Coronary Computed Tomography Angiography

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Chapter 8 Coronary Computed Tomography Angiography

Suhny Abbara, Stephan Achenbach

INTRODUCTION

Noninvasive cardiac imaging is of vital importance for the assessment of coronary artery disease.

The currently used modalities include nuclear perfusion imaging, stress echocardiography (ECHO), magnetic resonance imaging (MRI), and most recently coronary computed tomography (CT). The latter is the only test that routinely directly visualizes the coronary arteries (with the rare exception of coronary MRI). Nuclear imaging focuses on the detection of hypoperfused myocardium downstream from coronary stenoses; however, it will miss stenoses that have developed collateral pathways and may miss triple vessel disease as a consequence of “balanced ischemia.” Stress ECHO is focused on secondary wall motion abnormalities downstream from coronary stenoses that may be present during stress only because the limited blood flow at rest may be sufficient for the lower demand of the myocardium. Cardiac MRI can image the coronary arteries directly; however, to date this has not become a routine clinical tool. However, cardiac MRI is a powerful tool to depict first pass perfusion defects and to identify viable (alive) myocardium and scar tissue, as well as infarcted or fibrosed myocardium.

Coronary computed tomography angiography (CTA) has the potential to image cardiac function, cardiac perfusion, myocardial morphology (e.g., calcification indicating remote myocardial infarct [MI]), and the coronary arteries, all in one acquisition. Therefore, coronary CTA provides a potential alternative to the established noninvasive cardiac imaging technologies.

PATIENT SELECTION

In addition to the standard contraindications to CT scanning with iodinated contrast, there are additional factors that can be considered relative contraindications for coronary CT, or predictors of poor image quality and poor evaluability of coronary CTA. These factors may depend on the exact scanner type. In general, single source scanners do not allow routine imaging of coronary arteries in patients with atrial fibrillation or high heart rates and irregular heart rhythms. Dual-source technology has faster temporal resolution, and therefore the “penalty” for scanning patients with arrhythmias (penalty = poorer image quality) is less severe, and some preliminary data suggest that atrial fibrillation should not necessarily be considered a relative contraindication with that particular scanner type. In general, coronary CTA is challenged by contrast-to-noise restrictions (relatively noisy images) and by high-radiation doses. The higher the body mass index of a patient, the more likely the study is to be not entirely diagnostic (may contain segments of coronary arteries that are nonevaluable). Another challenge for coronary CT is the presence of extensive coronary calcium. Calcium causes blooming (looks bigger than it really is), which may make it impossible to evaluate the underlying lumen for presence of stenosis. In addition, the presence of calcium aggravates motion artifact. Because coronary arteries have small diameters, there are general challenges in terms of spatial resolution. An excellent maneuver to help overcome this challenge is administration of nitroglycerine before the CTA acquisition. Nitroglycerine dilates the coronary arteries and therefore improves image quality by reduction of the effect of volume averaging artifacts and improves the contrast-to-noise ratio of smaller vessels.

Putting it all together, the ideal patient for coronary CTA is a thin person who is not pregnant, not too young (radiation issue), and not too old (prevalence of heavy calcification), with a low and steady heart rate, with normal renal function, and otherwise no contraindication to iodinated contrast material, beta-blockers, or nitroglycerine. Additionally, this patient should have an unanswered clinical question, a question that coronary CTA has the potential to answer, and a question the answer to which may affect the patients’ clinical management. Clinical indications currently considered valid for coronary CTA will be reviewed later in the chapter. Please note that currently is the most important word of the last sentence; cardiac CT is a rapidly developing field (as are many others in medicine) and much of what may be accepted as standard of care today may be considered obsolete within a decade or even sooner.

Before discussing the clinical applications of cardiac CT, the technical limitations and pitfalls of this technique must be reviewed.

TECHNICAL DEVELOPMENT OF SCANNERS

Recent developments in mechanical cardiac CT have dramatically improved the ability of CT to visualize the heart and coronary arteries. The major improvements that have made this possible are fast gantry rotation speed and image reconstruction algorithms that allow us to use only a subset of projections from one (or possibly multiple) rotation off the gantry. Generally, half a gantry rotation is necessary to acquire all projections that generate an axial image. The temporal resolution is the time it takes to collect these projections and is calculated as one-half the gantry rotation speed. Thus, if the gantry rotation speed were 330 ms (gantry spins around the patient 3 times per second), then the temporal resolution is 1/2 × 330 ms equaling 165 ms. The temporal resolution is comparable to the shutter speed of a camera; the shorter (or faster) the shutter speed is, the more likely are you to generate motion free images of a rapidly moving object. Although 165 ms represents one of the fastest temporal resolutions of current 64-slice multidetector computed tomography (MDCT) systems, it is not fast enough to obtain motion-free images of the coronary arteries in all phases of the cardiac cycle. Therefore, image reconstruction is typically performed in mid to late diastole, where there is the least cardiac motion. Additionally, beta-blockers are administered before the scanning to reduce the patient’s heart rate to 60 beats per minute or below. This results in a longer diastolic rest period, less motion of the coronary arteries, and reduces the risk of motion artifact.

If the development of cardiac CT over the past decade is reviewed, it is noted that cardiac imaging was possible with 4-slice systems, but most research studies that were conducted removed a substantial number of coronary artery segments (up to 30%) from their analysis because of nonevaluability. This was in part as a result of presence of motion artifact (image blurring). The reason for the large amount of motion artifact with these systems is the slower gantry rotation speed of approximately 500 ms (resulting in a temporal resolution of 250 ms). The major improvement with 64-slice CT systems was in the reduction of the number of nonevaluable segments, which is in keeping with the improved temporal resolution. Typical gantry rotation speeds of 16-slice scanners are 420 ms to 370 ms (even though some remained at 500 ms at first), resulting in temporal resolutions typically ranging from 210 ms to 185 ms. This has caused the number of nonevaluable coronary artery segments to go down to on average approximately 6%. Keep in mind that temporal resolution and the resulting motion artifact is only one of the two major reasons for unevaluable segments, the other one being presence of dense coronary calcium. The next major step in the development of cardiac CT was the 64-slice MDCT generation. All major vendors have offered a 64-slice MDCT system, even though the number of slices is calculated in different ways. Some vendors have actually 64 equally sized detector rows within the gantry and have x-rays emitted from one focal spot in the x-ray tube. One vendor, however, uses 32 equally spaced detectors and two focal spots on the x-ray tube that alternate in emitting x-rays. Thus they acquire two different projections for each of the 32 detector rows, resulting in 64 individual projections.

Up to 64-slice technology, all vendors were going along the same route with their technical innovations. However, from here on, there are substantial differences in the newer generation of scanners. One vendor developed a two-x-ray tube and two-detector array system, that is, a dual-source system. This allows collecting 180 degrees worth of projections in only one quarter of an actual gantry rotation. Thus the temporal resolution is one fourth the gantry rotation speed (not one half). At a gantry rotation speed of 330 ms, this system has a temporal resolution of 83 ms, currently the fastest temporal resolution available.

Two other vendors have widened their detector arrays to 256 and 320 detector rows, which cover up to 16 cm of the chest with only one rotation. This eliminates the need for gating and some arrhythmia issues; however, the temporal resolution remains lower than for dual-source CT (gantry rotation/2), and therefore motion artifact remains an issue. Another vendor is improving the axial step-and-shoot acquisition method to allow substantial reduction in radiation dose in patients with low heart rates. However, patients with higher heart rates may not benefit from this acquisition method.

TECHNICAL PRINCIPLES

There are a number of technical aspects that are substantially different for cardiac CT compared to all other CT applications. They revolve around improving temporal resolution, synchronization of data acquisition with the cardiac cycle, and minimizing radiation dose to the patients. The latter is an important issue because cardiac CT has a substantially higher radiation dose compared to nongated chest CT. This is because of the small pitch that is used. To allow an image to be reconstructed at any location in z-axis and at any phase of the cardiac cycle, redundant data have to be acquired. This is generally achieved by using a pitch as small as 0.2 to 0.3. This means that each rotation around the patient overlaps to 80% with the previous, or in other words, each section through the heart may see x-rays from up to five consecutive rotations.

Retrospective Gating versus Prospective Triggering

There are two general approaches to cardiac synchronization of the CT acquisition. One is prospective and “observes” the electrocardiogram (ECG) for a small number of heart beats (or more accurately the peak of the R-wave or R-peak) and then anticipates when the next R peak is to occur. Given the anticipated time point of the future R peak, the scanner will then only acquire x-ray projections in a prespecified phase of the cardiac cycle (usually late in diastole where the heart is most quiescent). This approach is called prospective triggering because the x-ray tube is triggered to shoot in a predefined cardiac phase. Data acquisition is in an axial fashion, and the table only moves in between heartbeats and is stationary during x-ray transmission. This approach has the advantage of having a low radiation dose to the patient, but it has a number of disadvantages. One of the major disadvantages is that typically only one dataset (or few similar ones) can be acquired in the anticipated cardiac cycle, which may not turn out to be of optimal image quality (Fig. 8-1).

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FIGURE 8-1 Prospective triggering mode. This mode uses sequential axial (nonspiral) acquisitions during every other heart beat. X-rays are only emitted during a predefined cardiac window (typically diastole), and the tube is turned off during the reminder of the cardiac cycle. In the in-between heartbeats the table moves forward by the equivalent of the detector width (step-and-shoot mode).

The newer 256- and 320-slice scanners use a modified step-and-shoot mode. Because their detectors cover a large volume, in many cases the entire heart, no table motion is necessary to acquire a coronary CTA dataset. Having the tube current turned on for approximately one entire heartbeat allows acquiring a dataset for analysis of cardiac anatomy and function. Theoretically, x-ray exposure can be limited to a short segment in diastole if only coronary artery visualization, and if no information on function is desired. Multi-phase reconstruction (to improve temporal resolution) would, however, require data acquisition during several consecutive cardiac phases (Fig. 8-2).

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FIGURE 8-2 Whole organ coverage computed tomography. Current 320-slice scanners have a detector width of 16 cm in z-dimension, which allows acquisition of a coronary computed tomography angiography without the need to move the table. The x-ray tube is turned on only for the duration of one heartbeat. If whole chest coverage is desired, a step-and-shoot mode with only two steps is used.

A radically different approach is retrospective gating (Fig. 8-3). Retrospective gating allows acquiring unlimited complete datasets in any phase of the cardiac cycle. This approach uses a spiral CT acquisition, in which the x-ray current remains turned on during the entire scan. The user may then in retrospect define what phase of the cardiac cycle to reconstruct. The major advantage of this approach is that the interpreter may decide to try a different phase of the cardiac cycle if the initial reconstruction demonstrates motion artifact. Another advantage is the ability to “edit” the ECG. ECG editing allows the user to select heartbeats that should not be used for reconstructions (e.g., premature ventricular contractions [PVCs]), or to correct trigger points that were not placed on an R peak by the computer algorithm. The major disadvantage of retrospective gating is the high radiation dose to the patient. For this reason a number of dose reduction strategies were developed.

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FIGURE 8-3 Retrospective gating. This mode is a spiral computed tomography angiography acquisition, in which the tube emits x-rays during the entire cardiac cycles over a 5- to 20-second period (depending on z-axis coverage and scanner type). Data that were acquired during a specific cardiac phase (typically diastole) are then retrospectively selected to reconstruct the corresponding images. Any phase of the cardiac cycle (early systole to late diastole) can be reconstructed. Functional cine images can be obtained by reconstructing datasets every 10% of the cardiac cycle.

Tube Modulation

One of the most important dose reduction strategies in cardiac CT is ECG-correlated x-ray tube current modulation or short tube modulation. In this algorithm the scanner does perform a spiral acquisition using the retrospective gating method; however, it prospectively down regulates the x-ray tube current (typically in systole and usually down to approximately 20% of the maximum). This reduces the radiation dose to the patient during systole, but nevertheless allows for reconstructions of images in systole, if necessary. The penalty for reducing the tube current is a higher noise level. This approach is clearly a compromise trying to capture the advantages of both the prospective triggering and the retrospective gating acquisitions (Figure 8-4).

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FIGURE 8-4 Retrospective gating with x-ray tube modulation. This mode is a variant of retrospective gating, in which the tube emits the desired amount of x-rays during diastole (or any other predefined window), but in which the tube current is substantially reduced during the remainder of the cardiac cycle. This allows optimal noise levels during diastole, but results in increased noise in systole. This technique allows reduction of the radiation dose by up to 50% if the heart rate is low.

HALF-SCAN VERSUS MULTI-SEGMENT RECONSTRUCTION

To capture images without blurring from rapid cardiac motion, it is important to achieve a high temporal resolution. Temporal resolution refers to the time it takes to collect all the data (projections) to generate an axial source image. The time it takes to collect these data (the temporal resolution) is dependent on how fast the CT gantry spins around the patient. In conventional CT, the temporal resolution is equal to the gantry rotation speed. There are, however, ways to improve the temporal resolution for cardiac CT imaging. Two algorithms can be applied, the “half-scan” reconstruction algorithm, or the “multi-segment reconstruction” algorithm (Fig. 8-5A, B).

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FIGURE 8-5 Half-scan reconstructions compared to multi-segment reconstruction. A, The half-scan (partial scan or single segment) reconstruction algorithm takes advantage of the fact that only 180 degrees worth of projections are needed to generate an image. Note that 180 degrees refers to the x-ray tube position and that that results in projections covering 180 degrees plus the fan angle. This reconstruction mode results in reduction of the temporal resolution by 50% (one-half the gantry rotation speed) compared to conventional algorithms. B, The multi-segment reconstruction algorithm allows utilizations from projections that are collected from two to four subsequent heartbeats at identical cardiac phases. This assumes that the heart returns to identical coordinates during consecutive cardiac cycles. This mode allows collection of all necessary projections during a shorter time window with respect to the cardiac cycle length (improved temporal resolution), even though the overall time it takes to collect these projections is stretched over two to four heartbeats.

SCAN ACQUISITION

Beta-Blockers and Nitroglycerine

With current generation of single source scanners (64-slice, 256- or 320-slice MDCT), it is advisable to slow the heart rate by administration of beta-blockers. This can be done by oral administration, intravenous application, or a combination of both. The effect not only reduces the motion of the coronary within the image acquisition window (less motion artifacts), but it also results in a lower radiation dose if ECG based tube current modulation is used. The application of oral nitroglycerine immediately before scanning results in increased diameter of the coronary arteries and substantially improves evaluability.

Breath Holding and Positioning

It is important to practice inspiratory breath holding with the patient on the CT table before scanning. This has a number of important beneficial effects. First, the patients are familiar with the breath hold instructions and are more likely to perform optimal breath holds. Second, the physician or technologist can ensure that patients perform sufficient breath holds (long enough, no slow exhalation, etc.). Third, the heart rate varies between rest and breath holding, and the beta-blocker dosage can be adjusted to the heart rate that is seen during the breath holds.

Contrast Injection

There are a number of injection protocols that can be applied to coronary CTA. It is beyond the scope of this chapter to review all these. However, there are a number of common principles that are important to review. The intravenous catheter is ideally of large bore and placed in the right antecubital fossa. The right-sided injection is somewhat preferred because of a shorter route to the heart and because it avoids the crossing of dense contrast past the left internal mammary artery (LIMA) and the great vessels via the left innominate vein. This has the potential to cause streak artifact, which, for example, can hinder the assessment of a portion of a LIMA graft.

High flow rates (5 to 7 ml/sec) are needed for adequate opacification of the coronary arteries. Larger patients generally require faster flow rates (and larger overall volumes) to achieve the same opacification as smaller patients. Dual-head injection pumps are standard and allow a saline chaser bolus to follow the contrast injection (biphasic protocols). This allows for washing out the veins and right atrium.

Some sites prefer using triphasic protocols, in which the initial injection is contrast at a fast rate, say 5 ml/sec (phase 1), followed by a slower rate of contrast (e.g., 2 ml/sec), or a mixture of saline and contrast (phase 2), and lastly followed by pure saline (phase 3). The proposed advantage of these protocols is that they result in “better” opacification of the right ventricle, while still avoiding excessive mixing artifact. Thus, evaluation of right ventricular function and ventricular septal motion is thought to be improved. However, the value of triphasic over biphasic injection protocols has not been shown today.

Image Reconstruction (Best Phase Selection)

The initial reconstruction of images of coronary CTA datasets is usually performed in mid to late diastole in which the heart is most quiescent. Late diastole is usually less desirable because the atrial kick generates rapid motion of the coronary arteries, especially the right and left circumflex coronary artery. The field of view of the axial source slices is limited to the heart to maximize in-plane resolution (12 to 15 cm). Slice thickness is 0.6 to 1 mm, and most centers use an overlap of about one third of the slice thickness. (e.g., 0.75-mm slices with 0.5-mm spacing). Typically the images are initially analyzed for presence of coronary motion artifact, and if motion blurring is found, additional reconstructions are necessary. Late systolic reconstructions may be helpful if right coronary artery (RCA) motion cannot otherwise be eliminated by reconstructing various diastolic phases. During interpretation the interpreter may need to jump back and forth between different phases because each segment of the coronary arteries may be best displayed in different phases. In the presence of stents or calcium that makes interpretation difficult, reconstructions are repeated with sharper high-contrast kernels. Additional reconstructions of a larger field of views are performed with thicker slices for a review of extra cardiac structures.

CORONARY ANATOMY

The RCA originates from the right sinus of Valsalva. The first RCA branch is the conus branch, which supplies the myocardium of the right ventricular outflow tract (RVOT). Occasionally the conus branch may have a separate ostium from the right sinus. The RCA gives rise to anterior right ventricular (RV) free wall branches and acute marginal branches that run along the angle that the anterior and inferior RV free walls form. The RCA is dominant in ≈80% of cases and runs in the right atrioventricular groove up to the crux of the heart (the point of the inferior cardiac surface where the atria and ventricles meet), where it bifurcates into a posterior descending artery (PDA) that runs within the inferior interventricular groove, and a posterior left ventricular branch (PLV) that supplies the inferior left ventricular (LV) wall (Figures 8-6, 8-7, 8-8). The PLV often gives rise to a small atrioventricular nodal branch at the crux of the heart. If the RCA is nondominant, it usually does not reach the crux of the heart, and the PDA and PLV are supplied by the left circumflex coronary artery (LCX).

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FIGURE 8-6 Normal right coronary artery (RCA). A, Invasive angiogram of normal RCA (white arrows) shows conus branch (open arrow) arising from proximal RCA, acute marginal branch (solid black arrow), and bifurcation of RCA into posterior descending artery (arrowhead) and posterior left ventricular branch (open curved arrow). B, Volume-rendered reconstruction of coronary computed tomography angiography in a different patient shows normal right coronary artery (white arrow), small conus branch (black open arrow), acute marginal branch (white open arrow), posterior descending artery (arrowhead), and posterior left ventricular branch (open curved arrow).

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FIGURE 8-7 Right dominant system. Inferior volume-rendered view of distal right coronary artery (arrows) shows bifurcation into posterior descending artery (arrowhead), and posterior left ventricular branch (open curved arrow), indicating a right dominant system. The distal left circumflex artery (curved arrow) is small. LV, left ventricle; RV, right ventricle.

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FIGURE 8-8 Normal right coronary artery (RCA). Maximum intensity projection (MIP) and curved multiplanar reconstruction images of normal RCA. A, MIP image of the RCA is referred to as the “C-view,” because the RCA (white arrows) may frequently be visualized in its entirety and resemble the letter C. Side branches are typically only partially visualized because they leave the image plane. Note: conus branch (black open arrow), posterior descending artery (arrowhead), and posterior left ventricular branch (curved arrow). The atrioventricular node branch can be seen ascending from the cranial most portion of the PLV. B, Curved multiplanar reconstruction image of the computed tomography angiography shows a centerline reconstruction of the entire right coronary artery (arrows) but distorts all other anatomy. Note: conus branch (black open arrow), acute marginal branches (black arrows), posterior descending artery (arrowhead), and posterior left ventricular branch (curved arrows).

The left main coronary artery (LM) origin is usually more cephalad compared to the RCA ostium. The LM originates from the left sinus of Valsalva and bifurcates within 2 cm of its origin into the left anterior descending artery (LAD) and LCX (Figures 8-9, 8-10). Occasionally there is no LM, and the LAD and LCX both originate directly from the left sinus of Valsalva (Fig. 8-11). An LM trifurcation is a situation in which there is a third branch arising from the LM between the LAD and LCX (Fig. 8-12). This branch is called ramus intermedius.

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FIGURE 8-9 Normal left main coronary artery (LM). A, Invasive angiogram of normal LM (black arrow) shows left anterior descending artery (white arrows) giving rise to diagonal branches and septal perforators, and nondominant left circumflex artery (curved arrows) giving rise to two large obtuse marginal branches (arrowheads). B, Volume-rendered reconstruction of coronary computed tomography angiography (different patient) shows normal left main coronary artery (black arrow), left anterior descending artery (white arrows) giving rise to two small diagonal branches (open arrow), and left circumflex artery (curved arrows) giving rise to a large proximal obtuse marginal branch (arrowheads).

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FIGURE 8-10 Normal coronary anatomy. Volume-rendered reconstruction of coronary computed tomography angiography shows the opposite end of the spectrum of normal (compared to Figure 8-9) with a large branching first diagonal branch (open white arrows) and a small left circumflex artery with no obtuse marginal branches. Note the left anterior descending artery (white arrows).

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FIGURE 8-11 Absent left main coronary artery. Volume-rendered reconstruction of coronary computed tomography angiography shows a variation of normal anatomy with absence of a left main coronary artery (LM) and the left circumflex artery (LCX) and the left anterior descending artery (LAD) arising directly from the left sinus of Valsalva (black arrows). Note LAD (white arrows) with diagonal branches (open arrows) and LCX (curved arrows) with obtuse marginal branch (arrowhead).

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FIGURE 8-12 Ramus intermedius. Volume-rendered reconstruction of coronary computed tomography angiography shows left main coronary artery (black arrow) trifurcating into a ramus intermedius (black arrowheads), left anterior descending artery (white arrow), and left circumflex artery (curved arrow). Note: diagonal branch (open arrow) and obtuse marginal branch (white arrowhead).

The LAD runs in the anterior interventricular groove and gives rise to septal perforators that perfuse the ventricular septum and to diagonal branches that supply the anterior LV wall. The distal LAD commonly wraps around the apex, where it may form collaterals to the PDA.

The LCX runs in the left atrioventricular groove and gives rise to obtuse marginal branches and posterolateral branches. If the PDA and PLV arise from the LCX, then the system is considered left dominant (Fig. 8-13). Codominance is present if both the RCA and the LCX provide a PDA branch.

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FIGURE 8-13 Left dominant coronary arterial system. Volume-rendered reconstructions of coronary computed tomography angiography (CTA) shows dominant left circumflex artery (curved arrows) giving rise to a posterior left ventricular branch (open curved arrow) and terminating as a posterior descending artery (arrowhead).

CLINICAL ROLE OF CORONARY COMPUTED TOMOGRAPHY ANGIOGRAPHY

Detection of Coronary Artery Stenoses

The most important role for cardiac CT is to detect stenoses in the coronary arteries that likely will be hemodynamically significant and therefore will be the cause of the patient’s symptoms. The gold standard against which CT is usually measured is conventional angiography, where stenoses that have greater than or equal to 70% of luminal narrowing are considered likely hemodynamically relevant or “significant stenoses.” The only exception is the LM, where greater than or equal to 50% stenosis is considered a significant stenosis. That the angiographic stenosis degree is only a surrogate for hemodynamic significance must be kept in mind. The best way of obtaining that information is via direct flow measurements (fractional flow reserve [FFR]), which are almost never obtained because of the required higher degree of invasiveness and the associated increased cost and complication risk. In any case, when interpreting CTA datasets, we are looking for that CTA equivalent of a greater than or equal to 50% stenosis in the LM and greater than or equal to 70% stenosis everywhere else.

Clinical applications of coronary CTA will critically depend on its accuracy for detection of significant stenoses. Numerous recent studies have assessed the accuracy of coronary CTA for stenosis detection in comparison to invasive, catheter-based coronary angiography. Using 40-slice CT, 64-slice CT, or dual-source CT, the sensitivity for the detection of coronary artery stenoses ranged from 86% to 100%, and the specificity ranged from 91% to 98%. Accuracy values are not uniform across all groups of patients. Several trials have convincingly shown that high heart rates and extensive calcification negatively influence accuracy. Usually, false-positive findings will occur if image quality is degraded and specificity will therefore be affected worst.

A recent meta-analysis has carefully analyzed and summarized the accuracy data that are available for coronary CTA with various generations of CT technology. The authors have demonstrated a significant increase in the sensitivity and specificity for stenosis detection as scanner technology progressed from 4-slice to 16-slice and 64-slice equipment (Table 8-1). For 64-slice CT, the analysis indicates a pooled sensitivity of 93% and specificity of 96% based on a per-segment analysis and sensitivity of 99% and specificity of 93% based on a per-patient analysis (363 patients total). These results confirm the high accuracy of coronary CTA for the detection of coronary artery stenoses, but it has to be taken into account that this applies to somewhat selected patients with stable sinus rhythm and usually a low heart rate, ability to cooperate and perform at least a 10-second breath hold, and absence of renal failure, previously implanted coronary stents, or previous bypass surgery. Also, studies were performed in single, experienced academic centers.

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TABLE 8-1 Sensitivity and specificity of coronary computed tomography angiography for the detection of coronary artery stenoses in comparison to invasive coronary angiography: Meta-analysis of pooled data

Rights were not granted to include this table in electronic media. Please refer to the printed book.

(From Vanhoenacker PK, Heijenbrok-Kal MH, Van Heste R, et al.: Diagnostic performance of multidetector CT angiography for assessment of coronary artery disease: meta-analysis, Radiology 44:419-428, 2007.)

Across all published studies, the negative predictive value of coronary CTA was uniformly high, ranging from 93% to 100%. This indicates that coronary CTA will be able to reliably rule out coronary artery stenoses in patients comparable to those that were included in these published trials if image quality is good. It has to be taken into account, however, that both the positive and negative predictive value will be influenced by the pretest likelihood of disease in the patient that is investigated—the negative predictive value will be lower for patients with a high pretest likelihood (which means that a negative result is more likely to be incorrect), whereas in patients with a low pretest likelihood, a positive result is more likely to represent a false-positive finding. As a consequence, the clinical use of coronary CTA—as that of any other diagnostic test—will substantially depend on the patient group that is investigated. In a recent publication by Meijboom and colleagues, the diagnostic accuracy of coronary CTA was analyzed in the context of the pretest likelihood of disease. It was clearly shown that the diagnostic value of CTA was highest in patients with a relatively low pretest likelihood of disease and lowest in those patients in whom the clinical presentation suggested a high likelihood that coronary stenoses would be present. Thus, clinical applications of CTA seem to be most beneficial whenever the clinical situation implies a relatively low pretest likelihood of coronary disease, but still requires further workup to rule out significant coronary stenoses. In clinical cardiology, this situation, both in the setting of stable symptoms and acute chest pain, is not infrequent.

Clinical Applications

As previously outlined, coronary CTA has high accuracy for the detection of coronary artery stenoses, if it is performed with adequate equipment and interpreted with sufficient experience. However, despite the impressive image quality—which continues to improve—CT imaging will not constitute a general replacement for invasive, catheter-based diagnostic coronary angiography in the foreseeable future. Arrhythmias, most prominently atrial fibrillation, high heart rates, severe calcification, and contraindications to iodinated contrast agent are problematic and will preclude CT angiography in many patients who require a workup for coronary artery disease. Furthermore, in patients with diffuse, severe disease or with small coronary arteries (e.g., as frequently encountered in patients with diabetes), the spatial resolution of CT may not be sufficiently high to allow reliable interpretation the coronary system. For challenging cases like these, invasive angiography will remain the best diagnostic option.

Nevertheless, there is an important clinical role for coronary CTA. One of the biggest strengths of cardiac CT is its high negative predictive value. When assessing for a coronary artery stenosis, a negative CT (with good image quality) has an extremely low chance for having missed a hemodynamically significant coronary stenosis.

Therefore, CT is most useful to rule out coronary artery stenoses with a high degree if certainty in a population in which coronary artery disease is a clinical consideration, but in which the clinical suspicion for stenosis is not too high.

This is said because of a number of factors: On one hand, if the pretest likelihood of coronary obstruction is high, or if there is established coronary artery disease, then the image quality is frequently reduced as a result of presence of substantial calcification and possibly smaller luminal diameters. Both factors decrease the accuracy of CTA. On the other hand, interventions cannot be performed during CT, but radiation and contrast material are administered, and therefore the patient may lose valuable time and receive unnecessary contrast and radiation because the likelihood of having to undergo a catheter-based angiogram anyway is high. The other extreme are asymptomatic patients. It has not been shown that there is any benefit from performing coronary CTA in asymptomatic persons. Even if a high-grade stenoses were detected, and consequently treated (e.g., by stent placement), this does not necessarily translate to a benefit for the patient because revascularization of asymptomatic stenoses has not been shown to improve survival. The major benefit of revascularization is improvement of symptoms; because asymptomatic patients do not have any symptoms, it is hard to make them feel better.

Thus, CTA is a good choice in patients who are symptomatic and who have a low to intermediate clinical suspicion for actually having a coronary artery stenosis (especially if nuclear stress testing cannot be performed or is inconclusive). If CT clearly shows absence of coronary artery stenoses, any further testing is not necessary. If, on the other hand, stenoses are expected with high probability, the patient should rather be directed toward invasive angiography.

The potential value of cardiac CT to exclude obstructive coronary artery disease in patients with special circumstances (e.g., cardiomyopathies, preoperative clearance) has been recently examined. In many of these special circumstances, invasive angiography is usually performed to rule out stenoses, but often shows clean coronaries. For example, coronary CTA has been demonstrated to have excellent accuracy (100% in a small series) for exclusion of stenoses in patients with cardiomyopathies. In patients with heart failure of unknown etiology, 16-slice CT had a sensitivity of 99% for detection of coronary artery stenoses when compared to invasive angiography.

Other data has recently become available that demonstrates the high accuracy of coronary CTA patients with left bundle branch block (sensitivity of 97%, specificity of 95%, and negative predictive value of 97% for stenoses >50%).

One untapped potential of cardiac CT is its use for cardiac clearance before major noncardiac or noncoronary cardiac surgery. Few studies have targeted this population to date, but preliminary data is promising. For example, one study examined 70 patients with aortic valve stenosis with 64-slice CT and invasive angiography before surgery. The study showed that not a single patient with significant coronary stenosis would have been missed if cardiac CT alone were used for preoperative evaluation (sensitivity = 100%). In a similar study 64-slice CT was used to evaluate 50 patients before surgery for aortic valve regurgitation and found also a sensitivity of 100% and a specificity of 95% for the identification of patients with coronary artery stenoses. Because invasive coronary angiography is currently routinely performed in such patients with the only aim to rule out coronary artery stenoses, coronary CT angiography may become an important substitute for invasive testing in many of these patients.

Acute Chest Pain

At the time of writing of this chapter, there is no established role for coronary CTA in the clinical emergency department setting. However, numerous single and multicenter studies that have been recently published, or are on the way, and it is likely that in the near future (maybe at the time you are reading this text) there will be a defined role for cardiac CT in the acute chest pain setting.

This role will be limited to patients who do not rule in for a myocardial infarction based on their ECG and laboratory findings. The reason for this is that if there were typical ST-segment changes and a documented cardiac enzyme leak, this patient needed to proceed to the catheter lab for revascularization and the performance of a coronary CT would waste valuable time. The prevalence of those patients who are admitted with a rule out MI diagnosis is relatively low. Preliminary data suggest that cardiac CT may drastically reduce the amount of unnecessary admissions, without risking to wrongfully discharging patients from the emergency department.

Coronary CTA has the potential to reliably differentiate patients who have significant coronary artery stenoses from patients without obstructive coronary, allowing for rapid triage with a quick and noninvasive test.

Stents

Evaluation of coronary artery stents with coronary CTA remains challenging. It is possible to visualize stent lumen if the CT is of excellent quality, and if the contrast opacification is high, the noise level is low, motion and respiratory artifacts are absent, and the stent is large and in a proximal location. Such a combination of favorable conditions, however, is often not present.

In general, CT for assessment of stents should be discouraged. Exceptions are if one has the knowledge of a large stent diameter (3.5 or greater tend to be evaluable) and proximal location. Other exceptions are if the stent location is in one vessel, but the clinical interest is in another. In a patient with questionable inferior wall ischemia on a nuclear perfusion test it would be of most interest if the RCA were patent, and if there were a nonevaluable stent in this patients LAD, it may not matter clinically.

A number of recent studies regarding the ability to assess stents for restenosis revealed many factors that influence the accuracy CT. These include the stent type and diameter, the overall quality of the dataset, and the location (LM stents are often evaluable, even though in the United States this vessel is rarely stented). The accuracy of CT in those stents that are deemed to be evaluable ranges from only 75% up to 95% depending on the study. Because these numbers are not great, given the fact that many stents are not evaluable at all, most experts discourage the use of coronary CTA to assess patients with implanted stents.

Bypass Grafts

Imaging of patients with bypass grafts by CT angiography has been possible since the days of electron beam CT. The relative large diameter and little motion of saphenous vein grafts results in reliably good image quality, compared to the native coronary arteries. Imaging of internal mammary artery (IMA) grafts is more difficult because they are much smaller and often are accompanied by numerous metal clips that cause artifacts.

A number of studies have shown that graft occlusions and stenoses can be detected with high diagnostic accuracy by CTA (high 90% to 100%). However, the native arteries in graft patients are harder to evaluate compared to nongrafted patients. One reason for that is that coronary artery disease is more likely present (≈100% of patients—it is the reason why they received the grafts), which comes with a much higher calcium burden. The presence of calcium, plus surgical material and potentially stents makes assessment of the native coronary arteries very difficult.

A recent 64-slice CT study found a sensitivity and specificity of only 86% and 76%, respectively, for the detection of stenoses in the native coronary arteries after patients with previous bypass surgery.

On the other hand, there are a number of complications that can occur with bypass grafts. CT is an excellent tool to depict such complications. The most common complication is graft stenosis or occlusion. Occluded graft can be completely invisible on CT, with the exception of a small out-pouching at the aortic anastomosis site. It is important to obtain information from a surgical report in which it is possible to correlate with the CT appearance. A mismatch usually indicates a graft occlusion. Perhaps the most often missed occlusion is a partial graft occlusion that affects only the distal portion of a graft that has more than one distal anastomosis. A jump graft is one that has a side-to-side anastomosis first (e.g., to a diagonal branch), but continues to anastomose to a second vessel more distally (e.g., end-to-side anastomosis to an obtuse marginal branch). Occasionally, the graft segment to the initial side to side anastomosis remains patent, whereas the distal portion completely occludes. The latter can be difficult to discern, but information from an operative note will make this an easy call. Stenoses of bypass grafts are readily detectable, and the reported accuracy is high.

If a LIMA graft is present, it may be useful to also interrogate the left subclavian artery for stenosis because a stenosis here may lead to myocardial ischemia as well.

Another potential complication of grafts is formation of aneurysms. Unlike invasive angiography, CT has the advantage to visualize the graft lumen plus potentially present mural thrombus. The larger the aneurysm diameter, the higher the chance of rupture. Graft aneurysms are readily detected with gated cardiac CT, but are also detectable with nongated CT. A mass adjacent to the ventricles in a patient with sternotomy wires in place should raise the concern for graft aneurysm. If there is any doubt, a gated CT should be obtained before considering a biopsy.

When evaluating grafts, it is important to critically evaluate the native coronary arteries as well. Naturally we expect obstructive disease proximal to the anastomotic sites. These lesions are the reason the patient received the grafts in the first place. It is important to evaluate any vessel that may not be protected by a graft and that may have formed a stenosis in the interim. It is also important to judge the vessels downstream from the anastomosis for interval development of stenosis.

Another issue to be aware of is the location of the grafts and the right heart with respect to the sternum. This becomes important in the setting of planned reoperations. Occasionally grafts may be pasted immediately against the midline sternum or the sternal wires, which carries the risk for graft injury during resternotomy. Surgeons value this kind of information because it may prevent significant operative morbidity. A sagittal oblique plane through the sternum and the adjacent grafts is best to communicate these anatomical relationships. Occasionally the space between anterior RV free-wall and sternum can be extremely tight, which increases the risk for cardiac perforation during the opening of the chest.

IMAGING CORONARY ATHEROSCLEROTIC PLAQUE

One of the unique abilities that coronary CT has is that it allows for noninvasive visualization of nonstenotic, noncalcified coronary atherosclerotic plaque, which is often referred to a “soft” plaque. Next to the subjective nature of the term soft, this is not a good choice in nomenclature because there are two fundamentally different categories of noncalcified plaques, the fibrous plaque and the thin-cap fibroatheroma (“vulnerable plaque,” or lipid-rich plaque). Although the latter would likely feel soft if one were to touch it, this cannot be said for fibrous plaques (these should feel rather firm). Because the lipid-rich plaques are potentially dangerous—they are prone to plaque rupture and myocardial infarction, and because fibrous plaques are considered stable—it would be wrong to lump them together and to refer to both as soft plaques. The proper terminology is noncalcified plaque.

There are a small number of studies that have determined the accuracy of coronary CTA for identification of noncalcified plaque, as compared to the gold standard of intravascular (intracoronary) ultrasound (IVUS or ICUS). The overall sensitivity for detection of such noncalcified plaques ranges from ≈80% to 90%, suggesting that finding these plaques with CT is possible. However, the goal to predict which plaques are more likely to cause future harm (the “vulnerable plaques”) has not quite been met. Although the average CT attenuation within fibrous plaques is higher than within lipid-rich plaques (mean Hounsfield units [HU]: 91-116 compared to 47-71), the variability (standard deviation) of density measurements within plaque types is too large, and density measurements are too heavily influenced by contrast attenuation of surrounding structures to allow prospective classification of plaques as lipid rich versus fibrous or as stable versus vulnerable.

There has been some retrospectively collected evidence in patients with acute coronary syndromes that showed a higher prevalence of noncalcified plaque and positive remodeling in lesions that were responsible for cardiac events compared to patients that had stable angina. However, we have to keep in mind that the CT scans were performed after the ischemic event, and plaque rupture and that thrombus formation may be partially responsible for the CT appearance.

Coronary Anomalies and Fistula

Congenital anomalies of the coronary arteries are found in approximately 1% to 2% of the general population. A number of imaging techniques may depict coronary anomalies, including MRI, CT, and invasive angiography. Today, cardiac CT is considered the gold standard test for evaluation of coronary anomalies, and often the test of choice for the characterization of coronary fistulas. The major reason for this is that CT can readily depict the proximal coronary arteries in a three-dimensional dataset with high isovolumetric spatial resolution and high contrast, and at the same time will show the surrounding structures. Although coronary magnetic resonance angiography is promising, it does not produce three-dimensional datasets at equal spatial resolution, is much more technically challenging, and is usually only available at specialized centers.

A number of studies have demonstrated the effectiveness of CT for characterizing the key features of coronary anomalies. These features include the origin of the abnormal vessel and its course. When determining benign versus malignant character of a coronary anomaly, it is important to determine whether the abnormal coronary artery courses between the ascending aorta and the aortic root. Those anomalies that course between the two great vessels are usually said to be at risk of coronary compression and sudden cardiac death. There is consensus that passage of the LM with right-sided origin between the aorta and pulmonary artery constitutes a malignant coronary anomaly. A variant of this coronary anomaly that is considered “less malignant” is an abnormal LAD or LM that originates from the right sinus of Valsalva but passes below the PA. In this variant, the abnormal vessel actually runs through myocardium just below the origin of the PA, and further distal it runs in the ventricular septum, before surfacing to the left and anterior to the aorta. The fact that the vessel courses through myocardium is considered somewhat “protective”: Frequently found is an anomalous RCA arising from the left sinus of Valsalva and turning right to pass between the PA and ascending aorta. There is no uniform opinion as to whether this situation constitutes a malignant variant. Additional features that are considered predictors of risk of sudden cardiac death are a slitlike ostium of the abnormal vessel, and an intramural (aortic wall) course of the abnormal vessel at its ostium (which virtually always results in a slitlike ostium).

The benign variants of coronary anomalies are a heterogeneous group. The most common coronary anomaly is an abnormal origin of the LCX from the RCA or via a separate ostium from the right sinus of Valsalva, that then turns posterior and inferiorly to wrap around the posterior surface of the aortic root to eventually assume the normal course of the LCX in the left atrioventricular groove. These anomalies do not bear any risk of compression and sudden cardiac death because the course of the abnormal LCX is behind the aorta and not in between the aorta and PA (which would be anterior to the aorta and posterior to the PA). Another group of benign variants are those in which the abnormal vessel (e.g., LM or LAD) arises from the right sinus of Valsalva or RCA, and then runs anterior to the PA or the RV outflow tract.

The necessity for contrast agent injection and radiation exposure remain limitations of CT imaging as compared to magnetic resonance angiography, and as a result, magnetic resonance angiography should be considered to assess coronary anomalies in young patients and those with known contrast reactions. Coronary CTA is otherwise considered the method of choice for the workup of known or suspected anomalous coronary arteries because of the degree of reliability with which high resolution diagnostic image quality datasets are obtained. Therefore, workup of coronary anomalies has been classified as a clinically “appropriate” indication in a recent multi-society expert consensus statement on appropriateness criteria for cardiac CT.

Coronary artery fistulas have a variable appearance on cross-sectional imaging or catheter angiography. The appearance depends on the number and site of abnormal connections between the coronary arterial system and a cardiac chamber or lower pressure vascular system. If the fistula connects to a cardiac chamber, such as the right ventricle, it is referred to as coronary-cameral (camera, Latin for “chamber”) fistula. General imaging features include one or multiple feeding vessels, which are usually dilated and tortuous. Often there is an aneurysmal dilatation just proximal to the abnormal connection with the lower pressure system. One of the more common types is a fistula between branches of the coronary arteries and the anterior surface of the PA. These may have one or multiple feeders from LAD branches, the conus branch of the RCA, or mediastinal branches of the aorta. Fistulas can occur with the right and left atria and ventricles or the coronary venous system. One extreme type of coronary fistula is if the entire left coronary artery system arises from the PA. This anomaly is referred to as anomalous left coronary artery arising from the pulmonary artery or ALCAPA, or as Bland-White-Garland syndrome. If the RCA has the abnormal origin, this entity is referred to as ARCAPA. In these anomalies, a steal phenomenon from normal coronary arteries via the abnormal vessel into the lower pressure PA develops, resulting in massive dilatation of the involved coronary arteries (Figures 8-14, 8-15, 8-16, 8-17).

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FIGURE 8-14 Anomalous origin of right coronary artery (RCA). A, Maximum intensity projection image of coronary computed tomography angiography shows anomalous origin of RCA from left sinus of Valsalva (open arrow) instead of the right (black arrow points to site of expected RCA origin). The RCA has an interarterial course between pulmonary artery (PA) and aorta. Note: extensive atherosclerotic calcification of left anterior descending artery and RCA. B, Maximum intensity projection image of coronary computed tomography angiography (different patient) shows same pathology. Arrowheads point to interarterial segment of anomalous vessel. Black arrow points to the normal site of right coronary artery ostium at the right sinus of Valsalva.

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FIGURE 8-15 Anomalous left main coronary artery (LM). Maximum intensity projection image of coronary computed tomography angiography shows abnormal origin of LM (arrow) from right sinus of Valsalva. The LM courses leftward through the myocardium between the right ventricular outflow tract and the aortic root. Although this variant is in the malignant category, its transmyocardial course through a portion of the septum is considered somewhat protective compared to cases where there is a truly interarterial course. Note: no coronary artery arises from left sinus of Valsalva (black arrowhead); right coronary artery (black arrow).

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FIGURE 8-16 Benign anomalous left circumflex artery (LCX). Volume-rendered images of coronary computed tomography angiography show abnormal origin of LCX (arrows) from right sinus of Valsalva. The circumflex courses behind the aortic root (benign variant) toward the left. Note that there is no left main coronary artery and the left anterior descending artery (LAD) arises directly from the left sinus. RCA, right coronary artery.

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FIGURE 8-17 Coronary to pulmonary artery (PA) fistula. Volume-rendered images of coronary computed tomography angiography show a tangle of vessels on the surface of the PA with feeders from the conus branch (white arrow) and a diagonal branch (grey arrow). An aneurysm (black arrowhead) at the site of the anomalous connection is commonly associated with fistulas to lower pressure chambers or vessels.

CURRENT INDICATION FOR CORONARY COMPUTED TOMOGRAPHY ANGIOGRAPHY

Coronary CTA is most appropriate to rule out significant coronary artery stenoses from atherosclerotic disease (Figures 8-18, 8-19, 8-20). However, CT is also very useful in the evaluation of aneurysms and bypass grafts (Figures 8-21, 8-22, 8-23, 8-24, 8-25, 8-26). Although the CT evaluation of stents should be discouraged if the stent diameter is 3.5 mm or less, in the presence of larger stents CT may determine the patency, occlusion, or in-stent restenosis (Figures 8-27, 8-28, 8-29).

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FIGURE 8-18 Proximal left anterior descending artery (LAD) stenosis. Coronary computed tomography angiography shows stenosis of proximal LAD (white arrow) on maximum intensity projection (MIP) image (top left) and volume-rendered image (top right). Note that the volume-rendered image only displays the luminal narrowing, whereas the MIP image shows the underlying lesion, a predominantly noncalcified plaque. The invasive angiogram (bottom) confirms a high-grade proximal LAD stenosis (black arrow).

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FIGURE 8-19 Right coronary artery stenosis. A, Coronary computed tomography angiography maximum intensity projection image (“C” view) shows a densely calcified plaque (red arrow) followed by noncalcified plaque (blue arrow). The short-axis images reveal that despite the calcium a severe stenosis can be excluded at that site; however, there is severe luminal narrowing caused by the noncalcified plaque. Yellow arrow shows normal segment distal to stenosis. B, Selective right coronary artery (RCA) invasive angiogram confirms high-grade stenosis in mid RCA (black arrow).

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FIGURE 8-20 Mild mid right coronary artery (RCA) and severe distal RCA stenoses. Coronary computed tomography angiography (left) shows noncalcified plaque causing mild to moderate luminal narrowing of the mid RCA (arrow) and a mixed plaque causing significant stenosis in the distal RCA (arrowheads) just proximal to its bifurcation into posterior descending artery and posterior left ventricular branch. These findings are confirmed on the invasive angiogram (right).

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FIGURE 8-21 Left main compression as a result of an enlarged pulmonary artery (PA). Volume-rendered (top left) and multiplanar reformatted (top right and bottom left) images from coronary computed tomography angiography show persistent ductus arteriosus (PDA; black arrows) and secondary marked dilatation of the PA. The PA exerts mass effect (arrowheads) on the aortic root and coronary arteries. Note: ostial compression and significant luminal narrowing of ostial left main coronary artery (white arrow). The angiogram (bottom right) confirms aortic root compression (arrowheads) and left main stenosis (white arrow).

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FIGURE 8-22 Right coronary artery (RCA) aneurysm. Volume-rendered reconstruction of a coronary computed tomography angiography shows fusiform aneurysm of proximal to mid RCA (arrow). The left main coronary artery, left anterior descending artery, and proximal left circumflex artery are of normal caliber.

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FIGURE 8-23 Left anterior descending artery (LAD) aneurysm. Coronary computed tomography angiography shows aortic root replacement with reimplanted coronary arteries. There is a large proximal LAD aneurysm with mural thrombus (arrows). Invasive angiography may only assess the luminal diameter and underestimate the true diameter of coronary aneurysms. LM, left main coronary artery.

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FIGURE 8-24 Normal aortic to coronary artery bypass graft (CABG). Axial image (top left) of coronary computed tomography angiography shows origin and distal portion of reversed saphenous vein CABG (arrows). The curved multiplanar reconstruction (top right) and volume-rendered image (bottom left) show a typical course of the graft (courses above the pulmonary artery) and a patent distal anastomosis (arrows) to an obtuse marginal branch. A left internal mammary artery graft to the left anterior descending artery (LAD) is occluded. Selective graft injection during invasive angiogram (bottom right) shows patent distal anastomosis (arrow) and retrograde filling of left circumflex artery, left main coronary artery, and LAD.

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FIGURE 8-25 Bypass graft occlusion. Volume-rendered images of coronary computed tomography angiography show an occluded aortocoronary vein graft (formerly to right coronary artery) as evident by a remaining stump (arrows) on the anterior aortic surface. A patent left internal mammary graft to the left anterior descending artery is present and demonstrates the typical appearance of multiple adjacent surgical clips. A patent aortocoronary vein graft (arrowheads) to the obtuse marginal branch is present.

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FIGURE 8-26 Giant bypass graft aneurysms. Anterior view volumerendered image of coronary computed tomography angiography shows three aortocoronary vein grafts (black arrows) with dilated lumens. The white arrows denote the adventitial borders of the aneurysm, illustrating the large amount of mural thrombus. Note mass effect on main pulmonary artery and pulmonary vessels from left giant aneurysm.

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FIGURE 8-27 Patent coronary stent. Curved multiplanar reconstruction of coronary computed tomography angiography shows a short 3.5-mm proximal left circumflex artery stent in two orthogonal views. The stent is patent without computed tomography evidence of neointimal hyperplasia.

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FIGURE 8-28 Stent occlusion. Coronary computed tomography angi-ography shows stent within proximal right coronary artery (RCA) with occlusion of native RCA proximal to the stent (blue arrow) and occlusion of proximal half of stent (yellow arrow). The distal RCA is severely diseased, but some contrast is noted in the posterior left ventricular branch and distal RCA, likely from collaterals.

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FIGURE 8-29 In-stent restenosis. Soft kernel reconstruction (top) and sharp kernel reconstruction (middle) of coronary computed tomography angiography shows proximal left anterior descending artery (LAD) stent with in-stent restenosis. Note: the softer kernel reconstruction exhibits less image noise, but also less image contrast, making it difficult to assess the lumen within the stent. The sharper kernel reconstruction allows for better assessment of the stent lumen, despite a higher noise level. The correlating invasive angiogram (bottom) confirms in-stent restenosis (arrow).

Appropriateness Criteria

Recently published official documents that were developed jointly by multiple medical societies, including cardiology and radiology societies, offer recommendations as to the appropriate clinical use of coronary CTA (Table 8-2).

TABLE 8-2 “Appropriate” indications for computed tomography coronary angiography according to an expert consensus document

Detection of CAD with prior test results—Evaluation of chest pain syndrome
Uninterpretable or equivocal stress test result (exercise, perfusion, or stress echo)
Detection of CAD: symptomatic—Evaluation of chest pain syndrome
Intermediate pretest probability of CAD, ECG uninterpretable or unable to exercise
Detection of CAD: symptomatic—Acute chest pain
Intermediate pretest probability of CAD, no ECG changes, and serial enzymes negative
Evaluation of coronary arteries in patients with new-onset heart failure to assess etiology
Evaluation of suspected coronary anomalies

CAD, coronary artery disease; ECG, electrocardiogram.

(From Hendel RC, Patel MR, Kramer CM, et al: ACCF/ACR/SCCT/SCMR/ASNC/NASCI/SCAI/SIR 2006 appropriateness criteria for cardiac computed tomography and cardiac magnetic resonance imaging: a report of the American College of Cardiology Foundation Quality Strategic Directions Committee Appropriateness Criteria Working Group, American College of Radiology, Society of Cardiovascular Computed Tomography, Society for Cardiovascular Magnetic Resonance, American Society of Nuclear Cardiology, North American Society for Cardiac Imaging, Society for Cardiovascular Angiography and Interventions, and Society of Interventional Radiology, J Am Coll Cardiol 48:1475-1497, 2006).

Among those documents is a consensus statement that lists “Appropriateness Criteria” for cardiac CT and magnetic resonance scanning, which assigns an appropriateness score to a number of potential indications for coronary CTA. Indications for coronary CTA that are considered appropriate include its use to rule out coronary artery stenoses in symptomatic patients who (1) are unable to exercise (and therefore cannot have an exercise stress test), (2) have an uninterpretable ECG, (3) have an uninterpretable or equivocal stress test, or (4) need further characterization of anomalous coronary arteries.

The use of coronary CTA for patients with new-onset heart failure and for patients who present with acute chest pain and an intermediate pretest likelihood of coronary artery disease, but who have a normal ECG and absence of enzyme elevation, is considered potentially appropriate.

A Scientific Statement on the Assessment of Coronary Artery Disease by CT that was issued by the American Heart Association describes the role for cardiac CT as follows: “Especially in the context of ruling out stenosis in patients with low to intermediate pretest likelihood of disease, CT coronary angiography may develop into a clinically useful tool. CT coronary angiography is reasonable for the assessment of obstructive disease in symptomatic patients (Class IIa, Level of Evidence: B).”

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