The most common method of emission tomography in the world is myocardial perfusion SPECT. The reason is that cardiovascular diseases are the leading causes of death (ahead of tumors), especially in highly industrialized countries (WHO, 2004). It is crucial to identify the cases in which the plaques in coronary arteries cause such a serious stenosis that revascularization (either by angioplasty or by bypass graft surgery) is necessary.

The principle is that stenosed coronary arteries cannot dilate on demand (e. g., when the patient performs physical exercise or takes vasodilator medicine) as healthy coronary arteries do. During exercise, the relative perfusion of myo­cardial regions may be different from when at rest. If the myocardium evenly perfused in a resting state shows a relatively hypoperfused area after stress, it indicates ischemia (a relative shortage of the blood supply) (see Figure 12.11). The patients suffering from active ischemia are those whose cardiac pumping function will probably improve after revascularization, and who are at risk of further cardiac events if left untreated. For myocardial perfusion SPECT, we most frequently use Tc-99m-labeled methoxy isobutyl isonitrile (MIBI) or tetro — fosmin, and less frequently thallium chloride labeled by the potassium-analog Tl-201.