Figure 27a. Attenuation correction artifact. (a) Attenuation-corrected axial fused FDG PET-CT image shows a focus of hypermetabolism in the left axilla (arrow). (b) Attenuation-uncorrected fused FDG PET-CT image obtained at the same level shows lack of activity in the intensely enhancing (high-attenuation) left axillary vein (arrow), which is on the side of contrast material injection. This artifact is due to overcorrection by the attenuation correction software, which uses CT data for attenuation correction.