Li M. et al., 2021: Tri-modality cavitation mapping in shock wave lithotripsy
Li M, Sankin G, Vu T, Yao J, Zhong P.
Department of Biomedical Engineering, Duke University, Durham, North Carolina 27708, USA.
Department of Mechanical Engineering and Materials Science, Duke University, Durham, North Carolina 27708, USA.
Shock wave lithotripsy (SWL) has been widely used for non-invasive treatment of kidney stones. Cavitation plays an important role in stone fragmentation, yet it may also contribute to renal injury during SWL. It is therefore crucial to determine the spatiotemporal distributions of cavitation activities to maximize stone fragmentation while minimizing tissue injury. Traditional cavitation detection methods include high-speed optical imaging, active cavitation mapping (ACM), and passive cavitation mapping (PCM). While each of the three methods provides unique information about the dynamics of the bubbles, PCM has most practical applications in biological tissues. To image the dynamics of cavitation bubble collapse, we previously developed a sliding-window PCM (SW-PCM) method to identify each bubble collapse with high temporal and spatial resolution. In this work, to further validate and optimize the SW-PCM method, we have developed tri-modality cavitation imaging that includes three-dimensional high-speed optical imaging, ACM, and PCM seamlessly integrated in a single system. Using the tri-modality system, we imaged and analyzed laser-induced single cavitation bubbles in both free field and constricted space and shock wave-induced cavitation clusters. Collectively, our results have demonstrated the high reliability and spatial-temporal accuracy of the SW-PCM approach, which paves the way for the future in vivo applications on large animals and humans in SWL.
J Acoust Soc Am. 2021 Feb;149(2):1258. doi: 10.1121/10.0003555. PMID: 33639826
Cavitation is a well-known phenomenon in SWL and is hypothesized to cause tissue lesions. On the other hand, cavitation is made responsible for fine fragmentation of kidney stones and thus a welcome tool in SWL. From this point of view, the question “of what to do with cavitation” requires a balanced answer, which is not given by the authors. Their approach starts a step before and puts a lot of effort on recognizing, visualizing and analysing single laser generated cavitation bubbles and small clusters shock wave generated bubbles. They monitor the time history of generation, expansion and collapse of each single bubble by three different experimental modalities, high speed optical imaging, passive cavitation mapping (PCM) by recording acoustical signals from collapsing bubbles and active cavitation mapping (ACM) by visualizing bubbles by modified ultrasound imaging techniques. PCM and ACM have the potential to be integrated in a clinical shock wave lithotripter whereas the optical method is restricted to laboratory experiments due opacity of the human tissue to visible light and bulky optical set ups including high-speed cameras.
The authors could demonstrate a high spatial-temporal accuracy of the life cycle of distinct cavitation bubbles and developed tools to visualize and recognize bubble fields as expected in real lithotripter devices. The final goal of the authors: “…to determine the spatiotemporal distributions of cavitation activities to maximize stone fragmentation while minimizing tissue injury” is a challenging task requiring innovative and intelligent answers, since (according to the introduction statement) reduction of cavitation would reduce fragmentation efficiency simultaneously. The solution to that problem might be a bit easier if the previously made statement: “cavitation plays an important role in stone fragmentation” could be replaced by the term: “cavitation plays a minor role in stone fragmentation.”
Assuming that this true, one may look for possibilities to reduce cavitation and minimize tissue lesions without affecting fragmentation efficiency. The paper: 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, indicates, that the role of cavitation is of minor importance in SWL. Accordingly, reduction of cavitation, if possible, will not hamper fragmentation efficiency. Further research should focus on technical solutions to reduce cavitation.