Noninvasive Quantitation of Myocardial Infarction

We sought to quantitate infarct size using radioactive imaging techniques. Infarcts were created in closed chest dogs. Using a scintillation camera interfaced to a computer, infarct images were made in the anterior, left lateral, LAO, and RAO projections, 48 hours after infarction and 75 to 90 min following the intravenous injection of 15 mCi of Technetium 99m pyrophosphate (Tc-PYP). Images were computer enhanced and area was calibrated with a radioactive grid source of known dimensions. Image radioactivity was normalized for decay and dose corrected for body weight. Animals were sacrificed two hours following the injection of TcPYP. Postmortem images were also computer enhanced and calibrated. Gross infarct area and weight were estimated and transmural biopsies were evaluated for Tc-PYP activity and analyzed for creatine phosphokinase (CPK) content. Contiguous biopsies were pathologically analyzed and graded. There was a negative correlation between tissue Tc-PYP activity and CPK content (r = -0.89). Pathologic severity worsened with increased Tc-PYP activity and diminished CPK content. There was a good correlation between gross infarct area and image infarct area, both in vivo (r = 0.79), and at postmortem examination (r = 0.95). Gross infarct weight also correlated well with image infarct activity in vivo (r = 0.83 in the RAO view) and at postmortem examination (r = 0.87). An additional correlation between gross infarct weight and in vivo image infarct area (r = 0.92 in the LAO view) appeared most promising for future clinical evaluation. These experimental relationships are analyzed and future patient application of these imaging techniques are considered.