REGIONAL THALLIUM UPTAKE IN IRREVERSIBLE DEFECTS - MAGNITUDE OF CHANGE IN THALLIUM ACTIVITY AFTER REINJECTION DISTINGUISHES VIABLE FROM NONVIABLE MYOCARDIUM

Background. Thallium reinjection immediately after stress-redistribution imaging identifies ischemic but viable myocardium in as many as 50% of the regions characterized by conventional redistribution imaging as irreversibly injured. However, we have previously shown that some regions in which irreversible defects persist despite reinjection are metabolically active, and hence viable, by positron emission tomography. In the current study, we determined whether the severity of reduction in thallium activity within irreversible defects on redistribution images and the magnitude of change in regional thallium activity after reinjection can further discriminate viable from nonviable myocardium in such defects. Methods and Results. We studied 150 patients with coronary artery disease by exercise thallium tomography using the rest-reinjection protocol. The three sets of images (stress, redistribution, and reinjection) were then analyzed quantitatively. The increase in regional thallium activity from redistribution to reinjection was computed, normalized to the increase observed in a normal region, and termed "differential uptake." Of the 175 myocardial regions designated to have irreversible thallium defects on conventional 3-4-hour redistribution images, 132 had only mild-to-moderate reduction in thallium activity (51-85% of normal activity), and 43 had severe reduction in thallium activity (less-than-or-equal-to 50% of normal activity). Thallium reinjection resulted in enhanced relative activity in 60 of 132 (45%) of the mild-to-moderate irreversible defects and 22 of 43 (51%) of the severe irreversible defects, leaving roughly half of these defects remaining irreversible after reinjection. However, in regions that appeared to remain irreversible despite reinjection, the magnitude of differential uptake differed between mild-to-moderate (74 +/- 14%) and severe (35 +/- 16%) irreversible defects (p < 0.001). All regions with mild-to-moderate defects demonstrated > 50% differential uptake after reinjection. In contrast, all except two of the regions with severe irreversible defects demonstrated differential uptake of < 50%. Fifteen patients also underwent positron emission tomography at rest with F-18-fluorodeoxyglucose (FDG) and O-15-water. FDG uptake was present in 91% of regions with mild-to-moderate reduction in thallium activity, and the results of differential uptake and FDG data were concordant in 81% of these regions. Conclusions. These data indicate that the magnitude of thallium uptake after reinjection differs between mild-to-moderate and severe irreversible defects on standard 3-4-hour redistribution images. The substantial differential uptake of thallium (> 50%) after reinjection in mild-to-moderate defects, even when relative thallium activity does not increase appreciably (and the defect appears to remain irreversible), coupled with preserved metabolic activity by positron emission tomography, supports the concept that such mild-to-moderate irreversible defects represent viable myocardium.