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99mTc-sestamibi and thallium imaging have similar accuracy when used for diagnostic purposes, but whether sestamibi provides accurate information regarding myocardial viability in patients with chronic coronary artery disease has not been established. Since there is minimal redistribution of sestamibi over time, it may overestimate nonviable myocardium in patients with left ventricular dysfunction, in whom blood flow may be reduced at rest.We studied 54 patients with chronic coronary artery disease with a mean ejection fraction of 34±14%. Patients underwent stress/redistribution/reinjection thallium tomography and, within a mean of 5 days, same-day rest/stress sestamibi imaging using the same exercise protocol and with patients achieving the same exercise duration. Of the 111 reversible thallium defects on either the redistribution or reinjection study, 40 (36%) were determined to be irreversible on the rest/stress sestamibi study, whereas only 3 of 63 irreversible thallium defects despite reinjection (5%) were classified to be reversible by sestamibi imaging. The concordance regarding reversibility of myocardial defects between thallium stress/redistribution/reinjection and same day rest/ stress sestamibi studies was 75%. A subgroup of 25 patients also underwent positron emission tomography (PET) studies with 15O-labeled water and [18F]fluorodeoxyglucose (FDG) at rest after an oral glucose load. As in the overall group of 54 patients, there was concordance between thallium and sestamibi imaging regarding defect reversibility in 51 of 73 regions (70%). In the remaining 22 discordant regions (30%), 18 (82%) appeared irreversible by sestamibi imaging but were reversible by thallium imaging. Myocardial viability was confirmed in 17 of 18 regions, as evidenced by normal FDG uptake (10 regions) or FDG/blood flow mismatch (7 regions) on PET. These regions were present in 16 of the 25 patients studied (64%). We then explored methods to improve the sestamibi results. First, when the 18 discordant regions with irreversible sestamibi defects were further analyzed according to the severity of defects, 14 (78%) demonstrated only mild-tomoderate reduction in sestamibi activity (51% to 85% of normal activity), suggestive of predominantly viable myocardium, and the overall concordance between thallium and sestamibi studies increased to 93%. Second, when an additional 4-hour redistribution image was acquired in 18 patients after the injection of sestamibi at rest, 6 of 16 discordant irreversible regions (38%) on the rest/stress sestamibi study became reversible, thereby increasing the concordance between thallium and sestamibi studies to 82%.These data indicate that same-day rest/stress sestamibi imaging will incorrectly identify 36% of myocardial regions as being irreversibly impaired and nonviable compared with both thallium redistribution/reinjection and PET. However, the identification of reversible and viable myocardium can be greatly enhanced with sestamibi if an additional redistribution image is acquired after the rest sestamibi injection or if the severity of reduction in sestamibi activity within irreversible defects is considered.