1. A method of forming X-ray images (B) from at least two series of projection data sets (P 1, P 2 ) successively acquired along different trajectories (T 1, T 2 ), a respective 3D data set (S 1, S 2 ) being formed from each series of projection data sets (P 1, P 2 ) and a transformation rule (F), describing the location in space of the 3D data sets (S 1, S 2 ) relative to one another, being determined in that voxels in one 3D data set (S 1 ) are selected and their location in the other 3D data set (S 2 ) is determined by means of a suitable similarity measure, and X-ray images (B) being formed from the 3D data sets (S 1, S 2 ) combined by way of the transformation rule (F).