Persistent twenty-four hour SPECT thallium-201 defects, plasma thallium-201 concentrations and PET metabolic viability.

Previous studies have shown that defects on four hour thallium-201 redistribution images often exhibit late reversibility, suggesting that the thallium-201 scintigraphic assessment of myocardial viability might be influenced by delayed redistribution imaging. To assess tissue metabolic activity in segments with late thallium-201 defects, positron emission tomography (PET) with 13NH3 and 18FDG was performed in 26 coronary artery disease patients with left ventricular dysfunction undergoing twenty-four hour SPECT thallium-201 scintigraphy. In 13 patients, plasma thallium-201 levels were obtained at the time of SPECT study and integrated tracer concentrations were determined one, two, four and twenty-four hours following injection. On circumferential profile image analysis of the PET images, ischemia was defined by preserved glucose metabolism in hypoperfused myocardium while infarction was identified by concordant reductions in both perfusion and glucose metabolism. Nineteen patients had stress-redistribution SPECT studies and seven had rest-redistribution SPECT studies. Using a semi-quantitative scoring system, four experienced observers visually identified 100 fixed, 17 partially reversible and twelve completely reversible segmental SPECT thallium-201 defects. On PET, metabolic activity was identified in 51 (51%) fixed defects (21 PET ischemia, 30 PET normal) and nine (53%) partially reversible defects (five PET ischemia, four PET normal). 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 metabolic activity on PET did not depend on whether a stress or rest thallium-201 study was performed, or on whether the plasma thallium-201 integral concentration was high or low relative to mean values at any time point. Despite delayed redistribution imaging, PET imaging identifies glucose metabolic activity, and therefore residual tissue viability, in the majority of fixed twenty-four hour thallium-201 defects.