cardiovascular disorders
Cardiac Imaging Tests
Positron Emission Tomography (PET) in Cardiac Diagnosis
PET with CT (PET-CT) can demonstrate myocardial perfusion and metabolism and is increasingly being used to assess myocardial viability or to assess myocardial perfusion after an equivocal single-photon emission CT (SPECT) study or in patients with severe obesity. Perfusion agentsare radioactive nuclides that are used to trace the amount of blood flow entering a specific region and are therefore useful in unmasking myocardial perfusion deficits not evident at rest. They include carbon-11 (C-11) carbon dioxide, oxygen-15 (O-15) water, nitrogen-13 (N-13) ammonia, and rubidium-82 (Rb-82). Only Rb-82 does not require an on-site cyclotron. Metabolic agentsare radioactive analogs of normal biologic substances that are taken up and metabolized by cells. They include Fluorine-18 (F-18)–labeled deoxyglucose (FDG) C-11 acetate FDGdetects the enhancement of glucose metabolism under ischemic conditions, and can thus distinguish ischemic but still viable myocardium from scar tissue. Sensitivity is greater than with technetium-99m myocardial perfusion imaging, possibly making FDG imaging useful for selecting patients for revascularization and for avoiding such procedures when only scar tissue is present. This use may justify the greater expense of PET. Half-life of F-18 is long enough (110 min) that FDG can often be produced off-site. Techniques that enable FDG imaging to be used with conventional SPECT cameras may make this type of imaging widely available. FDG has also been used to detect inflammatory cardiovascular disorders (eg, infected pacemaker wires, aortic vasculitis, cardiac sarcoidosis). Carbon-11 acetateuptake appears to reflect overall oxygen metabolism by myocytes. Uptake does not depend on such potentially variable factors as blood glucose levels, which can affect FDG distribution. C-11 acetate imaging may better predict postintervention recovery of myocardial function than FDG imaging. However, because of its 20-minute half-life, C-11 must be produced by an on-site cyclotron. 