Nuclear Medicine & CAD

Nuclear Medicine Offers Sophisticated Diagnostics for Coronary Artery Disease

Nuclear medicine techniques use radioactive particles to "label" certain agents 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 resting blood flow to the heart, and during stress. The stress portion of the study is often accomplished with exercise. The radioisotope is injected immediately after vigorous exercise. The "stress" may also be induced with the injection of certain medications (particularly adenosine or dipyridamole) if the patient is unable to exercise. At rest, the radioisotope is usually evenly distributed throughout the heart (Figure 1). With stress, if the coronary arteries are normal or not severely narrowed, the "stress" images will also show even accumulation of the radioisotope tracer in the heart. However, if moderate-to-severe coronary artery narrowing is present, the stress images may show that the tracer fails to reach certain areas of the heart because the vessels leading to these regions will not permit the increase in blood flow the heart muscle typically demands during exercise (Figure 2). The areas of heart muscle affected by moderate-to-severe CAD will appear as "defects" in the nuclear image that are readily contrasted with the more normal-appearing rest images. Remember that coronary artery narrowing of approximately 60-70% must be present before this pattern will be seen. This means that CAD may be present even if a nuclear scan is normal.



Myocardial (myocardial means heart muscle) SPECT studies are very powerful tools for evaluating suspected CAD. Numerous investigations over a number of years have confirmed the usefulness of these techniques for risk factor assessment. When a myocardial SPECT study is normal, patients have a low risk for subsequent cardiac events. Even in patients at intermediate risk for CAD, the annual risk for death or MI in patients with normal myocardial perfusion SPECT examinations is no higher than 0.6%. In contrast, when the scan is moderately to severely abnormal, the annualized risk of death or MI rises to approximately 6%. Nevertheless, no test is perfect, and myocardial SPECT has its limitations. One limitation: defects that simulate heart muscle blood flow problems, usually referred to as perfusion abnormalities. Occasionally the diaphragm can partially block the transmission of the radioactivity from the injected radioisotope, creating the appearance of a perfusion defect; the same problem can occur in patients with large breasts. Often such problems, or artifacts, can be recognized, but occasionally they result in equivocal interpretations, which often lead to diagnostic uncertainty and the need for further, often invasive (see below) testing. Unlike SPECT, PET tests using a radioactive agent called rubidium do not suffer from this limitation.

Finally, if a patient has moderately severe CAD in all the coronary arteries in a fairly equal fashion, the difference between the rest and stress images can be quite minimal and may not be recognized - this situation is referred to as balanced ischemia (ischemia means reduced blood flow to an organ, often as a result of CAD). In this case, the SPECT test may be incorrectly interpreted as normal.