Nuclear Perfusion Imaging
Nuclear medicine techniques use radioactive particles to “label” certain compounds so that they can be viewed once they are administered to a patient. Many such nuclear techniques exist, but the most common nuclear techniques used for cardiac imaging are 201Thallium (201Tl) and 99mTechnetium (99m Tc) sestamibi (Cardiolite) or tetrofosmin (Myoview) single photon emission computed tomography (SPECT) and positron emission tomography (PET).
All three of these tests use radioactive agents called radioisotopes, injected into the bloodstream, to assess blood flow to the heart. These tests are performed “at rest,” to assess the adequacy of blood flow to the heart in a non-exertional state, and also during stress. The stress portion of the study is often performed with exercise. “Stress” imaging is performed by injecting the radioisotope immediately after vigorous exercise. Alternatively, for patients who cannot exercise, the “stress” may be induced with the injection of certain medications (particularly adenosine or dipyridamole).
At rest, the radioisotope typically shows an even distribution throughout the heart. Following stress, if the coronary arteries are either normal or not severely narrowed, the “stress” images will also show even accumulation of tracer in the heart that matches the “rest” images fairly closely. The areas of heart muscle affected by moderate-to-severe coronary artery narrowing will appear as “defects” in the nuclear image that are readily contrasted with the more normal-appearing rest images. Defects seen on both rest and stress images are usually related to scars in the heart muscle.
Myocardial SPECT studies are very powerful tools for the evaluation for suspected coronary artery disease. Many investigations over a number of years have confirmed the usefulness for patients with suspected coronary artery disease. When a myocardial SPECT study is normal, patients have a low risk for subsequent cardiac events. On the other hand, when SPECT studies show moderate-to-severely abnormalities, the annualized risk of cardiac-related death or myocardial infarction may be as high as 6 percent.