Considerations of 201Tl as a myocardial radionuclide imaging agent in man

Recent reports have suggested the use of intravenous 201Tl (thallium-201) for myocardial imaging with the gamma scintillation camera. In order to better appreciate the possible utility of this agent in humans we examined its distribution and kinetics in 13 patients and in six mongrel dogs, three with experimental coronary artery occlusion. In addition, 201Tl was compared to 86Rb (rubidium-86) in 84 rats. In the rat heart, the concentration of 201Tl was 30 percent higher than that of 86Rb ten minutes after injection. Moreover, myocardium-to-blood ratios for 201Tl averaged 51:1, but only 32:1 for 86Rb ten minutes after administration. In the dog heart, the distribution of 201Tl paralleled that of radioiodinated (131I) albumin particles injected into the left atrium and, thus, appears to be related to regional blood flow. Its concentration in ischemic regions decreased to 32.3 percent of the normally perfused myocardium. In the patients with a recent or old myocardial infarction, areas of decreased 201Tl uptake were easily identified and corresponded in location to that by ECG. Repeat scans 24 hours after the initial injection showed a significant retention of 201Tl by the myocardium. 201Tl blood levels in humans 15 minutes after injection were low (averaging 1.06 percent +- 0.41 percent SD of the total dose per liter) and these levels decreased with a biological half-life of 3.1 +- 0.7 days. Twenty-four hour urinary excretion rates ranged from 0.6 to 6.5 percent of the total dose and appeared related to urinary flow and the concentration of 201Tl in blood. Because of the higher target to background ratios, 201Tl compares favorably with radioactive rubidium. 201Tl in diagnostic doses remained without detectable adverse effects and appears promising as an agent for visualizing abnormal regional myocardial perfusion in patients with coronary artery disease