BASIC SCIENCES

Thallium-201 chloride is the most commonly used radiopharmaceutical for assessing myocardial perfusion. Studies using radioactive microspheres and contrast coronary angiography have demonstrated the ability of Tl-201 todetect regionsofmyocardial ischemia (1—4). Myocardial perfusion scans using Tl-20l have proven useful in the early detection of myocardial infarction (5,6), evaluation of results ofcoronary artery revascu larization surgery, (7), and the assessment of regional ischemia of the myocardium during exercise stress testing (8,9). A significant technical limitation in interpreting the Tl-20l myocardial scintigram is the relatively high background radioacti@ity in structures adjacent to the heart ( 10). The relatively low heart-to-background ratio of radioactivity (2: 1 at rest) contributes to poor image quality and thus to difficulty with assessing myocardial perfusion. Although exercise increases the heart-to background ratio and thus image quality, certain pa tients are able to achieveonly modestlevelsof exercise. Image interpretation is further complicated by the r