In our practice, the individual delay time is determined by injection of a 20 mL contrast media test bolus at 5 mL/s (64-slice CT) or 6 mL/s (DSCT), followed by 50 mL of saline using a dual-syringe injector (Stellant D, Medrad). Repeated scanning at the same z-position at the level of the aortic root is performed to monitor the arrival and passage of the test bolus. The peak time of test bolus enhancement is used as the delay time. The contrast volume for the actual coronary CTA scan is individually computed according to the following formula: Volume (mL)=scan time (s)×5 (64-slice CT) or ×6 (DSCT). The injector is preprogrammed to deliver 50 mL of a 30% contrast media/70% saline mixture during the second phase of injection, followed by a final 30 to 50 mL saline chaser, all injected at 5 mL/s (64-slice CT) or 6 mL/s (DSCT), respectively.
Single-segment Versus Multisegment Reconstruction Algorithms
For considerations regarding use of single-segment versus multisegment reconstruction, please see above discussion on rate control (“Rate Control for Single-Segment Reconstruction with Single Source CT Scanners” section). In general, we avoid the use of multisegment reconstruction in patients with heart rates of less than 80 bpm with 64-slice CT and entirely with DSCT.
Choosing the Optimal Reconstruction Interval
For the assessment of cardiac morphology, a phase with minimal cardiac motion is preferably chosen for placement of the image reconstruction interval. To define the starting point of the reconstruction interval within the cardiac cycle, absolute and relative approaches are available on most cardiac CT scanner types. With an absolute approach, each image reconstruction interval is placed in the cardiac cycle with a predefined temporal distance (eg, 400 ms) before or after an R-peak in the ECG. With a relative approach, the starting point of the image reconstruction interval is defined as a certain percentage (eg, 60%) of the duration of the cardiac cycle. We use the relative, percentage-based approach for 64-slice CT and an absolute approach for DSCT. If available, a preview function is preferably used for determining the optimal reconstruction phase with the least cardiac motion. Typically, the preview series consists of 20 images (Fig. 6), reconstructed at 20 different RR positions in 5% increments (0% to 95% RR) at the same z-position at the mid-level of the heart. The phase that shows the least motion artifacts in both, the left and RCA system is chosen for the image reconstruction. In cases where the right and left coronary artery show diverging motion patterns, more than one reconstruction is performed for optimized visualization of both arterial systems. If a preview function is not available, a first image reconstruction of the data set can be performed at 60% RR (Fig. 2), which has been shown to result in diagnostic image quality in most patients,13 especially at slower, regular heart rates. With the improved temporal resolution of newer scanners, late systole with total cardiac contraction (ie, 30% to 40% RR) has emerged as a second suitable time-point for image reconstruction (Fig. 3), where cardiac motion is at a minimum. In our experience, image reconstruction during late systole yields diagnostic results in most patients with a faster heart rate and is especially well suited for visualization of the RCA.
Field of View
To maximize spatial resolution, the smallest possible field of view should be chosen that encompasses the entire anatomy of the heart, for performing image reconstruction at CT coronary angiography. In addition, for each coronary CTA study, we perform a full field of view reconstruction of the entire chest along the acquired z-volume with 3-mm section thickness and a lung algorithm, to assess for incidental lung pathology. For specialized applications, such as “triple rule-out” scanning, we perform the 2 reconstructions described above and a third reconstruction with 1-mm section thickness, a vascular algorithm and a field of view that encompasses the entire chest, to evaluate for vascular pathology of the pulmonary circulation and the thoracic aorta.
Generally, to avoid artifacts, thin-section MDCT data should be reconstructed with a section width that is slightly wider than the collimated section width.14 For example, if the scan was acquired with 0.6-mm collimated section width, the next higher available reconstruction thickness (eg, 0.75 mm) should be chosen for the image reconstruction. Forty to sixty percent increment is ordinarily used for the image reconstruction at coronary CTA, which in our experience results in a somewhat crisper and sharper delineation of the coronary artery tree but does not necessarily improve diagnostic accuracy compared to contiguous image reconstruction without overlap.
Reconstruction Algorithm (“Kernel”)
Most CT scanners used for coronary CTA offer a dedicated reconstruction filter (kernel) for the image reconstruction of cardiac CT studies. Typically, these kernels maintain a degree of edge enhancement, to provide the spatial resolution necessary to visualize small vascular detail. Ideally, the kernels are also optimized to suppress image noise as much as possible, to improve the visual impression and maintain contrast resolution for evaluation of the myocardium and the vessel wall. For the evaluation of coronary artery stents, it is recommendable to use a kernel with even stronger edge enhancing characteristics and greater spatial resolution. This approach suppresses beam hardening artifacts to some extent and provides better delineation of metallic stent struts (Fig. 7) than the algorithms which are routinely used at coronary CTA. This approach may also somewhat increase the diagnostic yield in the presence of heavy calcifications, which pose a similar problem as dense stent struts for the evaluating luminal integrity. Although dedicated reconstruction algorithms improve visualization of coronary artery stents, our ability to assess for stent patency with CT is extremely variable and depends on the overall quality of the data set and the size and material of the stent. Because of this variability, reliable assessment of stents cannot be expected on a routine basis and we discourage use of CT for dedicated stent follow-up.
IMAGE DISPLAY FOR LESION DETECTION AND GRADING
For cardiac applications, the role of advanced, dedicated image display, and analysis tools is considerably greater than for general CT applications. Review of the individual transverse source images, however, cannot be abandoned and must be a part of the diagnostic process in each case. The transverse source images are richest in information regarding incidental mediastinal findings, artifacts (Fig. 4), and the overall atherosclerotic plaque burden within the coronary artery tree. Every postprocessing step necessarily and by design reduces the available information for the sake of more intuitive image visualization.
Depending on the particular indication for performing cardiac CT, we employ slightly different strategies for our diagnostic approach. When assessing bypass grafts or the left atrium and pulmonary veins in the context of ablation therapy for cardiac arrhythmia, a 3-dimensional volume rendered model is used for quick initial orientation, for example regarding the type and course of bypass grafts or the general configuration of the pulmonary venous return. This is followed by the review of transverse source images for the detection and grading of graft lesions or pulmonary vein stenosis and also additional or alternative findings in the chest. For suspected coronary artery stenosis, the transverse source images are initially reviewed, to obtain general information on the presence, location, and composition (calcified vs. noncalcified) of atherosclerotic lesions15 and also consequences of ischemic disease, such as myocardial perfusion deficits or scarring (Fig. 8). Once lesions are detected, stenosis severity is evaluated by using simple visualization tools that enable a more comprehensive, condensed display of the data set. Multiplanar reformats (MPRs, see below) are easy to use basic tools and are available on most CT scanners. For improved detection and grading of coronary artery lesions, we use dedicated visualization and analysis tools (see below), whenever interpreting a scan performed for suspected stenotic disease. Different from the evaluation of bypass grafts and pulmonary veins, there is a little diagnostic value in performing 3-dimensional volume rendered displays for suspected coronary artery disease, as lesions are frequently obscured or overestimated, depending on the rendering parameters. In our practice, 3-dimensional rendering is exclusively used for intuitive communication of our findings to referring physicians and patients.
For visualization of the coronary artery tree at contrast enhanced CT coronary angiography, MPRs16 are widely used and recommended as a robust and easy to perform secondary visualization tool for data viewing. Because of the isotropic (equal voxel dimensions in x-axis, y-axis and z-axis) or near isotropic nature of high-resolution CT acquisitions, image data can be rearranged in arbitrary imaging planes with comparable image quality as in the original transverse section.
MPRs serve the purpose of enabling views of coronary artery lesions from different angles and perspectives, which enables better assessment of stenosis severity and residual perfused lumen than can be appreciated by only a single projection. This is of particular importance in the presence of severe calcifications, where a single view often fails to display residual lumen in the vicinity of a heavily calcified plaque (Fig. 9).
Advanced Visualization Tools
Advanced software tools have become available and are being continuously refined that facilitate viewing and analysis of large volume data sets. The common rationale of most of these software platforms is to provide a means for rapid analysis of the coronary artery tree for the detection and grading of stenosis. Typically, the first step of postprocessing after review of the transverse sections and MPRs (Fig. 10A) consists in automated sculpting of the chest wall to enable an unobstructed view of the heart (Figs. 9C, 10B). Threshold-dependent or contour-dependent extraction of the coronary arteries from the contrast enhanced data set is then performed (Fig. 10C). Most software applications enable unraveling of the tortuous course of the extracted coronary artery, which affords intuitive visualization of the entire vessel, typically as an automatically generated MPR (Figs. 9B, 10D). Lastly, most available software platforms provide tools for quantitative evaluation of stenosis severity (Fig. 10E) based on cross-sectional measurements of vessel diameter or area. Naturally, the accuracy of such tools for stenosis grading is directly related to the image quality and spatial resolution of the original acquisition and subject to the inherent limitations of coronary CTA for assessing stenosis severity. Therefore, as with any automated assessment in medicine, the measurement results of vessel analysis tools should not be trusted blindly, but the experience and acumen of the physician is still required to validate the results in the appropriate clinical context.
We use a standardized template for reporting findings at coronary CTA. With the exception of a more detailed discussion of the coronary arteries and other cardiac structures, our coronary CTA reports are not fundamentally different from general radiology reports and include all pathology, variations, and changes that are visible on the different reconstruction series. In the Procedure section, we include the items that are pertinent to appropriate billing in our local health care environment. These items may be different in other geographical areas, but generally include the section thickness, use of retrospective ECG-gating, contrast volume and injection speed, medications used, and image postprocessing methods, such as MPRs or 3-dimensional reconstructions. In the Findings section, we begin with describing general cardiac and great vessel anatomy, commenting on the myocardium (thickness, areas of infarction, scars etc), the cardiac chambers, valves, pericardium, pulmonary veins, pulmonary arteries, and aorta. A section is dedicated to incidental findings in the chest wall, mediastinum, and lung, for example, the description and classification of lung nodules including recommendations for the follow-up according to standard clinical practice.17 In the cardiac section, we report on the presence and location of cardiac devices, catheters etc. When coronary artery bypass grafts are present, we describe the type, origin, course, site of anastomosis, the presence, location, and degree of graft stenosis and also the quality of the run-off within the grafted vessel distal to the anastomosis. The presence and course of anomalous coronary arteries is noted, and also the coronary supply type (right-dominant, left-dominant or codominant). Finally, each coronary artery (left main, left anterior descending artery, circumflex, RCA) is separately commented on with regards to the presence and type of atherosclerotic plaque burden (calcified vs. noncalcified). For reporting the site of stenosis, use of the American Heart Association/American College of Cardiology segmental model, which is widely employed for research purposes, has proved less useful for routine clinical interpretation. We rather use the common terminology found in routine catheter reports, that describe lesions as located in the proximal, mid, or distal portion of the respective main coronary arteries or their side branches (left anterior descending artery: diagonals and septals, circumflex: obtuse marginals, RCA: acute marginals). We use our visualization methods (see above) to determine the severity of stenosis as the percentage of luminal obstruction, on the basis of cross-sectional measurements of vessel diameter or area.
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Keywords:© 2007 Lippincott Williams & Wilkins, Inc.
coronary CTA; multidetector-row CT