-D projection images show the temporal course of the distribution of a contrast medium in an examination object containing a vascular system and the surroundings thereof. Each projection image has pixels with pixel values defined by the same areas of the examination object a computer determines a 2-D evaluation image having pixels corresponding to those of the projection images and assigns each pixel in a sub-area, to one of three types, perfused part of the surroundings or non-perfused part of the surroundings assigns, an extent of a perfusion in the pixels of the evaluation image assigned the type of perfused part of the surroundings to the respective pixel. The type and extend are determined from the temporal course of the pixel values of the pixels of the projection image which is ina two-dimensional evaluation core defined by a respective pixel of the evaluation image.