Schlieren observation of therapeutic field in water surrounded by cranium radiated from 500 kHz ultrasonic sector transducer

Standing-wave formation in water surrounded by a section of a human cranium, produced by a transcranial 500 kHz ultrasonic beam was observed optically. The ultrasonic beam was generated from a prototype sector-scan phased-array transducer, designed for transcranial enhancement of thrombolysis with tissue plasminogen activator (tPA). The amplitude distribution and the wavefronts of the ultrasonic field were observed in schlieren images. The stripe patterns of the standing waves were clearly seen near the sites of reflection in these images under certain acoustic conditions. No standing wave patterns were detected in basically the same arrangement with a sector-scan phased-array transducer operating at 2 MHz. These findings suggest that standing waves may be formed in the tissue at the positions of reflection by transcranial insonation of a human brain at a relatively low ultrasonic frequency, typically less than 1 MHz. This suggests further the possibility of inducing adverse cavitational effects in brain tissue.