CLEARANCE OF TL-201 FROM THE PERIPHERAL-BLOOD - COMPARISON OF IMMEDIATE AND STANDARD TL-201 REINJECTION

As several reinjection procedures have shown encouraging results in te rms of imaging, we investigated whether the kinetics of thallium-201 w ould differ between the standard stress-redistribution-reinjection app roach and the stress-immediate reinjection approach. In 53 consecutive patients with undiagnosed chest pain, 75 MBq (2 mCi) Tl-201 was injec ted at maximal exercise. In 26 of these patients (group I), 37 MBq (1 mCi) Tl-201 was reinjected immediately after completing the exercise i mages (the immediate reinjection procedure) and in 27 patients (group 1I), 37 MBq (1 mCi) Tl-201 was reinjected after completing 3-h redistr ibution images (the standard reinjection procedure), Mean peak Tl-201 blood activity after exercise was 17.7+/-12.5 kBq/ml (4.8+/-3.4 mCi/ml ) for group I versus 16.4+/-9.2 kBq/ml (4.4+/-2.5 mCi/ml) for group II (NS). The relative increase in Tl-201 blood activity after reinjectio n of half the initial dose [37 MBq (1 mCi)] exceeded 50% of the initia l peak in both groups. The relative amount of Tl-201 delivered to the myocardium was assessed by the area under the curve after both exercis e and reinjection, and was 117%+/-72% for group I and 112%+/-73% forgr oup II (NS). Blood clearance of Tl-201 was at least biexponential. Mea n early decay constants (lambda(1)) after exercise and reinjection wer e 0.30+/-0.18 min(-1) and 0.22+/-0.046 min(-1) respectively for group I (T-1/2 2.3 min and 3.2 min respectively, NS), and 0.30+/-0.12 min(-1 ) and 0.24+/-0.07 min(-1) respectively for group II (T-1/2 2.3 min(-1) and 2.9 min respectively, NS). For both procedures no significant dif ferences were found between lambda(1) after exercise and lambda(1) aft er injection. The mean late clearance (lambda(2)) from the blood was 0 .032+/-0.056 min(-1) and 0.012+0.012 min(-1) respectively for group I (T-1/2 21.6 min and 57.7 min respectively, NS), and 0.036+/-0.030 min( -1) and 0.014+/-0.014 min(-1) respectively for group II (T-1/2 19.3 mi n and 49.5 min respectively, NS). Also, no significant differences wer e found between lambda(2) after exercise for both groups and between l ambda(2) after reinjection for both groups. We conclude that reinjecti on of 37 MBq (1 mCi) Tl-201 (half the initial dose) results in a relat ive increase in the initial peak and a relative increase in the amount of Tl-201 delivered to the myocardium of more than 50% for both the s tandard and the immediate reinjection procedure. The clearance of Tl-2 01 from the blood was not influenced by exercise or by the time of rei njection. Based on Tl-201 kinetics as measured in the peripheral blood , there is no reason to postpone reinjection until 3-4 h following exe rcise.