Morphologic correlates of technetium 99m stannous pyrophosphate imaging of acute myocardial infarcts in dogs

To obtain insight into the mechanism(s) responsible for the direct visualization of acute myocardial infarcts by myocardial scintigraphy with technetium 99m stannous pyrophosphate (⁹⁹Tc(m) PYP), scintigraphic and morphologic studies were performed in 22 dogs subjected to occlusion of the proximal left anterior descending coronary artery (LAD). Grossly visible myocardial infarcts occurred in ten of 11 dogs with LAD occlusion for one day, five with LAD occlusion for two days, two with LAD occlusion for seven days and two with LAD occlusion for 13 days. Rare, microscopic foci of necrosis were observed in one dog with LAD occlusion for one day, and no lesions were present in two dogs subjected to temporary LAD occlusion for eight minutes and reflow for 24 hours. In the latter three dogs, ⁹⁹Tc(m) PYP myocardial scintigrams were negative. In the 19 dogs with gross infarcts, ⁹⁹Tc(m) PYP myocardial scintigrams were strongly positive at one and two days after LAD occlusion, much less positive at seven days and faintly positive at 13 days after occlusion. Positive myocardial scintigrams in most dogs showed 'doughnut' patterns, with marked peripheral concentration of radioactivity around central zones of much lower activity. On histologic examination, the one and two day old infarcts exhibited subendocardially located central zones and surrounding peripheral zones, both of which showed distinctive histopathological and histochemical features, including the selective occurrence in the peripheral zones of calcified muscle cells with ultrastructurally demonstrable apatite like crystals in mitochondria. Selective occurrence of high tissue levels of ⁹⁹Tc(m) PYP radioactivity also was demonstrated in the peripheral zones of four infarcts. Hearts with older infarcts (seven and 13 days) showed progressive replacement of necrotic myocardium by granulation tissue and progressive reduction in calcium deposits in the areas of damage. The data obtained in this study establish a temporal and topographical relationship between calcium accumulation in acute myocardial infarcts and ⁹⁹Tc(m) PYP uptake responsible for scintigraphic detection of the lesions with this radionuclide in dogs subjected to proximal LAD occlusion.