A diagnostic imaging system includes a rotating gantry (16) which defines a subject receiving aperture (18). A rotatable source of high energy penetrating radiation (20) and corresponding high energy radiation detector (26) are disposed across the subject receiving aperture (18). A plurality of nuclear detector heads (30a, 30b) are movably attached to the rotating gantry (16) in order to detect low energy radiation emitted by a radiopharmaceutical injected into the subject (12). Each of the nuclear detector heads (30a, 30b) within the system include a variable axial radiation shield (40a, 40b) disposed adjacent a radiation receiving face (38) on the detector head. The variable axial radiation shield (40a, 40b) includes a plurality of substantially parallel vanes (42) pivotally connected to each nuclear detector head (30a, 30b) for movement between an open configuration (FIG. 2) and a closed, radiation blocking configuration (FIG. 3).