Quantitation of Perfused Myocardial Mass Using Tl-201 and Emission Computed Tomography

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Abstract
We determined the accuracy with which perfused myocardial mass can be measured using thallium-201 and emission computed tomography in a canine model of acute coronary artery occlusion. Eight dogs underwent permanent coronary artery ligation. Transaxial emission tomography was performed using the Harvard multidetector scanning single photon emission computed tomography system beginning 10 minutes after the intravenous injection of 2 mCi of thallium-201 and 20 minutes after occlusion. After imaging, the animals were killed and the heart sectioned into 1–2-gram specimens for in vitro counting to determine the quantity of myocardium perfused greater than 75% of the maximum concentration in the left ventricle or septum. Perfusion defects were detected in five of the seven animals with perfusion defects ranging in size from 3.7 to 20.3 grams. One dog with a 1.2-g defect at the apex and another with a 6.5-g defect at the base of the left ventricle were not detected. In one of the operated dogs, no perfusion defect was detected by in vitro measurement or by imaging. Correlation between the measurement of perfused myocardial mass by emission tomography and by in vitro measurement was excellent when a variable threshold was used for defining the borders of the myocardium (for myocardium perfused ≥ 75% of maximum, r = 0.83, weight in vitro= 22.6 + 0.69 weight ECT; Sy.x = 7.3). The calculated weight of perfused myocardium was heavily dependent on the threshold when fixed values were used for defining the myocardial borders. Changes in threshold of 5% resulted in a decrease in apparent infarct size by more than 20% for thresholds ranging from 50% to 65%. Emission computed transaxial tomography with thallium-201 can be used to accurately determine the quantity of perfused myocardial mass; the accuracy of the technique is heavily dependent upon the definition of the myocardial border.

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