Myocardial imaging with thallium-201 for assessment of regional myocardial perfusion and viability after intracoronary thrombolytic therapy.

Whereas coronary arteriography demonstrates success of reflow or recanalization after intracoronary thrombolysis, myocardial perfusion imaging with thallium-201 (201Tl) permits assessment of viability of reperfused myocytes. The initial distribution of 201Tl in the myocardium immediately after intravenous injection is the result of both blood flow delivery of the radionuclide to the heart and the extraction fraction for 201Tl. If 201Tl is administered during a period of coronary occlusion, a scintigraphic defect will be observed in the territory of the occluded vessel. If, subsequently, blood flow is restored, as with reperfusion, and cellular kinetics transport of 201Tl returns to normal, myocardial regions initially deprived of 201Tl will demonstrate delayed redistribution and the defect will tend to normalize within several hours. In a sustained coronary occlusion, the defect will persist after 201Tl injection. Specific imaging protocols that have been or can be used to assess thrombolytic therapy are reviewed. If intracoronary streptokinase infusion is associated with a substantial enhancement of perfusion and preservation of cellular integrity in the ischemic zone, delayed 201Tl images obtained several hours after intravenous injection of the radionuclide will demonstrate significantly more 201Tl activity than the images before reflow. Direct intracoronary injections of 201Tl down the infarct vessel have also been used to evaluate the benefit of thrombolytic therapy by the demonstration of increased 201Tl uptake after reperfusion in the ischemic zone. Finally, computer methods for subtracting serial images obtained after two separate intravenous 201Tl injections administered before and after streptokinase infusion might be used to derive a "functional image" of the ischemic region that would quantitatively be proportional to the amount of myocardial salvage achieved.