Effects of ultrasound parameters on the acoustic characteristics of phase-change droplets

Phase-change droplets (PCDs) can be vaporized into bubbles by ultrasound (acoustic droplet vaporization, ADV) as promising diagnostic and therapeutic agents. The substantial volume expansion during the droplet-to-bubble transition with duration of a few microseconds may produce low-frequency shock wave, serving as the acoustic characteristics for monitoring and control of ADV. In this study we investigated the influence of ultrasound parameters including acoustic frequency, pulse duration, and pressure upon producing low-frequency shock wave from ADV process. Moreover, optimization of shock wave dose for pulse sequences were evaluated and measured by the total volume of vaporized droplets with the ADV efficiency experiments. The infused droplets were exposed to 3.5- and 10-MHz high-intensity focused ultrasound (HIFU) Hanning window weighted pulses with pulse durations from 3 to 50 cycles at varying pressures from 4 to 12 MPa. A 0.5-MHz transducer (for shock wave dose estimation) and a 1-MHz (for quantitative analysis) were confocally-arranged with the HIFU transducers for passive acoustic detection of low frequency shock wave. Increasing the pulse duration and pressure, the shock wave dose enhance during the variation of varying pulse length from 3 to 15 cycle, then decreased from 15 to 50 cycle. However, the effect of pulse durations of ADV efficiency at 10 MPa was insignificant. By changing pulse duration, result confirmed that no significant but significant on ADV efficiency measurement and shock wave dose evaluation, respectively. In the future, the techniques of the optimization low frequency had the potential application for physical studies in the development of droplet-specific imaging for real-time monitoring of therapeutic process.