Attenuation can be measured in PET scanners by performing a transmission scan, during which an external radioactive source rotates around the scanner field of view emitting photons along the lines of response measured in the PET emission scan (Phelps 1975). By comparing the measurements with and without the subject in the scanner, attenuation can be calculated for each line of response. However, PET/MR scanners usually cannot fit  the transmission hardware inside the MR bore and correct for attenuation by other means.