Linear scanning x-ray apparatus generates a collimated fan beam which is moved together with a detector relative to a subject to generate a composite x-ray image of the subject. An adjustable collimator varies the width of the imaging beam in the scan direction. A control system responds to adjustment of the collimator to combine output signals of groups of two or more pixels in the detector, in order to optimize the contrast resolution of the image signals for a given spatial resolution. The invention provides a collimator for use in the apparatus.