Biasiori-Poulanges L. et al., 2024: Cavitation cloud formation and surface damage of a model stone in a high-intensity focused ultrasound field.

Thursday, 15 August 2024

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Biasiori-Poulanges L, Lukić B, Supponen O.
Institute of Fluid Dynamics, Department of Mechanical and Process Engineering, ETH Zurich, Sonneggstrasse 3, Zurich 8092, Switzerland.
European Synchrotron Radiation Facility, CS 40220, Grenoble F-38043, France.
Institute of Fluid Dynamics, Department of Mechanical and Process Engineering, ETH Zurich, Sonneggstrasse 3, Zurich 8092, Switzerland.

Abstract

This work investigates the fundamental role of cavitation bubble clouds in stone comminution by focused ultrasound. The fragmentation of stones by ultrasound has applications in medical lithotripsy for the comminution of kidney stones or gall stones, where their fragmentation is believed to result from the high acoustic wave energy as well as the formation of cavitation. Cavitation is known to contribute to erosion and to cause damage away from the target, yet the exact contribution and mechanisms of cavitation remain currently unclear. Based on in situ experimental observations, post-exposure microtomography and acoustic simulations, the present work sheds light on the fundamental role of cavitation bubbles in the stone surface fragmentation by correlating the detected damage to the observed bubble activity. Our results show that not all clouds erode the stone, but only those located in preferential nucleation sites whose locations are herein examined. Furthermore, quantitative characterizations of the bubble clouds and their trajectories within the ultrasonic field are discussed. These include experiments with and without the presence of a model stone in the acoustic path length. Finally, the optimal stone-to-source distance maximizing the cavitation-induced surface damage area has been determined. Assuming the pressure magnitude within the focal region to exceed the cavitation pressure threshold, this location does not correspond to the acoustic focus, where the pressure is maximal, but rather to the region where the acoustic beam and thereby the acoustic cavitation activity near the stone surface is the widest.

Ultrason Sonochem. 2024 Jan;102:106738. doi: 10.1016/j.ultsonch.2023.106738. Epub 2023 Dec 22. PMID: 38150955 FREE PMC ARTICLE

Comments 1

Othmar Wess on Thursday, 15 August 2024 11:00

The mechanism of stone fragmentation in shock wave lithotripsy is still under discussion even more than 40 years after the first human treatment in 1980. Today. a two-step theory is widely used: First, direct spallation of shock wave energy (Hopkinson`s effect) for coarse fragmentation and second, fine fragmentation by cavitation (P. Zhong). While the first step is well explored, the second step, cavitation, is under intensive investigation.
The theoretical and experimental approach of the authors deals with the behaviour of cavitation bubbles generated by ultrasound fields (no shock waves). They suppose:
“Upon exposure to the negative phase of the pulsed shock wave
(i.e., the tensile component), cavitation bubbles are formed near the stone surface. These bubbles then scatter and absorb the energy of the
subsequent pulses until they violently collapse. The collapse of individual
bubbles may initiate a high-speed jet, which can develop towards
neighbouring boundaries, that is, in the direction of the stone, or in the
direction of the acoustic wave propagation”
In real lithotripsy procedures, however, the subsequent pulses are usually released with a repetition frequency of 0.5-3Hz. That means that the shock wave generated cavitation bubbles have collapsed already long before (within 1 ms,) the next shock wave is released.
Anyhow, the bubbles collapse violently under radiation of a tiny spherical shock wave, which has an impact on the fragmentation process and erode the surface of the stone. This effect alone would require extensive time to gain a complete fragmentation of a solid stone.
Under laboratory conditions we can control (enhance or reduce) cavitation by degassing of water or by taking tap water with more content of gas.. It is difficult under clinical conditions
The authors suppose, that cavitation is necessary and its contribution to fragmentation essential. However, dominant are mechanical forces created by direct impact of shock waves or ultrasound like in case shock wave lithotripsy and burst wave lithotripsy. Burst wave lithotripsy makes use of forces generated by interference bursts of ultrasound waves and is interrupted when significant cavitation occurs and the risk of tissue lesions rises.
Cavitation is regarded as beneficial and necessary for fragmentation but, on the other hand, cavitation is responsible for tissue lesions and puncturing small vessels with the risk of hematoma.
The paper investigates the behaviour of bubbles generated by ultrasound. Current lithotripters use shock waves instead of ultrasound, which may be similar to ultrasound, but in some regards significantly different. Burst wave lithotripsers use ultrasound burst waves, but there increased cavitation are regarded dangerous.
The authors show, how cavitation generated by ultrasound can improve erosion of stones, but I don’t see an impact on shock wave – or burst wave lithotripsy.
Future research should focus on best fragmentation and minimal tissue lesions. The question is not yet answered: Should cavitation be increased to enhance fragmentation or reduced to avoid tissue lesions?

Othmar Wess

The mechanism of stone fragmentation in shock wave lithotripsy is still under discussion even more than 40 years after the first human treatment in 1980. Today. a two-step theory is widely used: First, direct spallation of shock wave energy (Hopkinson`s effect) for coarse fragmentation and second, fine fragmentation by cavitation (P. Zhong). While the first step is well explored, the second step, cavitation, is under intensive investigation.
The theoretical and experimental approach of the authors deals with the behaviour of cavitation bubbles generated by ultrasound fields (no shock waves). They suppose: 
“Upon exposure to the negative phase of the pulsed shock wave
(i.e., the tensile component), cavitation bubbles are formed near the stone surface. These bubbles then scatter and absorb the energy of the
subsequent pulses until they violently collapse. The collapse of individual
bubbles may initiate a high-speed jet, which can develop towards
neighbouring boundaries, that is, in the direction of the stone, or in the
direction of the acoustic wave propagation”
In real lithotripsy procedures, however, the subsequent pulses are usually released with a repetition frequency of 0.5-3Hz. That means that the shock wave generated cavitation bubbles have collapsed already long before (within 1 ms,) the next shock wave is released. 
Anyhow, the bubbles collapse violently under radiation of a tiny spherical shock wave, which has an impact on the fragmentation process and erode the surface of the stone. This effect alone would require extensive time to gain a complete fragmentation of a solid stone. 
Under laboratory conditions we can control (enhance or reduce) cavitation by degassing of water or by taking tap water with more content of gas.. It is difficult under clinical conditions
The authors suppose, that cavitation is necessary and its contribution to fragmentation essential. However, dominant are mechanical forces created by direct impact of shock waves or ultrasound like in case shock wave lithotripsy and burst wave lithotripsy. Burst wave lithotripsy makes use of forces generated by interference bursts of ultrasound waves and is interrupted when significant cavitation occurs and the risk of tissue lesions rises. 
Cavitation is regarded as beneficial and necessary for fragmentation but, on the other hand, cavitation is responsible for tissue lesions and puncturing small vessels with the risk of hematoma.
The paper investigates the behaviour of bubbles generated by ultrasound. Current lithotripters use shock waves instead of ultrasound, which may be similar to ultrasound, but in some regards significantly different. Burst wave lithotripsers use ultrasound burst waves, but there increased cavitation are regarded dangerous. 
The authors show, how cavitation generated by ultrasound can improve erosion of stones, but I don’t see an impact on shock wave – or burst wave lithotripsy.
Future research should focus on best fragmentation and minimal tissue lesions. The question is not yet answered: Should cavitation be increased to enhance fragmentation or reduced to avoid tissue lesions?

Othmar Wess

The mechanism of stone fragmentation in shock wave lithotripsy is still under discussion even more than 40 years after the first human treatment in 1980. Today. a two-step theory is widely used: First, direct spallation of shock wave energy (Hopkinson`s effect) for coarse fragmentation and second, fine fragmentation by cavitation (P. Zhong). While the first step is well explored, the second step, cavitation, is under intensive investigation. The theoretical and experimental approach of the authors deals with the behaviour of cavitation bubbles generated by ultrasound fields (no shock waves). They suppose: “Upon exposure to the negative phase of the pulsed shock wave (i.e., the tensile component), cavitation bubbles are formed near the stone surface. These bubbles then scatter and absorb the energy of the subsequent pulses until they violently collapse. The collapse of individual bubbles may initiate a high-speed jet, which can develop towards neighbouring boundaries, that is, in the direction of the stone, or in the direction of the acoustic wave propagation” In real lithotripsy procedures, however, the subsequent pulses are usually released with a repetition frequency of 0.5-3Hz. That means that the shock wave generated cavitation bubbles have collapsed already long before (within 1 ms,) the next shock wave is released. Anyhow, the bubbles collapse violently under radiation of a tiny spherical shock wave, which has an impact on the fragmentation process and erode the surface of the stone. This effect alone would require extensive time to gain a complete fragmentation of a solid stone. Under laboratory conditions we can control (enhance or reduce) cavitation by degassing of water or by taking tap water with more content of gas.. It is difficult under clinical conditions The authors suppose, that cavitation is necessary and its contribution to fragmentation essential. However, dominant are mechanical forces created by direct impact of shock waves or ultrasound like in case shock wave lithotripsy and burst wave lithotripsy. Burst wave lithotripsy makes use of forces generated by interference bursts of ultrasound waves and is interrupted when significant cavitation occurs and the risk of tissue lesions rises. Cavitation is regarded as beneficial and necessary for fragmentation but, on the other hand, cavitation is responsible for tissue lesions and puncturing small vessels with the risk of hematoma. The paper investigates the behaviour of bubbles generated by ultrasound. Current lithotripters use shock waves instead of ultrasound, which may be similar to ultrasound, but in some regards significantly different. Burst wave lithotripsers use ultrasound burst waves, but there increased cavitation are regarded dangerous. The authors show, how cavitation generated by ultrasound can improve erosion of stones, but I don’t see an impact on shock wave – or burst wave lithotripsy. Future research should focus on best fragmentation and minimal tissue lesions. The question is not yet answered: Should cavitation be increased to enhance fragmentation or reduced to avoid tissue lesions? Othmar Wess

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