Radiation exposure varies widelyin 64-slice cardiac CT protocols

An international clinical trial involving 50 healthcare facilities and nearly 2000 patients has found that physicians often do not apply available dose reduction strategies in procedures, resulting in a wide variation in radiation exposure.

Investigators involved in the Prospective Multicenter Study on Radiation Dose Estimates of Cardiac CT Angiography in Daily Practice I study (PROTECTION I) were encouraged by the median 12-mSv effective dose for the 1965 patients scanned in selected one-month periods from February to December 2007. Participants were drawn from 21 academic and 29 community hospitals in Europe, the U.S. and Canada, Asia, and South America.

Results were published in the 4 Feb., 2009, issue of the Journal of the American Medical Association.

Trial authors were buoyed by results they say establish a benchmark for expected dosages for assessing coronary arteries, bypass grafts, or chest pain protocols for the combined visualization of the coronary and pulmonary arteries and thoracic aorta.

“The lesson of this trial is that a lot of things can be done to lower radiation,” said the trial’s senior researcher Prof. Stephan Achenbach, director of cardiac imaging research at the University of Erlangen in Germany. “It’s a very positive trial. It shows that you can do something about radiation.”

Dosage was highly variable, however, according to first author Dr. Jörg Hausleiter, a cardiologist at the Germany Heart Hospital in Munich. The per-center median dose length product (DLP), a measure of integrated dose during an entire CT exam, ranged from 331 mGy/cm to 2146 mGy/cm.

Patient dosages were lowest in the U.S. and Canada, Germany, and Western Europe. They were higher in East Asia and Australia, the Middle East, and South America.

Dose reduction strategies had a major effect on patient exposure, but except for tube current modulation, they were not generally used, Hausleiter said. Automatic exposure control tube current modulation, which resulted in a 25% DLP reduction, was applied for 73% of patients.

Utilization of tube current and kVp modifications according to patient weight and height using an automatic exposure control feature depended on the make and model of equipment. More than half of Philips and Toshiba 64-slice scanner users employed automatic exposure control. Utilization was lower for GE (32.3%) and Siemens (21.3%) 64-slice scanner users. About 45% of Siemens dual-source scanner users used the feature. A 100-kV tube setting produced a 46% dose reduction and was used for 5% of cases, though clinical trials establishing its benefits are new, according to coauthor Cynthia McCollough, a professor of radiological physics at the Mayo Clinic in Rochester, Minnesota.

“Many technologists have yet to learn you have to turn up the mAs to maintain good image quality when you turn down the kVs,” she said.

Sequential scanning, a prospective ECG-triggered sequence that halts exposure outside a specific time window, was credited with a 78% dose reduction and performed for 6% of cases. It also has only recently been evaluated in published clinical trials, McCollough said.

There were several other independent factors:
• patient weight (relative effect on DLP, 5%);
• absence of stable sinus rhythm (type of heart rhythm, 10% effect);
• scan length (a 1-cm increase in the scan length was associated with a 5% increase in DLP);
• facility experience in cardiac CT (1% reduction);
• cardiac CTA patient volume; and
• type of 64-slice CT system (for highest versus lowest dose system, 97% effect).

Overall, the study illustrates how the gradual implementation of new techniques can lower patient radiation, said Dr. Joseph Schoepf, director of CT research and development at the Medical University of South Carolina in the U.S.

“New technical developments are already on the horizon that hold the potential of greatly reducing radiation concerns,” he said.

In an accompanying editorial in JAMA, Dr. Andrew J. Einstein of Columbia University College of Physicians and Surgeons in New York City wrote that the results underscore the need to perform a patient-specific benefit-risk analysis before performing high-dose cardiac CT angiography.

The potential value of dose-reduction methods should serve as a wake-up call to cardiac CT laboratories that do not routinely use these methods, according to Einstein. He expressed support for quality improvement programs to reduce the variability of radiation exposure seen among the participating PROTECTION I sites.