A multiple modality imaging system includes a cone-beam computed tomography scanner which acquires CT projection data of a subject in an examination region and a nuclear imaging scanner which concurrently/subsequently acquires nuclear projection data of the subject in the examination region. A CT reconstruction processor is programmed to perform the steps of: in the CT projection data, defining a field-of-view (FoV) with a voxel grid in a trans-axial direction determining the subjects maximum trans-axial extents generating an extending FoV by extending the voxel grid of the FoV to at least one extended region outside the FoV that encompass at least the determined maximum trans-axial extents and all attenuation in the trans-axial direction and iteratively reconstructing the CT projection data into an attenuation map of the extended FoV. At least one nuclear reconstruction processor is programmed to correct the acquired nuclear projection data based on the iteratively reconstructed attenuation map.
