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During clinical procedures, the lithotripter shock wave (LSW) that is incident on the stone and resultant stress field is often asymmetric due to the respiratory motion of the patient. The variations of the LSW-stone interaction and associated fracture pattern were investigated by photoelastic imaging, phantom experiments, and three-dimensional fluid-solid interaction modeling at different lateral locations in a lithotripter field. In contrast to a T-shaped fracture pattern often observed in the posterior region of the disk-shaped stone under symmetric loading, the fracture pattern gradually transitioned to a tilted L-shape under asymmetric loading conditions. Moreover, the model simulations revealed the generation of surface acoustic waves (SAWs), i.e., a leaky Rayleigh wave on the anterior boundary and Scholte wave on the posterior boundary of the stone. The propagation of SAWs on the stone boundary is accompanied by a progressive transition of the LSW reflection pattern from regular to von Neumann and to weak von Neumann reflection near the glancing incidence and, concomitantly, the development and growth of a Mach stem, swirling around the stone boundary. The maximum tensile stress and stress integral were produced by SAWs on the stone boundary under asymmetric loading conditions, which drove the initiation and extension of surface cracks into the bulk of the stone that is confirmed by micro-computed tomography analysis.

A damage model suggested by the Tuler–Butcher concept of dynamic accumulation of microscopic defects is obtained from experimental data on microcrack formation in synthetic kidney stones. Experimental data on appearance of microcracks is extracted from micro-computed tomography images of BegoStone simulants obtained after subjecting the stone to successive pulses produced by an electromagnetic shock-wave lithotripter source. Image processing of the data is used to infer statistical distributions of crack length and width in representative transversal cross-sections of a cylindrical stone. A high-resolution finite volume computational model, capable of accurately modeling internal reflections due to local changes in material properties produced by material damage, is used to simulate the accumulation of damage due to successive shocks. Comparison of statistical distributions of microcrack formation in computation and experiment allows calibration of the damage model. The model is subsequently used to compute fracture of a different aspect-ratio cylindrical stone predicting concurrent formation of two main fracture areas as observed experimentally.

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.

In shock wave lithotripsy (SWL), acoustic pulses with a leading compression wave followed by a tensile wave are delivered into the patient’s body using a water-filled coupling cushion. Cavitation-related acoustic energy loss in the coupling unit depends critically on water conditions, e.g. dissolved gas concentration and exchange flow rate. We have systematically investigated the attenuation mechanism in the coupling water via pressure measurements and cavitation characterization. In non-degassed water the bubble cluster became progressively dense (i.e., proliferated because of gas diffusion into bubbles and splitting of bubbles into many daughter bubbles) in shock waves delivered at 1 Hz leading to reduction in the tensile wave duration from a nominal value of 4.6 to 1.8 µs. To reduce cavitation in the coupling water along the beam path, we have used a continuous jet flow to remove residual daughter bubbles between consecutive shocks. As a result, stone fragmentation efficiency was increased from 16±4% t...

Carefully timed tandem microbubbles have been shown to produce directional and targeted membrane poration of individual cells in microfluidic systems, which could be of use in ultrasound-mediated drug and gene delivery. This study aims at contributing to the understanding of the mechanisms at play in such an interaction. The dynamics of single and tandem microbubbles between two parallel plates is studied numerically and analytically. Comparisons are then made between the numerical results and the available experimental results. Numerically, assuming a potential flow, a three-dimensional boundary element method (BEM) is used to describe complex bubble deformations, jet formation, and bubble splitting. Analytically, compressibility and viscous boundary layer effects along the channel walls, neglected in the BEM model, are considered while shape of the bubble is not considered. Comparisons show that energy losses modify the bubble dynamics when the two approaches use identical initial conditions. The initial conditions in the boundary element method can be adjusted to recover the bubble period and maximum bubble volume when in an infinite medium. Using the same conditions enables the method to recover the full dynamics of single and tandem bubbles, including large deformations and fast re-entering jet formation. This method can be used as a design tool for future tandem-bubble sonoporation experiments.

Previous studies have demonstrated that stone comminution decreases with increased pulse repetition frequency as a result of bubble proliferation in the cavitation field of a shock wave lithotripter (Pishchalnikov et al 2011 J. Acoust. Soc. Am. 130 EL87-93). If cavitation nuclei remain in the propagation path of successive lithotripter pulses, especially in the acoustic coupling cushion of the shock wave source, they will consume part of the incident wave energy, leading to reduced tensile pressure in the focal region and thus lower stone comminution efficiency. We introduce a method to remove cavitation nuclei from the coupling cushion between successive shock exposures using a jet of degassed water. As a result, pre-focal bubble nuclei lifetime quantified by B-mode ultrasound imaging was reduced from 7 to 0.3 s by a jet with an exit velocity of 62 cm s(-1). Stone fragmentation (percent mass2 mm) after 250 shocks delivered at 1 Hz was enhanced from 22 ± 6% to 33 ± 5% (p = 0.007) in water without interposing tissue mimicking materials. Stone fragmentation after 500 shocks delivered at 2 Hz was increased from 18 ± 6% to 28 ± 8% (p = 0.04) with an interposing tissue phantom of 8 cm thick. These results demonstrate the critical influence of cavitation bubbles in the coupling cushion on stone comminution and suggest a potential strategy to improve the efficacy of contemporary shock wave lithotripters.

The mechanism responsible for the development of autocyclic circular breakdowns in electrolyte as a result of the electric discharge within a hole in a dielectric plate has been experimentally studied. It is demonstrated that the breakdowns are caused by an axisymmetric electrohydrodynamic process involving the formation of a circular bubble breakdown, which acts as a current interrupter in the stage of expansion and as a switch in the stage of collapse, which is followed by the circular breakdown. On the whole, a self-adjusting antiphase process of bubble generation dynamics is developed in the system, whereby each preceding bubble collapses, in the field of pressure due to the subsequent bubble breakdown. This collapse proceeds at an accelerated rate and is accompanied by the fractionation of bubbles.

The early stage of the dynamics of interacting cavitation bubbles in a transmitted spherically focused pulsed wave consisting of compression and rarefaction phases is studied. The cavitation is investigated away from the liquid boundaries for negative pressures up to −42 MPa with a decrease rate of −40 MPa/μs by high-speed microscopic filming (100 million frames per second and a spatial resolution of 5–50μm/pixel) and pressure measurements. It is demonstrated that, according to the kinematics of size variation, the bubbles are separated into two fractions: expanding and collapsing ones. Experimental data provide grounds to assume that the formation of a two-fraction bubble cluster occurs on account of different threshold values of pressure in the rarefaction wave for two characteristic sizes of nuclei, namely, micronuclei (d I < 10μm) and nanonuclei (“bubstons,” d II ∼ 1 nm), which become detectable only when the rarefaction wave amplitude exceeds the critical one. Pulsed compression of small bubbles in the process of transformation of a rarefaction wave into a compression wave occurs under the effect of a cluster’s internal pressure up to 20 MPa and proceeds with the conservation of the spherical shape of the second-fraction bubbles. It is demonstrated that the velocity of the center of mass of a bubble reaches its peak value close to the moment of bubble collapse. The translational and radial dynamics of a bubble are studied in a numerical experiment using the Rayleigh-Plesset equation and taking into account the viscous force and the pressure gradient. The results of measuring the translational shift may be useful for estimating the minimum bubble radius by comparing the numerical results with the experiment.

To elucidate mechanisms of cavitation action on a surface with microcapillary discontinuity and refine the model of the sonocapillary effect, the force acting on a cylinder is correlated with the height of a liquid in the capillary under cavitation, both quantities being measured at the same point of the ultrasonic field. It is found that the force acting on the cylinder is directed toward a cavitation cluster stabilized at the end face of the cylinder. This force can be enhanced with another cylinder placed on the side of the cluster coaxially with the first one. The dynamics of the cavitation cluster depending on the cylinder spacing is investigated in the case when one of the cylinders is on a pendulum suspension. The conditions for self-sustained oscillations of the pendulum are found. The dependence of the attracting force between the cylinders on the ultrasonic frequency and cylinder spacing is derived. The end-face-averaged pressure exerted on a cylinder of diameter 1.2 mm may reach 0.16 kPa, and the intracapillary pressure attains 0.89 kPa. Thus, the sonocapillary effect may be explained, at least partially, by a counterpressure arising from cluster-capillary interaction. This effect can be used in designing cavitation sensors and ultrasonic sources.

The application of an electric discharge at numerous holes in a dielectric diaphragm for generating spherically focused shock waves in a salt solution is described. The design of the presented transducer makes the erosion of the electrodes impossible, since they are placed beyond the discharge zone; as a result, the point of focus stabilizes. The dependences on the discharge voltage, the number and total area of the holes in the diaphragm, and the electrolyte concentration are obtained for the pressure-wave amplitude at the focus; the extreme values are ∼40 and –12 MPa in the compression and rarefaction waves, respectively. The possibility of comminuting stones with masses of

The dynamics of luminescence generation and cavitation in the field of spherically focused and plane shock-acoustic waves in water, glycerin, and their mixtures is studied. It is shown that in the spherical focusing of shock-acoustic waves, the first pulse of luminescence correlates with cavitational rupture of the liquid. It is assumed that the registered luminescence is caused by electrokinetic processes upon formation and growth of cavitational bubbles.

Previous studies have demonstrated that treatment strategy plays a critical role in ensuring maximum stone fragmentation during shockwave lithotripsy (SWL). We aimed to develop an optimal treatment strategy in SWL to produce maximum stone fragmentation.Four treatment strategies were evaluated using an in-vitro experimental setup that mimics stone fragmentation in the renal pelvis. Spherical stone phantoms were exposed to 2100 shocks using the Siemens Modularis (electromagnetic) lithotripter. The treatment strategies included increasing output voltage with 100 shocks at 12.3 kV, 400 shocks at 14.8 kV, and 1600 shocks at 15.8 kV, and decreasing output voltage with 1600 shocks at 15.8 kV, 400 shocks at 14.8 kV, and 100 shocks at 12.3 kV. Both increasing and decreasing voltages models were run at a pulse repetition frequency (PRF) of 1 and 2 Hz. Fragmentation efficiency was determined using a sequential sieving method to isolate fragments less than 2 mm. A fiberoptic probe hydrophone was used to characterize the pressure waveforms at different output voltage and frequency settings. In addition, a high-speed camera was used to assess cavitation activity in the lithotripter field that was produced by different treatment strategies.The increasing output voltage strategy at 1 Hz PRF produced the best stone fragmentation efficiency. This result was significantly better than the decreasing voltage strategy at 1 Hz PFR (85.8% vs 80.8%, P=0.017) and over the same strategy at 2 Hz PRF (85.8% vs 79.59%, P=0.0078).A pretreatment dose of 100 low-voltage output shockwaves (SWs) at 60 SWs/min before increasing to a higher voltage output produces the best overall stone fragmentation in vitro. These findings could lead to increased fragmentation efficiency in vivo and higher success rates clinically.

The dynamic interaction between laser-generated tandem bubble and individual polystyrene particles of 2 and 10 μm in diameter is studied in a microfluidic channel (25 μm height) by high-speed imaging and particle image velocimetry. The asymmetric collapse of the tandem bubble produces a pair of microjets and associated long-lasting vortices that can propel a single particle to a maximum velocity of 1.4 m/s in 30 μs after the bubble collapse with a resultant directional displacement up to 60 μm in 150 μs. This method may be useful for high-throughput cell sorting in microfluidic devices.

The dynamic interaction of a shockwave (modelled as a pressure pulse) with an initially spherically oscillating bubble is investigated. Upon the shockwave impact, the bubble deforms non-spherically and the flow field surrounding the bubble is determined with potential flow theory using the boundary-element method (BEM). The primary advantage of this method is its computational efficiency. The simulation process is repeated until the two opposite sides of the bubble surface collide with each other (i.e. the formation of a jet along the shockwave propagation direction). The collapse time of the bubble, its shape and the velocity of the jet are calculated. Moreover, the impact pressure is estimated based on water-hammer pressure theory. The Kelvin impulse, kinetic energy and bubble displacement (all at the moment of jet impact) are also determined. Overall, the simulated results compare favourably with experimental observations of lithotripter shockwave interaction with single bubbles (using laser-induced bubbles at various oscillation stages). The simulations confirm the experimental observation that the most intense collapse, with the highest jet velocity and impact pressure, occurs for bubbles with intermediate size during the contraction phase when the collapse time of the bubble is approximately equal to the compressive pulse duration of the shock wave. Under this condition, the maximum amount of energy of the incident shockwave is transferred to the collapsing bubble. Further, the effect of the bubble contents (ideal gas with different initial pressures) and the initial conditions of the bubble (initially oscillating vs. non-oscillating) on the dynamics of the shockwave-bubble interaction are discussed.

Understanding the dynamics of bubble oscillation in tissue-constrained media such as within blood vessels is important for many current and potential therapeutic ultrasound applications. Cavitation is a primary mechanism responsible for vessel rupture and tissue injury in shock wave lithotripsy [1]. In sonoporation cavitation can be used to increase permeability of biological membranes. Particularly, ultrasound contrast agents are widely used for imaging of blood vessels and for enhancement of ultrasound-mediated gene delivery [2]. Modeling of non-linear oscillations of bubbles in acrylic capillaries in a high-intensity focused ultrasound field revealed a clear dependence of bubble displacement and fragmentation on tube diameter [3]. However, the effect of elastic boundary on bubble dynamics may differ significantly from that of a rigid boundary [4, 5]. In this study, experimental investigation of the dynamics of bubble oscillation in an elastic tube was performed and preliminary results from tubes of different inner diameters are presented.Copyright © 2007 by ASME

The interaction of a lithotripter shock wave (LSW) with laser-generated single vapor bubbles in water is investigated using high-speed photography and pressure measurement via a fiber-optic probe hydrophone. The interaction leads to nonspherical collapse of the bubble with secondary shock wave emission and microjet formation along the LSW propagation direction. The maximum pressure amplification is produced during the collapse phase of the bubble oscillation when the compressive pulse duration of the LSW matches with the forced collapse time of the bubble.

A previously introduced model for stone comminution in shock wave lithotripsy is extended to include damage produced by cavitation. At the macroscopic, continuum level a 3D elasticity model with time-varying material constants capturing localized damage provides the overall stress field within kidney stone simulants. Regions of high stress are identified and a mesoscopic crack propagation is used to dynamically update localized damage. The crack propagation model in turn is linked with a microscopic grain dynamics model. Continuum stresses and surface pitting is provided by a multiscale cavitation model (see related talk). The overall procedure is capable of tracking stone fragments and surface cavitation of the fragments through several levels of breakdown. Computed stone fragment distributions are compared to experimental results. [Work supported by NIH through 5R37DK052985-18.]

Cavitation damage is responsible for initial pitting of kidney stone surfaces, damage that is thought to play an important role in shock wave lithotripsy. We introduce a multiscale model of the formation of cavitation bubbles in water, and subsequent breakdown. At a macroscopic, continuum scale cavitation is modeled by the 3D Euler equations with a Tait equation of state. Adaptive mesh refinement is used to provide increased resolution at the liquid/vapor boundary. Cells with both liquid and vapor phases are flagged by the continuum solver for mesoscale, kinetic modeling by a lattice Boltzmann description capable of capturing non-equilibrium behavior (e.g., phase change, energetic jet impingement). Isolated and interacting two-bubble configurations are studied. Computational simulation results are compared with high-speed experimental imaging of individual bubble dynamics and bubble–bubble interaction. The model is used to build a statistical description of multiple-bubble interaction, with input from cavitation cloud imaging. [Work supported by NIH through 5R37DK052985-18.]

We will present a computational model of the focusing of an electromagnetically induced pressure pulse by an acoustic lens and subsequent shock wave formation. Linear elasticity equations for the lens are solved simultaneously with Euler hydrodynamic equations for water considered to be a compressible medium with a Tait equation of state. The adaptive mesh refinement model allows both three‐dimensional and two‐dimensional axisymmetric computations. A number of coupling approaches at the lens‐water interface are investigated by comparison to experimental results: transfer only of pressure boundary condition, coupling of displacement velocities, and a buffer linear elasticity region in the water immediately adjacent to lens. The model is validated against single‐medium measurements (water or lens material), and the complete experimental shock wave formation process. Initial results from a crack propagation model in stone simulants placed at the lithotripter focus will also be presented.

A method to characterize shock wave lithotripters by examining the potential for cavitation associated with the lithotripter shock wave (LSW) has been developed. The method uses the maximum radius achieved by a bubble subjected to a LSW as a representation of the cavitation potential for that region in the lithotripter. It is found that the maximum radius is determined by the work done on a bubble by the LSW. The method is used to characterize two reflectors: an ellipsoidal reflector and an ellipsoidal reflector with an insert. The results show that the use of an insert reduced the -6 dB volume (with respect to peak positive pressure) from 1.6 to 0.4 cm(3), the -6 dB volume (with respect to peak negative pressure) from 14.5 to 8.3 cm(3), and reduced the volume characterized by high cavitation potential (i.e., regions characterized by bubbles with radii larger than 429 µm) from 103 to 26 cm(3). Thus, the insert is an effective way to localize the potentially damaging effects of shock wave lithotripsy, and suggests an approach to optimize the shape of the reflector.

The interaction of laser‐generated single cavitation bubbles (maximum diameter Dmax=1.34 mm) near a silicon rubber membrane with a shock wave (39–75 MPa in peak pressure and 1.7 μs in compressive pulse duration) is investigated. The interaction leads to directional, forced collapse of the bubble with microjet formation along shock wave propagation direction. Maximum jet penetration into the membrane is produced during the bubble collapse phase with optimal shock wave arrival time and stand‐off distance s, a condition that ensures that the forced collapse time of the bubble matches with the compressive duration of the shock wave to generate presumably the maximum axial jet velocity toward the boundary. A maximum penetration depth of 0.5 mm is observed at dimensionless proximity γ (=2s/Dmax)=0.7 when the bubble establishes contact with the boundary during expansion. At larger γ (≥1.2), jet impact is significantly attenuated by the interposing water layer separating the collapsing bubble and the membrane sur...

The change in the current in an electrochemical cell is studied simultaneously with the dynamics and luminescence of a solitary cavitation bubble produced by focusing a shock wave near a disk anode 0.6 mm in diameter in an NaCl solution. The current is maximum at the instant of the bubble collapse. The increase in the current is due to the mass transfer near the electrode intensified by the hydrodynamic flow around the bubble.