Study of the viscous heating effect of particle enhancers in focused ultrasound based on the theory of two-phase media

In focused ultrasound surgery (FUS) for tumour ablation, ensuring the safety and efficiency of the procedure is challenging. Currently, researchers are investigating the effects of bioparticles such as hydroxyapatite to improve the acoustic properties of the treatment region, aiming to increase tumour ablation efficiency. In this study, we aim to elucidate the intricate thermal effects of the visco-inertial transfer mechanisms between the nanoparticles and the surrounding tissue. We introduced a two-phase acoustic attenuation model to simulate the acoustic attenuation coefficient and temperature rise in tissue containing nanoparticles. The acoustic attenuation coefficient revealed a progressive increase in viscous losses, which can be converted into heat as the volume fraction of particles increases. A larger density difference between nanoparticles and surrounding tissue results in greater viscous losses. Moreover, when the particle radius falls within a specific range, the viscous losses reach the maximum values. The thermal accumulation analysis revealed that a medium containing particles ranging from tens of nanometres to one micrometre in size at a volume fraction of 1 %–3 % could achieve over twice the thermal accumulation efficiency of a pure medium. The experimental results of the biomimetic model, consistent with the numerical simulation results, indicate that the viscous heating effect is predominantly observed during the initial stage of irradiation, specifically within the first 5 s. These findings can contribute to improving treatment outcomes and expanding the applicability of FUS to different tumour types.