Image Postprocessing in Cardiac Computed Tomography

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CHAPTER 12 Image Postprocessing in Cardiac Computed Tomography

The role of CT in the evaluation of cardiac disease is no longer the subject of possible applications, but more in the realm of determining positive and negative predictive values as cardiac CT angiography becomes a central part of cardiac imaging. Whether the application is to determine coronary artery stenosis in native vessels, to determine patency of bypass grafts or stents, or to detect anomalous anatomy, CT has been shown in many cases to be the study of choice. In the emergency department, the power of a 100% negative predictive value promises to lead to a paradigm shift in emergency department triage for patients with chest pain.¹^,² Similarly, a gated CT acquisition has become the standard of care to evaluate aortic root pathology, including valvular disease, suspected dissection, Marfan syndrome, and Loeys-Dietz syndrome.³^–⁵ Other applications, such as preoperative assessment and postoperative evaluation in congenital heart disease, are becoming a well-accepted application for CT angiography.⁶^,⁷

Despite more than 400 articles on the subject and a body of literature that continues to grow, few published data have focused on the technical aspects of different processing techniques for the evaluation of the heart and coronary arteries.⁸^–¹³ The “how-to” of data acquisition (16 vs. 64 vs. 128 detector scanners)¹⁴^–¹⁶ is addressed in detail in the literature, as are contrast delivery techniques (test bolus vs. bolus triggering vs. timed injection),¹⁷^–¹⁹ but the analysis of the resultant CT data has been the topic of only a few articles of note. This chapter provides a systematic approach to data analysis, with emphasis on how to use each postprocessing tool to its greatest advantage (Figs. 12-1 through 12-4), to interpret most accurately an isotropic multidetector CT volume of the heart.

FIGURE 12-1 A-F, This set of images shows the relative utilities of each rendering technique. A-D, The proper study protocol has yielded a clear view of the right coronary artery (arrowheads) with MIP imaging, but with MIP it is hard to be certain that one is actually imaging the origin of the right coronary artery. E and F, With VRT, this is not an issue, and the color-coded VRT images provide a better three-dimensional perspective. The VRT images are the same projection, but use different rendering parameters.

FIGURE 12-2 A-D, VRT (A-C) and CPR (D) techniques are used in this case to define an anomalous left coronary artery (arrow) arising off the posterior aspect of the main pulmonary artery. The unusual location was impossible to define on routine axial scans or with routine MPR imaging.

FIGURE 12-3 A-G, In this case, evaluation of an anomalous right coronary artery off the left cusp reveals the value of each technique. A, On a single axial image, one can define the vessel (arrow), but would need a series of individual images to delineate its course. B, With MPR images rendered in the optimal perspective, the origin of the right coronary artery is seen. C, MIP images show the vessel coursing between the aortic root (AO) and pulmonary outflow tract (P), but the full course of the vessel is hard to see without use of very thin slabs. D-G, VRT is ideal for complex vascular anatomy, and color (D and E) or gray-scale mapping (F and G) can be used.

FIGURE 12-4 Schematic representation of the value of CPR in imaging complex vascular anatomy. A, Axial sections show only segments of the artery in cross section. B, Multiplanar reconstruction in the orthogonal plane displays only various oblique cross sections of the artery. C, CPR generated by center-line tracking captures and displays the entire artery.

TECHNICAL REQUIREMENTS

With respect to data analysis, the most comprehensive article has been by Ferencik and colleagues.¹¹ Two important statements from this article are as follows:

“The evaluation of multidetector CT coronary angiography with interactive image display methods, especially interactive oblique MPRs [multiplanar reformats], permit higher diagnostic accuracy than evaluation of prerendered images (curved MPR, curved MIP [maximum intensity projection], or VRT [volume-rendering technique] images).”

“Interactive evaluation of multidetector CT coronary angiography data sets on a workstation should thus be the preferred way of interpretation.”

The key point made by this article was that the method of data set evaluation (interactive interrogation of the volume by the interpreting physician vs. review of preset images generated by an individual not performing the interpretation) is perhaps even more important than the postprocessing tools used. Interacting with the data set using only axial images and MPRs was more efficacious than review of preset three-dimensional images. When preset images are reviewed, the information seen depends on the skill of the individual (interpreting physician or technologist) who performed the postprocessing. We have found that in nearly every case, errors in postprocessing can result in key mistakes in data interpretation. Interactivity means that the interpreting physician must process the images himself or herself. The following rules are suggested to do this successfully:

1 Interpreting physicians should learn how to use a workstation for cardiac-specific applications.

2 The ideal workstation should have dedicated tools for cardiac evaluation, including but not limited to automated vessel segmentation (with minimal user time), stenosis calculation software, vessel tracking software, four-dimensional motion studies, excellent and easy-to-use filming, archiving, and networking solutions.

3 Interpreting physicians should be involved in the full scope of the imaging process, ranging from protocol design to quality assurance.

4 Interpreting physicians must be familiar with the various rendering techniques, including MPR, MIP, and VRT, and must know how to use these techniques, their advantages, and their disadvantages.

5 Another area of practical concern is the quality of the data set to be analyzed. The ability to acquire routinely a high-quality data set is mandatory to doing a quality interpretation. Although scan protocols are beyond the scope of this chapter, they cannot be overemphasized. The proper gating technique (retrospective or perspective), control of the patient’s heart rate, and timing of delivery of contrast material (contrast agent and saline flush) and data acquisition must be optimized.

Let us assume that the patient has been scanned, the data have been processed into the appropriate slice thickness (0.6 to 0.75 mm) and interscan spacing (0.4 to 0.5 mm), and the data have been sent to the workstation. Now what? That question is guided in part by individual preference and by workflow, and partly dictated by the workstation available to the user. Regardless of the workstation specifics, some general principles can be implemented across almost all current platforms.

TECHNIQUE

Axial Images

Although a cardiac CT scan typically is reviewed at least using multiplanar reconstruction and three-dimensional renderings, the axial images (see Fig. 12-3) remain a crucial part of the study workflow. At some sites with highly experienced interpreting physicians, interpretation is typically limited to the axial images with sparing use of additional views in selected cases. On a workstation with a composite display of axial, coronal, and sagittal images combined with a three-dimensional image, our strategy for axial images is to scroll interactively through the data set in what the scanner selects as the optimal phase (0% to 90% of R–R interval on retrospective gated study) for coronary artery visualization. We then do the following:

1 Scroll through the data set to define the quality of the study, including contrast delivery timing, lack of motion artifact, and coronary artery opacification. Based on a quick look, one can quickly ascertain how good the study is technically. We also ensure that the “optimal” phase was selected because occasionally multiple phases need to be processed and viewed.

2 Scroll through the heart in a global sense with analysis of chamber size, and a look for any abnormalities including the pericardium and the paracardiac regions. We also note the size of the ascending and descending aorta.

3 Scroll through the coronary arteries, and look for the origin of each vessel and its branching pattern. Vessels such as the sinoatrial nodal branch and atrioventricular nodal branch are often best seen on the axial images. We follow each coronary artery to look at lumen diameter and determine the presence or absence of calcified or noncalcified plaque. Axial images often nicely define noncalcified plaque especially when lipid-rich. Axial images also define the presence and extent of calcification, although a wide window often is needed in the presence of extensive calcification.

The limitation of axial images is that one may need to look at 200 to 350 individual slices to get through a single vessel, such as the left anterior descending coronary artery. Vessels with off-axis courses, such as the right coronary artery, are best defined when one is looking beyond the axial plane. A more global presentation of an entire coronary artery is impossible without a curved planar reconstruction (CPR) (see Fig. 12-4) or three-dimensional display.

Our experience is that referring physicians desire three-dimensional maps to correlate with what they typically see on classic catheter angiography. Axial CT images in this regard are not helpful and do not provide similar information.

Although we typically scroll through the classic axial display interactively, other physicians believe that reading studies can be noninteractive, and that there are no data to prove otherwise. This controversy needs to be addressed in a future study. One issue is the lack of a well-designed, controlled study on the use of different techniques for coronary CT angiography analysis. This is in contrast to many articles published on reading techniques (e.g., two-dimensional vs. three-dimensional) for virtual colonoscopy.

Although most physicians review the axial images as slabs in the plane acquired, one helpful hint is to draw a line through the plane of the aortic valve, and scroll up and down through this plane. This practice often provides a perfect en face display of the proximal coronary arteries.

Multiplanar Reconstruction—Coronal and Sagittal Images

The use of coronal and sagittal reconstructions is common in a range of CT applications, and in cardiac CT its role is also defined. With isotropic data, the individual cardiac chambers, aorta, and pulmonary vasculature can benefit from these displays, and they are routinely used. Whether it is for suspected aortic dissection or suspected pulmonary embolism, these displays are essential. On most workstations, the thickness of the reconstruction can be interactively adjusted (Fig. 12-5), but when slabs are reduced to beyond a few millimeters, the images become fuzzy and are of no value. This is in contrast to MIP images, where often less data may prove to be useful for these large-volume applications. One issue with coronal and sagittal reconstruction and evaluation of the coronary arteries is that because the vessels have a nonlinear pathway, only short segments can be analyzed at a time. When the left anterior descending coronary artery is analyzed with a coronal or axial display, one must constantly change the angle of the reconstruction plane to follow the vessel. Changing the angle can be done interactively, but potentially results in more room for error and less than ideal image displays. For analyzing selected segments of a vessel, this technique might be perfect. If there is a suspect area on the axial views, the coronal or sagittal view may help clarify and quantify this potential narrowing. Key points include the following:

1 Coronal and sagittal imaging may be useful, but it depends on the vessel tracking capabilities of the workstation. Ideally, a specific point is selected, and viewing is adjusted from that perspective.

2 Interactivity is key.

3 The oblique planes are valuable for additional viewing of selected suspicious areas, which often arise after review of the axial images.

4 With coronal, sagittal, and oblique images, one needs to be cautious in grading stenosis and not be fooled by partial averaging. We do not attempt to grade stenosis based only on these images because of this pitfall.

5 We have found that these reconstructions are best for larger vessels, as discussed previously.

6 Many sites now do preprocessed batched coronal and sagittal images at 1- to 3-mm intervals. This may mean a data set of 30 to 120 images that can be scrolled through on a picture archiving and communication system (pacs). Although this technique may be sufficient for routine abdominal or chest CT interpretation, interpreting physicians must recognize the limited utility of this technique for evaluation of the coronary arteries because of the vessel size (typically 3 to 5 mm) and ectatic course of the coronary vessels.

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