Imaging centers can, if they choose, become accredited by the American College of Radiology. That requires, among other things, scanning a phantom, a device that simulates a body part. Technologists must also be certified, and there are standards for supervising physicians. And the scanners must be regularly assessed to ensure they are properly functioning.

But many centers are not accredited, although the percentage is not known because there is no national registry of imaging centers.

Accrediting will be partly addressed by a little noticed aspect of a wide-ranging Medicare law passed last year. After it goes into effect in 2012, Medicare will pay only for scans done at accredited centers. But imaging experts say the law fixes only part of the problem. High-tech scanning is complicated, and there is no consensus on objective measures to ensure quality. Even with the new law, there is still little assurance that scans will be appropriately ordered and interpreted or that a scanner will be up to date.

Image A poor-quality scan of a ligament, left, and one of high quality. Many scans are done by machines that are a decade old. Credit... Images From Dr. Freddie Fu/University of Pittsburgh

Radiologists are struck by the wide variation in the quality of scans, and they say there is little patients can do other than to ask why the scan is necessary and, if it is, to ask about accreditation, the credentials of the person reading the scan and the age of the scanner.

“The studies I see coming from the outside vary from marginal quality to very good quality,” said Dr. Chris Beaulieu, a Stanford radiology professor. “Some of it is related to equipment, and some is related to people with very good equipment who don’t know how to use it right. And on the interpretation side, there is also a very wide range of quality or accuracy, in my opinion.”

Interpretation can be crucial, Dr. Beaulieu added. “A good radiologist can sometimes accurately read scans off of a lower-quality scanner,” he said. “I see that all the time. A good radiologist and a lower-quality scan could be better than a bad radiologist and a good scan.”