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In order to evaluate the usefulness of thallium-201 (201TI) myocardial scintigraphy in delineating the location size of prior myocardial infarction, 32 patients were evaluated at a mean of 7±2 months after infarction with a 12-lead ECG, resting 201TI myocardial scintigram, biplane left ventriculogram coronary angiograms. From the left ventriculogram, asynergy was quantified as percent abnormally contracting segment (% ACS), the percent of end-diastolic circumference which was either akinetic or dyskinetic. Using a computerized planimetry system, we expressed 201TI perfusion defect as a percentage of total potential thallium uptake.Of 21 patients with ECG evidence of prior transmural infarction, a 201TI defect was present in 20 (95%), angiographic asynergy was present in all 21 (100%). The site of prior infarction by ECG agreed with the 201TI defect location in 24 of 32 patients (75%) with site of angiographic asynergy in 23 of 32 patients (72%). Scintigraphic defects were present in only four of 10 patients (40%) with ACS ≤ 6%, but scintigraphic defects were found in 20 of 22 patients (91%) with ACS ≥ 6% (p ≤ 0.01). Thallium defect size correlated marginally with angiographic left ventricular ejection fraction (r = −0.60) but correlated closely with angiographic % ACS (r = 0.80). Thallium defect size was similar among patients with one-, two-, or three-vessel coronary artery disease (.70% stenosis), but thallium defect size was larger in patients with electrocardiographic evidence of transmural infarction (p ≤ 0.01) or pulmonary capillary wedge pressure ≥ 12 mm Hg (p ≤ 0.001). Thus, resting 201T1 myocardial scintigraphy is useful in localizing quantifying the extent of prior myocardial infarction, but is insensitive to small infarcts (ACS ≤ 6%).