Modelling lipid-coated microbubbles at subresonance frequencies

We present a computational study of a lipid-coated SonoVue microbubble, excited at subresonance frequencies (200-1500 kHz) and pressure amplitudes (100-1500 kPa) frequently considered for focused ultrasound applications. The bubble dynamics are modelled with the Rayleigh-Plesset equation and the Gilmore equation, together with the Marmottant model for the lipid monolayer coating. Below the onset of inertial cavitation, a linear regime bounded by the Blake pressure is identified in which the maximum pressure at the bubble wall is linearly proportional to the mechanical index. In the nonlinear regime the maximum pressure at the bubble wall is readily predicted by the maximum bubble radius, and both the Rayleigh-Plesset and Gilmore equations predict the onset of sub- and ultraharmonic frequencies of the acoustic emissions compared to in vitro experiments. Accounting for the lipid coating turns out to be critical for the accurate prediction of the bubble behaviour.