overlaid with the same or with a second 2d dsa and compared then the local threshold value is again increased or reduced in the differing pixels optionally the local threshold values are spatially smoothed these steps are repeated until a maximum match between computed projection image and dsa is reached or until one or more local threshold values deviate so widely from the original threshold value that no further improvement is to be expected a non claimed embodiment will now be explained in greater detail with reference to fig7 and 8 fig7 shows a section through a vessel tree model which has been computed from the 3d image data set 6 of fig5 and 6 in the area shown the vessel tree model includes the center line 16 of a vessel as well as the elliptical diameter of this vessel 34 the cross section of the vessel is thus defined by this in the vessel tree model and represented by the surface shaded from bottom left to top right a projection image 14 is computed from the vessel tree model in a similar manner to the first exemplary embodiment with reference to the known system geometry in that the outlines of the vessel tree model 34 are projected starting from the x ray focus 7.