MRA is a new method for looking at arteries and veins without standard angiography, which requires inserting a catheter into a blood vessel, injecting a contrast material, and obtaining X-ray images. Current MRA techniques, however, do not depict the lumen (cavity) of small vessels well enough to accurately determine the extent of their narrowing. This study will test image processing methods with the eventual goal of improving MRA accuracy to the point that it can replace X-ray catheter angiography for diagnosing atherosclerotic disease.