Through a technique known as single photon emission computed tomography (SPECT), a special gamma camera detects the rays emitted from a radionuclide substance (e.g., thallium) that is injected into the body. These rays produce clear images of heart tissue on a video monitor. A physician evaluates the heart’s function after exertion and at rest.

Because the radionuclide substance is absorbed by healthy tissue at a different rate than by diseased or damaged tissue, these images can give the physician specific information about the function and viability of the heart muscle and its ability to keep up with the increased demands of physical stress. “Cold spots,” or areas of disease or damage, will usually indicate lack of blood flow, vessel obstruction and/or muscle scarring. These tests are used to help physicians select candidates for surgery and/or catheter-based procedures, and also to measure the extent of a recent heart attack.

During hard exercise, heart muscle that cannot obtain adequate blood flow (perfusion) will show less radioactivity – producing less of a contrast on the monitor – than heart muscle that is receiving normal perfusion. Results can indicate any of the following:

Patients who are unable to exercise may receive a pharmacological stress test. In this test, the patient is injected with a medication such as dobutamine. The medication causes the heart to react as if the person were exercising, though the patient is actually at rest. Other medications that perform this function include adenosine and persantine. Some drugs increase the heart rate, and other drugs increase blood flow by causing the coronary arteries to widen.