Traditionally, CT (including helical CT) was used

primarily to examine the pericardium and the thoracic and abdominal aorta. The closer one got to the heart, the poorer the images became because of motion artifact. Most cardiac imaging was done with electron-beam tomography, primarily for calcium scoring, an application that remains controversial. The arteries themselves were out of reach because of their constant movement: the rest period of the right and left coronary arteries is as short as 66 milliseconds.

The introduction of four-slice multidetector CT scanners greatly expanded the utility of CT for studying the heart and the chest vasculature. For example, Roos et al,¹ University Hospital, Zurich, Switzerland, noted that multidetector studies with electrocardiographic gating greatly reduced motion artifacts in studies of the thoracic aorta, and Rubin et al,² Stanford University, Stanford, Calif, found that four-channel CT of the aorta was 2.6 times as fast and 4.1 times as efficient as one-channel helical CT and required less than half as much contrast medium. Lawler et al³ described the value of four-slice CT scans with three-dimensional volume rendering for examination of the pulmonary and thoracic veins.

The introduction of 16-slice CT scanners has brought CT another step forward in speed and potential (see box). These scanners may obviate some traditional cardiac imaging studies and enable a hospital to make better use of its cardiac catheterization laboratory. They also permit a more comprehensive assessment of many conditions and improve the follow-up care for some treatments. For example, 16-slice CT provides imaging of the higher thoracic aorta with a clarity previously obtainable only with echocardiography or MRI, but with the advantages of a full field of view (and without the contraindications associated with a magnet). Numerous other studies are improved or enabled, altering the indications for traditional cardiac imaging procedures. Richard D. White, MD, Head, Section of Cardiovascular Imaging, Cleveland Clinic, summarized the impact of a 16-slice scanner on cardiology by saying that it "has been such a dramatic change that we now realize we were doing very little with CT in the past."

Dysrhythmia Imaging

One application of 16-slice scanners is assessment and treatment planning for patients with hypertrophic cardiomyopathies, relatively common genetic disorders that predispose to dysrhythmias or sudden death if not identified and corrected.⁴ Another example is arrhythmogenic right ventricular dysplasia (ARVD), in which replacement of myocardium with fatty or fibrous fatty tissue creates electrical instability. MRI has traditionally been the primary means of evaluation, but the greater speed and resolution of the 16-slice scanner has made it the first choice, with MRI assuming a new role. "If the CT study is negative for ARVD, we stop," White says. "If it is equivocal or positive, we go to MRI, but we can get a more streamlined examination consisting solely of dynamic imaging. If the patient comes to us after a positive MRI study, we use CT to monitor the patient before and after the placement of a defibrillator. We also use the 16-slice scanner when it is appropriate to screen the family, and this saves a great deal of time. We recently saw four boys whose mother had ARVD. The imaging of the whole family took about an hour, whereas it would have consumed the better part of a day if we had used MRI."

Atrial fibrillation is an increasingly common diagnosis.^5,6 The disorder has been difficult to control, let alone cure.⁵ In 1998, Haissaguerre et al⁷ of the Hôpital Cardiologique du Haut-Leveque, Bordeaux-Pessac, France, who had examined 45 patients with drug-resistant atrial fibrillation, reported that, in 94%, the aberrant impulses arose 2 to 4 cm inside at least one of the pulmonary veins. The report opened a new target for ablation.⁸ Multidetector CT can be critical to the procedure. "In the past, imaging was important in atrial fibrillation mainly for examining the left atrium for clots," White notes. "We now routinely evaluate these patients before and three or four times after radiofrequency ablation of the impulse sources in the pulmonary veins. We help in planning the procedure and then look for any adverse consequences, such as pulmonary vein stenosis. Of course, the scanner can also look for clots, so you can avoid the otherwise almost mandatory transesophageal echocardiogram. The dysrhythmia does not help you, but it does not stop you from obtaining diagnostic images."

CT Coronary Angiography

Even with four-slice multidetector CT scanners, coronary angiography was largely restricted to slender patients with heartbeats that were slow (less than 65 beats per minute) and regular.⁹ Nevertheless, the potential value of CT coronary angiography was clear, in that the study had a negative predictive value for significant stenoses of 96% to 99%, as judged by catheter angiography.^10,11 Moreover, the arterial branches could be examined, as previously possible only using catheter angiography.¹² The 16-slice scanner still cannot rival the spatial and temporal resolution of catheter angiography, but that does not preclude one from exploiting it to triage patients for whom catheterization studies are being considered. "With multidetector CT, we can determine whether the patient has a large amount of atherosclerotic disease warranting further evaluation, probably with simultaneous angioplasty or stenting. The greatest strength of the CT study is its negative predictive value. We can say confidently that a patient does not need a catheter angiogram because nothing of significance will be found, and nothing will be done. The catheterization laboratory is evolving into a place of therapy more than diagnosis,"  White says.

Multidetector CT also has an important role in the diagnostic work-up of patients with atypical chest pain. "We often find that such patient does not have stenotic disease, yet needs the attention a preventive cardiologist," White says. "Picking up early atherosclerosis is important, because we know that 60% to 70% of myocardial infarctions occur in the absence of significant vessel narrowing."

There is considerable interest in methods of using imaging to characterize plaque, arising in part from the view that the composition of a plaque is more important than its size as a determinant of acute coronary events.¹³ Noncalcified vulnerable plaques are of greatest concern. In 2000, Kopp¹⁴ presented some tantalizing data on plaque characterization using a four-slice CT scanner with retrospective cardiac gating. In a series of 20 patients, CT identified 37 of the 40 plaques found using intravascular ultrasonography. One section of plaque from each patient was characterized as soft, intermediate, or calcified, and there was a perfect correlation between the two studies in making this determination. Can 16-slice scanners do even better? "If the concern is calcified versus noncalcified plaque, we can do that now," White says. "There have been some efforts to use CT numbers derived from tiny areas to say that this is a more fibrotic or a more fatty plaque, but the measurements are subjective. The subject is intriguing, but there has not yet been enough experience to characterize plaque reliably."

Nonetheless, 16-slice CT can, to some extent, replace intravascular ultrasonography in identifying worrisome areas of arterial remodeling. A team at the Cleveland Clinic recently compared the 16-detector system with intravascular ultrasonography in characterizing mildly stenotic plaque. "The correlation seems to be quite good," White reports. "This is important, because monitoring could be done by CT for many more patients than by intravascular ultrasonography."

The Cleveland Clinic offers CT calcium scoring as part of a preventive cardiology program. The multidetector scanner is preferred to electron-beam tomography because of the greater versatility of the machine. "Electron-beam scanners do cardiac imaging well; in some ways, perhaps, better than the multidetector scanner. Their shortcoming is that if there is an important related issue in the same patient, such as possible carotid or renal artery stenosis, the electron-beam scanner is not as good. If you want to take control of the patient's diagnostic work-up, there is a distinct advantage to the multidetector scanner," White says.

Future Uses

The growing popularity of using volumetric imaging data as a substitute for endoscopy may encompass the blood vessels with the availability of 16-slice CT scanners. The first efforts at virtual angioscopy employed electron-beam scanners, but were not successful in many patients. With a four-slice multidetector CT scanner, Schroeder et al¹⁵ at four institutions in Germany and the United Kingdom were successful in all 14 of their patients, who had chronic stable angina and at least one stenosis of more than 75%. To create the images of the vessel lumen, the contrast medium was removed by excluding voxels of 100 to 200 Hounsfield units (HU), a practice that does not affect the depiction of the vessel walls (80 to 100 HU) or vascular calcium (more than 250 HU). All of the severe and calcified lesions were visible, although noncalcified lesions of intermediate severity were difficult to distinguish from the vessel wall. The authors noted that the radiation dose necessary for the study was undesirably high and that the reconstruction took approximately an hour, in part because data from different parts of the cardiac cycle were required to reconstruct the various coronary vessels. Faster scanners (such as the 16-slice machines, which were not available at that time) can be expected to overcome many of these problems, and virtual angioscopy might become a useful clinical tool.¹²

Fusion imaging is yet another potential use of multidetector CT data. White and his colleague Randolph M. Setser, DSc, have discussed an integrated approach to the evaluation of ischemic heart disease in which coregistered displays of CT and MRI images will permit simultaneous characterization of vascular lesions and determination of the size and distribution of myocardial necrosis.¹⁶

Paying for Itself

In White's experience, the 16-slice scanners quickly show their utility, so justifying their acquisition has been less difficult than for MRI, for example. A university medical center is not the only site that might benefit from a 16-slice scanner. As Gaylord has written, even though such a scanner with a cardiac software package may cost more than $1 million, "If the volume of additional patients from coronary [CT angiography] meets the promise of this new technology, the cost difference should be affordable even to smaller hospitals."¹⁷

Judith Gunn Bronson is a contributing writer for Decisions in Imaging Economics.

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