Incidental CT findings on PET/SPECT with clinical and potential future risks

Coregistration CT in PET/CT and SPECT/CT is a transmissive imaging technique performed immediately before the emission study with the patient in the same position, eliminating temporal mismatches typical of software-based image fusion. Its primary purposes are attenuation correction through a densitometric map and anatomical localization of PET or SPECT abnormalities. Although coregistration CT can improve the specificity of nuclear medicine findings, it is not a full diagnostic CT because it is acquired with lower tube current, free breathing, and without contrast, limiting its ability to detect small or subtle lesions. With the widespread adoption of hybrid scanners, incidental morphological findings on coregistration CT have become increasingly common. These findings—whether or not they have a PET/SPECT correlate—are unrelated to the clinical indication but may represent clinically relevant or potentially progressive conditions. Unlike radiology, where structured models such as the ACR’s actionable incidental findings system guide management, no formal guidelines exist for handling incidental CT findings in PET/SPECT. Nuclear Medicine physicians, despite not being radiologists, should recognize CT abnormalities that may pose clinical risk and communicate them to the appropriate specialists as “findings of possible pathological significance,” without providing definitive diagnostic interpretation. This atlas supports such recognition by illustrating key CT patterns associated with patient risk.