Wang JC et al, 2014: Suppressing bubble shielding effect in shock wave lithotripsy by low intensity pulsed ultrasound.
Wang JC, Zhou Y
School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore
Extracorporeal shock wave lithotripsy (ESWL) has been used as an effective modality to fragment kidney calculi. Because of the bubble shielding effect in the pre-focal region, the acoustic energy delivered to the focus is reduced. Low pulse repetition frequency (PRF) will be
applied to dissolve these bubbles for better stone comminution efficiency. In this study, low intensity pulsed ultrasound (LIPUS) beam was aligned perpendicular to the axis of a shock wave (SW) lithotripter at its focus. The light transmission was used to evaluate the compressive wave and cavitation induced by SWs without or with a combination of LIPUS for continuous sonication. It is found that bubble shielding effect becomes dominated with the SW exposure and has a greater significant effect on cavitation than compressive wave. Using the combined wave scheme, the improvement began at the 5th pulse and gradually increased. Suppression effect on bubble shielding is independent on the trigger delay, but increases with the acoustic intensity and pulse duration of LIPUS. The peak negative and integral area of light transmission signal, which present the compressive wave and cavitation respectively, using our strategy at PRF of 1Hz are comparable to those using SW alone at PRF of 0.1Hz. In addition, high-speed photography confirmed the bubble activities in both free field and close to a stone surface. Bubble motion in response to the acoustic radiation force by LIPUS was found to be the major mechanism of suppressing bubble shielding effect. There is a 2.6-fold increase in stone fragmentation efficiency after 1000 SWs at PRF of 1Hz in combination with LIPUS. In summary, combination of SWs and LIPUS is an effective way of suppressing bubble shielding effect and, subsequently, improving cavitation at the focus for a better outcome.
Ultrasonics. 2015 Jan;55:65-74. doi: 10.1016/j.ultras.2014.08.004. Epub 2014 Aug 19
Gas bubbles and cavitation bubbles in particular have a significant impact on shock wave propagation. The energy of shock waves passing clouds of bubbles will be reduced considerably. Pre focal bubble clouds will decrease fragmentation efficiency (bubble shielding effect) On the other hand, shock waves will generate cavitation bubbles by tensile waves predominantly in the focal region. Those cavitation bubbles close to stone surface contribute to fragmentation by impact of microjets of the collapsing bubble towards the stone surface. In order to reduce the bubble shielding effect, usually degased water is used for shock wave transmission. The authors intend to reduce bubble shielding in the pre-focal area to avoid energy loss in the focal area.
They elaborate the effect of bubbles in the blast path of a piezoelectric shock wave source which is not designed for lithotripsy but for orthopaedic applications. With a focal length of only 40 mm kidney stones, usually, cannot be focused. It is not clear, what type of bubbles may be reduced by LIPUS. Cavitation bubbles generated by shock waves collapse or dissolve, according to the authors, within 300 µs. “No visible bubble was found after 300 µs” Therefore 1000 ms later (pulse repetition time) no cavitation bubble should exist anyhow. Nevertheless fragmentation could be increased 2.6 fold.
We do not understand the mechanism behind and if at all, how the LIPUS- technique could be implemented in a clinical SWL device. An explanation by the authors would be appreciated.