1. From the Mallinckrodt Institute of Radiology (T.J.F., R.T., E.R., D.R.L., J.P.H., K.J.F.), Division of Biostatistics (A.S.), and Department of Pathology and Immunology (I.N.), Washington University School of Medicine, 510 S Kingshighway Blvd, Campus Box 8131, St Louis, MO 63110.