1. Acoustics and Ionising Radiation Team; National Physical Laboratory; Hampton Road Teddington TW11 0LW United Kingdom
2. Department of Medical Physics and Biomedical Engineering; University College London; Gower Street London WC1E 6BT United Kingdom
3. Department of Physics; Université de Montréal; 2900 boul. Édouard-Montpetit Montréal QC H3T 1J4 Canada
4. Massachusetts General Hospital; 55 Fruit St Boston MA 02114 USA