916-65 Clearance of Thallium-201 from the Blood After Peak Exercise: No Difference in Washout Kinetics Between Early and Late Reinjection

Early reinjection (ER) of Thallium-201 (TI) following post exercise (Ex) imaging has been proposed as an alternative time-saving approach for the evaluation of patients (pts) with ischemic heart disease. However the influence of shortening the interval between Ex and RI on TI kinetics has not been established before, The aim of the study was to determine the influence of Ex and the time interval between Ex and RI at rest on TI clearance from the peripheral blood. A total of 53 pts were studied. In all pts TI (75 MBq) was injected at maximal Ex. In 26 pts 37 MBq was reinjected immediately after completing the Ex-images and in the remaining 27 pts TI (37 MBq) was reinjected after completing 3-h redistribution images. Venous blood samples were taken at 2-minute intervals during the first 30min after TI administration and residual TI activity was measured shortly before RI. From blood activity curves peak activity, residual activity and decay constants were calculated.ResultsEx-injection (75 MBq)Early RI (37 MBq)Late RI (37 MBq)P-valuepeak activity (% of inilial peak)10095±5576±59n.s.residual activity (% of Initial peak)8±66±7n.s.λ1, min-1(1-10 min)0.17±0.090.14±0.050.13±0.03n.s.λ2, min-1(11–30 min)0.029±0.050.007±0.0150.003±0.011n.s.Conclusions1. Increase of TI blood concentration after RI of half the initial dose often exceeds half the initial peak activity. 2. Tl clearance from the blood is monoexponential both after Ex and Rl with comparable decay constants and is not influenced by Ex or the time interval between Ex and RI. 3. Based on these findings TI can be reinjected immediately after completing Ex-images without influencing washout kinetics of TI