Characterization of a Perirectal Artifact in 18F-FDG PET/CT

Assessing tumor involvement in the rectal region can sometimes be complicated by what appears to be an artifact on F-18-FDG PET/CT images. This artifact manifests as a high-intensity region on the PET image, extending posterior to the bladder in the area around the rectum. The aim of this study was to describe this artifact, which-as far as we are aware-has not been previously reported, and to investigate its cause. Methods: One hundred F-18-FDG PET/CT studies (ordered-subsets expectation maximization reconstruction, CT attenuation correction) of patients with no known malignancy in the pelvis were retrospectively reviewed. Localized regions of apparently high uptake posterior to the bladder were considered an artifact when there was a discrepancy between attenuation-corrected (asymmetric appearance) and non-attenuation- corrected images (symmetric appearance). In addition, an experiment was performed using a body phantom containing 2 cylindric inserts simulating the bladder and a region of low-attenuation rectal gas. Attenuation-corrected images were reconstructed with different amounts of spatial misregistration intentionally introduced between the CT and PET images. Results: The artifact was observed in 15 of 100 patient studies and had a mean maximum standardized uptake value of 4.8 +/- 2.7. When fused with sequentially acquired CT images, the artifact always appeared to be in the perirectal region near the bladder and an area of rectal gas. The phantom study indicated this artifact was consistent with an attenuation-correction problem caused by misregistration between CT and PET. Movement of gas within the rectum can cause an air pocket to be present during the PET acquisition at a location where CT indicated soft tissue. The resulting localized overcorrection for attenuation at the margin of the rectum and the extremely high activity concentration in the nearby bladder contributed to the artifact. Conclusion: Movement of gas within the rectum between acquisition of CT and PET images can lead to an artifact in attenuation-corrected PET images in the perirectal region. An awareness of this artifact and reference to non-attenuation-corrected images will aid in the interpretation of F-18-FDG pelvis studies.