Ex-vivo study of cavitation-enhanced heating by combining low (40 kHz) and high (566 kHz) frequency ultrasound

The objective of this study is to demonstrate the cavitation-enhanced heating via combining LF ultrasound during an ultrasound thermal treatment. Two ultrasound transducers were used: a 566-kHz spherical transducer was used to generate the thermal lesion (TL) while another 40-kHz planar transducer was positioned orthogonally with its axis across the focus of the high-frequency one. The emission and backscattered signals from the bubble activity were also recorded during the heating process by a needle hydrophone. Compared to the TL formed by exposure to the focused transducer only, its size increased up to 200% and 140% in the axial direction and cross section, respectively. The enlarged TL, rather than being in tadpole shape, was symmetrical in the focal direction, which implied that the cavitation-enhanced TL could be generated without losing its penetration ability. This study demonstrates an easy and effective approach to enlarge the final size of a TL by applying LF ultrasound simultaneously, and would be important if a large-volume and deep-sited TL by ultrasound ablation was necessary.