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Figure 3-18. Multidetector computed tomographic (MDCT) coronary angiography showing obstructive multivessel disease.

A, Volume-rendered maximum-intensity projection of the aortic root and coronary arteries.

B, Curved multiplanar reconstruction of the left circumflex coronary artery (LCX). C, Series of cross-sectional images obtained from the mid-LCX at 1-mm intervals. Arrows indicate areas of severe stenosis caused predominantly by noncalcified (dark) atherosclerotic plaques.

approach, the sensitivity, specificity, and positive and negative predictive values for detecting greater than 50% luminal stenoses were 89%, 65%, 13%, and 99%, respectively. In a patient-based analysis, the respective values for detecting subjects with at least one "positive" segment were 98%, 54%, 50%, and 99%. The high number of nonevaluable and false-positive segments in this study indicated that 16-slice MDCT may lead to an excessive number of catheterizations or additional functional testing if applied indiscriminately. Nevertheless, given its high sensitivity and high negative predictive value, 16-slice MDCT may be very useful for excluding coronary disease in selected patients in whom a false-positive or inconclusive stress test result is suspected. It is anticipated that the lower number of nonevaluable segments seen with 64-slice scanners will result in higher specificity and positive predictive values, allowing wider implementation of this test.

It is likely that there always will be a discrepancy between MDCT and invasive coronary angiography for the quantitative assessment of luminal stenosis. Unlike angiography, MDCT highlights both the lumen and the wall vessel plaque. Therefore, MDCT may be more comparable to intravascular ultrasound

(IVUS) than to invasive angiography. In addition, MDCT can provide an infinite number of projections because of its 3D nature. In many cases, a presumed false-positive MDCT finding may actually represent a false-negative coronary angiogram, if adequate projections in the latter test were not obtained (Fig. 3-20).

Accurate assessment of previously stented coronary vessels remains an important limitation to MDCT coronary angiography.43 A noninvasive, accurate test for in-stent restenosis would be invaluable for patients with postintervention chest pain. This is particularly true because the widespread use of drug-eluting stents reduces the incidence of in-stent restenosis, thereby reducing the yield from repeat invasive coronary angiography. In a study using 16-slice MDCT, only 126 (64%) of 232 stents could be evaluated.44 Smaller stents, in vessels smaller than 3 mm, were harder to accurately evaluate. Internal luminal diameter is often underestimated. More recently, another study evaluated 13 stented segments using 64-slice MDCT.39 Two stents with severe stenosis were accurately identified; two other stents with moderate stenosis were called normal by MDCT; and four stents with no angiographic restenosis were thought to be stenotic by MDCT. The ability to evaluate the lumen of stented vessels depends on the type and diameter of the stent. Practical delineation of in-stent stenosis remains difficult in lumens smaller than 3 mm in diameter.

MDCT has been proposed for the evaluation of coronary artery bypass grafts (CABG) (Fig. 3-21). A

Figure 3-19. Angiographic left anterior projection of the left coronary artery and branches obtained by catheterization in the patient shown in Figure 3-18. Arrows indicate severe stenotic lesions in the left anterior descending coronary artery and the left circumflex coronary artery.

study using 16-slice MDCT reported successful visualization in all bypass conduits after exclusion of three subjects due to poor overall image quality.45 The reported sensitivity, specificity, and positive and negative predictive values for detecting total graft occlusion were 96%, 95%, 81%, and 99%, respectively. However, evaluation of the distal anastomosis site was possible only in 75% of the cases. Motion artifacts and interference by surgical clips often limit the assessment of vessel anastomosis. Therefore, determining total vein graft occlusion is straightforward, but quantifying moderate stenoses can be difficult. Analysis of the native vessels is often more difficult in patients with previous CABG, due to poor run-off, more extensive calcification, and smaller lumen size. This can potentially limit the diagnostic utility of MDCT angiography in this setting. MDCT angiography may also help characterize the 3D location of preexisting coronary grafts in relationship to each other and to the chest wall in patients undergoing repeat sternotomy. In patients with previous CABG, MDCT should be considered as complementary to invasive angiography for those in whom direct catheterization carries a risk, such as patients with suspected atheroma or patients in whom a high contrast load should be avoided. MDCT may also be useful in symptomatic patients with recent CABG in whom graft occlusion is suspected.

Evaluation of Atherosclerotic Plaque Morphology by MDCT

Until recently, IVUS was the only diagnostic tool capable of detecting the presence, extent, and composition of atherosclerotic plaques in the coronary

Figure 3-20. A patient with a large mixed atherosclerotic plaque at the left main ostium (arrowheads). Quantitative analysis indicated 65% stenosis by multidetector computed tomography (MDCT) coronary angiography (right) versus 34% by invasive angiography (left), probably owing to difference in angiographic projections. (From Garcia MJ, Lessick J, Hoffmann MHK: Accuracy of 16-row multidetector computed tomography for the assessment of coronary artery stenosis. JAMA 2006;296:403-411.)

Figure 3-20. A patient with a large mixed atherosclerotic plaque at the left main ostium (arrowheads). Quantitative analysis indicated 65% stenosis by multidetector computed tomography (MDCT) coronary angiography (right) versus 34% by invasive angiography (left), probably owing to difference in angiographic projections. (From Garcia MJ, Lessick J, Hoffmann MHK: Accuracy of 16-row multidetector computed tomography for the assessment of coronary artery stenosis. JAMA 2006;296:403-411.)

Figure 3-21. Multidetector computed tomographic (MDCT) coronary angiography obtained in a patient with previous coronary artery bypass grafts. A, Volume-rendered projection of the heart. Arrows indicate the stump of an occluded bypass to the circumflex and stents previously deployed in this vessel. The graft is not visualized because of the lack of contrast opacification. B, Oblique sagittal maximum-intensity projection shows a series of staples, corresponding to an occluded left internal thoracic graft to the left anterior descending coronary artery (LAD). C, Curved multiplanar reconstruction of a saphenous vein bypass graft to the LAD. The arrows indicate a stent in the proximal segment and the location of anastomosis to the distal LAD.

Figure 3-21. Multidetector computed tomographic (MDCT) coronary angiography obtained in a patient with previous coronary artery bypass grafts. A, Volume-rendered projection of the heart. Arrows indicate the stump of an occluded bypass to the circumflex and stents previously deployed in this vessel. The graft is not visualized because of the lack of contrast opacification. B, Oblique sagittal maximum-intensity projection shows a series of staples, corresponding to an occluded left internal thoracic graft to the left anterior descending coronary artery (LAD). C, Curved multiplanar reconstruction of a saphenous vein bypass graft to the LAD. The arrows indicate a stent in the proximal segment and the location of anastomosis to the distal LAD.

arteries in vivo.46,47 However, the wide application of IVUS as a screening tool for risk assessment is impractical because of the need for and high cost of invasive catheterization. In contrast to invasive coronary angiography, MDCT angiography is also capable of imaging the vessel wall. Recent studies have documented the ability of MDCT to visualize atherosclerotic coronary plaques48-50 and to differentiate calcified from noncalcified lesions based on Hounsfield unit values.51 In a series of 22 patients clinically referred for IVUS, MDCT correctly identified the presence of coronary atherosclerotic plaques in 41 of 50 affected segments.50 Whether MDCT could be used in clinical practice as a screening test remains to be proven, but in selected patients at low-to-intermediate risk, it could potentially help to justify lifelong aggressive preventive intervention. MDCT plaque characterization could also potentially help in devising optimal revascularization strategies.

Cardiac CT, like invasive angiography, involves radiation exposure. The "effective dose," expressed in milli-Sieverts (mSv), depends on multiple factors, including volume of acquisition required, duration of the scan, and radiation energy level used. The volume of acquisition is typically 12 to 16 cm for coronary angiography and 18 to 25 cm for angiogra-phy for coronary bypass conduits. The radiation energy level required to obtain adequate image quality also depends on the weight of the patient. Obese individuals require a larger amount of energy owing to scattering and attenuation. Current generation systems with 64 detectors provide a typical dose range in the order of 8 to 14 mSv for MDCT coronary angiography with the use of tube current modulation. This compares to 2 to 6 mSv for invasive angi-ography, 15 to 25 mSv for nuclear stress myocardial perfusion studies, and 3.6 mSv from yearly background radiation exposure. Therefore, careful analysis of the long-term risk from radiation versus potential benefits derived should be taken into consideration when indicating this study, particularly in younger individuals who are at higher risk.

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