1. Ultrasound and Underwater Acoustics, National Physical Laboratory 1 , Hampton Road, Teddington, TW11 0LW, United Kingdom
2. Department of Medical Physics and Biomedical Engineering, University College London 2 , Malet Place Engineering Building, Gower Street, London, WC1E 6BT, United Kingdom
3. Optoelectronics Research Centre, University of Southampton 3 , Southampton, SO17 1BJ, United Kingdom