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: obtaining a three-dimensional calibration data set representing a three-dimensional calibration model of a calibration object in the three-dimensional scan reference frame, wherein the calibration model is determined from scan data generated by a three-dimensional calibration x-ray scan of the calibration object, wherein during the three-dimensional calibration x-ray scan an x-ray unit is moved along a three-dimensional scanning path relative to the calibration object, and at least a portion of the three-dimensional scanning path lies in the x-ray apparatus reference frame; obtaining x-ray source relative position data that includes information on the position of an x-ray source of the x-ray unit relative to an x-ray unit marker device attached to the x-ray unit when actual two-dimensional x-ray images are produced; obtaining a two-dimensional calibration data set that describes a first of the actual two-dimensional x-ray images produced by irradiating at least the calibration object in a first position of the x-ray source in the x-ray apparatus reference frame, and which also describes at least one of: a) a second of the actual two-dimensional x-ray images produced by irradiating at least the calibration object, which is spatially fixed as compared to the first two-dimensional x-ray image, in a second position of the x-ray source in the x-ray apparatus reference frame different from the first position; or b) the dimensions of the calibration object; obtaining an x-ray unit data set that describes the position of the x-ray unit marker device when the at least one actual two-dimensional x-ray image is generated; and determining the calibration information using an adapting method based on the x-ray source relative position data, the x-ray unit data set and the two-dimensional calibration data set, said calibration information including information on the spatial relationship between the x-ray apparatus reference frame and the three-dimensional scan reference frame.