Stress myocardial perfusion imaging is the most commonly used procedure in the nuclear cardiology laboratories and is useful in the diagnosis of coronary artery disease, risk stratication, assessment of therapy and myocardial viability". Various combinations of stress perfusion imaging are shown in Table 1. Since introduction in 1973, thallium-201 has been extensively used and is gradually replaced by technetium-labelled perfusion imaging agents"´. When viability is in question, the agent of choice becomes thallium. There has been also improvement in the computer and camera system, i.e. faster and more memory and single-photon emission computed tomography (SPECT) since the early 80´s. The advantages of SPECT are better assessment of perfusion abnormality as to its location, extent and severity since there is no interference with super-imposition and overlapping of abnormal and normal myocardium.
Exercise imaging
Previous studies reported that exercise thallium imaging is superior to exercise ECG in diagnosing coronary artery disease" ; however, these studies may be biased in favor of thallium imaging because of inclusion of previous myocardial infarction. We have analyzed 321 patients with normal resting ECG, SPECT thallium and coronary angiography within 3 months of each others´. The sensitivity of SPECT was 81 % and that of ST changes, 42 % (P < 0.0001).
When exercise testing is either not feasible or suboptimal, pharmacologic stress testing is an alternative"´. This is important, especially if one con. siders that one third of patents do not reach adequate heart rate in the exercise laboratory. We have shown that submaximal exercise compromises the ability;of detecting coronary artery disease, the presence of is"´ chemic abnormality, extent and severity of abnormal ity"´. Abnormal thallium images were less common in the submaximal exercise group in the total patients (73% submaximal vs 88% maximal, P<0.002)
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in the subgroups of patients with one(52% vs 74%3 two (84 % vs 88 %)and three(79 % vs 98%) vessel CAD. In coronary occlusion models patients undergo ing angioplasty, cascade of events following coro occlusion is: a decrease in coronary flow/metabo-> lism, diastolic dysfunction, systolic wall motion ab`
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normality, ECG changes and, lastly, angina pectoris. Thus, the techniques which rely on the detection´of? flow disparity would be more sensitive than the techniques relying on wall motion abnormality for ECG changes. =
Pharmacokinetics of thallium-201
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Lee et al. studied thallium biokinetics during exert cise, adenosine, dipyridamole and dobutamine stress _ testing in 15 normal volunteers". Absolute myocardial thallium activity was greater after pharmacologic testing than following exercise. The myocardial thallium clearance was lower with pharmacologic testing than after exercise. The thallium uptake and clearance in the lung and liver were also greater in the pharmacologic stress testing than exercise. Therefore, diagnostic criteria for quantitative analysis of myocardial perfusion imaging must be specific for the type of stress"´. With nitrogen-13-labelled ammonia and positron emission tomography, quantitative measurements of absolute myocardial flow have
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