1. A method of determining the spatial transformation between an object and a three-dimensional representation of the object derived from a data set, the method comprising: acquiring an X-ray image of the object which is situated in an examination zone, selecting a sub-volume of the examination zone, deriving from a part of the data set which is limited to the sub-volume a pseudo-projection image sub-volume on a two-dimensional zone with defined projection geometry deriving a respective difference image either from the X-ray image and the pseudo-projection image or from equally filtered versions of the X-ray image and the pseudo projection image, in such a manner that corresponding structures in these images cancel one another in the difference image, calculating a similarity measure which is a measure dependent on the structuredness in at least a part of the difference image, repeating the steps of deriving a pseudo-projection image, deriving a difference image, and calculating a similarity measure with varied projection parameters until the best registration is obtained on the basis of the similarity measure, determining the spatial transformation while utilizing the projection parameters of the pseudo-projection image which registers best with the X-ray image.