The origins of nonlinear enhancement in ex vivo tissue during high intensity focused ultrasound (HIFU) ablation

Thermal ablation by High Intensity Focused Ultrasound (HIFU) is the only technique for noninvasive treatment of tumours, but suffers from a long treatment times and varied treatment outcomes. Knowledge of the nonlinear parameter B/A can resolve these by improving treatment planning, and correlation between enhanced nonlinear signals and the presence of HIFU lesions has the potential to monitor treatment acoustically in real-time. It remains unclear whether the nonlinear enhancement is due to changes in the properties of treated tissue or cavitation. This study improves the signal-to-noise ratio of the standard finite-amplitude insertion technique to measure the B/A of ex-vivo bovine liver. The technique creates plane wave conditions by measuring the waveform in the planar near field of a large transducer then uses the Burgers equation to find B/A. Measurements are taken during heating in a water bath and HIFU exposure. Cavitation is monitored during the HIFU exposures, so that the effect of cavitation on acoustic properties can be assessed and the difference between HIFU heating and slower, water bath heating. Water bath results disagree with previously reported data showing a doubling in B/A, finding a modest increase post-heating, making mechanical effects of ultrasound the likely cause of nonlinear enhancement. © 2013 Acoustical Society of America