When compensating for truncated patient scan data acquired by a multi-modal PET/CT or PET/MR imaging system (14, 16), such as occurs when a patient is larger than a field of view for an anatomical imaging device, a segmented contour of a non-attenuation-corrected (NAC) PET image is used to identify a contour of the truncated region. An appropriate tissue type is used to fill in truncated regions of a truncated CT or MR image for the attenuation map. The corrected attenuation map is then used to generate an attenuation-corrected PET image of the patient or a region of interest. Alternatively, the system can be employed in PET/CT or PET/MR imaging scenarios where two modalities are performed sequentially (e.g., not simultaneously), and thus the contour derived from the PET scan can be compared to the CT or MR image to infer potential subject motion between the PET and CT or MR scans. Additionally, the system can be employed in PET imaging scenarios where the contour derived from the NAC PET image is used as emission boundary for scatter correction using single-scatter simulation, in which a tail-fitting procedure utilizes an emission boundary to define pure-scatter tails (e.g., in the absence of true coincidence events).