1. From the Departments of Radiology (S.A.N., E.A.H., P.K.S.) and Internal Medicine (A.P.C., E.A.H.), Carver College of Medicine, and Departments of Biomedical Engineering (E.A.H.) and Electrical and Computer Engineering (P.K.S.), College of Engineering, The University of Iowa, 4312 Seamans Center, Iowa City, IA 52242.