High-intensity focused ultrasound therapies utilizing cavitation bubbles sometimes cause a “refocused phenomenon” where ultrasound waves reflected by the bubbles are re-reflected by the transducer and return to the focus. This effect significantly influences bubble behavior. This study quantitatively analyzes this refocused acoustic field using the optical shadowgraph method. The shadowgraph method visualizes an acoustic field by capturing the phase shift of light as optical intensity changes. The optical phase shift is caused by variations in the refractive index of water, which are induced by the acoustic field. In our experiment, 1-MHz ultrasound was sonicated with exposure times ranging from 250 to 1000 μs, and a stainless-steel rod was placed at the focus as a stable acoustic reflector to simulate the reflection from cavitation bubbles. To validate the accuracy of our measurements, the results were compared with acoustic pressure measured by a fiber-optic hydrophone. At a low driving voltage of 60 V, where cavitation bubbles were not generated, we observed a three-stage pressure increase of approximately 1.36-fold. The stepwise increase resulted from the interference between the incident and refocused waves. At a higher driving voltage of 160 V, we observed the generation and growth of bubbles at the locations and timings consistent with the pressure increases caused by this interference pattern. Work supported by the AIE Program at Tohoku University.
Ono et al. (Wed,) studied this question.