Supplemental transmission information is used in PET imaging with a hybrid PET/MR system. The magnetic field of the MR portion is used to direct positrons from one or more sources outside or inside the PET field of view to within the PET field of view. An oblique target or targets create an annihilation source within the PET field of view from the positron beam or beams. The resulting radiation may be detected. In combination with measurements made with the sources shielded (e.g., no positron beam-target annihilation sources), the attenuation or other characteristics outside the uniform region of the MR field of view is determined, such as calculating attenuation of arms of a patient for attenuation correction in PET imaging.