Moghimnezhad M. et al., 2021: Multiphysics Analysis of Ultrasonic Shock Wave Lithotripsy and Side Effects on Surrounding Tissues.
Moghimnezhad M, Shahidian A, Andayesh M.J.
MSc, Department of Mechanical Engineering, K. N . Toosi University of Technology, Tehran, Iran.
PhD, Department of Mechanical Engineering, K. N. Toosi University of Technology, Tehran, Iran.
BSc, Department of Mechanical Engineering, K. N. Toosi University of Technology, Tehran, Iran.
Background: Today, the most common method for kidney stone therapy is extracorporeal shock wave lithotripsy. Current research is a numerical simulation of kidney stone fragmentation via ultrasonic shock waves. Most numerical studies in lithotripsy have been carried out using the elasticity or energy method and neglected the dissipation phenomenon. In the current study, it is solved by not only the linear acoustics equation, but also the Westervelt acoustics equation which nonlinearity and dissipation are involved.
Objective: This study is to compare two methods for simulation of shock wave lithotripsy, clarifying the effect of shock wave profiles and stones' material, and investigating side effects on surrounding tissues.
Material and methods: Computational study is done using COMSOL Multiphysics, commercial software based on the finite element method. Nonlinear governing equations of acoustics, elasticity and bioheat-transfer are coupled and solved.
Results: A decrease in the rise time of shock wave leads to increase the produced acoustic pressure and enlarge focus region. The shock wave damages kidney tissues in both linear and nonlinear simulation but the damage due to high temperature is very negligible compared to the High Intensity Focused Ultrasound (HIFU).
Conclusion: Disaffiliation of wave nonlinearity causes a high incompatibility with reality. Stone's material is an important factor, affecting the fragmentation.
Biomed Phys Eng. 2021 Dec 1;11(6):701-712. doi: 10.31661/jbpe.v0i0.1182. eCollection 2021 Dec. PMID: 34904067. FREE ARTICLE
The authors investigate propagation and focussing of shock waves as used in lithotripsy applications. They compare theoretical approaches of a linear computational method with a nonlinear method including heat dissipation factors. The different results are applied on the impact on two types of kidney stones, Calcium Oxalate Monohydrate (COM) and Uric acid stones.
Since shock waves feature essential non-linear effects, the according non-linear calculation results, as expected, in a better description of the real shock wave behaviour. Another interesting result of the calculated heat dissipation is, again as expected, negligible compared to high intensity ultrasound (HIFU)
If I understand the conclusion regarding the side effects to the surrounding tissue correctly, the authors claim, that a larger focus may cause increased side effects because shock waves may pass the stone and affect adjacent tissue. Some people argue the opposite, that a larger focus may be favourable because of a reduced peak pressure and lower energy density, which reduces tissue lesions. Unfortunately, fragmentation efficiency is also reduced. It is an ongoing discussion around. Under normal clinical conditions, we cannot avoid shock waves passing the stone impinging soft tissue in the surrounding of the stone. However, the effect of shockwaves is minor on soft tissue compared to hard stones or bones due to the different reflexion conditions. Based on the mismatch of the acoustic impedances reflexion of shock waves at soft tissue interfaces generate minor forces compared to reflexion at hard interfaces given by soft tissue and stones.