Alavi Tamaddoni H. et al, 2019: Enhanced shockwave lithotripsy with active cavitation mitigation
Alavi Tamaddoni H, Roberts WW, Hall TL.
Department of Biomedical Engineering, University of Michigan, 2131 Carl A. Gerstacker Building, 2200 Bonisteel Boulevard, Ann Arbor, Michigan 48109, USA.
Department of Urology, University of Michigan, 4444 Medical Science Building 1, 1301 Catherine Street, Ann Arbor, Michigan 48109, USA.
Department of Biomedical Engineering, University of Michigan, 2107 Carl A. Gerstacker Building, 2200 Bonisteel Boulevard, Ann Arbor, Michigan 48109, USA.
The goal of this study was to examine acoustical mechanisms that manipulate cavitation events in order to improve the efficacy of shockwave lithotripsy (SWL) at higher rates. Previous work has shown that applying low amplitude acoustic pulses immediately after each shockwave (SW) can force cavitation bubbles to coalesce and enhance SWL efficacy. In this study, the effects of applying low amplitude acoustic pulses at different time delays is investigated before and after each SW, which would result in different interactions among residual microbubbles producing forced coalescence and dispersion. Utilizing forced coalescence and dispersion was hypothesized to mitigate the shielding effect of residual bubbles, further improving efficacy particularly for higher SWL rates. A set of in vitro experiments was performed in a water tank so that the behavior of bubbles, coalescence and dispersion, could be observed with a high-speed camera. Model kidney stones were treated by a clinical Dornier lithotripter with firing rates of 30 shocks/min and 120 shocks/min, along with an in-house made transducer to generate low amplitude acoustic pulses fired at different pressures and time delays. The average percentage of untreated stone fragments greater than 2 mm was 15.81% for 120 shocks/min without mitigation and significantly reduced to 0.19% for the optimum mitigation protocol.
J Acoust Soc Am. 2019 Nov;146(5):3275. doi: 10.1121/1.5131649.
Diverse publications confirm lower shock rates of 30-60 SW/min are more efficient than higher rates of 90-120 SW/min. Since shock rates significantly determine the total length of a lithotripsy procedure high, shock rates are most welcome. The authors hypothesize persisting pre-focal cavitation bubbles may shield or attenuate the amplitude of subsequent lithotripsy SWs and reduce efficacy of SWL at higher rates. The lifespan of SW-generated cavitation bubbles, however, is in the range of 1ms and therefore much shorter than the time interval of subsequent SWs in the range of 1s. The question is why higher shock rates are less efficient than slower rates although SW-generated cavitation bubbles collapsed and disappeared long before consequent SWs are released.
The authors hypothesise the existence of micronized bubbles after collapse of previous cavitation bubbles lasting in the range of 1s, which is long enough to have an impact on subsequent SWs fired. The micronized bubbles are supposed to attenuate SWs and, in turn, reduce efficiency.
The goal the work is to apply a low amplitude acoustic pulse to actively remove pre-focal residual cavitation bubbles through forced coalescence and/or forcing the residual bubble nuclei to disperse from the propagation path away or to the targeted area before arrival of the next therapy pulse.
Although the mechanism of forced coalescence requires further clarifications the results of low amplitude acoustic bubble removal pulses, as reported in the paper, increases the efficiency of higher rate SWs close to superior values of low rate SW-delivery. The technique presented seems to lay the ground for faster and saver SWL in clinical applications. It is not yet clear how this approach can be matched with clinical requirements. The space required for the acoustic pulse generator e.g. would significantly reduce the focal distance of a clinical lithotripter and would limit the use for not to obese patients.