Cardiac CT edges closer toward gold standard

CT technology advances have gone a long way toward improving noninvasive cardiac imaging. Radiologists with access to 64-slice and dual-source scanners are now evaluating how these new tools can take cardiac CT to the next stage.

The rapid evolution of CT has resulted in significant gains to both spatial and temporal resolution, according to Dr. Christoph Becker, an associate professor and chief of CT at the Grosshadern Clinic in Munich. Just 10 years ago, the radiologists at Grosshadern were working with an electron-beam CT system with a spatial resolution of 3 mm.

"With 64-slice and dual-source CT, we are getting 0.6 mm or better," he said. "With this level of spatial resolution, we are getting very close to the cardiac catheter, which has a resolution of approximately 0.2 mm."

In practice, this translates into better visualization of anatomy. For example, aortic valve leaflets can hardly be seen on four-slice CT. They are far sharper and clearer on 64-slice images (Figure 1). Coronary artery anatomy can also be visualized more clearly, with images revealing the different layers in vessel wall lesions. Problems with blooming artifacts are also reduced.

Shorter scanning times yield other benefits. Whole-heart imaging with four-slice CT used to take 40 seconds, which could be quite uncomfortable for patients, he said. With 64-slice CT, it now takes just 10 seconds to perform the same exam. Using dual-source CT, the same exam can be conducted in five seconds.

Contrast protocols will need to change to adapt to the faster scans. Grosshadern-based radiologists administer 80 mL of contrast at 5 mL/sec for a 10-second scan, whereas they previously injected 120 mL of contrast at 3 mL/sec. This technique is aiding visualization of the coronary arteries much farther into the periphery. Side branches and diagonal branches can also be more clearly observed.

Many papers comparing 64-slice CT with cardiac catheter techniques for detecting coronary stenoses have now been published, Becker told delegates at a Siemens-sponsored symposium at the 2006 European Congress of Radiology. The data generally indicate a negative predictive value of 100% for CT. Positive predictive values range from 66% to 87%. So while cardiac catheterization is unlikely to find a lesion not already identified using cardiac CT, 64-slice CT images may overestimate the degree of stenosis.

One possible reason for the false-positive readings is that interpreting radiologists scan studies too quickly when making their diagnoses. In such a circumstance, large plaques can looked like occlusions. Such overinterpretation may be avoided in the future by the use of quantitative postprocessing tools that more accurately assess the degree of stenosis.

The improved temporal resolution of advanced CT systems is additionally aiding the assessment and triage of patients with acute coronary syndrome, Becker said. Motion-free images can be obtained even at relatively high heart rates, removing the need to administer drugs (Figure 2). His team is now trying to determine the optimum time point in the cardiac cycle for image acquisition.

"With dual-source CT, we can produce volume-rendered images for almost the entire cardiac cycle with no motion artifacts," Becker said.

Impressive trial results have led some radiologists and cardiologists to recognize cardiac CT as a new standard of care for patients with an intermediate risk for acute coronary syndrome, even without the reassurance of a randomized multicenter trial (Figure 3). Researchers at Erasmus Medical Center in Rotterdam, the Netherlands, have confirmed that 64-slice CT rarely fails to diagnose significant coronary artery stenoses. On a per-segment basis among 35 patients with stable angina, its sensitivity and specificity for detecting 50% or greater stenosis were 99% and 96%, respectively. On a per-patient basis, the sensitivity and specificity were 100% and 90%, respectively (Euro J Radiol 2006;16(3):575-582).

Abstracts from the 2006 American College of Cardiology meeting that demonstrated the potential of cardiac CT in screening chest pain patients are encouraging, said Dr. Stefan Achenbach, president of the Society of Cardiovascular Computed Tomography. But it is too early to draw conclusions from these pilot studies, although they did show that cardiac CT speeds diagnosis and allows for earlier discharge of patients who have an intermediate or low risk of myocardial infarction.

"This could be a tremendous application because about three million patients report to emergency rooms with chest pain every year,"Achenbach said.

With the spotlight focused on cardiac CT, important cardiac MR findings have not attracted as much attention as in previous years. But the field is moving forward with discoveries that could expand MR's role for evaluation of dilated cardiomyopathy and myocarditis.

Researchers at Royal Brompton Hospital in London established a relationship between widespread patchy fibrosis in the myocardium detected with delayed-enhancement contrast MR and the risk of sudden cardiac death among patients with dilated cardiomyopathy. The delayed enhancement strategy revealed a midwall pattern of circumferential fiber, and follow-up outcome analysis showed fibrotic patients were significantly more likely to die or be hospitalized than patients without fibrosis, according to principal investigator Dr. Ravi Assomull. The work was presented at the 2006 Society of Cardiovascular Magnetic Resonance meeting.

Another research team, headquartered at the Robert Bosch Medical Center in Stuttgart, Germany, discovered that delayed-enhancement MR patterns are associated with viral causes of myocarditis. Imaging studies compared with histology showed that herpesvirus tended to affect the septum and was more aggressive than parvovirus, which concentrated in the lateral wall.

Others have documented that T2-weighted contrast MR can delineate reversible and irreversibly infarcted myocardial tissue following coronary occlusion (Circulation 2006; 114:1865-1870). The protocol, performed on dogs, promises a convenient alternative to technetium- 99m sestamibi for identifying such salvageable regions.

Lastly, a joint U.S. National Institute on Aging/Icelandic Health Association study has found that unrecognized myocardial infarction among elderly people is more common than previously thought. Delayed-enhancement MR revealed previous infarction among 21.7% of 458 Icelandic subjects. By comparing this finding against hospital and registry records, Dr. Andrew Arai, principal investigator of cardiovascular imaging at the U.S. National Heart, Lung, and Blood Institute's cardiac energetics lab, concluded that the prevalence of unrecognized infarction was 12.5%.