TUESDAY, Feb. 3 (HealthDay News) -- Exposure to radiation varies widely among people who undergo computed tomography (CT) scans, a new study has found.

The study, published in the Feb. 4 issue of the Journal of the American Medical Association, comes a day after the American Heart Association urged that CT scans be used judiciously to minimize exposure to ionizing radiation. That recommendation came in an advisory published in Circulation.

"It is a coincidence," said Dr. Thomas C. Gerber, an associate professor of medicine and radiology at the Mayo Clinic in Jacksonville, Fla. "These projects have been going on for months or years. The timing ultimately was a coincidence." Gerber was an author of the study and lead author of the advisory.

The two reports show the intense interest in 64-slice CT scans, so called because they produce that many simultaneous images of the heart. A CT scan gives a radiation dose equivalent to 600 chest X-rays.

The Circulation report noted that medical imaging using X-rays accounted for half the medical radiation dose to which people in the United States were exposed in 2006, and that its use was growing by 5 percent to 10 percent a year.

The JAMA report found that the radiation exposure of 1,965 people having 64-slice cardiac CT scans at 50 medical centers in a number of countries, including the United States, varied more than sixfold. Radiation doses ranged from 331 mGy x cm (a measure of absorbed radiation) to 2,146 mGy cm.

Sloppiness was not the cause of the wide range of exposure seen in the study, Gerber said. "We actually thought it would be wider," he said. "It has nothing to do with technician competence. It's that there is not one protocol of radiation setting that applies to all patients."

The amount of radiation depends on such factors as what is being scanned, Gerber said -- all or part of the heart. It can also depend on the heart rate of the person being scanned, the regularity of the heart rate and other factors.

Obesity is one of those factors, said Dr. Andrew J. Einstein, an assistant professor of clinical medicine in radiology at Columbia University, who wrote an editorial accompanying the JAMA report.