Han CS et al., 2019: Dual-frequency setting for urinary stone fragmentation during shock wave lithotripsy: an in vitro study
Han CS, Vetter JM, Endicott R, Chevinsky M, Zafar A, Venkatesh R.
Washington University School of Medicine, St. Louis, MO, USA.
Midwest Stone Institute, St. Louis, MO, USA.
Washington University School of Medicine, St. Louis, MO, USA.
Extracorporeal shock wave lithotripsy (SWL) is less invasive compared to the other invasive modalities of stone treatment that are gaining popularity. Hence, methods to improve the efficacy of SWL are desirable. We studied the effectiveness of dual frequency on the efficacy of stone fragmentation, but minimizing treatment time. A phantom 10 mm spherical BegoStone was fragmented in vitro in a kidney model using an electromagnetic lithotripter (Storz MODULITH®SLX-F2). A total of 78 stones were fragmented each with 3000 shocks at 60 Hz or 120 Hz or a dual frequency (DF) of 60-120 Hz. For the DF setting, the first 1000 shocks were delivered at 60 Hz and the next 2000 at 120 Hz. Total weight and number of significant fragments of > 3 mm (TWSF and TNSF, respectively) and also > 2 mm was measured. Results: The mean TWSF was 0.1, 0.16, and 0.08 g for 60 Hz, 120 Hz, and DF 60-120 Hz, respectively. The TWSF of DF 60-120 Hz was significantly lower than that of 120 Hz (p = 0.02), but same as the 60 Hz (p = 0.32). The mean TNSF of > 3 mm was 2.6, 3.0, and 2.0 for 60 Hz, 120 Hz, and DF 60-120 Hz, respectively, without significant differences between each setting. However, increasing trend of TWSF, TW2 mm and TN2 mm was seen in the order of DF, 60 Hz and 120 Hz (p = 0.019, p = 0.004 and 0.017, respectively). Treatment time for 60 Hz, 120 Hz, and DF 60-120 Hz was 50, 25, and 34 min, respectively. Dual-frequency setting produced effective stone fragmentation compared to the recommended 60 Hz, while decreasing treatment time. DF variation is one other factor that may be tailored for effective stone comminution and needs clinical evaluation.
Urolithiasis. 2019 Oct 17. doi: 10.1007/s00240-019-01162-w. [Epub ahead of print]
I wonder how the authors developed the hypothesis evaluated in the experiments. I find the theory formulated by them not very conclusive: “… once enough crack lines through which damage is spread from surface into the core of the stone are formed at lower frequency rate , we hypothesize that subsequent shock waves at a higher firing rate are sufficient to complete further fragmentation of the stone into passable smaller fragments (i.e. fragments smaller than 2 or 3 mm in sizes).”
Otherwise this is a simple, nice study, well done with practical consequences.
1 Zhu S, Cocks FH, Preminger GM et al (2002) The role of stress waves and cavitation in stone comminution in shock wave lithotripsy.
Ultrasound Med Biol 28(5):661–671
The study is one of several other studies confirming the strategy of slow (60 shocks per min) vs. fast (120 shocks per min) shock wave delivery for better fragmentation efficacy. (I assume the indication 60 Hz and 120 Hz throughout the whole paper is an error since the lithotripter involved in the study cannot deliver shock waves in such high frequencies. The numbers should be taken as 60 min-1 and 120 min-1 accordingly).
Although the fragmentation results at DF (Dual Frequency) and 60 min-1 are superior, a faster shock wave rate is preferable since it can significantly reduce the treatment time, a desirable feature in clinical routine. If we know the reason why fast shock wave delivery is inferior to slow rates, we may find means to improve fragmentation efficiency and speed up delivery rates simultaneously.
Several authors look at cavitation bubbles as the main reason. Shock waves usually generate cavitation bubbles in the focal zone of a lithotripter and persisting bubbles may attenuate subsequent shockwaves by absorption and scatter. However, the bubbles generated by the tensile part of a shock wave grow and collapse within ≤ 1 millisecond (ms), which means, they fade away long before the next shock wave arrives 500-1000 ms later. An explanation published by Tamaddoni et al. (2019)  focuses on collapsing cavitation bubbles which create (invisible?) micro-nuclei persisting seconds (not ms) giving rise to an increased production of bubbles by the subsequent shock wave which, in turn, draws energy out of the tensile part of the shock wave. Accordingly, the following shock waves are less powerful and fragmentation efficacy is diminished. They reported that a so called bubble coalescing low amplitude acoustic pulse could remove the long persistent secondary bubbles and increase fragmentation efficacy vitro. The bubble removing acoustic pulse is hypothesized to mitigate shielding by forced bubble coalescence and dispersion. This mechanism is not jet well understood.
Pishchalnikov et al. (2006)  identify small particles released by shock waves from a artificial stone model persisting seconds in the vicinity of the shock wave focus. These particles may act as cavitation nuclei for the subsequent shock waves and generation of shielding cavitation bubbles.
Our own fragmentation experiments (see Fig.1) (Wess and Mayer 2018)  confirm particles being expelled from the stone surface that persist for seconds until they follow gravity and move out of the focal zone. Those particles may directly shield subsequent shock waves or may draw energy out of the tensile wave as hypothesized by Pishchalnikov .
Fig 1: A 1 cm cube of Plaster of Paris is hit by a shock wave from the right. The stone is suspended by two filaments and can move horizontally. It is pushed a few centimetres out of its resting position (white bar) by momentum transfer from the shock wave to the stone. The picture is taken approximately 250 ms after shock wave impact and shows small particles and dust floating in the focal area. Cavitation bubbles are not visible in this picture since they collapsed long before (≤ 1 ms).
If the expelled and floating particles are the reason for shielding and reduction of fragmentation efficacy, lower shock wave delivery rates may benefit from the extended time span for clearing the medium from the particles by gravity and enhance efficacy compared to higher rates.
Active removal of the particles between subsequent shocks may be done by adding a flow to the medium to drain the particles out of the focal zone. A low amplitude acoustic pulse may have a similar effect on particles to push them out of the focal area and clean the medium before the next shock wave is delivered .
We should not forget that fragmentation processes feature beside delivery rates significant statistical fluctuations due to various reasons such coupling, energy settings, quality of localization etc. The DF strategy developed in the paper, however, seems to be a practical method to shorten treatment time without affecting fragmentation efficacy.
 Hedieh Alavi Tamaddoni, William W. Roberts, and Timothy L. Hall Enhanced shockwave lithotripsy with active cavitation mitigation. Citation: The Journal of the Acoustical Society of America146, 3275 (2019); doi: 10.1121/1.5131649View online: https://doi.org/10.1121/1.5131649View Table of Contents: https://asa.scitation.org/toc/jas/146/5Published by the Acoustical Society of America
 Yuri A. Pishchalnikov, James A. McAteer, James C. Williams Jr., Irina V. Pishchalnikova, and R. Jason VonDerHaar
Why stones break better at slow shock wave rate than at fast rate: In vitro study with a research electrohydraulic lithotripter
J Endourol. 2006 August ; 20(8): 537–541.
 Wess, O.J., Mayer, J. Fragmentation of brittle material by shock wave lithotripsy. Momentum transfer and inertia: a novel view on fragmentation mechanisms. Urolithiasis 48, 137–149 (2020). https://doi.org/10.1007/s00240-018-1102-6