Thallium-201 kinetics in nonischemic canine myocardium
Myocardial thallium-201 (201Tl) kinetics have not been precisely defined because reliable techniques to determine continuous activity in vivo have not been available. In this study, an implantable miniature cadmium telluride radiation detection device was inserted through the left ventricular apex, positioned against the endocardium, and used for continuous on-line monitoring of myocardial 201Tl activity for 260-810 minutes in 18 dogs. Blood was serially sampled and counted. Microsphere-determined myocardial blood flow was measured before administration of 201Tl and was not significantly different from that measured at the end of the experiment in 11 of 18 dogs. Thallium was administered intravenously. Myocardial and blood activity curves were analyzed using a nonlinear least-squares estimation of the decay constant λ (min-1). Myocardial activity reached 80% of peak within 1 minute, peaked in a mean time of 23.7 ± 17.9 minutes (±SD), then decreased monoexponentially with a mean λ = 0.00176 ± 0.00054 min-1 (half-time [T( 1/2 )] = 437.2 ± 126.3 minutes) (r = 0.95-0.99). Blood activity decreased triexponentially with a mean λ1 = 0.36782 ± 0.20141 min-1 [T( 1/2 ) = 2.4 ± 1.1 minutes], mean λ2 = 0.06134 ± 0.02973 min-1 [T( 1/2 ) = 14.1 ± 7.1 minutes] and mean λ3 = 0.00177 ± 0.00075 min-1 [T( 1/2 ) = 445.5 ± 201.6 minutes] (r = 0.99-1.00). The mean final blood λ3 was almost identical to the mean myocardial λ. To determine the relationship between myocardial and blood clearance rates of 201Tl after i.v. and intracoronary administration. 201Tl was administered by the intracoronary route in 4 dogs. Myocardial 201Tl activity peaked in a mean time of 2.0 ± 0.8 minutes, then decreased monoexponentially with a mean λ = 0.00833 ± 0.00115 min-1 [T( 1/2 ) = 84.2 ± 12.5 minutes] (r = 0.94-0.98). Thallium clearance from the myocardium after intracoronary 201Tl was significantly greater than after i.v. 201Tl (p < 0.00001). The data suggest that 201Tl washout after peak activity is reached in nonischemic myocardium is monoexponential. The rate of 201Tl clearance from the myocardium is related to the rate of 201Tl clearance from the blood after i.v. administration of the tracer.