Aug. 27--Every year, about four million Americans under age 65 undergo medical imaging procedures that bombard them with more radiation than the safety limit set for workers in the health-care and nuclear industries, a new study concludes.

The research is the latest to estimate the dangers of this country's exploding use of CT scans and other diagnostic imaging that involve radiation. The United States has the world's highest per-person imaging rate, and previous research suggests that up to 2 percent of cancers may be linked just to CT scanning.

"We don't want to scare patients or have them refuse necessary procedures," Reza Fazel, a cardiologist at Emory University in Atlanta who led the study, said in an interview. "The risk to an individual patient is very small with these doses. But when you apply a small risk to the entire population, it becomes a concern."

The study, published in today's New England Journal of Medicine, is accompanied by an editorial that calls for rigorous studies of the benefits of medical imaging -- and explicit warnings to patients about the risks.

Michael S. Lauer, a cardiologist at the National Heart, Lung and Blood Institute, cited a study showing that the number of nuclear stress tests nearly tripled from 1993 to 2001.

Yet no large, controlled studies have shown that this barrage of imaging "prolongs life, improves quality of life, or reduces long-term medical costs," Lauer wrote.

There is no direct proof that radiation from medical imaging causes cancer. Instead, scientists estimate the risk by drawing on studies of atomic-bomb survivors and workers exposed to medical or nuclear radiation.

In the United States, those workers are carefully monitored to limit their risks. For example, they may wear special badges that absorb and track daily radiation exposure. The Nuclear Regulatory Commission, which oversees radiation use, restricts them to 100 millisieverts (mSv) every five years (20 mSv per year), with a maximum of 50 mSv in any one year. The sievert is a unit that reflects the biological effects of radiation, such as DNA damage.

What makes risk estimation so tricky is that radiation exposure has a cumulative effect, and organs such as the thyroid and breast are extra sensitive. Children are not only extra sensitive but also have decades longer for damaging effects to progress to cancer.

The new study used health-insurance data to see how many X-rays, CT scans, and nuclear-imaging tests were done on almost a million adults under age 64 in five cities across the country from 2005 through 2007.

Although most of them were exposed to less than 3 mSv per year, about 19,000 adults received 21 to 50 mSv per year, and almost 2,000 received more than 50 mSv.

Generalizing these finding to the whole population, the researchers found that 4 million Americans are being hit with more radiation than workers are allowed to get.

And no one -- least of all patients -- is keeping track.

William Muhr, a radiologist at South Jersey Radiology's nine outpatient clinics as well as in the ER of Virtua Health hospitals, described one troublesome scenario:

"A patient goes to the ER with chest pain. A cardiac CT scan shows he's OK. Six months later, the patient goes to another ER with chest pain, but doesn't say he already had a scan."

Compounding the problem, doctors rarely discuss the issue of radiation, and patients rarely ask about it.

"Radiation exposure is very tough to explain," said Ethan Halpern, vice chair of radiology research at Thomas Jefferson University Hospital "It may sound old-fashioned, but I think the doctor has to make the judgment."

But advances in technology have complicated that judgment call, especially for heart problems.

For decades, physicians relied on a nuclear heart scan, also called a nuclear stress test or myocardial perfusion imaging, to look for blocked arteries and damaged heart muscle. The test, which involves injecting a radioactive tracer into the patient's bloodstream, delivers an average 16 mSv of radiation -- close to the 20 mSv annual limit for occupational exposure.

Over the last five years, a newer scan, called cardiac CT or coronary angiography, has become common. Instead of taking a single X-ray, it takes dozens of X-ray "slices" that a computer combines into a three-dimensional image of the heart and blood vessels.

Cardiac CT has become the preferred scan for an emergency room patient with chest pain because it can rule out not only a heart attack, but also a deadly clot in the lungs, and a rip in the aorta, a main heart artery. The dose from a "triple rule-out" CT: at least as much as a nuclear scan.

And hospitals are just one source of hefty radiation doses. In the new study, most CT and nuclear scans were done in outpatient clinics.

Robert Steiner, a Temple University radiologist, serves on an expert committee that is now developing guidelines for appropriate use of cardiac-imaging technologies.

The problem is, the technology has evolved faster than studies supporting its use.

"We don't really know how successful any of these technologies are at prolonging life," Steiner said. "All of us are looking very closely at ways of cutting the dose."

So are machine manufacturers. They now tout radiation-reduction features, such as CT scanners that turn on and off in a heartbeat -- literally -- so the heart is imaged only when it is relatively still.

At Jefferson, Halpern also gives patients a drug that slows their heart rates during cardiac CT. This has improved image quality while cutting the radiation dose in half, to about 9 mSv.

Of course, insurers are also pushing to reduce the use of imaging, Halpern said: "And that's good. Every Tom, Dick, and Harry who walks into the ER with chest pain shouldn't be getting a CT."

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Contact staff reporter Marie McCullough at 215-854-2720 or mmccullough@phillynews.com.

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