Abstract
Ultrasou¬nd has been used in a variety of clinical settings and therapy, which can improve efficiency of therapy by microbubbles (MBs), including contrast imaging, gas embolotherapy, cancer detection and drug delivery. However, the short lifetime of MBs in the circulation limits the clinical applications. Comparing to MBs, phase-change droplets (PCDs) are much stable in the circulation. PCDs will triggered vaporize into bubbles under ultrasound sonication, and this transient process is called acoustic droplet vaporization (ADV). ADV are related to the clinical diagnosis and therapy, if understanding of the physical mechanisms of ADV process, and can be monitored in real-time of ADV will be a great help for applications. The aim of this study is to find out the mechanism during ADV by investigating the acoustic signatures of phase-change droplets and figured out their correlations with the physical behaviors observed with high-speed optical imaging. In the acoustic detection, we use pass cavitation detection (PCD) and active cavitation detection (ACD) to clarify the mechanism during ADV. The research are divided into single and group droplet vaporization.5 MHz HIFU were transmitted to induce ADV of single droplet in a 200-μm agar tube at the mutual focus. A 2.25-MHz or 25-MHz US transducer was confocally-arranged with the mutual focus for simultaneous PCD and ACD. PCD was conducted to passively acquire the acoustic signatures emitted during ADV, whilst ACD was conducted to interrogate the morphological evolution of bubbles using a low-pressure US pulse. Finally, we use the characteristic signatures to develop M-mode imaging. In the results, a broadband shock wave was observed in the beginning of PCD signal. Since internal gas nucleation is the necessary 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 with acoustic detection. Keywords: phase-change droplets (PCDs)、acoustic droplet vaporization (ADV)、pass cavitation detection (PCD) 、active cavitation detection (ACD)