1. An X-ray examination apparatus which includes an X-ray source and an X-ray detector, the apparatus comprising: means for forming a series of projection images at respective orientations of the X-ray source and the X-ray detector relative to a predetermined frame of reference; means for calibrating the orientations relative to the frame of reference; and means for reconstructing a basic three-dimensional data set from the series of projection images, wherein the positions of the X-ray source and the X-ray detector relative to the frame of reference are calibrated for one or more additional directions of observation.