Abstract: During computer tomography examinations, ring-like artefacts occasionally occur because the X-ray source becomes hot, so that the X-ray focus changes its position in space. Because the distance between the detector and the diaphragm which stops the radiation beam is several times larger than the distance between the X-ray focus and the diaphragm, the shifting of the radiation beam on the detector is even greater. If the detector consists of detection elements which are not exactly identical, it may occur that the sensetivities of the detection elements apparently change with respect to each other due to such a shift, the reconstruction then giving rise to said artefacts. In accordance with the invention, the position where the radiation beam is incident on the detector is measured and the diaphragm for the stopping of the radiation beam is readjusted so that the radiation is each time incident in the same area.