Use of advanced imaging machines in hospital emergency departments tripled between 1998 and 2007, resulting in higher costs and longer emergency room stays, U.S. researchers said on Tuesday.

They said the rate of growth is untenable, and needs to be tempered by the extra costs and risks associated with the use of computed tomography or CT scans, which expose patients to radiation.

Overuse of the costly scans has been a concern of policymakers for the past few years, prompting cuts in Medicare reimbursement. Recent studies on radiation risks from computed tomography, or CT scans, and several cases of accidental radiation overdoses have drawn new focus on medical imaging.

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.

For this study, a team led by Dr. Frederick Korley of Johns Hopkins Medicine in Baltimore combed through a nationwide sample of emergency department data collected between 1998 to 2007.

They found patients with injury-related conditions were three times more likely to get a CT or a magnetic resonance imaging or MRI scan in 2007 than they were in 1998.

During the same period, however, the diagnosis of life-threatening conditions -- such as fractures of the cervical spine or bleeding in the brain -- rose just slightly.

They also found that patients who got an advanced scan spent about two hours longer in the emergency department than other patients.

And there was virtually no change in the number of patients who were admitted to the hospital during the study period.

We didn't notice just a small increase in the utilization of advanced imaging. It was a really significant increase without a corresponding increase in the diagnosis of life-threatening illness, said Korley, whose study appears in the Journal of the American Medical Association.

It implies there is a potential amount of overuse or use that is not directly yielding any meaningful clinical results, he said.

He said in some cases, there may be a benefit to giving a patient a scan to rule out a potential life-threatening illness, such as a brain bleed in person who has had a head trauma. That person can then leave the emergency department with peace of mind, Korley said.

Korley said costs of a CT scan vary widely from state to state, ranging from a few hundred dollars to a few thousand.