Size and pressure dependent activity of lipid coated bubbles and finite element simulation of the propgation of focused ultrasound through bubbly media

Abstract

Use of nanodroplets as alternative to bubbles is limited to high pressure applications due to their high vaporization threshold (>1 MPa). For low pressure applications (e.g <800 kPa) and stable bubble activity, size isolated micron or sub-micron bubbles may be used to tackle the pre-focal bubble activity and attenuation. Numerical simulations of the Marmottant model were ran for bubble sizes of 0.45, 1, 2, and 4 μm in response to 1 MHz ultrasound with pressures between 10 and 700 kPa. All agents were volume matched to 20 μl/kg of Definity considering inter-bubble interactions. The pressure-dependent attenuation and the total acoustic power (TAP) were calculated for each population. Finite element simulations (FEMS) were run by taking account the pressure dependent attenuation and sound speed. Using size isolated bubbles an experimental passive cavitation case study was performed for the same exposure conditions and sizes. Numerical results show that TAP and attenuation of the bubbles are size dependent. Bigger bubbles have stronger responses at lower pressures. However, smaller agents exhibit a size-dependent pressure threshold behavior (PT) above which their attenuation and TAP grow stronger than their bigger counterparts in qualitative agreement with experiments. FEMS show that the PT of oscillations may be used to reduce pre-focal attenuation for ultrasound propagation with minimal loss.