Features May 2011 Issue

Understand the Benefits and Risks of Cardiac Imaging Tests

Radiation exposure is a concern with some of these diagnostic tests, so have frank discussion with your doctor about what tests are necessary for you.

Heart disease remains America’s number one killer, and medical technology is in hot pursuit of ways to diagnose patients earlier and more accurately and treat them less invasively. This has resulted in an explosion of cardiac imaging tests and procedures performed every year. In many instances, there is little doubt these tests save lives. However, all but magnetic resonance imaging (MRI) and some ultrasound-based procedures expose patients to radiation.

 

Patients and physicians alike have growing concerns about the long-term cancer risks from radiation delivered through CT angiography, myocardial perfusion imaging, diagnostic cardiac catheterization, percutaneous coronary procedures and electrophysiology procedures. Here are the facts to help you balance the potential risks against the known benefits, when your doctor orders an imaging study of your heart.

Is Radiation Dangerous?

"Studies of Hiroshima data concluded that if you are exposed to a lot of radiation, you and your progeny will get cancer and birth defects. Having said that, have there been any studies definitively linking CT scans or other radiology tests to cancer? The answer is no," says Milind Desai, MD, a Cleveland Clinic cardiologist with a joint appointment in the Imaging Institute.

This being said, a safe limit of radiation exposure is unknown. No one avoids radiation completely—the average U.S. resident receives 3 mSv (millisieverts, a standard unit of radiation exposure measurement) per year from the earth (radon being the most common culprit), atmosphere and other sources. A single round-trip flight from New York to Los Angeles results in a radiation exposure of 0.03 mSv.

 

A 2-D and a 3-D image (below) of a right coronary artery in the same patient taken with coronary CT angiography. Due to improvements in modern dose-saving strategies, the patient only received 2.9 mSv of radiation. Imaging with conventional X-ray cardiac catheterization would have likely exposed the patient to 4-7 mSv of radiation.

Hospitals limit staff exposure to 50 mSv per year. Patients who undergo cardiac tests rarely receive 50 mSv from a single test. But there’s another issue to consider: "Radiation exposure is cumulative, so if you have multiple scans, the risk is additive. This makes it impossible to know the risks from exposure to a single radiology test," says Dr. Desai.

Variation Among Cardiac Tests

When it comes to radiation exposure, CT has received most of the bad press, due to the skyrocketing number of CT scans being performed. The use of CT angiography (CTA) in coronary disease has also increased dramatically. However, on the bright side, with the advent of new technology and imaging protocols, the amount of radiation patients are exposed to when undergoing a cardiac CTA has dropped dramatically..

"Five years ago, a diagnostic-quality CT study required about15 mSv. Today, we can do a coronary CTA at the same or better quality with 1-3 mSv—less than 1 mSv in some patients," says Dr. Desai.

Angiography, a moving X-ray-based procedure performed during a cardiac catheterization, exposes patients to 4-7 mSv of radiation. Longer angiography-based procedures, such as stenting or electrophysiology studies, produce two to five times this amount, simply because multiple X-rays are required.

At the top end of the spectrum are the myocardial perfusion studies and PET scans, which have also grown in popularity. These tests produce 10-40 mSv of radiation, depending on the radioactive tracer used.

In contrast, magnetic resonance imaging (MRI), like ultrasound, emit no radiation.

Minimizing Exposure

Patients can protect themselves by being informed consumers (see "What You Should Know"). However, it it primarily up to the physicians who order the test and perform them to minimize radiation exposure.

"They need to stop and ask, ‘Is this test necessary and appropriate?’ If the test is appropriate, it is the responsibility of the imaging specialist to select a protocol that minimizes radiation," says Dr. Desai.

If the request is inappropriate, the imaging specialist must decide whether or not to carry out the test.

"In many instances, having a conversation with the referring physician may result in choosing the appropriate test, which, in the case of magnetic resonance imaging (MRI) or an ultrasound-based study, would avoid radiation. I call the referring physician and recommend a different test, if I feel is the appropriate thing to do. Most of the time, they listen and are appreciative!" says Dr. Desai.

For the past two years, Cleveland Clinic has voluntarily entered the radiation dose from each CT scan in each patient’s medical record, which patients can access by logging on to a secure web site. Such reporting is rare. However, in 2012, California will become the first state to require radiation doses from CT scans to be recorded and reported.

Benefit Versus Risk

Based on a patient’s history and physical examination, the cardiologist must select the best test to answer clinical questions. Many different imaging tests are available for patients with coronary artery disease (CAD), and radiation is only one aspect to consider. The patient’s age and gender also play roles in determining appropriateness.

"The benefit of a particular diagnostic modality should outweigh the potential risks. For example, the likelihood of CAD is so low in a 30-year-old woman with chest pain that I would not subject her to a radiation-laden test. But in a 60-year-old male executive with multiple risk factors that place him at intermediate risk, the benefits of the study may outweigh the perceived risk," Dr. Desai explains.

"Most radiation-emitting scans are performed on older patients, who are more likely to die from heart disease than from radiation-related cancer. Nevertheless, we all feel that the fewer radiation-exposing procedures a patient has, the better," he says.