Targeting of stones and identification of stone fragmentation in shock wave lithotripsy

In shock wave lithotripsy (SWL), acoustic shock waves (SWs) are generated outside the body and applied to break a stone into fragments small enough to pass from the urinary tract. Tissue injury that is proportional to the number of SWs applied accompanies most if not all SWL treatments. However, there is currently little feedback available regarding whether a SW hits the stone, which can move with patient respiration, or if the stone is breaking. To target stones, a prototype targeting system based on vibro-acoustography was built to detect kidney stones and provide feedback to gate the application of SWs. The targeting system was characterized by numerical calculation using the Rayleigh Integral and by experimental measurement. The use of targeting to gate the application of SWs was compared to a constant rate method in which SWs were triggered at 1 Hz. Targeting increased percent fragmentation of model stones and would decrease the number of SWs necessary to complete a SWL treatment. To identify stone fracture, SW scattering from intact and fractured stone models was calculated numerically with a linear elastic model and measured experimentally in vitro with a broadband receiver. A dominant effect in SW scattering, termed 'resonant acoustic scatter' (RAS), was attributable to the reverberation of elastic waves within the stone models. The frequency of RAS was inversely proportional to stone size. A new frequency analysis method was developed and used to indicate fracture in a clear and simple way. To identify stone fragmentation, SW scattering from cement stone models was measured experimentally in vitro. Stone fragments were sieved after SW treatment to measure percent fragmentation. Scatter signals were processed in time and frequency, and the output was compared to percent fragmentation. Signal processing output correlated positively with fragmentation and the frequency of detected scatter was inversely proportional to stone size. The work presented here is primarily experimental and conducted in vitro to prove the concepts. If applied to clinical lithotripsy, targeting could reduce the number of SWs required for full fragmentation, and the identification of fracture and fragmentation could help determine the endpoint of therapy.