Abstract: The invention relates to a device for determining local absorption differences in a cross-sectional slice of an object. The device comprises an X-ray source and an X-ray detector which is directed toward the X-ray source. The detector comprises a number of plate-shaped collimating elements which are directed toward the X-ray source. On the respective lines of projection of these collimating elements toward the source, there are arranged radiation absorbing elements. The length of each absorbing element, measured in the direction of the X-ray source, is smaller than the length of each collimating element. The width of each absorbing element, measured in a direction at right angles and in the sectional slice, are larger than those of the collimating elements. Apparent variations in the sensitivity of the X-ray detector, due to a lateral shift of the X-ray focus in the X-ray source, are thus substantially mitigated.