KINETIC-PROPERTIES OF TC-99M-Q12 IN CANINE MYOCARDIUM

Background Tc-99m-Q12 is a new Tc(III) perfusion imaging agent that permits prompt myocardial visualization in humans. We postulated that Tc-99m-Q12 myocardial activity is related to actual myocardial blood flow during conditions of myocardial ischemia and pharmacological coronary artery vasodilation and that Tc-99m-Q12 shows little or no myocardial redistribution as long as 4 hours after intravenous injection. Methods and Results In seven anesthetized, open chest dogs, the left circumflex coronary artery was occluded, and dipyridamole (0.32 or 0.56 mg/kg) was infused into the right atrium, followed by 10 mCi of Tc-99m-Q12. Myocardial blood flow was measured by radiolabeled microspheres. The animals were euthanized, and a total of 315 myocardial samples were assayed in a well counter for Tc-99m activity. One week later, radiolabeled microsphere activity was determined, and myocardial blood flow was calculated. Tc-99m activity (y) was related to myocardial blood flow (x) from 0 to 2 mL.g(-1).min(-1) by the relation y = 0.64x + 0.35 (r = .88, P = .0001). In 14 additional anesthetized, open chest dogs, an occluder was placed around the left circumflex coronary artery, and an ischemic level of circumflex blood flow was maintained constant over 4 hours as measured by an ultrasonic flowmeter. Dipyridamole (0.56 mg/kg) was infused intravenously beginning 15 minutes after coronary occlusion and then followed by 10 mCi of Tc-99m-Q12. Gamma camera images, hemodynamics, microsphere blood flow, and endocardial biopsies (latter in six dogs) were performed at 30, 60, 120, and 240 minutes after Tc-99m-Q12 injection. Myocardial blood flow in the distribution of the left anterior descending artery decreased by 29.6% from 30 to 240 minutes (P<.05), whereas left circumflex blood flow increased by 40.4% from 30 to 120 minutes (P<.05) as the dipyridamole hemodynamic effects dissipated. Nevertheless, the ratio of myocardial perfusion defect zone counts to normal myocardial zone counts remained constant over 4 hours, as did the technetium counts from the needle biopsy endocardial samples. Conclusions Over a limited range of myocardial blood flows from 0 to approximately 2 mL.g(-1).min(-1), Tc-99m-Q12 myocardial activity is related to actual myocardial flow at the time of tracer injection. Tc-99m-Q12 shows no evidence of myocardial redistribution.