Nuclear Imaging - Diagnosing Heart Disease: Heart Disease

Content provided by the Faculty of the Harvard Medical School

Nuclear imaging

The principle behind nuclear imaging, also known as radionuclide testing, is similar to that of an exercise tolerance test: By pushing the heart to work harder, the test induces ischemia in people with hidden heart disease. Images are usually taken with single photon emission computed tomography (SPECT), in which a camera that records x-ray images circles around the heart, taking pictures from many angles. A computer uses this information to build a more detailed and precise image of the heart.

Because nuclear imaging is more expensive and invasive and takes longer than an exercise tolerance test, it's not usually the first test used for diagnosing coronary artery disease. It's used primarily for people with abnormal EKGs who either have inconclusive exercise test results or can't do a conventional exercise test because arthritis, stroke, lung disease, or other ailments restrict their mobility.

During a nuclear-imaging test, the heart can be made to work harder in one of two ways: Either you exercise on a treadmill (see "Exercise tolerance test") or you are injected with a medication that can induce ischemia (see "Pharmacologic stress test," above). Regardless of whether a treadmill or medications are used to stress the heart, you also get an injection of a tracer, a slightly radioactive substance that circulates in the bloodstream. These radioactive particles in the bloodstream are what the SPECT scanner detects when it tracks blood flow through the heart.

The tracers are deposited wherever blood is able to reach living heart muscle. Therefore, the tracers will concentrate in areas of the heart muscle that have a good blood supply, while areas with no blood supply, such as scars from a previous heart attack, won't collect the radioactive particles. You wait a short time for the tracers to reach your heart, then lie down on a bed as the scanner rotates around you taking pictures.

SPECT images show where blood is flowing in the heart and, most importantly, regions that lack blood flow, indicating damage or blockages. A second series of images is taken later on, when the heart is at rest. When comparing the two sets of images, doctors look to see whether abnormalities present under stress disappear once the stress has passed. This difference would suggest that the heart muscle is in danger from coronary artery disease. In people with coronary artery disease, ischemia occurs under periods of stress, but not otherwise. Any abnormalities that appear on both sets of images indicate old heart damage, perhaps from a prior heart attack.

Last updated:	May 03, 2007