Fan Ching-Hsiang, Kao Wei-Fu, Kang Shih-Tsung, Ho Yi-Ju, Yeh Chih-Kuang
IEEE Trans Ultrason Ferroelectr Freq Control. 2021 Apr;68(4):1051-1061. doi: 10.1109/TUFFC.2020.3032441. Epub 2021 Mar 26.
Acoustic droplet vaporization (ADV) provides the on-demand production of bubbles for use in ultrasound (US)-based diagnostic and therapeutic applications. The droplet-to-bubble transition process has been shown to involve localized internal gas nucleation, followed by a volume expansion of threefold to fivefold and inertial bubble oscillation, all of which take place within a few microseconds. Monitoring these ADV processes is important in gauging the mechanical effects of phase-change droplets in a biological environment, but this is difficult to achieve using regular optical observations. In this study, we utilized acoustic characterization [i.e., simultaneous passive cavitation detection (PCD) and active cavitation detection (ACD)] to investigate the acoustic signatures emitted from phase-change droplets ADV and determined their correlations with the physical behaviors observed using high-speed optical imaging. The experimental results showed that activation with three-cycle 5-MHz US pulse resulted in the droplets (diameter: 3.0- [Formula: see text]) overexpanding and undergoing damped oscillation before settling to bubbles with a final diameter. Meanwhile, a broadband shock wave was observed at the beginning of the PCD signal. The intense fluctuations of the ACD signal revealed that the shock wave arose from the inertial cavitation of nucleated small gas pockets in the droplets. It was particularly interesting that another shock-wave signal with a much lower acoustic frequency (< 2 MHz) was observed at about [Formula: see text] after the first half signal. This signal coincided with the reduction of the ACD signal amplitude that indicated the rebound of the transforming bubble. Since internal gas nucleation is a crucial process of ADV, the first half signal may indicate the occurrence of an ADV event, and the second half signal may further reveal the degrees of expansion and oscillation of the bubble. These acoustic signatures provide opportunities for monitoring ADV dynamics based on the detection of acoustic signals.
声滴汽化（ADV）可按需产生气泡，用于基于超声（US）的诊断和治疗应用。已表明液滴到气泡的转变过程涉及局部内部气体成核，随后体积膨胀三到五倍并伴有惯性气泡振荡，所有这些都在几微秒内发生。监测这些ADV过程对于衡量相变液滴在生物环境中的机械效应很重要，但使用常规光学观察很难实现。在本研究中，我们利用声学表征[即同时进行被动空化检测（PCD）和主动空化检测（ACD）]来研究相变液滴ADV发出的声学特征，并确定它们与使用高速光学成像观察到的物理行为的相关性。实验结果表明，用三周期5-MHz超声脉冲激活会导致液滴（直径：3.0 - [公式：见原文]）过度膨胀并经历阻尼振荡，然后稳定为最终直径的气泡。同时，在PCD信号开始时观察到一个宽带冲击波。ACD信号的强烈波动表明该冲击波源于液滴中成核的小气穴的惯性空化。特别有趣的是，在第一个半信号后约[公式：见原文]处观察到另一个声学频率低得多（<2 MHz）的冲击波信号。该信号与ACD信号幅度的降低相吻合，这表明正在转变的气泡发生了反弹。由于内部气体成核是ADV的关键过程，第一个半信号可能表明ADV事件的发生，第二个半信号可能进一步揭示气泡的膨胀和振荡程度。这些声学特征为基于声学信号检测监测ADV动力学提供了机会。
