Localized detection of cavitation generated by lithotripsy shock waves

Abstract

Extracorporeal shock wave lithotripters generate intense cavitation which plays a role in both kidney stone destruction and tissue damage. The cavitation can be detected by listening for the characteristic ‘‘double-bang‘‘ acoustic emission, radiated by the bubbles during their rapid expansion and collapse phases. Previous workers have used a single focused transducer to detect the double-bang signature. However, the focal spot of the transducers is typcially 5 cm long and individual cavitation events can not be localized. A system has been developed that uses two confocal transducers (1 MHz, 20 cm radius of curvature, 10 cm diameter) to detect cavitation events in a localized volume. The system can be run in two modes: passive or active. In the passive mode both transducers listen to acoustic emissions generated by the cavitation field. In the active mode one transducer sends a tone and the second transducer listens to the sound scattered by any bubbles present in the focal region. In both cases the region to which the detectors are sensitive is approximately a sphere 5 mm in diameter. The passive system shows bubble rebound times far longer than predicted by theory for a single bubble. Scattering indicates that bubbles persist in the focal region for at least 6 ms. [Work supported by NIH.]