1. From the Department of Diagnostic Physics (B.H.Ø., A.C.T.M.) and Division of Radiology and Nuclear Medicine (R.G., P.S.), Oslo University Hospital, Building 20, Gaustad, PO Box 4959, Nydalen, 0424 Oslo, Norway; and Institute of Clinical Medicine (B.H.Ø., P.S.) and Department of Physics (A.C.T.M.), University of Oslo, Oslo, Norway.