In a combined system, a magnetic resonance (MR) scanner includes a magnet (10, 110) configured to generate a static magnetic field (B0) at least in a MR examination region (12) from which MR data are acquired. Radiation detectors (40, 41, 140) are configured to detect gamma rays generated by positron-electron annihilation events in a positron emission tomography (PET) examination region (70). The radiation 5 detectors include electron multiplier elements (60, 160) having a direction of electron acceleration (ae) arranged substantially parallel or anti-parallel with the static magnetic field (B0). In some embodiments, the magnet is an open magnet having first and second spaced apart magnet pole pieces (14, 15) disposed on opposite sides of a magnetic 10 resonance examination region, and the radiation detectors include first and second arrays (40, 41) of radiation detectors disposed with the first and second spaced apart magnet pole pieces.