<p>A positron CT scanner comprises a plurality of detectors (110) arranged in multi-layered rings and slice diaphragms (130) for restricting the visual fields of the detectors, so that the visual fields intersect a set axis during a scan. The spacing between diaphragms substantially corresponds to a certain number of detector cells, each of which can determine the positions of incident rays in the direction of the set axis during a scan. The slice diaphragms (130) are movable relative to the detector cells (110) in the direction of the set axis during a scan. <IMAGE></p>
