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Myocardial Uptake and Kinetic Properties of Technetium-99m-Q3 in Dogs

From the Division of Cardiology, Department of Internal Medicine, The Eugene L. Saenger Radioisotope Laboratory, Department of Radiology, The Department of Pharmacology and Cell Biophysics, The Department of Chemistry, University of Cincinnati, Cincinnati, Ohio
Mallinckrodt Medical, Inc., St. Louis, Missouri

Correspondence: For correspondence or reprints contact: Myron C. Gerson, MD, University of Cincinnati, Division of Cardiology, Mail Location # 542, Cincinnati, Ohio 45267.

ABSTRACT

We postulated that ^99mTc-Q3, a cationic imaging agent, produces myocardial activity related to myocardial blood flow during myocardial ischemia 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 circumflex coronary artery was acutely ligated, and dipyridamole (0.32, 0.56 or 0.84 mg/kg) was infused into the right atrium, followed by 10 mCi of ^99mTc-Q3. Myocardial blood flow was measured by radiolabaled microspheres. The animals were euthanized and 357 myocardial samples were assayed in a well counter for ^99mTc activity. One week later, radiolabeled microsphere activity was counted and myocardial blood flow calculated. In nine Group 2 dogs, a variable occluder was placed around the left circumflex coronary artery and an ischemic level of circumflex blood 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 ^99mTc-Q3. Gamma camera images were acquired at 5, 15, 30, 60, 120 and 240 min following ^99mTc-Q3 injection. Microsphere blood flow and endocardial biopsies (n = 6 dogs) were performed at 30, 60, 120 and 240 min following ^99mTc-Q3 injection. Results: In the Group 1 animals, ^99mTc activity (y) was related to myocardial 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 perfusion 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 discrimination between perfusion defect and normal myocardial regions beginning at 5 min following ^99mTc-Q3 injection. Conclusions: Over a range of myocardial blood flows from 0 to 6.1 ml/min/g, ^99mTc-Q3 myocardial activity is related to myocardial flow at the time of tracer injection. Technetium-99m-Q3 shows no evidence of myocardial redistribution over a 4-hr period.

Key Words: myocardial perfusion • radionuclide imaging • technetium-99m