An imaging system comprising: positron emission tomography detectors substantially encircling an examination region; electronics (50, 50 ob) operatively coupled with the PET detectors to perform coincidence detection of probative radiation comprising 511 keV gamma rays generated by electron-positron annihilation events occurring in the examination region; radiation shielding (60, 62) disposed adjacent the PET detectors to reduce interaction of non-probative radiation with the PET detectors, at least a portion of the radiation shielding (60, 62) comprising an electrically non-conductive and non-ferromagnetic heavy atom oxide material, and a magnetic resonance scanner comprising a main magnet (12, 14), a magnetic field gradient assembly (20, 20', 22, 24), and a radio frequency coil, the magnetic resonance scanner configured to acquire imaging data from a magnetic resonance examination region at least partially overlapping the examination region surrounded by the PET detectors, wherein the magnetic resonance scanner is a bore-type scanner, the PET detectors are arranged as an annular ring concentric with a bore of the bore-type scanner, and the magnetic resonance scanner further comprises: a cylindrical radio frequency screen (88, 288) arranged coaxially with the bore, the magnetic field gradient assembly (20, 20', 22, 24) being disposed outside of the cylindrical radio frequency screen, wherein the cylindrical radio frequency screen has an annular groove or channel receiving the annular ring of PET detectors. positron emission tomography detectors substantially encircling an examination region; electronics (50, 50 ob) operatively coupled with the PET detectors to perform coincidence detection of probative radiation comprising 511 keV gamma rays generated by electron-positron annihilation events occurring in the examination region; radiation shielding (60, 62) disposed adjacent the PET detectors to reduce interaction of non-probative radiation with the PET detectors, at least a portion of the radiation shielding (60, 62) comprising an electrically non-conductive and non-ferromagnetic heavy atom oxide material, and a magnetic resonance scanner comprising a main magnet (12, 14), a magnetic field gradient assembly (20, 20', 22, 24), and a radio frequency coil, the magnetic resonance scanner configured to acquire imaging data from a magnetic resonance examination region at least partially overlapping the examination region surrounded by the PET detectors, wherein the magnetic resonance scanner is a bore-type scanner, the PET detectors are arranged as an annular ring concentric with a bore of the bore-type scanner, and the magnetic resonance scanner further comprises: a cylindrical radio frequency screen (88, 288) arranged coaxially with the bore, the magnetic field gradient assembly (20, 20', 22, 24) being disposed outside of the cylindrical radio frequency screen, wherein the cylindrical radio frequency screen has an annular groove or channel receiving the annular ring of PET detectors. a cylindrical radio frequency screen (88, 288) arranged coaxially with the bore, the magnetic field gradient assembly (20, 20', 22, 24) being disposed outside of the cylindrical radio frequency screen, wherein the cylindrical radio frequency screen has an annular groove or channel receiving the annular ring of PET detectors.