A diagnostic imaging system comprising: a stationary gantry (14; 214) a rotating gantry rotatably supported by the stationary gantry, the rotating gantry defining a subject receiving aperture (18; 218); a source of penetrating radiation (20; 220) mounted on the rotating gantry which transmits transmission radiation through a subject (12; 212) disposed in a subject receiving aperture (18; 218), the radiation source (20; 220) being mounted for rotation around the subject receiving aperture (18; 218); a detector (26; 226) adapted for detecting transmission radiation transmitted by the source (20; 220) after passage of the radiation through the subject (12; 212) in the subject receiving aperture (18; 218); at least one nuclear detector head (40a, 40b; 240a-240c), distinct from the detector (26; 226), mounted for rotation around the subject receiving aperture (18; 218), which detector head (40a, 40b; 240a-240c) and adapted to detect emission radiation emitted by a radiopharmaceutical, at least one reconstruction processor (66, 74) which reconstructs transmission radiation received by the two-dimensional flat panel radiation detector (26; 226) into a transmission volumetric image representation and emission radiation received by the nuclear detector head (40a, 40b; 240a-240c) into an emission volumetric image representation; and a fusion processor which combines the transmission and emission volumetric image representations together, characterized in that: the detector is a two dimensional flat panel radiation detector, the nuclear detector head includes a radiation shielding means (42a, 42b; 242a-242c) disposed adjacent a radiation receiving face to shield the nuclear detector head (40a, 40b; 240a-240c) from transmission radiation originating from the source of penetrating radiation (20; 220), by selectively restricting and permitting radiation to strike the radiation receiving face. a stationary gantry (14; 214) a rotating gantry rotatably supported by the stationary gantry, the rotating gantry defining a subject receiving aperture (18; 218); a source of penetrating radiation (20; 220) mounted on the rotating gantry which transmits transmission radiation through a subject (12; 212) disposed in a subject receiving aperture (18; 218), the radiation source (20; 220) being mounted for rotation around the subject receiving aperture (18; 218); a detector (26; 226) adapted for detecting transmission radiation transmitted by the source (20; 220) after passage of the radiation through the subject (12; 212) in the subject receiving aperture (18; 218); at least one nuclear detector head (40a, 40b; 240a-240c), distinct from the detector (26; 226), mounted for rotation around the subject receiving aperture (18; 218), which detector head (40a, 40b; 240a-240c) and adapted to detect emission radiation emitted by a radiopharmaceutical, at least one reconstruction processor (66, 74) which reconstructs transmission radiation received by the two-dimensional flat panel radiation detector (26; 226) into a transmission volumetric image representation and emission radiation received by the nuclear detector head (40a, 40b; 240a-240c) into an emission volumetric image representation; and a fusion processor which combines the transmission and emission volumetric image representations together, characterized in that: the detector is a two dimensional flat panel radiation detector, the nuclear detector head includes a radiation shielding means (42a, 42b; 242a-242c) disposed adjacent a radiation receiving face to shield the nuclear detector head (40a, 40b; 240a-240c) from transmission radiation originating from the source of penetrating radiation (20; 220), by selectively restricting and permitting radiation to strike the radiation receiving face. the detector is a two dimensional flat panel radiation detector, the nuclear detector head includes a radiation shielding means (42a, 42b; 242a-242c) disposed adjacent a radiation receiving face to shield the nuclear detector head (40a, 40b; 240a-240c) from transmission radiation originating from the source of penetrating radiation (20; 220), by selectively restricting and permitting radiation to strike the radiation receiving face.