2-D projection images show the variation over time of the distribution of a contrast medium in an examination subject. Each projection image comprises pixels having pixel values corresponding to one another in the projection images that are determined by identical areas of the examination subject. A computer subdivides an image that is to be displayed from the projection images into parcels in one perfusion region. For each parcel, the computer determines a characteristic value and, based on the characteristic value, a projection color. It assigns the projection color to the parcel. The characteristic value is determined based on the pixel values occurring in the parcel of the projection image or their differences from the pixel values of a corresponding parcel of another projection image. The computer outputs a subarea of the projection image containing the perfusion region. It represents each parcel of the perfusion region in its assigned projection color.