With the advent of technetium 99m-labeled myocardial blood flow agents, there is a need for a simple technique for quantitation of infarcted or jeopardized myocardium (IM). This study provides an in vitro validation of a simple technique based upon the analysis of three short-axis slices through the heart following emission computed tomography. All acquisitions were performed using a static cardiac phantom containing pertechnetate Tc 99m. Activity in the phantom was adjusted so that the count density and myocardial-to-background ratio were comparable to those observed in patients. Plastic insets (range of sizes = 4%-72% of myocardium) were used to simulate transmural infarctions. Eighteen studies were acquired, each over 180 degrees into a 64 x 64 matrix. Data were reconstructed using a Ramp Hanning filter with cut off at 0.7 times the Nyquist frequency. Short-axis slices of the myocardium were then generated, and representative apical (A), mid-ventricular (MV), and basal (B) slices were selected. For each slice, a circumferential profile was generated, and the average radius (R) was measured. The fraction (F) of the profile falling below a threshold value was considered to represent IM. Total IM was given by % IM = 100 x (RB FB + RMV FMV + 0.67 RA FA)/(RB + RMV + 0.67 RA), where the subscripts to R and F refer to the relevant short-axis slices. For a threshold set at 60% of peak, measured IM agreed closely with true IM (R2 = 0.98, measured IM = 1.01 x true IM - 1.35). Measurement of % IM was not distorted by variations in slice radius or in slice selection.(ABSTRACT TRUNCATED AT 250 WORDS)