Although the benefits of the medical imaging scans are unquestioned, so is their overuse. A pained chuckle rose from the audience when Mettler flashed a slide of a Web site offering gift certificates for CT scans.

Between 1993 and 2006, the number of CT scans grew by more than 10 percent a year, to 62 million scans annually. Multislice CT scanners deliver higher doses of radiation than single-slice scanners, though it's hard to estimate the doses that patients receive because different machines in the hands of different practitioners deliver doses that vary by as much as a factor of 10.

The widely varying dose demonstrates the need for standardization, said Richard L. Morin, Ph.D., of the Mayo Clinic in Jacksonville, Fla., chairperson of the ACR's Dose Index Registry Committee, who gave an afternoon presentation on regulations and ethics governing research using human subjects. He noted that with the exception of mammograms, the size of the doses delivered during scans is largely unregulated.

If you're concerned about the dose of the scans, the safest place to be is in a study, where doses are regulated, Morin said. "If you want your kid to get low-dose scans, get him into a research study," he said.

In one case, that of CT colonography, the requirements for a low-dose scan for a research study led to the wider adoption of the low-dose scan in other situations. The strict regulation of studies means that human subjects are now well protected, Morin said.

Mettler and James A. Brink, MD, of Yale University, who was the conference keynote speaker, both advocated more quality standards in medical scanning as one antidote to the increasing dose of radiation, though it's unclear what form those requirements should take. Regulation of mammography has been cumbersome, according to Mettler. "But it's upped the quality for women," Mettler said.

Brink suggested that what's needed now are diagnostic algorithms that indicate when a test ought to be used. Some facilities have begun incorporating such guidelines into the computer systems used to order tests. Also, many facilities and many private payers have already incorporated ACR Appropriateness Criteria® into their imaging decision-making progress as well.

A large part of the problem may be the lack of knowledge on the part of patients and doctors, especially nonradiologists, about the potential dangers of radiation. Mettler noted that many emergency room physicians routinely order multiple scans on trauma patients without stopping to consider the amount of radiation that a patient may be exposed to.

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