Fovargue DE, Mitran S, Smith NB, Sankin GN, Simmons WN, Zhong P

Department of Mathematics, University of North Carolina at Chapel Hill, 329 Phillips Hall, CB 3250, Chapel Hill, North Carolina 27599, USA

Abstract

A multiphysics computational model of the focusing of an acoustic pulse and subsequent shock wave formation that occurs during extracorporeal shock wave lithotripsy is presented. In the electromagnetic lithotripter modeled in this work the focusing is achieved via a polystyrene acoustic lens. The transition of the acoustic pulse through the solid lens is modeled by the linear elasticity equations and the subsequent shock wave formation in water is modeled by the Euler equations with a Tait equation of state. Both sets of equations are solved simultaneously in subsets of a single computational domain within the BEARCLAW framework which uses a finite-volume Riemann solver approach. This model is first validated against experimental measurements with a standard (or original) lens design. The model is then used to successfully predict the effects of a lens modification in the form of an annular ring cut. A second model which includes a kidney stone simulant in the domain is also presented. Within the stone the linear elasticity equations incorporate a simple damage model.

J Acoust Soc Am. 2013 Aug;134(2):1598-609. doi: 10.1121/1.4812881

PMID:23927200 [PubMed - in process]

PMCID:PMC3745489 [Available on 2014/8/1]

Othmar Wess
am Montag, 04. November 2013 14:45

Daniel Fovargue et al. developed a novel computational model to calculate the acoustic field of an electromagnetic lithotripter with focusing by aid of an acoustic lens. The transition of the acoustic pulse from the electromagnetic actuator through water, the solid lens and subsequent focusing in water requires demanding computational tools presented in this paper. The theoretical results are validated against experimental findings with a standard lithotripter lens configuration. So far the authors have done an excellent job establishing a basis for calculation of alternative lens designs and their predicted shock wave field parameters such as peak pressure, energy distribution and focal dimensions.

Obviously, the reason to look for an alternative lens design and according modified shock wave field is to improve lithotripter performance with respect to both, fragmentation efficiency and reduction of possible side effects. What,

however, is the target for an improved lens design? Which are the optimal shock wave field parameters? Here, I doubt about the starting point of the paper, the frequently repeated claim of a larger focal zone with lower peak pressure being the most important prerequisite for superior lithotripter performance. The popular rationale for a supposed superiority of the HM3, e.g., based on its larger focus is – in my mind – not substantiated if a detailed analysis of physical experiments and literature is done. This is not the place to do this analysis. If at all, possible superior clinical results of the HM3 may be, amongst others, due to simpler and fail-safe coupling in an open water bath compared to dry coupling by aid of a coupling cushion of modern lithotripters.

The second part of the paper presents a novel lens design which is supposed to improve lithotripter performance with respect to fragmentation efficiency and reduction of medical side effects. The paper does not clearly show either of the two aspired advantages. This is particulary remarkable since for comparison the new lens requires almost 50% increased electrical energy input in order to provide comparable acoustic field parameters in the focal area.

More important would be the reduction of side effects under equal fragmentation power. However, this could not be shown in this paper. It cannot be shown by simple in-vitro experiments or by theoretical considerations. The range of less than 1% significant hematoma in all types of lithotripters which cannot be handled by conservative means, requires large controlled clinical studies with large numbers of treatments to prove possible technical progress with respect to this parameter.

To summarize, the computational tools developed in this paper enable an excellent prediction of shock wave field parameters of alternative lens designs. It is still questionable and needs further clinical proof, whether the presented new lens design really holds the promised advantages.

Othmar Wess