1. From the Departments of Radiology (A.K.N., R.J.M., D.F.K., L.J.E., J.S.M.), Department of Laboratory Medicine and Pathology (A.M.B., D.L.M., P.J.J.), Department of Health Sciences Research (T.M.G.), and Department of Neurosurgery, College of Medicine, Mayo Clinic, 200 1st St SW, Rochester, MN 55905.