A known limitation of CT-based attenuation correction (CTAC) is the creation of artifacts that mimic increased FDG uptake in the location of metallic prostheses. Calculated attenuation correction (CAC) uses a constant value for the attenuation coefficient of brain tissue at 511 keV and corrects the reconstructed data for attenuation effects. Comparison of CTAC and NAC PET images should always be the first step when evaluating possible metallic artifacts on CTAC PET brain images. If the etiology responsible for the increased FDG uptake remains unclear, reprocessing using CAC is an additional useful tool to differentiate between artifactual and true FDG uptake. Copyright © 2006 by Lippincott Williams & Wilkins.