1. Department of Electrical Engineering, Research Institute for Smart Energy, The Hong Kong Polytechnic University, Hung Hom, Kowloon 999077, Hong Kong, China
2. Faculty of Science and Engineering, The University of Manchester, Manchester M13 9PL, England, U.K.
3. Centre for Medical and Industrial Ultrasonics, James Watt School of Engineering, University of Glasgow, Glasgow G12 8QQ, Scotland, U.K.