Single-photon perfusion imaging for the assessment of myocardial viability.

The identification of viable myocardium is an important consideration for patient selection prior to interventional procedures, especially in patients with severe left ventricular dysfunction. Myocardial perfusion imaging may reflect viability, because tracer uptake requires adequate perfusion, cellular integrity and metabolic function. The underestimation of myocardial viability noted with traditional stress and redistribution thallium imaging has lead to the development of alternative protocols to detect viable myocardium, such as late (24-hr) imaging and the thallium reinjection method. Rest-redistribution thallium imaging may be a useful procedure for predicting recovery of ventricular function following revascularization. Quantitative analysis of thallium activity provides important information, because mild or moderate defects are usually metabolically active as determined by PET. Administration of adjunct medications, such as ribose or nitroglycerin, can increase the detection of reversible perfusion abnormalities. Technetium-99m perfusion agents offer great promise for perfusion imaging, but their role in the detection of myocardial viability is not well defined. Underestimation of myocardial viability has been described with 99mTc-sestamibi scintigraphy; however, use of quantitative perfusion image analysis or the addition of functional data provided by gated tomography or first-pass imaging may enhance the assessment of viability. Although the optimal method for the detection of myocardial viability with single-photon agents is not apparent, current methods have substantially improved discrimination between nonviable and viable myocardium. In certain instances, however, metabolic imaging with single-photon fatty acid analogs or positron-emitting radionuclides may be necessary.