Clinical Applications of 3D CT Imaging in Thoracic Pathology M. Prokop

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Thoracic vascular imaging is moving away from catheter angiography towards non-invasive techniques, and in par- ticular computed tomography angiography (CTA). The three-dimensional imaging capabilities of CT and its ease of use have been the main reason for this change.

However, 3D imaging is not limited to the vascular sys- tem; it also has useful but underused applications in rou- tine chest CT, high-resolution CT (HRCT), or trauma cases. This article will present the basic rules for data ac- quisition for 3D imaging in chest CT and CTA, briefly discuss the various techniques for exploring a 3D data set, and finally focus on clinical applications that can im- prove chest imaging in daily routine as well as in situa- tions in which it is important to gain as much informa- tion as possible about the three-dimensional extent of the anatomy and pathology of interest.

Acquisition Technique

Optimal three-dimensional imaging requires a near isotropic data acquisition. In CT, this is possible with thin-section imaging, in which the in-plane and through plane resolution are similar. In practice, this is synony- mous with thin sections (ideally <1.25mm thickness).

Single-Slice CT

Isotropic imaging with single-slice CT is limited to very short scan ranges (e.g., 6 cm for protocols with 1-mm collimation, pitch 2, and a 30-s scan duration). It may be used in the evaluation of focal bronchial abnormalities or tumors, but in clinical practice it is seldom applied.

For the aorta, pulmonary artery, and bronchial system, a scanning protocol with 3-mm collimation and a pitch between 1.6 and 2 can be applied if the scan range is lim- ited to only part of the chest (top of the diaphragms to 1 cm above the aortic arch). Despite a scan range of <20 cm, the scan duration should be >30 s. This requires meticu- lous patient instruction to avoid breathing artifacts.

For routine imaging, a 5-mm collimation with a pitch around 1.7 is necessary to cover the whole chest within one breath-holding period. Again, proper patient in-

struction is mandatory. However, this protocol will suf- fer from substantial anisotropy (effective section thick- ness 6-7mm) and will only suffice in the assessment of gross abnormalities, e.g., of the aorta, or for general anatomic orientation.

Reconstruction increments should ideally be half the effective section thickness, i.e., 0.7-1 mm for the near- isotropic protocol, 2 mm for evaluation of the central or- gans, and 3-4 mm for the whole chest.

Multislice CT

Isotropic imaging of the whole chest is possible with all multislice scanners (4-slice and more). At thin collima- tion (1-1.25 mm), 4-slice scanners still require scan du- rations between 20 and 30 s, which makes them vulnera- ble to breathing artifacts towards the end of the scan. The 10- to 64-slice scanners require only 5-15s, even at sub- millimeter collimation. However, since scans are now much faster, breathing artifacts may occur towards the beginning of the scan if the patient has not fully stopped breathing when the scan is started. Still, in very dyspne- ic or uncooperative patients, no breath-hold should be at- tempted because artifacts will be major if patients con- tinue breathing after a failed breath-holding maneuver.

Instead, such patients should be asked to hyperventilate before the scan (if possible) and then told to breathe shal- lowly. Breathing artifacts with fast scanners will be mi- nor, and 3D evaluation will still be possible.

In general, a collimation between 0.5 and 1.25 mm is used with multislice scanners. The pitch factor should be set around 1.5 for 4-slice units, around 1.25 for 10- to 16- slice scanners, and around 0.9 for 32- to 64-slice scan- ners. Cone-beam artifacts are reduced at lower pitch and the increase in scan speed with more detector rows is such that a minor reduction in pitch can be afforded.

As with single-slice CT, reconstruction increments should theoretically be about half the effective section thickness but they need not be smaller than the pixel size in the scan plane. In practice, this pixel size be- comes the determining factor for the choice of the re- construction increment: at a field of view of between 250 and 350 mm, the pixel will vary between 0.5 and

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Clinical Applications of 3D CT Imaging in Thoracic Pathology

M. Prokop

Department of Radiology, University Medical Center, Utrecht, The Netherlands

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0.7 mm (512 matrix). For practical reasons, we set the reconstruction increment to 0.7 mm for all but small or pediatric patients.

The reconstructed section thickness can be chosen (almost) arbitrarily with multislice scanners. However, it has to be equal to or larger than the slice collimation.

If the chosen section thickness is identical to the colli- mation, then the image noise will usually be substan- tially higher - a situation that should be avoided for 3D imaging. As a consequence images of between 0.8 and 1.0 mm thickness should be reconstructed for 10- to 64-slice scanners and images of 1.25- to 1.6-mm thick- ness for 4-slice scanners. Since image noise will in- crease substantially in the imaging of large patients, even thicker sections (1.5-2.5 mm) and/or additional smoothing filter kernels should be applied in such sit- uations.

With 16- to 64-slice scanners, ECG-gating becomes an option. This technique has been developed for imaging of the heart but it can also be applied to 3D imaging of the thoracic aorta, which otherwise will suffer from substan- tial pulsation artifacts, especially in young patients or those with high blood pressure, aortic valve disease, or acute aortic dissections. Since the use of ECG gating will increase the scan duration substantially, the slice colli- mation will have to be increased with 16-slice units. With 64-slice scanners, ECG-gated imaging of the whole chest can be achieved within 20-35 s.

Rendering Techniques

The most common techniques for exploring 3D data in CT are multiplanar reformations (MPRs) and maximum intensity projections (MIPs). Minimum intensity projec- tions (MinIPs) or volume rendering techniques (VRTs) are reserved for more specialized applications. During the course various tips and tricks will be demonstrated for optimizing these viewing techniques with respect to im- age quality but also with respect to time-efficient use in clinical practice.

Multiplanar Reformations

Multiplanar reformations (MPRs) are the basis for 3D image evaluation with CT. They allow for interactive ex- ploration of a CT data set similar to that obtained with ul- trasound (but after data acquisition has been accom- plished). MPRs should be standard for a large variety of clinical imaging tasks.

One of the most important tasks of MPR is noise re- duction: by increasing the MPR thickness, image noise can be controlled without losing in-plane resolution. This is usually the first step in the process of interactive analy- sis of a 3D CT data set.

MPR can be used to display the craniocaudal extent of disease, to improve anatomic delineation, for tu- mors, the spine, aortic arch abnormalities (Fig. 1), the

bronchial system, or the lung parenchyma, to name but a few.

MPR can either be incorporated into a standard proto- col (e.g., coronal and sagittal MPR for staging of bron- chogenic carcinoma) and be performed by technologists or be done interactively by the radiologists during image evaluation. In general, we combine both approaches: rou- tine MPR for most indications and additional interactive MPR wherever necessary.

Maximum Intensity Projections

Maximum intensity projections (MIPs) are most popular for vascular imaging (CTA) but are also useful for evalu- ation of the lung parenchyma. For the whole scan vol- ume, a suitable subvolume, or an image slab, MIPs dis- play the maximum CT number in the viewing direction and therefore optimize the display of high-density struc- tures, such as contrast-enhanced vessels, but also lung nodules and bones.

MIPs require the choice of a subvolume that is free from structures whose densities are higher than that of the structure to be visualized. In practice, MIPs for the chest are usually applied to thin slabs of 5- to 20-mm thickness, which can be interactively moved through the data volume much in the same way as MPRs.

MIPs are less useful for the aorta, but play a role in evaluating the supra-aortic vessels, the bronchial arteries, and, most importantly, the pulmonary arteries (Fig. 2).

Recently, MIPs have turned out to be most efficient when used to search for pulmonary nodules, be it in the case of metastases, lung cancer screening, or simply as a final step in any chest CT. MIPs have also been successfully employed for identifying and classifying subtle HRCT findings (Fig. 3).

Fig. 1.Parasagittal reformation of the aorta in a trauma patient.

Note the irregularity of the aortic lumen at a typical position in the proximal descending aorta, indicative of traumatic injury. Also note the extensive mural hematoma in the descending aorta

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Minimum Intensity Projections

Minimum intensity projections (MinIPs) display pixels with the lowest density in the viewing directions. Their use has been advocated in assessing the central bronchial system. However, because MinIPs lack depth informa- tion, they turn out to be quite useless for most such clin- ical applications. Instead, for evaluation of lung density changes, such as mosaic patterns, and for the display of bronchiectases, MinIPs are well-suited because they sup- press the anatomic background (vessels) and only display the lung parenchyma and dilated bronchi.

Volume-Rendering Techniques

These allow for a vast variety of display options, be it for the vessels, the skeleton, the bronchi, or the lung parenchyma. Volume rendering images can yield surface as well as density information and can be easily color- coded.

Volume-rendering techniques (VRTs) are the best ap- proach to visualizing the thoracic aorta because depth in- formation is preserved and 3D orientation is facilitated.

Bone removal can improve the display of paravertebral portions of the aorta; but in general some parts of the skeleton should remain in the image to facilitate anatom- ic orientation. Complex anatomy is preferentially dis- played by VRTs (Fig. 4).

VRTs can be targeted to display density gradients, such as at the aortic or bronchial wall, and can thus be used for luminal evaluation. With densities color-coded in the range of the lung parenchyma, VRT improves the visualization of subtle variations in lung density, such as occur in mosaic perfusion or bronchiolitis obliterans.

Shaded Surface Displays

This older technique can be used as an alternative to VRTs but it is less versatile and more prone to artifacts.

For this reason, shaded surface displays (SSDs) are hard- ly used anymore.

Virtual Endoscopy

Virtual endoscopy is a technique that uses VRT or SSD combined with perspective rendering to gain endoscopy- like images of interior surfaces. While it is very useful

Fig. 2.Maximum intensity projection (MIP) in a patient with bron-

chogenic cancer and encasement of the pulmonary arteries on the right (arrowheads) as well as pulmonary embolus on the left (arrow)

Fig. 3. Comparison of a 1-mm thick axial high-resolution CT (HRCT) image (a) and a 10-mm thick maximum intensity projec- tion (MIP) (b) for the differentiation of centrilobular abnormalities.

Correct classification of the tree-in-bud phenomenon is much easier on MIP images

a b

Fig. 4. Volume rendering of an aberrant arterial supply of a pulmonary sequester. Note the excellent anatomic orientation

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for evaluation of the colon, virtual endoscopy plays a minor role in the chest. Neither virtual bronchoscopy nor virtual angioscopy have made their way into routine clin- ical practice.

Image Noise

Noise is the most important enemy of 3D evaluation tools other than MPR. Too much noise will increase the back- ground density in MIPs and decrease the background in MinIPs. While this effect is less critical for MIPs, it can substantially deteriorate the evaluation of smaller struc- tures on MinIPs. On MIPs, however, irregular contours of high-density may occur due to image noise. Similar ef- fects are seen on images obtained with VRTs.

Noise will increase with thinner sections, use of high- er-resolution filters for image reconstruction, and espe- cially in obese patients. In fact, every additional 4 cm of soft tissue roughly cuts the number of photons that hit the CT detector in half and will increase the noise by the square root of 2 (i.e. by ca. 40%).

The image quality achieved with MIPs, MinIPs, or VRTs can therefore be improved by using lower-resolu- tion filters or by increasing the thickness of the recon- structed sections. Some manufacturers offer edge-pre- serving filters that reduce noise without sacrificing im- age details. If thick MPRs are first reconstructed perpen- dicular to the viewing plane (i.e., coronal planes for the AP viewing direction), then noise can be reduced without sacrificing spatial resolution in the viewing direction.

Clinical Applications Oncologic Imaging

Staging of primary tumors of the chest requires excel- lent anatomic orientation. For this reason, coronal and sagittal MPR should be routinely reconstructed. This is best done in a standardized fashion by CT technolo- gists. Interactive MPR will only be necessary in com- plex cases. Such an approach ensures correct assign- ment of tumors to pulmonary lobes and improves the evaluation of tumor invasion into the mediastinum or chest wall. For vascular encasement and stenoses, MIPs are advantageous.

The search for lung metastases is substantially im- proved if ~10-mm thick MIPs, rather than axial sections, are evaluated. This is best done interactively at a work- station. Since vessel cross-sections resemble nodules on axial sections, small and central metastases can easily be missed. With MIPs, vessels retain their tubular shape while the contrast with nodules remains intact. Thus, MIPs facilitate the detection of difficult nodules and therefore should be adopted as the standard approach to this imaging task.

In lung cancer screening, MIPs play the same role. For small nodules, growth may be the main criterion to dif-

ferentiate malignant from benign disease. Three-dimen- sional measurements by automated segmentation and volumetry are much more reliable than diameter mea- surements in the determination of growth.

High-Resolution CT

If HRCT is done using volumetric data acquisition rather than (classical) discontinuous scanning, coronal MPRs should be reconstructed on a standard basis by CT tech- nologists. This improves evaluation of the extent of dis- ease and facilitates the comparison of inspirational depth on inspiratory and (low-dose) expiratory scans.

MIPs of ~10-mm thickness improve the differentiation between centrilobular nodules and tree-in-bud sign (Fig.

3) and allow better detection of micronodules in sar- coidosis or pneumoconiosis patients, as well as identifi- cation of the initial signs of interstitial lung disease that may be missed on axial sections.

For the identification of mosaic patterns in ground-glass disease, chronic embolism, or air trapping, MinIPs of 5- to 10-mm thickness remove anatomic background (i.e., small- er vessels) and make the detection of density difference much easier. For the detection of discrete density changes, color-coded VRTs applied to analysis of the lung parenchy- ma (20-30mm thickness) can be very advantageous.

For peripheral bronchiectases, MinIPs (~10-mm thick) well-demonstrate the extent of the disease, and MIPs (al- so ~10-mm thick) better display small mucous plugs.

Standard Chest CT

Standard chest CT relies on axial sections. Additional coronal or sagittal MPR are useful but not mandatory on a routine basis. However, if interactive evaluation is eas- ily available (e.g., good integration in a PACS worksta- tion), thin-slab MIPs in axial directions should be evalu- ated in order not to miss small nodules. Sagittal MPRs can be helpful in order to evaluate the vertebral column.

When performed on a regular basis, it is surprising how many nodules or vertebral abnormalities would have been missed if they had been evaluated on axial sections only.

Interactive evaluation using MIPs or MPRs can be helpful in patients with more complex pathology: MIPs for the evaluation of vascular structures and discrete parenchymal abnormalities, and MPRs for the assess- ment of spatial relationships to the chest wall, medi- astinum, and lung segments.

Computed Tomography Angiography

CT angiography (CTA) profits most from 3D rendering techniques, even if single-slice scanning has been used.

Aorta

Evaluation of the aorta requires standard reconstruction

of coronal and oblique MPRs (Fig. 1) that are adapted to

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the position of the aortic arch (very much like the posi- tion of parasagittal slabs in magnetic resonance angiog- raphy). MIPs play no major role in CT, mainly because VRTs yield superior image quality and retain depth in- formation. In addition, only rough segmentation is need- ed for VRTs, while MIPs require the removal of all su- perimposing bony structures. Bone removal for VRTs can be limited to cutting away the sternum and removal of the lateral portions of the chest. Simple cutting functions suf- fice for this task. The result is a superior demonstration of the aorta from the arch to its descending portion. It is useful for evaluating aneurysms as well as dissections or congenital abnormalities, such as coarctation.

In case of an ECG-gated acquisition, less pulsation ar- tifacts are seen in the ascending aorta and even the eval- uation of the (proximal) coronaries may become feasible.

Four-dimensional evaluation (MPRs) allows valvular function and dynamic processes, such as obstruction of the true lumen during diastole in acute dissection, to be examined.

Pulmonary CTA

This technique relies on the evaluation of thin axial sec- tions. The first step is to exclude those intraluminal em- boli that only obstruct the lumen incompletely. They are usually found in more central locations and are more eas- ily detected than complete occlusions of smaller periph- eral vessels. The latter are subject to partial volume ef- fects (even with new multislice scanners) and therefore may have a slightly decreased contrast enhancement.

Differentiation from occlusions may therefore be difficult and is facilitated by MIP (Fig. 2). By evaluating MIPs of

~10-mm thickness, both in the axial and coronal plane, it is much easier to differentiate between real occlusions and partial volume effects: for this purpose one has to compare the density of a suspicious vessel with that of a

‘normal’ vessel of similar size and orientation. If the den- sity is substantially lower, then a peripheral embolus should be suspected.

MPRs can be helpful for differentiating lymph nodes from occluded central vessels but are rarely necessary with thin-section imaging.

MinIPs and color-coded VRTs of the lung parenchyma improve the detection of signs of mosaic perfusion in pa- tients with suspected chronic thrombembolic pulmonary hypertension.

Volume rendering becomes important for examination of pulmonary arteriovenous malformations or congenital abnormalities, such as aberrant veins or pulmonary se- questration (Fig. 4).

Trauma

The evaluation of chest trauma mainly profits from sagit- tal MPRs of the spine, which in turn facilitates identifica- tion of vertebral fractures. For this reason, sagittal (and coronal) reformations should be reconstructed routinely by

CT technologists. Coronal reformations may help further classify vertebral fractures and identify sternal fractures.

Parasagittal MPRs are vital for establishing the extent of (and sometimes detecting) aortic tears, which can then be better visualized with VRTs.

MIPs approximately 1 cm in thickness can be helpful in analyzing the extent of severe trauma involving chest contusion in order to identify active contrast extravasa- tion that may otherwise escape detection.

For further evaluation of the data (not a first-line task), coronal curved planar reformations (CPRs) of the sternum or vertebral column can optimize the display of these re- gions. VRTs can be helpful for visualizing complex frac- tures but also provide a good overview of fractured ribs.

Conclusions

Modern CT techniques require 3D exploration for opti- mized use of the available information. Of these, thick MPRs are the most important, but MIPs, MinIPs, and VRTs should be used regularly in clinical practice for imaging tasks as diverse as nodule detection, subtle find- ings in HRCT, CTA of the aorta and pulmonary arteries, and evaluation of thoracic trauma, to name but a few. This review has discussed the efficient use of these techniques in clinical practice.