1. From the Departments of Radiology (J.B.G., T.J.K., L.J.E., J.M.M., D.R.J.), Laboratory Medicine and Pathology (R.A.V., C.G.), and Neurology (D.R.J.), Mayo Clinic, 200 First St SW, Rochester, MN 55905; and Department of Biomedical and Neuromotor Sciences, University of Bologna, Bologna, Italy (C.G.).