Comparison of the Myocardial Uptake of a Technetium-Labeled Isonitrile Analogue and Thallium

The myocardial transmicrovascular transport of thallium-201 (201T1) and technetium-99m hexakis(2-methoxyisobutylisonitrilc) (MIBI) were compared during variable blood flow levels in nine blood-perfused, isolated rabbit hearts. Seventeen injections of radiolabeled albumin and EDTA as well as 201Tl and MIBI were performed by indicator-dilution techniques. When coronary flow was varied from 0.52 to 3.19 ml/g/min, myocardial extraction for MIBI averaged 0–38 ± 0.09 (SD) whereas W1T1 myocardial extraction averaged 0.73 ± 0.10 (p <0.001). Net extraction, which was calculated using end points of 1.8–4.9 minutes, averaged 0.41 ± 0.15 for MIBI and was less than the 201T1 net extraction of 0.57 ± 0.13 (p <0.001). The mean capillary permeability-surface area product for MIBI (0.44 ± 0.13 ml/g/min) was one third of 201Tl (1.30 ± 0.45 ml/g/min; p <0.001). However, parenchymal cell permeability-surface area product for MIBI (47.58 ± 25.85 ml/g/min) was much higher than 201T1 (6.52 ± 6.51 ml/g/min; p <0.0001), and apparent cellular volume of distribution for MIBI (15.15 ± 3.31 ml/g) was also higher than 201Tl (10.19 ± 4.00 ml/g; p <0.01). These data suggest that capillary permeability for 201T1 is greater than MIBI, but the reverse is true at the parenchymal cell wall. In addition, a new blood-perfused preparation is used for indicator-dilution techniques, and previously developed modeling analyses are also extended to these experiments.

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