Dynamics of bubble oscillation in constrained media and mechanisms of vessel rupture in SWL

Modified shock waves for extracorporeal shock wave lithotripsy: a simulation based on the Gilmore formulation

Extracorporeal shock wave lithotripsy (SWL) is a reliable therapy for the treatment of urolithiasis. Nevertheless, improvements to enhance stone fragmentation and reduce tissue damage are still needed. During SWL, cavitation is one of the most important stone fragmentation mechanisms. Bubbles with a diameter between about 7 and 55μm have been reported to expand and collapse after shock wave passage, forming liquid microjets at velocities of up to 400m/s that contribute to the pulverization of renal calculi. Several authors have reported that the fragmentation efficiency may be improved by using tandem shock waves. Tandem SWL is based on the fact that the collapse of a bubble can be intensified if a second shock wave arrives tenths or even a few hundredths of microseconds before its collapse. The object of this study is to determine if tandem pulses consisting of a conventional shock wave (estimated rise time between 1 and 20ns), followed by a slower second pressure profile (0.8μs rise time), have advantages over conventional tandem SWL. The Gilmore equation was used to simulate the influence of the modified pressure field on the dynamics of a single bubble immersed in water and compare the results with the behavior of the same bubble subjected to tandem shock waves. The influence of the delay between pulses on the dynamics of the collapsing bubble was also studied for both conventional and modified tandem waves. For a bubble of 0.07mm, our results indicate that the modified pressure profile enhances cavitation compared to conventional tandem waves at a wide range of delays (10-280μs). According to this, the proposed pressure profile could be more efficient for SWL than conventional tandem shock waves. Similar results were obtained for a ten times smaller bubble.

Blood-brain barrier disruption and vascular damage induced by ultrasound bursts combined with microbubbles can be influenced by choice of anesthesia protocol

Numerous animal studies have demonstrated that ultrasound bursts combined with a microbubble-based ultrasound contrast agent can temporarily disrupt the blood-brain barrier (BBB) with little or no other apparent effects to the brain. As the BBB is a primary limitation to the use of most drugs in the brain, this method could enable a noninvasive means for targeted drug delivery in the brain. This work investigated whether BBB disruption and vessel damage when overexposure occurs can be influenced by choice of anesthesia protocol, which have different vasoactive effects. Four locations were sonicated transcranially in each brain of 16 rats using an unfocused 532 kHz piston transducer. Burst sonications (10 ms bursts applied at 1 Hz for 60 s) were combined with intravenous Definity (10 μl/kg) injections. BBB disruption was evaluated using contrast-enhanced MRI. Half of the animals were anesthetized with i.p. ketamine and xylazine, and the other half with inhaled isoflurane and oxygen. Over the range of exposure levels tested, MRI contrast enhancement was significantly higher (p < 0.05) for animals anesthetized with ketamine/xylazine. Furthermore, the threshold for extensive erythrocyte extravasation was lower with ketamine/xylazine. These results suggest that BBB disruption and/or vascular damage can be affected by vascular or other factors that are influenced by different anesthesia protocol. These experiments may also have been influenced by the recently reported findings that the circulation time for perfluorocarbon microbubbles is substantially reduced when oxygen is used as the carrier gas.

Mechanisms of primary blast-induced traumatic brain injury: insights from shock-wave research

Traumatic brain injury caused by explosive or blast events is traditionally divided into four phases: primary, secondary, tertiary, and quaternary blast injury. These phases of blast-induced traumatic brain injury (bTBI) are biomechanically distinct and can be modeled in both in vivo and in vitro systems. The primary bTBI injury phase represents the response of brain tissue to the initial blast wave. Among the four phases of bTBI, there is a remarkable paucity of information about the cause of primary bTBI. On the other hand, 30 years of research on the medical application of shockwaves (SW) has given us insight into the mechanisms of tissue and cellular damage in bTBI, including both air-mediated and underwater SW sources. From a basic physics perspective, the typical blast wave consists of a lead SW followed by supersonic flow. The resultant tissue injury includes several features observed in bTBI, such as hemorrhage, edema, pseudoaneurysm formation, vasoconstriction, and induction of apoptosis. These are well-described pathological findings within the SW literature. Acoustic impedance mismatch, penetration of tissue by shock/bubble interaction, geometry of the skull, shear stress, tensile stress, and subsequent cavitation formation, are all important factors in determining the extent of SW-induced tissue and cellular injury. Herein we describe the requirements for the adequate experimental set-up when investigating blast-induced tissue and cellular injury; review SW physics, research, and the importance of engineering validation (visualization/pressure measurement/numerical simulation); and, based upon our findings of SW-induced injury, discuss the potential underlying mechanisms of primary bTBI.

Shock wave technology and application: an update

CONTEXT

The introduction of new lithotripters has increased problems associated with shock wave application. Recent studies concerning mechanisms of stone disintegration, shock wave focusing, coupling, and application have appeared that may address some of these problems.

OBJECTIVE

To present a consensus with respect to the physics and techniques used by urologists, physicists, and representatives of European lithotripter companies.

EVIDENCE ACQUISITION

We reviewed recent literature (PubMed, Embase, Medline) that focused on the physics of shock waves, theories of stone disintegration, and studies on optimising shock wave application. In addition, we used relevant information from a consensus meeting of the German Society of Shock Wave Lithotripsy.

EVIDENCE SYNTHESIS

Besides established mechanisms describing initial fragmentation (tear and shear forces, spallation, cavitation, quasi-static squeezing), the model of dynamic squeezing offers new insight in stone comminution. Manufacturers have modified sources to either enlarge the focal zone or offer different focal sizes. The efficacy of extracorporeal shock wave lithotripsy (ESWL) can be increased by lowering the pulse rate to 60-80 shock waves/min and by ramping the shock wave energy. With the water cushion, the quality of coupling has become a critical factor that depends on the amount, viscosity, and temperature of the gel. Fluoroscopy time can be reduced by automated localisation or the use of optical and acoustic tracking systems. There is a trend towards larger focal zones and lower shock wave pressures.

CONCLUSIONS

New theories for stone disintegration favour the use of shock wave sources with larger focal zones. Use of slower pulse rates, ramping strategies, and adequate coupling of the shock wave head can significantly increase the efficacy and safety of ESWL.

Blood vessel deformations on microsecond time scales by ultrasonic cavitation

Transient interactions among ultrasound, microbubbles, and microvessels were studied using high-speed photomicrography. We observed liquid jets, vessel distention (motion outward against the surrounding tissue), and vessel invagination (motion inward toward the lumen). Contrary to current paradigms, liquid jets were directed away from the nearest vessel wall and invagination exceeded distention. These observations provide insight into the mechanics of bubble-vessel interactions, which appear to depend qualitatively upon the mechanical properties of biological tissues.

A tissue phantom for visualization and measurement of ultrasound-induced cavitation damage

Many ultrasound studies involve the use of tissue-mimicking materials to research phenomena in vitro and predict in vivo bioeffects. We have developed a tissue phantom to study cavitation-induced damage to tissue. The phantom consists of red blood cells suspended in an agarose hydrogel. The acoustic and mechanical properties of the gel phantom were found to be similar to soft tissue properties. The phantom's response to cavitation was evaluated using histotripsy. Histotripsy causes breakdown of tissue structures by the generation of controlled cavitation using short, focused, high-intensity ultrasound pulses. Histotripsy lesions were generated in the phantom and kidney tissue using a spherically focused 1-MHz transducer generating 15 cycle pulses, at a pulse repetition frequency of 100 Hz with a peak negative pressure of 14 MPa. Damage appeared clearly as increased optical transparency of the phantom due to rupture of individual red blood cells. The morphology of lesions generated in the phantom was very similar to that generated in kidney tissue at both macroscopic and cellular levels. Additionally, lesions in the phantom could be visualized as hypoechoic regions on a B-mode ultrasound image, similar to histotripsy lesions in tissue. High-speed imaging of the optically transparent phantom was used to show that damage coincides with the presence of cavitation. These results indicate that the phantom can accurately mimic the response of soft tissue to cavitation and provide a useful tool for studying damage induced by acoustic cavitation.

Blood vessel rupture by cavitation

Cavitation is thought to be one mechanism for vessel rupture during shock wave lithotripsy treatment. However, just how cavitation induces vessel rupture remains unknown. In this work, a high-speed photomicrography system was set up to directly observe the dynamics of bubbles inside blood vessels in ex vivo rat mesenteries. Vascular rupture correlating to observed bubble dynamics were examined by imaging bubble extravasation and dye leakage. The high-speed images show that bubble expansion can cause vessel distention, and bubble collapse can lead to vessel invagination. Liquid jets were also observed to form. Our results suggest that all three mechanisms, vessel distention, invagination and liquid jets, can contribute to vessel rupture.

Morphological changes of the neural cells after blast injury of spinal cord and neuroprotective effects of sodium beta-aescinate in rabbits

OBJECTIVE

Explosive blast neurotrauma is becoming more and more common not only in the military population but also in civilian life due to the ever-present threat of terrorism and accidents. However, little attention has been offered to the studies associated with blast wave-induced spinal cord injury in the literatures. The purpose of this study is to report a rabbit model of explosive blast injury to the spinal cord, to investigate the histological changes, focusing especially on apoptosis, and to reveal whether beta-aescinate (SA) has the neuroprotective effects against the blast injury.

METHODS

Adult male New Zealand white rabbits were randomly divided into sham group, experimental group and SA group. All rabbits except the sham group were exposed to the detonation, produced by the blast tube containing 0.7 g cyclotrimethylene trinitramine, with the mean peak overpressure of 50.4 MP focused on the dorsal surface of T9-T10 level. After evaluation of the neurologic function, spinal cord of the rabbits was removed at 8 h, 1, 3, 7, 14 or 30 days and the H&E staining, EM examination, DNA gel electrophoresis and TUNEL were progressively performed.

RESULTS

The study demonstrated the occurrence of both necrosis and apoptosis at the lesion site. Moreover, the SA therapy could not only improve the neurologic outcomes (P<0.05) but also reduce the loss of motoneuron and TUNEL-positive rate (P<0.05).

CONCLUSIONS

In the rabbit model of explosive blast injury to the spinal cord, the coexistent apoptotic and necrotic changes in cells was confirmed and the SA had neuroprotective effects to the blast injury of the spinal cord in rabbits. This is the first report in which the histological characteristics and drug treatment of the blast injury to the spinal cord is demonstrated.

Additive effect of low-frequency ultrasound and endothelial monocyte-activating polypeptide II on blood-tumor barrier in rats with brain glioma

Brain glioma is a malignant tumor which needs surgery followed by chemotherapy. Low-frequency ultrasound (LFU) and Optison could open blood-tumor barrier (BTB) selectively and noninvasively and thus increase the permeability of BTB. Endothelial monocyte-activating polypeptide II (EMAP-II) induces cytoskeletal remodeling in endothelial cells. In this study, we asked whether LFU, Optison, and/or EMAP-II used in combination have additive effects on increasing the permeability of BTB by tight junction (TJ)-associated protein-dependent manner and thus help understand the possible mechanisms for TJ-based drug delivery to the central nervous system through BTB. Evans Blue assay was used to measure the permeability of BTB in rat model of C6 glioma. The mRNA and protein levels of TJ-associated proteins, claudin-5, occludin, and ZO-1, were determined. Results showed that Evans blue content significantly increased and the mRNA and protein levels of claudin-5, occludin, and ZO-1 significantly reduced after the treatment in groups treated with EMAP-II and LFU combined with or without Optison (LFU+EMAP-II and LFU+Optison+EMAP-II groups) and in the group treated with LFU and Optison (LFU+Optison group). In conclusion, LFU and EMAP-II used in combination have additive effects on increasing the permeability of BTB and remodeling of TJ-associated proteins.

Effect of lithotripter focal width on stone comminution in shock wave lithotripsy

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

Shock wave lithotripsy: advances in technology and technique

Shock wave lithotripsy (SWL) is the only noninvasive method for stone removal. Once considered as a primary option for the treatment of virtually all stones, SWL is now recognized to have important limitations that restrict its use. In particular, the effectiveness of SWL is severely limited by stone burden, and treatment with shock waves carries the risk of acute injury with the potential for long-term adverse effects. Research aiming to characterize the renal response to shock waves and to determine the mechanisms of shock wave action in stone breakage and renal injury has begun to suggest new treatment strategies to improve success rates and safety. Urologists can achieve better outcomes by treating at slower shock wave rate using a step-wise protocol. The aim is to achieve stone comminution using as few shock waves and at as low a power level as possible. Important challenges remain, including the need to improve acoustic coupling, enhance stone targeting, better determine when stone breakage is complete, and minimize the occurrence of residual stone fragments. New technologies have begun to address many of these issues, and hold considerable promise for the future.

Acoustic measurement of bubble size in an inkjet printhead

The volume of a bubble in a piezoinkjet printhead is measured acoustically. The method is based on a numerical model of the investigated system. The piezo not only drives the system but it is also used as a sensor by measuring the current it generates. The numerical model is used to predict this current for a given bubble volume. The inverse problem is to infer the bubble volume from an experimentally obtained piezocurrent. By solving this inverse problem, the size and position of the bubble can thus be measured acoustically. The method is experimentally validated with an inkjet printhead that is augmented with a glass connection channel, through which the bubble was observed optically, while at the same time the piezocurrent was measured. The results from the acoustical measurement method correspond closely to the results from the optical measurement.

Lithotripsy

Shock wave lithotripsy (SWL) is the process of fragmentation of renal or ureteric stones by the use of repetitive shock waves generated outside the body and focused onto the stone. Following its introduction in 1980, SWL revolutionized the treatment of kidney stones by offering patients a non-invasive procedure. It is now seen as a mature technology and its use is perceived to be routine. It is noteworthy that, at the time of its introduction, there was a great effort to discover the mechanism(s) by which it works, and the type of sound field that is optimal. Although nearly three decades of subsequent research have increased the knowledge base significantly, the mechanisms are still controversial. Furthermore there is a growing body of evidence that SWL results in injury to the kidney which may have long-term side effects, such as new onset hypertension, although again there is much controversy within the field. Currently, use of lithotripsy is waning, particularly with the advent of minimally invasive ureteroscopic approaches. The goal here is to review the state of the art in SWL and to present the barriers and challenges that need to be addressed for SWL to deliver on its initial promise of a safe, effective, non-invasive treatment for kidney stones.

Myocardial injury induced by ultrasound-targeted microbubble destruction: evidence for the contribution of myocardial ischemia

Ultrasound-targeted microbubble destruction (UTMD) can cause left ventricular (LV) dysfunction and tissue alterations in rats when high ultrasound (US) energy and long duration of imaging are used. However, the mechanism underlying these alterations remains unclear. The aim of the present work was to investigate the possible role of ischemia in the pathogenesis of the UTMD-induced LV damages in rats. To address this issue, rat hearts were exposed in situ to perfluorocarbon-enhanced sonicated dextrose albumin (PESDA) and US at peak negative pressures of 0.6, 1.2 or 1.8 MPa for 1, 3, 9, 15 or 30 min. Blood pressure and electrocardiogram were continuously recorded during insonation. LV function was assessed before and immediately after US exposure, as well as at 24 h and 7 d. At each time point, groups of rats were euthanized and their hearts were harvested for morphologic analysis. Rats exposed to either PESDA alone or US alone showed no functional or morphologic abnormalities. By contrast, rats exposed to both PESDA and US exhibited transient LV dysfunction, transient ST-segment elevation, premature ventricular contractions, microvascular ruptures, contraction band necrosis and morphologic tissue damage. These bio-effects were spontaneously and completely reversible by one week, except in the groups exposed to the highest peak negative pressure for the longest duration, in which mild dysfunction persisted and interstitial fibrosis developed. In conclusion, simultaneous exposure of rat hearts to PESDA and US in vivo results in significant bio-effects that are similar to myocardial ischemia, including transient regional LV dysfunction, transient ST-segment elevation and myocyte contraction band necrosis.

Hydrogel based tissue mimicking phantom for in-vitro ultrasound contrast agents studies

Ultrasound medical imaging (UMI) is the most widely used image analysis technique, and often requires advanced in-vitro set up to perform morphological and functional investigations. These studies are based on contrast properties both related to tissue structure and injectable contrast agents (CA). In this work, we present a three-dimensional structure composed of two different hydrogels reassembly the microvascular network of a human tissue. This phantom was particularly suitable for the echocontrastographic measurements in human microvascular system. This phantom has been characterized to present the acoustic properties of an animal liver, that is, acoustic impedance (Z) and attenuation coefficient (AC), in UMI signal analysis in particular; the two different hydrogels have been selected to simulate the target organ and the acoustic properties of the vascular system. The two hydrogels were prepared starting from cellulose derivatives to simulating the target organ parenchyma and using a PEG-diacrylate to reproduce the vascular system. Moreover, harmonic analysis was performed on the hydrogel mimicking the liver parenchyma hydrogel to evaluate the ultrasound (US) distortion during echographic measurement. The phantom was employed in the characterization of an experimental US CA. Perfect agreement was found when comparing the hydrogel acoustical properties materials with the corresponding living reference tissues (i.e., vascular and parenchimal tissue).

Ultrasonic excitation of a bubble inside a deformable tube: implications for ultrasonically induced hemorrhage

Various independent investigations indicate that the presence of microbubbles within blood vessels may increase the likelihood of ultrasound-induced hemorrhage. To explore potential damage mechanisms, an axisymmetric coupled finite element and boundary element code was developed and employed to simulate the response of an acoustically excited bubble centered within a deformable tube. As expected, the tube mitigates the expansion of the bubble. The maximum tube dilation and maximum hoop stress were found to occur well before the bubble reached its maximum radius. Therefore, it is not likely that the expanding low pressure bubble pushes the tube wall outward. Instead, simulation results indicate that the tensile portion of the acoustic excitation plays a major role in tube dilation and thus tube rupture. The effects of tube dimensions (tube wall thickness 1-5 microm), material properties (Young's modulus 1-10 MPa), ultrasound frequency (1-10 MHz), and pressure amplitude (0.2-1.0 MPa) on bubble response and tube dilation were investigated. As the tube thickness, tube radius, and acoustic frequency decreased, the maximum hoop stress increased, indicating a higher potential for tube rupture and hemorrhage.

Kidney damage in extracorporeal shock wave lithotripsy: a numerical approach for different shock profiles

In shock-wave lithotripsy--a medical procedure to fragment kidney stones--the patient is subjected to hypersonic waves focused at the kidney stone. Although this procedure is widely applied, the physics behind this medical treatment, in particular the question of how the injuries to the surrounding kidney tissue arise, is still under investigation. To contribute to the solution of this problem, two- and three-dimensional numerical simulations of a human kidney under shock-wave loading are presented. For this purpose a constitutive model of the bio-mechanical system kidney is introduced, which is able to map large visco-elastic deformations and, in particular, material damage. The specific phenomena of cavitation induced oscillating bubbles is modeled here as an evolution of spherical pores within the soft kidney tissue. By means of large scale finite element simulations, we study the shock-wave propagation into the kidney tissue, adapt unknown material parameters and analyze the resulting stress states. The simulations predict localized damage in the human kidney in the same regions as observed in animal experiments. Furthermore, the numerical results suggest that in first instance the pressure amplitude of the shock wave impulse (and not so much its exact time-pressure profile) is responsible for damaging the kidney tissue.

Progress and problems in the application of focused ultrasound for blood-brain barrier disruption

Advances in neuroscience have resulted in the development of new diagnostic and therapeutic agents for potential use in the central nervous system (CNS). However, the ability to deliver the majority of these agents to the brain is limited by the blood-brain barrier (BBB), a specialized structure of the blood vessel wall that hampers transport and diffusion from the blood to the brain. Many CNS disorders could be treated with drugs, enzymes, genes, or large-molecule biotechnological products such as recombinant proteins, if they could cross the BBB. This article reviews the problems of the BBB presence in treating the vast majority of CNS diseases and the efforts to circumvent the BBB through the design of new drugs and the development of more sophisticated delivery methods. Recent advances in the development of noninvasive, targeted drug delivery by MRI-guided ultrasound-induced BBB disruption are also summarized.

Liposomes in ultrasonic drug and gene delivery

Liposome-based drug and gene delivery systems have potential for significant roles in a variety of therapeutic applications. Recently, liposomes have been used to entrap gas and drugs for ultrasound-controlled drug release and ultrasound-enhanced drug delivery. Echogenic liposomes have been produced by different preparation methods, including lyophilization, pressurization, and biotin-avidin binding. Presently, significant in vivo applications of liposomal ultrasound-based drug and gene delivery are being made in cardiac disease, stroke and tumor therapy. Translation of these vehicles into the clinic will require a better understanding of improved physical properties to avoid rapid clearance, as well as of possible side effects, including those of the ultrasound. The aim of this review is to provide orientation for new researchers in the area of ultrasound-enhanced liposome drug and gene delivery.

Potential for cavitation-mediated tissue damage in shockwave lithotripsy

PURPOSE

Shockwave lithotripsy (SWL) injures renal tissue, and cavitation has been reported to mediate some of these effects. Much of the work characterizing the cavitation injury of SWL has been performed in small animals or in vitro. We describe experiments that promote cavitation during SWL and estimate the spatial distribution of the resulting hemorrhagic lesion in a large-animal (porcine) model of clinical lithotripsy.

MATERIALS AND METHODS

The lower pole calix of the left kidney in female farm pigs was targeted for SWL with a Dornier HM3 lithotripter. Intraventricular injections of polystyrene microspheres were made before and at intervals during lithotripsy to blanket systemic circulation with cavitation nuclei. Following SWL, the abdominal viscera were inspected and the kidneys were processed for morphologic analysis.

RESULTS

Extensive surface hemorrhage occurred over both the targeted and contralateral kidneys, along with widespread petechial hemorrhage over the spleen, intestines, and peritoneum. The targeted kidneys developed subcapsular hematomas. Histology revealed focal and diffuse damage to the targeted kidneys and vascular rupture in both kidneys with complete necrosis of the walls of intralobular arteries and veins.

CONCLUSIONS

These results demonstrate the potential for unfocused shockwaves to damage blood vessels outside the focal zone of the lithotripter when the vasculature is seeded with cavitation nuclei. The wide distribution of damage suggests that the acoustic field of a lithotripter delivers negative pressures that exceed the cavitation threshold far off the acoustic axis. The findings underscore that conditions permissive for cavitation can lead to dramatic sequelae during SWL.

Suppression of shocked-bubble expansion due to tissue confinement with application to shock-wave lithotripsy

Estimates are made of the effect of tissue confinement on the response of small bubbles subjected to lithotriptor shock pressures. To do this the Rayleigh-Plesset equation, which governs the dynamics of spherical bubbles, is generalized to treat a bubble in a liquid region (blood), which is in turn encased within an elastic membrane (like a vessel's basement membrane), beyond which a Voigt viscoelastic material models the exterior tissue. Material properties are estimated from a range of measurements available for kidneys and similar soft tissues. Special attention is given to the constitutive modeling of the basement membranes because of their expected importance due to their proximity to the bubble and their toughness. It is found that the highest expected values for the elasticity of the membrane and surrounding tissue are insufficient to suppress bubble growth. The reduced confinement of a cylindrical vessel should not alter this conclusion. Tissue viscosities taken from ultrasound measurements suppress bubble growth somewhat, though not to a degree expected to resist injury. However, the higher reported viscosities measured by other means, which are arguably more relevant to the deformations caused by growing bubbles, do indeed significantly suppress bubble expansion.

Effect of an entrained air bubble on the acoustics of an ink channel

Piezo-driven inkjet systems are very sensitive to air entrapment. The entrapped air bubbles grow by rectified diffusion in the ink channel and finally result in nozzle failure. Experimental results on the dynamics of fully grown air bubbles are presented. It is found that the bubble counteracts the pressure buildup necessary for the droplet formation. The channel acoustics and the air bubble dynamics are modeled. For good agreement with the experimental data it is crucial to include the confined geometry into the model: The air bubble acts back on the acoustic field in the channel and thus on its own dynamics. This two-way coupling limits further bubble growth and thus determines the saturation size of the bubble.

The risk of exposure to diagnostic ultrasound in postnatal subjects: nonthermal mechanisms

This review examines the nonthermal physical mechanisms by which ultrasound can harm tissue in postnatal patients. First the physical nature of the more significant interactions between ultrasound and tissue is described, followed by an examination of the existing literature with particular emphasis on the pressure thresholds for potential adverse effects. The interaction of ultrasonic fields with tissue depends in a fundamental way on whether the tissue naturally contains undissolved gas under normal physiologic conditions. Examples of gas-containing tissues are lung and intestine. Considerable effort has been devoted to investigating the acoustic parameters relevant to the threshold and extent of lung hemorrhage. Thresholds as low as 0.4 MPa at 1 MHz have been reported. The situation for intestinal damage is similar, although the threshold appears to be somewhat higher. For other tissues, auditory stimulation or tactile perception may occur, if rarely, during exposure to diagnostic ultrasound; ultrasound at similar or lower intensities is used therapeutically to accelerate the healing of bone fractures. At the exposure levels used in diagnostic ultrasound, there is no consistent evidence for adverse effects in tissues that are not known to contain stabilized gas bodies. Although modest tissue damage may occur in certain identifiable applications, the risk for induction of an adverse biological effect by a nonthermal mechanism due to exposure to diagnostic ultrasound is extremely small.

The acute and long-term adverse effects of shock wave lithotripsy

Shock wave lithotripsy (SWL) has proven to be a highly effective treatment for the removal of kidney stones. Shock waves (SWs) can be used to break most stone types, and because lithotripsy is the only noninvasive treatment for urinary stones, SWL is particularly attractive. On the downside SWL can cause vascular trauma to the kidney and surrounding organs. This acute SW damage can be severe, can lead to scarring with a permanent loss of functional renal volume, and has been linked to potentially serious long-term adverse effects. A recent retrospective study linking lithotripsy to the development of diabetes mellitus has further focused attention on the possibility that SWL may lead to life-altering chronic effects. Thus, it appears that what was once considered to be an entirely safe means to eliminate renal stones can elicit potentially severe unintended consequences. The purpose of this review is to put these findings in perspective. The goal is to explain the factors that influence the severity of SWL injury, update current understanding of the long-term consequences of SW damage, describe the physical mechanisms thought to cause SWL injury, and introduce treatment protocols to improve stone breakage and reduce tissue damage.

Ultrasound-driven microbubble oscillation and translation within small phantom vessels

The use of ultrasound radiation force to manipulate microbubbles in blood vessels has attracted recent interest as a method to increase the efficiency of ultrasonic molecular imaging and drug delivery. However, recent studies indicate that microbubble oscillation is diminished within small blood vessels, and therefore we investigate microbubble oscillation and translation within 12 microm vessels using high-speed photography. With each 0.1- to 1-MPa ultrasound pulse, microbubbles (radius of 1, 1.5 and 2 microm) within 12 microm tubes translate 5 to 10 times less than those within 200 microm tubes. Application of a pulse train with a high pulse repetition frequency displaces bubbles to the wall of 12- and 200-microm tubes within an interval ( approximately 1 s) that is reasonable for clinical translation. Modeling of coupled oscillation and translation for unconstrained microbubbles, based on a modified Rayleigh-Plesset (RP) and the trajectory equations, is compared with experimental observations and demonstrates agreement for the larger displacements observed within the 200 microm tubes. This study has implications for contrast-assisted ultrasound applications, aiding the manipulation of targeted microbubbles and for further theoretical understanding of the complex bubble dynamics within constrained vessel.

New concepts in shock wave lithotripsy

This monograph reviews the basic principles of shock wave lithotripsy. The focus is on new research on stone fragmentation and tissue injury and how this improved understanding of shock-wave technology is leading to modifications in lithotripsy that will allow this therapy to be a safer, more effective treatment for nephrolithiasis.

Cavitation threshold of microbubbles in gel tunnels by focused ultrasound

The investigation of inertial cavitation in micro-tunnels has significant implications for the development of therapeutic applications of ultrasound such as ultrasound-mediated drug and gene delivery. The threshold for inertial cavitation was investigated using a passive cavitation detector with a center frequency of 1 MHz. Micro-tunnels of various diameters (90 to 800 microm) embedded in gel were fabricated and injected with a solution of Optison(trade mark) contrast agent of concentrations 1.2% and 0.2% diluted in water. An ultrasound pulse of duration 500 ms and center frequency 1.736 MHz was used to insonate the microbubbles. The acoustic pressure was increased at 1-s intervals until broadband noise emission was detected. The pressure threshold at which broadband noise emission was observed was found to be dependent on the diameter of the micro-tunnels, with an average increase of 1.2 to 1.5 between the smallest and the largest tunnels, depending on the microbubble concentration. The evaluation of inertial cavitation in gel tunnels rather than tubes provides a novel opportunity to investigate microbubble collapse in a situation that simulates in vivo blood vessels better than tubes with solid walls do.

A cumulative shear mechanism for tissue damage initiation in shock-wave lithotripsy

Evidence suggests that inertial cavitation plays an important role in the renal injury incurred during shock-wave lithotripsy. However, it is unclear how tissue damage is initiated, and significant injury typically occurs only after a sufficient dose of shock waves. Although it has been suggested that shock-induced shearing might initiate injury, estimates indicate that individual shocks do not produce sufficient shear to do so. In this paper, we hypothesize that the cumulative shear of the many shocks is damaging. This mechanism depends on whether there is sufficient time between shocks for tissue to relax to its unstrained state. We investigate the mechanism with a physics-based simulation model, wherein the basement membranes that define the tubules and vessels in the inner medulla are represented as elastic shells surrounded by viscous fluid. Material properties are estimated from in-vitro tests of renal basement membranes and documented mechanical properties of cells and extracellular gels. Estimates for the net shear deformation from a typical lithotripter shock (approximately 0.1%) are found from a separate dynamic shock simulation. The results suggest that the larger interstitial volume (approximately 40%) near the papilla tip gives the tissue there a relaxation time comparable to clinical shock delivery rates (approximately 1 Hz), thus allowing shear to accumulate. Away from the papilla tip, where the interstitial volume is smaller (approximately 20%), the model tissue relaxes completely before the next shock would be delivered. Implications of the model are that slower delivery rates and broader focal zones should both decrease injury, consistent with some recent observations.

Direct observations of ultrasound microbubble contrast agent interaction with the microvessel wall

Many thousands of contrast ultrasound studies have been conducted in clinics around the world. In addition, the microbubbles employed in these examinations are being widely investigated to deliver drugs and genes. Here, for the first time, the oscillation of these microbubbles in small vessels is directly observed and shown to be substantially different than that predicted by previous models and imaged within large fluid volumes. Using pulsed ultrasound with a center frequency of 1 MHz and peak rarefactional pressure of 0.8 or 2.0 MPa, microbubble expansion was significantly reduced when microbubbles were constrained within small vessels in the rat cecum (p<0.05). A model for microbubble oscillation within compliant vessels is presented that accurately predicts oscillation and vessel displacement within small vessels. As a result of the decreased oscillation in small vessels, a large resting microbubble diameter resulting from agent fusion or a high mechanical index was required to bring the agent shell into contact with the endothelium. Also, contact with the endothelium was observed during asymmetrical collapse, not during expansion. These results will be used to improve the design of drug delivery techniques using microbubbles.

Controlled cavitation in microfluidic systems

We report on cavitation in confined microscopic environments which are commonly called microfluidic or lab-on-a-chip systems. The cavitation bubble is created by focusing a pulsed laser into these structures filled with a light-absorbing liquid. At the center of a 20 microm thick and 1 mm wide channel, pancake-shaped bubbles expand and collapse radially. The bubble dynamics compares with a two-dimensional Rayleigh model and a planar flow field during the bubble collapse is measured. When the bubble is created close to a wall a liquid jet is focused towards the wall, resembling the jetting phenomenon in axisymmetry. The jet flow creates two counter-rotating vortices which stir the liquid at high velocities. For more complex geometries, e.g., triangle- and square-shaped structures, the number of liquid jets recorded correlates with the number of boundaries close to the bubble.

A new electromagnetic shock-wave generator “SLX-F2” with user-selectable dual focus size: ex vivo evaluation of renal injury

Storz Medical AG (Kreutzlingen/Switzerland) has developed a new electromagnetic shockwave (SW) generator, the "SLX-F2", which allows the user to choose between a small-focus, high-pressure treatment regime or a wide-focus, low-pressure option. The aim of this study was to investigate, under standardized conditions, the impact of these two different treatment regimes on SW-induced renal injury. SW-induced renal injury was investigated by using the standardized model of the perfused ex vivo kidney. SWs were applied under ultrasound control in the parenchyma of a kidney pole. Different SW numbers (20, 50, 125, 250, 500, 1,000) were applied in three groups: group A was treated with a wider focus (80 MPa), groups B (60 MPa) and C (120 MPa) with a smaller focus (each parameter setting was repeated ten-fold). Disintegration capacity (measured by crater volume in cubes of plaster of Paris) was the same in groups A and C. After SW exposure, barium sulphate suspension was perfused through the renal artery. The maximum diameter (mm) of the extravasation in the cortex, representing the extent of vascular injury, was measured on X-ray mammography films. H&E staining was performed. In all three groups (A, B, C) a higher number of SWs caused the diameter of the extravasate to increase, with statistical significance appearing at 1,000 shots versus 20 shots (p < 0.05). Vascular injury was not influenced by the focal size and positive peak pressure at identical SW numbers applied. Histology of the focal area showed gap-like defects. Our ex vivo data show that renal vascular injury is independent of the focal diameter of the SW generator at the same peak positive pressure and disintegration power. This confirms the in vivo findings that show renal injury caused by SW as being related to the number of SWs administered. Clinical studies are needed to investigate whether there is any advantage to offering both treatment regimes in one SW machine-for example, by using the "wide-focus, low-pressure" option for kidney stones and the "small-focus, high-pressure" regimen for stones in the ureter. The renal injury caused by either regime remains comparable.

Pressure-dependent effect of shock waves on rat brain: induction of neuronal apoptosis mediated by a caspase-dependent pathway

OBJECT

Shock waves have been experimentally applied to various neurosurgical treatments including fragmentation of cerebral emboli, perforation of cyst walls or tissue, and delivery of drugs into cells. Nevertheless, the application of shock waves to clinical neurosurgery remains challenging because the threshold for shock wave-induced brain injury has not been determined. The authors investigated the pressure-dependent effect of shock waves on histological changes of rat brain, focusing especially on apoptosis.

METHODS

Adult male rats were exposed to a single shot of shock waves (produced by silver azide explosion) at overpressures of 1 or 10 MPa after craniotomy. Histological changes were evaluated sequentially by H & E staining and terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling (TUNEL). The expression of active caspase-3 and the effect of the nonselective caspase inhibitor N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (Z-VAD-FMK) were examined to evaluate the contribution of a caspase-dependent pathway to shock wave-induced brain injury. High-overpressure (> 10 MPa) shock wave exposure resulted in contusional hemorrhage associated with a significant increase in TUNEL-positive neurons exhibiting chromatin condensation, nuclear segmentation, and apoptotic bodies. The maximum increase was seen at 24 hours after shock wave application. Low-overpressure (1 MPa) shock wave exposure resulted in spindle-shaped changes in neurons and elongation of nuclei without marked neuronal injury. The administration of Z-VAD-FMK significantly reduced the number of TUNEL-positive cells observed 24 hours after high-overpressure shock wave exposure (p < 0.01). A significant increase in the cytosolic expression of active caspase-3 was evident 24 hours after high-overpressure shock wave application; this increase was prevented by Z-VAD-FMK administration. Double immunofluorescence staining showed that TUNEL-positive cells were exclusively neurons.

CONCLUSIONS

The threshold for shock wave-induced brain injury is speculated to be under 1 MPa, a level that is lower than the threshold for other organs. High-overpressure shock wave exposure results in brain injury, including neuronal apoptosis mediated by a caspase-dependent pathway. This is the first report in which the pressure-dependent effect of shock wave on the histological characteristics of brain tissue is demonstrated.

Enhancement of vascular permeability with low-frequency contrast-enhanced ultrasound in the chorioallantoic membrane model

PURPOSE

To characterize the effect of low-frequency contrast material-enhanced ultrasound on the vascular endothelium and to determine the parameters and techniques required to deliver a therapeutic agent by using the chorioallantoic membrane (CAM) model.

MATERIALS AND METHODS

All in vivo animal procedures were conducted with institutional Animal Care and Use Committee approval. Extravasation of 8.5-nm-diameter fluorescein isothiocyanate-labeled dextran was evaluated in the vasculature of a chick CAM model. Intravital microscopy was performed during contrast-enhanced ultrasound exposure (1.00 or 2.25 MHz); results were compared with results of electron microscopy of the insonated regions. Data acquired after insonation with greater mechanical stress (n = 30 animals) (mechanical index [MI] > 1.3) and with lower mechanical stress (n = 86 animals) (MI < 1.13) were compared with measurements in control conditions (n = 46 animals). The diameter of affected vessels; number of extravasation sites; extravasation rate, area, and location; and changes in endothelial cells and basement membrane were evaluated. Differences were tested with analysis of variance or the Student t test.

RESULTS

After ultrasound application, convective transport of the model drug was observed through micron-sized openings with a mean fluid velocity of 188.6 microm/sec in the low-stress class and 362.5 microm/sec in the high-stress class. Electron microscopy revealed micron-sized focal endothelial gaps and disseminated blebs, vacuoles, and filopodia extending across tens of microns. The threshold pressure for extravasation was 0.5 MPa for a transmitted center frequency of 1.00 MHz (MI = 0.5) and 1.6 MPa for a frequency of 2.25 MHz (MI = 1.06); thus, the frequency dependence of the threshold was not predicted simply by the MI.

CONCLUSION

Low-frequency contrast-enhanced ultrasound can increase vascular permeability and result in convective extravasation of an 8.5-nm-diameter model drug.

Assessment of shock wave lithotripters via cavitation potential

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

Interaction of lithotripter shockwaves with single inertial cavitation bubbles

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

Interaction between shock wave and single inertial bubbles near an elastic boundary

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

Progressive increase of lithotripter output produces better in-vivo stone comminution

BACKGROUND AND PURPOSE

Shockwave lithotripsy (SWL) has become a first-line intervention for treatment of nephrolithiasis. However, few studies have examined the effects of modifications in the method of shockwave energy administration on comminution efficiency. We propose that a gradual increase in output voltage will produce superior stone fragmentation in comparison with a constant or a decreasing output voltage by optimizing the stress wave and cavitation erosion forces on renal calculi.

MATERIALS AND METHODS

BegoStone phantoms were implanted in the renal pelvis of 11 pigs that underwent SWL at a pulse repetition rate of 1 Hz. Animals in the increasing strategy group (N = 4) were subjected to 18, 20, and 22 kV for 600, 600, and 800 shocks, respectively. The second group (N = 4) received a decreasing strategy of 22, 20, and 18 kV for 800, 600, and 600 shocks, respectively. The third group (N = 3) received all 2000 shocks at 20 kV, mimicking the clinical protocol.

RESULTS

A progressively decreasing strategy and constant output voltage produced a mean comminution efficiency, or percentage of stone fragments <2 mm, of 89.0% +/- 3.3% and 87.6% +/- 1.7%, respectively. The mean comminution efficiency was improved to 96.5% +/- 1.4% by using the increasing strategy (P = 0.01).

CONCLUSIONS

A progressive increase in lithotripter output voltage during SWL can produce greater stone fragmentation than protocols employing constant or decreasing output voltage.

Why stones break better at slow shockwave rates than at fast rates: in vitro study with a research electrohydraulic lithotripter

BACKGROUND AND PURPOSE

Stones break better when the rate of shockwave (SW) delivery is slowed. It has been hypothesized that the greater cavitation accompanying a fast rate shields pulse propagation, thus interfering with the delivery of SW energy to the stone. We tested this idea by correlating waveforms measured at the SW focus with cavitation viewed using high-speed imaging.

MATERIALS AND METHODS

A series of U30 gypsum stones held in a 2-mm mesh basket were exposed to 200 SWs at 30 or 120 SW/min from a research electrohydraulic lithotripter (HM3 clone). Waveforms were collected using a fiberoptic probe hydrophone. High-speed imaging was used to observe cavitation bubbles in the water and at the stone surface.

RESULTS

Stone breakage was significantly better at 30 SW/min than at 120 SW/min. The rate had little effect on SW parameters in the water free field. In the presence of particulates released from stones, the positive pressure of the SW remained unaffected, but the trailing tensile phase of the pulse was significantly reduced at 120 SW/min.

CONCLUSIONS

Cavitation bubbles do not persist between SWs. Thus, mature bubbles from one pulse do not interfere with the next pulse, even at 120 SW/min. However, cavitation nuclei carried by fine particles released from stones can persist between pulses. These nuclei have little effect on the compressive wave but seed cavitation under the influence of the tensile wave. Bubble growth draws energy from the negative-pressure phase of the SW, reducing its amplitude. This likely affects the dynamics of cavitation bubble clusters at the stone surface, reducing the effectiveness of bubble action in stone comminution.

Oscillatory interaction between bubbles and confining microvessels and its implications on clinical vascular injuries of shock-wave lithotripsy

This paper presents a detailed study of the oscillation characteristics of a bubble confined inside a deformable microvessel, whose size is comparable with the bubble size. The vessel's compliance is characterized by a nonlinear relation between the intraluminal pressure and the expansion ratio of the vessel radius, which represents the variation of the vessel stiffness with the pressure of the filling liquid. In this analysis, an initially spherical bubble evolves into an ellipsoid, and the asymmetric oscillation appears immediately after the driving pressure is applied and magnifies with oscillation cycles. Compared with the symmetric oscillation in an unconstrained environment, the vessel constraint makes the bubble contract significantly more and subsequently expand in a more violent rebound, inducing substantially larger peaks of the intraluminal pressure exerted on the vessel wall. A larger initial bubble/vessel radius ratio leads to not only a larger peak but also a higher oscillation frequency of the intraluminal pressure, which are the two most dominating parameters in determining the vessel's failure under cyclic loading. The numerical results have further shown that an increase of the vessel wall stiffness strengthens the asymmetric effect, i.e., a larger peak of the intraluminal pressure with a higher oscillation frequency, and so does a larger pre-existing pressure in the liquid filling the vessel. These findings imply that the asymmetric effect is one of the primary mechanisms for clinical injuries of capillary and small blood vessels and for the higher risk of pediatric and hypertension patients in shock wave lithotripsy.

The effect of reflector geometry on the acoustic field and bubble dynamics produced by an electrohydraulic shock wave lithotripter

A theoretical model for the propagation of shock wave from an axisymmetric reflector was developed by modifying the initial conditions for the conventional solution of a nonlinear parabolic wave equation (i.e., the Khokhlov-Zabolotskaya-Kuznestsov equation). The ellipsoidal reflector of an HM-3 lithotripter is modeled equivalently as a self-focusing spherically distributed pressure source. The pressure wave form generated by the spark discharge of the HM-3 electrode was measured by a fiber optic probe hydrophone and used as source conditions in the numerical calculation. The simulated pressure wave forms, accounting for the effects of diffraction, nonlinearity, and thermoviscous absorption in wave propagation and focusing, were compared with the measured results and a reasonably good agreement was found. Furthermore, the primary characteristics in the pressure wave forms produced by different reflector geometries, such as that produced by a reflector insert, can also be predicted by this model. It is interesting to note that when the interpulse delay time calculated by linear geometric model is less than about 1.5 micros, two pulses from the reflector insert and the uncovered bottom of the original HM-3 reflector will merge together. Coupling the simulated pressure wave form with the Gilmore model was carried out to evaluate the effect of reflector geometry on resultant bubble dynamics in a lithotripter field. Altogether, the equivalent reflector model was found to provide a useful tool for the prediction of pressure wave form generated in a lithotripter field. This model may be used to guide the design optimization of reflector geometries for improving the performance and safety of clinical lithotripters.

Targeted Therapeutic Applications of Acoustically Active Microspheres in the Microcirculation

The targeted delivery of intravascular drugs and genes across the endothelial barrier with only minimal side effects remains a significant obstacle in establishing effective therapies for many pathological conditions. Recent investigations have shown that contrast agent microbubbles, which are typically used for image enhancement in diagnostic ultrasound, may also be promising tools in emergent, ultrasound-based therapies. Explorations of the bioeffects generated by ultrasound-microbubble interactions indicate that these phenomena may be exploited for clinical utility such as in the targeted revascularization of flow-deficient tissues. Moreover, development of this treatment modality may also include using ultrasound-microbubble interactions to deliver therapeutic material to tissues, and reporter genes and therapeutic agents have been successfully transferred from the microcirculation to tissue in various animal models of normal and pathological function. This article reviews the recent studies aimed at using interactions between ultrasound and contrast agent microbubbles in the microcirculation for therapeutic purposes. Furthermore, the authors present investigations involving microspheres that are of a different design compared to current microbubble contrast agents, yet are acoustically active and demonstrate potential as tools for targeted delivery. Future directions necessary to address current challenges and advance these techniques to clinical practicality are also discussed.

Cavitation selectively reduces the negative-pressure phase of lithotripter shock pulses

Measurements using a fiber-optic probe hydrophone, high-speed camera, and B-mode ultrasound showed attenuation of the trailing negative-pressure phase of a lithotripter shock pulse under conditions that favor generation of cavitation bubbles, such as in water with a high content of dissolved gas or at high pulse repetition rate where more cavitation nuclei persisted between pulses. This cavitation-mediated attenuation of the acoustic pulse was also observed to increase with increasing amplitude of source discharge potential, such that the negative-pressure phase of the pulse can remain fixed in amplitude even with increasing source discharge potential.

Shock wave lithotripsy at 60 or 120 shocks per minute: a randomized, double-blind trial

PURPOSE

The rate of shock wave administration is a factor in the per shock efficiency of shock wave lithotripsy (SWL). Experimental evidence suggests that decreasing shock wave frequency from 120 shocks per minute results in improved stone fragmentation. To our knowledge this study is the first to examine the effect of decreased shock wave frequency in patients with renal stones.

MATERIALS AND METHODS

Patients with previously untreated radiopaque stones in the renal collecting system were randomized to SWL at 60 or 120 shocks per minute. They were followed at 2 weeks and 3 months. The primary outcome was the success rate, defined as stone-free status or asymptomatic fragments less than 5 mm 3 months after treatment.

RESULTS

A total of 220 patients were randomized, including 111 to 60 shocks per minute and 109 to 120 shocks per minute. The 2 groups were comparable in regard to age, sex, body mass index, stent status and initial stone area. The success rate was higher for 60 shocks per minute (75% vs 61%, p = 0.027). Patients with larger stones (stone area 100 mm or greater) experienced a greater benefit with treatment at 60 shocks per minute. The success rate was 71% for 60 shocks per minute vs 32% (p = 0.002) and the stone-free rate was 60% vs 28% (p = 0.015). Repeat SWL was required in 32% of patients treated with 120 shocks per minute vs 18% (p = 0.018). Fewer shocks were required with 60 shocks per minute (2,423 vs 2,906, p <0.001) but treatment time was longer (40.6 vs 24.2 minutes, p <0.001). There was a trend toward fewer complications with 60 shocks per minute (p = 0.079).

CONCLUSIONS

SWL treatment at 60 shocks per minute yields better outcomes than at 120 shocks per minute, particularly for stones 100 mm or greater, without any increase in morbidity and with an acceptable increase in treatment time.

Cavitation detection during shock-wave lithotripsy

A system was built to detect cavitation in pig kidney during shock-wave lithotripsy (SWL) with a Dornier HM3 lithotripter. Active detection using echo on B-mode ultrasound, and passive cavitation detection using coincident signals on confocal orthogonal receivers, were used to interrogate the renal collecting system (urine) and the kidney parenchyma (tissue). Cavitation was detected in urine immediately upon shock-wave (SW) administration in urine or urine plus X-ray contrast agent but, in native tissue, cavitation required hundreds of SWs to initiate. Localization of cavitation was confirmed by fluoroscopy, sonography and by thermally marking the kidney using the passive cavitation detection receivers as high-intensity focused ultrasound sources. Cavitation collapse times in tissue and native urine were about the same, but less than in urine after injection of X-ray contrast agent. The finding that cavitation occurs in kidney tissue is a critical step toward determining the mechanisms of tissue injury in SWL.

Cavitation cluster dynamics in shock-wave lithotripsy: part 1. Free field

The spatiotemporal dynamics of cavitation bubble growth and collapse in shock-wave lithotripsy in a free field was studied experimentally. The lithotripter was equipped with two independently triggerable layers of piezoceramics. The front and back layers generated positive pressure amplitudes of 30 MPa and 15 MPa, respectively, and -10 MPa negative amplitude. The time interval between the launch of the shock waves was varied from 0 and 0.1 s, covering the regimens of pulse-modification (regimen A, delay 0 to 4 micros), shock wave-cavitation cluster interaction (B, 4 micros to 64 micros) and shock wave-gas bubble interaction (C, 256 micros to 0.1 s). The time-integrated cavitation activity was most strongly influenced in regimen A and, in regimen B, the spatial distribution of bubbles was altered, whereas enhancement of cavitation activity was observed in regimen C. Quantitative measurements of the spatial- and time-integrated void fractions were obtained with a photographic and light-scattering technique. The preconditions for a reproducible experiment are explained, with the existence of two distinct types of cavitation nuclei, small particles suspended in the liquid and residuals of bubbles from prior cavitation clusters.