A computer-implemented method of processing a three-dimensional image data set of two-dimensional X-ray images from an X-ray apparatus, comprising the steps of: • Providing a three-dimensional image data set of at least one examination area in which anatomical structures are present, • segmenting the three-dimensional image data set to provide a three-dimensional vascular structure model describing a vascular structure and a three-dimensional bone structure model describing a bone structure, • acquiring, by means of the X-ray apparatus, a first two-dimensional X-ray image containing at least partially the bone structure and at least partially the vascular structure, under an acquisition geometry and the presence of a first contrast agent concentration, • Acquiring, by means of the X-ray apparatus, a second two-dimensional X-ray image of the examination area, which contains a second contrast medium concentration different from the first X-ray image, wherein the acquisition of the second X-ray image takes place under the same acquisition geometry as the acquisition of the first X-ray image, • subtracting the first and second X-ray images to produce and provide a subtraction image, the subtraction image including a vascular structure, • Determining an optimal projection geometry using a threepart objective function, ▪ wherein a first part of the objective function represents a similarity between a forward projection (31; 32; 33) of the bone structure model of the three-dimensional image data set and the bone structure in the first and/or second X-ray image under variation of an assumed projection geometry (P i), ▪ wherein a second part of the objective function represents a similarity between the forward projection (31; 32; 33) of the vessel structure model of the three-dimensional image data set and the vessel structure of the subtraction image under variation of the assumed projection geometry, ▪ and wherein a third part of the objective function represents a similarity between all image information of the forward projection (31; 32; 33) of the three-dimensional image data set and all image information of the first and/or second X-ray image under variation of the assumed projection geometry; wherein • the optimal projection geometry results from a minimisation or maximisation of the objective function. • Providing a three-dimensional image data set of at least one examination area in which anatomical structures are present, • segmenting the three-dimensional image data set to provide a three-dimensional vascular structure model describing a vascular structure and a three-dimensional bone structure model describing a bone structure, • acquiring, by means of the X-ray apparatus, a first two-dimensional X-ray image containing at least partially the bone structure and at least partially the vascular structure, under an acquisition geometry and the presence of a first contrast agent concentration, • Acquiring, by means of the X-ray apparatus, a second two-dimensional X-ray image of the examination area, which contains a second contrast medium concentration different from the first X-ray image, wherein the acquisition of the second X-ray image takes place under the same acquisition geometry as the acquisition of the first X-ray image, • subtracting the first and second X-ray images to produce and provide a subtraction image, the subtraction image including a vascular structure, • Determining an optimal projection geometry using a threepart objective function, ▪ wherein a first part of the objective function represents a similarity between a forward projection (31; 32; 33) of the bone structure model of the three-dimensional image data set and the bone structure in the first and/or second X-ray image under variation of an assumed projection geometry (P i), ▪ wherein a second part of the objective function represents a similarity between the forward projection (31; 32; 33) of the vessel structure model of the three-dimensional image data set and the vessel structure of the subtraction image under variation of the assumed projection geometry, ▪ and wherein a third part of the objective function represents a similarity between all image information of the forward projection (31; 32; 33) of the three-dimensional image data set and all image information of the first and/or second X-ray image under variation of the assumed projection geometry; wherein ▪ wherein a first part of the objective function represents a similarity between a forward projection (31; 32; 33) of the bone structure model of the three-dimensional image data set and the bone structure in the first and/or second X-ray image under variation of an assumed projection geometry (P i), ▪ wherein a second part of the objective function represents a similarity between the forward projection (31; 32; 33) of the vessel structure model of the three-dimensional image data set and the vessel structure of the subtraction image under variation of the assumed projection geometry, ▪ and wherein a third part of the objective function represents a similarity between all image information of the forward projection (31; 32; 33) of the three-dimensional image data set and all image information of the first and/or second X-ray image under variation of the assumed projection geometry; wherein • the optimal projection geometry results from a minimisation or maximisation of the objective function.