In this research study the investigators will acquire a PET/CT image of the body (neck to mid-thigh) using best-practice diagnostic imaging protocols, typically involving 64-slice CT systems, 1x1x3 mm voxel dimensions, use of non-ionic contrast agents, and breath-holding for motion correction. Subjects will also receive a single MRI with injected contrast of the brain using best-practice diagnostic imaging protocols, typically involving a 3-Tesla clinical MRI system.