This study will determine the usefulness of magnetic resonance imaging (MRI) for examining the heart or blood vessels. MRI uses a magnetic field and radio waves to produce images of body tissues and organs. The subject lies on a table that can slide in and out of the scanner (a metal cylinder), wearing earplugs to muffle loud noises that occur during the scanning process. MRI of the heart and blood vessels, called magnetic resonance angiography (MRA), is a developing diagnostic method that permits evaluation of arteries and veins without the use of x-rays or invasive catheterization required by conventional angiography.