From the Departments of Radiology (J.A., J.S.M., M.A.C., C.R.F., S.H., D.R.J., B.S., G.T., D.D., R.K., D.F.K., R.J.M.), Molecular Pharmacology and Experimental Therapeutics (K.M.W., D.S.C.), Biochemistry and Molecular Biology (J.L.S.), Laboratory Medicine and Pathology (P.J.J., J.A.B.), and Neurosurgery, College of Medicine (D.F.K.), Mayo Clinic, 200 1st St SW, Rochester, MN 55905.