Heart CT Means High Radiation Exposure

The test packs a dose equal to 600 X-rays, but doctors say it is a useful scan.

February 4, 2009, 6:31 AM

Feb. 4, 2009— -- A test commonly ordered by cardiologists across the country to diagnose heart disease exposes patients to radiation doses equivalent to 600 chest X-rays, researchers have found.

The test is called a cardiac CT angiogram, or cardiac CTA for short, and it is a relatively new way to look for potentially dangerous blockages in the heart arteries.

The study, released Tuesday in the Journal of the American Medical Association, is the first to look at radiation exposure from this test, and it is important because of the risk for developing cancer from radiation exposure.

"The strength of [this study] is that it is the first available survey of radiation doses in cardiac CTA," said Dr. Thomas Gerber, study author and associate professor of medicine and radiology at the Mayo Clinic in Rochester, Minn. "That information wasn't known, but it is conceivably important because of the risks of ... radiation."

However, the authors and experts in the field caution that this study should not send patients and doctors fleeing from an important way to diagnose heart disease.

"Not all cardiac CT is evil," said Gerber. "It is valuable in patients who have symptoms of heart disease."

The risk of dying from cancer because of the radiation exposure from one cardiac CTA is difficult to estimate, but the American Heart Association puts the additional risk at around 0.05 percent in a statement released in the journal Circulation.

The actual radiation dose here may also not be as big as it initially sounds.

Radiation is commonly measured in units called millisieverts, or mSv. Authors studied almost 2,000 patients who had a cardiac CTA, and they found that the average radiation dose from these CT scans was 12 mSv.

Gerber said, "This is four times the annual background radiation [that everyone gets] from substances [in the environment] such as radon."

So in other words, an encounter with a heart CT scan equals four years worth of the radiation one would encounter in their day-to-day lives.

"It is important for patients' understanding to compare the test dose to annual background radiation ... instead of to chest X-rays," said Dr. Gilbert Raff, director of the Ministrelli Center for Advanced Cardiovascular Imaging at the William Beaumont Hospital. "Modern technology has reduced the chest X-ray [radiation] dose so substantially that it is no longer a fair or reasonable target to use."

It is also important for patients to recognize that the radiation dose from cardiac CTA is comparable to the radiation exposure with other common tests used by cardiologists to look at blockages in the heart arteries.

Gerber says patients getting a stress test with imaging (where pictures are taken of the heart during exercise) get about the same dose of radiation. A cardiac catheterization procedure, in which a cardiologist puts a catheter and contrast dye into the heart arteries and takes pictures, typically exposes patients to about half as much radiation, but it is an invasive procedure and carries other risks.

"I think the most important ... lesson is that the dose is comparable to noninvasive imaging with stress scanning, which is the most common test we use in patients with ... chest pain," Raff said.

Dr. Christopher Cannon, senior investigator for the TIMI study group at the Brigham and Women's Hospital agreed. "We need to review all cardiac tests, not just CT angiograms."

The study also examined several techniques that are used to reduce radiation exposure when doing a cardiac CTA.

Researchers found that, while these techniques were associated with a significantly reduced radiation dose, not all centers were using these methods consistently. Importantly, these radiation reducing techniques did not affect the quality of the image obtained.

"One objective of this type of study [is] to raise awareness and make sure that [we] use all the means that we have available to reduce radiation dose," Gerber said. "The techniques available today are better than those studied in 2007. We [now] have revolutionary new technology that will probably reduce radiation dose from an [average] of 12 mSv to an [average] of 3 mSv."

Dr. Armin Zadeh, associate director of Cardiac CT at the Johns Hopkins University agreed. "The take-home message is that if you use cardiac CT responsibly, you can achieve good results with radiation exposures, which are very reasonable compared to other imaging tests."