Effect of overpressure and pulse repetition frequency on cavitation in shock wave lithotripsy

Shock Wave Therapy in Plastic Surgery: A Review of the Current Indications

BACKGROUND

Extracorporeal shock wave therapy (ESWT) represents a promising, non-invasive management strategy supporting the treatment of a variety of conditions related to plastic surgery.

OBJECTIVES

This literature review aimed to give a systematic overview of current applications, its mechanism of action, and its potential to provide tangible therapies in plastic surgery.

METHODS

The databases PubMed (National Institute of Health, Bethesda, MD), Embase (via Ovid [Elsevier, Amsterdam, the Netherlands]), and the Cochrane Library (Cochrane, London, UK) were searched for articles published up to June 1, 2021. Clinical studies of any design including ESWT in the context of plastic surgery were included. Two reviewers extracted data, and 46 articles were analyzed after application of the inclusion and exclusion criteria.

RESULTS

Forty-six included studies (n = 1496) were categorized into the following broad themes: cellulite/body contouring/skin rejuvenation, burns/scar treatment, diabetic foot ulcers/chronic wound, and future perspectives of ESWT. Overall, applications of ESWT were heterogenous, and the majority of studies reported effectiveness of ESWT as an alternative treatment technique. Flawed methodology and differences in technical standards limit the outcome and conclusion of this review.

CONCLUSIONS

There is yet insufficient evidence to support the effectiveness of any specific intervention included in this review; however, all included studies reported improvements in key outcomes. Where reported, ESWT displayed a good safety profile with no serious adverse events. Further research is needed to provide more evidence to delineate the indications of ESWT in plastic surgery.

Functional and Morphological Changes Associated with Burst Wave Lithotripsy-Treated Pig Kidneys

Purpose: Burst wave lithotripsy (BWL) is a new technique for comminution of urinary stones. This technology is noninvasive, has a low positive pressure magnitude, and is thought to produce minor amounts of renal injury. However, little is known about the functional changes related to BWL treatment. In this study, we sought to determine if clinical BWL exposure produces a functional or morphological change in the kidney. Materials and Methods: Twelve female pigs were prepared for renal clearance assessment and served as either sham time controls (6) or were treated with BWL (6). In the treated group, 1 kidney in each pig was exposed to 18,000 pulses at 10 pulses/s with 20 cycles/pulse. Pressure levels related to each pulse were 12 and -7 MPa. Inulin (glomerular filtration rate, GFR) and para-aminohippuric acid (effective renal plasma flow, eRPF) clearance was measured before and 1 hour after treatment. Lesion size analysis was performed to assess the volume of hemorrhagic tissue injury created by each treatment (% FRV). Results: No visible gross hematuria was observed in any of the collected urine samples of the treated kidneys. BWL exposure also did not lead to a change in GFR or eRPF after treatment, nor did it cause a measurable amount of hemorrhage in the tissue. Conclusion: Using the clinical treatment parameters employed in this study, BWL did not cause an acute change in renal function or a hemorrhagic lesion.

Maximizing mechanical stress in small urinary stones during burst wave lithotripsy

Unlike shock wave lithotripsy, burst wave lithotripsy (BWL) uses tone bursts, consisting of many periods of a sinusoidal wave. In this work, an analytical theoretical approach to modeling mechanical stresses in a spherical stone was developed to assess the dependence of frequency and stone size on stress generated in the stone. The analytical model for spherical stones is compared against a finite-difference model used to calculate stress in nonspherical stones. It is shown that at low frequencies, when the wavelength is much greater than the diameter of the stone, the maximum principal stress is approximately equal to the pressure amplitude of the incident wave. With increasing frequency, when the diameter of the stone begins to exceed about half the wavelength in the surrounding liquid (the exact condition depends on the material of the stone), the maximum stress increases and can be more than six times greater than the incident pressure. These results suggest that the BWL frequency should be elevated for small stones to improve the likelihood and rate of fragmentation.

Effects of sub-atmospheric pressure and dissolved oxygen concentration on lesions generated in ex vivo tissues by high intensity focused ultrasound

Background

Acoustic cavitation plays an important role in the medical treatment using high-intensity focused ultrasound (HIFU), but unnecessarily strong cavitation also could deform the morphology and enlarge the size of lesions. It is known that the increase of ambient hydrostatic pressure (P_stat) can control the acoustic cavitation. But the question of how the decrease of P_stat and dissolved oxygen concentration (DOC) influence the strength of cavitation has not been thoroughly answered. In this study, we aimed to investigate the relationship among the P_stat, DOC and the strength of cavitation.

Methods

Ex vivo bovine liver tissues were immersed in degassed water with different DOC of 1.0 mg/L, 1.5 mg/L and 2.0 mg/L. Ultrasound (US) of 1 MHz and the spatial and temporal average intensity (I_sata) of 6500 W/cm² was used to expose two groups of in vitro bovine livers for 2 s; one group was under atmospheric pressure (P_stat = 1 bar) and the other was under sub-atmospheric pressure (P_stat = 0.1 bar). Acoustic cavitation was detected by a passive cavitation detector (PCD) during the exposure process. Echo signals at the focal zone of HIFU were monitored by B-mode ultrasound imaging before and after exposure. The difference between two pressure groups was tested using paired sample t-test. The difference among different DOC groups was evaluated by one-way analysis of variance (ANOVA).

Results

The results demonstrated a significant difference of broadband acoustic emissions from the cavitation bubbles, echo signals on B-mode image, morphology of lesions under various conditions of ambient pressure and DOC. The lesion volume in tissue was increased with the increase of ambient pressure and DOC.

Conclusion

Cavitation could be suppressed through sub-atmospheric pressure and low DOC level in liver tissue, which could provide a method of controlling cavitation in HIFU treatment to avoid unpredictable lesions.

Effect of Overpressure on Acoustic Emissions and Treated Tissue Histology in ex Vivo Bulk Ultrasound Ablation

Bulk ultrasound ablation is a thermal therapy approach in which tissue is heated by unfocused or weakly focused sonication (average intensities on the order of 100 W/cm²) to achieve coagulative necrosis within a few minutes exposure time. Assessing the role of bubble activity, including acoustic cavitation and tissue vaporization, in bulk ultrasound ablation may help in making bulk ultrasound ablation safer and more effective for clinical applications. Here, two series of ex vivo ablation trials were conducted to investigate the role of bubble activity and tissue vaporization in bulk ultrasound ablation. Fresh bovine liver tissue was ablated with unfocused, continuous-wave ultrasound using ultrasound image-ablate arrays sonicating at 31 W/cm² (0.9 MPa amplitude) for either 20 min at a frequency of 3.1 MHz or 10 min at 4.8 MHz. Tissue specimens were maintained at a static overpressure of either 0.52 or 1.2 MPa to suppress bubble activity and tissue vaporization or at atmospheric pressure for control groups. A passive cavitation detector was used to record subharmonic (1.55 or 2.4 MHz), broadband (1.2-1.5 MHz) and low-frequency (5-20 kHz) acoustic emissions. Treated tissue was stained with 2% triphenyl tetrazolium chloride to evaluate thermal lesion dimensions. Subharmonic emissions were significantly reduced in overpressure groups compared with control groups. Correlations observed between acoustic emissions and lesion dimensions were significant and positive for the 3.1-MHz series, but significant and negative for the 4.8-MHz series. The results indicate that for bulk ultrasound ablation, where both acoustic cavitation and tissue vaporization are possible, bubble activity can enhance ablation in the absence of tissue vaporization, but can reduce thermal lesion dimensions in the presence of vaporization.

Effects of shockwave therapy on pain and disability in individuals with rotator cuff tendinopathy: a systematic review protocol

OBJECTIVE

The objective of this study is to systematically review randomized controlled trials comparing the effect of shockwave therapy with other forms of interventions on pain and disability in individuals with rotator cuff tendinopathy.

INTRODUCTION

Shoulder pain is a common musculoskeletal complaint in which rotator cuff tendons are usually affected. Shockwave therapy is a novel conservative treatment option for rotator cuff-related shoulder pain. A systematic review in 2004 reported conflicting results on the effectiveness of shockwave therapy in treating chronic rotator cuff tendonitis, whereby it is effective for calcific rotator cuff tendonitis but not for non-calcific ones. Hence, it is timely to review this topic with the addition of results from studies published since 2004.

INCLUSION CRITERIA

This review will consider randomized controlled trials that have evaluated shockwave therapy delivered via radial or focused extracorporeal means in individuals with rotator cuff tendinopathy without restrictions of race, gender, and age. Studies comparing individuals receiving shockwave therapy of varying dosages will be excluded in this review.

METHODS

A three-step search strategy will be used to locate studies published in English from databases. No search restrictions will be applied regarding the year of publication. Study selection, assessment of methodological quality, and data extraction will be conducted by two independent reviewers. Data will be pooled in a statistical meta-analysis, where possible. A funnel plot will be generated to detect any potential publication bias. The quality of the evidence will be analyzed using the Grading of Recommendations, Assessment, Development, and Evaluation approach.

SYSTEMATIC REVIEW REGISTRATION NUMBER

PROSPERO CRD42020160166.

Mechanically Induced Cavitation in Biological Systems

Cavitation bubbles form in soft biological systems when subjected to a negative pressure above a critical threshold, and dynamically change their size and shape in a violent manner. The critical threshold and dynamic response of these bubbles are known to be sensitive to the mechanical characteristics of highly compliant biological systems. Several recent studies have demonstrated different biological implications of cavitation events in biological systems, from therapeutic drug delivery and microsurgery to blunt injury mechanisms. Due to the rapidly increasing relevance of cavitation in biological and biomedical communities, it is necessary to review the current state-of-the-art theoretical framework, experimental techniques, and research trends with an emphasis on cavitation behavior in biologically relevant systems (e.g., tissue simulant and organs). In this review, we first introduce several theoretical models that predict bubble response in different types of biological systems and discuss the use of each model with physical interpretations. Then, we review the experimental techniques that allow the characterization of cavitation in biologically relevant systems with in-depth discussions of their unique advantages and disadvantages. Finally, we highlight key biological studies and findings, through the direct use of live cells or organs, for each experimental approach.

Extracorporeal shock wave therapy for treating primary dysmenorrhea: A randomized controlled trial

BACKGROUND

There are scanty data to apply radial extracorporeal shock wave therapy (rESWT) on the acupuncture points in the lower abdomen to reduce the menstrual pain. This trial aimed to test the rESWT safety and efficacy for treating primary dysmenorrhea (PD).

METHODS

Forty-four young-women with PD were randomly assigned to one of the three groups: to receive rESWT on the acupuncture points during the follicular phase (Group A, n = 15) or during the luteal phase (Group B, n = 14), or to apply heat patch to the acupuncture points during the follicular phase as the control (Group C, n = 15) over three menstrual cycles. The pain severity (using 0-to-10 visual analog scale), the pain duration (hours), plasma PGF2α prostaglandin F2alpha and prostaglandin E2 (PGE2), self-rating anxiety scale and menstrual blood loss were assessed before and after interventions.

RESULTS

The pain severity and duration significantly decreased in all groups after interventions. Although the reduced pain duration was not different among the groups, the reduced pain severity was more significant (P = .003) in Groups A (-53.8 ± 33.7%) and B (-59.3 ± 36.7%) than in Group C (-18.7 ± 27.1%). The rESWT intervention did not change plasma prostaglandins in Group A, although there was a decreased prostaglandin F2alpha (-20.5 ± 32.9%) in Group B or a decreased PGE2 (-18.9 ± 17.8%) in Group C. The anxiety level showed no change after intervention. The menstrual blood volume reduced slightly after intervention and the change of menstrual blood loss in Group B was significant (P = .038).

CONCLUSION

The rESWT applications on the abdominal acupuncture points safely and effectively reduced the menstrual pain, which was not associated with the prostaglandin changes. The rESWT-reduced pain seemed equally effective with the intervention applied during the follicular phase or luteal phase of the menstrual cycle. Heat patch placed on the abdominal acupuncture points also reduced the pain severity and duration, indicating that the improved blood flow could effectively alleviate the menstrual pain with PD. The changes in anxiety level and menstrual blood loss were slight after intervention.

A dynamical model of oncotripsy by mechanical cell fatigue: selective cancer cell ablation by low-intensity pulsed ultrasound

The method of oncotripsy, first proposed in Heyden & Ortiz (Heyden & Ortiz 2016 J. Mech. Phys. Solids 92, 164-175 (doi:10.1016/j.jmps.2016.04.016)), exploits aberrations in the material properties and morphology of cancerous cells in order to ablate them selectively by means of tuned low-intensity pulsed ultrasound. We propose the dynamical model of oncotripsy that follows as an application of cell dynamics, statistical mechanical theory of network elasticity and 'birth-death' kinetics to describe the processes of damage and repair of the cytoskeleton. We also develop a reduced dynamical model that approximates the three-dimensional dynamics of the cell and facilitates parametric studies, including sensitivity analysis and process optimization. We show that the dynamical model predicts-and provides a conceptual basis for understanding-the oncotripsy effect and other trends in the data of Mittelstein et al. (Mittelstein et al. 2019 Appl. Phys. Lett. 116, 013701 (doi:10.1063/1.5128627)), for cells in suspension, including the dependence of cell-death curves on cell and process parameters.

Confocal lens focused piezoelectric lithotripter

This work focuses on the evaluation of a type of piezoelectric lithotripter with similar dimensions of a commercial lithotripter and composed of either 3 or 4 large lens focused piezoelectric transducers set either in a confocal coplanar C-shape or a confocal spherical shape. Each transducer is made with a 92 mm diameter 220 kHz flat piezoelectric ceramic disc and a 3D printed acoustic lens. Both confocal setups pressure field were measured with a fiber optic hydrophone, and in vitro fragmentations of 13 mm diameter and 14 mm length cylindrical model stones were done in a 2 mm mesh basket. The acoustic characterization of the three transducers confocal setup revealed a disc shaped focal volume, with a 2.2 mm width on one axis and a 9.6 mm width on the other, and a peak positive pressure of 40.9 MPa and a peak negative pressure of -16.9 MPa, while the focus of the four transducers confocal setup was similar to a traditional narrow focus high pressure lithotripter with a focus width of 2.1 mm, and a peak positive pressure of 71.9 MPa and peak negative pressure of -24.3 MPa. Both confocal setups showed in vitro fragmentation efficiency close to a commercial electroconductive lithotripter.

In Vitro Evaluation of Urinary Stone Comminution with a Clinical Burst Wave Lithotripsy System

Objective: Our goals were to validate stone comminution with an investigational burst wave lithotripsy (BWL) system in patient-relevant conditions and to evaluate the use of ultrasonic propulsion to move a stone or fragments to aid in observing the treatment endpoint. Materials and Methods: The Propulse-1 system, used in clinical trials of ultrasonic propulsion and upgraded for BWL trials, was used to fragment 46 human stones (5-7 mm) in either a 15-mm or 4-mm diameter calix phantom in water at either 50% or 75% dissolved oxygen level. Stones were paired by size and composition, and exposed to 20-cycle, 390-kHz bursts at 6-MPa peak negative pressure (PNP) and 13-Hz pulse repetition frequency (PRF) or 7-MPa PNP and 6.5-Hz PRF. Stones were exposed in 5-minute increments and sieved, with fragments >2 mm weighed and returned for additional treatment. Effectiveness for pairs of conditions was compared statistically within a framework of survival data analysis for interval censored data. Three reviewers blinded to the experimental conditions scored ultrasound imaging videos for degree of fragmentation based on stone response to ultrasonic propulsion. Results: Overall, 89% (41/46) and 70% (32/46) of human stones were fully comminuted within 30 and 10 minutes, respectively. Fragments remained after 30 minutes in 4% (1/28) of calcium oxalate monohydrate stones and 40% (4/10) of brushite stones. There were no statistically significant differences in comminution time between the two output settings (p = 0.44), the two dissolved oxygen levels (p = 0.65), or the two calyx diameters (p = 0.58). Inter-rater correlation on endpoint detection was substantial (Fleiss' kappa = 0.638, p < 0.0001), with individual reviewer sensitivities of 95%, 86%, and 100%. Conclusions: Eighty-nine percent of human stones were comminuted with a clinical BWL system within 30 minutes under conditions intended to reflect conditions in vivo. The results demonstrate the advantage of using ultrasonic propulsion to disperse fragments when making a visual determination of breakage endpoint from the real-time ultrasound image.

Study of acoustic and low-pressure exposure on the temperature of the evaporation of a liquid with free interface before it freezes

On the basis of the first law of thermodynamics, a thermodynamic physical and mathematical model of the process of evaporation and freezing in a partially filled closed volume of a liquid under acoustic exposure (AE) and pressure reduction has been developed. The acoustic effect on the liquid is taken as a component on the heating of the liquid in the closed volume. The experimental studies performed under AE (amplitude 2.0 μm, frequency 25 kHz), initial temperature, and mass of the liquid 300.85 K, 7 g, respectively, the pressure decreases from 101 to 0.5 kPa showed a close coincidence of the actual and calculated moments of freezing start liquid surfaces, as well as close calculated and experimental values of liquid temperatures during the experiment.

High-speed video microscopy and numerical modeling of bubble dynamics near a surface of urinary stone

Ultra-high-speed video microscopy and numerical modeling were used to assess the dynamics of microbubbles at the surface of urinary stones. Lipid-shell microbubbles designed to accumulate on stone surfaces were driven by bursts of ultrasound in the sub-MHz range with pressure amplitudes on the order of 1 MPa. Microbubbles were observed to undergo repeated cycles of expansion and violent collapse. At maximum expansion, the microbubbles' cross-section resembled an ellipse truncated by the stone. Approximating the bubble shape as an oblate spheroid, this study modeled the collapse by solving the multicomponent Euler equations with a two-dimensional-axisymmetric code with adaptive mesh refinement for fine resolution of the gas-liquid interface. Modeled bubble collapse and high-speed video microscopy showed a distinctive circumferential pinching during the collapse. In the numerical model, this pinching was associated with bidirectional microjetting normal to the rigid surface and toroidal collapse of the bubble. Modeled pressure spikes had amplitudes two-to-three orders of magnitude greater than that of the driving wave. Micro-computed tomography was used to study surface erosion and formation of microcracks from the action of microbubbles. This study suggests that engineered microbubbles enable stone-treatment modalities with driving pressures significantly lower than those required without the microbubbles.

Performance of five ultrasonic transducers modified for efficient atomization

Performance of five ultrasonic transducers modified with different shapes of electrodes is investigated for efficient atomization. A circular silver electrode, which is conventionally used as standard on a transducer, is chemically replaced with five shapes of silver electrodes (circular, toroidal, singlet, doublet, and cross). Each electrode reflects its precise shape on the water surface and statically forms characteristic three-dimensional geometry of a water column. The modified electrode also affects the dynamics of this water column, generating two types of wobbling on the column and inducing three types of atomization depending on the shape of the electrodes. Statistical analysis indicates that the shape of the electrode on an ultrasonic transducer affects the speed of atomization, showing that the singlet electrode exhibits the highest speed of atomization (4 mg/s). The mechanisms of atomization are analyzed from the viewpoint of energy transformation with reference to mass transformation of the oscillating liquid, indicating that the vibration energy of the transducer is transferred to the water film through resonance, consuming this vibration energy with four kinds of energy such as kinetic energy of atomized mists and work function of atomization, which are defined in this study. This analysis clarifies why the speed of atomization increases with a decreasing amount of water on the transducer. Application of the appropriate shape of an electrode will greatly contribute to the fields of engineering, medicine, and biology where various types of atomization are highly desired as the situation demands.

The Impact of Dust and Confinement on Fragmentation of Kidney Stones by Shockwave Lithotripsy in Tissue Phantoms

Objective: The goal was to test whether stone composition and kidney phantom configuration affected comminution in extracorporeal shockwave lithotripsy (SWL) laboratory tests. Confinement may enhance the accumulation of dust and associated cavitation bubbles in the fluid surrounding the stone. It is known that high shockwave delivery rates in SWL are less effective because bubbles generated by one shockwave do not have sufficient time to dissolve, thereby shielding the next shockwave. Materials and Methods: Experiments were conducted with a lithotripter coupled to a water bath. The rate of comminution was measured by weighing fragments over 2 mm at 5-minute time points. First, plaster and crystal stones were broken in four phantoms: a nylon wire mesh, an open polyvinyl chloride (PVC) cup, a closed PVC cup, and an anatomical kidney model-the phantoms have decreasing fluid volumes around the stone. Second, the fluid volume in the kidney model was flushed with water at different rates (0, 7, and 86 mL/min) to remove dust. Results: The efficiency of breakage of stones decreases for the dust emitting plaster stones (percentage of breakage in 5 minutes decreased from 92% ± 2% [n = 3] in wire mesh to 19% ± 3% [n = 3] in model calix) with increasing confinement, but not for the calcite crystal stones that produced little dust (percentage of breakage changed from 87% ± 3% [n = 3] in wire mesh to 81% ± 3% [n = 3] in kidney model). Flushing the kidney phantom at the fastest rate improved comminution of smaller plaster stones by 27%. Conclusions: Phantoms restricting dispersion of dust were found to affect stone breakage in SWL and in vitro experiments should replicate kidney environments. The dust around the stone and potential cavitation may shield the stone from shockwaves and reduce efficacy of SWL. Understanding of stone composition and degree of hydronephrosis could be used to adapt patient-specific protocols.

Experimental observations and numerical modeling of lipid-shell microbubbles with calcium-adhering moieties for minimally-invasive treatment of urinary stones

A novel treatment modality incorporating calcium-adhering microbubbles has recently entered human clinical trials as a new minimally-invasive approach to treat urinary stones. In this treatment method, lipid-shell gas-core microbubbles can be introduced into the urinary tract through a catheter. Lipid moities with calcium-adherance properties incorporated into the lipid shell facilitate binding to stones. The microbubbles can be excited by an extracorporeal source of quasi-collimated ultrasound. Alternatively, the microbubbles can be excited by an intraluminal source, such as a fiber-optic laser. With either excitation technique, calcium-adhering microbubbles can significantly increase rates of erosion, pitting, and fragmentation of stones. We report here on new experiments using high-speed photography to characterize microbubble expansion and collapse. The bubble geometry observed in the experiments was used as one of the initial shapes for the numerical modeling. The modeling showed that the bubble dynamics strongly depends on bubble shape and stand-off distance. For the experimentally observed shape of microbubbles, the numerical modeling showed that the collapse of the microbubbles was associated with pressure increases of some two-to-three orders of magnitude compared to the excitation source pressures. This in-vitro study provides key insights into the use of microbubbles with calcium-adhering moieties in treatment of urinary stones.

A small-angle scattering environment for in situ ultrasound studies

Ultrasonic devices are common tools in laboratory and industrial settings to produce cavitation events for cleaning, emulsification, cell lysis and other materials applications. Effects of sonication at the macroscopic scale can be visible while effects at the molecular and nano-scales are not easily probed and, therefore, not fully understood. We present a new small angle scattering sample environment designed specifically to study structural changes occurring in various types of dispersions at the nano-scale due to ultrasonic acoustic waves. The sample environment features two face-to-face high-intensity focused ultrasound transducers coaxially aligned and normal to the neutron/X-ray beam propagation direction. A third broadband transducer is fixed beneath the scattering volume to acoustically monitor for cavitation events. By correlating acoustic data to scattering data, measured structural changes can be correlated to changes in parameters such as frequency, acoustic pressure, or cavitation pressure threshold. Several example applications of colloidal systems effectively influenced by ultrasound fields are also presented to demonstrate the capabilities of the device and to motivate future work on in situ scattering analysis of ultrasound materials processing methods.

Using the cavitation collapse time to indicate the extent of histotripsy-induced tissue fractionation

Histotripsy is an ultrasonic tissue ablation method based on acoustic cavitation. It has been shown that cavitation dynamics change depending on the mechanical properties of the host medium. During histotripsy treatment, the target-tissue is gradually fractionated and eventually liquefied to acellular homogenate. In this study, the change in the collapse time (t _col) of the cavitation bubble cloud over the course of histotripsy treatment is investigated as an indicator for progression of the tissue fractionation process throughout treatment. A 500 kHz histotripsy transducer is used to generate single-location lesions within tissue-mimicking agar phantoms of varying stiffness levels as well as ex vivo bovine liver samples. Cavitation collapse signals are acquired with broadband hydrophones, and cavitation is imaged optically using a high-speed camera in transparent tissue-mimicking phantoms. The high-speed-camera-acquired measurements of t _col validate the acoustic hydrophone measurements. Increases in t _col are observed both with decreasing phantom stiffness and throughout histotripsy treatment with increasing number of pulses applied. The increasing trend of t _col throughout the histotripsy treatment correlates well with the progression of lesion formation generated in tissue-mimicking phantoms (R ²  =  0.87). Finally, the increasing trend of t _col over the histotripsy treatment is validated in ex vivo bovine liver.

Ultrasound-Induced Bubble Clusters in Tissue-Mimicking Agar Phantoms

Therapeutic ultrasound can drive bubble activity that damages soft tissues. To study the potential mechanisms of such injury, transparent agar tissue-mimicking phantoms were subjected to multiple pressure wave bursts of the kind being considered specifically for burst wave lithotripsy. A high-speed camera recorded bubble activity during each pulse. Various agar concentrations were used to alter the phantom's mechanical properties, especially its stiffness, which was varied by a factor of 3.5. However, the maximum observed bubble radius was insensitive to stiffness. During 1000 wave bursts of a candidate burst wave lithotripsy treatment, bubbles appeared continuously in a region that expanded slowly, primarily toward the transducer. Denser bubble clouds are formed at higher pulse repetition frequency. The specific observations are used to inform the incorporation of damage mechanisms into cavitation models for soft materials.

Multiphase fluid-solid coupled analysis of shock-bubble-stone interaction in shockwave lithotripsy

A novel multiphase fluid-solid-coupled computational framework is applied to investigate the interaction of a kidney stone immersed in liquid with a lithotripsy shock wave (LSW) and a gas bubble near the stone. The main objective is to elucidate the effects of a bubble in the shock path to the elastic and fracture behaviors of the stone. The computational framework couples a finite volume 2-phase computational fluid dynamics solver with a finite element computational solid dynamics solver. The surface of the stone is represented as a dynamic embedded boundary in the computational fluid dynamics solver. The evolution of the bubble surface is captured by solving the level set equation. The interface conditions at the surfaces of the stone and the bubble are enforced through the construction and solution of local fluid-solid and 2-fluid Riemann problems. This computational framework is first verified for 3 example problems including a 1D multimaterial Riemann problem, a 3D shock-stone interaction problem, and a 3D shock-bubble interaction problem. Next, a series of shock-bubble-stone-coupled simulations are presented. This study suggests that the dynamic response of a bubble to LSW varies dramatically depending on its initial size. Bubbles with an initial radius smaller than a threshold collapse within 1 μs after the passage of LSW, whereas larger bubbles do not. For a typical LSW generated by an electrohydraulic lithotripter (p_max  = 35.0MPa, p_min  =- 10.1MPa), this threshold is approximately 0.12mm. Moreover, this study suggests that a noncollapsing bubble imposes a negative effect on stone fracture as it shields part of the LSW from the stone. On the other hand, a collapsing bubble may promote fracture on the proximal surface of the stone, yet hinder fracture from stone interior.

Effects of different hydrostatic pressure on lesions in ex vivo bovine livers induced by high intensity focused ultrasound

It is well-known that acoustic cavitation associated with the high intensity focused ultrasound (HIFU) treatment often would change the morphology and size of lesions in its treatment. In most studies reported in literature, high ambient hydrostatic pressure was used to suppress the cavitation completely. Investigation of the effects by varying the ambient hydrostatic pressure (P_stat) is still lacking. In this paper, the effects of HIFU on lesions in ex vivo bovine liver specimens under various P_stat are systematically investigated. A 1MHz HIFU transducer, with an aperture diameter of 70mm and a focal length of 55mm, was used to generate two groups US exposure of different acoustic intensities and exposure time (6095W/cm²×8s and 9752W/cm²×5s), while keeping the same acoustic energies per unit area (48760J/cm²). The peak acoustic negative pressures (p^-) of the two groups were p₁^-=9.58MPa and p₂^-=10.82MPa, respectively, with the difference p_d^-=p₂^--p₁^-=1.24MPa. A passive cavitation detection (PCD) was used to monitor the ultrasonic cavitation signal during exposure of the two groups. The US exposures were done under the following ambient hydrostatic pressures, P_stat: atmospheric pressure, 0.5MPa, 1.0MPa, 1.5MPa, 2.0MPa, 2.5MPa and3.0MPa, respectively. The result of PCD showed that there was a statistically significant increase above background noise level in broadband emissions at dose of 9752W/cm²×5s, but not at dose of 6095W/cm²×8s under atmospheric pressure; i.e., the acoustic cavitation took place for p₂^- but not for p₁^- when under atmospheric pressure. The results also showed that there was no statistically difference of the morphology and size of lesions for 6095W/cm²×8s exposure under the aforementioned different ambient hydrostatic pressures. But the lesions generated at 9752W/cm²×5s exposure under P_stat=atmospheric pressure, 0.5MPa, 1.0MPa (all of them are less than p_d^-), were larger than those under 1.5MPa, 2.0MPa, 2.5MPa and 3.0MPa (all of them are over than p_d^-) which were consistence with 6095W/cm²×8s group. It was concluded that when P_stat>p_d^-, the acoustic cavitation was suppressed and prompted that there was no need to elevate P_stat higher than p^- to suppress the acoustic cavitation in tissue, just need P_stat higher than p_d^-.

Double-Strand Breaks in Genome-Sized DNA Caused by Ultrasound

DNA double-strand breaks (DSBs) caused by ultrasound were evaluated in a quantitative manner by single-molecule fluorescence microscopy. We compared the effect of time-interval (or pulse) sonication to that of continuous wave (CW) sonication at a fixed frequency of 30 kHz. Pulses caused fewer DSBs than CW sonication under the same total input ultrasound energy when the pulse repetition period was above the order of a second. In contrast, pulses caused more DSBs than CW sonication for pulse widths shorter than a second. These effect of ultrasound on DNA were interpreted in terms of the time-dependent decay in the probability of breakage during the duration of a pulse. We propose a simple phenomenological model by considering a characteristic decay in the probability of DSBs during single-pulse sonication, which reproduces the essence of the experimental trend. In addition, a data analysis revealed a characteristic scaling behavior between the number of pulses and the number of DSBs.

Metastable Liquid Cavitation Control (With Memory) Apparatus, Methodology, and Results: For Radiation Detection, Reactor Safety, and Other Industrial Applications

We present a method to simultaneously pressurize fluid filled containers from outside and within, results of experiments with temporary 2 h of fluid precompression followed by overpressure removal before testing for cavitation strength and sensitivity to neutron radiation of multi-mL quantities of widely used unfiltered and undegassed liquids, such as water, ethanol, and dodecane (a surrogate jet fuel), enclosed within containers using glass, epoxy, and steel. We found that in contrast to prior methods involving laborious degassing and purification, a straightforward one-step approach using only a modest 2 h precompression treatment at a pressure of 0.7+ MPa enabled us, reproducibly, to reach directly the highest attainable “negative” (subvacuum) pressures attainable in our apparatus (−0.7  MPa)—enabling efficient sensitivity to neutron-type radiation. Cavitation strength results are explained on theoretical grounds. However, surprisingly using the technique of this paper, the 2-h precompressed (unfiltered, undegassed) fluid also retained memory of this property, after the overpressure was removed, even 3 months later—thereby suggesting that active cavitation nuclei suppression can be extended to long periods of time. Successful results for cavitation suppression (in the absence of ionizing radiation) through −0.7  MPa were also attainable for fluids in simultaneous contact with a combination of glass, steel, and epoxy surfaces. The relative importance of cavitation strength retention at liquid–wall interfaces versus within the bulk of the fluids is reported along with implications for high-efficiency nuclear particle detection and spectroscopy, and nuclear fission water reactor safety thermal-hydraulic assessments for blowdown transients.

Effect of extracorporeal shock wave therapy on the shoulder joint functional status of patients with calcific tendinitis

[Purpose] This study aimed to analyze the effect of extracorporeal shock wave therapy on the shoulder function of patients with calcific tendinitis through a 12-week follow-up. [Subjects and Methods] A total of 34 patients with calcific tendinitis participated in this study. In the extracorporeal shock wave therapy group, 18 patients received 6-week extracorporeal shock wave therapy and 12-week follow-up. The Constant-Murley scale was used to evaluate shoulder joint function. [Results] Analysis of variance showed a significant difference between the measurement periods. The independent t-test showed significant differences between the groups at 2, 6, and 12 weeks. [Conclusion] Extracorporeal shock wave therapy can be an effective treatment method for calcific tendinitis that affects patients’ shoulder function.

Review on Lithotripsy and Cavitation in Urinary Stone Therapy

Cavitation is the sudden formation of vapor bubbles or voids in liquid media and occurs after rapid changes in pressure as a consequence of mechanical forces. It is mostly an undesirable phenomenon. Although the elimination of cavitation is a major topic in the study of fluid dynamics, its destructive nature could be exploited for therapeutic applications. Ultrasonic and hydrodynamic sources are two main origins for generating cavitation. The purpose of this review is to give the reader a general idea about the formation of cavitation phenomenon and existing biomedical applications of ultrasonic and hydrodynamic cavitation. Because of the high number of the studies on ultrasound cavitation in the literature, the main focus of this review is placed on the lithotripsy techniques, which have been widely used for the treatment of urinary stones. Accordingly, cavitation phenomenon and its basic concepts are presented in Section II. The significance of the ultrasound cavitation in the urinary stone treatment is discussed in Section III in detail and hydrodynamic cavitation as an important alternative for the ultrasound cavitation is included in Section IV. Finally, side effects of using both ultrasound and hydrodynamic cavitation in biomedical applications are presented in Section V.

Focused Ultrasound and Lithotripsy

Shock wave lithotripsy has generally been a first choice for kidney stone removal. The shock wave lithotripter uses an order of microsecond pulse durations and up to a 100 MPa pressure spike triggered at approximately 0.5-2 Hz to fragment kidney stones through mechanical mechanisms. One important mechanism is cavitation. We proposed an alternative type of lithotripsy method that maximizes cavitation activity to disintegrate kidney stones using high-intensity focused ultrasound (HIFU). Here we outline the method according to the previously published literature (Matsumoto et al., Dynamics of bubble cloud in focused ultrasound. Proceedings of the second international symposium on therapeutic ultrasound, pp 290-299, 2002; Ikeda et al., Ultrasound Med Biol 32:1383-1397, 2006; Yoshizawa et al., Med Biol Eng Comput 47:851-860, 2009; Koizumi et al., A control framework for the non-invasive ultrasound the ragnostic system. Proceedings of 2009 IEEE/RSJ International Conference on Intelligent Robotics and Systems (IROS), pp 4511-4516, 2009; Koizumi et al., IEEE Trans Robot 25:522-538, 2009). Cavitation activity is highly unpredictable; thus, a precise control system is needed. The proposed method comprises three steps of control in kidney stone treatment. The first step is control of localized high pressure fluctuation on the stone. The second step is monitoring of cavitation activity and giving feedback on the optimized ultrasound conditions. The third step is stone tracking and precise ultrasound focusing on the stone. For the high pressure control we designed a two-frequency wave (cavitation control (C-C) waveform); a high frequency ultrasound pulse (1-4 MHz) to create a cavitation cloud, and a low frequency trailing pulse (0.5 MHz) following the high frequency pulse to force the cloud into collapse. High speed photography showed cavitation collapse on a kidney stone and shock wave emission from the cloud. We also conducted in-vitro erosion tests of model and natural kidney stones. For the model stones, the erosion rate of the C-C waveform showed a distinct advantage with the combined high and low frequency waves over either wave alone. For optimization of the high frequency ultrasound intensity, we investigated the relationship between subharmonic emission from cavitation bubbles and stone erosion volume. For stone tracking we have also developed a non-invasive ultrasound theragnostic system (NIUTS) that compensates for kidney motion. Natural stones were eroded and most of the resulting fragments were less than 1 mm in diameter. The small fragments were small enough to pass through the urethra. The results demonstrate that, with the precise control of cavitation activity, focused ultrasound has the potential to be used to develop a less invasive and more controllable lithotripsy system.

Investigation into the mechanisms of tissue atomization by high-intensity focused ultrasound

Ultrasonic atomization, or the emission of a fog of droplets, was recently proposed to explain tissue fractionation in boiling histotripsy. However, even though liquid atomization has been studied extensively, the mechanisms underlying tissue atomization remain unclear. In the work described here, high-speed photography and overpressure were used to evaluate the role of bubbles in tissue atomization. As static pressure increased, the degree of fractionation decreased, and the ex vivo tissue became thermally denatured. The effect of surface wetness on atomization was also evaluated in vivo and in tissue-mimicking gels, where surface wetness was found to enhance atomization by forming surface instabilities that augment cavitation. In addition, experimental results indicated that wetting collagenous tissues, such as the liver capsule, allowed atomization to breach such barriers. These results highlight the importance of bubbles and surface instabilities in atomization and could be used to enhance boiling histotripsy for transition to clinical use.

Molecular mechanisms of the effect of ultrasound on the fibrinolysis of clots

BACKGROUND

Ultrasound accelerates tissue-type plasminogen activator (t-PA)-induced fibrinolysis of clots in vitro and in vivo.

OBJECTIVE

To identify mechanisms for the enhancement of t-PA-induced fibrinolysis of clots.

METHODS

Turbidity is an accurate and convenient method, not previously used, to follow the effects of ultrasound. Deconvolution microscopy was used to determine changes in structure, while fluorescence recovery after photobleaching was used to characterize the kinetics of binding/unbinding and transport.

RESULTS

The ultrasound pulse repetition frequency affected clot lysis times, but there were no thermal effects. Ultrasound in the absence of t-PA produced a slight but consistent decrease in turbidity, suggesting a decrease in fibrin diameter due solely to the action of the ultrasound, likely caused by an increase in protofibril tension because of vibration from ultrasound. Changes in fibrin network structure during lysis with ultrasound were visualized in real time by deconvolution microscopy, revealing that the network becomes unstable when 30-40% of the protein in the network was digested, whereas without ultrasound, the fibrin network was digested gradually and retained structural integrity. Fluorescence recovery after photobleaching during lysis revealed that the off-rate of oligomers from digesting fibers was little affected, but the number of binding/unbinding sites was increased.

CONCLUSIONS

Ultrasound causes a decrease in the diameter of the fibers due to tension as a result of vibration, leading to increased binding sites for plasmin(ogen)/t-PA. The positive feedback of this structural change together with increased mixing/transport of t-PA/plasmin(ogen) is likely to account for the observed enhancement of fibrinolysis by ultrasound.

Combined short and long-delay tandem shock waves to improve shock wave lithotripsy according to the Gilmore-Akulichev theory

Extracorporeal shock wave lithotripsy is a common non-invasive treatment for urinary stones whose fragmentation is achieved mainly by acoustic cavitation and mechanical stress. A few years ago, in vitro and in vivo experimentation demonstrated that such fragmentation can be improved, without increasing tissue damage, by sending a second shock wave hundreds of microseconds after the previous wave. Later, numerical simulations revealed that if the second pulse had a longer full width at half maximum than a standard shock wave, cavitation could be enhanced significantly. On the other side, a theoretical study showed that stress inside the stone can be increased if two lithotripter shock waves hit the stone with a delay of only 20 μs. We used the Gilmore-Akulichev formulation to show that, in principle, both effects can be combined, that is, stress and cavitation could be increased using a pressure pulse with long full width at half maximum, which reaches the stone within hundreds of microseconds after two 20 μs-delayed initial shock waves. Implementing the suggested pressure profile into clinical devices could be feasible, especially with piezoelectric shock wave sources.

Ultrasonic atomization of liquids in drop-chain acoustic fountains

When focused ultrasound waves of moderate intensity in liquid encounter an air interface, a chain of drops emerges from the liquid surface to form what is known as a drop-chain fountain. Atomization, or the emission of micro-droplets, occurs when the acoustic intensity exceeds a liquid-dependent threshold. While the cavitation-wave hypothesis, which states that atomization arises from a combination of capillary-wave instabilities and cavitation bubble oscillations, is currently the most accepted theory of atomization, more data on the roles of cavitation, capillary waves, and even heat deposition or boiling would be valuable. In this paper, we experimentally test whether bubbles are a significant mechanism of atomization in drop-chain fountains. High-speed photography was used to observe the formation and atomization of drop-chain fountains composed of water and other liquids. For a range of ultrasonic frequencies and liquid sound speeds, it was found that the drop diameters approximately equalled the ultrasonic wavelengths. When water was exchanged for other liquids, it was observed that the atomization threshold increased with shear viscosity. Upon heating water, it was found that the time to commence atomization decreased with increasing temperature. Finally, water was atomized in an overpressure chamber where it was found that atomization was significantly diminished when the static pressure was increased. These results indicate that bubbles, generated by either acoustic cavitation or boiling, contribute significantly to atomization in the drop-chain fountain.

Dependence of pulsed focused ultrasound induced thrombolysis on duty cycle and cavitation bubble size distribution

In this study, we investigated the relationship between the efficiency of pulsed, focused ultrasound (FUS)-induced thrombolysis, the duty cycle (2.3%, 9%, and 18%) and the size distribution of cavitation bubbles. The efficiency of thrombolysis was evaluated through the degree of mechanical fragmentation, namely the number, mass, and size of clot debris particles. First, we found that the total number and mass of clot debris particles were highest when a duty cycle of 9% was used and that the mean diameter of clot debris particles was smallest. Second, we found that the size distribution of cavitation bubbles was mainly centered around the linear resonance radius (2.5μm) of the emission frequency (1.2MHz) of the FUS transducer when a 9% duty cycle was used, while the majority of cavitation bubbles became smaller or larger than the linear resonance radius when a 2.3% or 18% duty cycle was used. In addition, the inertial cavitation dose from the treatment performed at 9% duty cycle was much higher than the dose obtained with the other two duty cycles. The data presented here suggest that there is an optimal duty cycle at which the thrombolysis efficiency and cavitation activity are strongest. They further indicate that using a pulsed FUS may help control the size distribution of cavitation nuclei within an active size range, which we found to be near the linear resonance radius of the emission frequency of the FUS transducer.

Microbubbles for Medical Applications

Ultrasound contrast agent (UCA) suspensions contain encapsulated microbubbles with radii ranging from 1 to 10 micrometers. The bubbles oscillate to the driving ultrasound pulse generating harmonics of the driving ultrasound frequency. This feature allows for the discrimination of non-linear bubble echoes from linear tissue echoes facilitating the visualization and quantification of blood perfusion in organs. Targeting the microbubbles to specific receptors in the body has led to molecular imaging application with ultrasound and targeted drug delivery with drug-loaded microbubbles. Traditional UCA production methods offer high yield but poor control over the microbubble size and uniformity. Medical ultrasound transducers typically operate at a single frequency, therefore only a small selection of bubbles resonates to the driving ultrasound pulse. Here we discuss recent lab-on-a-chip based production and sorting methods that have been shown to produce highly monodisperse bubbles, thereby improving the sensitivity of contrast-enhanced ultrasound imaging and molecular imaging with microbubbles. Moreover, monodisperse UCA show great potential for targeted drug delivery by the well-controlled bubble response.

Removal of residual nuclei following a cavitation event using low-amplitude ultrasound

Microscopic residual bubble nuclei can persist on the order of 1 s following a cavitation event. These bubbles can limit the efficacy of ultrasound therapies such as shock wave lithotripsy and histotripsy, because they attenuate pulses that arrive subsequent to their formation and seed repetitive cavitation activity at a discrete set of sites (cavitation memory). Here, we explore a strategy for the removal of these residual bubbles following a cavitation event, using low-amplitude ultrasound pulses to stimulate bubble coalescence. All experiments were conducted in degassed water and monitored using high-speed photography. In each case, a 2-MHz histotripsy transducer was used to initiate cavitation activity (a cavitational bubble cloud), the collapse of which generated a population of residual bubble nuclei. This residual nuclei population was then sonicated using a 1 ms pulse from a separate 500-kHz transducer, which we term the bubble removal pulse. Bubble removal pulse amplitudes ranging from 0 to 1.7 MPa were tested, and the backlit area of shadow from bubbles remaining in the field following bubble removal was calculated to quantify efficacy. It was found that an ideal amplitude range exists (roughly 180 to 570 kPa) in which bubble removal pulses stimulate the aggregation and subsequent coalescence of residual bubble nuclei, effectively removing them from the field. Further optimization of bubble removal pulse sequences stands to provide an adjunct to cavitation-based ultrasound therapies such as shock wave lithotripsy and histotripsy, mitigating the effects of residual bubble nuclei that currently limit their efficacy.

Flow rate and duty cycle effects in lysis of Chlamydomonas reinhardtii using high-energy pulsed focused ultrasound

To consider microalgae lipid biofuel as a viable energy source, it is a necessity to maximize algal cell lysis, lipid harvest, and thus biofuel production versus the energy used to lyse the cells. Previous techniques have been to use energy consumptive ultrasound waves in the 10-40 kHz range in a stationary exposure environment. This study evaluated the potential of using 1.1 MHz ultrasound pulses in a new flow through type chamber on Chlamydomonas reinhardtii as a model organism for cell breakage. The ultrasound was generated using a spherically focused transducer with a focal length of 6.34 cm and an active diameter of 6.36 cm driven by 20 cycle sine-wave tone bursts at varied pulse repetition frequencies. First, variations in flow rate were examined at a constant duty cycle of 3.6%. After assessing flow rates, the duty cycle was varied to further explore the dependence on the tone burst parameters. Cell lysis was assessed by quantifying protein and chlorophyll release into the supernatant as well as by lipid extractability. Appropriate flow rates with higher duty cycles led to statistically significant increases in cell lysis relative to controls and other exposure conditions.

Lysis of Chlamydomonas reinhardtii by high-intensity focused ultrasound as a function of exposure time

Efficient lysis of microalgae for lipid extraction is an important concern when processing biofuels. Historically, ultrasound frequencies in the range of 10-40 kHz have been utilized for this task. However, greater efficiencies might be achievable if higher frequencies could be used. In our study, we evaluated the potential of using 1.1 MHz ultrasound to lyse microalgae for biofuel production while using Chlamydomonas reinhardtii as a model organism. The ultrasound was generated using a spherically focused transducer with a focal length of 6.34 cm and an active diameter of 6.36 cm driven by 20 cycle sine-wave tone bursts at a pulse repetition frequency of 2 kHz (3.6% duty cycle). The time-average acoustic power output was 26.2 W while the spatial-peak-pulse-average intensity (ISPPA) for each tone burst was 41 kW/cm(2). The peak compressional and rarefactional pressures at the focus were 102 and 17 MPa, respectively. The exposure time was varied for the different cases in the experiments from 5s to 9 min and cell lysis was assessed by quantifying the percentage of protein and chlorophyll release into the supernate as well as the lipid extractability. Free radical generation and lipid oxidation for the different ultrasound exposures were also determined. We found that there was a statistically significant increase in lipid extractability for all of the exposures compared to the control. The longer exposures also completely fragmented the cells releasing almost all of the protein and chlorophyll into the supernate. The cavitation activity did not significantly increase lipid oxidation while there was a minor trend of increased free radical production with increased ultrasound exposure.

Improving the lens design and performance of a contemporary electromagnetic shock wave lithotripter

The efficiency of shock wave lithotripsy (SWL), a noninvasive first-line therapy for millions of nephrolithiasis patients, has not improved substantially in the past two decades, especially in regard to stone clearance. Here, we report a new acoustic lens design for a contemporary electromagnetic (EM) shock wave lithotripter, based on recently acquired knowledge of the key lithotripter field characteristics that correlate with efficient and safe SWL. The new lens design addresses concomitantly three fundamental drawbacks in EM lithotripters, namely, narrow focal width, nonidealized pulse profile, and significant misalignment in acoustic focus and cavitation activities with the target stone at high output settings. Key design features and performance of the new lens were evaluated using model calculations and experimental measurements against the original lens under comparable acoustic pulse energy (E+) of 40 mJ. The -6-dB focal width of the new lens was enhanced from 7.4 to 11 mm at this energy level, and peak pressure (41 MPa) and maximum cavitation activity were both realigned to be within 5 mm of the lithotripter focus. Stone comminution produced by the new lens was either statistically improved or similar to that of the original lens under various in vitro test conditions and was significantly improved in vivo in a swine model (89% vs. 54%, P = 0.01), and tissue injury was minimal using a clinical treatment protocol. The general principle and associated techniques described in this work can be applied to design improvement of all EM lithotripters.

Acoustic bubble removal to enhance SWL efficacy at high shock rate: an in vitro study

Rate-dependent efficacy has been extensively documented in shock wave lithotripsy (SWL) stone comminution, with shock waves (SWs) delivered at a low rate producing more efficient fragmentation in comparison to those delivered at high rates. Cavitation is postulated to be the primary source underlying this rate phenomenon. Residual bubble nuclei that persist along the axis of SW propagation can drastically attenuate the waveform's negative phase, decreasing the energy which is ultimately delivered to the stone and compromising comminution. The effect is more pronounced at high rates, as residual nuclei have less time to passively dissolve between successive shocks. In this study, we investigate a means of actively removing such nuclei from the field using a low-amplitude acoustic pulse designed to stimulate their aggregation and subsequent coalescence. To test the efficacy of this bubble removal scheme, model kidney stones were treated in vitro using a research electrohydraulic lithotripter. SWL was applied at rates of 120, 60, or 30 SW/min with or without the incorporation of bubble removal pulses. Optical images displaying the extent of cavitation in the vicinity of the stone were also collected for each treatment. Results show that bubble removal pulses drastically enhance the efficacy of stone comminution at the higher rates tested (120 and 60 SW/min), while optical images show a corresponding reduction in bubble excitation along the SW axis when bubble removal pulses are incorporated. At the lower rate of 30 SW/min, no difference in stone comminution or bubble excitation was detected with the addition of bubble removal pulses, suggesting that remnant nuclei had sufficient time for more complete dissolution. These results corroborate previous work regarding the role of cavitation in rate-dependent SWL efficacy, and suggest that the effect can be mitigated via appropriate control of the cavitation environment surrounding the stone.

Cavitation-induced streaming in shock wave lithotripsy

Cavitation generated by lithotripter shock waves (SWs) in non-degassed water was studied using a 60 frames-per-second camcorder-recording the migration of microbubbles over successive SWs. Lithotripter SWs were produced using a Dornier DoLi-50 electromagnetic lithotripter at 0.5 and 2 Hz pulse repetition frequency (PRF). Cavitation was affected by PRF and by the power level (PL) of the lithotripter. At slow PRF, such as shots fired many seconds apart, cavitation was relatively sparse and bubble clouds flowed in the direction of SW propagation. When PRF was increased, the bubble clouds generated by one SW were amplified by subsequent SWs. Cloud amplification was accompanied by an apparent change in the pattern of bubble migration. Whereas bubbles continued to enter the field of view from the prefocal side, the main bubble cloud remained near the focal point. This was due to a streaming of bubbles opposite to the direction of SW propagation. Increasing the PL grew the cavitation field and enhanced the flow of bubbles opposite to the direction of SW propagation. Stepping up the PL acted to push the broad cloud progressively prefocally (toward the SW source), shifting the position of the plane at which the opposing directional bubble flows collided. (NIH DK43881).

Evidence for trapped surface bubbles as the cause for the twinkling artifact in ultrasound imaging

The mechanism of the twinkling artifact (TA) that occurs during Doppler ultrasound imaging of kidney stones was investigated. The TA expresses itself in Doppler images as time-varying color. To define the TA quantitatively, beam-forming and Doppler processing were performed on raw per channel radio-frequency data collected when imaging human kidney stones in vitro. Suppression of twinkling by an ensemble of computer-generated replicas of a single radio frequency signal demonstrated that the TA arises from variability among the acoustic signals and not from electronic signal capture or processing. This variability was found to be random, and its suppression by elevated static pressure and return when the pressure was released suggest that the presence of bubbles on the stone surface is the mechanism that gives rise to the TA.

Assessment of a modified acoustic lens for electromagnetic shock wave lithotripters in a swine model

PURPOSE

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.

MATERIALS AND METHODS

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.

RESULTS

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).

CONCLUSIONS

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.

Turbulent water coupling in shock wave lithotripsy

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 mass <2 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.

An efficient treatment strategy for histotripsy by removing cavitation memory

Cavitation memory effects occur when remnants of cavitation bubbles (nuclei) persist in the host medium and act as seeds for subsequent events. In pulsed cavitational ultrasound therapy, or histotripsy, this effect may cause cavitation to repeatedly occur at these seeded locations within a target volume, producing inhomogeneous tissue fractionation or requiring an excess number of pulses to completely homogenize the target volume. We hypothesized that by removing the cavitation memory, i.e., the persistent nuclei, the cavitation bubbles could be induced at random locations in response to each pulse; therefore, complete disruption of a tissue volume may be achieved with fewer pulses. To test the hypothesis, the cavitation memory was passively removed by increasing the intervals between successive pulses, ∆t, from 2, 10, 20, 50 and 100, to 200 ms. Histotripsy treatments were performed in red blood cell tissue phantoms and ex vivo livers using 1-MHz ultrasound pulses of 10 cycles at P-/P+ pressure of 21/59 MPa. The phantom study allowed for direct visualization of the cavitation patterns and the lesion development process in real time using high-speed photography; the ex vivo tissue study provided validation of the memory effect in real tissues. Results of the phantom study showed an exponential decrease in the correlation coefficient between cavitation patterns in successive pulses from 0.5 ± 0.1 to 0.1 ± 0.1 as ∆t increased from 2-200 ms; correspondingly, the lesion was completely fractionated with significantly fewer pulses for longer ∆ts. In the tissue study, given the same number of therapy pulses, complete and homogeneous tissue fractionation with well-defined lesion boundaries was achieved only for ∆t ≥ 100 ms. These results indicated that the removal of the cavitation memory resulted in more efficient treatments and homogeneous lesions.

Characteristics of the secondary bubble cluster produced by an electrohydraulic shock wave lithotripter

This study investigated the characteristics of the secondary bubble cluster produced by an electrohydraulic lithotripter using high-speed imaging and passive cavitation detection techniques. The results showed that (i) the discrepancy of the collapse time between near a flat rigid boundary and in a free field of the secondary bubble cluster was not as significant as that by the primary one; (ii) the secondary bubble clusters were small but in a high bubble density and nonuniform in distribution, and they did not expand and aggregate significantly near a rigid boundary; and (iii) the corresponding bubble collapse was weaker with few microjet formation and bubble rebound. By applying a strong suction flow near the electrode tip, the production of the secondary shock wave (SW) and induced bubble cluster could be disturbed significantly, but without influence on the primary ones. Consequently, stone fragmentation efficiency was reduced from 41.2 ± 7.1% to 32.2 ± 3.5% after 250 shocks (p < 0.05). Altogether, these observations suggest that the secondary bubble cluster produced by an electrohydraulic lithotripter may contribute to its ability for effective stone fragmentation.

A comparison of light spot hydrophone and fiber optic probe hydrophone for lithotripter field characterization

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.

A reduced-order, single-bubble cavitation model with applications to therapeutic ultrasound

Cavitation often occurs in therapeutic applications of medical ultrasound such as shock-wave lithotripsy (SWL) and high-intensity focused ultrasound (HIFU). Because cavitation bubbles can affect an intended treatment, it is important to understand the dynamics of bubbles in this context. The relevant context includes very high acoustic pressures and frequencies as well as elevated temperatures. Relative to much of the prior research on cavitation and bubble dynamics, such conditions are unique. To address the relevant physics, a reduced-order model of a single, spherical bubble is proposed that incorporates phase change at the liquid-gas interface as well as heat and mass transport in both phases. Based on the energy lost during the inertial collapse and rebound of a millimeter-sized bubble, experimental observations were used to tune and test model predictions. In addition, benchmarks from the published literature were used to assess various aspects of model performance. Benchmark comparisons demonstrate that the model captures the basic physics of phase change and diffusive transport, while it is quantitatively sensitive to specific model assumptions and implementation details. Given its performance and numerical stability, the model can be used to explore bubble behaviors across a broad parameter space relevant to therapeutic ultrasound.

Effect of static pressure on acoustic energy radiated by cavitation bubbles in viscous liquids under ultrasound

The effect of static pressure on acoustic emissions including shock-wave emissions from cavitation bubbles in viscous liquids under ultrasound has been studied by numerical simulations in order to investigate the effect of static pressure on dispersion of nano-particles in liquids by ultrasound. The results of the numerical simulations for bubbles of 5 μm in equilibrium radius at 20 kHz have indicated that the optimal static pressure which maximizes the energy of acoustic waves radiated by a bubble per acoustic cycle increases as the acoustic pressure amplitude increases or the viscosity of the solution decreases. It qualitatively agrees with the experimental results by Sauter et al. [Ultrason. Sonochem. 15, 517 (2008)]. In liquids with relatively high viscosity (∼200 mPa s), a bubble collapses more violently than in pure water when the acoustic pressure amplitude is relatively large (∼20 bar). In a mixture of bubbles of different equilibrium radius (3 and 5 μm), the acoustic energy radiated by a 5 μm bubble is much larger than that by a 3 μm bubble due to the interaction with bubbles of different equilibrium radius. The acoustic energy radiated by a 5 μm bubble is substantially increased by the interaction with 3 μm bubbles.