1. Faculty
of Science and Technology, Norwegian University of Life Sciences, P.O. Box 5003, 1432 Ås, Norway
2. Department
of Physics, University of Oslo, P.O. Box 1048 Blindern, 0316 Oslo, Norway
3. Department
of Medical Physics, Oslo University Hospital, P.O. Box 4953 Nydalen, 0424 Oslo, Norway