Physiological influences on perfusion imaging in transient myocardial ischaemia: importance of early distribution of thallium-201

We tested the hypothesis that visualisation of defects on thallium-201 (201Tl) myocardial perfusion images (MPI) depends on the duration of the ischaemic state between 201Tl injection and the time of reperfusion of an occluded coronary artery. Praecordial imaging with a gamma camera was performed in 24 anaesthetised, open-chest dogs with transient coronary occlusion. Results indicated that if the duration of the ischaemic state after 201Tl injection was less than 3 min before reperfusion, then the MPI 5 to 15 min after 201Tl injection was falsely negative (201Tl activity in zone (IZ)/normal zone (NZ)greater than 0.85). Dogs which were ischaemic more than 5 min always had MPI defects 5 to 15 min after 201Tl injection (IZ/NZ201Tl ratio less than 0.85). MPI results (201Tl IZ/NZ) 15 min after 201Tl injection were determined by the duration of the ischaemic state after 201Tl injection (r = -0.86) because prolonged ischaemia allowed 201Tl to distribute from blood to myocardium before reperfusion: 201Tl (IZ/NZ( = 0.356 +/- 1.00 (fraction of total 201Tl remaining in blood at the end of the ischaemic state), r = 0.94.