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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 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...

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.

In this study, a foam mask is introduced to elongate the beam width in the focal plane of a shock wave lithotripter, and a respiration gating beam steering strategy is proposed to accurately target stones, which are movable because of patient respiratory motion, radiation force, hydraulics, gravity, etc. The mask is steered to revolve the elongated elliptical focal zone of the acoustic field, so as to cover as much as possible the stone moving area. The beam steering strategy provides the most opportunity to deliver the shock wave energy straightly on the moving stones but not to the peripheral tissues, enhancing the stone fracture efficacy whilst reducing tissue injury. Simulation results with specific stone motion models are given to proof the advantages of the elongated focal zone in tracking stone target, and show that the stone overlap in the focal zone is improved significantly and competitively with the beam steering strategy, compared with no beam steering.

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

Nonlinear processes accompanying the focusing of a microsecond acoustic pulse produced by an electromagnetic source shaped as a spherical segment are investigated. The processes are considered to be far from the boundaries of a liquid, in the absence of cavitation. Detailed measurements of the pressure field by a fiber-optic sensor and high-speed photography of the shock front are performed. The pressure field is found to be determined by the nonlinear effects that occur in the course of the propagation of the initial converging compression wave and an edge rarefaction wave. The peak pressure amplitudes at the focus are 75 and −42 MPa for the compression and rarefaction waves, respectively, at the maximum voltage of the pulse generator in use. The measured length of the compression wave front is equal to the response time of the sensor (8 ns). The pressure amplitude is shown to be limited by the irregularity of the propagation of a shock wave in the form of Mach’s disk. At the focus, the pressure gradient across the radiator axis reaches 0.5 atm/μm, while the diameter of the focal spot is 2.5±0.2 mm. The focus of the edge rarefaction wave formed due to diffraction is located closer to the radiator than the focus of the compression wave, which may facilitate the study of the biological effect of cavitation independently of the shear motion of the medium.

A modified version of a flat electromagnetic converter for enhancing the amplitude of the pressure of a shock wave using the Laptev-Trishin method and reducing the duration of an acoustic pulse is described. The interference of a focused edge diffraction rarefaction wave and a wave reflected from a free surface is used to form a narrow space-time negative-pressure region. The conditions for generating a single cavitation bubble near the free surface are found.

A transducer for measuring the velocity of shock wave propagation in salt water is described. The transducer operation is based on the depolarization of a double electrical layer exposed to an acoustic pulse. The time lag of the electrolysis current step at a disk anode was used for measuring the velocity of the propagation of the pressure disturbance. It was shown experimentally that the suggested method provides a measurement accuracy of 2% for the shock-wave front velocity, and base length of 5 mm. The transducer withstands repeated exposure to cavitation and shock waves at a pressure of up to 70 MPa.

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.

The acoustic lens of the Modularis electromagnetic shock wave lithotripter (Siemens, Malvern, Pennsylvania) was modified to produce a pressure waveform and focal zone more closely resembling that of the original HM3 device (Dornier Medtech, Wessling, Germany). We assessed the newly designed acoustic lens in vivo in an animal model.Stone fragmentation and tissue injury produced by the original and modified lenses of the Modularis lithotripter were evaluated in a swine model under equivalent acoustic pulse energy (about 45 mJ) at 1 Hz pulse repetition frequency. Stone fragmentation was determined by the weight percent of stone fragments less than 2 mm. To assess tissue injury, shock wave treated kidneys were perfused, dehydrated, cast in paraffin wax and sectioned. Digital images were captured every 120 μm and processed to determine functional renal volume damage.After 500 shocks, the mean ± SD stone fragmentation efficiency produced by the original and modified lenses was 48% ± 12% and 52% ± 17%, respectively (p = 0.60). However, after 2,000 shocks, the modified lens showed significantly improved stone fragmentation compared to the original lens (mean 86% ± 10% vs 72% ± 12%, p = 0.02). Tissue injury caused by the original and modified lenses was minimal at a mean of 0.57% ± 0.44% and 0.25% ± 0.25%, respectively (p = 0.27).With lens modification the Modularis lithotripter demonstrates significantly improved stone fragmentation with minimal tissue injury at a clinically relevant acoustic pulse energy. This new lens design could potentially be retrofitted to existing lithotripters, improving the effectiveness of electromagnetic lithotripters.

The performance of a newly developed light spot hydrophone (LSHD) in lithotripter field characterization was compared to that of the fiber optic probe hydrophone (FOPH). Pressure waveforms produced by a stable electromagnetic shock wave source were measured by the LSHD and FOPH under identical experimental conditions. In the low energy regime, focus and field acoustic parameters matched well between the two hydrophones. At clinically relevant high energy settings for shock wave lithotripsy, the measured leading compressive pressure waveforms matched closely with each other. However, the LSHD recorded slightly larger |P–| (p < 0.05) and secondary peak compressive pressures (p < 0.01) than the FOPH, leading to about 20% increase in total acoustic pulse energy calculated in a 6 mm radius around the focus (p = 0.06). Tensile pulse durations deviated ∼5% (p < 0.01) due to tensile wave shortening from cavitation activity using the LSHD. Intermittent compression spikes and laser light reflection artifacts have been correlated to bubble activity based on simultaneous high-speed imaging analysis. Altogether, both hydrophones are adequate for lithotripter field characterization as specified by the international standard IEC 61846.

The 3rd‐generation electromagnetic (EM) shock wave lithotripters often have narrow focal width and high peak pressure compared to the original Dornier HM‐3. In addition, the pressure waveform produced by a typical EM lithotripter has a secondary compressive wave following the tensile component that suppresses lithotripter pulse induced cavitation, which may impact negatively on stone comminution. These characteristic changes in the modern EM lithotripters may contribute in part to their reduced effectiveness observed clinically. To overcome these two drawbacks, we have designed a new acoustic lens for the Siemens Modularis EM lithotripter that produces an idealized pressure waveform similar to that of the HM‐3 with broad focal width and low peak pressure. At acoustic pulse energy of 53 mJ, the new lens design enlarges the −6 dB focal width of the Modularis by 47% while significantly reducing the second compressive wave in the lithotripter pulse throughout its focal plane. After 2000 shocks, in vitro commi...

Using a reflector insert, the original HM-3 lithotripter field at 20 kV was altered significantly with the peak positive pressure (p(+)) in the focal plane increased from 49 to 87 MPa while the -6 dB focal width decreased concomitantly from 11 to 4 mm. Using the original reflector, p(+) of 33 MPa with a -6 dB focal width of 18 mm were measured in a pre-focal plane 15-mm proximal to the lithotripter focus. However, the acoustic pulse energy delivered to a 28-mm diameter area around the lithotripter axis was comparable ( approximately 120 mJ). For all three exposure conditions, similar stone comminution ( approximately 70%) was produced in a mesh holder of 15 mm after 250 shocks. In contrast, stone comminution produced by the modified reflector either in a 15-mm finger cot (45%) or in a 30-mm membrane holder (14%) was significantly reduced from the corresponding values (56% and 26%) produced by the original reflector (no statistically significant differences were observed between the focal and pre-focal planes). These observations suggest that a low-pressure/broad focal width lithotripter field will produce better stone comminution than its counterpart with high-pressure/narrow focal width under clinically relevant in vitro comminution conditions.

The effect of beam size on stone comminution in the Dornier HM‐3 lithotripter were investigated using a modified reflector (MR), which was developed to produce an acoustic field with higher peak pressure and smaller beam size compared to the original reflector (OR) of the HM‐3. The acoustic fields produced at 20 kV by the MR in the focal plane, and by the OR in the focal and pre‐focal plane (z = −15 mm) were characterized using a light spot hydrophone. The efficiencies of stone comminution in a mesh holder were similar, but the corresponding values produced by using the MR in a membrane holder were significantly lower than those produced by the OR in the focal and pre‐focal planes. These results suggest that a broad beam size could increase stone comminution efficiency when fragments are spreading out or moving due to respiratory motion in a large area during SWL. In contrast, when stone fragments are confined and well aligned to lithotripter focus the beam size may not influence significantly the treatme...

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.

The focusing of laser-generated shock waves by a truncated ellipsoidal reflector was experimentally and numerically investigated. Pressure waveform and distribution around the first (F(1)) and second foci (F(2)) of the ellipsoidal reflector were measured. A neodymium doped yttrium aluminum garnet laser of 1046 nm wavelength and 5 ns pulse duration was used to create an optical breakdown at F(1), which generates a spherically diverging shock wave with a peak pressure of 2.1-5.9 MPa at 1.1 mm stand-off distance and a pulse width at half maximum of 36-65 ns. Upon reflection, a converging shock wave is produced which, upon arriving at F(2), has a leading compressive wave with a peak pressure of 26 MPa and a zero-crossing pulse duration of 0.1 mus, followed by a trailing tensile wave of -3.3 MPa peak pressure and 0.2 mus pulse duration. The -6 dB beam size of the focused shock wave field is 1.6 x 0.2 mm(2) along and transverse to the shock wave propagation direction. Formation of elongated plasmas at high laser energy levels limits the increase in the peak pressure at F(2). General features in the waveform profile of the converging shock wave are in qualitative agreement with numerical simulations based on the Hamilton model.

Despite recent innovations in lithotripter design and technology, the first generation Dornier HM-3 electrohydraulic lithotripter remains the most effective clinical, non-invasive modality for comminuting stones. Understanding the fundamental differences between the HM-3 and more clinically practicable lithotripters is critical to achieving meaningful improvements in future models. This study aims to further clarify the role of the second, weaker pulse of the HM-3 in stone comminution by introducing a similar pulse in the pressure profile of an electromagnetic lithotripter. Additionally, this study investigates the potential for this tandem shock technique to improve comminution in electromagnetic lithotripters through optimization of the interpulse delay time.

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 laser-generated single inertial bubbles (collapse $\text{time}=121\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{s}$) near a silicon rubber membrane with a shock wave ($55\phantom{\rule{0.3em}{0ex}}\mathrm{MPa}$ in peak pressure and $1.7\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{s}$ in compressive pulse duration) is investigated. The interaction leads to directional, forced asymmetric collapse of the bubble with microjet formation toward the surface. Maximum jet penetration into the membrane is produced during the bubble collapse phase with optimal shock wave arrival time and stand-off distance. Such interaction may provide a unique acoustic means for in vivo microinjection, applicable to targeted delivery of macromolecules and gene vectors to biological tissues.

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.

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.

The third‐generation electromagnetic (EM) shock wave lithotripters often have narrow focal width and high‐peak pressure compared to the original HM‐3. In addition, the pressure waveform produced by a typical EM lithotripter has a secondary compressive wave that suppresses lithotripter pulse induced cavitation, which is an important mechanism for stone comminution. These features have been contributed to the reduced effectiveness of the modern EM lithotripters. To overcome these drawbacks, we have designed a new acoustic lens for the Siemens Modularis EM lithotripter that produces an idealized pressure waveform similar to the HM‐3 with broad focal width and low‐peak pressure. At acoustic pulse energy of 53 mJ, the new lens design enlarges the −6 dB focal width of the Modularis by 47% while significantly reducing the second compressive wave throughout its focal plane. After 2000 shocks, in vitro comminution produced by the original and new lens designs are 100% and 99% at the lithotripter focus, and (49.7±15.0)% and (75.3±8.74)% (p < 0.001) at 10 mm off axis, respectively. Corresponding values in vivo are (63.4±17.4)% and (83.3±8.5)% (p < 0.01), demonstrating the significant performance improvement provided by the new lens design. [Work supported by NIH Grant Nos. R01DK052985 and S10RR016802.]

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.