Acoustic and optical observation of nonlinear oscillation of nanobubbles exposed to focused ultrasound

Abstract Microbubbles are applied not only for diagnostic purposes but also as acoustic sensitizers in therapies such as drug delivery, blood-brain barrier opening, and sonodynamic therapy. Their dynamics are well understood due to their optically observable size. In contrast, sub-micron nanobubbles (NBs) are difficult to visualize, leaving their physical dynamics unclear. This study investigated the acoustic response of NBs under 1.1 MHz focused ultrasound. We examined nonlinear oscillation of NBs under various acoustic pressures with two different sonication durations using acoustic and optical evaluations. During shorter sonication duration of 200 μs, increasing acoustic pressure enhanced harmonic components and induced broadband noise, corresponding to optically confirmed bubble oscillation. Sonication duration of 5 ms caused significant temporal decrease in bubble echo signals. These results suggest that correlating acoustic spectra with high-speed images enables the evaluation of cavitation phenomena generated from NBs.