Simulating ultrasound propagation through a contaminated nanobubble population

Recent papers have reported the ability of nanobubbles to exhibit a comparable or superior acoustic response to microbubbles. Such a claim is significant, as the ability of nanobubbles to extravasate, while remaining visible to conventional ultrasound imaging, could lead to vast applications in terms of diagnostic and therapeutic treatments. The paradoxical performance of these nanobubble populations is, however, disputed by classical acoustic theory, indicating that only nonlinear propagation effects in the harmonic content could be responsible for the higher than expected contrast:background ratio observed. An alternative explanation could be that the large scattering detected is caused by a small proportion of larger microbubbles within the nanobubble suspension. In this study, we test these ideas by adapting an efficient fully nonlinear model of wave propagation through bubbly media to model ultrasound propagation through a relatively high concentration of coated nanobubbles and resolve the harmonic content in the backscattered signal. Such computations require an efficient method of computing the radial dynamics of each bubble size present at every spatial location at each time step while incorporating a nonlinear shell that rapidly buckles and ruptures during large oscillations. The theoretical results presented will be discussed together with recent experimental data.