Andrew J. Coleman
Min J. Choi
John E. Saunders
Medical Phys. Directorate, Guy's and St. Thomas' Hospital Trust, London SE1 7EH, England
A clinical cavitation detection system has been developed that monitors the amplitude-time variation of the 1-MHz component of the broadband acoustic emission expected to accompany bubble collapse during clinical extracorporeal shockwave lithotripsy [C. C. Church, J. Acoust. Soc. Am. 86, 215--227 (1989)]. The detector is based on a hand-help piezoceramic focal bowl hydrophone that is acoustically coupled to the patients' skin. A commercial electromagnetic positioning device (Fastrak, Polhemus Inc., Vermont) is used to enable the reception zone of the hydrophone to be directed under ultrasound image guidance. The system has been tested in the clinic by monitoring acoustic emission from positions around the beam focus of a clinical shockwave lithotripter during routine lithotripsy. The detected acoustic emission shows features characteristic of those obtained from cavitation observed in vitro and arises from within a region of similar dimensions to that of the hyper-echoic region that can be simultaneously observed in the B-scan image. The study provides evidence that acoustic emission from cavitation in tissue can be detected noninvasively during clinical lithotripsy.