1. Department
of Physics, University of Oslo, P.O. Box 1048, Blindern, N-0316 Oslo, Norway
2. The
Intervention Centre, Oslo University Hospital, P.O. Box 4950, Nydalen, N-0424 Oslo, Norway
3. Department
of Medical Physics, Oslo University Hospital, P.O. Box 4950, Nydalen, N-0424 Oslo, Norway