1. From the Departments of Radiology (A.A., V.G., P.B., A.D.) and Neuroradiology (P.V.), Mayo Clinic, 4500 San Pablo Rd, Jacksonville, FL 32224; Mallinckrodt Institute of Radiology, Washington University School of Medicine, St. Louis, Mo (N.J.M.); and Department of Radiology, Mayo Clinic, Rochester, Minn (G.B.).