Kobayashi K et al, 2011: Shock wave-bubble interaction near soft and rigid boundaries during lithotripsy: numerical analysis by the improved ghost fluid method
Kobayashi K, Kodama T, Takahira H
Division of Mechanical and Space Engineering, Faculty of Engineering, Hokkaido University, Kita 13, Nishi 8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan
In the case of extracorporeal shock wave lithotripsy (ESWL), a shock wave-bubble interaction inevitably occurs near the focusing point of stones, resulting in stone fragmentation and subsequent tissue damage. Because shock wave-bubble interactions are high-speed phenomena occurring in tissue consisting of various media with different acoustic impedance values, numerical analysis is an effective method for elucidating the mechanism of these interactions. However, the mechanism has not been examined in detail because, at present, numerical simulations capable of incorporating the acoustic impedance of various tissues do not exist. Here, we show that the improved ghost fluid method (IGFM) can treat shock wave-bubble interactions in various media. Nonspherical bubble collapse near a rigid or soft tissue boundary (stone, liver, gelatin and fat) was analyzed. The reflection wave of an incident shock wave at a tissue boundary was the primary cause for the acceleration or deceleration of bubble collapse. The impulse that was obtained from the temporal evolution of pressure created by the bubble collapse increased the downward velocity of the boundary and caused subsequent boundary deformation. Results of this study showed that the IGFM is a useful method for analyzing the shock wave-bubble interaction near various tissues with different acoustic impedance.
Phys Med Biol. 2011 Oct 7;56(19):6421-40. doi: 10.1088/0031-9155/56/19/016. Epub 2011 Sep 15
PMID: 21918295 [PubMed - in process]
Cavitation is an important phenomenon in shock wave lithotripsy (SWL) and is considered a major mechanism for tissue damage as well as for stone fragmentation. Shock waves generate cavitation bubbles which may collapse near acoustic interfaces such as water (urine)/soft tissue and water/stone boundaries by micro jet formation directed towards the interface. This process is numerically elucidated by Kobayashi et al. using an improved ghost fluid method (IGFM). This numerical method is applied on existing bubbles generated by previously fired shock waves and their forced collapse by successional shock waves near rigid (stone) or soft tissue boundaries (liver, fat, gelatine). The paper corresponds to the clinical situation of shock wave application with high repetition rate (e.g. 2 Hz, corresponding 500 ms). Since most cavitation bubbles may collapse by itself within 1000 ms the clinically relevant consequence is to perform SWL with lower repetition rates (