1. A method comprising: receiving a time sequence of magnetic resonance images for a region, each image having a plurality of pixels and wherein a contrast agent is introduced into the region at an introduction time occurring during the time sequence; 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; 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; calculating a mask image as a function of corresponding pixels of each image of the first subset; calculating an arterial image as a function of corresponding pixels of each image of the second subset; and using a processor 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.