Slow regional clearance of myocardial thallium -201 in the absence of perfusion defect: Contribution to detection of individual coronary artery stenoses and mechanism for occurrence

Abstract

As a result of the 'spatially relative' nature of perfusion defect analysis of stress-redistribution thallium-201 (201Tl) scintigrams, hypoperfused myocardial segments may not appear as perfusion defects but they may demonstrate a slow washout rate of 201Tl that can be analyzed in a 'spatially nonrelative' manner. Quantitative analysis of perfusion defects and slow washout rate of 201Tl was performed on scintigrams from 116 consecutive patients with adequate exercise tests, defined as achievement of 85% or more of age-predicted maximum heart rate or development of angina or ST segment depression. A total of 232 diseased and 116 nondiseased vessels were found in the patients. Additional analysis of slow washout rate significantly (p < .05) improved sensitivity for detection of disease in the left anterior descending (from 74% to 82%), left circumflex (from 40% to 61%), and right coronary arteries (from 78% to 90%) without significant loss of specificity. This improvement resulted from the additional detection of 32 of 232 (14%) diseased vascular distributions when the isolated slow washout rate of 201Tl was also determined. In nine of these 32, isolates slow washout rate was associated with another region with isolated slow washout rate in the contralateral myocardial segment. Coronary arteriography revealed similar degrees of stenosis in the vessels supplying these segment pairs. In 13 of 32 cases, a perfusion defect and a slow washout rate were found in the contralateral segment. Coronary angiography revealed that 10 of these 13 segments with perfusion defects were supplied by vessels in which there was a greater degree of stenosis than that in the vessel supplying the segment with isolated slow washout rate. For the remaining 10 of 32, the contralateral myocardial segment was normal. The initial 201Tl distribution in all 10 myocardial segments in which isolated slow washout rates were noted showed slightly decreased regional 201Tl activity that was just within the normal range. The coronary artery supplying the myocardial segment with an isolated slow washout rate had stenosis of a greater degree thant hat supplying the contralateral myocardial segment in nine of 10 cases. The finding of isolated slow washout rates in these patients with adequate exercise tests was highly specific (95%) for segmental disease. We conclude that in patients with adequate exercise tests the occurrence of slow regional myocardial washout of 201Tl in the absence of perfusion defect significantly improves the detection of segmental myocardial hypoperfusion. This improvement appears to be due to the spatially nonrelative nature of 201Tl washout rate analysis, which complements the spatially relative perfusion defect analysis for the assessment of diseased myocardial segments.