Development of a Cavitation Bubble Observation System and Application to Study Expansion Waves and Shock Waves

Ryo Tachikawa, Akira Tsukamoto, Yuki Monden, Keiichi Nakagawa, Takehiro Ando, Etsuko Kobayashi, Takashi Ushida, Ichiro Sakuma
Vol. 4 (2015) p. 96-104

Minimally invasive therapies by applying cavitation bubbles, such as histotripsy and gene transfer, have recently attracted attention. As a method to generate bubbles, the use of expansion waves is expected to be more effective than using shock waves or ultrasound. However, few studies have confirmed the effectiveness of expansion waves in generating bubbles. Previous studies have investigated this issue using biological targets. When contrast agents were injected prior to exposure to expansion waves, a larger hemorrhage was observed after the exposure. However, without contrast agents, little difference in hemorrhage was observed. Thus, at least with biological targets, it is still uncertain whether cavitation bubbles are generated more effectively by expansion waves or by shock waves. In this study, we developed a novel cavitation bubble observation system, and used it to observe bubble dynamics created by either expansion waves or shock waves. A spherical wave generated by electric discharge was focused using two reflectors: one was made of stainless steel for focusing shock waves, and the other was made of polydimethylsiloxane for focusing expansion waves. Using the new system, we clearly observed the generation and growth of bubbles. Comparing the dynamics of bubbles produced by expansion waves with those produced by shock waves, both generation and growth of bubbles were more remarkable with expansion waves. To understand the impact of albumin, which is present in vivo, on the generation of bubbles, bovine serum albumin (BSA) concentration at the focus region was altered. However, BSA concentration had little effect on the effectiveness of the expansion waves. Moreover, several parameters of the expansion wave, such as peak negative pressure and full width of half maximum (FWHM), were investigated to clarify which was the main contributor to the observed promotion of bubble dynamics. The results showed that peak negative pressure contributed substantially to the generation of bubbles when produced by expansion waves. In contrast, FWHM contributed mainly to the growth of bubbles. These results provide insight to understand the mechanisms of cavitation and may lead to clinical applications of expansion waves.