Coronary CT angiography finally finds itself an affordable home

August 1, 2007

A year ago, research regarding 64-slice CT angiography focused on feasibility. Feasibility is no longer an issue. Rather, a wealth of evidence presented at this year's European Congress of Radiology attests to the fact that coronary CTA is a powerful and useful tool to evaluate patients suspected of coronary artery disease who are at intermediate risk.

A year ago, research regarding 64-slice CT angiography focused on feasibility. Feasibility is no longer an issue. Rather, a wealth of evidence presented at this year's European Congress of Radiology attests to the fact that coronary CTA is a powerful and useful tool to evaluate patients suspected of coronary artery disease who are at intermediate risk. It is within this niche patient group-those who would otherwise undergo invasive catheter angiography-that coronary CTA is finding an affordable home. Researchers are also venturing outside this niche to determine where else cardiac CT might have value.

In one study, Dr. Gudrun Feuchtner and colleagues at the Medical University Innsbruck in Austria found coronary CTA was a cost-effective pretest to exclude stenosis greater than 50% in intermediate risk patients with clinically suspected CAD. In one part of the study, researchers evaluated 22 patients whose ECG-stress test was mild-pathologic. CT ruled out coronary stenosis greater than 50% in 17 of them. Not performing catheter angiography on these patients netted a savings of nearly Euro 19,000. Factoring in CT's one false positive and two true positives, the total amount saved by using CT as a second noninvasive test after a positive ECG-stress was around Euro 16,600.

Overall, the group clocked sensitivity (100%), specificity (78%), positive predictive value (80%), and negative predictive value (100%) similar to those of published references, Feuchtner said.

Many coronary CTA studies are problematic because they concentrate only on significant stenosis, not hemodynamically relevant stenosis ( > 70%); they don't use quantitative catheter angiography as a reference standard; and their patient population is too homogeneous, according to Dr. Christoph Herzog, a radiologist at Johann Wolfgang Goethe University in Frankfurt, Germany.

Herzog and colleagues at Medical University of South Carolina conducted a study that sought to remedy those problems. They included 100 patients with a variety of cardiac conditions such as angina pectoris, bypass grafts, stents, and elevated troponin levels. They used quantitative coronary angiography as the reference and sought results for both thresholds of stenosis.

CT detected all patients with significant and hemodynamically relevant stenosis. For all stenoses greater than 50%, CTA performed reliably on a per-patient basis (100% accuracy). Its diagnostic accuracy decreased slightly on a per-segment and per-vessel basis (both 96% accuracy), primarily because of limited spatial resolution, Herzog said. Stenoses greater than 70% were detected with greater accuracy than stenoses greater than 50%, but the difference was not statistically significant.

Researchers are also testing to determine what incremental value CT has in various patient populations. Dr. Filippo Cademartiri, who is a radiologist at Erasmus University Medical Center in the Netherlands, and colleagues at University Hospital Azienda in Parma, Italy, evaluated CT's ability to detect potential culprit lesions in patients with acute myocardial infarction and a catheter angiogram negative for significant stenosis. In 19 of 30 patients, CTA identified a potential culprit lesion associated with positive remodeling. Of these, 13 lesions were associated with segmental alterations of left ventricular motion found with echocardiography. Researchers suggested that therapy for these patients can be adapted based on this risk stratification.

MRI may also have a role for plaque characterization. Dr. Mai-Lin Oei and colleagues at the University of Munster in Germany used an inversion recovery MRI sequence to detect intraplaque hemorrhage in plaques identified with CTA. Work such as this has been conducted in the carotids but not in the coronaries, Oei said. Of the 43 plaques evaluated, five appeared hyperintense on IR-MRI, indicating intraplaque hemorrhage.

The average signal-to-noise ratio for the hyperintense plaques was three times that of the hypointense lesions, and the average contrast-to-noise ratio of the hyperintense lesions was seven times that of the hypointense plaques.

No one knows how to treat these vulnerable lesions once they are detected and characterized, said moderator Dr. U. Joseph Schoepf in an interview after the session.

"We don't have enough experience in vivo. We extrapolate from pathology findings more or less to know what we should do with these lesions. Nobody knows whether it would be worthwhile to intervene. A scientific rationale to support such an approach is not there," Schoepf said.

One of the last frontiers to be broached by the newer CT scanners is cardiac function. Schoepf's group at the Medical University of South Carolina presented a study that compared 64-slice CT functional analysis with SPECT myocardial perfusion imaging in 42 patients with suspected or known CAD. Researchers found a good (r = 0.68) correlation between CT functional analysis and SPECT for detection of regional functional abnormalities.

There's no rush to replace SPECT perfusion imaging with CT functional studies, said lead author Dr. Christian Thilo. Because CTA and quantitative coronary angiography correlated well with each other, however, he suggested that CTA may replace catheter angiography in a myocardial perfusion-based algorithm for workup of patients with known or suspected CAD.

"There is no doubt that functional analysis based on CT is accurate in comparison with clinical reference standards. The problem is that CT is probably not the right test to look at cardiac function, per se," Schoepf said.

Radiologists at his institution perform cardiac functional analysis on every CT scan because the information is inherently available. He says it's the ethical thing to do.

"We apply all the radiation to look at the coronary arteries. So I find it ethical to utilize the data to the utmost extent and provide the referring physician with a comprehensive report on structure and function on every patient," he said.