1. A method of correcting a PET or SPECT image of an imaged subject for attenuation, the method comprising: performing a volumetric attenuation imaging scan of the subject with an X-ray source and an X-ray detector to generate volumetric X-ray attenuation imaging data; using the volumetric X-ray attenuation imaging data to generate a gamma ray attenuation map; performing a gamma ray imaging scan of the subject with a gamma ray detector to generate PET or SPECT imaging data; and using the gamma ray attenuation map to correct the PET or SPECT imaging data for attenuation, and to generate an attenuation-corrected PET or SPECT image.