Abstract
Histotripsy is a technique using ultrasound pulses to liquefy bio-tissues, which is less invasive comparing with other tumor therapy. As the cavitation nuclei, microbubbles (MBs) had been reported can improve the treatment efficiency and also enhance the ultrasound image contrast during the treatment for guiding the treatment location. However, the short lifetime of MBs in the circulation limits the clinical application of this method. Comparing to MBs, phase-change droplets (PCDs) are stable in the circulation. PCDs will triggered vaporize into bubbles under ultrasound sonication, and this transient process is called acoustic droplet vaporization (ADV). Previous studies showed that surrounding structure could be damaged during ADV and the possibly mechanism of the erosion behavior is inertial cavitation, which makes PCDs have potential to be used in histotripsy. The aim of this study is to find out the mechanism of tissue damage during ADV occurred by using PCDs vaporing in tissue-mimicking phantoms. Moreover, by varying the acoustic parameters of ultrasound pulses, the shape and size of eroded region can be control and then optimizing the dose of ultrasound exposure to increase the safety and possibility of the therapy at clinical use. In this research, the erosion behavior of ADV was observed using an acousto-optics system during PCDs vaporizing within the vessel of tissue-mimicking phantoms. The erosion rates were also quantified under different experiment coditions. The backscatter signals of the ADV were then collected and quantified to verify the effects of bubbles created by ADV to the erosion behavior. By processing the characterization wideband signal of inertial cavitation, the intensity and the time of inertial cavitation appeared were determined to find out the role of inertial cavitation in the erosion process. It is found that the intensity in the near field will increased while bubble density in the focused region increased, which makes the direction of erosion opposite to the ultrasound transmission direction. Meanwhile, the ultrasound intensity will be attenuated thus prevent distal side of phantom from damage. Besides, the inertial cavitation only happened at the first few cycle of vaporization pulses. This result showed that short pulses are much effective comparing to long pulses in tissue erosion. Nevertheless, the thermal effect caused by short pulses is relative lower, and the resolution of ultrasound image is better using short pulses rather than long pulses. In the case of irradiating continuously and other parameters fixed, the level of erosion rate increase was the most significant while pulse duration increase.