1. A system comprising: a processor adapted to receive a time sequence of magnetic resonance images for a region, each image having a plurality of pixels and wherein arrival of a contrast agent into the region at an introduction time occurs during the time sequence; a selector module for selecting a first subset of at least one image from the time sequence of magnetic resonance images, each image of the first subset corresponding to a time prior to the introduction time and for selecting a second subset of at least two images from the time sequence of magnetic resonance images, each image of the second subset corresponding to a time subsequent to the introduction time; and wherein the processor includes instructions to calculate a mask image as a function of corresponding pixels of each image of the first subset and to calculate an arterial image as a function of corresponding pixels of each image of the second subset; and wherein the processor includes instructions to generate a composite image as a function of a difference between each pixel of the mask image and a corresponding pixel of the arterial image.