Subjects with any metallic objects within them just prior to MR imaging (cardiac or neural pacemaker, aneurysm clips [metal clips on the wall of a large artery], metallic prostheses [including heart valves and cochlear implants] or shrapnel fragments. Welders and metal workers are also at risk for injury and may not take part in the study because of possible small metal fragments in the eye of which they may be unaware.