1. From the Department of Diagnostic Physics (A.B., C.L., I.R.G.), Intervention Centre (S.A.S.V., P.K.H.), and Department of Radiology (P.D.T.), Oslo University Hospital, Rikshospitalet, Sognsvannsveien 20, N-0372 Oslo, Norway; Faculty of Mathematics and Natural Sciences, Institute of Physics (A.B) and Faculty of Medicine, Institute of Clinical Medicine (P.K.H, C.L.), University of Oslo, Oslo, Norway.