Coronary CTA produces good results with better ones in the works

As evidence supporting 16-slice coronary CT angiography trickles in, researchers have already set their sights on 64-slice scanners.

The latest study using 16-slice CTA appeared in the May 25 issue of the Journal of the American Medical Association. Dr. Martin H.K. Hoffmann and colleagues at University Hospital in Ulm, Germany, examined 103 patients with known or suspected coronary artery disease. Compared with catheter angiography, MSCT scored well in detecting lesions stenosed greater than 50%.

Since that study's completion, Hoffmann and colleagues have evaluated nearly 100 coronary CTA cases on a 40-slice scanner and have recently upgraded to a 64-slice machine. Image quality on the 40-slice CT is only slightly better, Hoffmann said, but the machine provides a substantial boost to robustness.

Breathing artifacts are almost gone, provided that the patient has performed breath-holding rehearsals. Motion artifacts are alleviated as well, due to better utilization of the multicycle algorithm, which increases temporal resolution, he said.

With the 16-slice scanner, only 73% of scans were completely artifact-free. Preliminary data from the 40-slice studies suggest that the number of excellent quality images will jump to 85%. Hoffmann doesn't anticipate much more improvement to image quality with the 64-slice scanner, unless the rotational time - the main parameter determining temporal resolution - increases below 300 msec.

In the JAMA study, researchers recorded segment-based sensitivity, specificity, and negative predictive values of 95% and greater. Positive predictive value dipped to 87%.

Regarding CTA false positives, Hoffmann said that catheter angiography is just as likely to suffer from false negatives, when compared with intravascular ultrasound as a standard of reference. IVUS is the best method to delineate a cross-sectional cut of the coronary artery, but it is highly invasive and therefore restricted to stringent indications of plaque imaging.

"IVUS may serve as the true gold standard in the future to test MSCT and coronary catheterization angiography against one another, and we may find cath to be inferior to delineate complex lesion anatomy," he said.

Dr. Mario J. Garcia wrote a commentary accompanying Hoffmann's study that praised the work while sounding several caveats for coronary CTA. Hoffmann's patients were consecutively acquired, thereby eliminating selection bias, and researchers excluded very few (6.4%) segments from analysis due to poor image quality.

Stents and calcifications still present a problem for MSCT, often leading to overestimation of severity, the reason for many of Hoffmann's false positives. Hoffmann's high per-segment sensitivity (95%) and specificity (98%) are excellent, however, when compared with other indirect methods used to detect coronary stenosis such as conventional stress tests, Garcia said.

He pointed out that if CTA had been used as the first test, nearly half the patients could have avoided catheter angiography, and only two with significant disease would have been missed.

Garcia, a cardiologist at the Cleveland Clinic Foundation, told Diagnostic Imaging that Hoffmann's results would only improve with a 64-slice machine. Preliminary data from other studies suggest that calcification and stents are better imaged on 64-slice scanners, resulting in the exclusion of fewer segments for poor image quality and fewer false positives.

The radiation dose in the Hoffmann study, however, was three times that of a diagnostic catheter angiogram. This can be problematic when imaging young patients and women of childbearing age, he said.

Garcia also called for outcome and cost analysis studies before coronary CTA becomes routine clinical practice.

"I am concerned that many practices are moving too quickly to adopt the technology in the absence of guidelines that define performance and interpreting standards," he said.

The newly formed Society for Cardiovascular Computed Tomography (SCCT) has begun efforts to establish guidelines for credentialing, said Garcia, an SCCT board member.

Additionally, a multicenter trial comparing CTA with catheter angiography is wrapping up. The Coronary Assessment by Computed Tomographic Scanning and Catheter Angiography Trail enrolled more than 200 patients from 12 different hospitals worldwide. A third person in a remote location is analyzing the CT images - all performed on 16-slice scanners,

"If Hoffmann's excellent results can be replicated in smaller institutions with less experienced physicians and a patient population potentially more difficult to image, then we can prove the technology," Garcia said.

The current JAMA study and multiple other single-center studies show that the patient with atypical clinical presentation of chest pain suspected for coronary heart disease will benefit from an MSCT scan, Hoffmann said. Patients with arrhythmia, obesity, and insufficient breath-holding capacity should be excluded.

"The benefit seems to be especially high for those patients with intermediate pretest probability," he said.

For more information from the Diagnostic Imaging archives:

Cardiac CT images reveal serious problems outside heart

Vendors at ECR reveal new strategies in MR, IT, CAD, and CTA

Calling all cards

Additional CT slices improve coronary artery resolution, reduce radiation exposure