1. Acoustics and Ionising Radiation Team; National Physical Laboratory; Teddington TW11 0LW UK
2. Department of Medical Physics and Biomedical Engineering; University College London; London WC1E 6BT UK
3. Départment de physique; Université de Montréal; Montréal QC H3T 1J4 Canada
4. Department of Radiation Oncology; Massachusetts General Hospital and Harvard Medical School; Boston MA 02114 USA
5. Centre de recherche du Centre hospitalier de l'Université de Montréal; Montréal QC H2X 0A9 Canada