A CT scanner has an X-ray source for producing a fan beam that is rotatable relative to the axis of the scanner in a plane perpendicular to the axis, and a plurality of X-ray detectors mounted in a non-rotatable helical detector ring whose axis is colinear with the scannner axis for reeiving X-rays from the fan beam, the detectors extending 360.degree. around the axis. The detector ring has an inner surface facing the axis of the helix, and the detectors are mounted on the inner surface so that their photosensitive areas face the axis of the helix. At any given axial position of the detector ring, a given group of detectors will have an unobstructed view of certain of the X-ray sources through the space between adjacent leaves of the helix.