1. From the Department of Radiology (J.F.P.), Tufts Medical Center, Tufts University School of Medicine Boston MA; Department of Biostatistics (M.J.P.), Boston University School of Public Health, Framingham Heart Study, Boston MA; Saint Elizabeth's Hospital (D.H.O.), Tufts University School of Medicine, Boston, MA; Departments of Mathematics and Statistics (R.B.D.), Boston University School of Medicine, Boston MA.