POSITRON EMISSION TOMOGRAPHY DETECTS METABOLIC VIABILITY IN MYOCARDIUM WITH PERSISTENT 24-HOUR SINGLE-PHOTON EMISSION COMPUTED-TOMOGRAPHY TL-201 DEFECTS

Background. Four-hour Tl-201 redistribution images underestimate myocardial viability in patients with coronary artery disease (CAD). Because 4-hour defects often redistribute late, delayed imaging may enhance assessment of tissue viability. Myocardial metabolic activity was therefore assessed with positron emission tomography (PET) in 26 CAD patients with impaired ventricular function (ejection fraction, 32.1+/-13.9%) and 24-hour single-photon emission computed tomography (SPECT) Tl-201 defects. Methods and Results. On circumferential profile analysis, PET ischemia was defined by preserved glucose metabolism in hypoperfused myocardium, and PET infarction was defined by concordant reductions in perfusion and metabolism. On 19 stress-redistribution and seven rest-redistribution SPECT studies, four observers visually scored Tl-201 activity in eight segments on a scale from 0 (normal) to 3 (complete defect). Using an improvement in visual score greater-than-or-equal-to 0.75 to define redistribution, there were 100 fixed, 17 partially reversible, and 12 completely reversible defects. PET identified tissue metabolic activity in 51 (51%) segments with fixed defects (21 PET ischemia, 30 PET normal) and nine (53%) segments with partially reversible defects (five PET ischemia, four PET normal). When grouped by 24-hour score, the proportion of fixed defects with metabolic activity varied from 84% (scores less-than-or-equal-to 1.4) to 15% (scores > 2.6). For partially reversible defects, only 53% with scores < 2.0 and one of two with scores greater-than-or-equal-to 2.0 were considered metabolically viable on PET. Of 12 completely reversible defects, six (50%) were normal, five (42%) had PET ischemia, and one (8%) bad PET infarction. The proportion of fixed defects with metabolic activity did not depend on whether a rest or stress study was performed or on the change in visual score used to define Tl-201 redistribution (0.25, 0.50, 0.75, and 1.00). Conclusions. In CAD patients, PET identifies glucose metabolic activity in the majority of fixed 24-hour Tl-201 defects. However, very severe (near-complete) 24-hour Tl-201 defects are less likely to exhibit metabolic activity on PET imaging than are defects with less-pronounced reductions in segmental Tl-201 activity.