simulation study on the dynamics of cavitation bubbles in multi-frequency ultrasound

Background: high-intensity focused ultrasound (hifu) therapy is an effective minimally invasive treatment technique.objective: this work aimed to present a theoretical foundation for transient cavitation control in hifu treatment and investigate cavitation bubbles in multi-frequency ultrasound.material and methods: in this theoretical study, the nonlinear vibrations of bubbles in different mediums (water, urine, kidney, and muscle) were simulated using gilmore-akulichev and modified keller-miksis equations. the dynamic changes of bubble radius during irradiation by multi-frequency combined ultrasound were analyzed, and the effects of multi-frequency ultrasound combinations and frequency differences on the maximum and minimum values of bubble expansion radius and bubble collapse time were investigated.results: at the same highest frequency, the triple-frequency produced the largest bubble expansion radius (rmax) while the single-frequency resulted in the smallest bubble expansion radius (rmin). at the same lowest frequency, the single-frequency had the biggest bubble expansion radius and the triple-frequency had the smallest bubble expansion radius. compared to the combination with a large frequency difference at high frequency, the triple-frequency combination with a small frequency difference at low frequency exhibited a noticeably larger rmax, but rmin showed the opposite behavior. rmax/rmin decreased for the same ultrasonic combination when the medium viscosity increased. the bubble expansion radius ratio rmax/rmin was positively correlated with the bubble collapse time. conclusion: there was a strong correlation between the frequency difference and the multi-frequency ultrasound combination and the maximum and minimum values of the cavitation bubble radius and the collapse time.