作者单位:Meharry Med Coll, Sch Med, Dalton, GA USA[1]Washington Univ, Sch Med, Div Biostat, St Louis, MO 63110 USA[2]Washington Univ, Sch Med, Mallinckrodt Inst Radiol, 510 S Kingshighway Blvd,Campus Box 8131, St[3]Univ Calif San Diego, San Diego, CA 92103 USA[4]