[en] A CT scanner comprises a rotatable gantry carrying an X-ray source which provides a fan-shaped beam of X-rays and an X-ray detector which scans the X-ray intensity along a line in the plane of the beam. The gantry is rotatable about an axis relative to a couch which supports the patient under investigation. The X-ray detector comprises two concentric X-ray masks exposed around a scintillator tube, the two masks have helical X-ray transmissive apertures, the helices being of opposite senses so that rotation of the masks in opposite directions causes the exposed area of the scintillator tube to move along its length. (author)