Medical imaging has transformed the practice of medicine as imaging studies increasingly replace more invasive, and often more costly, techniques for any number of indications. However, the amount of radiation the U.S. population receives from medical imaging has risen 750 percent in the last 25 years, according to the preliminary results of a report of the medical subgroup of the National Council on Radiation Protection and Measurement (NCRP).

Medical imaging may now be the largest source of radiation exposure in the United States, topping natural sources, said Fred A. Mettler Jr. M.D., of the New Mexico Federal Regional Medical Center. The collective annual dose of radiation from radiology and nuclear medicine is 930,000 person-Sv, he estimated, while the dose from natural background sources may be less than 900,000 person-Sv.

"I don't think most radiologists have a clue about how much this has grown," Mettler said in an interview after the presentation. He presented the results Monday, at this year's National Council on Radiation Protection & Measurements conference in Arlington, Va. The full report is likely to be published in 2008.

Also, the American College of Radiology (ACR) just released the ACR White Paper on Radiation Dose in Medicine, a far-reaching and extensive set of recommendations designed to counteract medical and societal trends that have contributed to any increased radiation dosage that Americans may experience as this beneficial technology advances.

The paper advocates 33 separate measures, including a plan to educate patients, doctors, and others in the principles of radiation safety and appropriate use of imaging, as well as a dose index registry, which is now in progress. The ACR has long been involved in other efforts to make medical imaging safe, such as publishing guidelines and technical standards and offering accreditation programs and a patient education Web site, www.radiologyinfo.org, a cooperative effort with the Radiological Society of North America. (www.rsna.org)

The size of the increase in the collective annual dose was a surprise to many who attended the conference. The increase stems from the growth in the number of scans being done, and from larger doses delivered by new kinds of scans. CT scans deliver the largest portion of the annual collective dose, 440,000 person-Sv, with chest and abdominal and pelvic scans delivering the largest shares, 17 percent and 58 percent, respectively, of that number. Nuclear medicine procedures account for 220,000 person-Sv of the collective dose, with cardiac studies accounting for more than 85 percent of that total.

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.

In order to alleviate this lack of knowledge regarding radiation safety, the ACR white paper advocates that nonradiologist providers receive more radiation physics training as part of their medical education.

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