ABSTRACTPreviously, technetium-99m-stannous pyrophosphate (99mTc-PPi) has been used to localize and estimate the size of myocardial infarcts in animals after permanent coronary artery occlusion. This study tested the hypothesis that 99mTc-PPi accurately sizes myocardial infarctions produced by temporary coronary artery occlusion and reflow in dogs. Three groups of dogs were studied: group A underwent 3 hr of occlusion followed by 2 hr of reperfusion, with 99mTc-PPi injected 10 min after reflow (n = 10); group B underwent 3 hr of occlusion followed by 2 hr of reperfusion, with 99mTc-PPi injected 90 min after reflow (n = 11); and group C underwent 3 hr of occlusion followed by reflow with 99mTc-PPi injected at 10 min and again at 48 hr after reflow (n = 5). Myocardial slices from group A and B dogs were imaged in vitro. Group C dogs were imaged with single photon-emission computed tomography (SPECT) in vivo, and myocardial slices were imaged in vitro at the conclusion of the study. The extent of myocardial infarction was defined with triphenyltetrazolium chloride (TTC) staining, and coronary blood flow was estimated with radioactive microspheres. In addition, transmural myocardial tissue samples were taken from the center of the myocardial infarction, the lateral portion of the myocardial infarction, the normal myocardium adjacent to the lateral aspect of the infarcts, and from the normal myocardium and counted for 99mTc-PPi activity. A significant correlation was found between infarct size determined by areas of increased 99mTc-PPi uptake and that estimated from TTC staining for both group A (r = .89) and group B animals (r = .98). However, the intercepts and slopes relating estimates of infarct size from 99mTc-PPi uptake and TTC staining indicated that 99mTc-PPi overestimated size of infarction when injected early after reflow (group A), especially that of small infarcts (.5 g). Results in group C dogs confirmed that, in the same dogs, estimates of infarct size were smaller when 99mTc-PPi was injected relatively late as compared with early after reperfusion. Thus, 99mTc-PPi injected 90 min or later after reperfusion provides an excellent means to identify and size myocardial infarcts produced by temporary coronary artery occlusion and reflow in this canine preparation. 99mTc-PPi uptake early, but not late, after reperfusion probably occurs in some myocardial cells that are severely injured but viable after reperfusion, and possibly in cells with excess cytosolic calcium that recover with longer periods of reperfusion.