Focused ultrasound to expel calculi from the kidney: safety and efficacy of a clinical prototype device

Steerable ultrasonic propulsion of rigid objects based on circular pressure modulation of a focused sectorial transducer array

As a common disease of human urinary system, the high prevalence and incidence rate of renal calculus have brought heavy burden to society. Traditional ultrasonic lithotripsy struggles with the comprehensive elimination of residual fragments and may inadvertently inflict renal damage. Although focused ultrasound can propel stones by the acoustic radiation force (ARF) with minimal tissue damage and enhanced passage rate, it is still lack of the accurate control for calculi at different locations. A circular pressure modulation approach for steerable ultrasonic propulsion of rigid objects is developed based on a sector-array of focused transducers. The ARF exerted on on-axis rigid spheres (stones) is derived based on acoustic scattering. It is proved that the ARF of focused fields exhibits an axial distribution of increasing followed by decreasing with the peak slightly beyond the focus. As the sphere radius increases, the ARF exerted on spheres at the focus increases accordingly with a decreasing growth rate. Inclined propulsion can be realized by the circular binary pressure modulation with the deflection increased by expanding the angle of power-off sector sources. The maximum deflection angle approaching 60° is determined by the F-number and element number of the sector-array. Experimental propulsions of steel balls are conducted using an 8-element sector-array with motion trajectories captured by a high-speed camera. Distributions of the motion speed and acceleration for steel balls of different radii are calculated through image processing. The ARF of mN level and the deflection angle of 12° are demonstrated by the successful propulsion of steel balls. This research provides an effective and flexible approach of steerable stone propulsion using an ultrasonic power supply without the complex control in amplitude or phase and the high-precision motion of the sector-array, hence promoting the practical application in non-invasive treatment of stones.

Physical Techniques to Remove Residual Stone Fragments in the Urinary System

BACKGROUND

Although significant progress has been made in treatment techniques for renal and ureteral calculi, residual fragments (RF) persisting long after treatment pose a serious threat to patients' health. A variety of novel physical techniques and extraction devices are currently being developed to promote the removal of RF from the urinary system, and a series of in vivo experiments have demonstrated their safety and efficacy.

SUMMARY

External physical vibration lithecbole, magnetic extraction, biocompatible stone adhesive-based methods, and ultrasonic propulsion technologies are examples of innovative therapies that can promote the clearance of RF and improve the stone-free rate. In conclusion, the physical treatment of these RF needs to be optimized and improved. They are a promising technique for improving the efficiency of endovascular urology, and further in vivo studies are needed to confirm their safety and efficacy.

KEY MESSAGES

We have summarized the literature on removal of RF by physical methods in recent years, especially the new progress.

Application of ultrasound imaging in the treatment of urinary tract stones

Urinary tract stones are a common clinical condition that affect millions of individuals worldwide. The management of these stones has evolved significantly over the past 70 years, and ultrasound imaging has emerged as a valuable tool for diagnosis, treatment planning, and follow-up. This review aims to provide an overview of the application of ultrasound imaging in the treatment of urinary tract stones, highlighting its advantages, limitations, and current advancements in the field.

Recent Advances in the Science of Burst Wave Lithotripsy and Ultrasonic Propulsion

Nephrolithiasis is a common, painful condition that requires surgery in many patients whose stones do not pass spontaneously. Recent technologic advances have enabled the use of ultrasonic propulsion to reposition stones within the urinary tract, either to relieve symptoms or facilitate treatment. Burst wave lithotripsy (BWL) has emerged as a noninvasive technique to fragment stones in awake patients without significant pain or renal injury. We review the preclinical and human studies that have explored the use of these two technologies. We envision that BWL will fill an unmet need for the noninvasive treatment of patients with nephrolithiasis.

Bellagio II Report: Terrestrial Applications of Space Medicine Research

AbstractINTRODUCTION: For over 50 yr, investigators have studied the physiological adaptations of the human system during short- and long-duration spaceflight exposures. Much of the knowledge gained in developing health countermeasures for astronauts onboard the International Space Station demonstrate terrestrial applications. To date, a systematic process for translating these space applications to terrestrial human health has yet to be defined.METHODS: In the summer of 2017, a team of 38 international scientists launched the Bellagio ll Summit Initiative. The goals of the Summit were: 1) To identify space medicine findings and countermeasures with highest probability for future terrestrial applications; and 2) To develop a roadmap for translation of these countermeasures to future terrestrial application. The team reviewed public domain literature, NASA databases, and evidence books within the framework of the five-stage National Institutes of Health (NIH) translation science model, and the NASA two-stage translation model. Teams then analyzed and discussed interdisciplinary findings to determine the most significant evidence-based countermeasures sufficiently developed for terrestrial application.RESULTS: Teams identified published human spaceflight research and applied translational science models to define mature products for terrestrial clinical practice.CONCLUSIONS: The Bellagio ll Summit identified a snapshot of space medicine research and mature science with the highest probability of translation and developed a Roadmap of terrestrial application from space medicine-derived countermeasures. These evidence-based findings can provide guidance regarding the terrestrial applications of best practices, countermeasures, and clinical protocols currently used in spaceflight.Sides MB, Johnston SL III, Sirek A, Lee PH, Blue RS, Antonsen EL, Basner M, Douglas GL, Epstein A, Flynn-Evans EE, Gallagher MB, Hayes J, Lee SMC, Lockley SW, Monseur B, Nelson NG, Sargsyan A, Smith SM, Stenger MB, Stepanek J, Zwart SR; Bellagio II Team. Bellagio II report: terrestrial applications of space medicine research. Aerosp Med Hum Perform. 2021; 92(8):650669.

First In-Human Burst Wave Lithotripsy for Kidney Stone Comminution: Initial Two Case Studies

Purpose: To test the effectiveness (Participant A) and tolerability (Participant B) of urinary stone comminution in the first-in-human trial of a new technology, burst-wave lithotripsy (BWL). Materials and Methods: An investigational BWL and ultrasonic propulsion system was used to target a 7-mm kidney stone in the operating room before ureteroscopy (Participant A). The same system was used to target a 7.5 mm ureterovesical junction stone in clinic without anesthesia (Participant B). Results: For Participant A, a ureteroscope inserted after 9 minutes of BWL observed fragmentation of the stone to <2 mm fragments. Participant B tolerated the procedure without pain from BWL, required no anesthesia, and passed the stone on day 15. Conclusions: The first-in-human tests of BWL pulses were successful in that a renal stone was comminuted in <10 minutes, and BWL was also tolerated by an awake subject for a distal ureteral stone. Clinical Trial NCT03873259 and NCT02028559.

Efficacy and Safety of External Physical Vibration Lithecbole After Extracorporeal Shock Wave Lithotripsy or Retrograde Intrarenal Surgery for Urinary Stone: A Systematic Review and Meta-analysis

Introduction: The current study evaluated the efficacy and safety of external physical vibration lithecbole (EPVL) after extracorporeal shock wave lithotripsy (SWL) or retrograde intrarenal surgery (RIRS) for urolithiasis. Methods: Publicized literature was systematically searched from EMBASE, Cochrane Library, PubMed, ScienceDirect, ClinicalTrials.gov, and Web of Science up to February 2020. Fixed-effects or random-effects model was chosen in risk ratio (RR) calculation according to heterogeneity. Quality of evidence was estimated under the guidance of Cochrane handbook. Stone expulsion rate, stone-free rates (SFRs), and complication rates were set as end points. Results: Six randomized controlled trials, including 853 patients, were eligible for analysis. EPVL significantly increased SFR within 3 weeks (RR = 1.17, 95% CI: 1.06-1.29, P = 0.001) and above 3 weeks (RR = 1.19, 95% CI: 1.03-1.37, P = 0.02) after SWL. EPVL also improved SFR within 3 weeks (RR = 1.84, 95% CI: 1.35-2.49, P < 0.0001) and above 3 weeks (RR = 1.53, 95% CI: 1.33-1.77, P < 0.00001) after RIRS. Besides, EPVL can significantly increase SFRs for stones in renal pelvis, lower calix, and multiple locations (all P-value <0.05). Although the overall complication rate was not significantly higher in EPVL + RIRS group, it was found to be 1.38 times higher in EPVL+SWL group (RR = 1.38, 95% CI: 1.06-1.79, P = 0.02), especially the incidence of flank pain (RR = 3.11, 95% CI: 1.02-9.46, P = 0.05). Conclusions: EPVL is effective and safe with high SFRs (especially in renal pelvis, lower calix, and multiple locations) after SWL or RIRS and lower overall complication rate after RIRS in patients with urolithiasis. However, the overall complication rate (especially the incidence of flank pain) was higher after EPVL + SWL.

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.

Quantitative Assessment of Effectiveness of Ultrasonic Propulsion of Kidney Stones

Purpose: Ultrasonic propulsion is an investigative modality to noninvasively image and reposition urinary stones. Our goals were to test safety and effectiveness of new acoustic exposure conditions from a new transducer, and to use simultaneous ureteroscopic and ultrasonic observation to quantify stone repositioning. Materials and Methods: During operation, ultrasonic propulsion was applied transcutaneously, whereas stone targets were visualized ureteroscopically. Exposures were 350 kHz frequency, ≤200 W/cm2 focal intensity, and ≤3-second bursts per push. Ureteroscope and ultrasound (US) videos were recorded. Video clips with and without stone motion were randomized and scored for motion ≥3 mm by independent reviewers blinded to the exposures. Subjects were followed with telephone calls, imaging, and chart review for adverse events. Results: The investigative treatment was used in 18 subjects and 19 kidneys. A total of 62 stone targets were treated ranging in size from a collection of "dust" to 15 mm. Subjects received an average of 17 ± 14 propulsion bursts (per kidney) for a total average exposure time of 40 ± 40 seconds. Independent reviewers scored at least one stone movement ≥3 mm in 18 of 19 kidneys (95%) from the ureteroscope videos and in 15 of 19 kidneys (79%) from the US videos. This difference was probably because of motion out of the US imaging plane. Treatment repositioned stones in two cases that would have otherwise required basket repositioning. No serious adverse events were observed with the device or procedure. Conclusions: Ultrasonic propulsion was shown to be safe, and it effectively repositioned stones in 95% of kidneys despite positioning and access restrictions caused by working in an operating room on anesthetized subjects.

Innovations in Ultrasound Technology in the Management of Kidney Stones

This article reviews new advances in ultrasound technology for urinary stone disease. Recent research to facilitate the diagnosis of nephrolithiasis, including use of the twinkling signal and posterior acoustic shadow, have helped to improve the use of ultrasound examination for detecting and sizing renal stones. New therapeutic applications of ultrasound technology for stone disease have emerged, including ultrasonic propulsion to reposition stones and burst wave lithotripsy to fragment stones noninvasively. The safety, efficacy, and evolution of these technologies in phantom, animal, and human studies are reviewed herein. New developments in these rapidly growing areas of ultrasound research are also highlighted.

Nanosizing Noncrystalline and Porous Silica Material-Naturally Occurring Opal Shale for Systemic Tumor Targeting Drug Delivery

Opal shale, as a naturally occurring and noncrystalline silica material with porous structure, has the potential to be a drug delivery carrier. In this study, we obtained opal shale nanoparticles (OS NPs) through the techniques of ultrasonic emulsion and differential centrifugation. The OS NPs exhibited markedly lower cytotoxicity than crystalline mesoporous silica nanoparticles. The highly porous structure and the strong adsorbability endowed OS NPs with the ability of loading and sustained release of doxorubicin (DOX). DOX-loaded OS NPs improved tumor cellular uptake and antiproliferation compared with free drug. Interestingly, OS NPs possessed strong binding with the nuclear envelope, which can be beneficial to the nucleus localization and apoptosis inducing of loaded DOX. We further demonstrated the tumor passive targeting ability, prolonged blood circulation, and enhanced antitumor effect with limited in vivo toxicity. Our results suggest that OS NPs can be applied for tumor targeting drug delivery, which may have a significant influence on the development of silica-based drug delivery system.

The effect of shear waves in an elastic sphere on the radiation force from a quasi-Gaussian beam

Ultrasound beams are capable of exerting radiation force on scattering or absorbing obstacles. Previously, our team developed a technology to reposition kidney stones using this approach. It is convenient to study the influence of different parameters using a theoretical model based on a spherical shape stone and a quasi-Gaussian acoustic beam. In such an approach, only two geometrical parameters are involved, namely the beam width and the sphere diameter. The radiation force depends on their ratio, as well as on the elastic properties of the liquid and the stone. In this work, numerical modeling was performed to calculate the force acting on an elastic sphere using previously developed theory. Our numerical modeling indicates that the force on a stone is strongest when the beam is slightly wider than the stone. Also, the force created by a narrow beam appears to be the strongest when the beam is targeted to the side of the sphere. These peculiarities of the radiation force are explained by more effective generation of shear waves inside the stone resulting from their effective coupling with the acoustic waves in liquid at the stone edge.

Characterizing the Acoustic Output of an Ultrasonic Propulsion Device for Urinary Stones

A noninvasive ultrasound (US) system to facilitate the passage of small kidney stones has been developed. The device incorporates a software-based US platform programmed with brightness mode and Doppler for visualizing stones, plus long duration focused pulses for repositioning stones using the same transducer. This paper characterizes the acoustic outputs of the ultrasonic propulsion device. Though the application and outputs are unique, measurements were performed based on the regulatory standards for both diagnostic US and extracorporeal lithotripters. The extended length of the pulse, time varying pressure output over the pulse, the use of focused targeting, and the need to regulate the output at shallow depths, however, required modifications to the traditional acoustic measurement methods. Output parameters included spatial-peak intensities, mechanical index (MI), thermal index, pulse energy, focal geometry, and target accuracy. The imaging and Doppler operating modes of the system meet the Food and Drug Administration acoustic power and intensity limits for diagnostic US device. Push mode operates at a maximum MI of 2.2, which is above the limit of 1.9 for diagnostic US, but well below any lithotripsy device and an ISPTA of 548 mW/cm2, which is below the 720-mW/cm2 limit for diagnostic US.

Integrating nanomedicine and imaging

Biomedical engineering and its associated disciplines play a pivotal role in improving our understanding and management of disease. Motivated by past accomplishments, such as the clinical implementation of coronary stents, pacemakers or recent developments in antibody therapies, disease management now enters a new era in which precision imaging and nanotechnology-enabled therapeutics are maturing to clinical translation. Preclinical molecular imaging increasingly focuses on specific components of the immune system that drive disease progression and complications, allowing the in vivo study of potential therapeutic targets. The first multicentre trials highlight the potential of clinical multimodality imaging for more efficient drug development. In this perspective, the role of integrating engineering, nanotechnology, molecular imaging and immunology to yield precision medicine is discussed.This article is part of the themed issue 'Challenges for chemistry in molecular imaging'.

Effect of Stone Size and Composition on Ultrasonic Propulsion Ex Vivo

OBJECTIVE

To evaluate in more detail the effectiveness of a new designed more efficient ultrasonic propulsion for large stones and specific stone compositions in a tissue phantom model. In the first clinical trial of noninvasive ultrasonic propulsion, urinary stones of unknown compositions and sizes up to 10 mm were successfully repositioned.

MATERIALS AND METHODS

The study included 8- to 12-mm stones of 4 different primary compositions (calcium oxalate monohydrate, ammonium acid urate, calcium phosphate, and struvite) and a renal calyx phantom consisting of a 12 mm × 30 mm well in a 10-cm block of tissue-mimicking material. Primary outcome was the number of times a stone was expelled over 10 attempts, with ultrasonic propulsion burst duration varying from 0.5 seconds to 5 seconds.

RESULTS

Overall success rate at expelling stones was 95%. All calcium oxalate monohydrate and ammonium acid urate stones were expelled 100% of the time. The largest stone (12 mm) became lodged within the 12-mm phantom calyx 25% of the time regardless of the burst duration. With the 0.5-second burst, there was insufficient energy to expel the heaviest stone (0.88 g), but there was sufficient energy at the longer burst durations.

CONCLUSION

With a single burst, ultrasonic propulsion successfully moved most stones at least 3 cm and, regardless of size or composition, expelled them from the calyx. Ultrasonic propulsion is limited to the stones smaller than the calyceal space, and for each burst duration, related to maximum stone mass.

Safety and Effectiveness of a Longer Focal Beam and Burst Duration in Ultrasonic Propulsion for Repositioning Urinary Stones and Fragments

PURPOSE

In the first-in-human trial of ultrasonic propulsion, subjects passed collections of residual stone fragments repositioned with a C5-2 probe. Here, effectiveness and safety in moving multiple fragments are compared between the C5-2 and a custom (SC-50) probe that produces a longer focal beam and burst duration.

MATERIALS AND METHODS

Effectiveness was quantified by the number of stones expelled from a calyx phantom consisting of a 30-mm deep, water-filled well in a block of tissue mimicking material. Each probe was positioned below the phantom to move stones against gravity. Single propulsion bursts of 50 ms or 3 s duration were applied to three separate targets: 10 fragments of 2 different sizes (1-2 and 2-3 mm) and a single 4 × 7 mm human stone. Safety studies consisted of porcine kidneys exposed to an extreme dose of 10-minute burst duration, including a 7-day survival study and acute studies with surgically implanted stones.

RESULTS

Although successful in the clinical trial, the shorter focal beam and maximum 50 ms burst duration of the C5-2 probe moved stones, but did not expel any stones from the phantom's 30-mm deep calyx. The results were similar with the SC-50 probe under the same 50 ms burst duration. Longer (3 s) bursts available with the SC-50 probe expelled all stones at both 4.5 and 9.5 cm "skin-to-stone" depths with lower probe heating compared to the C5-2. No abnormal behavior, urine chemistry, serum chemistry, or histological findings were observed within the kidney or surrounding tissues for the 10 min burst duration used in the animal studies.

CONCLUSIONS

A longer focal beam and burst duration improved expulsion of a stone and multiple stone fragments from a phantom over a broad range of clinically relevant penetration depths and did not cause kidney injury in animal studies.

Use of Ultrasound in Pediatric Renal Stone Diagnosis and Surgery

Pediatric urolithiasis is on the rise globally and incidence rates have risen by 6-10% annually over the past couple of decades. Given the increasing incidence, high likelihood of recurrence, and long life expectancy of children, the use of ionizing radiation in the diagnosis, management, and follow up of pediatric urolithiasis has been scrutinized recently and many institutions and organizations have emphasized the use of non-ionizing imaging modality such as ultrasound. This review examines the use of ultrasound in the diagnosis and treatment of pediatric urolithiasis. Specifically, the role of ultrasound in shockwave lithotripsy, percutaneous nephrolithotomy, and, more recently, ureteroscopy will be examined.

Ultrasonic propulsion of kidney stones

PURPOSE OF REVIEW

Ultrasonic propulsion is a novel technique that uses short bursts of focused ultrasonic pulses to reposition stones transcutaneously within the renal collecting system and ureter. The purpose of this review is to discuss the initial testing of effectiveness and safety, directions for refinement of technique and technology, and opinions on clinical application.

RECENT FINDINGS

Preclinical studies with a range of probes, interfaces, and outputs have demonstrated feasibility and consistent safety of ultrasonic propulsion with room for increased outputs and refinement toward specific applications. Ultrasonic propulsion was used painlessly and without adverse events to reposition stones in 14 of 15 human study participants without restrictions on patient size, stone size, or stone location. The initial feasibility study showed applicability in a range of clinically relevant situations, including facilitating passage of residual fragments following ureteroscopy or shock wave lithotripsy, moving a large stone at the ureteropelvic junction with relief of pain, and differentiating large stones from a collection of small fragments.

SUMMARY

Ultrasonic propulsion shows promise as an office-based system for transcutaneously repositioning kidney stones. Potential applications include facilitating expulsion of residual fragments following ureteroscopy or shock wave lithotripsy, repositioning stones prior to treatment, and repositioning obstructing ureteropelvic junction stones into the kidney to alleviate acute renal colic.

Evaluation of dusting versus basketing - can new technologies improve stone-free rates?

Over the past two decades, the management of upper-tract urinary stones has dramatically changed towards an increase in the use of ureteroscopic treatment. This change has been driven by technological advances such as the creation of flexible ureteroscopes with reduced calibre (which now have digital, disposable and dual flexion capability) and holmium lasers with increased power. Two basic principles exist when treating stones ureteroscopically: either creating stone dust and small fragments (<1-2 mm) to theoretically enable spontaneous passage of the small particles or stone fragmentation that enables safe extraction of the stone pieces with a basket or grasper in an efficient manner. Each method has unique advantages and disadvantages, but, ultimately, surgeon preference, stone size, composition, location and intrarenal and/or ureteral anatomy determine which technique is used. To date, clinical trials comparing these two techniques are lacking.

DEVELOPING COMPLETE ULTRASONIC MANAGEMENT OF KIDNEY STONES FOR SPACEFLIGHT

Bone demineralization, dehydration, and stasis put astronauts at an increased risk of forming kidney stones in space. The incidence of kidney stones and the potential for a mission-critical event are expected to rise as expeditions become longer and immediate transport to Earth becomes more problematic. At the University of Washington, we are developing an ultrasound-based stone management system to detect stones with S-mode™ ultrasound imaging, break stones with burst wave lithotripsy (BWL™), and reposition stones with ultrasonic propulsion (UP™) on Earth and in space. This review discusses the development and current state of these technologies, as well as integration on the flexible ultrasound system sponsored by NASA and the National Space Biomedical Research Institute.

First in Human Clinical Trial of Ultrasonic Propulsion of Kidney Stones

PURPOSE

Ultrasonic propulsion is a new technology using focused ultrasound energy applied transcutaneously to reposition kidney stones. We report what are to our knowledge the findings from the first human investigational trial of ultrasonic propulsion toward the applications of expelling small stones and dislodging large obstructing stones.

MATERIALS AND METHODS

Subjects underwent ultrasonic propulsion while awake without sedation in clinic, or during ureteroscopy while anesthetized. Ultrasound and a pain questionnaire were completed before, during and after propulsion. The primary outcome was to reposition stones in the collecting system. Secondary outcomes included safety, controllable movement of stones and movement of stones less than 5 mm and 5 mm or greater. Adverse events were assessed weekly for 3 weeks.

RESULTS

Kidney stones were repositioned in 14 of 15 subjects. Of the 43 targets 28 (65%) showed some level of movement while 13 (30%) were displaced greater than 3 mm to a new location. Discomfort during the procedure was rare, mild, brief and self-limited. Stones were moved in a controlled direction with more than 30 fragments passed by 4 of the 6 subjects who had previously undergone a lithotripsy procedure. The largest stone moved was 10 mm. One patient experienced pain relief during treatment of a large stone at the ureteropelvic junction. In 4 subjects a seemingly large stone was determined to be a cluster of small passable stones after they were moved.

CONCLUSIONS

Ultrasonic propulsion was able to successfully reposition stones and facilitate the passage of fragments in humans. No adverse events were associated with the investigational procedure.

Histotripsy Lesion Formation Using an Ultrasound Imaging Probe Enabled by a Low-Frequency Pump Transducer

When histotripsy pulses shorter than 2 cycles are applied, the formation of a dense bubble cloud relies only on the applied peak negative pressure (p-) exceeding the "intrinsic threshold" of the medium (absolute value of 26-30 MPa in most soft tissues). It has been found that a sub-threshold high-frequency probe pulse (3 MHz) can be enabled by a sub-threshold low-frequency pump pulse (500 kHz) where the sum exceeds the intrinsic threshold, thus generating lesion-producing dense bubble clouds ("dual-beam histotripsy"). Here, the feasibility of using an imaging transducer to provide the high-frequency probe pulse in the dual-beam histotripsy approach is investigated. More specifically, an ATL L7-4 imaging transducer (Philips Healthcare, Andover, MA, USA), pulsed by a V-1 Data Acquisition System (Verasonics, Redmond, WA, USA), was used to generate the high-frequency probe pulses. The low-frequency pump pulses were generated by a 20-element 345-kHz array transducer, driven by a custom high-voltage pulser. These dual-beam histotripsy pulses were applied to red blood cell tissue-mimicking phantoms at a pulse repetition frequency of 1 Hz, and optical imaging was used to visualize bubble clouds and lesions generated in the red blood cell phantoms. The results indicated that dense bubble clouds (and resulting lesions) were generated when the p- of the sub-threshold pump and probe pulses combined constructively to exceed the intrinsic threshold. The average size of the smallest reproducible lesions using the imaging probe pulse enabled by the sub-threshold pump pulse was 0.7 × 1.7 mm, whereas that using the supra-threshold pump pulse alone was 1.4 × 3.7 mm. When the imaging transducer was steered laterally, bubble clouds and lesions were steered correspondingly until the combined p- no longer exceeded the intrinsic threshold. These results were also validated with ex vivo porcine liver experiments. Using an imaging transducer for dual-beam histotripsy can have two advantages: (i) lesion steering can be achieved using the steering of the imaging transducer (implemented with the beamformer of the accompanying programmable ultrasound system), and (ii) treatment can be simultaneously monitored when the imaging transducer is used in conjunction with an ultrasound imaging system.

Non-invasive measurement of the temperature rise in tissue surrounding a kidney stone subjected to ultrasonic propulsion

Transcutaneous focused ultrasound (US) is used to propel kidney stones using acoustic radiation force. It is important to estimate the level of heating generated at the stone/tissue interface for safety assessment. An in-vitro experiment is conducted to measure the temperature rise in a tissue-mimicking phantom with an embedded artificial stone and subjected to a focused beam from an imaging US array. A novel optical-imaging-based thermometry method is described using an optically clear tissue phantom. Measurements are compared to the output from a fine wire thermocouple placed on the stone surface. The optical method has good sensitivity, and it does not suffer from artificial viscous heating typically observed with invasive probes and thermocouples.

Extracorporeal shockwave lithotripsy for upper tract urolithiasis

PURPOSE OF REVIEW

For the last three decades, extracorporeal shockwave lithotripsy (SWL) has been the mainstay of management of urolithiasis; recognized widely by patients and physicians for its noninvasive approach and good outcomes. Recent challenges by endoscopic approaches have driven ongoing research to refine indications, define outcomes and explore innovations.

RECENT FINDINGS

Utilization of SWL remains high, despite increasing utilization of endoscopic approaches. Patient selection is critical--outcomes with percutaneous nephrolithotomy and ureteroscopy after failed SWL are not as good as those obtained in patients who have not had prior SWL. A structured training in ultrasound localization and proper patient positioning can have dramatic impacts on stone-free results. Stone size, location, Hounsfield unit stone attenuation and stone volume remain important predictors of outcomes. Renal cysts may negatively impact outcomes with SWL.

SUMMARY

These recent studies highlight important considerations for patient selection, SWL technique and follow-up for patients undergoing SWL. New technologies hold promise but require further study.

Preclinical safety and effectiveness studies of ultrasonic propulsion of kidney stones

OBJECTIVE

To provide an update on a research device to ultrasonically reposition kidney stones transcutaneously. This article reports preclinical safety and effectiveness studies, survival data, modifications of the system, and testing in a stone-forming porcine model. These data formed the basis for regulatory approval to test the device in humans.

MATERIALS AND METHODS

The ultrasound burst was shortened to 50 ms from previous investigations with 1-s bursts. Focused ultrasound was used to expel 2- to 5-mm calcium oxalate monohydrate stones placed ureteroscopically in 5 pigs. Additionally, de novo stones were imaged and repositioned in a stone-forming porcine model. Acute safety studies were performed targeting 2 kidneys (6 sites) and 3 pancreases (8 sites). Survival studies followed 10 animals for 1 week after simulated treatment. Serum and urine analyses were performed, and tissues were evaluated histologically.

RESULTS

All ureteroscopically implanted stones (6/6) were repositioned out of the kidney in 14 ± 8 minutes with 13 ± 6 bursts. On average, 3 bursts moved a stone more than 4 mm and collectively accounted for the majority of relocation. Stones (3 mm) were detected and repositioned in the 200-kg stone-forming model. No injury was detected in the acute or survival studies.

CONCLUSION

Ultrasonic propulsion is safe and effective in the porcine model. Stones were expelled from the kidney. De novo stones formed in a large porcine model were repositioned. No adverse effects were identified with the acute studies directly targeting kidney or pancreatic tissue or during the survival studies indicating no evidence of delayed tissue injury.

Focused ultrasound to displace renal calculi: threshold for tissue injury

BACKGROUND

The global prevalence and incidence of renal calculi is reported to be increasing. Of the patients that undergo surgical intervention, nearly half experience symptomatic complications associated with stone fragments that are not passed and require follow-up surgical intervention. In a clinical simulation using a clinical prototype, ultrasonic propulsion was proven effective at repositioning kidney stones in pigs. The use of ultrasound to reposition smaller stones or stone fragments to a location that facilitates spontaneous clearance could therefore improve stone-free rates. The goal of this study was to determine an injury threshold under which stones could be safely repositioned.

METHODS

Kidneys of 28 domestic swine were treated with exposures that ranged in duty cycle from 0%-100% and spatial peak pulse average intensities up to 30 kW/cm(2) for a total duration of 10 min. The kidneys were processed for morphological analysis and evaluated for injury by experts blinded to the exposure conditions.

RESULTS

At a duty cycle of 3.3%, a spatial peak intensity threshold of 16,620 W/cm(2) was needed before a statistically significant portion of the samples showed injury. This is nearly seven times the 2,400-W/cm(2) maximum output of the clinical prototype used to move the stones effectively in pigs.

CONCLUSIONS

The data obtained from this study show that exposure of kidneys to ultrasonic propulsion for displacing renal calculi is well below the threshold for tissue injury.

Content and face validation of a curriculum for ultrasonic propulsion of calculi in a human renal model

PURPOSE

Ultrasonic propulsion to reposition urinary tract calculi requires knowledge about ultrasound image capture, device manipulation, and interpretation. The purpose of this study was to validate a cognitive and technical skills curriculum to teach urologists ultrasonic propulsion to reposition kidney stones in tissue phantoms.

MATERIALS AND METHODS

Ten board-certified urologists recruited from a single institution underwent a didactic session on renal ultrasound imaging. Subjects completed technical skills modules in tissue phantoms, including kidney imaging, pushing a stone through a translucent maze, and repositioning a lower pole calyceal stone. Objective cognitive and technical performance metrics were recorded. Subjects completed a questionnaire to ascertain face and content validity on a five-point Likert scale.

RESULTS

Eight urologists (80%) had never attended a previous ultrasound course, and nine (90%) performed renal ultrasounds less frequently than every 6 months. Mean cognitive skills scores improved from 55% to 91% (p<0.0001) on pre- and post-didactic tests. In the kidney phantom, 10 subjects (100%) repositioned the lower pole calyceal stone to at least the lower pole infundibulum, while 9 (90%) successfully repositioned the stone to the renal pelvis. A mean±SD (15.7±13.3) pushes were required to complete the task over an average of 4.6±2.2 minutes. Urologists rated the curriculum's effectiveness and realism as a training tool at a mean score of 4.6/5.0 and 4.1/5.0, respectively.

CONCLUSIONS

The curriculum for ultrasonic propulsion is effective and useful for training urologists with limited ultrasound proficiency in stone repositioning technique. Further studies in animate and human models will be required to assess predictive validity.

Focused Ultrasonic Propulsion of Kidney Stones

Introduction: Our research group is studying a noninvasive transcutaneous ultrasound device to expel small kidney stones or residual post-treatment stone fragments from the kidney.1-3 The purpose of this study was to evaluate the efficacy and safety of ultrasonic propulsion in a live porcine model. Materials and Methods: In domestic female swine (50-60 kg), human stones (calcium oxalate monohydrate) and metalized glass beads (2-8 mm) were ureteroscopically implanted.4 Target stones and beads were placed in the lower half of the kidney and a reference bead was placed in the upper pole. Ultrasonic propulsion was achieved through a single ultrasound system that allowed targeting, stone propulsion, and ultrasound imaging using a Philips HDI C5-2 commercial imaging transducer and a Verasonics diagnostic ultrasound platform. Stone propulsion was achieved through the delivery of 1-second bursts of focused, ultrasound pulses, which consist of 250 finely focused pulses 0.1 milliseconds in duration. Stone propulsion was then observed using fluoroscopy, ultrasound, and visually with the ureteroscope. The kidneys were then perfusion-fixed with glutaraldehyde, embedded in paraffin, sectioned, and stained. Samples were histologically scored for injury by a blinded independent expert. Using the same pulsing scheme, while varying acoustic intensities, an injury threshold and patterns of injury were determined in additional pigs.5,6 Results: Stones were successfully implanted in 14 kidneys. Overall, 17 of 26 (65)% stones/beads were moved the entire distance to the renal pelvis, ureteropelvic junction (UPJ), or proximal ureter. The average procedure time for successfully repositioned stones was 14.2±7.9 minutes with 23±16 push bursts. No gross or histologic damage was identified from the ultrasound propulsion procedure. Under this pulsing scheme, a maximum exposure of 2400 W/cm2 was delivered during each treatment. An intensity threshold of 16,620 W/cm2 was determined at which, above this level, tissue injury consistent with emulsification, necrosis, and hemorrhage appeared to be dose dependent. Conclusions: Ultrasonic propulsion is effective with most stones being relocated to the renal pelvis, UPJ, or proximal ureter in a timely fashion. The procedure appears safe with no evidence of injury. The acoustic intensities delivered at maximum treatment settings are well below the threshold at which injury is observed. The angle and alignment of directional force are the most critical factors determining the efficacy of stone propulsion. We are now pursuing FDA approval for a human feasibility study. No competing financial interests exist. Runtime of video: 5 mins 44 secs Acknowledgments: This work was supported by NIH DK43881, DK092197, NSBRI through NASA NCC 9-58, the Coulter Foundation, and the University of Washington. This material is the result of work supported by resources from the VA Puget Sound Health Care System, Seattle, Washington. We are very grateful for the help of a large team at the University of Washington and the Consortium for Shock Waves in Medicine, which we cannot list in detail.

Focused ultrasonic propulsion of kidney stones: review and update of preclinical technology

INTRODUCTION

A noninvasive tool to reposition kidney stones could have significant impact in the management of stone disease. Our research group has developed a noninvasive transcutaneous ultrasound device. A review and update of the current status of this technology is provided. DISCUSSION OF TECHNOLOGY: Stone propulsion is achieved through short bursts of focused, ultrasonic pulses. The initial system consisted of an eight-element annular array transducer, computer, and separate ultrasound imager. In the current generation, imaging and therapy are completed with one ultrasound system and a commercial probe. This generation allows real-time ultrasound imaging, targeting, and propulsion. Safety and effectiveness for the relocation of calyceal stones have been demonstrated in the porcine model. ROLE IN ENDOUROLOGY: This technology may have applications in repositioning stones as an adjunct to lithotripsy, facilitating clearance of residual fragments after lithotripsy, expelling de novo stones, and potentially repositioning obstructing stones. Human trials are in preparation.

Comparison of tissue injury from focused ultrasonic propulsion of kidney stones versus extracorporeal shock wave lithotripsy

PURPOSE

Focused ultrasonic propulsion is a new noninvasive technique designed to move kidney stones and stone fragments out of the urinary collecting system. However, to our knowledge the extent of tissue injury associated with this technique is not known. We quantitated the amount of tissue injury produced by focused ultrasonic propulsion under simulated clinical treatment conditions and under conditions of higher power or continuous duty cycles. We compared those results to extracorporeal shock wave lithotripsy injury.

MATERIALS AND METHODS

A human calcium oxalate monohydrate stone and/or nickel beads were implanted by ureteroscopy in 3 kidneys of live pigs weighing 45 to 55 kg and repositioned using focused ultrasonic propulsion. Additional pig kidneys were exposed to extracorporeal shock wave lithotripsy level pulse intensity or continuous ultrasound exposure 10 minutes in duration using an ultrasound probe transcutaneously or on the kidney. These kidneys were compared to 6 treated with an unmodified Dornier HM3 lithotripter (Dornier Medical Systems, Kennesaw, Georgia) using 2,400 shocks at 120 shock waves per minute and 24 kV. Histological analysis was performed to assess the volume of hemorrhagic tissue injury created by each technique according to the percent of functional renal volume.

RESULTS

Extracorporeal shock wave lithotripsy produced a mean ± SEM lesion of 1.56% ± 0.45% of functional renal volume. Ultrasonic propulsion produced no detectable lesion with simulated clinical treatment. A lesion of 0.46% ± 0.37% or 1.15% ± 0.49% of functional renal volume was produced when excessive treatment parameters were used with the ultrasound probe placed on the kidney.

CONCLUSIONS

Focused ultrasonic propulsion produced no detectable morphological injury to the renal parenchyma when using clinical treatment parameters but produced injury comparable in size to that of extracorporeal shock wave lithotripsy when using excessive treatment parameters.