Cardiac CT: Is multidetector imaging a paradigm shift?

Calcium scoring has been used for more than a decade to directly visualize atherosclerotic changes of the coronary arteries. Coronary calcium is an independent predictor of cardiac events, although the relationship is not linear. An asymptomatic individual with a calcium score in the 80% rank has more than 15 times the relative risk of a serious cardiac event compared with an individual without coronary calcium.² Moreover, the negative predictive value of a normal calcium scoring test (absence of any calcium) in the setting of atypical chest pain approaches 100%.^3,4

Conventionally, calcium scoring has been obtained with EBT. The Agatston score, which is the product of the number of pixels with calcification and the density weighting of the calcification on a sliding scale, is the most widely used metric and can be calculated using standard software.⁵ A threshold of 130 HU is typically used to indicate calcification (Figure 1). Results are normalized for age and gender, and a percentile rank is generated based on a large database.⁶

Much interest has centered on whether MDCT provides as accurate and reproducible a study as EBT. A recent study by Becker et al⁷ found a strong correlation (r = 0.9) between EBT and a 4-slice scanner among 100 patients who underwent both studies. Increasingly, there is recognition that the Agatston score, which is based on a conventional model of 3-mm-thick axial sections, does not take into account the volumetric nature of MDCT. Two newer measurements, calcium volume and calcium mass, better account for these volumetric characteristics and, thus, may provide more reproducible calcium scores among different scanners. A recent critical analysis of the literature suggested that MDCT correlates well with EBT for calcium scoring, with the possible exception of patients with low levels of coronary calcium.⁸

Cardiac CT provides accurate delineation of the cardiac chamber size and wall thickness. Valve thickness and calcification are readily assessed and the mitral and aortic valves are clearly depicted (Figure 2). The right-sided valves may be more difficult to delineate due to contrast-related streak artifact. A thrombus in the apex of the left ventricle and the left atrial appendage is visualized easily and can be quantified (Figure 2). Cardiac tumors, such as myxoma and metastatic disease, are clearly delineated (Figure 3). A variety of congenital heart diseases are readily diagnosed (Figure 4).⁹

One of the indications for which cardiac CT has been used increasingly is in the evaluation of the left atrium and pulmonary veins as part of planning atrial fibrillation ablation therapy. The size and number of veins can be assessed (Figure 4).¹⁰ Radiofrequency ablation is associated with a 5% rate of pulmonary vein stenosis, which is well demonstrated on CT .¹¹

The use of submillimeter resolution, ECG gating, and breath-holding allows reliable visualization of the proximal coronary arteries and often permits assessment of the more distal coronary tree. In patients with higher heart rates (>70 beats per minute), beta-blocker administration optimizes coronary artery evaluation. Typically, images are reconstructed at 40% to 90% of the cardiac cycle, and the best images are selected by visual inspection. The optimal image set for the left and right coronary system may differ. Coronary artery images may be viewed using curved planar, thin-slab, or volumetric techniques (Figure 5). Newer software permits extraction of the coronary arteries and assessment of the extent of coronary artery stenosis using a preselected reference point.

Several studies have evaluated coronary artery stenosis on CT angiography using coronary angiography as the standard reference technique. Early studies using a 4-slice CT scanner revealed sensitivity and specificity values of 80% to 95% but had a relatively high number of nonevaluable segments (up to 28%).^12,13 With more recent technology, the number of nonevaluable segments has decreased, with continued high sensitivity) (Figures 6 and 7).^14,15 At least two multicenter trials using current CT technology are planned.

Cardiac CT is useful to evaluate coronary stents and bypass grafts. The location of the coronary stent is readily identified. Thrombosis within the stent can be depicted, although the stent material may interfere with luminal visualization.¹⁶ Absence of fiow distal to the stent is an indication of stent occlusion, although the presence of distal fiow, which may be due to collateral sources, does not guarantee stent patency. After coronary bypass surgery, patency of both internal mammary artery and saphenous vein bypass grafts is also well demonstrated (Figure 8). Sensitivity and specificity of 95% to 100% for detecting occlusion has been reported.^17,18 Assessment of bypass graft stenosis is associated with somewhat lower sensitivity and specificity. In general, the distal graft anastamosis with the native coronary artery is more difficult to visualize than is the proximal portion. Complications, such as saphenous venous graft aneurysm, are readily depicted.

Coronary artery anomalies occur in 1% of patients and may be associated with sudden death if the anomalous vessel passes between the aorta and pulmonary artery. In this setting, prophylactic bypass graft may be indicated. The anomaly is typically discovered at coronary angiography, but the precise course with respect to the great vessels is often ambiguous. Cardiac CT reliably delineates both the origin of the anomalous vessel and its subsequent course (Figure 9). Miscellaneous coronary abnormalities that can be visualized on cardiac CT include coronary aneurysms and coronary arteriovenous fistulas.

Current MDCT scanners provide software packages that permit assessment of ejection fraction, stroke volume, wall-motion, and wall thickness and provide a detailed output display. Although an automated edge-detection program provides an initial estimate of cardiac margins, some operator input is usually necessary to define the proper endo- and epicardial borders (Figure 10). A cardiac ejection fraction similar to that of other methods may be obtained with cardiac CT, but some concern exists that it may systematically underestimate the correct value due to temporal resolution that is inadequate to define end-systole. Notwithstanding this potential limitation, limited investigations indicate a strong correlation between ejection fraction calculated with CT and by MRI.^19-21

Similar to cardiac MRI, ischemia may cause differential perfusion of the myocardium on CT. Decreased attenuation is observed in ischemic areas and may be subendocardial or transmural (Figure 11).²² It is not clear whether acute ischemia has a different appearance from chronic ischemia or infarction. It is also uncertain what the role of delayed imaging, analogous to that used to assess myocardial viability on MRI, might be.

Over the last 5 years, the technology curve for cardiac CT has been much steeper than that for MRI. Currently, CT features superior spatial resolution, less artifact-related image degradation, ability to image in the presence of cardiac pacemakers and defibrillators, and lower cost. Even more important, perhaps, is that CT is more intuitive to interpret. However, MRI remains superior in many respects. It has superior temporal resolution, does not rely on iodinated contrast, with its attendant risks of allergies and contrast nephropathy, has no ionizing radiation, and has the capability to measure fiow velocities and to assess valvular regurgitation, which CT does not. Finally, the use of MRI to assess myocardial viability is well established, whereas its evaluation by CT has yet to be proven.

One of the disadvantages of cardiac CT compared with competing technologies, such as MRI, is its use of ionizing radiation. The radiation is highly dependent on the protocol used in cardiac CT. For prospective gating generally used for calcium scoring, in which imaging occurs for only a portion of the cardiac cycle, the radiation dose is comparatively low. Typically, doses in the 1 to 2 mSv range are reported. For retrospective gating used for coronary stenosis assessment, much higher doses of 8 to 12 mSv are reported.^23,24 The higher dose is due to the necessity of imaging throughout the cardiac cycle. By comparison, the radiation dose of an uncomplicated coronary angiography is 4 to 6 mSv. Efforts to reduce the high dose accumulated in retrospective gating, termed dose modulation, are directed at reducing the tube current during parts of the cardiac cycle, particularly systole, where poorer image quality can be tolerated because assessment of the coronary arteries is suboptimal. With dose modulation, a 30% to 50% dose reduction can be achieved.²⁵