Thallium 201 single photon emission computed tomography (SPECT)
Thallium 201 SPECT imaging is one of the oldest methods of assessing myocardial viability, though of late it is seldom used because of short half life of Thallium 201 and the free availability of Technetium 99, the alternate tracer. Initial uptake of Thallium 201 into the myocardium is proportional to the blood flow of the region. But the washout is dependent on the difference in the concentration in blood and myocardium cell as well as the integrity of the sodium potassium adenosine triphosphatase (Na/K ATPase) pump. This is because Thallium 201 is a potassium analogue. Hence in regions of lower uptake (reduced blood supply), if the washout is slower, it indicates viable myocardium. Only viable myocardium is able to retain the tracer and take up circulating Thallium 201 using the intact membrane transport. In non viable myocardium, cell membrane integrity is lost and washout is more rapid. Hence a redistribution of tracer indicates viability while a fixed perfusion defect indicates loss of viability .
Imaging protocols for Thallium 201 SPECT
In stress-redistribution, injection is given at peak stress (usually treadmill exercise) and initial imaging done, followed by delayed imaging after 3-4 hours. This technique may underestimate myocardial viability.
Rest redistribution is also similar, with initial baseline imaging followed by delayed imaging after 3-4 hours. This method can also underestimate viability.
In the stress-redistribution-reinjection method, a reinjection of the isotope is given immediately after acquiring the delayed images. Other methods include assessment of late redistribution at 24 hours and nitrate enhanced stress-redistribution-reinjection. Second injection avoids the low counts and image quality at 24 hours.