Technetium-99m-tetrofosmin to assess myocardial blood flow: experimental validation in an intact canine model of ischemia.

Abstract

Technetium-99m-tetrofosmin is a 99mTc-labeled perfusion tracer demonstrating promise for myocardial perfusion imaging. To determine if 99mTc-tetrofosmin tracks myocardial flow over a pathophysiologic range, the initial myocardial uptake and clearance of 99mTc-tetrofosmin relative to microsphere flow were evaluated in a canine model of ischemia during pharmacological vasodilatation. Six open-chest dogs were subjected to complete left anterior descending coronary artery occlusion. Dogs were injected with 99mTc-tetrofosmin and radiolabeled microspheres during pharmacological stress. Coincident with radiotracer injection, dynamic planar imaging and arterial sampling were performed to assess 99mTc-tetrofosmin clearance from blood, myocardium, lung and liver. Fifteen minutes after injection, hearts were excised for well counting of myocardial 99mTc-tetrofosmin activity and flow. Myocardial 99mTc-tetrofosmin activity correlated linearly with microsphere flow (r = 0.84). Relative 99mTc-tetrofosmin activity underestimated flow at higher flow ranges (> 2.0 ml/min/g) and overestimated flow in low flow ranges (< 0.2 ml/min/g). Technetium-99m-tetrofosmin cleared rapidly from the blood and was retained in the myocardium. Resting target-to-background activity ratios (heart:lung = 3.57 +/- 1.01; heart:liver = 0.58 +/- 0.04) were acceptable 10 min after injection. Our experimental data support both the validity of 99mTc-tetrofosmin as a myocardial perfusion tracer and the use of early poststress 99mTc-tetrofosmin imaging for the assessment of myocardial perfusion in man.