A sequence of groups of projection images shows an object under examination comprising a vascular system and its environment. A computer determines a 2-dimensional evaluation image having a plurality of pixels based on combination images determined from the projection images of a group. The combination images have a plurality of pixels with pixel values. The sequence of the combination images shows the time characteristic of the distribution of a contrast medium in the object. The pixels of the evaluation image correspond to those of the projection images. The computer assigns each pixel, at least in a part area of the evaluation image, a type that is characteristic of whether the respective pixel corresponds to a vessel of the vascular system, a perfusion area or a background. It performs the assignment of the type on the basis of the time characteristic of the pixel values of the combination images.