A CT scanner according to the present invention is particularly useful for scanning the spine and extremities, such as knees, and ankles, especially while the patient is in an upright position. The CT scanner generally includes a source and detector that are rotatable about a generally upright axis. The source and detector are also moved along the upright axis during rotation to perform a helical scan. The source and detector are mounted to an inner ring, which is rotatably mounted within an outer ring. The outer ring is fixedly mounted to a carriage that is movable along an upright rail.