MYOCARDIAL SCINTIGRAPHY FOR ASSESSING MYO CARDIAL VIABILITY

Fundamental principles: Myocardial scintigraphy is a metabolic approac h to myocardial viability visualizing the localization, the extent and to some degree the quantity of non-functional yet viable myocardial t issue. Potential for functional recovery cannot be ascertained directl y from the scintigram but can be inferred from commonly observed behav ior after blood Row has been restored. Myocardial scintigraphy is thus fundamentally different from other functional exploration methods suc h as echocardiography or nuclear magnetic resonance imaging which can detect residual contractile capacity unmasked by inotropic stimulation . It must be remembered however that such 'forced' contractility may n ot necessarily be expressed spontaneously after revascularization and that, however detected, truly viable myocardium may not recover normal contractility after reperfusion when associated with non-transmural i nfarction or diffuse fibrosis. PET and Thallium 201 scans: positron em ission tomography (PET) is the gold standard. Accomplished after admin istration of an isotope labeled substance (18-fluoro-deoxyglucose, FDG ), the PET scan Visualizes metabolic activity in viable myocardium. Sp ecial equipment is however required and facilities are limited, partic ularly in France. Thallium 201 scans can be acquired with conventional gamma cameras and protocols have been widely developed with nearly eq uivalent performance in certain situations of doubtful residual viabil ity after post-infarction thrombolysis or angioplasty. It must be note d however that in such cases, search for homolateral or contralateral ischemia may be the main objective rather than the detection of residu al viability A 3-step thallium 201 scintigraphy protocol with stress, 4-hr redistribution then imaging after reinjection is usually sufficie nt to document ischemia or viability warranting revascularization. The problem is quite different for patients with major myocardial dysfunc tion and histological remodeling due to hypokinetic dilated cardiomyop athy. In such types of myocardium, chances of recovering inotropic cap acity are quite limited and detecting viable tissue would be technical ly difficult; however with a proper protocol (without stress, resting images late after injection), thallium 201 scintigraphy can be helpful . Performance: Data in the literature shows that isotopic techniques l ack specificity by overestimating the extent of viable tissue capable of recovering contractility. Actually this could be seen as an advanta ge since the consequences of missing even a small chance for revascula rization warrant risking an ineffective procedure for a patient whose only alternative is heart transplantation. This situation explains why 18-FDG PET exploration should be performed even if the thallium scint igram leaves very little room for hope of recovering viable myocardium in patients with terminal disease. Perspectives: Isotopic exploration of the myocardium is a moving field and routine practice can expect t o benefit from research conducted in pioneer centers, The future offer s two main perspectives: the development of metabolic tracers giving m ore precision than thallium 201 (for example isotope-labeled fatty add s); and technical advances in conventional gamma cameras more adapted to the physical characteristics of 18-FDG used for PET scans. Scintigr aphy is an indispensible tool for metabolic exploration of the myocard ium. Only nuclear magnetic resonance spectroscopy may provide comparab le results.