1. Department of Radiology, Wake Forest School of Medicine, Winston-Salem, North Carolina 27157, United States
2. Department of Pathology, Section on Comparative Medicine, Wake Forest School of Medicine, Winston-Salem, North Carolina 27157, United States
3. Department of Radiology, Columbia University School of Medicine, New York, New York 10032, United States
4. Department of Gerontology, Wake Forest School of Medicine, Winston-Salem, North Carolina 27157, United States