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Passive cavitation mapping, which generates images using bubble acoustic emission signals, has been increasingly used for monitoring and guiding focused ultrasound surgery. Passive cavitation mapping can be used as an adjunct to magnetic resonance imaging to provide crucial information on the safety and efficacy of focused ultrasound surgery. The most widely used algorithm for passive cavitation mapping is delay-and-sum. One of the major limitations of delay-and-sum is its suboptimal computational efficiency. Although frequency-domain delay-and-sum can partially resolve this issue, such an algorithm is not suitable for imaging the evolution of bubble activity in real time and for cases in which cavitation events occur asynchronously. This study investigates a transient angular spectrum approach for passive cavitation mapping. The working principle of this approach is to back propagate the received signal to the domain of interest and reconstruct the spatial-temporal wave field encoded with the bubble location and collapse time. The transient angular spectrum approach is validated using an in silico model and water bath experiments. It is found that the transient angular spectrum approach yields similar results to delay-and-sum, but it is one order of magnitude faster. The results obtained by this study suggest that the transient angular spectrum approach is promising for fast and accurate passive cavitation mapping.

Shock wave lithotripsy (SWL) has been widely used for non-invasive treatment of kidney stones. Cavitation plays an important role in stone fragmentation, yet it may also contribute to renal injury during SWL. It is therefore crucial to determine the spatiotemporal distributions of cavitation activities to maximize stone fragmentation while minimizing tissue injury. Traditional cavitation detection methods include high-speed optical imaging, active cavitation mapping (ACM), and passive cavitation mapping (PCM). While each of the three methods provides unique information about the dynamics of the bubbles, PCM has most practical applications in biological tissues. To image the dynamics of cavitation bubble collapse, we previously developed a sliding-window PCM (SW-PCM) method to identify each bubble collapse with high temporal and spatial resolution. In this work, to further validate and optimize the SW-PCM method, we have developed tri-modality cavitation imaging that includes three-dimensional high-speed optical imaging, ACM, and PCM seamlessly integrated in a single system. Using the tri-modality system, we imaged and analyzed laser-induced single cavitation bubbles in both free field and constricted space and shock wave-induced cavitation clusters. Collectively, our results have demonstrated the high reliability and spatial-temporal accuracy of the SW-PCM approach, which paves the way for the future in vivo applications on large animals and humans in SWL.

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.

The dynamics, stabilization, and acoustic spectra of a bubble cluster in different liquids are investigated under the condition of ultrasonic cavitation. Experimental data for the dynamics of a spherical ultrasonic cluster near the end face of a rod, capillary, or pressure sensor placed in the antinode of a pressure standing wave at the center of a single-wave spherical piezoelectric concentrator (piezoelectric sphere) are presented. The variation of the cluster size with the parameters of the ultrasonic field and properties of the liquid is studied. It is found that the shape, collapse dynamics, and stability of the cavitation cluster have a significant influence on the acoustocapillary effect. It is shown that the maximal acoustocapillary effect and sonoluminescence are observed when a stable cluster with spherically symmetric collapse dynamics is provided at the end of a capillary in a 50% solution of glycerol. Using a small-size piezotransducer placed at the center of the sphere, the acoustic pressure is measured and acoustic spectra are studied for different voltages across the piezosphere and during the formation of variously shaped cavitation clusters. In the case of fully developed cavitation and a spherical cluster, the acoustic spectra contain subharmonic components, the cavitation noise factor rises to 35%, and the maximum of the noise envelope shifts toward higher frequencies.

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.

Kidney stones are a common and painful affliction, but fortunately physics offers some relief: Sound can be used to break up the stones, without invasive surgery. Efficacy of this ``lithotripsy'' treatment, however, depends strongly on cavitation, which can both attenuate the sound waves and provoke significant tissue damage. In this collaboration between a physicist, engineers, and materials scientists, our understanding of this process is advanced through measurements and modeling, providing insight into the relations between bubble dynamics and lithotripsy effectiveness.

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 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 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

To assess the role of stress waves and cavitation in comminuting residual fragments during shock wave lithotripsy (SWL), cylindrical 4 × 4 mm BegoStone phantoms were treated in an electromagnetic lithotripter either at the focus (z = 0, p+ = ∼45 MPa) or pre-focally (z = −30 mm, p+ = ∼24 MPa). The treatment was performed with the stone immersed either in degassed water or in Butanediol, which has similar acoustic impedance to water but much higher viscosity to suppress cavitation. At the focus, the first fracture was observed after 26 ± 9 shocks, both in water and Butanediol (p = 0.7). However, when stones were moved pre-focally where comparable cavitation is produced (based on high speed imaging), the average shock number required for the initial fracture was increased to 66 ± 10 in water and 122 ± 20 in Butanediol (p = 0.002). Below −40 mm prefocally (p+

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.]

Results are presented of experimental investigations of the dynamics of the electrical conductivity of an electrolyte when one of the electrodes is exposed to a weak shock pulse. It is shown that the electrical conductivity in the cell increases as a result of damage to the electric double layer.

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 excitation of impulse acoustic waves in a liquid upon the deformation of a cable under the action of pondermotive forces is described. It is shown that rarefaction and compression waves of microsecond duration can be focused with the use of a ring transducer manufactured from a coaxial cable. Acoustic pulses, in which a rarefaction wave preceded a compression wave, were excited upon the passage of a current pulse through the cable shield. When the current was passed through the central conductor, a compression wave preceded a rarefaction wave. High-speed photography revealed the presence of cavitation bubbles at the focus of the radiator. This work shows the possibility of studying the dynamics of bubbles at the stage of their expansion from cavitation nuclei in a rarefaction wave and the dynamics of such processes as aspherical collapse, the formation of a cumulative jet, and bubble fragmentation in a compression wave.