Kinetics of technetium-99m-Sestamibi and thallium-201 in a transient ischemic myocardium animal model: insight into the 'redistribution' phenomenon.

This study was designed to determine the redistribution potential for technetium-99m (99mTc)-hexakis-2-methoxy isobutyl isonitrile (99mTc-Sestamibi) as compared to thallium-201 (201Tl) in a transiently ischemic swine heart model. The left anterior descending coronary artery (LAD) was totally occluded for 10 min. One minute prior to the release of the LAD, 99mTc-Sestamibi, 201Tl and a set of 95Nb-radiolabeled microspheres (15 microns) were injected. A second set of 51Cr-radiolabeled microspheres was injected prior to sacrifice in order to document reflow. Animals were sacrificed at different times post-LAD release (ranging from 1 min to 4 h). The left ventricle was sectioned into 0.2- to 0.5-gram pieces for the gamma spectroscopic counting of the 99mTc-Sestamibi, 201Tl and radiolabeled microspheres. Linear regression analysis of radiotracer localization versus microsphere-determined regional myocardial blood flow (rMBF) demonstrates an initial slight filling in of 99mTc-Sestamibi into transiently ischemic zones, with subsequent stable kinetics up to 4 h (i.e., it does not further redistribute). In comparison, the ischemic to normal ratios for 201Tl activity increase progressively in a time-dependent manner. These differences between 99mTc-Sestamibi and 201Tl might be explained on the basis of their blood clearance kinetics and/or their net clearance from normal and ischemic zones of the heart. It is concluded that 99mTc-Sestamibi is a stable and reliable indicator for rMBF over time, and that the lack of normalization of 99mTc-Sestamibi into transient ischemic zones will necessitate two separate injections for differentiation between ischemia and persistent defects.