Los Angeles ? As many as two-thirds of adults underwent a medical test in the last few years that exposed them to radiation and in some cases, a potentially higher risk of cancer, a study in five areas of the U.S. suggests.

It is the latest big attempt to measure how much radiation Americans are getting from sometimes unnecessary medical imaging.

Though the annual average radiation exposure from X-rays, CT scans and other tests was low, researchers found about 20 percent were exposed to moderate radiation doses and 2 percent were exposed to high levels. “Super X-rays” to check for heart problems accounted for nearly a quarter of the radiation people received.

“Given the growing use of medical imaging procedures, our findings have important implications for the health of the general population,” the researchers reported in today’s New England Journal of Medicine.

The study did not directly address whether medical imaging is being overused, but some doctors are concerned that advanced tests like CT scans are being over-prescribed.