A computer receives a temporal sequence of x-ray images of an examination region of an examination object. The examination region includes a blood vessel system and tissue supplied with blood. A detection time is assigned in each instance to the x-ray images. The x-ray images correspond locally with one another in terms of pixels and each display a distribution of a contrast agent in the examination region at the respective detection time. The computer determines the temporal course of the temporal derivation of the data values and/or of the average value of the data values of the pixels located in the evaluation region for at least one evaluation region which is standard for all x-ray images. It assigns a type to the evaluation region as a function hereof.