1. A method for determining calibration information which includes information on a spatial relationship between an x-ray apparatus reference frame and a three-dimensional scan reference frame, comprising the following steps: a three-dimensional calibration data set is provided which represents a three-dimensional calibration model of a calibration object in the three-dimensional scan reference frame, wherein the calibration model has been determined from scan data generated by a three-dimensional calibration x-ray scan, wherein in the three-dimensional calibration x-ray scan, an x-ray unit is moved along a three-dimensional scanning path relative to the calibration object, wherein at least a portion of the three-dimensional scanning path lies in the x-ray apparatus reference frame; x-ray source relative position data is provided which includes information on the relative position of an x-ray source relative to an x-ray unit marker device attached to the x-ray unit when actual two-dimensional x-ray images are produced; a two-dimensional calibration data set is provided which describes a first of the actual two-dimensional x-ray images which has been produced by means of irradiating at least the calibration object in a first position ( FIG. 1 ) of the x-ray source in the x-ray apparatus reference frame, and which also describes: a) a second of the actual two-dimensional x-ray images which has been produced by means of irradiating at least the calibration object, which is spatially fixed as compared to the first two-dimensional x-ray image, in a second position ( FIG. 2 ) of the x-ray source in the x-ray apparatus reference frame; and/or b) the dimensions of the calibration object; an x-ray unit data set is provided which describes the position of the x-ray unit marker device when the at least one actual two-dimensional x-ray image is generated; and the calibration information is determined by means of an adapting method on the basis of the x-ray source relative position data, the x-ray unit data set and the two-dimensional calibration data set.