TL-201 IMAGING FOR DETECTION OF MYOCARDIA L VIABILITY IN THE SETTING OF STUNNED AND HIBERNATING MYOCARDIUM

Myocardial uptake of thallium-201 is not only a function of regional m yocardial blood flow, but also reflects cellular uptake by intact cell membranes and thus can not be merely regarded a signal of perfusion b ut also of structural cell integrity. Especially in the setting of sev erely depressed left ventricular function evidence of viable but dysfu nctional myocardium has impact on recovery potential and prognosis aft er myocardial revascularization. Hibernating myocardium, a reversible chronically ischemic state of reduced aerobic metabolism and depressed contractile function, may be identified after injection of thallium-2 01 at rest and rest-redistribution SPECT imaging. Since thallium-201 u ptake in initial defect areas may occur as a function of time allowed for redistribution, even partial late uptake may be considered a relia ble signal for viable, but hibernating tissue, 75% of which demonstrat ing contractile recovery after revascularization. Uptake of thallium-2 01 at rest or with redistribution, thus, is indicative of myocardial v iability irrespective of function. Conversely, lack of thallium-201 up take after stress and redistribution does not always indicate necrosis , since 45 to 83% of myocardium with no uptake may improve function af ter revascularization. These defects, however, often resolve with rein jection of thallium-201 and subsequent imaging at rest. This observati on led to a triphasic imaging protocol including conventional rest-red istribution imaging and a third set of images after reinjection of 1 m Ci thallium-201 at rest. This concept ensures uptake of thallium-201 i n 31 to 49% of presumably persistant defects and increases sensitivity for detection of viable tissue. In 80 to 87% of areas with reinjectio n thallium-201 uptake function improved after revascularization compar ed to 0 to 8% of segments with no uptake at all. Redistribution imagin g should not be omitted for logistical reasons, since important inform ation not only on ischemia but also on viability may be lost. Useful i maging protocols for detection of both ischemia and viability comprize either a sequence of stress, redistribution and reinjection imaging o r a series of stress, reinjection and 24 hour redistribution images; b oth protocols have similar sensitivity for detection of tissue viabili ty. In the setting of stunned myocardium mainly after thrombolytic the rapy the assessment of residual viability may be important for both ad ditional therapeutic and prognostic reasons. Thallium-201 uptake prefe rentially using a rest-redistribution protocol may help to differentia te viable from non-viable myocardium: however thallium imaging should be performed after the hyperemic phase following successful thrombolys is (greater-than-or-equal-to 24 to 48 hours after thrombolysis). A det ailed overview of various protocols to assess myocardial viability is outlined in Table 1. In summary, thallium-201 uptake after injection o f tracer at rest or after stress (pharmacologic such as vasodilatory s tress induced by dipyridamole or adenosine or ergometric), at a given time to allow redistribution and delayed uptake of tracer or with rein jection of activity may indicate myocardial tissue viability. Thus tha llium-201 imaging with special emphasis on viability may often be a co st-effective alternative to metabolic imaging with PET. With the use o f new thallium-201 imaging protocols (biphasic and triphasic) and quan titative methods the net uptake of thallium-201 may be a reliable and clinically useful parameter to assess tissue viability in the setting of both stunned and hibernating myocardium and. thus. may carry progno stic information with functional improvement after revascularization.