A dual passive cavitation detector for localized detection of lithotripsy-induced cavitation in vitro

A passive cavitation detector (PCD) identifies cavitation events by sensing acoustic emissions generated by the collapse of bubbles. In this work, a d ual passive cavitation detector (dual PCD), consisting of a pair of orthogo nal confocal receivers, is described for use in shock wave lithotripsy. Cav itation events are detected by both receivers and can be localized to withi n 5 mm by the nature of the small intersecting volume of the focal areas of the two receivers in association with a coincidence detection algorithm. A calibration technique, based on the impulse response of the transducer, wa s employed to estimate radiated pressures at collapse near the bubble. Resu lts are presented for the in vitro cavitation fields of both a clinical and a research electrohydraulic lithotripter. The measured lifetime of the pri mary growth-and-collapse of the cavitation bubbles increased from 180 to 42 0 mu s as the power setting was increased from 12 to 24 kV. The measured li fetime compared well with calculations based on the Gilmore-Akulichev formu lation for bubble dynamics. The radiated acoustic pressure 10 mm from the c ollapsing cavitation bubble was measured to vary from 4 to 16 MPa with incr easing power setting; although the trends agreed with calculations, the pre dicted values were four times larger than measured values. The axial length of the cavitation field correlated well with the 6-dB region of the acoust ic field. However, the width of the cavitation field (10 mm) was significan tly narrower than the acoustic field (25 mm) as bubbles appeared to be draw n to the acoustic axis during the collapse. The dual PCD also detected sign als from "rebounds." secondary and tertiary growth-and-collapse cycles. The measured rebound time did not agree with calculations from the single-bubb le model. The rebounds could be fitted to a Rayleigh collapse model by cons idering the entire bubble cloud as an effective single bubble. The results from the dual PCD agreed well with images from high-speed photography. The results indicate that single-bubble theory is sufficient to model lithotrip sy cavitation dynamics up to time of the main collapse, but that upon colla pse bubble cloud dynamics becomes important. (C) 2000 Acoustical Society of America. [S0001-4966(00)02503-0].