Comparison of SPECT using technetium-99m agents and thallium-201 and PET for the assessment of myocardial perfusion and viability

Abstract

This report reviews the applications of tomographic imaging with current and new tracers in assessing myocardial perfusion and viability. Multiple studies with thallium-201 (Tl-201) single photon emission computed tomography (SPECT) imaging for the detection of coronary artery disease (CAD) have demonstrated high sensitivity, high rates of normalcy and high reproducibility. In assessing viability, fixed defects are frequently detected in viable zones in 4-hour studies with Tl-201 imaging. Redistribution imaging performed 18 to 72 hours after injection or reinjection of Tl-201 before 4-hour redistribution imaging has been shown to improve accuracy of viability assessment. Tl-201 SPECT studies are limited by the suboptimal physical properties of Tl-201, which result in variable image quality. The 2 new technetium-99m (Tc-99m) — labeled myocardial perfusion tracers offer the ability to inject much higher amounts of radioactivity, making it possible to assess ventricular function as well as myocardial perfusion from the same injection of radiotracer. Tc-99m sestamibi has very slow myocardial clearance, which allows for prolonged imaging time and results in image quality superior to that obtained with Tl-201 and Tc-99m teboroxime. The combination of minimal redistribution of Tc-99m sestamibi and high count rates makes gated SPECT imaging feasible, and also permits assessment of patients with acute ischemic syndromes by uncoupling the time of injection from the time of imaging. The combination of high image quality and firstpass exercise capabilities may lead to a choice of this agent over Tl-201 for assessment of chronic CAD. Tc-99m teboroxime is efficiently extracted by the myocardium in proportion to myocardial perfusion even at extremely high flow rates; however, this tracer demonstrates rapid myocardial washout, necessitating the completion of images very quickly after injection and generally resulting in suboptimal image quality. Positron emission tomography (PET) has been applied with both rest and stress myocardial perfusion studies using rubidium-82, which has recently become commercially available, or nitrogen-13 ammonia. Although PET is clearly established as highly accurate for noninvasive detection of CAD, there are still divergent views as to whether PET is more accurate than Tl-201 SPECT for these studies. No direct comparisons have been made with attenuation-corrected Tl-201 SPECT or with the newer Tc-99m myocardial perfusion imaging agents. With respect to myocardial viability, studies using PET with fluorine-18 fluorodeoxyglucose remain the “gold standard.” The principal limitations of this approach are the high costs of equipment and radiopharmaceuticals. The availability of multiple excellent agents for the scintigraphic assessment of myocardial perfusion is likely to greatly expand the use of nuclear cardiology techniques and to result in the ability to choose a particular agent to fit a given clinical situation.