PERSISTENT 24-HOUR SPECT TL-201 DEFECTS, PLASMA TL-201 CONCENTRATIONSAND PET METABOLIC VIABILITY

Previous studies have shown that defects on four hour thallium-201 red istribution images often exhibit late reversibility, suggesting that t he thallium-201 scintigraphic assessment of myocardial viability might be influenced by delayed redistribution imaging. To assess tissue met abolic activity in segments with late thallium-201 defects, positron e mission tomography (PET) with (NH3)-N-13 and (FDG)-F-18 was performed in 26 coronary artery disease patients with left ventricular dysfuncti on undergoing twenty-four hour SPECT thallium-201 scintigraphy. In 13 patients, plasma thallium-201 levels were obtained at the time of SPEC T study and integrated tracer concentrations were determined one, two, four and twenty-four hours following injection. On circumferential pr ofile image analysis of the PET images, ischemia was defined by preser ved glucose metabolism in hypoperfused myocardium while infarction was identified by concordant reductions in both perfusion and glucose met abolism. Nineteen patients had stress-redistribution SPECT studies and seven had rest-redistribution SPECT studies. Using a semi-quantitativ e scoring system, four experienced observers visually identified 100 f ixed, 17 partially reversible and twelve completely reversible segment al SPECT thallium-201 defects. On PET, metabolic activity was identifi ed in 51 (51%) fixed defects (21 PET ischemia, 30 PET normal) and nine (53%) partially reversible defects (five PET ischemia, four PET norma l). Of the twelve completely reversible thallium-201 defects, six (50% ) were normal on PET, five (42%) had PET ischemia and one (8%) had PET infarction. The relative number of fixed thallium-201 defects with me tabolic activity on PET did not depend on whether a stress or rest tha llium-201 study was performed, or on whether the plasma thallium-201 i ntegral concentration was high or low relative to mean values at any t ime point. Despite delayed redistribution imaging, PET imaging identif ies glucose metabolic activity, and therefore residual tissue viabilit y, in the majority of fixed twenty-four hour thallium-201 defects.