1. An imaging system comprising: a magnetic resonance imaging scanner including at least a main magnet and magnetic field gradient coils housed in or on a scanner housing acquires spatially encoded magnetic resonances in an imaging region; a plurality of solid state radiation detectors disposed in or on the scanner housing and arranged to detect gamma rays emitted from the imaging region; time-of-flight positron emission tomography (TOF-PET) processing configured to determine localized lines of response based on (i) locations of substantially simultaneous gamma ray detections output by the solid state radiation detectors and (ii) a time interval between said substantially simultaneous gamma ray detections; time-of-flight positron emission tomography (TOF-PET) reconstruction processing configured to reconstruct the localized lines of response to produce a TOF-PET image; and magnetic resonance imaging (MRI) reconstruction processing configured to reconstruct the acquired magnetic resonances to produce an MRI image.