Murat V. Kalayoglu, M.D., Ph.D.
Contributing Editor Non-invasive imaging technologies continue to revolutionize every subspecialty of medicine. Cardiology is no different; new imaging devices have shown enormous promise to diagnose a variety of cardiovascular abnormalities. Perhaps the most important such technology in cardiac diagnostics is the development of the multi-detector cardiac CT. Recent advances have allowed many cardiologists to experience excellent resolution and high accuracy using 16-slice cardiac CT scanners. However, the next generation of scanners enables 64 slice scans within seconds. Such unparalleled resolution, combined with short scan times, allow visualization of the heart and entire coronary tree within one breath.
The first multi-slice CT scanners were introduced in the early 1990s. The original Elscint-Twin scanner, introduced in 1994, allowed acquisition of two images simultaneously through a dual detector array. The 4-slice scanner was introduced in 1998; these scanners were capable of processing scans within 0.5 seconds. Such short scan times translated into shorter examination times. For example, examination of the heart using the 4-slice scanner was 8 times shorter compared with using the single slice system. Reduced scan times allowed processing of images with fewer artifacts caused by movement. For cardiac scanning, these motions were accentuated given that the scans were focused on the chest: patients could not hold their breath for the extended periods required to collect all data. Enhanced, clinically relevant images were therefore only possible with the introduction of the 16-slice scanners, which could collect all data within 20-25 seconds. Such reduced scan times allowed more patients to hold their breath for the entire duration of the scan period.
The introduction of 64-slice cardiac CT allows nearly all patients to be scanned with very high resolution. Scan times are now on the order of several seconds (usually 5-13 seconds); this means that even patients with severe pulmonary disease and congestive heart failure can hold their breath for the required length of time. Reduced time translates to minimal or no motion artifacts. Furthermore, higher number of slices means higher resolution; today’s 64-slice scanners are capable of performing 64 slices per rotation at less than 0.4 – 0.7 mm resolution. Such high resolution allows visualization of the entire coronary tree with extremely high accuracy and detail. Individual atheromatous plaques can be detected and characterized. Calcification can be visualized and used as an added variable in disease management.
A recent study, presented several weeks ago at the annual meeting of the American College of Cardiology, compared the capacity of 64-slice cardiac CT versus coronary angiography to detect significant coronary artery stenosis (defined as >50% lumen diameter reduction). 30 patients with stable angina or acute coronary syndrome were enrolled in the study; individuals unable to hold their breath for less than 15 seconds were excluded from the study. The heart was scanned following intravenous injection of contrast material and analyzed by two observers who were not aware of the results obtained from invasive angiography. Most patients received a beta blocker before the study, and all coronary arteries were compared between coronary CT and angiography. Compared with angiography, 64-channel cardiac CT showed a sensitivity of 96% and specificity of 89% when detecting significant stenosis. The authors concluded that 64-channel cardiac CT could reliably detect significant coronary stenoses in patients with stable angina or acute coronary syndrome (Mollet et. al., presentation #1054-83, ACC ‘05).
Despite the promise of 64-slice CT in non-invasive cardiac diagnostics, some cardiologists have expressed concern in their widespread use. For one, is the dose of X-ray radiation safe to use on everyone, i.e. as a screening test? Multi-scan CT delivers increased levels of focused radiation compared with single slice CT; although these slightly increased doses are justified for patients with existing or symptomatic disease, they may not be appropriate for everyone. In addition, if many patients without significant coronary disease are scanned, there may be a sizeable number of false positive tests; such false positives may drive up health care costs as cardiologists order additional tests to rule out significant disease. Finally, at approximately $700-1000 per scan, the costs of the scans themselves could increase health care costs. Despite these caveats, the technology is a major advance and continues to be refined. Prototypes for 128-slice and even 256-slice scanners are under development. Clinical use of multi-slice CT is increasing, and the 64-slice cardiac CT heralds the new age of non-invasive cardiac imaging.
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