Photoacoustic Cavitation of Nanodroplets: High Speed Observation and Acoustic Signal Analysis

Perfluorocarbon (PFC) nanodroplet has a longer lifespan and small volume to circulate freely in blood microcirculation. Its phase transition has great potential in localized therapy. Photoacoustic cavitation, a new approach involving synergetic laser and ultrasound to achieve the phase transition of PFC nanodroplets, could improve the spatial and temporal resolution and reduce the energy threshold compared to individual use of either modalities. In this study, we present a confocal microscopic system to induce photoacoustic cavitation of nanodroplets and study its characteristics by high speed observation and acoustic detection. Upon excitation by laser and ultrasound pulses, the liquid-gas phase transition of PFC nanodroplets and subsequent bubble dynamics were demonstrated. Two distinct bubble formation patterns were observed. It is associated with the size distribution of the droplets within the confocal volume of laser and ultrasound. Moreover, the detected acoustic signals suggested the presence of stable cavitation. Due to the reduced energy threshold of phase transition, the proposed method enables the theranostic applications of PFC nanodroplets at lower and safer laser/ultrasound energies in tissues or cells. Detection of high amplitude signals also revealed the potential of this method to generate strong photoacoustic signals for the photoacoustic imaging.