Acoustically detonated microbubbles using low frequency insonation: mechanism for low energy mechanical ablation

Noninvasive ultrasound surgery can be achieved using focused ultrasound to locally affect the targeted site without damaging intervening tissues. Histotripsy uses short and intense acoustic pulses to destroy the tissue via a purely mechanical effect. Here we show that coupled with low-frequency excitation, targeted microbubbles serve as mechanical therapeutic warheads that trigger potent mechanical effects at specific locations under the direction of focused ultrasound. Upon low frequency excitation (below 500 kHz), high amplitude microbubbles oscillations occur at substantially lower pressures as compared to higher MHz ultrasonic frequencies. Here, we used these enhanced oscillations to achieve cells and tissue fractionation at low pressures of hundreds of kilopascals (an order of magnitude less than the pressures used in standard histotripsy). We demonstrate these capabilities using two low frequencies of 250 kHz and 80 kHz in vitro, in cell cultures, and in ex vivo tissue. Specifically, we compare the oscillations of free vs. targeted microbubbles and show that cell-targeted microbubbles can effectively destroy cancer cells at low ultrasonic pressures. This technology provides a unique theranostic ultrasound-based technology for low energy mechanical ablation of tissues.