Heart imaging procedures can deliver a significant amount of radiation to patients, U.S. researchers said on Wednesday, urging patients and doctors to weigh the risks against the benefits.

They said nearly one in 10 adults under the age of 64 had a heart procedure involving radiation over a three-year period in five major healthcare markets.

"For many patients in the United States, there is a substantial cumulative radiation exposure from cardiac procedures," said Dr. Jersey Chen of Yale University School of Medicine, whose study appears in the Journal of the American College of Cardiology.

An advanced type of heart stress test called myocardial perfusion imaging, in which doctors inject a radioactive tracer in patients to test blood flow, accounted for 74 percent of radiation exposure from heart scans.

Heart catheterization and stenting — procedures in which thin tubes are fished through blood vessels to open blocked arteries — were the second biggest contributor to radiation exposure, Chen said.

More than half of the heart procedures using radiation were done in the doctor's own office, the team found.

"Policymakers have been concerned that there is a rise in physician office imaging and a rise in total use of imaging," Chen said in a telephone interview.

"I think there is legitimate concern that easy availability — convenience — makes the threshold for testing lower. Whether it is inappropriate or not, our study can't say."

While doctors disagree over how much, most agree that radiation can cause cancer, and researchers are growing concerned that an explosion in the use of medical imaging is making it more likely that patients may develop cancer.

A report last year by the National Council on Radiation Protection and Measurement found that Americans receive seven times more radiation from diagnostic scans than in 1980.

Chen's team, looking specifically at radiation exposure from heart imaging procedures, studied medical claims records from nearly 1 million patients aged 18 to 64 insured by United Healthcare.

They calculated the annual radiation dose based on three exposure ranges: less than 3 millisieverts a year, the average exposure level for people from the sun and environment; between 3 and 20 millisieverts a year and more than 20 millisieverts a year, the upper safety limit for workers exposed to radiation.

Of those who got heart procedures using radiation, most fell into the middle range but more than 3,000 patients got more than 20 millisieverts a year over the three-year period and 75 people got more than 50 millisieverts a year.

"The average patient who gets a nuclear stress test is going to get 16 millisieverts each time. It's going to increase their risk on a statistical basis," Chen said.

Dr. Pamela Douglas of Duke University in North Carolina and a former president of the American College of Cardiology said doctors need to pay attention to radiation but she said the benefits of these heart tests and procedures should be weighed against the risk that a cancer might develop down the road.

"Let's not get hysterical," she said in a telephone interview.

"Overall, patients with heart disease are doing way better. The procedures are helping people. If we can change them or tweak them to help reduce radiation exposure even more, that would be wonderful."