Bubble generation by standing wave in water surrounded by cranium with transcranial ultrasonic beam

Low-frequency ultrasound, typically less than 1 MHz, is suitable for enhancing thrombolysis because it penetrates the cranium effectively. However, intracerebral hemorrhages after transcranial insonation in clinical trials at 300 Hz have been reported. In this study, acoustic bubble formation in a standing wave with a 617 kHz ultrasonic beam in water surrounded by a contoured piece of a human cranium was detected by ultrasound B-mode imaging. This bubble formation was indirect evidence that standing-wave formation led to cavitational adverse effects in brain tissue at the place of reflection by transcranial insonation at a relatively low ultrasonic frequency. A way of suppressing cavitation after bubble formation was also investigated. The efficiency of nucleation of bubbles was highly dependent on pulse duration at a constant total acoustic power. The obtained result suggests that inertial cavitation can be suppressed while preserving the efficiency of thrombolysis by temporally changing the acoustic condition before resonant bubble formation.