Researchers confront strengths, limitations of 64-slice CTA

State-of-the-art 64-slice CT is proving its worth as a potent noninvasive tool for imaging coronary arteries, but it has some noteworthy limitations, according to research presented at the European Congress of Radiology in March.

One study from Germany looked at the use of 64-slice CT angiography as an alternative to coronary angiography in patients with low to moderate risk of coronary artery stenosis.

"Many patients have equivocal symptoms and an intermediate risk profile, so a reliable method for identification of significant artery stenosis would be of great value," said Dr. Thomas Schlosser of the University Hospital Essen.

The study involved 179 patients, 127 men and 52 women, with suspected coronary artery disease. Patients were imaged on a Siemens Somatom Sensation 64-slice system with reconstructed slice thickness of 0.6 mm.

The researchers defined significant stenosis as reduction of the lumen diameter of 50% or more. Of the 179 patients involved in the study group, two were not eligible to undergo 64-slice CT, due to insufficient heart rate reduction. Researchers were able to confidently rule out another 116 patients for high-grade coronary artery stenosis, and these patients were spared from undergoing an invasive angiography procedure.

The 61 remaining patients (41 men and 20 women) underwent invasive coronary angiography. CTA found 62 cases of significant stenosis on 915 segments. Coronary angiography confirmed 34 true positives and identified 28 false positives. These resulted from plaque formation that did not produce a significant reduction of the lumen and from severe calcifications.

"Presence of severe calcifications on CTA can result in overestimation of stenosis," Schlosser said.

Despite this limitation, 64-slice CTA's utility was strong. The technique had a sensitivity of 100%, specificity of almost 97%, positive predictive value of 54.8%, and negative predictive value of 100%.

"Sixty-four-slice CT reliably detects significant coronary artery stenosis in patients with low to moderate pretest probability of significant coronary artery disease. CTA appears to be helpful for selecting patients who need to undergo invasive angiography," Schlosser said.

In a study from the U.S., researchers at the Medical University of South Carolina examined 64-slice CT in coronary imaging overall, then by segment and by vessel. Results of the study, performed from December 2004 to November 2005, were presented by Dr. Giancarlo Savino of the MUSC radiology department.

The researchers evaluated performance for both stenosis greater than 50% and stenosis greater than 70%. They performed studies with retrospective ECG-gating and compared CTA results with coronary angiography findings.

Sensitivity and specificity with 64-slice CT were very good on a per-patient basis, indicating that the imaging technique may be used to rule out significant coronary artery disease.

In cases of more than 50% stenosis, CTA had the following results:

- accuracy of 99%;

- sensitivity of 100%;

- specificity of 98.3%;

- PPV of 97.7%; and

- NPV of 100%.

In stenosis greater than 70%, values were a bit lower, but still very good.

"Diagnostic accuracy for exclusion of critical coronary artery stenosis over 50% indicates that 64-slice CT may be beneficial in triage of patients with atypical angina," Savino said.

Results by segment were weaker. In segments with stenosis of over 50%, accuracy was 96%, sensitivity was 87.4%, and PPV was 69.8%. In segments with stenosis of over 70%, accuracy was 97.9%, sensitivity was 86.7%, and PPV was 49.1%.

The study shows that 64-slice scanning by segment is better in comparison with earlier-generation CT systems, but performance still has room for improvement.

"Sixty-four-slice CT coronary angiography provides sufficient sensitivity and negative predictive value to rule out significant coronary stenosis in patients with suspected CAD. But on a per-segment basis, sensitivity and diagnostic accuracy are still impaired, due to limited spatial resolution," Savino said.