1. From the Laboratory for Structural, Physiologic and Functional Imaging, Department of Radiology (A.C., M.C.L., A.J., F.W.W.), Department of Biostatistics and Epidemiology (W.G.), and Institute for Environmental Medicine and Department of Physiology (S.C.), University of Pennsylvania Perelman School of Medicine, 3400 Spruce St, Philadelphia, PA 19104.