters thick. The cysts may be round, or have bizarre shapes; they may coalesce and then become larger than 2Dmm (Fig. 9.64b, c). Subpleural cysts predispose to recurrent episodes of pneumothorax. Cysts as well as nodules show an upper lobe predominance (in 57%); the lung bases and the basal pleurocostal angle are relatively spared. The patients are prone to pneumothorax, probably from cyst rupture. There may be rarely patchy or diffuse ground-glass opacification, but it is never a prominent feature. The lung volume is usually increased. With disease progression the presence of nodules decreases while the whole lung parenchyma consists increasingly of thin-walled cysts up to the extent that intact lung parenchyma will be present only in the basal areas (vanishing lung disease). The nodular lesions and cyst-wall thickness may regress in response to therapy.

Most patients show no evidence of fibrosis or septal thickening. Only few show an irregular interface sign or a fine reticular network correlating with intralobular fibrosis, which differs from idiopathic pulmonary fibrosis in the relative sparing of the subpleural lung (Fig. 9.64 d). The differential diagnosis includes lymphangiomyomatosis (LAM), cystic bronchiectasis, idiopathic pulmonary fibrosis (IPF) and lymphocytic interstitial pneumonia (LIP) . The cysts found in LAM may have a similar appearance to those of histiocytosis but are not associated with nodular changes or an upper lobe lung distribution. In addition, the cysts in histiocytosis are less uniform than in LAM. The round shaped cystic air spaces-especially when close to a vessel-may mimic the appearance of cystic bronchiectasis (signet -ring sign), close inspection of serial sections above and below help to quite readily differentiate tubular from spherical structures. Unlike IPF the cysts show no predominant subpleural distribution. The cysts in histiocytosis are usually discrete rather than clustered, and most importantly the lung volumes are usually preserved or even increased rather than decreased as frequently seen with IPF.

363

9 Lungs and Tracheobronchial

System

Both, histiocytosis and LIP, are characterized by centrilobular nodules and cystic air spaces. Distinguishing features of histiocytosis from LIP include the absence of interlobular thickening and lymphadenopathy, both of which were found in 82% and 70%, respectively, in patients with LIP. The combination of pulmonary nodules and cysts is virtually diagnostic for pulmonary histiocytosis X. It was shown that a first choice diagnosis of histiocytosis had a 60% likelihood of being correct when based on the chest radiograph but a 90% likelihood of correctness with HRCT. Although the HRCT findings are quite suggestive, there are no other specific laboratory tests to further underline the diagnosis. Langerhans cells can be identified in specimens obtained by BAL. However, they can also be seen in cases of pulmonary fibrosis of other etiology. A definite diagnosis is only established after examination of lung tissue. While trans bronchial biopsy has a very low yield due to the focal nature of the disease, open or video-assisted thoracoscopic Iung biopsy is recommended. The extent of lung abnormality as determined by CT correlates well with the lung diffusion capacity. Serial HRCT scans could show that regression of the nodules leaves multiple thin walled cysts because of check-valve obstruction by bronchiole-centered granulomas, and these small cysts could only be detected by HRCT and not by radiography.

•

CT Morphology

The CT features are pathognomonic and consist of predominantly small, thin-walled cysts of varying size (2 mm to 5 ern) that are distributed uniformly throughout the lungs (Fig. 9.65). The cysts range from 2 mm to 5 cm in diameter, but rarely can become even larger. Their size tends to increase with disease progression. The cysts are primarily round in shape; only some of them are confluent. In most patients the intervening parenchyma is normal. However, there are also reports of cases showing patchy ground-glass opacities or increased interstitial markings in the pericystic lung parenchyma and even rarely signs oflung distortion. The cysts are distributed diffusely throughout the lungs; no lung zone is spared and upper and lower lobes are involved to a similar degree. The wall thickness of the cysts ranges from rarely perceptible to 4mm in thickness. Very rarely, few nodules may be seen. Chylothorax is present in 60% of cases, and spontaneous pneumothorax develops in 40%. Mediastinal lymphadenopathy is common (50%). Without treatment, the cystic transformation of the lung parenchyma will progress for a period of years, accompanied by a relatively mild fibrotic component. It should be noted that normal-appearing CT lung scans do not exclude LAM.

Pulmonary Lymphangioleiomyomatosis (LAM) and Tuberous Sclerosis

Lymphangiomyomatosis (LAM) and pulmonary involvement in tuberous sclerosis (BournevillePringle disease, in A %) are radiologically and pathologically identical. Pulmonary LAM is a rare disease based on the proliferation of atypical spindle cells in the lung parenchyma and in the walls of lymphatic vessels. Ruptures of pulmonary venules lead to episodes of hemoptysis and pulmonary hemorrhage, obstruction of pulmonary lymphatics by smooth muscles cause chylous pleural effusions. It most commonly affects women of childbearing age. A hormonal cause (estrogen) has been postulated.

a

b

Fig. 9.65 Cystic parenchymal changes, (a) Lymphangioleiomyomatosis (LAM) with complete structural transformation of the lung parenchyma and relatively uniform cysts that are evenly distributed throughout the parenchyma, (b) Lymphoid interstitial pneumonia (LIP) with ground glass pattern and cystic airspaces.

Chronic Thromboembolic Pulmonary Hypertension (CTEPH) 365 The differential diagnosis of diffuse lung diseases with primarily cystic lesions include histiocytosis X, centrilobular emphysema, end-stage interstitial fibrosis and lymphocytic inters titial pneumonia (U P) (Fig, 9,65 b), LAM can be reliably differentiated from histiocytosis by the diffuse lung involvement (in LAM the lung bases or costophrenic suici are not spared), the infrequency of nodules, and the regular shape of the cysts, Additionally LAM is not associated with smoking history, while more than 90% of patients with histiocytosis are smokers, Emphysematous cystic air spaces have no perceptible walls, In LAM, vessels are typically seen at the margins of the cysts rather than at the centers of air spaces as is characteristically seen with emphysema, Differential diagnosis, however, may be difficult especially in early stages of LAM, and in advanced stages of paraseptal emphysema, Complete absence of fibrotic changes or of signs of distortion facilitate the differential from IPF, However, in accordance with reported pathological findings, interstitial changes may be present in LAM more frequently than previously reported, Interstitial changes may be explained by additional factors such as intercurrent relapsing infections, Lymphocytic interstitial pneumonia (LIP, as seen in patients with HIV infection, Sjogren syndrome or Castleman's disease) also shows

Acquired

Disorders

of Pulmonary

cystic air spaces that have perceptible walls, The cysts, however, are less uniform in size and reportedly in a predominant subpleural location, They may also be scattered throughout the whole lung but rarely represent the dominant feature, Other findings usually seen in UP include ill-defined centrilobular nodules (3 mm and 30 mm with most being small) and diffuse ground-glass opacification of the lung parenchyma, They represent the key features for differential diagnosis: the virtual absence of centrilobular nodules in LAM allows for ready distinction, HRCT is significantly superior to chest radiography and conventional CT in determining the extent and distribution of air cysts, It should, however, be noted that normal CT findings do not rule out parenchyma! disease in patients with LAM, TheCT appearance of LAM is diagnostic in the correct clinical context The presence of many thin-walled, round orgeographiccystic air spaces scattered throughout both lungs in a young woman isvirtually pathognomonic, Definitediagnosts, however, would require open lung biopsy, CT correlates better than radiography with clinical and functional impairment The extent of cystic disease was found to correlate well with impairment of gas exchange and severity of airways obstruction,

Vasculature

Chronic Thromboembolic Pulmonary Hypertension (CTEPH) Pulmonary embolism is a common finding in hospitalized patients, but more than 80% ofpulmonaryemboli produce no clinical symptoms, If adequate recanalization does not occur, a chronic stage with pulmonary arterial hypertension will supervene, CT can demonstrate mural thrombi in the central pulmonary vessels, so it can aid in selecting patients for pulmonary thromboendarterectomy (see Chapter 23), The CTA technique should be used for this type of investiga-

tton, although signs of mosaic perfusion can be appreciated on unenhanced CT and HRCT images, CT Morphology In more than 70% of patients with chronic recurring pulmonary emboli, CTshows a patchwork of sharply demarcated areas of increased and decreased parenchyma! density caused by regional variations in perfusion ("mosaic pattern" or "mosaic perfusion") (Fig, 9,66), Vascular calibers appear increased in the higher-density areas and decreased in the lower-density areas: this distinguishes the mosaic pattern from ground-

9 Lungs and Tracheobronchial System

b

a

Fig. 9.66 Chronic pulmonary embolism, (a) CT A of the pulmonary arterial trunk in chronic embolism shows thrombi adherent to the vessel walls (arrowhead) and demonstrates a recent embolus in the left lower lobar artery (arrow), (b) "Mosaic perfusion" pattern in chronic

c pulmonary embolism with areas of decreased density (narrow vessels) and areas of compensatory hyperperfusion (dilated vessels), (c) Coronal MPR (4 x 1/6) demonstrating mosaic perfusion and peripheral vascular rarification.

glass opacities. Mosaic patterns due to air trapping are accentuated on expiratory scans. Accompanying pulmonary infarcts are most commonly found in the basal zones. The signs of pulmonary hypertension (main trunk of pulmonary artery> 29 mm in diameter with rapid tapering of distal vessels) are accompanied by wall irregularities, hypodense deposits, or nonenhancement of smaller vessels. Pulmonary Veno-occlusive Disease

Pulmonary veno-occlusive disease (PVOD) is a rare abnormality characterized by the combination of pulmonary arterial hypertension and postcapillary hypertension, chiefly pulmonary edema. There is evidence of repeated pulmonary venous thrombosis. The etiology is unknown. Associations with viral infections, bone marrow transplantation, some medications, the use of oral contraceptives, and radiation therapy to the thorax have been described. •

CT Morphology

The combination of dilated pulmonary arteries, signs of increased right ventricle load, and pulmonary edema is virtually diagnostic. CT shows interlobular thickening and areas of groundglass attenuation consistent with interstitial edema (Fig. 9.67).

Fig. 9.67 Veno-occlusive disease with faint alveolar edema and bronchial wall thickening.

Hepatopulmonary

Syndrome

Patients with liver cirrhosis may develop hypoxemia without intrinsic pulmonary disorder. The fact that liver transplantation is able to reverse the signs underline the functional etiology.

Chest Trauma •

CT Morphology

Arising from decreased vascular tone there is dilatation especially of the small pulmonary vessels and capillaries. With increasing severity of cirrhosis there is increasing occurrence of small arteriovenoUs shunts that are best appreciated in the subpleural area (Fig. 9.68).

a

b

Fig. 9.68 Hepatopulmonary syndrome with increased subpleural vascularity (a) and small arteriovenous shunts are best appreciated on a thin slab MIP (b).

Trauma and Postoperative Changes Chest Trauma

Chest injuries are responsible for s:25% of all trauma-related deaths. A blunt trauma (with an intact chest wall) is commoner that a penetrating trauma and is-most frequently related to deceleration forces due to a motor vehicle accident. In most cases, the plain chest radiograph is the initial investigation and CT plays a secondary role, mostly to clarify an abnormality suspected on the plain radiograph. Indications for a CT examination in chest trauma are relatively limited (Table 9.45). An acute indication for CT exists in patients with suspected injury to mediastinal structures, particularly the great vessels. Bleeding from arteries or brachiocephalic veins (unenhanced CT) and aortic injuries (see Chapter 24) can be confidently diagnosed with optimal spiral CT, which has replaced preoperative angiography. Other life-threatening injuries include a tracheal rupture, tension pneumothorax, hemothorax, and cardiac tamponade. Other indications for CT in chest trauma are the differentiation of effusion, hemorrhage, atelectasis, and contusion; localizing an opacity to a specific anatomic structure (lung vs. pleura); and defining the extent of an indeterminate pulmonary density on the chest radiograph. A pulmonary contusion leads to intra-

Table 9.45 Recommended chest trauma

diagnostic

procedures

Type of injury/location of injury

Diagnostic procedure of choice

Chest wall/skeleton

Chest radiograph

Hematothorax/ pneumothorax

Chest radiograph/ultrasound

Pulmonary contusion

Chest radiograph, followed by CT

Pulmonary laceration

Chest radiograph, followed by CT

Mediastinal

CT angiography

hematoma

Aortic injury

CT angiography

Cardiac injury

Echocardiography

Bronchial rupture

Bronchoscopy

Diaphragmatic

Combination of procedures in most cases

Indeterminate

rupture opacity

in

CT

parenchymal hemorrhage and its presence alone does not indicate CT. A more severe trauma causes disruption of the parenchyma resulting in laceration. The radiological diagnosis of diaphragmatic rupture is problematic, and often delayed. Diaphragmatic rupture is commoner on the left side than the right, and only a few cases are associated with the acute herniation of abdominal contents into the chest.

361

9 Lungs and Tracheobronchial System •

CT Morphology

Pulmonary contusion is manifested by focal, confl uent alveolar shadowing that may progress to an area of consolidation. Free intraparenchymal air is suggestive of pulmonary laceration. A pneumatocele is a rounded intraparenchymal air collection caused by air leakage and lung retraction following a laceration (Fig. 9.69). Pneumatoceles are usually small (less than 5mm) but may also range from 2 to 5 cm in diameter, rarely larger (up to 14cm) dependent on the size of laceration and can be demonstrated several hours after the injury. They do not have well-defined walls, and they may be filled with fluid (e.g., blood). Pneumatoceles and hematomas generally resolve without treatment. Hematomas may persist for some months and may produce a solitary nodule or mass that should not be mistaken for a malignant tumor. A tracheal or bronchial rupture is mostly associated with an injury to the upper bony thorax (e.g., the first three ribs). The tear may be partial or complete. Bronchial rupture occurs within 2.5 cm of the carina in 80% of patients. A right -sided rupture is more common than left sided. Hallmarks of a tracheobronchial rupture are a (frequently large) persistent pneumothorax, a persistent pneumomediastinum, an extrabronchial air collection at the rupture site,

a

b

Fig. 9.69 Chest trauma, (a) Trauma-induced laceration of the lung parenchyma (absence of pleura, arrow) with a broad pneumothorax. There is extensive chestwall emphysema but no mediastinal emphysema. (b) Cystic pneumatocele secondary to a pulmonary laceration. Areas of intralesional hemorrhage appear as

peripheral atelectasis that persists despite treatment, or sagging of the lung away from the hilum to the inferior and lateral hemithorax ("fallen lung" sign with complete disruption). Torsion of the lung is extremely rare; it occurs almost exclusively in children, and represents a surgical emergency. Torsion of the whole lung or of only a lobe usually results in pulmonary infarction. It is manifested by anatomic malposition of the hilar structures, the bronchial tree and shows pulmonary opacities due to atelectasis or infarction. Multiplanar reformats in the coronal or sagittal plane are most hel pful for diagnosis of a diaphragmatic rupture. Gas-containing bowel structures within the hemithorax, displacement of the nasogastric tube, elevation of the hemidiaphragm, and a pleural effusion are suggesting or associated findings. Adult Respiratory Distress Syndrome

Adult respiratory distress syndrome (ARDS) is a generalized, but heterogeneous, response to pulmonary injury. A great variety of mediators can precipitate ARDS, including sepsis, shock, trauma, pneumonia, aspiration, transfusion, and micro emboli. Often more than one factor is present. The pathomorphologic changes initially

c consolidations and ground-glass opacities, (c) Extensive pulmonary contusion with consolidation of the lower lobe. The intra-alveolar blood shows less contrast enhancement than atelectasis, with clear delineation of the vessels (CTangiogram). The peripheral air inclusions suggest the presence of a laceration.

Adult Respiratory Distress Syndrome

consist of a capillary permeability defect producing a noncardiogenic interstitial and alveolar edema, secondary leading to fibrosis and vascular obstruction resulting in ischemia and necrosis. Initial radiological manifestations may not appear until 12 hours after the onset of clinical manifestations (Table 9.46). Radiographically, three phases are differentiated: the exudative, proliferative, andfibroticphase. •

CT Morphology

Stage 1 ofARDS

(first 24 hours) is dominated by signs of interstitial edema (ground-glass opacity, prominent interstitial septa) and pulmonary hypertension (dilated central pulmonary arteries, enlarged right heart). Scanning in stage 2 (2 ro 7 days) shows widespread, patchy or diffusely confluent consolidations with a lower zone predominance and preservation of normal parenchyma! subdivisions. The incidence of pneumonic infiltration is around 70% in this stage with an at least doubled mortality risk. The diagnosis of pneumonia is equally difficult with radiography and CT. Bronchiectasis within areas of ground-glass opacification are regarded as early sign offibrosis. Signs of pulmonary interstitial emphysema (PIE) due to advanced alveolar wall destruction can be seen in prolonged cases of ARDS that have received continuous positive-pressure ventilation. CT reveals air collections around the Table 9.46

Stages in the evolution of ARDS

Stage 1 (early exudative stage)

First

-24 hours • Vasodilatation • Interstitial and alveolar edema (alveolar filling) • Capillary stasis with thrombosis leads to intersti-

tial thickening Stage 2 (intermediate stage) • Alveolar

shadowing

proliferative

2-7 days

due to hemorrhage

and

fibrin deposition

• Early (day 2 to 4) consolidation with air bronchogram • Late (day 4 to 7) patchy consolidation Stage 3 (chronic fibrotic stage) • Coarse reticular distortion • Pneumothorax

pattern, is frequent

interstitial

>7 fibrosis,

days lung

pulmonary veins and lymphatics within the interstitium earlier than chest films, air cysts up to 5 mm in diameter can be seen in a subpleural or perihilar location. When arranged in peribronchovascular chains, these cysts form "interstitial air tracks' that predispose to pneumothorax (32%), formation of bullae (30%) or pneumomediastinum (13%), and imply a poor prognosis. Stage 3 (after 7 days] is dominated by a coarse reticular pattern, signs of interstitial fibrosis, and signs of parenchyma! distortion. Pneumothorax and mediastinal emphysema are common associated findings. Long-term, sequelae are fibrotic changes predominantly in the ventral lung areas suggesting that parenchyma! consolidation has a protective effect against mechanical injury through ventilation. CT is superior to radiography in demonstrating the inhomogeneous distribution of atelectatic lung areas. The goal of ventilation with positive end-expiratory pressure (PEEP) is the recruitment of atelectatic alveolar air spaces in the dorsobasal lung areas. It was found that subtotal atelectases with reduced but still existing ventilation (representing ground-glass opacification in C1) are more readily recruited by PEEP as opposed to areas with complete atelectasis and non existing ventilation (consolidation on C1). CT studies in patients with ARDS under zero and positive end-expiratory pressure (ZEEP and PEEP) showed that alveolar recruitment takes place with increasing PEEP along a craniocaudal and ventrodorsal axis. The dorsal and basal areas are recruited last at the expense of overinflation of the ventral and cranial lungs (which increases the risk of damage and subsequent fibrosis). CT was suggested as visual guide for individually optimizing the mechanical ventilation parameters with respect to ventilation pressure and tidal volume, in addition to respiratory function parameters (Fig. 9.70).

369

9 Lungs and Tracheobronchial

System

Fig. 9.70 Severe ARDS with consolidation and ground glass opacification. There is alveolar recruitment as positive end expiratory pressure (PEEP) is increased from 15 mm Hg (a) to 25 mmHg (b). Note the pleural effusion and ventrodorsal density gradient (low-dose scan in a very obese patient).

Pneumonectomy

and Lobectomy

CT is the modality of choice for evaluating the post -pneumonectomy chest. CT is useful for detecting tumor recurrence and diagnosing complications (e.g., empyema). MRI has not shown any definite advantages for these applications. •

CT Morphology

CT shows the anatomic distortion caused by resective surgery, marked particularly by the rotation and ipsilateral displacement of mediastinal structures. These changes may be sufficiently pronounced, especially after a right-sided pneumonectomy, to cause angulation or compression

of the distal trachea or left main bronchus. The formation of a fluid collection that may persist for years after the resection is more common than complete obliteration of the empty thoracic cavity (60% vs. 40%). Recurrent tumor usually presents as a perihilar or paramediastinal mass of soft-tissue density. If an empyema develops, CT shows the lesion as a hyperdense fluid collection (>20HU) surrounded by a thickened, enhancing rim. A lobectomy results in distortion of the central tracheobronchial system with compensatory expansion and displacement of the remaining pulmonary lobes. After an upper lobectomy, for example, the apical segment of the lower lobe (segment 6) shifts upward to occupy the apical zone. After a lower lobectomy, the middle lobe or lingula (anteriorly) and the posterobasal segment of the upper lobe (posteriorly) occupy the vacated basal zone. Bronchoplastic techniques are used in central bronchial carcinomas that affect only one pulmonary lobe but extend locally beyond the lobar bronchus into the main bronchus. Such tumors may still be resected by lobectomy if the infiltrated part of the main bronchus is resected either as circular cuff or as wedge-shaped patch and the remaining bronchial surfaces are anastomosed directly (bronchial sleeve resection). Short-term complications are bronchial dehiscence, long-term complications are tumor recurrence and fi brotic stenosis (Fig. 9.71, see also 9.72 c).

Fig. 9.71 Coronal thin MPR (a) and 10 mm thick MPR (b) in a patient after upper lobe lobectomy and bronchial sleeve resection (4 x 1 /6). High grade anastomotic stenosis with prestenotic pouch-like bronchial dilatation and severe poststenotic fibrotic changes due to recurrent infection.

a

b

Lung Transplantation Lung Transplantation

Pulmonary densities seen in the postoperative period may signify a reimplantation response, an infection, or allograft rejection (Table 9.47).

Reimplantation Response The reimplantation response occurs in 40% of lung transplant recipients and is manifested within 8-12 hours after the transplantation. It is an edematous response caused by increased capillary permeability due to ischemia and impaired lymph clearance due to the disruption of lymphatic pathways. •

CT Morphology

The interstitial and alveolar edema are manifested on CT by accentuation or thickening of bronchial walls and interlobular septa, increased reticular markings, and ground-glass opacities. The changes usually resolve in 1 to 2 weeks. In protracted cases where adequate resolution does not occur, the changes may progress to "mntaldysfuncnon" (transplantfailure).

Acute Rejection Almost all patients experience at least one episode of acute rejection during the first three weeks, and many have from one to six episodes during the first three months. Acute rejection is manifested clinically by hypoxia and histologically by peribronchial and perivascular monocytic infiltrates. •

CT Morphology

HRCT has proven to be a more sensitive but less specific indicator of pulmonary infiltrates in the

Table 9.47

Chronic Rejection The incidence of chronic rejection is approximately 10% during the first year after lung transplantation and 20% during the second year. Chronic rejection is manifested clinically by obstructive ventilatory impairment and histologically by intraluminal granulations in the small bronchi with involvement of the alveolar airspaces and interstitium (obliterative bronchiolitis, see also p. 300). CT Morphology

Sensitive but nonspecific HRCT findings are decreased vascularity, bronchiectasis, nonseptal linear densities with a basal predilection, and a mosaic pattern (Fig.9.72b). Regions of air trapping are seen on expiratory scans. Additional signs are cylindrical bronchiectasis and a progressive increase in pulmonary opacity. Advanced cases show increasing signs of pulmonary fibrosis, which in rare cases may reach the proportions of progressive massive fibrosis (PMF).

Airway Complications Tracheal combined

anastomoses heart-lung

are performed transplantations.

CT manifestations of lung allograft rejection

Acute rejection • • • • •

setting of a rejection response. Findings consist of thickened bronchial walls and interlobular septa, diffuse air space shadowing (typically but not always in a symmetrical perihilar distribution) , and occasional ground-glass opacities and pleural effusions (Fig.9.72a). Because the morphologic changes are nonspecific, CT is not helpful in differentiating acute rejection from infection (especially CMY or Pneumocystis carinii).

Thickened bronchialwalls Thickened interlobularsepta Diffuse airspace shadowing (symmetric,perihilar) Ground-glass opacities Pleural effusion

Chronic rejection Thickened bronchialwalls Thickened interlobularsepta Cylindricalbronchiectasis Decreasedvascularity Pleural and interstitialfibrosis Focal peribronchialopacities(BOOP)

only in Single

371

372

9 Lungs and Tracheobronchial System

a

b

Fig. 9.72 Complications after lung transplantation. (a) Acute rejection: with diffuse alveolar damage (DAD): ground-glass opacities, prominent intralobular septa and interlobar effusion, (b) Chronic rejection: tubular bronchiectases and mosaic pattern with air trapping due to bronchiolitis obliterans. (c) Chronic re-

and double lung transplantations employ anastomoses in the area of the distal main bronchi near the origin of the upper lobe bronchus. Bronchial dehiscence is a frequent complication during the early postoperative days. It is caused by ischemia, since the bronchial segments bordering the anastomosis are supplied only by retrograde flow through the bronchial arteries and by diffusion. As healing occurs, sites of high-grade fibrotic stenosis may develop in the central tracheobronchial system. The diagnostic modality of choice is spiral CT using a high-resolution technique (1-2 mm). Spiral CT is superior to bronchoscopy for detection and localization.

d

c

jection: bronchiolitis obliterans with subpleural fibrosis. (d) Bronchial anastomotic dehiscences: (1) broad bronchial wall defect at the inferior border of the main bronchus (arrow) and (2) small fingerlike protrusions (arrowhead).

•

CT Morphology

Two forms of anastomotic dehiscence are recognized on CT. One is manifested by small peribronchial air collections with small "punctate" bronchial wall defects that resemble pseudodiverticula. This type heals without sequelae. The second form is manifested by shallow, pouch-like protrusions with broad bronchial wall defects that occur chiefly in the inferomedial aspect of the right main bronchus and extend inferiorly in continuity with the main bronchus, medial to the bronchus intermedius (Fig. 9.72 c). This type predisposes to bronchial scarring and stenosis.

10 Mediastinum, Pleura, and Chest Wall C. Schaefer-Prokop

Anatomy Examination Technique Enhancement Characteristics Differential Diagnosis of Mediastinal Masses Masses Predominantly Involving the Anterior Mediastinum Thymic Lesions: Hyperplasia, Thymoma, Thymic Carcinoma Extragonadal Germ Cell Tumors (p. 382) Goiter (p. 383) Parathyroid Adenoma (p. 383) Hodgkin's Disease (p. 383) Non-Hodqkin's Lymphoma (p. 384) Lymphangioma (p. 385) Mesenchymal Tumors (p. 385)

(p. 380)

Masses Predominantly Involving the Middle Mediastinum Lymphoma (p. 386) Tumors of the Tracheobronchial System (p. 386) Bronchogenic Cyst, Pericardial Cyst (p. 386) Paraganglioma (p. 386) Masses Predominantly

Involving the Posterior Mediastinum

Neurogenic Tumors (p. 387) Extramedullary Hematopoiesis (p. 388) Esophageal Tumors (p. 389) Esophageal Diverticula and Duplication Cysts (p. 389) Esophageal Varices, Azygos Continuation (p. 389) Meningoceles and Neuroenteric Cysts (p. 389) Lymph Node Enlargement Pancoast Tumor Tumors Involving the Chest Wall

Pleural Tumors

.

394

Benign Pleural Tumors (p. 394) Pleural Metastases (p. 394) Malignant Mesothelioma (p. 395) Pleural Lymphoma (p. 395) Superior Vena Cava Syndrome (p. 396) Diffuse Mediastinal Diseases

.,

397

Acute Mediastinitis (p. 397) Fibrosing Mediastinitis (p. 397) Mediastinal Lipomatosis (p. 398) Diffuse Pleural Diseases

.

398

Pleural Effusion, Hemothorax, Chylothorax (p. 398) Pleural Empyema (p. 399) ,Pleural Fibrosis, Asbestos-Associated Changes (p. 400) Trauma and Postoperative Findings

401

Pneumothorax (p. 401) Mediastinal Hematoma (p.402) Mediastinal Emphysema (p. 402) Diaphragmatic Rupture (p. 403) Cardiac Surgery (p. 404) Heart Transplantation (p. 404)

Computed tomography is the standard imaging technique for the examination of the mediastinum. It is most commonly used for the evaluation of suspected masses and lymph node staging in chest tumors (Table W.I). The pleura and chest wall are generally examined as part of a chest C'I, CT used to be inferior to MRI with respect to the evaluation of the relationship of abnormalities (e.g .. tumor infiltration) to the various mediastinal structures, the chest wall, or the pleura. This was due to the excellent soft-tissue contrast and the availability of multiplanar im-

aging with MRI. With the advent of spiral CT and, more recently, multislice CT, this has changed. Multiplanar imaging is now also available with CT, and the spatial resolution of multislice CT is often superior to MRI. In addition, calcifications and associated findings in the lung parenchyma can be better-detected with CT, With proper timing of the injection of intravenous contrast agents, the contrast of the various anatomical structures is usually sufficient with CT, leaving only a few cases to MR where the clinical question cannot be solved with CT.

Anatomy

Table 10.1

Indications

for CT scanning

of the mediastinum

Tumor diagnosis Suspected tumor (on chest radiograph) Tumor characterization T staging N

staqnq

Detection

or exclusion

of a mediastinal

Differential diagnosis Bronchial carcinoma,

primary

Bronchial

lymphoma,

carcinoma,

mass

mediastinal

tumors

testiculartumors

Diffuse disease Mediastinitis Superior vena cava syndrome Vascular disease See Chapter 24 Pleural

and chest-wall

lesions

Suspected empyema Differentiation of effusion/infiltrate/atelectasisltumor Asbestos-associated disease Chest

trauma

Anatomy The mediastinum is conveniently divided into three compartments-anterior, middle, and posterior-despite the absence of anatomic boundaries that would limit the spread of disease between the compartments, The ant'enor compartment is located anterior to the pericardium, ascending aorta, and superior vena cava and contains the thymus and may contain portions of the thyroid, The middle compartment contains the heart and great vessels and the central tracheobronchial system including the pulmonary hila, The postertor compartment is composed of the retrocardiac and paravertebral spaces, lt contains the descending aorta, esophagus, thoracic duct, and the azygos-hemiazygos venous system The differentiation of vascular structures and lymph nodes in these regions requires a sound knowledge of cross-sectional anatomy (Fig, 10.1), The size of the normal thymus is highly variable until about 20 years of age, If the thy-

mus can be visualized in young adults, it usually appears as an arrowhead-shaped structure (62%), lt is less common to find two thymic lobes (32%) or one isolated lobe (6%), The thymus undergoes involution from about 20 to 60 years of age and is replaced by fatty tissue, Normally, cannot delineate the visceral and parietal layers of the pleura, The pleural layers, endothoracic fascia, and deepest intercostal muscles blend together on CT scans to form a well-defined "pleural line" of 1-2 mm in thickness (Fig, lO.2), Normally, there are no structures of soft-tissue density along the inner contour of the ribs, Epipleural fat may be visible in obese patients and along marginal rib segments, Paravertebral portions of the pleural line often cannot be visualized, but segments of the intercostal veins can be identified as linear streaks of soft-tissue density, Epipleural fat and intercostal veins should not be mismterpreted as pleural thickening,

cr

375

376

10 Mediastinum, Pleura, and Chest Wall

SVCfRA

Fig. 10.1

anatomy. Pericard

RA -----

Schematic

representation

of

mediastinal

Ao Aorta AUL, VUL, BUL Upper lobar arteries, veins, and bronchi ALL, VLL, BLL Lower lobar arteries, veins, and bronchi Ai-io, 81-10 Segmental arteries and bronchi Bi Bronchus intermedius AZV Azygos vein BCA, BCV Brachiocephalic artery and vein CCA Common carotid artery E Esophagus PA Pulmonary artery RA, LA Right atrium, left atrium RV, LV Right ventricle, left ventricle RPA, LPA Right pulmonary artery, left pulmonary artery SCA Subclavian artery SVC Superior vena cava T Trachea

Contrast Administration

nI IJ

Extrapleural

fat

--

-Intercostal muscles

Vein Artery Nerve

sharp line (1-2 mm wide)

Muscles of chest wall

a Fig. 10.2

Paravertebral muscles

Schematic

Examination

Endothoracic fascia --1---\0:

Internal ---+-intercosta I muscle

---

Visceral and parietal pleura

External intercostal muscle

Extrapleural

fat

Intercosta I intimus muscle Intercostal

finepleural line on CT

)

fat

~

representation

b of the anatomy

of the chest wall.

Technique

Scanning of the mediastinum, pleura, or chest wall is usually done as part of a general thoracic CT examination (see Chapter 9). A more focused examination of tumors, hilar lymph nodes, or the chest wall benefits from coronal or sagittal reformations, and therefore should rely on thinsection spiral or multislice CT. Contrast enhancement is of subordinate importance because it mainly serves as a means of differentiating between vessels and solid structures (lymph nodes, tumors). Patient Preparation

No special patient preparation is needed. Having the patient swallow contrast material (e.g., highly viscous barium preparations such as Esophocat®) on the examination table may improve the identification and evaluation of the esophageal lumen. However, luminal opacification is very often incomplete and the approach therefore cannot be generally recommended.

and should be employed whenever available. For most indications, however, it is sufficient to reconstruct axial sections of 5-7 mm width for evaluating mediastinal abnormalities. Retrospective ECG-gating can be employed to reduce pulsation artifacts in regions adjacent to the heart. Cardiac gating techniques with a four detector-row scanner typically employ a pitch factor P*=1-2 (corresponding P=0.250.5). This leads to a substantial increase in radiation exposure and reduced volume coverage per time. Cardiac retrospective gating algorithms, therefore, cannot be generally recommended. Instead, special gating techniques that exclude the projection data of the period of maximum movement (late systole) from the multislice raw data set are better suited because they allow for pitch factors P of 0.75-0.875 (p*= 3-3.5 with 4slice scanners) and only slightly increase radiation exposure. This makes it possible to use 0.75-1.25 mm sections and still cover the mediastinum within one breath hold. Not all manufactures, however, do have such algorithms available.

Acquisition Parameters

Thin collimation reportedly improves the evaluation ofthe pulmonary hilum, the chest wall at the lung apex, and the diaphragm. Thin-section multislice CT (SC= 0.75-1.5 mm with 4- to 16slice scanners) is best suited for this purpose

Contrast Administration

Noncontrast CT is mainly required for evaluating hemorrhage, indeterminate mediastinal masses, or pleural thickening. It also has been suggested as an appropriate scanning technique

10 Mediastinum, Pleura, and Chest Wall

and investigating vascular disorders (Tables 10.2, 10.3). Most indications require enhancement only of the mediastinal vessels to be able to differentiate them from lymph nodes or tumors. For this indication relatively small amounts of contrast medium «90ml) need to be injected at moderate flow rates (s. 2 mlls). Inflammatory processes or evaluation of chest wall invasion are best detected after injection of 90-120 ml contrast material with a start delay of 30-50 s. Evaluation of tumor vascularity for characterization of suspected mediastinal lesions requires injection of higher volumes (100150 ml) at higher How rates (3-5 ml).

for patients with peripheral bronchogenic carcinoma since lymph nodes that exceed the size criterion of 1 cm short axis diameter can be detected even on noncontrast scans. However, a more subtle evaluation of such nodes may not be possible, and with the advent of more sophisticated morphologic criteria for lymph node differentiation, multislice scanning with contrast administration can be highly recommended. Intravenous contrast is helpful for differentiating vessels, lymph nodes, and other structures of soft-tissue density, for evaluating lesions of the chest wall, characterizing lesions,

Table 10.2

Suggested

protocols

for CT scanning

of the mediastinum,

pleura and chest wall

General Oral contrast

none or 1 tbi spoon Esophocatw

Patient position

Supine with elevated

arms

Scan range

Chest: from posterior

lung recess to lung apex

Respiratory phase

Inspiration

Windowing

Noncontrast CT: Contrast-enhanced Bone:

Scan parameters Bf Standard Volume

imaging

W/L=400/40 W/L= 400/60 W/L= 1500/300

CT:

1-slice SCITF/RI

4-slice

(HR kernel)

SC

16-slicea SC

axial SWIRl

5/8/4 T

2-3 i

1-1.Sf

5/4

3/5/2 t

1-1.25 t

0.75-1.25T

3/3

a

[e

MPR1• SWIRl

3/4 cor

Asbestos screening

2/25 i

Contrast

1-slice V/F/D

4-slice V+IM/FID

16-slice V+IM/FIDX'

Standard

70/2/20

70+50/2/20

50+30/2/30

Tumor staging

90/3/30

90+50/3/30

90+50/3/25A

Abscess,

90/2/60

90+50/3/60

90+50/3/75

injection

empyema

SC = slice collimation

1-1.25/251

(rnrn). TF = table feed (rnrn/rotation).

width (rnrn). MPR = multiplanar CM = contrast

c

material.

reformats,

RI = recon. increment

axial = axial sections,

Contrast concentration

0.5-0.75/25

Pitch P=TF/(NxSC)

b

MPR are reconstructed

C

read: single section mode HRCT with 0.5-1.5

dread:

(rnrn). f i = scanning direction,

cor = coronal. sag = sagittal

= 300 mg/ml iodine

V = volume of CM (rnl). N = volume of saline flush (rnl) F = flow rate (mils), a

D = start delay, X = trigger

should be chosen close to 1.5 with 4-slice scanners, 1.3-1.5 with from a "secondary

25A = start delay of 25 s after reaching

1-2/25

raw data set" with SWIRl ~ 1-1.5/0.7 mm collimation

or 0.5-0.8/0.5

and 10-20 mm increment

50 HU enhancement

region

16-slice scanners

in the desc. aorta

between sections

SW = effective section

Image Processing Table 10.3

Suggested

examination

technique Scan

Contrast

Injection

standard

standard

enhancement

standard/volumetric

standard/vascular

volumetric

vascular

standard

standard

standard/volumetric

standard/vascular

standard

late phase

DO of effusion/infiltrate/atelectasisltumor

standard

standard

Pleural calcifications

HRCT

Suspected Tumor

tumor

(on chest radiograph)

characterization

T staging N staging Pleural and chest-wall Suspected

Vascular

lesions

empyema (asbestosis)

Parameters

CTAsee

diseases

Image Processing Thick coronal reformations (3-5 mm) through the mediastinum improve the evaluation of mediastinal masses and lymph nodes. The central airways can best be evaluated with thicker semicoronal MPR (1 Omm section width) parallel to the course of the trachea (see Fig. 2.18). Tissue transition projections (TTP) of some 20 mm width are an alternative display tool for lesions of the

Enhancement

enhancement

enhancement enhancement enhancement

scanning enhancement

Chapter24

central airways (see Fig. 2.36). Thin-slab mlP (MinlP) also can also be used for display of the central airways but are usually inferior to thick MPTorTTP (see Figs. 2.16 and 2.18). Thin-slab MIP or volume rendering (VRT) can help to define vascular abnormalies (see Fig. 10.13). VRT may also be a helpful tool for preoperative visualization of mediastinal masses (see Figs. 10.10 and 10.12).

Characteristics

Ectopic thyroid tissue. parathyroid adenomas. and paragangliomas are examples of primary. hypervascular mediastinal masses that show intense enhancement following intravenous contrast administration. Foci of tumor necrosis (in germ cell tumors or lymphomas) appear as hypoattenuating areas after contrast administration. Cystic lesions (enteric. pleuropericardial. bronchogenic) do not show contrast enhancement (dOH attenuation difference between pre- and postcontrast scans).

Thickened pleural layers that enhance with intravenous contrast suggest a neoplastic or inflammatory processes. Tumors that have invaded the chest wall are more clearly delineated after contrast administration. depending on the vascularity of the tumor. Hypervascular chest wall lesions are rare but enhance earlier than hypovascular tumors in which the enhancement builds up in a later phase, often due to increased contrast leak into the interstitium.

Differential Diagnosis of Mediastinal Masses The main objective of CT is to evaluate the location and extent of a mediastinal mass. The presence of fat and cystic components together with size. shape. and precise location within the mediastinal compartments are the key features for differential diagnoses of mediastinal masses. Generally there is no difficulty in distinguishing tumor-like lesions from true neoplasms. When a

tumor is found, CT may help to narrow the differential diagnosis (Table 10.4). The patient's age, the presence of calcifications, and the degree of vascularity of the tumor provide additionalinformation.

379

380

70 Mediastinum, Table

10.4

Location

Pleura, and Chest Wall and CT morphology Anterior

Thymoma, thymic Goiter Lymph nodes Hodqkin's disease

Solid masses

Cystic

masses

Calcifications

Fatty

Vascular

lesions

Pseudomasses

Masses

carcinoma

masses

Middle Lymph

mediastinum node

Non-Hodqkin's

lymphoma

Castleman'stumor Goiter

Germ cell tumors Parathyroid adenoma Mesenchymal tumors

Paraqanqioma

Thymic cyst Teratoma Dermoid cyst Goiter

Bronchogenic cyst Pericardia I cyst

cell tumors

Lipoma Thymolipoma Teratoma Dermoid cyst Aortic

Predominantly

mediastinum

Neurogenictumor LYl"TlJh node Non-Hodgkin's lymphoma Goiter (rare) Esophageal tumors Extramedullary hemato-

nberculosis. silicosis

Neurogenic cyst Enterogenic cyst Pancreatic pseudocyst Meningocele Neuroblastoma

Goiter

Ganglioneuroma Neuroma Goiter

Lipomatosis

Lipoblastoma Neurogenictumors

aneurysm

Morgagni

Posterior

poiesis

Lymph node: sarcoidosis,

Teratoma Dermoid cyst Malignant germ Goiter

elements

of mediastinal

mediastinum

Aortic aneurysm Sinus of Val salva aneurysm Pulmonary artery aneurysm Anomalous pulmonary venous drainage Coronary artery aneurysm

Bochdaiek hernia Hiatal hernia Achalasia

hernia

Involving the Anterior

The commonest anteriormediastinal masses are thyroid or thymic lesions, teratomas, and lymphomas.Mesenchymaltumorsarerare.CTisused to investigate suspicious findings on chest radiographs and for the staging of known tumors. Thymic Lesions: Hyperplasia, Thymoma, Thymic Carcinoma

Thymic hyperplasia may be seen in thyrotoxicosis, Addison's disease, lupus erythematosus, Behcet disease, and Hashimoto thyroiditis. It frequently occurs as a rebound phenomenon in the period of recovery after stress reactions or chemotherapy, especially in young patients, following chemotherapy for various diseases (Hodgkin's disease, sarcoma, Wilms' tumor, ALL). The

Esophagealvarices Azygos vein anomalies

Mediastinum

"thymichyperplasia" seeninm yastheniagravisis actually a lymphoid response that is not always associated with enlargement ofthe gland. Benign and malignant thymomas represent a neoplastic proliferation ofthe thymic epithelium. The prognosis ofthymic epithelial tumors follows the classification of Levine and Rosai who differentiate between benign thymoma, invasive thymoma, and thymic carcinoma. Invasive thymoma is a malignant thymoma (category 1) with metastases and local invasiveness but it is cytoarchitecturally indistinguishable from benign thymomas. Thymiccarcinom aisamalignantthymoma (category 11) withdefinite cytologicalatypia. Thymomas are most commonly diagnosed in the fourth decade of life. The incidence of thymoma in patients with myasthenia gravis is ap-

Thymic Lesions: Hyperplasia, Thymoma, Thymic Carcinoma

proximately 15%. Benign tumors are indistinguishable from malignant lesions by their CT features alone, and all thymic tumors require surgical removal. In patients with myasthenia gravis nonionic contrast media should be used. When frank bulbar symptoms are present, care should be taken to be avoid a myasthenic crisis. The thymus is often involved in Hodgkin and non-Hodgkin lymphomas, mostly as combined thymic and mediastinal node disease and only rarely as isolated involvement. •

CT Morphology

Given the variability in thymic size, normal limits cannot be stated for patients under 20 years of age. After age 20, the maximum transverse diameter should be less than 15 mm. A diameter of 15 mm or more signifies thymic hyperplasia. Rebound hyperplasia after chemotherapy may resolve completely within a period of months, with 25% of patients showing persistent thymic enlargement by more than 50% of the original gland size. Benign thymomas typically appear as rounded or oval well-circumscribed structures of soft-tissue density located in front of the aorta in the upper anterior mediastinum (Fig.10. 3a). In rare cases it is located more caudally at the base of the heart. Calcifications occur in 25% of thymomas, regardless of whether they are benign or malignant. Primary cystic components are common. Approximately 30-40% of thymomas are classified as invasive or malignant, meaning that they show a locally invasive pattern of growth. The invasion of adjacent structures is manifested by pleural or pericardial thickening, effusion, or bony erosion of the sternum. The obliteration of

mediastinal fat planes is generally seen in invasive thymoma but may also be present in noninvasive tumors. Pleural extension as droplet spread without continuity with the primary tumor is seen in 15%. Thymic carcinomas do not show distinguishing CT features from invasive thymomas, yet they infiltrate more aggressively and mediastinal lymph nodes and extrathymic metastases are frequent (Fig. 10.3b). Thymic carcinoids present with an ectopic ACTH Cushing syndrome. They are indistinguishable from thymomas and diagnosed by open biopsy or surgical resection. Thymic cysts frequently develop after chemotherapy or after radiation therapy to the mediastinum. Intracystic hemorrhage is common, and thymic cysts show a highly variable CT density (from -10 to 80HLJ) depending on whether the cyst contains fresh or old blood, necrotic foci, or cholesterol. Involution of the thymus occurs between 20 and 60 years of age as the glandular tissue is replaced by fat. Thymolipomas are very rare tumors encountered in children and young adults. CT demonstrates a mass of predominantly fat density. Thymolipomas can reach a very large size and, being soft tumors, mold themselves to adjacent mediastinal structures without displacing them. The thymus is involved in approximately 30% of patients with Hodgkin 5 disease. Accompanying lymphadenopathy is almost invariably present. CT cannot distinguish rebound hyperplasia from normal thymic tissue or early invasion by Hodgkin's disease. There are no pleural implants in lymphomas.

Fig. 10.3 (a) Benign thymoma in a patient with myasthenia gravis. (b) Recurrence of malignant thymoma with a pleural drop metastasis. a

b

10 Mediastinum, Pleura, and Chest Wall

Extragonadal Germ Cell Tumors

Germ cell tumors (teratoid blastomas) arise from cells of the embryonic germ layers and comprise approximately 15% of all mediastinal masses. Germ cell tumors are commonly located in the thymus bed or within the thymus. While there is no sex predilection for the benign forms, the malignant forms predominantly affect young men (25% of cases). Germ cell tumors include benign and malignant teratoma, dermoid cysts, seminoma, the rare embryonal carcinoma, and choriocarcinoma. Teratomas contain elements of all three germ layers, whereas dermoid cysts have a (predominantly) epidermal origin. •

CT Morphology

Dermoid cysts can grow quite large. They have a rounded shape and are sharply demarcated by a capsule. They are composed of one or more cysts with well-defined walls that may show calcification. Chest radiographs and CT usually demonstrate an asymmetrical mass projecting into the upper anterior mediastinum on one side. Rarely it may extend into the posterior mediastinum or lung. The contents of the cyst show water or fat density. The detection of rudimentary teeth, foci of ossification, or a fat-fluid level is pathognomonic for a dermoid cyst. Teratomas originate from all three germ layer derivatives and show a mixture of CT densities ranging from fat and soft tissue to calcium. Dermoid cysts and cystic teratomas are on a continuum both histologically and in their CT features (Fig.lO.4a). Malignant teratoma is distinguished from benign forms by its ill-defined margins, large necrotic areas, and its compres-

sion and displacement of mediastinal vascular structures (Fig. 10.4b). Other malignant germ cell tumors are much rarer and present as solid masses that are more frequently lobulated than round. Seminomas characteristically are lobulated masses of homogeneous tissue density. This distinguishes them from other malignant germ cell tumors (chonocarcmoma, embryonal cell carcinoma, and mixed forms), which typically have a very heterogeneous appearance due to hemorrhage and necrosis but calcifications and fatty material are not seen. Criteria for malignancy are rapid growth, signs of local invasiveness, and the detection of distant metastases in lung, pleura, or bone. Invasion of mediastinal structures is difficult to assess, since a tumor may merely adhere to adjacent structures without invading them. The benign or malignant nature of the mass cannot be stated with certainty on CT criteria, and surgical removal is recommended. Larger tumors may displace or compress bronchial airways or the vena cava (10-30% of cases) . • In the case of extragonadal malignant germ cell tumors, the primary nature of the mediastinal mass is confirmed by noting the absence of gonadal and retroperitoneal tumor. • Residual masses of "mature" teratomatous tissue may persist following treatment, These masses may be cystic and, though benign, may enlarge over time.

Fig. 10.4 Cystic teratoma with an extensive solid component. causing posterior displacement of the mediastinal vessels (a). Large teratoma with extensive bone formation, a pleural effusion and a mediastinal shift to the left(b).

a

b

Hodgkin's Disease

Goiter

Parathyroid

Thyroid enlargement producing an intrathoracic mass may result from hyperplasia (iodine deficiency, hyperparathyroidism) or an inflammatory process (Hashimoto or Riedel thyroiditis) .

Orthotopic parathyroid glands are located posterior to the upper and lower poles of the thyroid gland. Ectopic adenomas may occur anywhere in the anterior upper mediastinum, and rarely also in the tracheoesophageal groove. MRI is often considered the imaging modality of choice owing to the distinctive signal characteristics of parathyroid tissue (high signal in T2weighted imaging with strong enhancement after contrast medium injection) but CT yields similar results in most cases.

•

CT Morphology

An intrathoracic goiter generally represents a downward extension of orthotopic thyroid tissue. Therefore, finding the pedicle connecting the intrathoracic mass with the thyroid gland will suggest the correct diagnosis. Rarely, the only connection is a narrow fibrous or vascular pedicle not visible on CT. Thyroid tissue enhances intensely after intravenous contrast administration (often by more than 40 HU). In 75-80% of cases, the intrathoracic goiter originates from the isthmus and lower poles of the thyroid and extends into the anterior mediastinum. In 20-25% of cases, the hyperplastic posterior portions of the thyroid gland pass lateral or posterior to the trachea or, very rarely, posterior to the esophagus, and extend into the posterior mediastinum (more common on the right side than the left). The intrathoracic thyroid mass commonly shows regressive changes with cystic elements, and circular, nodular, or eggshell patterns of calcification (Fig.W.5). Malignant transformation can be diagnosed only from secondary signs such as infiltrative changes and lymph node enlargement. The mass may cause significant displacement and narrowing of the trachea.

•

CT Morphology

Ectopic adenomas appear as soft-tissue attenuating or slightly hypoattenuating, round to oval masses of 0.5-3 cm diameter (mostly < 2 ern) in the thymic bed in the anterior upper mediastinum (see Fig.8.30b). Adenomas in the tracheoesophageal groove may be hard to differentiate from lymph nodes or veins on axial sections. Multiplanar reformations after contrast injection are helpful in these cases. Adenomas display a marked enhancement after contrast administration. Hodgkin's Disease

Hodgkin's disease (or Hodgkin's Lymphoma) has a bimodal age distribution, with peaks occurring at approximately 30 and 70 years of age. Most patients present with involvement of the cervical and supraclavicular lymph nodes. The prognosis depends on the histologic type and stage of the disease, and treatment is tailored to the extent of disease. CT is an essential part of tumor staging. •

Fig. 10.5 Goiter in the middle mediastinum. The mass shows regressive changes (cysts and calcifications) and has caused tracheal narrowing.

Adenoma

CT Morphology

Unlike non- Hodgkin lymphoma (NHL), Hodgkin's disease (HD) predominantly affects the anterior mediastinum and tends to spread from one group oflymph nodes to contiguous nodal groups. In the mediastinum, the prevascular and paratracheal lymph nodes of the anterior mediastinum are most commonly involved. Pulmonary involvement (approximately 10-"15%) is always associated with hilar lymphadenopathy. Concomitant enlargement of the thymus is common (30%). Pleural effusion and interstitial pulmonary edema may indicate venous and lymphogenous obstruction or direct tumor infiltration, while

384

10 Mediastinum, Pleura, and Chest Wall Fig. 10.6 (a) Hodgkin's disease in the anterior mediastinum. CT shows a large mass compressing the vena cava and invading the lung. (b) Hodgkin's lymphoma after therapy. There is a partially calcified residual mass in the region of the thymus.

a

b

pericardia! effusion usually signifies direct tumor invasion. In patients with pleural or pericardial involvement, the size of the anterior mediastinal mass usually exceeds 30% of the internal thoracic diameter (bulky disease, Fig. 10.6). Atelectasis may result from intrabronchial involvement (HD > NHL) or extrinsic bronchial compression (more common in children). Recurrences of Hodgkin's disease tend to occur at the original site after chemotherapy and outside the therapy port after irradiation. The absence of hilar adenopathy virtually excludes pulmonary involvement by Hodgkin's disease.

Non-Hodgkin's

Lymphoma

Non- Hodgkin lymphoma refers to a heterogeneous group of tumors with diverse histologi-

cal features. It is the third most common childhood malignancy after leukemia and eNS tumors butis unusual in children under 5 years of age. The adult form is most prevalent in the sixth and seventh decades. There is a 40 to 100 times greater risk for NHL in patients with immunodeficiency syndromes, after organ transplantation, and with collagen vascular diseases. Treatment depends on the histological classification. Frequently the disease is widespread at the time of diagnosis. •

CT Morphology

Non-Hodgkin lymphomas show a noncontiguous mode of spread with involvement of unusual lymph node groups. Involvement of the mammary and paracardiac nodes and isolated lymphomas in the posterior mediastinum are suggestive of non-Hodgkin lymphoma (Fig.lO. 7a). Extranodal disease is commoner than with Hodgkin's lymphoma (Figs.lO.7b, 10.8). Pulmonary lesions may appear as discrete

Fig. 10.7 (a) Extensive precardiac mass caused by non-Hodgkin lymphoma. (b) T-ALL with massive enlargement of the thymus and encasement of the superior vena cava (arrow).

a

b

Mesenchymal

a

c

b

Fig. 10.8 Non-Hodgkin's lymphoma in a 6-year-old child. Note the enlargement of the thymus (a), the axillary and supraclavicular lymph nodes (arrowheads). the

nodules, parenchymal infiltrates, glass pattern (see Chapter 9).

Tumors

or ground-

In up to 50% of cases, residual masses are seen after the successful treatment of malignant lymphomas. These masses are inactive and consist of sterile fibrous tissue. For distinguishing residual fibrotic masses from active tumor, CT relies on follow-ups showing lack of change in the size of the residual masses. An increase in size signifies an active lymphoma. MRI offers its own criteria of response to therapy: signal decreases on T2-weighted images with treatment, and residual fibrous masses showing low signals in Tl- and T2weighted images. However, so far MRI cannot reliably exclude focal vital lymphoma tissue within a residual mass.

invasion of the lower neck (b) and the stenosis of the trachea seen on a sagittal reformation of a low-dose (4 x 1 /6, CTDlvoi = 2.4 mGy) multislice CT data set (c).

•

CT shows a cystic mass, mostly unilocular, sometimes multilocular or septated, that molds to adjacent mediastinal vessels. The protein-rich content causes attenuation values of up to 40 HU. Unilateral or bilateral effusions may occur and may be chylous. Mesenchymal Tumors

Mesenchymal tumors may occur in all three mediastinal compartments. The benign forms most commonly occur in the anterior mediastinum, and the malignant forms (liposarcoma, fibrosarcoma) are commonest in the posterior mediastinum. •

Lymphangioma

Lymphangioma (cystic hygroma) is a rare congenital malformation of the lymphatic system consisting of lymph channels or cystic lymph spaces. In infants they are most commonly located in the anterior-superior mediastinum and frequently extend into the neck. In adults, lymphangiomas tend to occur in the lower anterior mediastinum and do not communicate with the neck.

CT Morphology

CT Morphology

Lipomas show characteristic fat attenuation on CT scans. They are soft tumors that model themselves to the mediastinal contours rather than displace adjacent organs. Other mesenchymal tumors such as fibroma, fibrosarcoma, and hemangiopericytoma can cause significant displacement. Hemangiomas are hypervascular masses that enhance intensely with intravenous contrast and often contain phleboliths.

385

10 Mediastinum, Pleura, and Chest Wall

Masses Predominantly

Involving the Middle Mediastinum

Lymphoma

Both Hodgkin's lymphoma and NHL may present as a mass in the middle mediastinum, although NHL is commoner than Hodgkin's disease at this location (see above). Tumors oftheTracheobronchial

System

See Chapter 9 (Lungs and Tracheobronchial tems, p. 303).

Sys-

Bronchogenic Cyst, Pericardial Cyst

Bronchogenic cysts are rare congenital anomalies of the tracheobronchial tree (see Chapter 9) that occur more frequently in the mediastinum than in the lungs. They have a fibrous capsule, frequently contain cartilage, are lined by respiratory epithelium, and contain mucoid material. Unlike intraparenchymal cysts they do not usually communicate with the bronchial tree. Pericardia! cysts are mesothelial anomalies (oLitpouching of the parietal pericardium without communication to the pericardial sac). MRI is often superior to CT in differentiating these cysts from mediastinal tumors. •

CT Morphology

Pericardia} cysts are most commonly found in the right cardiophrenic angle but may occur at any pericardial or pleuropericardial site (posterior cardiophrenic angle, superior pericardial recess). Most broncbogenic cysts are located adjacent to the carina on the right side (>50%). They may also occur adjacent to the trachea or esophageal wall or in the retrocardiac region (Fig.W.9). Both types of lesion typically range from 3 to Scm in size and display the features of a bland cyst with a round to oval shape, sharply defined margins, and homogeneous contents. The CT density is dose to that of water or may be higher, signifying mucus formation (pericardia! cyst: 20-50 HLJ; bronchogenic cyst: 0-50 HLJ). The lesions do not enhance after contrast administration.

Fig. 10.9 Bronchogenic cyst. The cyst. the upper border of which is below the carina, extends downward and to the right. terminating behind the left atrium.

Paraganglioma

Paragangliomas are neurogenic catecholamineproducing tumors that are malignant in 10% of cases. N on- functioning paragangliomas are called chemodectomas, functioning paragangliomas correspond to pheochromocytomas. Almost all chemodectomas are aortic body tumors. •

CT Morphology

Aortic body tumors are most often located at the angle of the ductus arteriosus, but can also occur lateral to the brachiocephalic artery, anterolateral to the aortic arch, or above and to the right of the right pulmonary artery. Pheochromocytomas are mostly found in the posterior mediastinum, in or adjacent to the heart or the pericardium, particularly in the wall of the left atrium or the interatrial septum. Paragangliomas are hypervascular and enhance intensely following intravenous contrast administration. Multicentric tumors are possible. Local invasion or metastases indicate malignancy (Fig.10 .10).

Neurogenic Tumors

a

b

c

Fig. 10.10 Malignant paraganglioma with encasement of the left coronary artery (4 x 1/6). There are two overlapping stents placed in the left main and left anterior descending coronary arteries (a). The tumor invades and displaces the pulmonary artery superiorly (b.

Masses

Predominantly

Involving

the Posterior

Neurogenic Tumors

•

With the exception of paragangliomas neurogenic tumors are found exclusively in the posterior mediastinum. Approximately 30% are malignant. Various histologic types are known (see Table 10.5). Neurogenic tumors can be divided into nerve sheath tumors (schwannoma, neurofibroma), ganglion cell tumors (malignant neuroblastoma and benign ganglioneuroma), and paraganglionic cell tumors.

Table 10.5

volume-rendered image). The extent of the tumor is excellently displayed on volume-rendered images of the heart (c).There is residual peripheral enhancement after radiation therapy indicating residual vital tumor tissue (arrowheads) .

Mediastinum

CT Morphology

CT typically shows a paravertebral mass (Fig.10.11) that is ellipsoidal or dumbbellshaped and may cause expansion of an intervertebral foramen. It is common to find sites of pressure erosion and deformation, usually involving the ribs. Foci of bone destruction without sclerotic borders indicate malignancy. Most neurogenic tumors have an oval shape and are of homogeneous soft -tissue attenuation.

Neurogenic tumors

Neuromas

Common

in neurofibromatosis,

show moderate

Neurofibroma

Frequently

expands

Schwannoma

Frequently

ellipsoidal

Paragangliomas

Very rare, show intense enhancement

the intervertebral

Chemodectoma

Frequently

near the aortic arch

Pheochromocytoma

Frequently

near the posterior

Sympathicoblastomas

Heterogeneous

Neuroblastoma

Malignant, age <8 years, or coarse calcifications

Ganglioneuroblastoma

Mixed form, age <10 years, traspinal component

contrast

enhancement

foramen

pericardium

pattern of enhancement large mass, usually metastasizes, dumbbell-shaped

contains

stippled

mass with an intra- and ex-

387

388

10 Mediastinum, Pleura, and Chest Wall Fig. 10.11 Neurofibroma with a typical dumbbell shape and expansion of the intervertebral foramen (a). Schwannoma presenting as a paravertebral mass compressing the adjacent lung (b). Multiple mediastinal masses in neurofibromatosis (c).

a

c

b

Some may show low attenuation areas that are due to lipid content, cystic degeneration, and entrapment of neural tissue. Central necrotic foci are rare. Except for paragangliomas that enhance strongly after contrast administration, most neurogenic tumors show moderate contrast enhancement and are virtually indistinguishable, based on their CT features. Relationship to vertebrae, ribs, and the spinal canal are essential for therapy planning and are best imaged by thin-section spiral and multislice CT, or by MRI. Extra medullary Hematopoiesis

Extramedullary hematopoiesis in the liver, spleen, and lymph nodes is common in various forms of anemia (sickle cell disease, spherocyto-

a

b

Fig. 10.12 Extramedullary hematopoiesis, manifested by paravertebral masses of soft-tissue density at the level of the lower thoracic spine (a). Coronal reforma-

sis, hemoglobinopathies) or bone marrow replacement disorders (leukemia, lymphoma, myelofibrosis). The collections of hematopoietic tissue may form extensive paravertebral masses, especially in patients with thalassemia or sickle cell disease. CT can differentiate the hematopoietic masses from malignant lesions. •

CT Morphology

Foci of extramedullary hematopoiesis typically appear on CT scans as convex paravertebral masses of soft-tissue density (Fig. 10.12). The masses usually extend downward for several vertebral segments below T7 and may have a subpleural component. Unlike neurogenic tumors, hematopoietic masses do not cause pressure erosion ofbone. Extramedullary hematopoiesis is accompanied by splenomegaly and skeletal changes due to bone marrow hyperplasia.

c tion (b) and volume-rendered image (4 x 1/6) of paraspinal masses due to extramedullary hematopoiesis in another patient (c).

Meningoceles and Neuroenteric Cysts Esophageal Tumors

See Chapter 15 (Gastrointestinal Tract).

be demonstrated by extending the examination to the upper abdomen. The azygos and hemiazygos veins are greatly dilated (equaling or surpassing the aortic diameter) and show homogeneous contrast enhancement (see Fig. 24.84).

Esophageal Diverticula and Duplication Cysts

See Chapter 15 (Gastrointestinal

Tract).

Esophageal Varices, Azygos Continuation

Esophageal varices in the lower half of the esophagus are caused by portal hypertension. Congenital or acquired occlusions ofthe inferior vena cava stimulate collateral circulation through the azygos venous system (azygos continuation). CT can positively distinguish the dilated veins from paravertebral masses.

Meningoceles and Neuroenteric Cysts

Cystic masses in the posterior mediastinum are very rare and may be caused by a meningocele, myelomeningocele, or neuroenteric cysts (which have pleural and alimentary components). Approximately 75% of intrathoracic meningoceles occur in association with neurofibromatosis. •

CT Morphology

CT demonstrates a well-circumscribed cystic mass in the posterior mediastinum. CSF -filled meningoceles may be unilateral or bilateral and • CT Morphology cause enlargement of the intervertebral foramina. Neurofibromatosis has a high associaEsophageal varices have a variable CT appearance depending on their size and the extent of tion with meningoceles as well as with solid neuthe underlying disease. Nonenhanced CT shows rofibromas, which can be distinguished by their nonspecific thickening of the esophageal wall CT attenuation. Thin-section spiral or multislice CT help detect the connection of the lesion with accompanied by nodular paraesophageal masses. On contrast-enhanced scans, the varices the subarachnoid space. MRI, however, is still the appear as tubular, tortuous structures that show technique of choice for this imaging task. Neuroenteric cysts are associated with bony marked pooling of contrast medium (Fig. 10.13). deformities of the thoracic spine (hemiverteEsophageal varices inside the esophageal lumen brae, butterfly vertebrae, scoliosis). Air incluhave to be distinguished from paraesophageal varices that are often more prominent and can- sions may be seen in neuroenteric cysts that not be detected by endoscopy. In patients with communicate with the esophagus. Differentiation is required from intramediunderlying liver disease, other venous collateral astinal pancreatic pseudocysts, which are rare vessels may be seen in the upper abdomen. In patients with azygos continuation, ab- (history), and from esophageal duplication sence or occlusion of the inferior vena cava can cysts.

Fig. 10.13 Varices appear as brightly enhancing tortuous veins in a submucosal location (a, esophageal varices, arrowheads) and adjacent to the esophageal wall (paraesophageal varices). The latter cannot be detected byendoscopy. Paraesophageal varices may present as a posterior mediastinal mass (b, volume-rendered image,4x2.5/15).

a

b

389

10 Mediastinum, Pleura, and Chest Wall

• For high specificity, a threshold of 15mm short axis diameter is suggested for subcarinal nodes and the azygos node, and 10 mm should be used for the other nodes. • Hilar (bronchopulmonary) lymph nodes should be regarded as suspicious if they bulge into the adjacent lung parenchyma (convex outer border; Fig. 10.15a). • With thin-section multislice CT, not only the cross-section but also the longitudinal shape of lymph nodes can be evaluated: round nodes should be considered suspicious even if they are between 5 and 10mm in size (Fig.10.15b). • The superior pericardial recess, which appears posterior to the ascending aorta, should not be misinterpreted as an enlarged lymph node (see Fig. 22.9). • Paracardiac and supradiaphragmatic lymph (Fig.10.15c) nodes are pathologic if their size exceeds 5 mm in diameter (except after previous upper abdominal surgery).

Lymph Node Enlargement

The anatomy and nomenclature of the mediastinal and hilar groups of lymph nodes are reviewed in Chapter 22 (see Fig. 22.2). CT is currently the technique of choice for evaluating the status of the mediastinal and hilar lymph nodes. MRI is not superior to CT for detecting enlarged nodes. In patients with extensive lymphoma, MRI or thin-section multislice CT provide images in any desired plane and are able to better define vascular structures. •

CT Morphology

Lymph nodes can be identified on CT when they are 2 to 5 mm in diameter. The differentiation between benign and malignant mediastinal lymph nodes is difficult (Fig. 10.14), because lymph node size is generally the only criterion available on CT scans. When a short axis nodal diameter of 1 cm is taken as the threshold, CT is approximately 60% sensitive and 70% specific in detecting malignant involvement. Similar outcomes are found for a long axis diameter of 15mm. A lower threshold will yield a higher sensitivity but at a reduced specificity. Neither CT nor MRI can provide a confident differentiation between benign and malignant nodes. Micrometastases are found in 5-15% of normal size lymph nodes, while 15-30% of moderately enlarged lymph nodes (10-15 mm) are free of metastases-an important possibility to consider in patients with accompanying pneumonia, COPD, or chronic heart failure.

a

b

Fig. 10.14 Various diseases can cause similar patterns of mediastinal lymph node enlargement: sarcoidosis (a), lymphoma (b), metastases from non-small cell lung

•

Differential Diagnosis

(Table 10.6) Between 80% and 90% of patients with sarcoidosis develop mediastinal lymphadenopathy (Fig. 1O.14a). The most frequent inflammatory causes of mediastinal lymph node enlargement are fungal infections and tuberculosis. Tuberculosis most often shows a characteristic peripheral enhancement of lymph nodes after contrast administration (Fig. 10.16a). Necrotic areas usually have a CT attenuation of below 30 HLJ.

c

d

cancer (c). Lymph node enlargement due to chronic left heart failure is more prominent in the perihilar region (d).

Lymph Node Enlargement

b

a

c

Fig. 10.15 Morphologic indicators of malignant lymph node involvement, (a) Bronchopulmonary nodes with a convex contour relative to the lungs suggest malignancy (arrow) while lymph nodes with a straight or concave interface to the lungs are very likely of benign eti-

Table 10.6

Differential

diagnosis

of mediastinal

ology. (b) Rounded shape of lymph nodes (long and short axis diameters nearly identical in all planes) makes malignant involvement more likely, (c) Supradiaphragmatic lymph nodes> 5 mm short axis diameter are considered pathologic.

lymph nodes (according

Calcified lymph nodes

Peripherally enhancing necrotic lymph nodes

Common:

Tuberculosis Sarcoidosis Silicosis Posttreatment Metastases (Hodgkin's disease, thyroid carcinoma)

Tuberculosis Fungal infection Lymphoma Metastases (seminoma, bronchial carcinoma)

Rare:

Histoplasmosis Amyloidosis Scleroderma Castleman's disease Metastases (mucinous carcinoma, osteosarcoma)

Most mediastinal lymph nodes are hypoattenuating relative to the mediastinal vessels when scanned with the start delay of20-40 s that is typical for most mediastinal CT examinations. At a later time some lymph nodes become almost isoattenuating to the vessels (indicating a relative hyperemiaduetoinflammationortumor), others remain hypoattenuating (indicating hypovascularization due to regressive changes or hypovasculartumors). Such a behavior is seen when the chest is scanned at a later phase, e.g., after having examined the upper abdomen first. There is no evidence yet as to whether the contrast enhancement patterns may be used for differentia-

to Naidich, Zerhouni, or

Hypervascular

and Siegelman) lymph nodes

Metastases (renal cell carcinoma, bronchial carcinoma, carcinoid tumor)

Sarcoidosis Castleman's disease Kaposi sarcoma

tion between benignormalignantdisease. Lymph nodes that already enhance after 20-30 s, however, are almost invariably malignant (Fig. W.1Gb; Table 10.5).Thepresence ofcalcific ations (Fig. W.1Gc) may help with the differential diagnosis only with additional clinical findings. Castleman disease (angiofollicular lymph node hyperplasia or giant lymph node hyperplasia) is characterized by well-defined masses in the mediastinum or proximal hilum, that are sometimes calcified. Mul ticentric forms present as multiple enlarged lymph nodes and have a poorer prognosis. Striking uniform contrast enhancement suggests the diagnosis.

391

10 Mediastinum, Pleura, and Chest Wall

b

a

Fig. 10.16 The etiology of mediastinal lymph nodes can be suspected from CT morphology only in conjunction with clinical data. (a) Rim enhancement of mediastinal lymph nodes in tuberculosis in a patient with

c low-grade fever, (b) Bilateral hilar and infracranial hypervascular lymph node metastases in a patient with Ewing's sarcoma, (c) Calcifications in pathologic lymph nodes in a patient with colorectal carcinoma.

Pancoast Tumor

ning, and coronal as well as sagittal reformations are mandatory for optimum evaluation of tumor extent (Fig.10.17b). When neurological symptoms are present, it should be possible to demonstrate a mass of soft-tissue attenuation lying adjacent to the vertebrae or in the supraclavicular fossa. Obliteration of the fat plate between the anterior and middle scalene muscles or lateral asymmetry of the scalene muscles • CT Morphology should raise suspicion of brachial plexus invaPancoast tumors show a characteristic location sion. Multislice CT can define the nerves only in in the superior pulmonary sulcus (Fig. 10.17). areas where there is sufficient perineural fat. They have a strong propensity for invading the Plexus evaluation is still the domain of MRI but ribs, vertebral column, esophagus, trachea, bra- multislice CT with sagittal reformations holds chiocephalic vessels, and cervical soft tissues. A the promise of similarly good results. thin-section technique should be used for scanPancoast tumor (superior sulcus tumor) is a bronchial carcinoma arising in the apex of the lung. The typical clinical manifestations are caused by invasion of the brachial plexus (shoulder and arm pain) or the sympathetic chain (Homer's syndrome). If multislice CT is not available, MRI is the technique of choice for images in the coronal and sagittal plane.

Fig. 10.17 (a) Pancoast tumor showing broad pleural adhesion and invasion of the mediastinal pleura (arrows). Scans at a higher level revealed encasement of the subclavian vessels, (b) Coronal MPR (4 x 1/6) of a small Pancoast tumor infiltrating the second rib (arrow).

a

b

Tumors Involving the Chest Wall

Tumors Involving the Chest Wall

Tumors of the chest wall are usually metastatic in origin. Most such tumors in adults are adenocarcinomas, while most childhood lesions are neuroblastomas or leukemic infiltrates. In patients with peripheral bronchogenic carcinoma, chest wall invasion marks a tumor as irresectable only if it is diffuse and associated with a malignant pleural effusion. Involvement of the chest wall by lymphoma (1.5%) generally arises from a mediastinal mass lesion. Primary tumors of the chest wall are rare (Table 10.7). Because of its superior soft-tissue contrast, MRI is often superior to CT in evaluating invasion of the soft tissues of the chest wall. •

Table 10.7 Primary the chest wall

tumors

and tumor-like

lesions

of

Cartilage or bone lesions Chondroma, chondrosarcoma Plasmacytoma Ewing's sarcoma Paget's disease Fibrous dysplasia Soft-tissue tumors Lipoma, liposarcoma Fibroma, fibrosarcoma, Desmoid tumors Rhabdomyosarcoma Hemangioma Neuroma

MFH

CT Morphology

Soft-tissue tumors cause a mass in the muscles of the chest wall with obliteration ofthe fat planes or the epipleural fat stripe. Usually these tumors are clearly delineated only on contrast-enhanced scans. Tumors of the bony structures cause local expansion of the bone or rib cartilage and may have extensive soft-tissue components. Invasion ofthe chest wall by a mediastinal or pulmonary tumor can be recognized by the obliteration of fat planes and by contrast enhancement. Loss ofthe narrow epipleural fat stripe is associated with chest wall invasion in 80% of patients with peripheral bronchogenic carci-

Intrapulmonary with pleural i

noma. Broad contact wi th the chest wall (> 3 ern) and an obtuse angle between the tumor and chest wall are not considered reliable indicators of invasion (Fig. 10.18). Unchanged position of a tumor on inspiratory and expiratory scans suggests adhesion to or invasion ofthe chest wall. Thickening ofepipleural (fatty) tissue at the site of contact between a lesion and the chest wall indicates pleural adhesion and a more long-standing process, although an initial chest wall invasion cannot be ruled out (Fig. 10.19).

I

Pleural

Intrapulmo

Fig. 10.18 Schematic comparison of the CT appearance of pleural lesions, peripleural intrapulmonary lesions, and lesions of the chest wall. The arrows indicate the "acute" pleural angles that are associated with intrapulmonary lesions.

Extra I with rit

··Ision

393

10 Mediastinum, Pleura, and Chest Wall

Fig. 10.19 Peripheral bronchogenic carcinoma with adhesion to the chest wall and mediastinal lymph node metastases (4 x 1/6). Note that there is a local thickening of the extrapleural soft tissue (arrow) in the region of contact, which has a distinctly different contrast enhancement pattern than the tumor. The arrowhead indicates encasement and infiltration of the upper lobe artery.

Pleural Tumors Benign Pleural Tumors

Approximately 70% of benign pleural tumors (benign mesothelioma, lipoma, fibroma) arise from the visceral pleura and only 30% arise from the parietal pleura. On the whole, benign pleural tumors are very rare. Most of them are fibrous tumors. •

CT Morphology

Benign pleural tumors form circumscribed nodular pleural thickenings (Fig. 10.20) that may reach considerable size and may even show locally invasive growth. They do not metastasize, however, and they have a good prognosis. Involvement of the mediastinal pleura can mimic a mediastinal tumor, and involvement of the interlobar fissure can mimic an intrapulmonary

tumor. Lipomas show fat density on CT. Benign fibrous tumors of the pleura t'benign mesotheliomas} most often arise from the visceral pleura and may be located within a fissure. They commonly form acute angles with the pleura with tiny amounts of adjacent pleural fluid mimicking focal pleural thickening. A relatively thin stalk may be present but usually cannot be seen on CT. Tumors tend to be hypervascular; non-homogeneities may be present only in large lesions. Tumors located within a chronic pleural empyema may be difficult to define. Pleural Metastases

Pleural metastases most commonly originate from the lung, breast, and ovary and are gen-

Fig. 10.20 Benign fibrous tumor of the pleura (4 x 1/6). The lesion is hard to evaluate on axial sections (a) but displays typical features (acute angle relative to the pleura) on sagittal reformations

(b). a

b

Pleural Lymphoma Fig. 10.21 (a) Pleural metastases from gastric carcinoma appear as fine nodular thickening that enhance after intravenous contrast administration. They also involve the mediastinal pleura, (b) Pleural metastases from a mucinous ovarian tumor appear as diffuse pleural nodularities with peripheral enhancement and hypoattenuating central areas containing mucinous material.

a

erally associated with malignant effusions. The diagnosis is established by the cytological analysis of aspirated pleural fluid. CT is useful for detecting solid tumor components. •

CT Morphology

CT typically demonstrates multiple foci ofnodular pleural thickening that show intense contrast enhancement (Fig. 10.21a). Involvement of the mediastinal pleura is highly suspicious for neoplastic disease (Fig.10.21b). Diffuse pleural thickening with contrast enhancement may occur but is entirely nonspecific. If no pleural calcifications are present, pleural metastases are morphologically indistinguishable from malignant mesothelioma by their CT features alone and only clinical features like a history of asbestos exposure may give clues to the diagnosis. Malignant Mesothelioma

The incidence of malignant mesothelioma is 300 times higher in asbestos-exposed patients than in the normal population. The latent period is 20 to 40 years, and the prognosis is poor. The main clinical symptoms are chest pain and shortness of breath. CT can define the extent of advanced tumors but is less reliable in patients with asbestosis and very early tumor stages. A definitive diagnosis requires core-biopsy. •

CT Morphology

All portions of the pleura may be involved. The tumor may cause extensive nodular thickening of the pleura or complete encasement of the

b

lung (Fig. 10.22). The tumor tissue usually shows heterogeneous enhancement on postcontrast scans. A hemorrhagic pleural effusion (>30HLJ) is often present. Advanced cases may present with pericardial effusion, rib destruction, contralateral involvement, thickening of the interlobar septa, and intrapulmonary metastases. Mesothelioma is distinguished from fibrothorax by the more irregular pleural thickening, which often shows a more intense contrast enhancement. In early cases the differentiation may be difficult (Fig. 10.22a). Pleural Lymphoma

Both Hodgkin' disease and non-Hodgkin lymphoma may involve the pleura. Pleural involvement is rarely the only initial manifestation of the disease. It occurs more frequently in conjunction with mediastinal nodes, thymic involvement, or parenchyma! pulmonary disease, or is due to recurrence of the lymphoma. A pleural effusion (30% patients with Hodgkin's disease) caused by lymphatic obstruction of mediastinal lymph nodes has to be differentiated from lymphomatous involvement of the pleural itself. Pleural involvement in patients with lymphoma alters radiation therapy and unrecognized disease increases the risk for treatment failure. Leukemia may also cause pleural thickening.

395

10 Mediastinum, Pleura, and Chest Wall

a

c

b

Fig. 10.22 Malignant mesothelioma. (a) CT initially demonstrated irregular posterior and mediastinal pleural thickening with calcified plaques that do not show significant enhancement after contrast administration. This resembles the findings seen in chronic effusion with incipient pleural thickening, (b) Scan two

months later shows marked progression of the neoplastic pleural thickening, which now is polynodular. Note the involvement of the mediastinal pleura, (c) Involvement of the major (arrowheads) and minor fissure (arrow) in another patient with extensive pleural mesothelioma (4 x 1/6, sagittal MPR).

Superior Vena Cava Syndrome

Superior vena cava syndrome may result from intravascular or extravascular obstruction of the superior vena cava. Approximately 80% of extrinsic obstructions are caused by malignant disease (bronchial carcinoma, primary mediastinal tumors, lymphoma), and only 20% have a benign cause (fibrosing mediastinitis, radiationinduced fibrosis, catheter-induced thrombosis). CT is useful for establishing the cause of the obstruction and, in acute cases, planning radiotherapy or vascular intervention.

•

CT Morphology

Postcontrast scans can differentiate extravascular causes (compression) from intravascular causes (thrombosis). The development of a collateral supply is manifested by dilatation of the azygos vein, superior intercostal veins, and small cutaneous veins that drain into the azygos-hemiazygos venous system, into pericardiophrenic veins, or rarely into the portal veins (Fig. 10.23).

Fig. 10.23 Superior vena cava syndrome with collateralization. Downhill varices in the esophagus (a). Collaterals via intercostal, pericardiophrenic, azygos and hemiazygos veins (b) seen on a lateral MIP(4X1/6).

a

b

Fibrosing Mediastinitis

Diffuse Mediastinal

Diseases

Acute Mediastinitis

In more than 90% of cases, acute mediastinitis is caused by an esophageal perforation that may be post-traumatic, spontaneous (Boerhaave syndrome), iatrogenic (after surgery or endoscopy), or neoplastic. The remaining 10% of cases are due to complications following sternotomy or an inflammatory process that has spread from the neck along fascial planes (visceral and posterior cervical space). Mediastinitis has a high mortality if untreated. Management include antibiotics, drainage or surgical debridement. •

CT Morphology

In mediastinitis there is a streaky or diffuse increase in CT attenuation in the mediastinal fat (Fig.lO .24a). Complete obliteration of normal mediastinal fat by material of water to softtissue density indicates a severe mediastinitis with edema and inflammatory infiltrates. Abscesses present as well-defined fluid collections with CT attenuation up to 30HL] that mayor may not have an hyperattenuating rim and may contain gas collections. Aspiration may be needed to determine the infection. Pleural effusion is common. Sternal osteomyelitis is characterized by demineralization and permeative destruction of the bone (see Fig. 23.58). It is not necessarily accompanied by increased attenuation or abscess formation in the surrounding fat and soft tissues. Following a median sternotomy, these findings are almost always associated with a

sternal dehiscence of more than 2 mm. Destruction of the sternum by malignant neoplasms is often accompanied by soft -tissue masses (e.g., in multiple myeloma or metastases from lung, breast, prostate, or renal cell carcinoma). Air and fluid collections may be seen for up to 20 days postoperatively, and rarely for up to 50 days, without signifying an inflammatory process. If follow-up examinations show new or enlarging fluid or air collections, the presence of an inflammatory process is highly probable (Fig.lO.24b). Normal CT findings exclude mediastinitis. False-positive CT diagnoses may occur.

Fibrosing Mediastinitis

A chronic granulomatous or fibrosing (sclerosing) mediastinitis may result from an infectious disease (tuberculosis, histoplasmosis, fungal infections, syphilis), a mediastinal hematoma, radiation therapy, or drug therapy (methysergide). Fibrosing mediastinitis is often associated with other fibrotic processes such as pseudotumors of the orbit, Riedel's struma, or retroperitoneal fibrosis (Ormond's disease). •

CT Morphology

The clinical and radiological features of fibrosing mediastinitis vary according to the location and the extent of the fibrosis. Postinfectious forms are marked by a self-perpetuating medi-

Fig. 10.24 Acute mediastinitis with diffuse increase in attenuation of the mediastinal fat in a patient after neck surgery (a). Mediastinal air and haziness of the mediastinal fat in a patient after liver surgery

(b). a

b

10 Mediastinum, Pleura, and Chest Wall

astinal fibrosis that may cause very little stranding or increase in attenuation of the perivascular fatty tissues, and may be difficult to identify. In other cases, however, the fibrosis forms solid tumor-like masses or ring-like zones of increased attenuation around the mediastinal structures. Approximately 10% of cases show obstruction of the vena cava (Fig. 10.25) or the central tracheobronchial system. Extensive calcifications may occur. Mediastinal Lipomatosis

The excessive deposition of mediastinal fat may occur as a nonpathologic process in obese individuals and in patients on cortisone therapy. CT can easily establish the cause of the mediastinal widening that is detected on chest radiographs. •

Fig. 10.25 CT in fibrosing mediastinitis (Ormond's disease) shows an indistinct peripheral increase in paratracheal and paracaval density and narrowing of the superior vena cava (arrow) by fibrotic encasement, leading to a superior vena caval syndrome. Note the intense opacification of the azygos vein due to the collateral circulation (arrowheads).

CT Morphology

CT shows a diffuse increase in mediastinal fat that is most pronounced in the upper mediastinum. Inhomogeneities, as opposed to uniform fat density, indicate a different etiology

such as fibrosis, hemorrhage, acute or chronic inflammation, tumor invasion, or postoperative or postirradiation changes.

Diffuse Pleural Diseases Pleural Effusion, Hemothorax, Chylothorax

Pleural effusion may consist oftransudate or exudate, depending on the protein content of the fluid. Chylothorax refers to an effusion of lymphatic fluid, hemothorax to an effusion that contains blood. Pneumonic infiltrates that border the pleura incite an parapneumonic effusion that progresses from an exudative to a fibrinopurulent stage as it becomes organized. This type of effusion is on a continuum with pleural empyema. A tuberculous pleural effusion results from the perforation of a subpleural pulmonary focus into the pleural space and usually develops 3 to 6 months after the primary infection. Malignant effusions are also exudative and are not" always associated with pleural thickening. Thus needle aspiration is often indicated. Hemorrhagic effusions usually have a neoplastic or traumatic etiology. The most frequent causes

of Chylothorax are lymphomas (> 50%) and trauma (25%). Effusions are a common associated finding in thoracic CT examinations but are not a primary indication for CT. •

CT Morphology

Pleural effusions with a volume of 15 ml or more can be detected with CT. Pleuritis sicca is not visible on CT scans. Free pleural effusions occur in the posterobasal region and form a crescentshaped collection that abuts the chest wall. Effusions that are loculated or encapsulated by fibrous septa may occur anywhere within the pleural cavity (Fig. 10.26). Fibrous septa or fibrin threads cannot be visualized with CT. Interlobar effusions have a typical biconvex shape and can present as a pseudotumor on chest radiographs. An air-fluid level can result from a bronchopleural fistula or prior therapeutic measures (surgery, aspiration biopsy, drainage).

Pleural Empyema

A cbylothorax has a CT density of approximately 0 HU and is usually quite extensive. Transudates are homogeneous and have nearwater attenuation « 15HLJ). Neither the pleura nor the effusion enhance with intravenous contrast. Attenuation values higher than 20 HU indicate an exudative, hemorrhagic, or purulent effusion. The effusion may enhance on late postcontrast scans due to diffusion of contrast medium into the effusion. There are borderline cases in which transudate and exudate are indistinguishable byCT. Withafi brinopurulenteffusion, the thickened pleura shows contrast enhancement in the absence of empyema (reactive pleura! thickening). It is common at this stage to find septations and local encapsulated collections (Fig. 10.26). Increased density and widening of the subpleural fat stripe signifies a long-standing process. In tuberculosis, pleural calcifications and extensive areas of pleural fibrosis are a common finding (d. Fig.10.28a). High CT numbers (>30HU) and heterogeneous attenuation are consistent with hemotborax. Sedimentation effects are seen if the blood in the pleural space does not coagulate. Pleural effusions are very commonly associated with compression atelectasis of adjacent lung areas. Compressed lung tissue floating in a free effusion appears on CT as a tag-like structure (see Fig. 9.13), a small portion of which may be fixed to the chest wall by adhesions. Multiple chest wall adhesions lead to corresponding taglike deformities of the atelectatic lung due to fibrous bands and indicate a septated effusion. Repositioning the patient (lateral decubitus) can be hel pful in distinguishing between free and encapsulated effusions, although ultrasound is the technique of choice for this indication. With a free effusion, atelectatic portions of the lower lobe in the posterior diaphragmatic recess may be mistaken for the diaphragm because of their curved shape, and anterior portions of the effusion may be misinterpreted as subphrenic fluid. A pleural effusion lies posterior (superior) to the diaphragm and extends medially adjacent to the spine. Ascites lies anterior (inferior) to the diaphragm and does not extend medially.

Fig. 10.26 borders.

Loculated

pleural effusion

with scalloped

Pleural Empyema

Pleural empyema is on a continuum with parapneumonic exudate (fibrinopurulent effusion). Most empyemas are accompanied by bronchopneumonic infiltrates (mostly Pneumococcus, and Staphylococcus pyogenes). Tuberculous pleural effusions progress to pleural empyema in approximately 20% of cases. CT can detect areas suspicious for empyema and can distinguish pleural empyema from a peripheral lung abscess. •

CT Morphology

The transition from exudative effusions to pleural empyema can be appreciated on CT scans. Empyema is associated with more intense contrast enhancement and greater thickening of the visceral and parietal pleural layers than exudates (the "split pleura sign", Fig.10.27a). With increasing duration of the empyema, there is increased thickening of the extrapleural fat stripe. Diffuse pleural thickening and calcification may be the end result after healing of a pleural empyema. Air inclusions (bubbles) signify infection by gas- forming bacteria or rarely an esophagopleural fistula, while an air-fluid level results from a bronchopleural fistula or previous needle procedure. Criteria for differentiating between pleural empyema and a subpleural lung abscess (Fig. 10.27 b) are listed in Table 10.8.

399

10 Mediastinum, Pleura, and Chest Wall Fig. 10.27 (a) Pleural empyema with thickened, markedly enhancing visceral (arrow) and parietal pleura ("split pleura sign"), (b) Peripheral lung abscess with an airfluid level. Note the acute pleural angles (arrows) compared with the pleural process in (a), confirming the intrapulmonary origin of the lesion.

a

b

Table 10.8 Features that distinguish pleural empyema from lung abscess Empyema

Peripheral lung abscess

Oval or crescent shape

Round or oval shape

Obtuse angle with chest wall

Acute angle with chest wall

Consistently associated with pleural thickening

Not consistently associated with pleural thickening

No air in lesion wall

Air in lesion wall

Bronchopleural fistula

Communicates with bronchus

Compresses contiguous lung

Distorts contiguous lung

Changes shape with position changes

Does not change shape with position changes

Common features: central air inclusions (depending on pathogen), peripheral contrast enhancement

Pleural Fibrosis, Asbestos-Associated Changes

Asbestos-associated pleural changes (pleural plaques) may not appear until several decades after the initial asbestos exposure. Approximately 3% of asbestos-exposed individuals develop an exudative pleural effusion during the first 10 years after exposure. The effusion may remain asymptomatic or it may cause pain and persist for more than a year. Many patients subsequently develop rounded atelectasis and diffuse pleural fibrosis with restrictive impairment of pulmonary function. Pleural mesothelioma, bronchial carcinoma, and tuberculosis show an increased incidence in asbestosexposed individuals. •

CT Morphology

Pleural plaques most commonly occur on the subcostal pleura and may be 1 to 15mm thick. Thickening of the pleura may be diffuse al-

though more commonly there is focal plaque formation. Plaques typically appear as sharpedged elevations ("mesa" sign) and tend to form calcifications that start at the center of the plaque and are therefore separated from the ribs by an non calcified zone (Fig.10.28b). The plaques spare the lung apices and costophrenic angles. Visceral pleural involvement, particularly within the fissures, and mediastinal pleural involvement are uncommon, and should raise the suspicion of a mesothelioma. Plaques may be associated with fibrosis of the subpleural fat, subpleural pulmonary fibrosis, and the formation of parenchymal bands and adjacent nodular opacities compatible with areas of early rounded atelectasis ("crows' feet" or "rounded atelectasis equivalents").

Pneumothorax Fig. 10.28 Pleural calcifications. (a) Extensive flocculent calcifications in pleuritis calcarea following tuberculosis, (b) Typical asbestosrelated pleural plaque, appearing as an isolated elevation ("mesa" sign) with central calcification (arrow). Note also the posterior pleural thickening.

a

The attachments of the serratus and external oblique muscles, epipleural fat, and intercostal vessels should not be mistaken for pleural plaques.

b

The term asbestosis is reserved for the pulmonary parenchyma! fibrosis caused by asbestos exposure and should not be used for asbestos-related pleural abnormalities.

Trauma and Postoperative Changes

CT is indicated in chest trauma for the evaluation of suspected injury to mediastinal structures, particularly the great vessels. CT angiography is a sensitive, noninvasive procedure that can replace arterial angiography for the diagnosis of aortic rupture (see p.915). In a postoperative setting CT is a problem-solving tool.

•

CT Morphology

CT can detect pneumothorax with high sensitivity by delineating the visceral pleura from the air- filled pleural space. In patients with recurrent pneumothorax, CT can be used to search for causative subpleural bullae, which may be congenital or may form in the setting of obstructive or fibrosing lung diseases (Fig. 10.29).

Pneumothorax

The presence of a pneumothorax is rarely a primary indication for CT. CT is used to evaluate recurrent spontaneous pneumothorax or insufficient pleural drainage (faulty tube placement, frequently in major fissures or in the lung parenchyma). It is also useful for differentiating a pneumatocele or a large bulla from a pneumothorax. Identification of even a small pneumothorax can be important in trauma patients about to undergo mechanical ventilation or general anesthesia for surgery. Therefore it appears useful always to evaluate the most cephalad scans of an abdominal CT examination in a trauma patient in lung window settings to identify otherwise occult pneumothorax.

Fig. 10.29 Patient with spontaneous pneumothorax. A coronal volume-rendered image (4x 1/6) demonstrates small apical bullae due to paraseptal emphy-

401

10 Mediastinum, Pleura, and Chest Wall

Mediastinal Hematoma

Mediastinal hematomas may result from central venous puncture, chest trauma, the spontaneous rupture of an aortic aneurysm, aortic dissection, or a coagulation disorder. If aortic injury is suspected, CT angiography should be performed as an adjunct to non contrast CT (see Chapter 24). •

CT Morphology

A fresh mediastinal

hemorrhage initially produces streak-like densities in the mediastinal fat. Only larger hematomas contain areas of increased attenuation. The approximate location of the bleeding site can be inferred from the site of the hematoma. Contrast extravasation into a fresh hemorrhage is rare and is usually an indication for immediate surgery. Bleeding into the pleural space signifies injury to the lung (Fig. 10.30) or intercostal arteries or an open or confined rupture of the aorta (ascending aorta on the right side, aortic arch or descending aorta on the left side). A (hemorrhagic) pericardia! effusion poses a risk of pericardial tamponade, especially if CT reveals a concomitant dissection of the ascending aorta. Rupture of the brachiocephalic veins due to blunt chest trauma leads to hematoma formation in the upper anterior mediastinum. Traumatic aortic aneurysms occur in up to 95% at the level of the ligamentum arteriosum and are associated with left-sided hematomas in the posterior mediastinum. Older hematomas show decreasing attenuation on CT scans and may have soft-tissue or water attenuation, making them difficult to distinguish from a solid mediastinal tumor (hematomas enhance by less than WHU after intravenous contrast administration).

Fig. 10.30 Massive chest trauma with injury of the right lung and acute intrapleural hemorrhage demonstrated after contrast injection.

surgery, intubation, or endoscopy). Mediastinal emphysema may be an indirect sign that a neoplasm has eroded the esophagus or tracheobronchial tree. Retroperitoneal and intraperitoneal free air can also track into the mediastinum. Mediastinal emphysema is rarely a primary indication for CT scanning. •

CT Morphology

CT can sensitively detect even the smallest mediastinal air collections, but the cause can be determined only when the CT scans are interpreted within the context of clinical findings (Fig. 10.31).

Mediastinal Emphysema

Mediastinal emphysema, known also as pneumomediastinum, usually signifies a tracheobronchial injury or subpleural alveolar rupture with air tracking along the pulmonary interstitium and lung root into the mediastinum. Possible causes are a sudden intra-alveolar pressure rise due to strenuous physical effort or mechanical ventilation (Macklin effect), bronchial rupture, esophageal rupture (Boerhaave syndrome), or therapeutic procedures (prior

A Fig. 10.31 Pneumomediastinum due to iatrogenic injury of the left main bronchus during difficult intubation. Soft tissue emphysema after insertion of a pleural drain.

Diaphragmatic

Rupture

~ a

b

Fig. 10.32 Traumatic diaphragmatic rupture on the left side with herniation of stomach and colon into the thoracic cavity, accompanied by extensive right-sided

pulmonary contusions. Note the exposed stomach wall and pneumothorax on the left side (5/8/4).

Air within the pulmonary interstitium associated with a pneumomediastinum is a sign of alveolar rupture. A traumatic rupture of the trachea or large bronchi occurs within 2.5cm of the carina in 85% of patients and leads to mediastinal emphysema. With rupture of a bronchus distal to the insertion of the pulmonary ligament a pneumothorax will form as well. A rupture is presumed to exist if a large pneumothorax persists despite adequate drainage, if the lung is completely collapsed, or if the lung collapses toward the chest wall rather than toward the hilum ("fallen lung" sign). Direct CT visualization of the rupture is rarely achieved and usually requires spiral or multislice scanning with thin slices (see Chapter 9).

angle. Tears of the central portion of the diaphragm are most difficult to evaluate. Multislice CT strongly improves evaluation of the diaphragm. Reformatted images in the coronal and sagittal plane from spiral and mul tislice CT data sets are extremely helpful to establish the diagnosis (Fig. 10.32). Indirect signs of a diaphragmatic tear are an apparent elevation of the diaphragm with pleural effusion, deformation of the hepatic or splenic contour by herniating viscera, and an unusually thin border separating the stomach, small intestine, or colon from the adjacent lung (lack of diaphragmatic coverage). In addition, thickening of the posterior portions of the diaphragm on the side of the rupture has been reported. This sign, however, must be distinguished from a relative hypertrophy of the diaphragm on the contralateral side, which may be due to phrenic nerve palsy.

Diaphragmatic Rupture

Diaphragmatic rupture is commoner on the left side than on the right. Visceral herniation into the chest is rarely seen in acute cases. The radiological diagnosis of diaphragmatic rupture is a difficult challenge, regardless of the modality used. Even with multiple imaging modalities, a definitive diagnosis must await operation in approximately 50% of cases. •

CT Morphology

A diaphragmatic tear can be directly visualized only on thin CT slices that cut the posterior part of the diaphragm almost at a perpendicular

Cardiac Surgery

CT can be used in postoperative cardiac patients to investigate a new occurrence of mediastinal widening, confirm or exclude suspected postoperative hemorrhage, or search for a focus of infection. •

CT Morphology

During the initial weeks after surgery, it may be difficult to distinguish infection-related changes from changes due to surgical manipulation.

403

10 Mediastinum, Pleura, and Chest Wall

However, a hypoattenuating mass that shows marked rim enhancement after contrast administration is suspicious for an abscess. Presternal or retrosternal edema during this period has no pathologic significance. It is manifested by stranding and increased density of the fat in the middle and anterior mediastinum. Fluid collections may be serous (lD20 HU). bloody (40-80 HU), or of mixed composition. Mediastinal and pericardial air inclusions may occur initially along with a small pericardial effusion. Enlarged mediastinal lymph nodes are a common finding. The abeve changes gradually resolve during the first three or four postoperative weeks and rarely persist for up to 50 days. In patients who have undergone a sternotomy, the osteotomy site should be fully reunioned, although small discontinuities are normal. A gap up to 2 mm wide may still be considered normal, but the risk of sternal instability increases with the width of the residual gap (see Fig. 23.58). Aortic pseudoaneuIJsms are a rare late complication « 1 % at 1 to 2 years after surgery) resulting from dehiscence of the suture line due to an acute or chronic infection. Typical sites of occurrence are the site of an aortotomy (for connection to the heart-lung machine) or an anastomosis between the aorta and a bypass (see Fig. 24.48). Constrictive pericarditis is another rare late complication (0.2% at months to years after surgery) manifested on CT by 3 to 20mm of pericardial thickening with manifestations of right heart failure: caval dilatation, flattening of the right ventricle with curvature of the septum to the left, pleural effusion, and ascites. Pericardial calcifications are rare in postoperative cases but occur in 50% of cases with other etiology (see Fig.23.52b).

Heart Transplantation Acute or chronic rejection after heart transplantation requires biopsy confirmation and is not an indication for CT. In cases of infection, which are responsible for up to 40% of postoperative mortality in heart transplant recipients, CT is a sensitive modality for the detection and localization of the focus (see Chapter 9). The findings are nonspecific, however, and cannot identify

the causative organism (most frequent problem organisms: gram-negative bacteria, viruses, fungi, protozoans). Long-term complications include lymphoma, carcinoma, and coronary sclerosis. CT Morphology Scanning of anastomotic sites in the ascending aorta or right atrium may reveal a constriction or an abrupt change in caliber. The lumen of the inferior vena cava is frequently enlarged. Unusual separation may be noted between the superior vena cava of the recipient and the ascending aorta of the donor or between the recipient pulmonary artery and the donor aorta. Pericardial effusion is frequently encapsulated and may persist for a period of weeks or months. Tricuspid insufficiency is a post-transplant complication that can be suspected from CT if an extreme dilatation of the right atrium including the valvular ring is present (see Fig. 23.57). Steroid therapy can lead to mediastinal lipomatosis and spinal osteoporosis, which is frequently associated with vertebral body compression.

i i 1lit, Liver M. Prokop, A. J. van der Molen

Anatomy Segmental Anatomy (p. 408) Vascular anatomy and variants Liver Attenuation (p. 411) Examination

(p. 409)

Technique

Patient Preparation (p. 412) Aquisition Technique (p.412) Intravenous Contrast Injection (p. 416) Intra-arterial Contrast Injection Techniques Perfusion CT(p.420)

(p. 419)

Enhancement Characteristics Hepatic Arterial Phase (HAP) (p.422) Portal Venous Phase (with CT AP) (p. 423) Portal Venous Phase (after i.v. Contrast Administration) Interstitial Phase (p. 424) Hepatic Excretion Phase (p. 425) Vascular Opacification (p. 425) Cystic Liver Lesions

(p. 423)

...........................••.......

Developmental (Simple) Hepatic Cyst (p. 425) Polycystic Liver Disease (p. 427) Biliary Hamartomas (p. 427) Mesenchymal Hamartoma (p. 427) Cystic Metastases (p. 428) Benign Tumors and Tumor-like Lesions Incidentally Found Lesions (p. 428) Focal Liver Steatosis (p. 428) Hemangioma (p. 430) Focal Nodular Hyperplasia (FNH) (p. 432) Hepatocellular Adenoma (HCA) (p. 434) Lipoma (p.435) Angiomyolipoma (p. 435) Inflammatory Pseudotumor (p. 435) Infantile Hemangioendothelioma (IHE) (p. 436) Hepatic Alveolar Echinococcosis (HAE) (p. 436)

.

Malignant

Tumors

Metastases (p. 437) Hepatocellular Carcinoma (HCe) (p. 441) Fibrolamellar Carcinoma (FLe) (p. 446) Cholangiocellular Carcinoma (CCC) (p. 446) Hepatoblastoma (p. 448) Undifferentiated Embryonal Sarcoma (p. 448) Epithelioid Hemangioendothelioma (EHE) (p. 449) Angiosarcoma (p. 450) Other Mesenchymal Tumors (p. 450) Lymphoma (p. 450) Post-transplant Lymphoproliferative Disease (PTLD) (p. 452) Diffuse liver Disease Fatty Infiltration (p. 452) Cirrhosis (p. 453) Primary Biliary Cirrhosis (p. 455) Hemochromatosis (p. 455) Radiation injury (p. 456) Miscellaneous diffuse liver diseases (p. 456) Infectious

Disorders

Hepatitis (p. 457) Abscesses (p. 457) Hydatid Disease (p. 459) Liver Tuberculosis (p. 459) Rare Infectious Diseases (p. 460) Vascular Disorders and Perfusion Abnormalities Perfusion Abnormalities (p. 461) Portal Hypertension (p. 462) Portal Venous Obstruction (p. 463) Hepatic Venous Outflow Obstruction (p. 465) Hepatic Arterial Thrombosis and Hepatic Infarction Rendu-Osler-Weber Disease (p. 467) Peliosis Hepatis (p. 469)

(p. 467)

Trauma Blunt Liver Trauma (p. 469) Penetrating Abdominal Trauma

(p.471)

Postoperative and Post-therapeutic Changes Liver Resection (p. 471) Liver Transplantation (p. 472) Transjugular Intrahepatic Portosystemic Minimal Invasive Therapies (p. 473)

Shunt (TIPS) (p.473)

11

The use of scintigraphy. such as " "Tc-sulfur colloid or IDA, is rapidly declining as these have been replaced by multiphasic spiral CT or MRI. Sulfur colloid scans can be used for focal fatty infiltration, regenerative nodules, and in characterizing primary tumors like focal nodular hyperplasia. Tc-red blood cell scan with SPECT is still a good technique for hemangioma. New techniques like 'In-octreotide imaging may have a niche in the work-up of neuroendocrine tumors, especially gastrinomas, or carcinoid tumors. The role for PET in clinical oncology is also increasing steadily, but the applications in the liver are still scarce. Image fusion of SPECT and PET data with CT data sets is still quite cumbersome but there are combined CTPET or CT-SPECT scanners-some even with multislice technology-becoming available that elegantly solve the problem of co-registration of data sets. While these scanners can provide excellent results, but still suffer from their substantial costs.

Computed tomography is a standard technique for imaging of the liver. It is mainly used for the evaluation of tumors but offers useful information also for a vast range of other diseases (Table 11.1). Ultrasound offers an economical. noninvasive alternative formany indications. but further evaluation by CT or MRI is frequently required in patients prior to surgery. Contrast-enhanced ultrasound may become a competitor in the diagnosis of focal lesions. but this technique is still difficult to perform for coverage of the entire liver. As compared to MRI. CT offers a consistently good image quality and requires only minimal patient cooperation. Complex invasive procedures such as CT hepatic arteriography (CTHA) or CT during arterial portography (CTAP) had been justified only if they had essential therapeutic implications (assessment of resect ability). At present. they are substituted for by biphasic multislice CT and dynamic Cd-enhanced MRI or MRI with liver-specific contrast media.

Table 11.1 Tumor

Indications

for CT examination

diagnosis:

of the

Indeterminate

liver

hapatic

lesion:

Differentiation lesion

Primary hepatic tumors: Hepatic

metastases:

Complications

" After surgery:

(hematoma,

resectability

Detection,

resectability

bilioma,

abscess)

Portal vein thrombosis Arterial anatomy Osiers disease

Other:

Quantitative

(before

intervention/surgery)

Abscess Trauma measurements:

Hemosiderosis (iron content) Liver and tumor volume

benign-malignant,

characterization

Detection,

Malperfusion Vascular:

The Liver

Anatomy

With a volume of 1400 to 1700ml, the liver is the largest parenchymal organ in the body. On average, it measures 13.5cm in its largest craniocauclal dimension. The size of a particular liver varies with the height and gender of the patient. Hepatomegaly is present in >80% of patients when the liver exceeds 15.5cm in its craniocaudal extent, or the volume of the liver is 2000 ml or more. The normal size ratio between the right and left hepatic lobes is 3 :2. •

Segmental

Anatomy

Knowledge of hepatic segmental anatomy is helpful to communicate CT findings to the surgeon. However, numerous approaches use the same names for different structures. The numbering system for the segmental anatomy is identical for most of these approaches. All approaches are only approximations of reality because there are marked variations in the individual anatomy of almost every patient. The liver is divided into four segments by the planes defined by the inferior vena cava (lVC) and the right, middle, and left liver veins. These define the anterior and posterior segment of the right hepatic lobe, and the medial and lateral segment of the left hepatic lobe. The right and left lobe are separated by the plane defined by the IVC, the middle liver vein, and the gallbladder. These vertically oriented segments are divided into subsegments by the horizontally oriented portal vein. In the most usual surgical classification (Bismuth) in North America, the portal venous Table 11.2

Segmental classification

Anatomic subsegment

Caudate lobe Left lateral superior subsegment Left lateral inferior subsegment Left medial subsegment Right Right Right Right

anterior inferior subsegment anterior superior subsegment posterior inferior subsegment posterior superior subsegment

Couninaud's

supply of the liver defines eight subsegments of the liver. The boundaries of these segments are grossly defined by the hepatic veins (in the craniocaudal direction) and by the plane of the portal vein (horizontal on the right, oblique on the left) (Fig. 11.1). Other numbering systems, such as the one by Couinaud, are more common in Europe. Table 11.2 gives an overview of the most important terminologies. Segment 1is the caudate lobe. Subsegments 11 and III are the portions of the left lobe that are 10-

Fig. 11.1 Bismuth classification of hepatic segments. The left, middle, and right hepatic veins divide the liver into four segments (Couinaud: sectors), which are further subdivided into cranial and caudal subsegments (Couinaud: segments) by the plane of the portal vein. Viewed from the front, the numbering of the (sub)segments proceeds in a clockwise direction (except for segment I and subsegment IVa).

of the liver (Soyer, 1993) Nomenclature Couinaud,

Bismuth,

Goldsmith and Woodburne,

1957

1982

1957

IV V VIII

III IV alb V VIII

Caudate lobe Left lateral segment Left lateral segment Left medial segment Right anterior segment Right anterior segment

VI VII

VI VII

Right posterior segment Right posterior segment

hemiliver, sector, and segment correspond to Bismuth's lobe, segment, and subsegment

Anatomy

cated to the left of the falciform ligament. Segment IV is the medial portion of the left hepatic lobe (the" quadrate lobe ") that is located between the falciform ligament and the vena cava-gallbladder line/middle hepatic vein. In the Bismuth classification segment IV is subdivided into subsegments IVa cranially and IVb caudally. The right hepatic lobe is divided into subsegments V through VlI 1, with a clockwise numbering system starting from the subsegment V inferiomedially next to the gallbladder (Figs. "11.1, 11.2). Common anatomic variants include a hypoplasia of segment IV b and a large left hepatic lobe that fills the left dome of the diaphragm. Complex congenital heart diseases may be associated with a symmetrical, centralized liver

(Fig. 11.3). A left-sided liver is part ofa complete or partial situs inversus. •

Vascular Anatomy and Variants

The common hepatic artery normally originates from the celiac trunk. After giving rise to the gastroduodenal artery behind the pylorus, it becomes the proper hepatic artery that passes through the hepatoduodenal ligament anteromedial to the portal vein and anterior to the common bile duct. In the liver hilum it bifurcates into the right (RHA) and left hepatic arteries (LHA). The middle hepatic artery that supplies segment IV originates with equal frequency from the right or left hepatic artery.

VII~ VII./.l

a

b

(

d

Fig. 11.2 Location of the hepatic segments on CT sections. In scans through the cranial portion of the liver, the segmental boundaries are defined by the vena cava and the main branches of the hepatic veins. At more caudal levels the cava-gallbladder line and the falciform

ligament define the central and left segmental planes. Often the plane of the portal vein can only be estimated. In case of doubt coronal reformations are helpful.

409

11 The Liver

patic ligament and reaches the left lobe via the fissure of the ligamentum venosum (>20% of all patients). Axial CT sections demonstrate such a vessel above the liver hil urn and posterior to the left lobe (Fig. T1.4a). Accessory and replaced arteries can be distinguished by searching for a normal left hepatic artery originating from the proper hepatic artery at the liver hil urn. Variations of the right hepatic artery are almost as common. They all present as an artery in the hepatoduodenal ligament posterior to the portal vein (Fig.l1.4b). By tracing this artery back to its origin, one can distinguish between a RHA arising from the aorta, the superior mesenFig. 11.3 Symmetric liver as an associated anomaly in a teric artery, or one directly from the celiac patient with Kartagener syndrome (immotile cilia synartery. By following it in both directions, it is drome) and incomplete situs inversus. Also note the just as easy to distinguish between an accessory right-sided location of the stomach and an aplasia of the or replaced artery, and to determine whether it intrahepatic segment of the inferior vena cava with azygos continuation. is just an aberrant RHA, or a common hepatic artery, or even the celiac artery that is displayed. Apart from the hepatic artery, there can be Variations in the arterial blood supply of other sources of arterial supply to the liver, which of periphthe liver occur in some 45% of individuals and are important in the vascularization are important for planning surgical or radiologieral hypervascular tumors (parasitic supply) and cal interventions. Various classifications have when the hepatic artery is occluded. The most been proposed. The one by Michels is most important is the right inferior phrenic artery, popular but is not very precise. Therefore, it is but also the left inferior phrenic, internal mammary, left gastric, or intercostal arteries may more advisable to properly describe variations give offbranches to the liver surface (Fig. "11.5). than to try to classify them. Aberrant arteries The main portal vein is formed by the concan either be a replaced (variation in origin) or accessory (additional vessel). Variants that in- fluence of the superior mesenteric vein and the splenic vein and runs posteriorly in the hepavolve more than one artery are common. The commonest variations involve an aber- toduodenal ligament. At the porta hepatis it branches in the right (RPV) and left portal veins rant left hepatic artery that runs in the gastrohe-

Fig. 11.4 The following patterns suggest an aberrant hepatic vascular supply, (a) An aberrant left hepatic artery (almost always from the left gastric artery) is present if there is artery entering the liver above the hilum and anterior to the caudate lobe. (b) An aberrant

right hepatic artery (most commonly from the superior mesenteric artery) is present ifthere is an artery supplying the liver posterior to the portal vein. Further analysis is required to determine the origin of the vessels and to distinguish accessory from displaced arteries.

Anatomy Fig. 11.5 Aberrant vascular supply to the liver in a patient with occlusion of the hepatic artery after embolization of a metastatic gastrinoma. Note the arterial revascularization (arrows) of the thrombosed of the portal vein (a) and collateralization (arrowheads) via phrenic artery branches (b).

a

(LPV). Most commonly, the RPV further divides in anterior and posterior segmental branches, while the LPV divides in superior and inferior segmental branches. Anatomical variants of the MPV (trifurcation, posterior from MPV, anterior from LPV) occur in up to 20% of patients, and variants of segmental branches are present in almost every individual. The anterior branch of the RPV is more constant than the posterior branch. The course of the main LPV almost always remains constant. There are usually three hepatic veins that course in the vertical fissures that separate the liver segments. The right hepatic vein (RHV) drains segments VI -VIII into the IVC. The middle hepatic vein (MHV) drains segments V and IV B, while the left hepatic vein (LHV) drains segments II -IV A. The MHV and LHV often form a common trunk before draining into the IVC. The caudate lobe (segment I) drains separately into the IVC via small veins at a more inferior level. This is the reason why it often hypertrophies in Budd-Chiari syndrome (occlusion of the large hepatic veins). There are numerous variations in the drainage pattern of the several segments to the hepatic veins. Other common variants include accessory right, middle, or left hepatic veins, and an accessory inferior RHV that may be important for atypical liver surgery because it separately drains segments V and VI. There are m ul tiple branching patterns ofthe LHV. •

b

liver attenuation decreases approximately 15 HU for each 10% increase in the proportion of fat. When the fat content reaches 10-15%, the intrahepatic vessels become isoattenuating to the parenchyma and cannot be seen on unenhanced scans (Fig.1l.6b). A further increase in fat content reverses the display of vascular structures, and the intrahepatic vessels appear hyperattenuating to the liver tissue on noncontrast scans (Fig. 11.6a) . In patients with hemochromatosis or hemosiderosis, liver attenuation increases in proportion to the iron load (Fig.11.6d). However, this attenuation increase depends strongly on the effective energy of the x-ray beam and varies with the tube voltage, scanner type, and patient diameter. A concomitant fatty infiltration of the liver may yield a falsely low CT attenuation. Table 11.3 lists other, rarer causes of increased liver attenuation. Table 11.3

of

increased

liver

attenuation

Iron:

Primary hemochromatosis Transfusion hemosiderosis Nutritional hemosiderosis

Copper:

Wilson's disease

Iodine:

Amiodarone

Gold:

Therapeutic thritis

Thorotrast:

Thorium-containing contrast medium (diffuse reticular distribution pattern)

Thallium:

Accidental

Liver Attenuation

Normal liver parenchyma has a CT attenuation of 55 to 65HU (Fig.H.Gc). This slightly higher attenuation as compared to other soft tissues is thought to be due to the glycogen content of the liver. When diffuse fatty infiltration is present,

Causes

(>70HU)

Arsenic: Acute massive

(antiarrhythmic

drug)

use in rheumatoid

or suicidal ingestion

Chronic poisoning protein storage

Glycogen storage diseases

ar-

411

77 The Liver

c

b

a

Fig. 11.6 CT densityofthe hepatic parenchyma: normal (c), hemochromatosis (d).

Examination

d

fatty liver (a, with hepatic metastases),

mild fatty infiltration (b),

Technique

CT scanning of the liver is usually a part of an upper abdominal or complete abdominal examination. With few exceptions (exclusion of hemorrhage, hemochromatosis), every CT of the liver should use intravenous contrast material (Table 11.4). A power injector is required. Table I1.S lists the scanning techniques that we recommend for specific clinical problems. Biphasic multislice CT and various MRI techniques increasingly substitute for procedures that require intra-arterial contrast application such as CT hepatic arteriography (CTHA) or CT during arterial portography (CTAP). Patient Preparation

Except in dire emergencies or CTAP studies, oral contrast medium should be administered. For upper abdominal studies, SOO-600ml of oral contrast medium given 30 minutes prior to the scan is sufficient, while whole abdominal examinations require administration of 1000IS00ml of oral contrast medium over 60-90 minutes before the examination. In trauma cases, SOOml given via nasogastric tube is beneficial, and extra 2S0ml portions may be added if the waiting time for CT is prolonged. Positive contrast material is still standard, but negative contrast material such as water or methylcellulose preparations are becoming in-

creasingly important for arterial phase liver examinations as part of a biphasic or multiphasic upper abdominal study, especially when multislice scanning is employed. Such negative contrast has the advantage that there is no superimposition of bowel and vessels for CTA, and there is a better display of the bowel walls. Acquisition Technique

Spiral scanning is the technique of choice for liver examinations, independent of the mode of contrast material application (Tables 11.4, 11.S). Spiral CT is the prerequisite for scanning the whole liver within the arterial phase of contrast enhancement, a phase that was inaccessible with older techniques such as dynamic incremental CT. Trade-offs and Parameter Optimization

Lesion detection in the liver depends on signalto-noise ratios and thus on the size of the lesion, its contrast, the amount of partial volume averaging, and the image noise. Large lesions usually do not pose a problem as long as there is an attenuation difference to normal liver parenchyma. Small lesions, however, suffer from partial volume effects that may further reduce

Acquisition Technique Table 11.4

Suggested parameters for examination

413

of the liver

General Oral contrast

Liver only: 0,51, administered 30 min prior to examination Abdomen: 1-1.51, administered 60 min prior to examination

Patient position

Supine with arms elevated

Scan range

Liver only: from diaphragm to inferior hepatic border Abdomen: from diaphragm to symphysis

Breath hold phase

Inspiration (Spiral-CT)

Windowing

Noncontrast or delayed CT: After I. v. contrast: CTHA, CTAP: Lipiodol-CT:

W/L=200/40 W/L=350/60 W/L= 500/100 (adjust as needed) W/L=300/40

Scan parameters

1-slice SCfTF/RI

Standard, precontrast

7/12/

Volumetric imaging

5/8/4

Perfusion CT

7-10/0

€ontrast injection

l-slice V/F/D

4-slice V/Ii/Ii)

16-slice 'if./F/f!)d

Abdominal screening

120/2/60

120 + 50/3/60

120 + 5012/205

Portal venous CT

150/4/70

150 + 50/5/60

150 + 50/5/205

Biphasic CT

150/4/25 + 70'

150 + 50/5/25 + 70'

150 + 50/5/10Aa 80 c.

Early delayed phase

3-5 min

3-5min

3-5 min

Late delayed phase

10-15 min

10-15 min

10-15min

Perfusion CT

50/5/10

50 + 50/5/10

50+ 50/5/10

CTHA

50/2/5 La.9

50/2/5 La.9

CTAP

120/2/30 La. 9

120/2/30 + 60

Lipiodol CT

5 ml Lipiodol i.a. h

5 ml Lipiodol i.a. h

Portal venous CT

150/4/70

150 + 50/5/60

4-slice' SC

t i

2-3

16-slice' SC

i

1-1.25

1-1.5

t

i

5-7.5/4-5

0.75-1.25

t

3/3 cor'

4/3

16 x 1.25/2/0'

4 x 5/0'

10-20/-

II f

50/2/15 i.a. 9 120/2/30 + 60 9

9

5 ml Lipiodol i.a. h 150 + 50/5/205

sc = slice collimation (mm), TF= table feed (rnrnjrotanon). RI = recon. increment (mm), T ~ scanning direction, width (mm), MPR = multiplanar reformats. axial = axial sections, cor coronal, sag = sagittal eM = contrast material, Contrast concentration = 300 mg/ml iodine (optional 400 mg/ml for 16-slice scanning) V= volume of eM (ml), N =volume of saline flush (ml), F" flow rate (mils). D = start delay, X = trigger region a Pitch P = $Cf(NxTF) should be chosen close to 1.5 with a-slice scanners. 1.3- 1.5 with 16-slice scanners b MPR are reconstructed from a "secondary raw data set" with SW/RI"1-1.5/0.7 { single section (perfusion) mode with no table increment, scans every 2-5 s dread: 205 (lOA) '" start delay 20 s (lOs) after reaching 50 HU enhancement in the spleen (desc. aorta) e read: biphasic scan (arterial + portal phase) after 25 s (lOA) for the pi for the scan, 70-80s for the 2"d scan =::

SW= effective section

=::

f 9

h

when 400 rnqfml is used for 16-slice scanning, the contrast volume can be reduced to 120 ml injection into proper heaptic artery (CTHA) or superior mesenteric/splenic artery (crAP); prior injection of vasodilator (optional); reduced iodine concentration of 150 mqjml (optional) examination performed approximately 14 days after Upiodol injection inot the proper heaptic artery

their contrast to surrounding low contrast will make such a ceptible to image noise: lesions noise and no longer be visible.

parenchyma. A lesion more susmay "drown" in For this reason,

there are three ways of improving lesion detection (and possibly also characterization): reduce partial volume effects, improve contrast. and limit image noise.

414

11 The Liver Table 11.5

Which CTtechnique

Screening for metastases hypovascular hypervascular Prior to resection Lesion of unknown

dignity

for which clinical indication? Phase after i. v. contrast

injection

i.a. contrast

application

Precontrast

arterial

portal

delayed

GAP

lipiodol

(+)

++

++ ++ ++

(+)

++

++

+

(+)

++

++

+

Suspected

HCC

(+)

++

++

+

Suspected

CCC

(+)

(+)

++

+

Post transplantation

(+)

++

++

Trauma

+

(+)

++

The recommended techniques are printed in blue. The arterial phase is always combined with the portal venous phase (biphasic well suited + ~ may provide additional (+) ~ may provide additional - ~ not recommended

G

++

spiral CT).

++ ~

information information

in selected

cases

Reduction oj partial volwlle averaging relies on the choice of a narrow section width and overlapping image reconstruction. Overlapping reconstruction increases the number of images to be viewed. documented. networked. and archived. There are two other important tradeoffs: thin-section scanning reduces the coverage per time. and it increases image noise substantially. The longer time that is required to cover the liver may become problematic for arterial phase imaging. Because there is only a narrow window for purely arterial phase imaging (see below). there will be varying degrees of overlap between the arterial and the portal phase of contrast enhancement during a single spiral acquisition. Timing. therefore. becomes crucial. In addition. the longer scan duration with thinsection imaging will increase the likelihood of breathing artifacts during the scan. Thinner sections also increase image noise. A reduction of section width by a factor of four from Srnrn to 2 mill will lead to a two-fold ( 4) increase in image noise. Lesion detection therefore becomes dose-dependent. To take full advantage of the increased lesion detection with thin sections. one has to substantially increase patient dose or use other means of noise reduction. This is underlined by a recent study that shows that evaluation of 1 mm mulrislice CT images no longer improves lesion detection but actually reduces it as compared to 2-4mm sections at a dose of 15mGy (CTDlvol). At a dose of

20 mGy. another study shows that the number of detected lesions can be substantially increased with 2.5 mrn section width as compared to 5 rnm or more. Characterization becomes a problem with small lesions. Here. the thinner section width is better because it improves morphologic evaluation. Delayed scans help differentiate between small cysts and tumors. Still. a substantial number of lesions of less than 8 mm diameter will remain undiscovered. It has been shown that the vast majority of such small lesions are benign. even in tumor patients. At the same time. the larger lesions seen in a patient with metastatic disease are only the tip of the iceberg. Depending on the primary. 1.6 to 4 times more metastases smaller than 1 ern have been shown to be present for every lesion> 1 ern. There is no consensus yet as to which section width and radiation dose is appropriate for which indication. Given the results mentioned above. however. we suggest reconstructing a moderate section width (S-7.5mm) at a reduced patient dose (CTDlvol= 6-10 mGy) for benign diseases or indeterminate lesions. a moderate section width (S-7.5mm) and moderate patient dose (8-12mGy) for known malignancies. and a narrow section width (2-4mm) and high dose (12-20 mGy) only for those patients in whom major surgery is planned. Dependent on body diameter in more obese patients. the emlvol has to be increased by a fac-

Acquisition

tor of 2-4 (see Chapter 5). In the near future. automated exposure control will become available. which will make it much easier to individualize the required patient dose. In addition. contrast material application should be optimized so that high liver-to-lesion contrast is guaranteed.

Single-Slice

CT

The main limiting factor for single-slice CT is coverage. To optimize coverage and section width. a pitch factor> 1.5 should be used. For most situations. 5 mm collirnation and a table feed of 8 mm (5/8/4 protocol) is a good solution. For improved scanning speed in the arterial phase. the table feed can be increased to 10 mm. If the spatial resolution has to be optimized, a 3/512 scanning protocol should be considered if a subsecond scanner is available. A table feed of 6mm is also possible but image quality of 3D reconstructions is visibly lower. Such a protocol can be important for planning surgery, but the protocol suffers from increased noise and therefore cannot be recommended for obese patients. In very obese individuals, the section collirnation may even be increased to 7 mm to obtain a s ufficie nt signal-to-noise ratio. Overlapping image reconstruction is necessary to take full advantage ofthe spiral CTacquisition. The number oflesions increases substantially when images are reconstructed in an overlapping fashion. Ideally, there should be 50-60% overlap with respect to the effective section thickness (section width) but in clinical practice, a somewhat lower overlap may suffice.

Multislice

CT

Multislice CT has similar trade-offs as singleslice spiral CT. It can be optimized either to near-isotropic resolution or to more homogeneous liver enhancement. Unless extremely rapid scanning of the liver is necessary « 5 s), a 5mm collirnation should not be used, even with 4-slice scanners This gives the user the choice between a 0.75-1.5 mm collirnation for nearisotropic resolution and a 2.5-4 mm collirnation for rapid scanning. For optimum speed, a pitch P > 1.35 should be used with either technique. A test bolus injection or bolus triggering becomes

Technique

mandatory with contrast injection to avoid missing the optimum perfusion phase. H

With

4-slice Scanners

a narrow collirnation of 1-1.25 mm {volumetric imaging}, multislice CT provides excellent multi planar reformations that are especially helpful before hepatic surgery (e.g., Fig. 11.19). This technique allows for excellent evaluation of the arterial and portal venous anatomy and the relationship of tumors to the liver hilum, the hepatic vessels, and the bile ducts. lt also yields the bestCTangiographic results for the arterial as well as the portal venous supply. However, contrast enhancement of the liver and vascular structures changes over time during arterial phase scanning; the most cranial sections will represent a relatively early arterial phase while the caudal sections will represent a late arterial/early portal venous phase. This problem is less pronounced in portal venous imaging because the portal phase lasts for a longer period. With a collirnation of 4x2.5 mm or even 4x5 mm (fast spiral scanning), the liver can be scanned in some 8-12 s or 4-6 s, respectively. This makes it possible to perform scans in an early and late hepatic arterial phase (see below). Contrast enhancement of the liver and vessels is more homogenous but spatial resolution of multiplanar reformations is lower. In patients with hepatomegaly, higher pitch factors (P" up to 8) or thicker collirnation (e.g., 4x3.75 mm) may be used to cover the liver in a sufficiently short time. As mentioned above, we suggest reconstructing a moderate section width (5-7.5 mm) for benign diseases, indeterminate lesions, or follow-up of known malignancies, and a narrow section width (2-4 mm) for patients before major surgery. CE scanners at present are limited as to which section widths can be reconstructed for a specific collirnation setting. This may require the use of 4x2.5 mm collirnation for the first group of indications if one wants to reconstruct axial 5mm sections (maximum section width with 4xl.25 mm collirnation is 2.5mm). Alternatively, a 4x1.25 mm protocol can be used and 5mm thick MPR can be reformatted from the resulting secondary raw data set of overlapping thin axial sections. Other scanners can use the thin-section protocol and reconstruct images of arbitrary section width (even 7 mm or above).

11 The Liver

In obese patients, thin-section imaging will lead to suboptimum results because of excessive noise. Such patients should be scanned using at least a 4x2.5mm protocol (4x3 mm or 4 x 3.75 mm, depending on the scanner type, will be better) and a section width of7-7.5 mm. In very obese patients, the collimation and section width may be increased even more. 8- and 16-slice Scanners

Newer multislice scanners with 8 or 16 detector rows allow for covering the whole upper abdomen in less than lOs, even with 1-1.5 mm section collimation. This makes it possible to use such a thin collimation and a pitch P >-1.35 as the standard for liver imaging. Thinner collimation (0.5-0.75 mm) is rarely necessary for the liver unless a detailed analysis of the vasculature or biliary system is required. With such thin collimation, higher pitch factors (P , 1.5) should be used. Like with 4-slice scanners, the reconstructed section width (either from the original raw data or an secondary raw data set of overlapping thin sections) depends on the clinical indication, the exposure dose and the size of the patient. Thick coronal and sagittal MPR (3-5 mm Width). especially when aligned parallel and perpendicular to the portal vein, are helpful for the evaluation of the porta hepatis and for the planning of partial liver resection.

Intravenous Contrast Injection While non-contrast CT is easily performed even on non-spiral CT scanners, contrast-enhanced CT of the whole liver was limited to scanning in the portal venous or later phases because the arterial phase of enhancement was too transient for capturing by conventional scanners. Spiral scanning allowed for rapid acquisition of the data volume within one breath hold, but arterial phase imaging has become a standard technique for the liver only with the possibility of quick repeat scanning of the same volume (biphasic CT). With multislice scanners, even more rapid data acquisition becomes possible and an early and late arterial phase of enhancement can be captured separately. Timing of contrast application is a crucial factor for optimum results.

Noncontrast

CT

Noncontrast CT has limited indications in modern practice. It is generally indispensable for evaluating suspected hemorrhage in trauma and is the best phase for documenting confluent fibrosis in cirrhosis, calcifications, and hemochromatosis. Most often however, it is used in the evaluation of hyper vascular liver tumors as part ofa multiphasic approach. Some of these tumors can best be detected on non contrast CT. We especially recommend them for patients with carcinoid or other endocrine tumors because lesion size can be assessed to better advantage. For hypovascular lesions, such as metastases of colorectal cancer, noncontrast CT is not required. Because of the low contrast of many liver lesions, noncontrast CT requires sufficiently high mAs settings depending on the size of the patient (CTOIvol 'IOmGy) and narrow ("liver") window settings for image documentation (150250/40). Blood vessels and lesions generally appear hypoattenuating compared to normal liver parenchyma. If differentiation between blood vessels and liver parenchyma is not possible because of slight fatty infiltration, noncontrast CT is not helpful for identifying lesions. Portal

Phase CT

For most indications in which the liver is just part ofa general work-up, scanning in the portal venous phase is considered sufficient. This is also the case in patients with known malignancies that cause hypovasculcir metastases, such as colorectal cancer. In a pre-surgical work-up, however, biphasic spiral scans should be preferred. For spiral CT in the portal venous phase, 100 to 200ml of contrast medium is administered by power injector (at 3-5 mlls). For optimum lesion-to-liver contrast, liver enhancement should be as high as possible. Peak enhancement increases with larger volumes of contrast material. Best results require injection of 150 ml or more, but there is a strong dependence on patient weight. At present, there is a trend to individualize contrast material injection, with 1.5-2 ml contrast material per kg body weight being a good compromise. There is some contradictory evidence in the literature as to whether flow rates influence he-

Intravenous Contrast Injection

patic parenchymal enhancement. Work by Chambers et a1. (1994) suggests that using injection rates of 3 mlls lead to an earlier and higher hepatic enhancement than rates of 2 mlls. Model calculations by Bae et a1. (1998) could show that peak enhancement is almost independent of flow rates and only depends on the total amount of iodine injected. As a consequence, the volume of contrast material probably cannot be reduced, even for 16-row multislice CT scanning. The duration of the phase of optimum liver enhancement increases with a biphasic injection protocol. The optimum parameters for a biphasic injection vary from patient to patient (because of different cardiac output and variations in splanchnic perfusion) but a combination of 50 ml contrast at 5 mlls, followed by 100mI at 3 mlls yields good results. With rapid scanning using either spiral or multislice technique, however, there is little advantage to such a protocol. As a rule of the thumb, a relative plateau of parenchymal enhancement is reached some 20 seconds after the end of contrast injection (Fig. 11.7), with peak enhancement occurring about lOs later. This enhancement plateau lasts for more than 30s although parenchymal enhancement may vary by up to 15HLJ during this phase. Thus, scanning is initiated about 50 to 100 seconds after the start of the injection depending on the volume and flow rate of the contrast medium. Because there may be substantial variations between patients, results are superior

Enhancement(H

E)

3001-

250

Aorta

200 150

Portal vein ~

100

~Live~veins_

50 a

------~ _ Liver parenchyma

30

40

50

60

70

80

90 Time (5)

Fig. 11.7 Schematic representation of vascular and hepatic enhancement after injection of 150 ml of contrast material at 4 mils.

when the scan delay is tailored to the individual circulation time (bolus triggering). Bolus triggering in the portal venous phase may either use a region of interest in the liver, spleen, or aorta, or rely on enhancement of the liver veins as a visual clue. Portal venous triggering is not always easy because the maximum enhancement of the liver parenchyma may vary strongly between patients, and there are variations in the time delay between start of aortic and optimum portal venous enhancement. Some authors have suggested using an ROI in the liver (enhancement of 50 HU as a trigger level) for starting the scan but more than 30% of patients may never reach such a level. We also suggest a trigger level of 50 HU enhancement, but use either the spleen (15-20 s delay for the portal venous phase, 30-40 s for the hepatic venous phase, see below) or the aorta (40-45 s for the portal venous phase, 55-65 s for the hepatic venous phase). For longer scan duration (e.g., with singleslice CT), we recommend scanning in the caudocranial direction because it delays imaging of the hepatic veins until they are more fully opacified. •

Biphasic Spiral CT

Biphasic spiral CT includes scanning in the arterial phase of contrast enhancement in addition to the portal venous phase. A biphasic examination is the standard technique for planning partial liver resections or whenever hypervascular lesions are suspected (see Tables 11.5, Table 11.6). Some authors advocate additional noncontrast scans in patients with hypervascular metastases but the number of additionally detected lesions is generally small « 5%). Biphasic CT is often sufficient to characterize focal liver lesions (see Figs. 11.12, 11.14-11.17),but a third phase after contrast injection increases the confidence oflesion characterization. Biphasic spiral CT uses similar scanning parameters to monophasic examination, except that scans are performed during both the arterial phase (start delay about 25 seconds) and the portal phase of enhancement (start delay about 60-90 seconds), in each case using a short scan duration. High flow rates improve the conspicuity of hypervascular lesions and good results are obtained with uniphasic flow rates of 4-5 mlls, although flow rates as high as 8 mlls have been advocated and yield excellent

11 The Liver

lesion-to-liver contrast (see Fig. 11.30). A secure venous access route is necessary for such high flow rates to avoid extravasation. Biphasic contrast injection protocols with a flow rate of 5mlls over 10-15 s and a lower flow rate of 3mlls over 20-25 s improve both arterial enhancement and the duration and homogeneity of the portal venous phase. High dose techniques using 150-200ml of a 300-370 mg IIml contrast agent injected at 5mlls have shown very promising results. comparable with spiral CTAP in detection of liver lesions. but with improved characterization. Bolus triggering further optimizes contrast enhancement of hepatic lesions. The trigger level chosen should not be at the top of the scan range because diaphragmatic movements may cause considerable artifacts in the aorta. The aorta at the level of the liver hilum is more suitable for triggering the scans. Arterial phase scans should start 5-10 s (5s for 20s scan duration. lOs for < lOs scan duration) after the enhancement in the trigger ROl exceeds 50HLJ above baseline. while portal phase scans are initiated a fixed time after the arterial phase has finished. Ideally. the inters can delay should be adapted to the duration of the arterial phase scan. We suggest aortic triggering with a 40-45 s delay for the portal venous phase and a 55-65 s delay for the hepatic venous phase (relative to the aortic trigger level). Triphasic Spiral CT

The term "triphasic spiral CT' is used inconsistently in the literature. It may indicate a noncontrast scan followed by a biphasic CT, or a true triphasic CT with three distinct enhancement phases. In this book. only the latter definition is applied. With Single-slice spiral CT, a triphasic examination consists of a scan in the hepatic arterial phase. portal venous phase. and a scan in the interstitial phase after contrast administration. Such a triphasic examination is performed identically to a biphasic CT but adds a third scan afterward with a start delay of either 3-5 minutes (early delayed. vascular equilibrium phase) or 10-15 minutes (late delayed. parenchymal equilibrium phase). A triphasic CT is helpful for the characterization of known hepatic lesions (hemangiomas. HCC. cholangiocarcinomas) and can improve the detection rate of hepatocellular carcinomas, especially in cir-

rhotic patients. Hemangiomas tend to demonstrate a blood-pool effect with an enhancement of the perfused portions of the tumor that is similar to the vascular space (better seen on early delayed scans). HCC may demonstrate a capsule, or may appear as a hypoattenuating lesion (better on late delayed scans). Peripheral cholangiocarcinomas may only be seen on delayed scans as a hyperattenuating lesion spreading parallel to the portal branches. Cysts remain hypoattenuating in all scan phases. An early delayed scan 3-5 minutes after contrast injection helps to identify hemangiomas and small cysts. • A late delayed scan after 10-15 minutes helps to detect small cysts and tumors with a large fibrotic component (e.g., cholangiocarcinoma, HCC with capsule, some nodular NHL). With multislice CT, early and late arterial phase scanning became possible, and a triphasic protocol has been suggested that includes these two phases followed by a later portal (= hepatic venous) phase. Rapid scanning is necessary to capture distinct phases. With 4-slice scanners, 4 x 2.5 mm to 4 x 3.75 mm collimation should be used. A high pitch factor is to be preferred over a wide collimation (GE scanners: HS instead of HQ. mode). Aortic bolus triggering is beneficial: the early arterial phase is started with a 3-5 s delay, the late arterial phase is started 5 s after the first phase ends. The hepatic venous phase is initiated some 20 s after the late arterial phase has ended. Such a protocol yields the best separation of arterial and venous anatomy (early arterial and hepatic venous phases). Lesion detection, however, does not profit from the early arterial phase because most hypervascular lesions are better (or only) detected during the late arterial phase. The protocol may therefore be recommended for a pre-surgical work-up but, because of the improved differentiation of detected lesions, we usually prefer a triphasic examination with an arterial phase scan initiated some lOs after contrast arrival in the aorta, an hepatic venous phase, and a (usually 3-5 min) delayed phase.

Intra-arterial Contrast Injection Techniques An early arterial phase is helpful for pre-surgical 3D reconstruction of the arterial supply but rarely improves tumor detection. Due to the increased radiation exposure. one should consider MRI with dynamic sequences after gadolinium or with liverspecific contrast agents as the first choice technique in younger patients with suspected benign liver tumors.

Intra-arterial

Contrast

Injection

Techniques

Intra-arterial contrast injection techniques have been developed to improve detection of liver lesions. but such techniques at present only hold fringe indications. CT hepatic arteriography (CTHA) may be used during arterial chemoembolization or before surgery of hepatocellular carcinomas (H CC). and CT during arterial portography (CTAP) may be used in critical preoperative situations if more modern CT or MR techniques are not available. •

CT Hepatic Arteriography

(CTHA)

CT hepatic arteriography is an interventional procedure in which a catheter is placed under fluoroscopic guidance into the proper hepatic artery. The patient is then transferred to the CT room and a scan (spiral or multislice CT) is performed during direct contrast injection into the hepatic artery. This procedure yields excellent and purely arterial contrast enhancement of hypervascular hepatic tumors. It is mostly performed in surgical candidates with liver cirrhosis and suspected hepatocellular carcinoma. nowadays often in conjunction with spiral CTAP. With a catheter in the hepatic artery. 7S ml of 1: 1 diluted contrast medium (l.e .. ISOmgll ml) is injected at a rate of 2-3 mlls. Scanning is initiated after 3-10 seconds using a technique that covers the entire liver in one breath hold. Replaced or aberrant hepatic arteries and perfusion abnormalities (cirrhosis) may give problems in the interpretation of the scan. Addition of a delayed phase (biphasic CTHA) is used for improved differentiation of tumor versus perfusion abnormalities. Injection of vasodilators like papaverine. Priscoline or prostaglandin Et has been reported to increase portal flow and to enable more confident differentiation of true le-

sions from pseudolesions due to systemic venous inflow tracts. The advent of multislice scanning probably makes the technique obsolete, because proper timing of the multislice scan and use of increased flow rates can reach high and almost selective arterial enhancement of the liver. CT during Arterial

Portography

(CTAP)

CTAP utilizes the principle of indirect splenic and mesenteric portography. It can detect hepatic lesions with high sensitivity by the selective delivery of contrast medium to the portal venous system (see Fig. 11.14). CTAP is used in the planning of partial hepatic resections (to exclude tumors in residual liver segments), but cirrhosis and large tumors can limit its effectiveness by altering portal venous hemodynamics. Spiral CT during arterial portography offers a detection rate of well above 90% for lesions ss 1 cm in diameter. For lesions smaller than \ cm, results are poorer. Specificity of CTAP is very limited, however, because all types of lesions appear hypoattenuating on these scans. Apart from biphasic multislice CT, MRI with liver-specific contrast agents (SPIO) has been shown to be equally accurate and more specific, and may replace CTAP in the future. The technique involves injection of ISO200ml of normal or 1: 1 diluted (I.e., ISO300 mg lIml) contrast medium at 3-S mlls through catheters selectively placed in the superior mesenteric artery, splenic artery, or both. Some authors claim better results with splenic artery injections. After a delay of 20-60 s spiral or multislice CT of the entire liver is performed during a breath hold. For improved characterization of lesions, biphasic studies during the true portal phase after 20s and late (recirculation) phase after 60s can be performed. Its specificity is still limited, however. The late scan is useful for distinguishing perfusion-related artifacts from lesions. Small rounded lesions that are conspicuous during the portal phase but not in the late phase are interpreted as neoplasms. Combination with CTHA is often performed for improved characterization, but there is controversy about which technique should be used first. As in CTHA, injection of va so dilating agents can enhance portal venous flow, leading to better consptcutty of small lesions and differentiation from pseudolesions.

11 The Liver

CTAP is a completely nonspecific method of lesion detection (except for cysts). Low-attenuation artifacts further reduce its specificity (Fig. 11.8).These artifacts may be caused by laminar flow in the portal vein (poor mixing of splenic and mesenteric blood) or by the compression of portal venous branches. Aberrant portal vessels that drain directly into the liver (e.g., the left gastric vein, the gallbladder veins) can mimic malignant lesions in segment IV or in the gallbladder bed. Compression of the liver parenchyma by ribs may cause hypoperfusion artifacts as well (see Perfusion Abnormalities, below). •

Lipiodol CT

Lipiodol CT is a technique that offers high specificity but only moderate sensitivity in the detection of HCC. Best results are achieved with hypervascular HCC tumors, especially those that are larger than 2 cm. In other cases, sensitivity is poorer. Prior to the CT examination, approximately 5 to 10ml of Lipiodol, an oil-like iodinated substance, is injected into the proper hepatic artery through a catheter under angiographic guidance (taking care to include accessory or aberrant hepatic arteries). Some 2-4 weeks later, a CT examination of the liver is performed without further administration of intravenous contrast material. By this time the marked uptake of Lipiodol by HCC nodules will contrast sharply

a

b

Fig. 11.8 Perfusion artifacts in CTAP may have a typical wedge-shaped appearance distal to a tumor with compression of portal venous branches (a), or they may be

with the very low Lipiodollevels in normal liver parenchyma. Hyperattenuating nodules that show essentially homogeneous high density are considered pathognomonic for HCC (see Fig. 11.34). A homogeneous high density pattern is not seen with all HCCs. Other hypervascular tumors may show nonspecific Lipiodol uptake. Spotty uptake of Lipiodol in the periphery of a lesion does not positively identify the lesionasHCC. Note that Lipiodol is a contrast agent for lymphography that is not approved for intraarterial injection in many countries [off-label use}.

Perfusion CT

Perfusion CT or Single-slice dynamic CT has (re) gained increased popularity for studying liver perfusion. Single-level CT scanning is performed while a bolus of 40-80 ml of contrast medium is administered by fast intravenous bolus injection (5-10 mlls) followed by a saline chaser-bolus (e.g., 40ml). Scans are started at 10-15 s after the begin of the injection and are acquired every 1 to 4 seconds for 60-120 seconds, then at larger intervals for up to 5 minutes after the start of the injection. Perfusion quantification relies on using slope measurements or

c caused by veins that drain directly into the liver, e.g., gastric veins anteriorto the portal vein in segment IV (b) or gallbladder veins into the gallbladder bed (c).

Perfusion CJ compartmental modeling. Several indices (transit time. hepatic perfusion index. peak perfusion values. distribution volume) can be measured and may be displayed in parametric images. To minimize radiation exposure. an 801
(injection rate 4-5mlls, 40ml saline chaser bolus), and the scans are acquired every 2 to 3 seconds for 30 seconds, then every 30 seconds for up to 5 minutes after the start of the injection. Generally a small delay of 10 seconds is used before the first scan. The scans can be repeated at a different level if multiple lesions are present, but residual enhancement will make these results somewhat less accurate. Small lesions may be missed du