title: Nonlinear acoustic properties of ex vivo bovine liver and the effects of temperature and denaturation.
abstract: Thermal ablation by high intensity focused ultrasound (HIFU) has a great potential for the non-invasive treatment of solid tumours. Due to the high pressure amplitudes involved, nonlinear acoustic effects must be understood and the relevant medium property is the parameter of nonlinearity B/A. Here, B/A was measured in ex vivo bovine liver, over a heating/cooling cycle replicating temperatures reached during HIFU ablation, adapting a finite amplitude insertion technique, which also allowed for measurement of sound-speed and attenuation. The method measures the nonlinear progression of a plane wave through liver and B/A was chosen so that numerical simulations matched the measured waveforms. To create plane-wave conditions, sinusoidal bursts were transmitted by a 100 mm diameter 1.125 MHz unfocused transducer and measured using a 15 mm diameter 2.25 MHz broadband transducer in the near field. Attenuation and sound-speed were calculated using a reflected pulse from the smaller transducer using the larger transducer as the reflecting interface. Results showed that attenuation initially decreased with heating then increased after denaturation, the sound-speed initially increased with temperature and then decreased, and B/A showed an increase with temperature but no significant post-heating change. The B/A data disagree with other reports that show a significant change and we suggest that any nonlinear enhancement in the received ultrasound signal post-treatment is likely due to acoustic cavitation rather than changes in tissue nonlinearity.
authors.affiliation: Department of Engineering Science, Institute of Biomedical Engineering, University of Oxford, Old Road Campus Research Building, Oxford, OX3 7DQ, UK.
authors.forename: E J
authors.initials: EJ
authors.lastname: Jackson
authors.forename: C-C
authors.initials: CC
authors.lastname: Coussios
authors.forename: R O
authors.initials: RO
authors.lastname: Cleveland
mesh_terms: D000162:Acoustics; D000818:Animals; D002417:Cattle; D057086:High-Intensity Focused Ultrasound Ablation / Q000295:instrumentation; D008099:Liver / Q000166:cytology*; D017711:Nonlinear Dynamics; D013696:Temperature; D013997:Time Factors; D014159:Transducers