To study the potential usefulness of technetium-99m hexakis-2-methoxy-2-methylpropyl-isonitrile (Tc-MIBI) as a cardiac perfusion imaging agent, the left circumflex coronary arteries of 12 dogs were partially occluded. Eight additional control dogs had no coronary artery stenosis. Myocardial Tc-MIBI activities in the left circumflex and left anterior descending zones were continuously monitored by miniature implantable radiation detectors for 4 hr after administration of the isotope. The dogs were then killed. Serial gamma camera images were also acquired during the study. Heart rate, arterial blood pressure, pressure distal to the stenosis, and cardiac output did not change significantly during the experiment. Microsphere-determined regional myocardial blood flow was significantly reduced in the left circumflex distribution in the 12 dogs with coronary artery stenoses. In the 12 dogs with left circumflex coronary artery stenoses, the 4 hr fractional Tc-MIBI clearances from the normal and ischemic zones were minimal and equivalent (0.15 + 0.05 SD vs 0.15 ± 0.07). In the eight control dogs, 4 hr fractional Tc-MIBI clearances from the left anterior descending and left circumflex artery zones were minimal and equivalent (0.11 ± 0.06 vs 0.10 + 0.07). Four hour fractional Tc-MIBI clearance from the blood was 0.98 ± 0.03 for the dogs with stenosis and 0.97 + 0.02 for the dogs without stenosis. One additional dog had complete occlusion of the left circumflex coronary artery followed by administration of Tc-MIBI and scandium-46-labeled microspheres. This heart was immediately sectioned and counted to determine the relationship of regional blood flow to Tc-MIBI distribution (r = .92). Furthermore, the final technetium activity ratio (ischemic divided by normal zone = 0.53 + 0.19) measured in a well counter for the 12 dogs with stenosis was not significantly different from the initial flow ratio at the time of Tc-MIBI administration (0.47 ± 0.20), confirming the absence of redistribution. Gamma camera images were of excellent quality for as l-ong as 4 hr after isotope administration. Thus its linear relationship to regional myocardial blood flow, the minimal myocardial washout and redistribution, and the 140 keV gamma photon make Tc-MIBI a promising new cardiac perfusion imaging agent. Circulation 77, No. 2, 491-498, 1988. THALLIUM -201 has been widely used for the assessment of myocardial perfusion and the diagnosis of coronary artery disease. Unfortunately, its physical properties are not ideal for gamma camera imaging,' and myocardial image quality has sometimes suffered from external or self attenuation. This has resulted in considerable interobserver variability in interpretation of thallium images. Technetium-99m (99mTc) is much better suited for gamma camera imaging because of its 140 keV gamma photon. Previous attempts to link

To evaluate the severity of coronary artery disease in patients with severe peripheral vascular disease requiring operation, we performed preoperative dipyridamole-thallium imaging in 54 stable patients with suspected coronary artery disease. Of the 54 patients, 48 had peripheral vascular surgery as scheduled without coronary angiography, of whom 8 (17 per cent) had postoperative cardiac ischemic events. The occurrence of these eight cardiac events could not have been predicted preoperatively by any clinical factors but did correlate with the presence of thallium redistribution. Eight of 16 patients with thallium redistribution had cardiac events, whereas there were no such events in 32 patients whose thallium scan either was normal or showed only persistent defects (P less than 0.0001). Six other patients also had thallium redistribution but underwent coronary angiography before vascular surgery. All had severe multivessel coronary artery disease, and four underwent coronary bypass surgery followed by uncomplicated peripheral vascular surgery. These data suggest that patients without thallium redistribution are at a low risk for postoperative ischemic events and may proceed to have vascular surgery. Patients with redistribution have a high incidence of postoperative ischemic events and should be considered for preoperative coronary angiography and myocardial revascularization in an effort to avoid postoperative myocardial ischemia and to improve survival. Dipyridamole-thallium imaging is superior to clinical assessment and is safer and less expensive than coronary angiography for the determination of cardiac risk.

Accurate prognostic information is important in determining optimal management of patients presenting for evaluation of chest pain. In this study, the ability of exercise thallium-201 myocardial imaging to predict future cardiac events (cardiovascular death or nonfatal myocardial infarction) was correlated with clinical, coronary and left ventricular angiographic and exercise electrocardiographic data in 139 consecutive, nonsurgically managed patients followed-up over a 3 to 5 year period (mean follow-up, 3.7 +/- 0.9), using a logistic regression analysis. Among patients without prior myocardial infarction (100 of 139), the number of myocardial segments with transient thallium-201 defects was the only statistically significant predictor of future cardiac events when all patient variables were evaluated. Among patients with myocardial infarction before evaluation (39 of 139), angiographic ejection fraction was the only significant predictor of future cardiac events when all variables were considered. This study suggests an approach to evaluate the risk of future cardiac events in patients with possible ischemic heart disease.

SUMMARY After a 4-minute i.v. dipyridamole infusion, 0.14 mg/kg/min, serial thallium-201 scans were obtained in 60 patients undergoing cardiac catheterization. Forty patients had significant (> 50% stenosis) coronary artery disease (CAD), and 20 patients had normal coronary arteries or trivial lesions. The images were graded qualitatively for thallium activity by three observers. Sensitivity was 93% (37 of 40) and specificity was 80% (16 of 20). The sensitivity and specificity of the thallium-201 study were not affected by the extent of CAD, the presence of Q waves, or propranolol therapy. Twenty-seven of 37 patients who had initial defects (73%) had complete thallium redistribution of one or more defects. Patient-by-patient analysis using a regression model of all patients showed that the fate of a segmental thallium defect predicted abnormal wall motion by angiography better than ECG Q waves. The presence of propranolol therapy or collaterals did not significantly affect the thallium redistribution results.We conclude that qualitative interpretation by multiple observers of thallium images after dipyridamole infusion is a highly sensitive and specific test for CAD. After dipyridamole, as with exercise stress, the extent of thallium redistribution is related to the degree of myocardial wall motion abnormality.PREVIOUS STUDIES have shown that exercise thallium-201 myocardial imaging can detect coronary artery disease (CAD).`1 In addition, thallium-201 redistribution shown by serial imaging can identify ischemic but viable myocardium.7-10 However, a standard maximal exercise stress test is not always possible for various reasons, including therapy with ,3 blockers, unstable anginal symptoms, poor physical condition and peripheral vascular disease. A method for evaluating myocardial perfusion and viability that does not require exercise would be valuable.Gould and co-workers"'-13 reported that dipyridamole given intravenously could lead to coronary vasodilation in normal vessels, but fixed coronary stenoses prevented or attenuated this response. The coronary flow response to dipyridamole was similar to that during exercise, but without the physiologic increase in myocardial oxygen demand. These and other investigators '46 have shown that in conjunction with a dipyridamole infusion, thallium-201 scans reliably detect CAD. 116 However, the significance of changes in thallium-201 distribution over time after dipyridamole infusion has not been investigated, and the impact of propranolol on the dipyridamole thallium-201 study has not been defined.Therefore, we sought to determine the significance

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