A Passive Acoustic Device for Real-Time Monitoring of the Efficacy of Shockwave Lithotripsy Treatment

Prediction of far-field acoustic emissions from cavitation clouds during shock wave lithotripsy for development of a clinical device

“…As expected, although the prediction of t c is close to that measured, the ratio m 2 /m 1 gives very poor agreement, being in fact greater than 1 (in common with most Gilmore simulations in the literature for lithotripter-induced cavity collapses; Church [40] and Leighton et al [17]). This is an important point.…”

“…This is sufficient to show the initial collapse (figure 4), and predict the emissions from this (figure 8b), but unlike the Gilmore model, it cannot (with current computational resources) go on to simulate the subsequent expansion of the compressed bubbles to many times their original size, followed by a second violent collapse some hundred microseconds or more later. This pair of violent collapses (first experimentally identified from the timing of the associated cavitation luminescence and correlated to the acoustic emissions from cavitation by Coleman et al [60]) dominates the far-field acoustic emission from cavitation during SWL, the first collapse of the cloud generating a first 'burst' of amplitude m 1 , and the second collapse generating a second burst of amplitude m 2 some time t c seconds later (exact definitions of m 1 , m 2 and t c are given by Leighton et al [17]). These are clearly shown, recorded from a patient using the sensor developed by this research programme, in figure 9a.…”

“…In introducing figure 9b, it was noted that the only source of asymmetry in the radiated field came from the finite time for the passage of the lithotripter shock wave across a spatially distributed bubble cloud (and even then it lacks bubble-bubble interactions that can affect emissions in clouds; [13,68]). In the free-Lagrange simulations, the ability to introduce a nearby boundary with real material properties was demonstrated [14], but even this cannot mimic the acoustical effects of the stone because the far-field pressures will also include emissions generated by the finite size of the stone (internal reflections-including resonances-and surface waves [17,69]). The passive sensor was designed to detect these in m 1 so that they could be included in the diagnosis.…”

“…The passive sensor was designed to detect these in m 1 so that they could be included in the diagnosis. Its high-pass filter has a low-frequency cut-off set at 297 kHz [17] to allow it to pick up stone scattering in m 1 , and to allow m 1 to be linked to respiration. None of these features are in the Gilmore simulations.…”

“…Justification for this necessary limitation is discussed by Jamaluddin et al [15] for propagation in the more highly absorbing human body tissue that occurs when the device is used in the clinic. While limited resources necessitate this approach, it was sufficient to allow development of the device that was then validated as useful in the clinic through empirical clinical trials [16,17].…”