Clinical roles of perfusion and metabolic imaging

Noninvasive assessment of regional myocardial perfusion is clinically important for early and accurate diagnosis of coronary artery disease. In addition, persistent metabolic alterations are often seen in post ischemic dysfunction after recovery of blood flow. Thus, prior ischemic insult may be identified as areas of altered metabolism despite normal perfusion (so-called ischemic memory imaging). Radionuclide imaging has great advantages over other imaging techniques based on the variety of radiopharmaceutical agents to probe regional cellular functions and biochemistry in vivo. Technetium-99m perfusion imaging agents provide excellent myocardial perfusion images which may enhance diagnostic accuracy in the study of coronary artery disease. In addition, greater photon flux from the tracer permits simultaneous assessment of regional perfusion and function with electrocardiogram-gated acquisition. Positron emission tomography (PET) enables metabolic assessment in vivo. Preserved fluorodeoxyglucose (FDG) uptake indicates ischemic but viable myocardium which is likely to improve regional dysfunction after revascularization. In addition, FDG-PET seems to be valuable for selecting a high risk subgroup. Recently, iodine-123 15-(p-iodophenyl)-3R, S-methyl pentadecanoic acid (BMIPP), a branched fatty acid analog, has become clinically available in Japan. Less uptake of BMIPP than thallium is often observed in the ischemic myocardium. This perfusion metabolic mismatch, which is also observed by FDG-PET, is identified with stunned or hibernating myocardium with regional dysfunction. Both are likely to recover afterwards. Severe ischemia is identified as reduced BMIPP uptake at rest, suggesting use as an ischemic memory imaging. These new techniques will provide insights into new pathological states in ischemic heart disease and help to select the optimal treatment for the patients.