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(²⁰¹T1) permits assessment of viability of reperfused myocytes. The initial distribution of ²⁰¹T1 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 ²⁰¹T1. If ²⁰¹T1 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 ²⁰¹T1 returns to normal, myocardial regions initially deprived of ²⁰¹T1 will demonstrate delayed redistribution and the defect will tend to normalize within several hours. In a sustained coronary occlusion, the defect will persist after ²⁰¹T1 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 ²⁰¹T1 images obtained several hours after intravenous injection of the radionuclide will demonstrate significantly more ²⁰¹T1 activity than the images before reflow. Direct intracoronary injections of ²⁰¹T1 down the infarct vessel have also been used to evaluate the benefit of thrombolytic therapy by the demonstration of increased ²⁰¹T1 uptake after reperfusion in the ischemic zone. Finally, computer methods for subtracting serial images obtained after two separate intravenous ²⁰¹T1 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.