Early redistribution of Thallium-201 after temporary ischemia

To define the time course of redistribution of thallium-201 (²⁰¹TI), ischemia was induced in seven pigs by temporary occlusion of the circumflex coronary artery. After 1 1/2 min of occlusion ²⁰¹TI and labeled microspheres were injected into the left atrium. Flow was re-established 4 min after occlusion. Prior to reflow, the relative activities of ²⁰¹TI and microspheres in the ischemic area were similar, but as early as 5 min after reflow the relative ²⁰¹TI activity was considerably higher than the relative microsphere activity and from 15 to 105 min after reflow, relative ²⁰¹TI activity (average 69% of that in normal myocardium) continued to be higher than relative microsphere activity (average 6% of normal). Myocardial arteriovenous differences for ²⁰¹TI were followed sequentially after ²⁰¹TI injection in normal dogs and in dogs with temporary coronary occlusions. The results suggested both loss of ²⁰¹TI from normal myocardium beginning 10 min after ²⁰¹TI injection and increased extraction of ²⁰¹TI from the blood pool immediately after release of a transient occlusion. Redistribution of ²⁰¹Tl therefore begins very soon after relief of myocardial ischemia and even a short delay in initiating myocardial imaging may decrease the sensitivity of the technique for detecting transient ischemia.