Differences in position of rectal gas between acquisition of CT and PET images can lead to a perirectal artifact on attenuation-corrected PET images.
The artifact is caused by localized attenuation over-correction at the margin of the rectum due to movement of gas: There is soft tissue attenuation in the rectum at CT acquisition, and gas during PET acquisition. The proximity of the high-signal urinary bladder results in a high-intensity region extending posterior to the bladder and around the rectum immediately adjacent to a region of rectal gas. The artifact is not observed when rectal gas is absent.
This artifact is reportedly seen in 15% of PET/CTs and has a maximum standardized uptake value of ~5.
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