TC-99M HEXAKIS 2-METHOXY-2-ISOBUTYL ISONITRILE AND TL-201 EXTRACTION, WASHOUT, AND RETENTION AT VARYING CORONARY FLOW-RATES IN RABBIT HEART

Technetium-99m hexakis 2-methoxy-2-isobutyl isonitrile (Tc-MIBI) and thallium-201 extraction, washout, and retention were investigated and compared in 20 isolated, isovolumic, retrograde blood-perfused rabbit hearts at flow rates ranging from 0.5 to 3.5 ml/g wet wt min-1 in the absence of tracer recirculation. Mean Tc-MIBI peak instantaneous extraction was lower (0.55±0.10, p<0.001) and more affected by flow rate (p<0.05) than 201Tl peak instantaneous extraction (0.83±0.06). In contrast, the rate of 201Tl washout was significantly faster (p<0.05) and initially more dependent on perfusion rate (p<0.05) than Tc-MIBI washout. Reflecting its higher peak instantaneous extraction, 201Tl retention was 55-79% higher immediately after isotope injection (p<0.001) than Tc-MIBI retention, and relative changes in maximal 201Tl net uptake correlated better (p<0.05) with relative flow changes than maximal Tc-MIBI net uptake. However, due to its faster washout rate, the superiority of thallium over Tc-MIBI as a perfusion indicator was lost within 10 minutes of tracer injection under the present single-pass experimental conditions. These data were interpreted to indicate that 1) Tc-MIBI is not as well extracted and has a slower washout rate than 201Tl; 2) varying the coronary flow rate has significant but divergent effects on the extraction, washout, and retention of Tc-MIBI and 201Tl; and 3) the present results support continued evaluation of Tc-MIBI as a possible perfusion indicator.