1. Dept. of Medical Phys. and Biomedical Eng., Univ. College London, Malet Pl. Eng. Bldg., Gower St., London WC1E 6BT, United Kingdom,
2. Dept. of Medical Phys. and Biomedical Eng., Univ. College London, London, United Kingdom
3. Dept. of Comput. Sci., Univ. College London, London, United Kingdom
4. Medical Phys. Dept., National Physical Lab., London, United Kingdom