MYOCARDIAL UPTAKE AND KINETIC-PROPERTIES OF TECHNETIUM-99M-Q3 IN DOGS

We postulated that Tc-99m-Q3, a cationic imaging agent, produces myoca rdial activity related to myocardial blood flow during myocardial isch emia and pharmacologic coronary artery vasodilation, and shows little or no myocardial redistribution over 4 hr after intravenous injection. Methods: In six Group 1 dogs, the chest was opened, the left circumfl ex coronary artery was acutely ligated, and dipyridamole (0.32, 0.56 o r 0.84 mg/kg) was infused into the right atrium, followed by 10 mCi of Tc-99m-Q3. Myocardial blood flow was measured by radiolabeled microsp heres. The animals were euthanized and 357 myocardial samples were ass ayed in a well counter for Tc-99m activity. One week later, radiolabel ed microsphere activity was counted and myocardial blood flow calculat ed. In nine Group 2 dogs, a variable occluder was placed around the le ft circumflex coronary artery and an ischemic level of circumflex bloo d flow was maintained constant over 4 hr as measured by an ultrasonic flow meter. Dipyridamole (0.56 mg/kg) was then infused into the right atrium followed by 10 mCi of Tc-99m-Q3. Gamma camera images were acqui red at 5, 15, 30, 60, 120 and 240 min following Tc-99m-Q3 injection. M icrosphere blood flow and endocardial biopsies (n = 6 dogs) were perfo rmed at 30, 60, 120 and 240 min following Tc-99m-Q3 injection. Results : In the Group 1 animals, Tc-99m activity (y) was related to myocardia l blood flow (x) from 0 to 6.1 ml/min/g by the relationship y = 0.83X + 0.18, r = 0.95, p = 0.0001. The scintigraphic ratio of myocardial pe rfusion defect zone counts-to-normal myocardial zone counts (0.54 +/- 0.05 at 30 min) remained constant over 4 hr, as did technetium counts from direct endocardial sampling. Scintigraphic count ratios allowed d iscrimination between perfusion defect and normal myocardial regions b eginning at 5 min following Tc-99m-Q3 injection. Conclusions: Over a r ange of myocardial blood flows from 0 to 6.1 ml/min/g, Tc-99m-Q3 myoca rdial activity is related to myocardial flow at the time of tracer inj ection. Technetium-99m-Q3 shows no evidence of myocardial redistributi on over a 4-hr period.