Regional variability in the myocardial clearance of thallium-201 and its importance in determining the presence or absence of coronary artery disease

There are several limitations in using absolute myocardial clearance of thallium-201 for the detection of coronary artery disease. Noncardiac factors such as peak exercise heart rate and blood level of thallium can affect its absolute myocardial clearance. However, because all myocardial segments in a given heart are exposed to the same noncardiac factors, a relative difference in myocardial clearance of thallium between segments could reflect the presence of coronary artery disease. Accordingly, myocardial clearance of thallium was analyzed in 370 patients. Patients in Group I (n = 45) had less than 1% probability of having coronary artery disease, patients in Group II (n = 44) had normal coronary arteries and patients in Group III (n = 281) had coronary artery disease. Although mean myocardial clearance of thallium in 15 myocardial segments in three views in Group I subjects was 3.4 ± 0.7 hours, the variability between the slowest and fastest clearing segments in the same subject was as much as 98%. This variability was systematic, suggesting technical reasons associated with imaging as the cause of the variability: 78% of the slowest clearing segments were basal whereas 53% of the fastest clearing segments were apical (p < 0.01). When Group II and HI patients were compared based on Group I values, the absolute myocardial clearance of thallium had a sensitivity and specificity of 92 and 16%, respectively. However, when myocardial clearance of thallium was considered abnormal in a segment only if the rate was 98% slower than that in the fastest clearing segment in the same patient, sensitivity and specificity changed to 69 and 86%, respectively (p < 0.01). When the latter values were provided for logistic regression analysis, myocardial clearance of thallium was at par with redistribution and lung/heart ratio for the detection of coronary artery disease. In conclusion: 1) there is significant regional variability in the myocardial clearance of thallium, even in normal subjects, probably as a result of factors related to imaging techniques; 2) when myocardial clearance of thallium is considered abnormal after comparison with the fastest clearing segment in the myocardium, its diagnostic utility improves significantly; and 3) although the initial impetus for using myocardial clearance of thallium to detect coronary artery disease was its ability to measure absolute values, its use as a relative variable optimizes its diagnostic utility.