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Oleg Sapozhnikov

Senior Principal Engineer

Email

olegs@apl.washington.edu

Phone

206-543-1385

Publications

2000-present and while at APL-UW

Fragmentation of urinary calculi in vitro by burst wave lithotripsy

Maxwell, A.D., B.W. Cunitz, W. Kreider, O.A. Sapozhnikov, R.S. Hsi, J.D. Harper, M.R. Bailey, and M.D. Sorensen, "Fragmentation of urinary calculi in vitro by burst wave lithotripsy," J. Urol., EOR, doi:10.1016/j.juro.2014.08.009, 2014.

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9 Aug 2014

Purpose
We have developed a new method of lithotripsy that uses short, broadly focused bursts of ultrasound rather than shock waves to fragment stones. This study investigated the characteristics of stone comminution by burst wave lithotripsy in vitro.

Materials and Methods
Artificial and natural stones (mean 8.2±3.0 mm, range 5–15 mm) were treated with ultrasound bursts using a focused transducer in a water bath. Stones were exposed to bursts with focal pressure amplitude 𕟮.5 MPa at 200 Hz burst repetition rate until completely fragmented. Ultrasound frequencies of 170 kHz, 285 kHz, and 800 kHz were applied using 3 different transducers. The time to achieve fragmentation for each stone type was recorded, and fragment size distribution was measured by sieving.

Results
Stones exposed to ultrasound bursts were fragmented at focal pressure amplitudes 𕟴.8 MPa at 170 kHz. Fractures appeared along the stone surface, resulting in fragments separating at the surface nearest to the transducer until the stone was disintegrated. All natural and artificial stones were fragmented at the highest focal pressure of 6.5 MPa with treatment durations between a mean of 36 seconds for uric acid to 14.7 minutes for cystine stones. At a frequency of 170 kHz, the largest artificial stone fragments were <4 mm. Exposures at 285 kHz produced only fragments <2 mm, and 800 kHz produced only fragments <1 mm.

Conclusions
Stone comminution with burst wave lithotripsy is feasible as a potential noninvasive treatment method for nephrolithiasis. Adjusting the fundamental ultrasound frequency allows control of stone fragment size.

Addressing nonlinear propagation effects in characterization of high intensity focused ultrasound fields and prediction of thermal and mechanical bioeffects in tissue

Khokhlova, V.A., P.V. Yuldashev, W. Kreider, O.A. Sapozhnikov, M.R. Vailey, T.D. Khokhlova, A.D. Maxwell, and L.A. Crum, "Addressing nonlinear propagation effects in characterization of high intensity focused ultrasound fields and prediction of thermal and mechanical bioeffects in tissue," J. Acoust. Soc. Am., 134, 4153, doi:10.1121/1.4831221, 2013.

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1 Nov 2013

Nonlinear propagation effects are present in most fields generated by high intensity focused ultrasound (HIFU) sources. In some newer HIFU applications, these effects are strong enough to result in the formation of high amplitude shocks that actually determine the therapy and provide a means for imaging. However, there is no standard approach yet accepted to address these effects. Here, a set of combined measurement and modeling methods to characterize nonlinear HIFU fields in water and predict acoustic pressures in tissue is presented. A characterization method includes linear acoustic holography measurements to set a boundary condition to the model and nonlinear acoustic simulations in water for increasing pressure levels at the source. A derating method to determine nonlinear focal fields with shocks in situ is based on the scaling of the source pressure for data obtained in water to compensate for attenuation losses in tissue. The accuracy of the methods is verified by comparing the results with hydrophone and time-to-boil measurements. Major effects associated with the formation of shocks are overviewed. A set of metrics for determining thermal and mechanical bioeffects is introduced and application of the proposed tools to strongly nonlinear HIFU applications is discussed.

An ultrasound system to identify and characterize kidney stones

Cunitz, B.W., B.L. Dunmire, M.D. Sorensen, R. Hsi, F. Lee, O.A. Sapozhnikov, J.D. Harper, and M. Bailey, "An ultrasound system to identify and characterize kidney stones," J. Acoust. Soc. Am., 134, 3976, doi:10.1121/1.4830485, 2013.

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1 Nov 2013

Ultrasound imaging has tissue and blood imaging modes. This report describes development of a kidney stone imaging mode. Two plane pulses generate a B-mode image. Overlaid in color are regions of high decorrelation between the pulses. Our previous data [UMB, 39, 1026-1038 (2013)] indicate the pulses excite bubbles on the stone surface, which causes the decorrelation. As such this mode automatically identifies stones in the image while scanning at a high frame rate. Further in a control box placed on the stone, highly focused beams are scanned across the stone and a harmonic B-mode image is produced to sharpen the lateral resolution. This mode is used to refine the size and shape of the stone. The first mode is used to aid visualization of stones. Our team is also using it to target and track stones that move with respiration during shock wave lithotripsy (SWL) and as an indicator of stone susceptibility to SWL since surface bubbles contribute to comminution. Improved stone sizing by the second mode aids treatment planning, and resolution of surface roughness is another indicator of stone fragility.

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Fragmentation of kidney stones in vitro by focused ultrasound bursts without shock waves

Maxwell, A.D., B.W. Cunitz, W. Kreider, O.A. Sapozhnikov, R.S. Hsi, M.D. Sorensen, J. D. Harper, and M.R. Bailey, "Fragmentation of kidney stones in vitro by focused ultrasound bursts without shock waves," J. Acoust. Soc. Am., 134, 4183, doi:10.1121/1.4831340, 2013.

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1 Nov 2013

Shock wave lithotripsy (SWL) is the most common procedure for treatment of kidney stones. SWL noninvasively delivers high-energy focused shocks to fracture stones into passable fragments. We have recently observed that lower-amplitude, sinusoidal bursts of ultrasound can generate similar fracture of stones. This work investigated the characteristics of stone fragmentation for natural (uric acid, struvite, calcium oxalate, and cystine) and artificial stones treated by ultrasound bursts. Stones were fixed in position in a degassed water tank and exposed to 10-cycle bursts from a 200-kHz transducer with a pressure amplitude of p ≤ 6.5 MPa, delivered at a rate of 40–200 Hz. Exposures caused progressive fractures in the stone surface leading to fragments up to 3 mm. Treatment of artificial stones at different frequencies exhibited an inverse relationship between the resulting fragment sizes and ultrasound frequency. All artificial and natural types of stones tested could be fragmented, but the comminution rate varied significantly with stone composition over a range of 12–630 mg/min. These data suggest that stones can be controllably fragmented by sinusoidal ultrasound bursts, which may offer an alternative treatment strategy to SWL.

Holography and numerical projection methods for characterizing the three-dimensional acoustic fields of arrays in continuous-wave and transient regimes

Kreider, W., A.D. Maxwell, P.V. Yuldashev, B.W. Cunitz, B. Dunmire, O.A. Sapozhnikov, and V.A. Khokhlova, "Holography and numerical projection methods for characterizing the three-dimensional acoustic fields of arrays in continuous-wave and transient regimes," J. Acoust. Soc. Am., 134, 4153, doi:10.1121/1.4831222, 2013.

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1 Nov 2013

The use of projection methods is increasingly accepted as a standard way of characterizing the 3D fields generated by medical ultrasound sources. When combined with hydrophone measurements of pressure amplitude and phase over a surface transverse to the wave propagation, numerical projection can be used to reconstruct 3D fields that account for operational details and imperfections of the source. Here, we use holography measurements to characterize the fields generated by two array transducers with different geometries and modes of operation. First, a seven-element, high-power therapy transducer is characterized in the continuous-wave regime using holography measurements and nonlinear forward-projection calculations. Second, a C5-2 imaging probe (Philips Healthcare) with 128 elements is characterized in the transient regime using holography measurements and linear projection calculations. Results from the numerical projections for both sources are compared with independent hydrophone measurements of select waveforms, including shocked focal waveforms for the therapy transducer. Accurate 3D field representations have been confirmed, though a notable sensitivity to hydrophone calibrations is revealed. Uncertainties associated with this approach are discussed toward the development of holography measurements combined with numerical projections as a standard metrological tool.

Kidney stone fracture by surface waves generated with focused ultrasound tone bursts

Sapozhnikov, O.A., A.D. Maxwell, W. Kreider, B.W. Cunitz, and M.R. Bailey, "Kidney stone fracture by surface waves generated with focused ultrasound tone bursts," J. Acoust. Soc. Am., 134, 4184, doi:10.1121/1.4831341, 2013.

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1 Nov 2013

Previous studies have provided insight into the physical mechanisms of stone fracture in shock wave lithotripsy. Broadly focused shocks efficiently generate shear waves in the stone leading to internal tensile stresses, which in concert with cavitation at the stone surface, cause cracks to form and propagate. Here, we propose a separate mechanism by which stones may fragment from sinusoidal ultrasound bursts without shocks. A numerical elastic wave model was used to simulate propagation of tone bursts through a cylindrical stone at a frequency between 0.15 and 2 MHz. Results suggest that bursts undergo mode conversion into surface waves on the stone that continually create significant stresses well after the exposure is terminated. Experimental exposures of artificial cylindrical stones to focused burst waves in vitro produced periodic fractures along the stone surface. The fracture spacing and resulting fragment sizes corresponded well with the spacing of stresses caused by surface waves in simulation at different frequencies. These results indicate surface waves may be an important factor in fragmentation of stones by focused tone bursts and suggest that the resulting stone fragment sizes may be controlled by ultrasound frequency.

Characterization of a multi-element clinical HIFU system using acoustic halography and nonlinear modeling

Kreider, W., P. Yuldashev, O.A. Sapozhnikov, N. Farr, A. Partanen, M. Bailey, and V.A. Khokhlova, "Characterization of a multi-element clinical HIFU system using acoustic halography and nonlinear modeling," IEEE Trans. Ultrason. Ferr. Freq. Control, 60, 1683-1698, doi:10.1109/TUFFC.2013.2750, 2013.

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1 Aug 2013

High-intensity focused ultrasound (HIFU) is a treatment modality that relies on the delivery of acoustic energy to remote tissue sites to induce thermal and/or mechanical tissue ablation. To ensure the safety and efficacy of this medical technology, standard approaches are needed for accurately characterizing the acoustic pressures generated by clinical ultrasound sources under operating conditions. Characterization of HIFU fields is complicated by nonlinear wave propagation and the complexity of phased-array transducers. Previous work has described aspects of an approach that combines measurements and modeling, and here we demonstrate this approach for a clinical phased-array transducer. First, low amplitude hydrophone measurements were performed in water over a scan plane between the array and the focus. Second, these measurements were used to holographically reconstruct the surface vibrations of the transducer and to set a boundary condition for a 3-D acoustic propagation model. Finally, nonlinear simulations of the acoustic field were carried out over a range of source power levels. Simulation results were compared with pressure waveforms measured directly by hydrophone at both low and high power levels, demonstrating that details of the acoustic field, including shock formation, are quantitatively predicted.

Rectified growth of histotripsy bubbles

Kreider, W., A.D. Maxwell, T. Khokhlova, J.C. Simon, V.A. Khokhlova, O. Sapzhnikov, and M.R. Bailey, "Rectified growth of histotripsy bubbles," Proc., Meetings on Acoustics, 19, 075035, doi:10.1121/1.4800326, 2013.

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2 Jun 2013

Histotripsy treatments use high-amplitude shock waves to fractionate tissue. Such treatments have been demonstrated using both cavitation bubbles excited with microsecond-long pulses and boiling bubbles excited for milliseconds. A common feature of both approaches is the need for bubble growth, where at 1 MHz cavitation bubbles reach maximum radii on the order of 100 microns and boiling bubbles grow to about 1 mm. To explore how histotripsy bubbles grow, a model of a single, spherical bubble that accounts for heat and mass transport was used to simulate the bubble dynamics. Results suggest that the asymmetry inherent in nonlinearly distorted waveforms can lead to rectified bubble growth, which is enhanced at elevated temperatures. Moreover, the rate of this growth is sensitive to the waveform shape, in particular the transition from the peak negative pressure to the shock front. Current efforts are focused on elucidating this behavior by obtaining an improved calibration of measured histotripsy waveforms with a fiber-optic hydrophone, using a nonlinear propagation model to assess the impact on the focal waveform of higher harmonics present at the source's surface, and photographically observing bubble growth rates.

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

Lu, W., O.A. Sapozhnikov, M.R. Bailey, P.J. Kaczkowski, and L.A. Crum, "Evidence for trapped surface bubbles as the cause for the twinkling artifact in ultrasound imaging," Ultrasound Med. Biol., 39, 1026-1038, doi:10.1016/j.ultrasmedbio.2013.01.011, 2013.

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1 Jun 2013

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

Radiation force of an arbitrary acoustic beam on an elastic sphere in a fluid

Sapozhnikov, O., and M.R. Bailey, "Radiation force of an arbitrary acoustic beam on an elastic sphere in a fluid," J. Acoust. Soc. Am., 133, 661-676, doi:10.1121/1.4773924, 2013.

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1 Feb 2013

A theoretical approach is developed to calculate the radiation force of an arbitrary acoustic beam on an elastic sphere in a liquid or gas medium. First, the incident beam is described as a sum of plane waves by employing conventional angular spectrum decomposition. Then, the classical solution for the scattering of a plane wave from an elastic sphere is applied for each plane-wave component of the incident field. The net scattered field is expressed as a superposition of the scattered fields from all angular spectrum components of the incident beam. With this formulation, the incident and scattered waves are superposed in the far field to derive expressions for components of the radiation stress tensor. These expressions are then integrated over a spherical surface to analytically describe the radiation force on an elastic sphere. Limiting cases for particular types of incident beams are presented and are shown to agree with known results. Finally, the analytical expressions are used to calculate radiation forces associated with two specific focusing transducers.

Ultrasonic atomization of tissue and its role in tissue fractionation by high intensity focused ultrasound

Simon, J.C., O.A. Sapozhnikov, V.A. Khokhlova, Y.-N. Wang, L.A. Crum, and M.R. Bailey, "Ultrasonic atomization of tissue and its role in tissue fractionation by high intensity focused ultrasound," Phys. Med. Biol. 57, 8061-8078, doi:10.1088/0031-9155/57/23/8061, 2012.

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7 Dec 2012

Atomization and fountain formation is a well-known phenomenon that occurs when a focused ultrasound wave in liquid encounters an air interface. High intensity focused ultrasound (HIFU) has been shown to fractionate a tissue into submicron-sized fragments in a process termed boiling histotripsy, wherein the focused ultrasound wave superheats the tissue at the focus, producing a millimetre-sized boiling or vapour bubble in several milliseconds. Yet the question of how this millimetre-sized boiling bubble creates submicron-sized tissue fragments remains. The hypothesis of this work is that the tissue can behave as a liquid such that it atomizes and forms a fountain within the vapour bubble produced in boiling histotripsy. We describe an experiment, in which a 2 MHz HIFU transducer (maximum in situ intensity of 24,000 W cm-2) was aligned with an air–tissue interface meant to simulate the boiling bubble. Atomization and fountain formation was observed with high-speed photography and resulted in tissue erosion. Histological examination of the atomized tissue showed whole and fragmented cells and nuclei. Air–liquid interfaces were also filmed. Our conclusion was that HIFU can fountain and atomize tissue. Although this process does not entirely mimic what was observed in liquids, it does explain many aspects of tissue fractionation in boiling histotripsy.

Disintegration of tissue using high intensity focused ultrasound: Two approaches that utilize shock waves

Maxwell, A., O. Sapozhnikov, M. Bailey, L. Crum, Z. Xu, B. Fowlkes, C. Cain, and V. Khokhlova, "Disintegration of tissue using high intensity focused ultrasound: Two approaches that utilize shock waves," Acoust. Today, 8, 24-37, doi:10.1121/1.4788649, 2012.

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1 Oct 2012

Surgery is moving more and more toward minimally-invasive procedures — using laparoscopic approaches with instruments inserted through tiny incisions or catheters placed in blood vessels through puncture sites. These techniques minimize the risks to the patient such as bleeding complications or infection during surgery. Taken a step further, high-intensity focused ultrasound (HIFU) can provide a tool to accomplish many of the same procedures without any incision at all. This article discusses the acoustics of histotripsy — including the processes of generation and focusing of intense ultrasound, the formation of cavitation clouds and rapid boiling in tissue, and the interactions of ultrasound shock waves with bubbles leading to tissue disintegration.

Nonlinear modeling as a metrology tool to characterize high intensity focused ultrasound fields

Khokhlova, V., P. Yuldashev, W. Kreider, O. Sapozhnikov, M. Bailey, and L. Crum, "Nonlinear modeling as a metrology tool to characterize high intensity focused ultrasound fields," J. Acoust. Soc. Am., 132, 1919, doi:10.1121/1.2755042, 2012.

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1 Sep 2012

High intensity focused ultrasound (HIFU) is a rapidly growing medical technology with many clinical applications. The safety and efficacy of these applications require accurate characterization of ultrasound fields produced by HIFU systems. Current nonlinear numerical models based on the KZK and Westervelt wave equations have been shown to serve as quantitatively accurate tools for HIFU metrology. One of the critical parts of the modeling is to set a boundary condition at the source. In previous studies we proposed using measurements of low-amplitude fields to determine the source parameters. In this paper, two approaches of setting the boundary condition are reviewed: The acoustic holography method utilizes two-dimensional scanning of pressure amplitude and phase and numerical back-propagation to the transducer surface. An equivalent source method utilizes one-dimensional pressure measurements on the beam axis and in the focal plane. The dimensions and surface velocity of a uniformly vibrating transducer then are determined to match the one-dimensional measurements in the focal region. Nonlinear simulations are performed for increasing pressure levels at the source for both approaches. Several examples showing the accuracy and capabilities of the proposed methods are presented for typical HIFU transducers with different geometries.

Shear waves in a cubic nonlinear inhomogeneous resonator

Krit, T.B., V.G. Andreev, and O.A. Sapozhnikov, "Shear waves in a cubic nonlinear inhomogeneous resonator," AIP Conference Proceedings Vol. 1474, 19th International Symposium on Nonlinear Acoustics, 21-24 May, Tokyo, Japan, 211-214, doi:10.1063/1.4749333, 2012.

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21 May 2012

We study finite-amplitude shear waves in one-dimensional resonator represented by a layer of rubber-like medium with inhomogeneities in the form of through holes made on the side face. The holes are parallel to the bases and perpendicular to the direction of vibrations. Two different configurations of the resonator: with holes at the bottom and at the top are studied. A rigid plate of finite mass is fixed on the upper surface. The lower boundary of the layer oscillates harmonically with a given acceleration. The equation of motion of particles in the resonator was found using the model of medium with one relaxation time, and a cubic dependence of the shear modulus of deformation. The measurements were performed in a resonator in the form of a rectangular parallelepiped of 15 mm thickness made of a rubber-like polymer plastisol. The linear shear modulus and shear viscosity of the polymer at the first resonant frequency were determined using the finite element method. The amplitudes of the oscillations in the resonator reached a point where the maximum shear strain in the resonator is 0.4 - 0.6, making it possible to observe nonlinear effects. The evolution of the resonance curves at different amplitudes of acceleration was investigated. A harmonic analysis of the acceleration profiles of the upper boundary was performed. The dependence of nonlinear effects on the holes position was studied.

An exact solution to the Helmholtz equation for a quasi-Gaussian beam in the form of a superposition of two sources and sinks with complex coordinates

Sapozhnikov, O.A., "An exact solution to the Helmholtz equation for a quasi-Gaussian beam in the form of a superposition of two sources and sinks with complex coordinates," Acoust. Phys., 58, 41-47, doi: 10.1134/S1063771012010216, 2012.

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3 Feb 2012

An exact solution to the Helmholtz equation is proposed. The solution describes a quasi-Gaussian beam with an arbitrary width and has the form of a superposition of sources and sinks with complex coordinates. It is shown that such a beam always lacks a component that propagates against the principal propagation direction. In addition, when the diameter of the beam exceeds the wavelength, the beam becomes directional in the broad sense: the radiation condition is satisfied with respect to the beam waist plane. For the beam under study, expressions for the angular spectrum and the spherical harmonic expansion coefficients are derived.

Mechanisms for saturation of nonlinear pulsed and periodic signals in focused acoustic beams

Karzova, M.M., M.V. Averiyanov, O.A. Sapozhnikov, and V.A. Khokhlova, "Mechanisms for saturation of nonlinear pulsed and periodic signals in focused acoustic beams," Acoust. Phys., 58, 81-89, doi: 10.1134/S1063771011060078, 2012.

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3 Feb 2012

Acoustic fields of powerful ultrasound sources with Gaussian spatial apodization and initial excitation in the form of a periodic wave or single pulse are examined based on the numerical solution of the Khokhlov-Zabolotskaya-Kuznetsov equation. The influence of nonlinear effects on the spatial structure of focused beams, as well as on the limiting values of the acoustic field parameters is compared. It is demonstrated that pressure saturation in periodic fields is mainly due to the effect of nonlinear absorption at a shock front, while in pulsed fields is due to the effect of nonlinear refraction. The limiting attainable values for the peak positive pressure in periodic fields turned out to be higher than the analogous values in pulsed acoustic fields. The total energy in a beam of periodic waves decreases with the distance from the source faster than in the case of a pulsed field, but it becomes concentrated within much smaller spatial region in the vicinity of the focus. These special features of nonlinear effect manifestation provide an opportunity to use pulsed beams for more efficient delivery of wave energy to the focus and to use periodic beams for attaining higher values of pressure in the focal region.

The dynamics of histotripsy bubbles

Kreider, W., M.R. Bailey, O.A. Sapozhnikov, V.A. Khokhlova, and L.A. Crum, "The dynamics of histotripsy bubbles," in Proc., 10th International Symposium on Therapeutic Ultrasound (ISTU 2010), 9-12 June, Tokyo, Japan, 427-430, doi:10.1063/1.3607944 (AIP Conf. Proc. 1359, 2011).

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9 Jun 2011

Histotripsy describes treatments in which high-amplitude acoustic pulses are used to excite bubbles and erode tissue. Though tissue erosion can be directly attributed to bubble activity, the genesis and dynamics of bubbles remain unclear. Histotripsy lesions that show no signs of thermal coagulative damage have been generated with two different acoustic protocols: relatively long acoustic pulses that produce local boiling within milliseconds and relatively short pulses that are higher in amplitude but likely do not produce boiling. While these two approaches are often distinguished as 'boiling' versus 'cavitation', such labels can obscure similarities. In both cases, a bubble undergoes large changes in radius and vapor is transported into and out of the bubble as it oscillates. Moreover, observations from both approaches suggest that bubbles grow to a size at which they cease to collapse violently. In order to better understand the dynamics of histotripsy bubbles, a single-bubble model has been developed that couples acoustically excited bubble motions to the thermodynamic state of the surrounding liquid. Using this model for bubbles exposed to histotripsy sound fields, simulations suggest that two mechanisms can act separately or in concert to lead to the typically observed bubble growth. First, nonlinear acoustic propagation leads to the evolution of shocks and an asymmetry in the positive and negative pressures that drive bubble motion. This asymmetry can have a rectifying effect on bubble oscillations whereby the bubble grows on average during each acoustic cycle. Second, vapor transport to/from the bubble tends to produce larger bubbles, especially at elevated temperatures. Vapor transport by itself can lead to rectified bubble growth when the ambient temperature exceeds 100C ('boiling') or local heating in the vicinity of the bubble leads to a superheated boundary layer.

Advantages and limitations of the fiber-optic probe hydrophone for characterization of shock waves in water

Pishchalnikov, Y.A., D.F. Gaitan, M.S. Einert, M.R. Bailey, O.A. Sapozhnikov, and J.A. McAteer, "Advantages and limitations of the fiber-optic probe hydrophone for characterization of shock waves in water," J. Acoust. Soc. Am., 129, 2677, doi:10.1121/1.3588972, 2011.

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1 Apr 2011

The fiber-optic probe hydrophone (FOPH) (RP Acoustics, Leutenbach, Germany) is the standard for shock wave measurement, as it is omnidirectional with a flat frequency response ranging from static pressure to several megahertz. The FOPH calibration is determined from the equation of state of water, the optical refractive index of the glass/water interface, and the dc level of reflected light. We tested the accuracy of this calibration by placing the sensitive tip of the FOPH under static pressure up to 140 MPa. The FOPH gave accurate readings of applied static pressures provided there were no defects in the fiber. Defects (cracks and chips) in the glass fiber were difficult to control and could occur during routine handling: stripping, cleaving, or mounting. Such defects led to spurious spikes in measured waveforms. Defects were also caused by cavitation damage to the fiber. In addition, cavitation bubbles on the fiber compressed the fiber and resulted in distorted waveform measurement. Thus, although the FOPH is omnidirectional and accurate from zero to tens of megahertz, it is also susceptible to minute defects in the fiber and to cavitation bubble collapse along the fiber.

Characterization of nonlinearly distorted ultrasound waves in water using broadband laser vibrometry

Sapozhnikov, O.A., B.W. Cunitz, and M.R. Bailey, "Characterization of nonlinearly distorted ultrasound waves in water using broadband laser vibrometry," J. Acoust. Soc. Am., 129, 2678, doi:10.1121/1.3588977, 2011.

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1 Apr 2011

Laser vibrometry is a practical method to detect surface displacement. The method enables a direct measurement of acoustic field parameters such as acoustic particle displacement or acoustic particle velocity. Unlike other sensors, e.g., hydrophones, laser vibrometers are completely non-contact. Such devices are capable of measuring displacements from centimeters to sub nanometers at frequencies from near dc to 10 s of megahertz and have been proven to establish a primary standard for calibrating hydrophones [Bacon, IEEE Trans. UFFC, 35 (1988)]. In this technique, an ultrasonic transducer radiates an acoustic wave which is detected by a thin plastic membrane - a pellicle. The pellicle is effectively transparent to the acoustic beam so that the vibration of the pellicle follows the particle motion in the sound wave, but is reflective to the optical beam of the vibrometer allowing for a measurement. The present talk will report on measurements of nonlinearly distorted sawtooth waves in water performed with two commercial Polytec laser vibrometers: a scanning 24 MHz bandwidth system and a non-scanning 600 MHz bandwidth system. It is shown that appropriately chosen optical targets - pellicle or thick glass block with flat sides - allow resolution of both shock front and the smooth part of the waveform.

Full-diffraction and parabolic axisymmetric numerical models to characterize nonlinear ultrasound fields of two-dimensional therapeutic arrays

Khokhlova, V.A., P.V.Yuldashev, M.V. Averiyanov, O.V. Bessanova, O.A. Sapozhnikov, and M.R. Bailey, "Full-diffraction and parabolic axisymmetric numerical models to characterize nonlinear ultrasound fields of two-dimensional therapeutic arrays," J. Acoust. Soc. Am., 129, 2404, doi:10.1121/1.3587828, 2011.

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1 Apr 2011

Numerical modeling has been shown to be an effective tool to characterize nonlinear pressure fields for single-element HIFU transducers, but it has not yet been applied for the much more complex three-dimensional (3-D) fields generated by therapeutic phased arrays. In this work, two approaches are presented to simulate nonlinear effects in the field of a 256-element focused array. A new full-diffraction approach includes rigorous 3-D simulations of the nonlinear wave equation with a boundary condition given at the elements of the array. A second simpler approach is based on the KZK model and a focused piston source as the boundary condition. The effective aperture and initial pressure of the piston source are set by matching linear simulations of the two models in the focal region. It is shown that as output power is increased, agreement in the focal waveforms of the two simulations, even when shocks were present, is maintained up to very high power outputs of the array. These results demonstrate the feasibility of using the simplified KZK model to evaluate the role of nonlinear effects in the fields of two-dimensional (2-D) phased arrays of clinical devices.

Holographic reconstruction of therapeutic ultrasound sources

Kreider, W., O.A. Sapozhnikov, M.R. Bailey, P.J. Kaczkowski, and V.A. Khokhlova, "Holographic reconstruction of therapeutic ultrasound sources," J. Acoust. Soc. Am. Vol. 129, 2403, doi: 10.1121/1.3587826, 2011.

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1 Apr 2011

Clinical therapeutic ultrasound systems rely on the delivery of known acoustic pressures to treatment sites. Assessing the safety and efficacy of these systems relies upon characterization of ultrasound sources in order to determine the acoustic fields they produce and to understand performance changes over time. While direct hydrophone measurements of intense acoustic fields are possible, data acquisition throughout a treatment volume can be time-consuming and is only applicable to the specific source conditions tested. Moreover, measuring intense acoustic fields poses challenges for the hydrophone. An alternate approach combines low-amplitude pressure measurements with modeling of the nonlinear pressure field at various transducer power levels. In this work, low-intensity measurements were acquired for several therapeutic transducers. Pressure amplitude and phase were measured on a plane near the test transducer; the Rayleigh integral was used to back-propagate the acoustic field and mathematically reconstruct relative vibrations of the transducer surface. Such holographic reconstructions identified the vibratory characteristics of different types of transducers, including a 256-element clinical array. These reconstructions can be used to define boundary conditions for modeling and to record characteristics of transducer performance.

Investigation of the effect of signal amplitude on twinkling artifact

Lu, W., B.W. Cunitz, O.A. Sapozhnikov, P.J. Kaczkowski, J.C. Kucewicz, N.R. Owen, M.R. Bailey, and L.A. Crum, "Investigation of the effect of signal amplitude on twinkling artifact," J. Acoust. Soc. Am., 129, 2376, doi: 10.1121/1.3587698, 2011.

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1 Apr 2011

Twinkling artifact on color Doppler ultrasound is the color labeling of hard objects, such as kidney stones, in the image. The origin of the artifact is unknown, but clinical studies have shown that twinkling artifact can improve the sensitivity of detection of stones by ultrasound. Although Doppler detection normally correlates changes in phase with moving blood, here the effect of amplitude on the artifact is investigated. Radio-frequency and in-phase and quadrature (IQ) data were recorded by pulse-echo ensembles using a software-programmable ultrasound system. Various hard targets in water and in tissue were insonified with a linear probe, and rectilinear pixel-based imaging was used to minimize beam-forming complexity. In addition, synthesized radio-frequency signals were sent directly into the ultrasound system to separate acoustic and signal processing effects. Artifact was observed both in onscreen and post-processed images, and as high statistical variance within the ensemble IQ data. Results showed that twinkling artifact could be obtained from most solid objects by changing the Doppler gain, yet signal amplitude did not have to be sufficiently high to saturate the receive circuits. In addition, low signal but high time gain compensation created the largest variance.

Miniature acoustic fountain mechanism for tissue emulsification during millisecond boiling in high intensity focused ultrasound fields

Simon, J.C., O.A. Sapozhnikov, V.A. Khokhlova, T.D. Khokhlova, M.R. Bailey, and L.A. Crum, "Miniature acoustic fountain mechanism for tissue emulsification during millisecond boiling in high intensity focused ultrasound fields," J. Acoust. Soc. Am., 129, 2478, doi:10.1121/1.3588151, 2011.

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1 Apr 2011

Feasibility of soft tissue emulsification using shock wave heating and millisecond boiling induced by high intensity focused ultrasound was demonstrated recently. However, the mechanism by which the bubbles emulsify tissue is not well understood. High-speed photography of such exposures in transparent gel phantoms shows a milimeter-sized boiling bubble, and histological analysis in tissue samples reveals sub-micron-sized fragments. Here, a novel mechanism of tissue emulsification by the formation of a miniature acoustic fountain within the boiling bubble is tested experimentally using a 2 MHz transducer generating up to 70 MPa positive and 15 MPa negative peak pressures at the focus. The focus was positioned at or 1-2 mm off the plane interface between air and various materials including degassed water, transparent gel, thin sliced muscle tissue phantom, and ex-vivo tissue. Pulsing schemes with duty factors 0.001-0.1, and pulse durations 0.05-500 ms were used. Violent removal of micron-sized fragments and substantial displacement of the phantom surface were observed through high-speed filming. At the end of each exposure, the resulting erosion of the phantom surface and subsurface area was photographed and related to the exposure parameters.

Modeling of radiation force imparted to an elastic sphere from an ultrasound beam of arbitrary structure

Sapozhnikov, O.A. and M.R. Bailey, "Modeling of radiation force imparted to an elastic sphere from an ultrasound beam of arbitrary structure," J. Acoust. Soc. Am., 129, 2377, doi:10.1121/1.3587700, 2011.

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1 Apr 2011

The radiation force created by an acoustic wave incident on an elastic sphere is studied theoretically. Elastic spheres with properties similar to kidney stones are considered. An acoustic wave is taken in the form of a high-intensity focused ultrasound beam of megahertz frequency, which is typical for transducers proposed for stone therapy. To study radiation force of beams with arbitrary structure, the source excitation is modeled as a sum of plane waves of various inclinations (angular spectrum representation). First, a plane acoustic wave scattering at the stone is modeled using the known solution in the form of a spherical harmonics series. Then superposition of such solutions is used to calculate the scattered field from a focused beam. Once the acoustic field is known, the radiation stress tensor is calculated on a surface surrounding the sphere. Finally, the net force acting on the sphere is calculated by integrating the radiation stress along the surface. Numerical calculations show that the direction and value of the radiation force acting on the sphere depend on the pressure field structure in the region where the scatterer is positioned.

Polyvinylidene flouride membrane hydrophone low-frequency response to medical shockwaves

Bailey, M.R., A.D. Maxwell, Y.A. Pishchalnikov, and O.A. Sapozhnikov, "Polyvinylidene flouride membrane hydrophone low-frequency response to medical shockwaves," J. Acoust. Soc. Am., 129, 2677, doi: 10.1121/1.3588971, 2011.

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1 Apr 2011

Lithotripsy shockwaves are particularly difficult to measure because of their wide signal bandwidth and large pressures. A polyvinylidene fluoride (PVDF) membrane hydrophone and preamplifier were built and tested. A broad-focus electromagnetic lithotripter was used to calibrate the PVDF hydrophone. A fiber optic probe hydrophone (FOPH) with known impulse response was used as a measurement standard for secondary calibration. A low-frequency circuit model for the PVDF membrane electrodes in an infinite conductive medium was developed. The model response was compared with signals recorded by the FOPH and PVDF hydrophone at different levels of water conductivity ranging from 1 to 1300 microseconds/cm. Measured waveforms were distorted by high-pass filtering effects of the water conductivity. The model results showed good agreement with the measured waveforms and provided a correction for the system. When the input impedance was altered appropriately or the hydrophone was submerged in a nonconductive fluid, the PVDF and FOPH waveforms appeared nearly identical. The PVDF hydrophone is capable of measuring lithotripsy shockwaves accurately when the low-frequency response is properly taken into account.

Ultrasonic atomization on the tissue-bubble interface as a possible mechanism of tissue erosion in histotripsy

Sapozhnikov, O.A., V.A. Khokhlova, and M.R. Bailey, "Ultrasonic atomization on the tissue-bubble interface as a possible mechanism of tissue erosion in histotripsy," J. Acoust. Soc. Am., 129, 2478, doi:10.1121/1.3588152, 2011.

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1 Apr 2011

When an intense ultrasound beam is directed at a free surface of a liquid, an acoustic fountain is produced that is typically accompanied by ejection of tiny droplets, i.e., liquid atomization. This phenomenon is usually attributed to instability of cavitation-produced capillary waves on the surface. In addition to capillary effects, a process called spallation may also contribute. Although the acoustic fountain is typically observed at a flat liquid surface, nothing prohibits the atomization from occurring at a curved surface. This brings about the possibility to create an acoustic fountain and droplet emission at the surface of a gas cavity in liquid or, similarly, in the bulk of soft biological tissue. The appropriate condition occurs when high-intensity ultrasound is focused in tissue and creates large (0.1 - 1 mm in diameter) bubbles due to acoustic cavitation or rapid boiling. To test this hypothesis, acoustic pressure distribution and the corresponding radiation force on the empty spherical cavity were calculated using finite difference modeling and spherical harmonic expansion. It is shown that in histotripsy regimes appropriate conditions appear for the atomization, which may be considered as a possible mechanism of tissue erosion.

Characterization of cylindrical ultrasonic transducers using acoustic holography.

Tsysar, S.A., Y.D. Sinenikov, and O.A. Sapozhnikov, "Characterization of cylindrical ultrasonic transducers using acoustic holography." Acoust. Phys., 57, 94-105, doi:10.1134/S1063771011010167, 2011.

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15 Jan 2011

We present the results of studying the vibrational velocity distribution over the surface of cylindrical ultrasound transducers by acoustic holography. We describe two approaches for acoustic holography: the spatial spectrum method and the Rayleigh integral method. In the case of cylindrical sources the spectral method has a specific feature in comparison to the case of quasi-plane sources: small-scale spectrum components having the form of evanescent (nonpropagating) waves near the source, turn into propagating waves at a certain distance from the source. The use of such a mixed type of waves makes it possible to increase the holographic resolution. To conduct holography of cylindrical sources by the Rayleigh integral method, a modification consisting in the superimposing of boundaries on the integration region is proposed. We present the results of numerical simulation and physical experiments on holography of small cylindrical piezoelectric transducers. We demonstrate that the proposed methods of holography make it possible to recover the vibration structure of source surfaces up to order of the wavelength scales.

Determination of the elastic properties of layered materials using laser excitation of ultrasound

Karabutov, A.A., A.A. Karabutov, and O.A. Sapozhnikov, "Determination of the elastic properties of layered materials using laser excitation of ultrasound," Phys. Wave Phenom., 18, 297-302, doi:10.3103/S1541308X10040126, 2010.

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6 Dec 2010

It is proposed to use ultrasonic signals excited by a laser pulse to investigate the elastic properties (impedances, speeds of sound, and densities) of layered media. The results of studying both a model medium with known parameters and a layered composite are reported. The experimental data are in good agreement with the known properties of the samples investigated.

Nonlinear propagation of spark-generated N-waves in air: Modeling and measurements using acoustical and optical methods

Yuldashev, P., S. Ollivier, M. Averiyanov, O. Sapozhnikov, V. Khokhlova, and P. Blanc-Benon, "Nonlinear propagation of spark-generated N-waves in air: Modeling and measurements using acoustical and optical methods," J. Acoust. Soc. Am., 128, 3321-3333, doi:10.1121/1.3505106, 2010.

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1 Dec 2010

The propagation of nonlinear spherically diverging N-waves in homogeneous air is studied experimentally and theoretically. A spark source is used to generate high amplitude (1.4 kPa) short duration (40 microseconds) N-waves; acoustic measurements are performed using microphones (3 mm diameter, 150 kHz bandwidth). Numerical modeling with the generalized Burgers equation is used to reveal the relative effects of acoustic nonlinearity, thermoviscous absorption, and oxygen and nitrogen relaxation on the wave propagation.

The results of modeling are in a good agreement with the measurements in respect to the wave amplitude and duration. However, the measured rise time of the front shock is ten times longer than the calculated one, which is attributed to the limited bandwidth of the microphone. To better resolve the shock thickness, a focused shadowgraphy technique is used. The recorded optical shadowgrams are compared with shadow patterns predicted by geometrical optics and scalar diffraction model of light propagation. It is shown that the geometrical optics approximation results in overestimation of the shock rise time, while the diffraction model allows to correctly resolve the shock width. A combination of microphone measurements and focused optical shadowgraphy is therefore a reliable way of studying evolution of spark-generated shock waves in air.

Ureteroscopic ultrasound technology to size kidney stone fragments: Proof of principle using a miniaturized probe in a porcine model

Sorensen M.D., A.R. Shah, M.S. Canney, O.A. Sapozhnikov, J.M. Teichman, and M.R. Bailey, "Ureteroscopic ultrasound technology to size kidney stone fragments: Proof of principle using a miniaturized probe in a porcine model," J. Endourol., 24, 939-942, 2010.

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1 Jun 2010

A prototype ultrasound-based probe for use in ureteroscopy was used for in vitro measurements of stone fragments in a porcine kidney. Fifteen human stones consisting of three different compositions were placed deep in the collecting system of a porcine kidney. A 2 MHz, 1.2 mm (3.6F) needle hydrophone was used to send and receive ultrasound pulses for stone sizing. Calculated stone thicknesses were compared with caliper measurements. Correlation between ultrasound-determined thickness and caliper measurements was excellent in all three stone types (r(2) = 0.90, p < 0.0001). All 15 ultrasound measurements were accurate to within 1 mm, and 10 measurements were accurate within 0.5 mm. A 3.6F ultrasound probe can be used to accurately size stone fragments to within 1 mm in a porcine kidney.

Measurement of shock N-waves using optical methods

Yuldashev, P., M. Averiyanov, V. Khokhlova, O. Sapozhnikov, S. Ollivier, and P. Blanc-Benon, "Measurement of shock N-waves using optical methods," In Proceedings, 10eme Congres Francais d'Acoustique, Lyon, 12-16 April, 6 pp. (Societe Francaise d'Acoustique, 2010).

12 Apr 2010

Standing waves in an elastic layer loaded with a finite mass

Andreev, V.G., T.B. Krit, and O.A. Sapozhnikov, "Standing waves in an elastic layer loaded with a finite mass," Phys. Acoust., 56, 168-173, doi: 10.1134/S1063771010020077, 2010.

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7 Apr 2010

Standing shear waves in a plane-parallel rubberlike layer fixed without slippage between two rigid plates with finite masses are investigated. The lower plate, which underlies the layer, oscillates in the direction parallel to its surface under an external harmonic force, whereas the upper plate freely overlies the layer. It is shown both theoretically and experimentally that such a system exhibits resonances at frequencies the values of which depend on the mass of the free plate and the shear modulus of the layer. The shapes of the resonance curves are calculated and measured for different values of parameters of the layer and different masses of the upper plate. From the measured resonance curves, it is possible to determine the dynamic shear modulus and the shear viscosity of the rubberlike material.

Potential temperature limitations of bubble-enhanced heating during HIFU

Kreider, W., M.R. Bailey, O.A Sapozhnikov, and L.A. Crum, "Potential temperature limitations of bubble-enhanced heating during HIFU," In Proceedings, Ninth International Symposium on Therapeutic Ultrasound, Aix-en-Provence, 24-26 September 2009, K. Hynynen and J. Souquet, eds., 367-370 (AIP, 2010).

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9 Mar 2010

During high-intensity focused ultrasound (HIFU) treatments in the absence of bubbles, tissue is heated by absorption of the incident ultrasound. However, bubbles present at the focus can enhance the rate of heating. One mechanism for such enhanced heating involves inertial bubble collapses that transduce incident ultrasound to higher frequencies that are more readily absorbed. Previously, it has been reported that bubble-enhanced heating diminishes as treatments progress.

The objective of this effort is to quantify how inertial bubble collapses are affected as the focal temperature rises during treatment. A model of a single, spherical bubble has been developed to couple the thermodynamic state of a strongly driven spherical bubble with the temperature of the surrounding liquid. This model allows for the dynamic transport of heat, vapor, and non-condensable gases to/from the bubble and has been demonstrated to fit experimental data from the collapses and rebounds of millimeter-sized bubbles over a range of temperature conditions. The responses of micron-sized, air-vapor bubbles in water were simulated under exposure to MHz/MPa HIFU excitation at various surrounding liquid temperatures. Each bubble response was characterized by the power spectral density of its radiated pressure in order to emulate a hydrophone measurement. Simulations suggest that bubble collapses are significantly attenuated at temperatures above about 70 deg C. For instance, the acoustically radiated energy at 80 deg C is an order of magnitude less than that at 20 deg C. Simulations that fully include the effect of vapor on bubbles excited during HIFU suggest that the efficacy of bubble-enhanced heating may be limited to temperatures below 70 deg C.

Therapeutic ultrasound: Recent trends and future perspectives

Crum, L., M. Bailey, J.H. Wang, V. Khokhlova, and O. Sapozhnikov, "Therapeutic ultrasound: Recent trends and future perspectives," In Physics Procedia, vol. 3 - International Congress on Ultrasonics, Santiago Chile, January 2009, Luis Gaete Garreton, ed., 25-34 (Elsevier, 2010).

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1 Jan 2010

Before ultrasound-imaging systems became widely available, ultrasound therapy devices showed great promise for general use in medicine. However, it is only in the last decade that ultrasound therapy has begun to obtain clinical acceptance. Recently, a variety of novel applications of therapeutic ultrasound have been developed that include sonothrombolysis, site-specific and ultrasound-mediated drug delivery, shock wave therapy, lithotripsy, tumor ablation, acoustic hemostasis and several others. This paper reviews a few selected applications of therapeutic ultrasound. It will address some of the basic scientific questions and future challenges in developing these methods and technologies for general use in our society. As a plenary presentation, its audience is intended for the ultrasound scientist or engineer, and thus is not presented at the level of the experienced medical ultrasound professional.

Group analysis of the Khokhlov-Zabolotskaya type equations

Sapozhnikov, O.A., "Group analysis of the Khokhlov-Zabolotskaya type equations," J. Acoust. Soc. Am., 126, 2200, doi:10.1121/1.3248606, 2009.

1 Oct 2009

Radiation pressure from ultrasound to help kidney stones pass

Lu, W., A. Shah, B.W. Cunitz, P.J. Kaczkowski, O.A. Sapozhnikov, and M.R. Bailey, "Radiation pressure from ultrasound to help kidney stones pass," J. Acoust. Soc. Am., 126, 2213, doi:10.1121/1.3248749, 2009.

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1 Oct 2009

Residual kidney stone fragments often remain months after treatment. These fragments may nucleate new stones and contribute to a 50% recurrence within 5 years. Here, a research focused ultrasound device was used to generate fragment motion with the goal of facilitating passage. Natural and artificial stones 1–8 mm in length were surgically placed in the urine space in pig kidneys. The ultrasound source was a 2.75-MHz, eight-element annular array with a 6-cm radius of curvature. At adjustable focal depths of 5–8 cm, the focal pressure beam width in water was about 2 mm, and peak pressure was about 4 MPa. Targeting was done by ultrasound using B-mode and twinkling artifact that stones produce in Doppler mode. The commercial imaging probe was placed within and oriented down the axis of the therapy probe. Ultrasound and fluoroscopy showed the stones moving in real-time under the influence of the focused ultrasound. Stones moved on the order of 1 cm/s away from the source and several stones moved several centimeters down the ureter. It appeared that stones were affected only when directly in the focal beam, perhaps indicating that radiation pressure not streaming caused the motion.

Ultrasonic holography of 3D objects

Tsysar, S.A., O.A. Sapozhnikov, "Ultrasonic holography of 3D objects," In Proceedings, International Ultrasonics Symposium, Rome, 20-23 September, 737-740, doi:10.1109/ULTSYM.2009.5441458 (IEEE, 2009).

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23 Sep 2009

The holographic approach used here relies on the principle of a time-reversal mirror and the Rayleigh integral. An ultrasonic beam consisting of long tone bursts is directed at a target object and the resulting acoustic field is measured at a large number of points surrounding the object. A computer-controlled positioning system is used to scan a small broadband hydrophone across a grid of measurement points in a single surface near the target. Object reconstruction is then accomplished numerically by back-propagating the acoustic field from measurement locations to a 3D region representing the object. Theoretically, the accuracy and the optimal parameters of the method were studied by modeling forward and backward propagation from a point scatterer. Experimentally set of 3-mm diameter plastic beads was investigated. Ultrasound frequencies from 1 to 1.5 MHz were considered, while hologram measurements were collected with grid spacings between 0.3 and 0.4 mm.

Both theory and measurements show that the spatial resolution of 3D ultrasonic holography is limited by diffraction effects. Discrete scatterers larger than a wavelength are well-resolved. Using this 3D ultrasonic holography method, it is possible to reconstruct the position and shape of objects or collections of objects that do not involve a significant amount of multiple scattering. Because the spatial resolution of the method has a typical diffraction limit on the order of a wavelength, improved spatial resolution can be achieved with higher frequencies.

The role of compressional pressure in the formation of dense bubble clouds in histotripsy

Maxwell, A.D., T.-Y. Wang, C.C. Cain, J.B. Fowlkes, Z. Xu, O.A. Sapozhnikov, and M.R. Bailey, "The role of compressional pressure in the formation of dense bubble clouds in histotripsy," In Proceedings, International Ultrasonics Symposium, Rome, 20-23 September, 81-84, doi:10.1109/ULTSYM.2009.5441398 (IEEE, 2009).

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20 Sep 2009

Histotripsy is a non-invasive ultrasound therapy which utilizes cavitation clouds to mechanically fractionate tissue. The mechanism by which bubble clouds form is important to understand the histotripsy process. We used high speed imaging with frame rates between 0.1-10 million fps to observe the progression of cloud formation. A 1 MHz spherically-focused transducer was used to apply single histotripsy pulses to optically-transparent gelatin tissue phantoms, with peak negative pressure of 19 MPa and 5-50 cycles. Dense bubble clouds were observed to first form at a distal position within the focus, and grow proximally towards the transducer, opposite the ultrasound propagation direction. Growth began from the site of single cavitation bubbles. Based on these observations, it was hypothesized that the shocked waveforms from histotripsy pulses scatter from single bubbles, which invert the shock and induce a large negative pressure in its vicinity. To test this hypothesis, the positive incident shock pressure was reduced without significantly affecting the negative pressure. When the peak positive pressure was lowered, the likelihood and size of bubble clouds initiating at the focus was greatly reduced. These results suggest that the positive pressure of the incident waveform is important for generating bubble clouds in histotripsy.

The mechanism of lesion formation by focused ultrasound ablation catheter for treatment of atrial fibrilation

Sinelnikov, Y.D., T. Fjield, and O.A. Sapozhnikov, "The mechanism of lesion formation by focused ultrasound ablation catheter for treatment of atrial fibrilation," Acoust. Phys., 55, 647-656, doi:10.1134/S1063771009040216, 2009.

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21 Jul 2009

The application of therapeutic ultrasound for the treatment of atrial fibrillation (AF) is investigated. The results of theoretical and experimental investigation of ultrasound ablation catheter are presented. The major components of the catheter are the high power cylindrical piezoelectric element and parabolic balloon reflector. Thermal elevation in the ostia of pulmonary veins is achieved by focusing the ultrasound beam in shape of a torus that transverses the myocardial tissue. High intensity ultrasound heating in the focal zone results in a lesion surrounding the pulmonary veins that creates an electrical conduction blocks and relief from AF symptoms. The success of the ablation procedure largely depends on the correct choice of reflector geometry and ultrasonic power.

We present a theoretical model of the catheter's acoustic field and bioheat transfer modeling of cardiac lesions. The application of an empirically derived relation between lesion formation and acoustic power is shown to correlate with the experimental data. Developed control methods combine the knowledge of theoretical acoustics and the thermal lesion formation simulations with experiment and thereby establish dosimetry that contributes to a safe and effective ultrasound ablation procedure.

Acousto-optic interaction in laser vibrometry in a liquid

Sapozhnikov, O.A., A.V. Morozov, and D. Cathignol, "Acousto-optic interaction in laser vibrometry in a liquid," Acoust. Phys., 55, 365-375, doi:10.1134/S1063771009030129, 2009.

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16 May 2009

It is demonstrated that, when the classical method of laser vibrometry is used for measurements in a liquid, it gives erroneous results with measurement errors reaching 100% or more. The vibration pattern observed in this case exhibits a false structure with a spatial scale identical to the wavelength of acoustic waves in the liquid. In addition, the laser vibrometer shows displacements in the regions where they are actually absent. In the transient mode of operation, the image displays nonexistent surface waves, which propagate with the velocity of sound in the liquid.

The origin of these distortions lies in the acoustooptic interaction that occurs in the condensed medium on the path of the probing laser beam. An analytic expression is obtained for the Green's function characterizing laser vibrometry in the cases of harmonic and pulsed excitation of the surface under investigation. It is shown that this function explains all the artifacts observed in laser vibrometry in a liquid and can be used to correct the measurement data.

Acoustic tomography of temperature distribution in a medium heated by a focused ultrasound beam

Tsysar, S.A., O.A. Sapozhnikov, and V.G. Andreev, "Acoustic tomography of temperature distribution in a medium heated by a focused ultrasound beam," Bull. Russ. Acad. Sci. Phys., 73, 529-532, doi:10.3103/S1062873809040212, 2009.

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10 May 2009

A new method is proposed for measuring temperature in the focal region of a high-intensity ultrasonic emitter on the assumption of axially symmetric temperature field. This method is based on solving the integral equation relating the temperature of the medium to the delay times of the probe ultrasound pulse intersecting the heated region in the transverse direction at different distances from the beam axis. The accuracy of the algorithm for calculating temperature with allowance for the finite experimental data set is analyzed. The calculation results are compared with the experimental data.

A Schlieren system for optical visualization of ultrasonic fields

Kaczkowski, P.J., M.R. Bailey, V.A. Khokhlova, and O.A. Sapozhnikov, "A Schlieren system for optical visualization of ultrasonic fields," J. Acoust. Soc. Am., 125, 2742, 2009.

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1 Apr 2009

Ultrasonic field mapping is an essential component of transducer characterization and of beam forming verification. Such measurements are commonly performed by displacing a hydrophone over a range of points within the field; these procedures can be time consuming. A calibrated hydrophone can provide accurate measurements of the field, subject to limitations of bandwidth and aperture of the device. A rapid qualitative 2D measurement of the spatial acoustic field can be obtained by optical means, in which the change in optical index due to the presence of acoustic pressure is imaged using a Schlieren approach.

This technique illuminates a transparent refracting acoustic medium using a plane collimated source and then focuses the transmitted light using a lens or mirror. In the absence of acoustic field, all of the light focuses to a small spot; acoustically induced refractive index perturbations cause some of the light to focus elsewhere. Obscuring the primary focal spot of unperturbed light with a mask permits imaging only the perturbations in the acoustic medium. We will describe a mirror-based Schlieren system for imaging continuous as well as pulsed fields and with color corresponding qualitatively to the intensity of the field.

Impact of temperature on bubbles excited by high intensity focused ultrasound

Kreider, W., M.R. Bailey, O.A. Sapozhnikov, and L.A. Crum, "Impact of temperature on bubbles excited by high intensity focused ultrasound," J. Acoust. Soc. Am., 125, 2742, doi:10.1121/1.3050272, 2009.

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1 Apr 2009

Bubble-enhanced heating is a current topic of interest associated with high intensity focused ultrasound (HIFU). For HIFU treatments designed to utilize acoustic radiation from bubbles as a heating mechanism, it has been reported that useful bubble activity diminishes at elevated temperatures. To better understand and quantify this behavior, a model has been implemented that couples the thermodynamic state of a strongly driven spherical bubble with thermal conditions in the surrounding liquid. This model has been validated over a range of temperature conditions against experimental data from the collapses and rebounds of millimeter-sized bubbles.

With this model, the response of a micron-sized bubble was simulated under exposure to MHz/MPa HIFU excitation, while various surrounding liquid temperatures were considered. Characterizing the bubble response through the power spectral density of pressure radiated from the bubble, model calculations suggest that bubble collapses are significantly attenuated at temperatures above about 70°C. For instance, the acoustically radiated energy at 80°C is an order of magnitude less than that at 20°C. These results suggest that the efficacy of bubble-enhanced heating may be limited to temperatures below 70°C. Moreover, temperature will affect hydrophone measurements used to passively assess cavitation activity.

Improved impulse response for hydrophone measurements in therapeutic ultrasound fields

Canney, M.S., V.A. Khokhlova, O.A. Sapozhnikov, Y.A. Pishchalnikov, A.D. Maxwell, M.R. Bailey, and L.A. Crum, "Improved impulse response for hydrophone measurements in therapeutic ultrasound fields," J. Acoust. Soc. Am., 125, 2740, 2009.

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1 Apr 2009

The accurate measurement of pressure waveforms in high intensity focused ultrasound (HIFU) fields is complicated by the fact that many devices operate at output levels where shock waves can form in the focal region. In tissue ablation applications, the accurate measurement of the shock amplitude is important for predicting tissue heating since the absorption at the shock is proportional to the shock amplitude cubed. To accurately measure shocked pressure waveforms, not only must a hydrophone with a broad bandwidth (>100 MHz) be used, but the frequency response of the hydrophone must be known and used to correct the measured waveform.

In this work, shocked pressure waveforms were measured using a fiber optic hydrophone and a frequency response for the hydrophone was determined by comparing measurements with numerical modeling using a KZK-type equation. The impulse response was separately determined by comparing a measured and an idealized shock pulse generated by an electromagnetic lithotripter. The frequency responses determined by the two methods were in good agreement. Calculations of heating using measured HIFU waveforms that had been deconvolved with the determined frequency response agreed well with measurements in tissue phantom.

Investigation of an ultrasound imaging technique to target kidney stones in lithotripsy

Shah, A., M. Paun, J. Kucewicz, O.A. Sapozhnikov, M. Dighe, H.A. McKay, M.D. Sorensen, and M.R. Bailey, "Investigation of an ultrasound imaging technique to target kidney stones in lithotripsy," J. Acoust. Soc. Am., 125, 2620, 2009.

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1 Apr 2009

Localization of kidney stones and targeting for lithotripsy can be challenges especially with ultrasound. However, twinkling artifact has been observed where Doppler ultrasound imagers assign color to the stone. We report a preliminary investigation from our observations in a porcine model and hypothesize why this artifact occurs. Glass beads, cement stones, and human stones were surgically placed into the renal collecting system through the ureter. The stones were imaged using several transducers and ultrasound imagers. In all cases, the twinkling artifact of the stone was observed, and its appearance and radiofrequency signature were unique from those of blood flow. Calcium oxalate monohydrate stones and smooth stones were not more difficult to image, contrary to previous reports. Increasing gain or placing the focal depth distal to the stone enhanced the artifact, but other user controls showed little effect. Twinkling started at the lateral edges of the stone and spread over the stone as gain was increased. The evidence supports the hypothesis that small motions induced by radiation force or elastic waves in the stone cause changes in received backscatter, particularly at imaging angles oblique to the stone surface.

Ureteroscopic ultrasound technology to size kidney stone fragments: Proof of principle using a miniaturized probe in a porcine model

Sorensen, M.D., A. Shah, M.S. Canney, O.A. Sapozhnikov, J.M.H. Teichman, and M.R. Bailey, "Ureteroscopic ultrasound technology to size kidney stone fragments: Proof of principle using a miniaturized probe in a porcine model," J. Acoust. Soc. Am., 125, 2622, 2009.

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1 Apr 2009

Background: the ability to measure stone fragment size could help prevent attempting to extract too large a stone fragment. We evaluated the ability of a 1.2 mm (3.6 French) ultrasound probe to measure stone fragments in a porcine kidney.

Methods: 15 human stones of three types (five each calcium oxalate, cystine, calcium phosphate) sized 3–7 mm were placed deep in a porcine kidney collecting system. The sound speed of each stone type was determined using a separate reference stone. A 2 MHz, 1.2 mm needle hydrophone was used to send and receive ultrasound pulses. Stone thickness was calculated as d=c*t/2 by determining the signal transit time through the stone and the stone sound speed. Calculated stone thicknesses were compared to digital caliper measurements.

Results: Stone size was determined for all 15 stones. Correlation between ultrasound-determined thickness and caliper measurements was excellent (r2=0.90, p<0.0001) with ultrasound performing well in all three stone types. All stone measurements were accurate within 1 mm, and ten (66%) stone measurements were accurate within 0.5 mm.

Conclusions: Ultrasound-based measurements are accurate and precise using a 3.6 French probe with stone fragments placed deep in a porcine kidney.

Effect of elastic waves in the metal reflector on bubble dynamics at the focus of an electrohydraulic lithotripter

Sapozhnikov, O.A., W. Kreider, and M.R. Bailey, "Effect of elastic waves in the metal reflector on bubble dynamics at the focus of an electrohydraulic lithotripter," Nelineinyi mir (Nonlinear World), 7, 575-580, 2009 (in Russian).

1 Jan 2009

Acoustic characterization of high intensity focused ultrasound fields: A combined measurement and modeling approach

Canney, M.S., M.R. Bailey, L.A. Crum, V.A. Khokhlova, and O.A. Sapozhnikov, "Acoustic characterization of high intensity focused ultrasound fields: A combined measurement and modeling approach," J. Acoust. Soc. Am., 124, 2406-2420, doi:10.1121/1.2967836, 2008.

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30 Oct 2008

Acoustic characterization of high intensity focused ultrasound (HIFU) fields is important both for the accurate prediction of ultrasound induced bioeffects in tissues and for the development of regulatory standards for clinical HIFU devices. In this paper, a method to determine HIFU field parameters at and around the focus is proposed. Nonlinear pressure waveforms were measured and modeled in water and in a tissue-mimicking gel phantom for a 2 MHz transducer with an aperture and focal length of 4.4 cm. Measurements were performed with a fiber optic probe hydrophone at intensity levels up to 24000 W/cm2. The inputs to a Khokhlov–Zabolotskaya–Kuznetsov-type numerical model were determined based on experimental low amplitude beam plots. Strongly asymmetric waveforms with peak positive pressures up to 80 MPa and peak negative pressures up to 15 MPa were obtained both numerically and experimentally. Numerical simulations and experimental measurements agreed well; however, when steep shocks were present in the waveform at focal intensity levels higher than 6000 W/cm2, lower values of the peak positive pressure were observed in the measured waveforms. This underrepresentation was attributed mainly to the limited hydrophone bandwidth of 100 MHz. It is shown that a combination of measurements and modeling is necessary to enable accurate characterization of HIFU fields.

Effect of elastic waves in the metal reflector on bubble dynamics at the focus of an electrohydraulic lithotripter

Sapozhnikov, O.A., W. Kreider, M.R. Bailey, V.A. Khokhlova, and F. Curra, "Effect of elastic waves in the metal reflector on bubble dynamics at the focus of an electrohydraulic lithotripter," J. Acoust. Soc. Am., 123, 3367-3368, 2008.

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1 May 2008

In extracorporeal electrohydraulic lithotripters, a hemi-ellipsoidal metal reflector is used to focus a spherical wave generated by an electrical discharge. The spark source is positioned at one of the ellipsoid foci (F1); this makes the reflected wave focused at the other focus (F2). Despite the common assumption that the reflector behaves as a rigid mirror, the true reflection phenomenon includes the generation and reverberation of elastic waves in the reflector, which reradiate to the medium. Although these waves are much lower in amplitude than the specularly reflected wave, they may influence cavitation at F2. To explore such effects, waves in water and a brass reflector were modeled in finite differences based on the linearized equations of elasticity. The bubble response was simulated based on a Rayleigh-type equation for the bubble radius. In addition, the role of acoustic nonlinearity was estimated by numerical modeling. It is shown that the elastic waves in the reflector give rise to a long "ringing" tail, which results in nonmonotonic behavior of the bubble radius during its inertial growth after shock wave passage. This numerical result is qualitatively confirmed by experimental observations of bubble behavior using high-speed photography.

Measurement of dynamic shear modulus in soft solids using laser vibrometry

Izosimova, M.Y., A.I. Korobov, E.V. Prokhorova, and O.A. Sapozhnikov, "Measurement of dynamic shear modulus in soft solids using laser vibrometry," J. Acoust. Soc. Am., 123, 3226, 2008.

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1 May 2008

Shear modulus is an important property of biological tissue and can be imaged for diagnostic purposes. A related application involves the acquisition of precise shear modulus measurements in excised soft tissue. Because the mechanical properties of tissue are frequency-dependent, it is important to develop methods that characterize the elasticity of soft solids at various frequencies. Here, two methods of measuring shear modulus are presented that employ laser vibrometry. At low frequencies (1–10 Hz), the tissue sample is fixed between plates so that the displacement of one plate induces a shear strain. Vibrometer measurements of the plate displacement define the induced strain so that shear modulus can be deduced if the applied shear force is known. At higher frequencies (0.1–10 kHz), the sample lays flat on a surface and an impulsive force is applied to the exposed surface. Using the vibrometer to measure displacements along the exposed surface enables estimation of the surface wave speed and the implied shear modulus. To demonstrate the capabilities of these techniques, experiments were performed using plastisol tissue phantom samples. These experiments indicated a shear modulus that was 50% greater at high frequencies (300 Hz) than at low frequencies (10 Hz).

Nonlinear propagation of spark-generated N-waves in atmosphere: Theoretical and experimental assessment of the shock front structure

Yuldashev, P.V., M.V. Averiyanov, V.A. Khokhlova, O.A. Sapozhnikov, O. Sebastien, and P. Blanc Benon, "Nonlinear propagation of spark-generated N-waves in atmosphere: Theoretical and experimental assessment of the shock front structure," J. Acoust. Soc. Am., 123, 3248, 2008.

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1 May 2008

Extensive outdoor and laboratory-scale experiments on sonic boom propagation in turbulent atmosphere have shown that shock wave amplitude and rise time are important parameters responsible for sonic boom annoyance. However, accurate measurement of the shock front structure with standard microphone remains a challenge due to the broadband spectrum of the N-wave shock front. In this work the experimental setup utilizing a spark source has been designed and built to investigate nonlinear N-wave propagation in homogeneous medium. Short duration (30µs) and high amplitude (1 kPa) spherically divergent N-waves were generated. In addition to acoustic measurements with 1/8" B&K microphones, the shadowgraphy method using short duration flash lamp (20 ns) and CCD camera was employed to assess the shock front structure at different distances from the spark. It is shown that the shock rise time measured by the shadowgraphy method was in a good agreement with the theoretical predictions and it was 10 times shorter than in microphone measurements. The widening of the shock in acoustic measurements was therefore due to the limited bandwidth of the microphone. The combination of modeling, acoustic and optical measurements provided an accurate calibration of the shock wave measuring system.

Simultaneous measurement of pressure and temperature in a focused ultrasound field with a fiber optic hydrophone

Canney, M.S., M.R. Bailey, V.A. Khokhlova, O.A. Sapozhnikov, and L.A. Crum, "Simultaneous measurement of pressure and temperature in a focused ultrasound field with a fiber optic hydrophone," J. Acoust. Soc. Am., 123, 3221, 2008.

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1 May 2008

The characterization of high intensity focused ultrasound (HIFU) fields is important for both clinical treatment planning as well as for regulation of HIFU medical devices. In previous work, we have used a 100-µm fiber optic probe hydrophone (FOPH) to measure pressure waveforms from a 2-MHz HIFU source with 42-mm aperture and 44-mm focal length. The formation of shock waves with peak positive pressure of up to 80 MPa were measured and modeled in transparent tissue-mimicking gel phantoms and boiling was achieved in milliseconds [Canney MS, et al., J. Acoust. Soc. Am., 120:3110 (2006)].

In this work, the FOPH was also used to measure temperature changes in tissue phantoms from HIFU at peak focal intensities of 5000–20,000 W cm2. Temperature measurements were obtained by first low-pass filtering the voltage signal measured from the FOPH to remove the acoustic part of the measurement. Then, calibration of voltage to temperature was performed using results from a separate calibration experiment. Experimental measurements were compared with numerical modeling using a KZK-type model for acoustic propagation coupled with a heat transfer model. In summary, temperatures of 100°C were measured at the HIFU focus in milliseconds, in agreement with modeling.

Calibration of PVDF hydrophones using a broad-focus electromagnetic lithotripter

Sapozhnikov, O.A., Y.A. Pishchalnikov, A.D. Maxwell, and M.R. Bailey, "Calibration of PVDF hydrophones using a broad-focus electromagnetic lithotripter," 2007 IEEE Ultrasonics Symposium, 28-31 October, New York, NY, 112-115 (IEEE: Piscataway, NJ, 2007).

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28 Oct 2007

Correct measurement of acoustic pressure is crucial in many applications, e.g., medical diagnostics and therapy, where the physical effect of ultrasound on tissue depends on specific parameters of the wave, such as positive and negative pressure, shock front thickness, and pulse duration. In our previous paper, we reported building a new low-cost PVDF hydrophone to detect broadband signals from lithotripters and high intensity focused ultrasound (HIFU) sources. The hydrophone was built to complement the fiber optic probe hydrophone (FOPH), which has become the de facto standard broadband hydrophone for high amplitude medical devices. Our PVDF membrane hydrophone is more sensitive than the FOPH, thereby making measurement possible when averaging is not an option. The goal of this research was to develop a reliable method of calibration by comparing the PVDF hydrophone to the FOPH in a repeatable broadband pressure field. The results showed that the PVDF membranes had similar, though not identical responses.

Experimental evidence for a growing surface wave and acoustic beam narrowing upon reflection from fluid-solid interfaces

Sapozhnikov, O.A., A.A. Karabutov, Jr., and V.G. Mozhaev, "Experimental evidence for a growing surface wave and acoustic beam narrowing upon reflection from fluid-solid interfaces," 2007 IEEE Ultrasonics Symposium, 28-31 October, New York, NY, 391-394, doi:10.1109/ULTSYM.2007.107 (IEEE: Piscataway, NJ, 2007).

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28 Oct 2007

The secular equation for acoustic waves at fluid-solid interfaces yields the common leaky wave and its complement. This complementary wave grows instead of decays with propagation and is time-reversed compared to the leaky wave. Moreover, this growing wave has not yet been observed experimentally, perhaps due to difficulty of its excitation. Experimental observation of this wave was one goal of our work. The second goal was to study mirror reflection of an acoustic beam of special shape when the incident angle is equal to the Rayleigh critical angle. An obliquely incident beam is known to split after reflection into two components: a specular beam and a broad beam generated by the leaky waves. The interference of these two components results in "Schoch displacement" of the reflected beam along the interface and overall beam broadening. Our hypothesis was that by time reversing the reflection at the critical angle, the reflection beam can be made narrower rather than broader.

Identification of kidney stone fragmentation in shock wave lithotripsy

Owen, N.R., M.R. Bailey, L.A. Crum, and O.A. Sapozhnikov, "Identification of kidney stone fragmentation in shock wave lithotripsy," 2007 IEEE Ultrasonics Symposium, 28-31 October, New York, NY, 323-326 (IEEE: Piscataway, NJ, 2007).

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28 Oct 2007

Identification of stone fragmentation, or comminution, during shock wave lithotripsy (SWL) would aid a urologist in determining the treatment endpoint, but there is currently little feedback available to do so. Here we report the measurement and analysis of SW scattering by kidney stone models in water to study the inverse relationship between stone size and scatter frequency. Stones were exposed to 20 SWs, 120 SWS, or 220 SWs to measure scatter and cause different levels of comminution. Measured scatter signals were processed in frequency to study the effect of stone comminution on the distribution of spectral energy. Comminution was measured by normalizing the mass of stone fragments, separated by size, to the mass of an intact stone. Output from frequency analysis was compared with percent mass comminution, and the shift of spectral energy to higher frequencies was proportional to the percent mass of stone fragments smaller than 2 mm.

Frequency analysis of shock wave scattering to identify kidney stone fragmentation in shock wave lithotripsy

Owen, N.R., M.R. Bailey, O.A. Sapozhnikov, and L.A. Crum, "Frequency analysis of shock wave scattering to identify kidney stone fragmentation in shock wave lithotripsy," Proceedings, 19th International Congress on Acoustics, 2-7 September, Madrid, Spain (2007).

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2 Sep 2007

Currently there is little feedback available in shock wave lithotripsy (SWL) to determine kidney stone fragmentation. The identification of fragmentation would aid a urologist in deciding to continue or stop treatment, and it could potentially reduce SW dose. Lithotripsy SWs strike stones with a broadband mechanical load. Reverberations excited within the stone are transmitted to surrounding fluid; a process termed resonant acoustic scatter (RAS). The frequency of RAS is inversely proportional to stone size. In experiment, variable SW treatments were applied to two types of stone models in vitro to produce different levels of fragmentation, which were measured by sieving dehydrated fragments and normalizing their mass to intact stone mass. RAS from selected SWs was measured with a broadband receiver and a new frequency analysis method was applied to display the redistribution of spectral energy. Mean percent mass for fragments smaller than 2 mm increased proportionally to the number of SWs applied. Amplitude of the frequency analysis output was directly proportional to fragmentation, and peak frequencies were inversely proportional to stone size. Results show promise that frequency analysis of RAS might provide feedback on fragmentation in SWL.

High-powered focused ultrasound fields in therapeutic medical applications: Modeling and measurements with a fiber optic hydrophone

Bailey, M.R., M.S. Canney, V.A. Kohkhlova, O.A. Sapozhnikov, and L.A. Crum, "High-powered focused ultrasound fields in therapeutic medical applications: Modeling and measurements with a fiber optic hydrophone," Proceedings, 19th International Congress on Acoustics, 2-7 September, Madrid, Spain (2007).

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2 Sep 2007

The goal of this work was to determine the acoustic waveform and beam width at the focus of a therapeutic ultrasound source both in water and in a tissue phantom. The source was a 2 MHz transducer of 45 mm focal length, 42 mm diameter, operating at 50 - 300 W acoustic power. Focal waveforms and beam widths calculated with a KZK-type model were in excellent agreement with values measured with a 100-µm, 100-MHz bandwidth fiber optic probe hydrophone (FOPH). Super focusing of the peak positive pressure and a proximal shift in the peak negative pressure were observed. Shocked distorted waveforms reached 70 MPa and - 15 MPa. Surface waves on the transducer were measured and included in the model but did not significantly affect the results obtained at focus. The change of the FOPH bandwidth to 30- MHz or the diameter of hydrophone to 500-µm resulted in 20% underestimation of the measured peak positive pressure but did not affect the measured negative peak pressure. Initiation of boiling was observed in tissue phantoms in milliseconds as predicted by weak shock theory due to absorption on the shocks. Work was supported by NIH DK43881, NSBRI SMS00402, and RFBR.

Use of scattering of ultrasound pulses and shock waves on kidney stones for imaging lithotripsy

Sapozhnikov, O.A., N.R. Owen, M.R. Bailey, A.I. Gromov, and L.A. Crum, "Use of scattering of ultrasound pulses and shock waves on kidney stones for imaging lithotripsy," Proceedings, 14th International Congress on Sound and Vibration, 9-12 July, Cairns, Australia (2007).

9 Jul 2007

Bubble proliferation in shock wave lithotripsy

Pishchalnikov, Y.A., J.A. McAteer, M.R. Bailey, J.C. Williams, Jr., and O.A. Sapozhnikov, "Bubble proliferation in shock wave lithotripsy," J. Acoust. Soc. Am., 121, 3081, 2007.

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1 May 2007

Stone breakage is less efficient when lithotripter shock waves (SWs) are delivered at 2 Hz compared to slower 0.5–1-Hz pulse repetition rates (PRFs). This correlates with increased number of transient cavitation bubbles observed along the SW path at fast PRF. The dynamics of this bubble proliferation throughout the bubble lifecycle is investigated in this report. Cavitation bubbles were studied in the free-field of a shock wave lithotripter using fine temporal and microscopic spatial resolution (high-speed camera Imacon-200). A typical cavitation bubble became visible (radius>10 μm) under the tensile phase of the lithotripter pulse, and at its first inertial collapse emitted a secondary SW and formed a micro-jet, which then could break up forming ~25 micro-bubbles. Subsequent rebound and collapse of the parent bubble appeared to produce a further 40–120 daughter bubbles visible following the rebound. Preexisting bubbles hit by the lithotripter SW also formed micro-jets and broke up into micro-bubbles that grew and coalesced, producing irregular-shaped bubbles that, in turn, broke into micro-bubbles upon subsequent inertial collapse. A conventional NTSC-rate camcorder was used to track cavitation bubbles from pulse-to-pulse, showing that a single bubble can give rise to a cavitation cloud verifying high-speed video results.

A passive technique to identify stone comminution during shock wave lithotripsy

Owen, N.R., O.A. Sapozhnikov, M.R. Bailey, L. Trusov, and L.A. Crum, "A passive technique to identify stone comminution during shock wave lithotripsy," American Institute of Physics Proceedings, 900, 364-367, doi:10.1063/1.2723597, 2007.

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5 Apr 2007

The identification of comminution during shock wave lithotripsy can be difficult using fluoroscopy or other imaging modalities. However, correct interpretation is necessary to determine if a stone is breaking and to evaluate the endpoint of therapy. Reported here is a passive method to detect acoustic signals generated by shock wave (SW) impact on a model stone and to correlate the spectrum of the detected signals to stone size. Acoustic scatter from model stones in an electrohydraulic lithotripter was measured in water with a passive, focused receiver before and after the application of either 20 SWs or 50 SWs. The five stones used for each case were dehydrated after the experiment, separated with 3 mm, 2 mm, and 1 mm sequential sieves, and weighed to quantify comminution. The detection method was first successfully used to differentiate broken and unbroken stones. Then the system tracked the decreasing size of particles and clearly showed the presence of particles smaller than 2 mm, which was considered passable size. Thus, the detection system gives feedback on whether stones are breaking and when they may be considered fully comminuted.

Advantage of a broad focal zone in SWL: synergism between squeezing and shear

Sapozhnikov, O.A., M.R. Bailey, A.D. Maxwell, B. MacConaghy, R.O. Cleveland, J.A. McAteer, and L.A. Crum, "Advantage of a broad focal zone in SWL: synergism between squeezing and shear," American Institute of Physics Proceedings, 900, 351-355, doi:10.1063/1.2723594, 2007.

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5 Apr 2007

Objective: The focal zone width appears to be a critical factor in lithotripsy. Narrow focus machines have a higher occurrence of adverse effects, and arguably no greater comminution efficiency. Manufacturers have introduced new machines and upgrades to broaden the focus. Still, little data exists on how focal width plays a role in stone fracture. Thus, our aim was to determine if focal width interacts with established mechanisms known to contribute to stone fracture. Method: A series of experiments were undertaken with changes made to the stone in an effort to determine which is most important, the shock wave (SW) reflected from the back end of the stone (spallation), the SW ringing the stone (squeezing), the shear wave generated at surface of the stone and concentrated in the bulk of it (shear), or SWs generated from bubble collapse (cavitation). Shock waves were generated by a Dornier HM3-style lithotripter, and stones were made from U30 cement. Baffles were used to block specific waves that contribute to spallation, shear, or squeezing, and glycerol was used to suppress cavitation. Numerical simulation and high-speed imaging allowed for visualization of specific waves as they traveled within the stone. Results: For brevity, one result is explained. A reflective baffle was placed around the front edge of a cylindrical stone. The proximal baffle prevented squeezing by preventing the SW from traveling over the stone, but permitted the SW entering the stone through the proximal face and did not affect the other mechanisms. The distal baffle behaved the same as no baffle. The proximal baffle dramatically reduced the stress, and the stone did not break (stone broke after 45±10 SWs without the baffle and did not break after 400 SWs when the experiment stopped). The result implies that since removing squeezing halted comminution, squeezing is dominant. However, there is much more to the story. For example, if the cylindrical stone was pointed, it broke with the point on the distal end but not with the point on the proximal end. In both cases, squeezing was the same, so if squeezing were dominant, both stones should have broken. But the pointed front edge prevents the shear wave. The squeezing wave and its product — the shear wave — are both needed and work synergistically in a way explained by the model. Conclusions: A broad focus enhances the synergism of squeezing and shear waves without altering cavitation's effects, and thus accelerates stone fracture in SWL.

Compression and amplification of an ultrasonic pulse reflected from a one-dimensional layered structure

Ponomarev, A.E., S.I. Bulatitski, and O.A. Sapozhnikov, "Compression and amplification of an ultrasonic pulse reflected from a one-dimensional layered structure," Acoust. Phys., 53, 127-135, doi:10.1134/S1063771007020030, 2007.

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23 Mar 2007

Compression of ultrasonic pulses reflected from layered structures is studied. A short pulse is emitted into water towards a structure consisting of solid plates backed with an air layer. Due to multiple reflections in the structure, the signal is elongated. The reflected signal is received by the same transducer and digitized. After that, the wave is reversed in time and emitted towards the layered structure for the second time; then, the reflected signal is received. Due to the invariance of the processes under the time reversal, the pulse is compressed by the structure: the reflected signal becomes shorter and acquires the waveform of the initial pulse. The possibility of an efficient compression of signals is demonstrated experimentally. Numerical simulations show that the use of more complex structures can considerably increase the compression ratio and produce short signals of a much higher amplitude than that emitted by the transducer. An efficient compression algorithm is proposed.

A mechanistic analysis of stone fracture in lithotripsy

Sapozhnikov, O.A., A.D. Maxwell, B. MacConaghy, and M.R. Bailey, "A mechanistic analysis of stone fracture in lithotripsy," J. Acoust. Soc. Am., 121, 1190-1202, 2007.

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1 Feb 2007

In vitro experiments and an elastic wave model were used to analyze how stress is induced in kidney stones by lithotripsy and to test the roles of individual mechanisms—spallation, squeezing, and cavitation. Cylindrical U30 cement stones were treated in an HM-3-style lithotripter. Baffles were used to block specific waves responsible for spallation or squeezing. Stones with and without surface cracks added to simulate cavitation damage were tested in glycerol (a cavitation suppressive medium). Each case was simulated using the elasticity equations for an isotropic medium. The calculated location of maximum stress compared well with the experimental observations of where stones fractured in two pieces. Higher calculated maximum tensile stress correlated with fewer shock waves required for fracture. The highest calculated tensile stresses resulted from shear waves initiated at the proximal corners and strengthened along the side surfaces of the stone by the liquid-borne lithotripter shock wave. Peak tensile stress was in the distal end of the stone where fracture occurred. Reflection of the longitudinal wave from the distal face of the stone—spallation—produced lower stresses. Surface cracks accelerated fragmentation when created near the location where the maximum stress was predicted.

The use of resonant scattering to identify stone fracture in shock wave lithotripsy

Owen, N.R., M.R. Bailey, L.A. Crum, O.A. Sapozhnikov, and L.A. Trusov, "The use of resonant scattering to identify stone fracture in shock wave lithotripsy," J. Acoust. Soc. Am., 121, EL41-EL47, doi:10.1121/1.2401266, 2007.

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1 Jan 2007

There is currently little feedback as to whether kidney stones have fractured during shock wave lithotripsy. Resonant scattering of the lithotripter shock wave was used here to differentiate intact and fractured stone models in water. Scattering, including reflection and radiation due to reverberation from within the stone, was calculated numerically with linear elasticity theory and agreed well with measurements made with a focused receiver. Identification of fracture was possible through frequency analysis, where scatter from fractured stones was characterized by higher energy in distinct bands. High-speed photography concurrent with measurement indicated the effect was not due to cavitation.

Parabolic equation for nonlinear acoustic wave propagation in inhomogeneous moving media

Averyanov, M.V., V.A. Khokhlova, O.A. Sapozhnikov, P.H. Blanc-Benon, and R.O. Cleveland, "Parabolic equation for nonlinear acoustic wave propagation in inhomogeneous moving media," Acoust. Phys., 52, 623-632, doi:10.1134/S1063771006060017, 2006.

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14 Nov 2006

A new parabolic equation is derived to describe the propagation of nonlinear sound waves in inhomogeneous moving media. The equation accounts for diffraction, nonlinearity, absorption, scalar inhomogeneities (density and sound speed), and vectorial inhomogeneities (flow). A numerical algorithm employed earlier to solve the KZK equation is adapted to this more general case. A two-dimensional version of the algorithm is used to investigate the propagation of nonlinear periodic waves in media with random inhomogeneities. For the case of scalar inhomogeneities, including the case of a flow parallel to the wave propagation direction, a complex acoustic field structure with multiple caustics is obtained. Inclusion of the transverse component of vectorial random inhomogeneities has little effect on the acoustic field. However, when a uniform transverse flow is present, the field structure is shifted without changing its morphology. The impact of nonlinearity is twofold: it produces strong shock waves in focal regions, while, outside the caustics, it produces higher harmonics without any shocks. When the intensity is averaged across the beam propagating through a random medium, it evolves similarly to the intensity of a plane nonlinear wave, indicating that the transverse redistribution of acoustic energy gives no considerable contribution to nonlinear absorption.

A sensitive, broadband polyvinylidine difluoride (PVDF) hydrophone for accurate characterization of shock waves

Maxwell, A.D., B.E. MacConaghy, O.A. Sapozhnikov, and M.R. Bailey, "A sensitive, broadband polyvinylidine difluoride (PVDF) hydrophone for accurate characterization of shock waves," J. Acoust. Soc. Am., 120, 3109-3110, 2006.

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1 Nov 2006

Few hydrophones are capable of measuring high-intensity fields such as shock waves accurately. One of the more reliable is the fiberoptic probe hydrophone. However, this system is expensive and insensitive. We created a new PVDF hydrophone and compared it with a fiberoptic system. The hydrophone consisted of a 25 µm thick PVDF membrane with a 0.5 mm active element and a preamplifier, which were each held in separate attached polycarbonate housings. The amplifier had adjustable gain and could account for membrane resonance to flatten the frequency response. A model of the frequency response for the system was developed, which agreed well with the measured response. Shock waves were measured in two Dornier HM-3 clones and an electromagnetic lithotripter. Measurements were also recorded using a 2 MHz focused piezoceramic source and a broadband PVDF source. Shock-wave measurements closely matched those recorded by the fiberoptic hydrophone and calculations made with a KZK-type model. Very little damage to the membrane was found after applying several thousand shock waves. This new membrane hydrophone is robust and sufficiently accurate to measure high-intensity fields, while greatly reducing cost, increasing sensitivity, and simplifying measurements of shock waves.

Bubble responses to lithotripsy shock waves

Kreider, W., M.R. Bailey, O.A. Sapozhnikov, and L.A. Crum, "Bubble responses to lithotripsy shock waves," J. Acoust. Soc. Am., 120, 3110, 2006.

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1 Nov 2006

The responses of bubbles subjected to a lithotripsy shock wave have been investigated numerically and experimentally to elucidate the role of heat and mass transfer in the underlying dynamics of strongly excited bubbles. Single spherical bubbles were modeled as gas–vapor bubbles by accounting for liquid compressibility, heat transfer, vapor transport, vapor trapping by noncondensable gases, diffusion of noncondensable gases, and heating of the liquid at the bubble wall. For shock-wave excitations, the model predicts bubble growth and collapse, followed by rebounds whose durations are significantly affected by vapor trapping. To experimentally test these predictions, bubble rebound durations were measured using passive cavitation detectors, while high-speed photographs were captured to evaluate the local cavitation field and to estimate radius–time curves for individual bubbles. Data were acquired for bubbles in water with varying temperature and dissolved gas content. Measurements verify that vapor trapping is an important mechanism that is sensitive to both temperature and dissolved gas content. While this work focuses primarily on individual bubbles, some bubble cloud effects were observed. Analysis with a simple multibubble model provides noteworthy insights.

Calculation and measurement of acoustic scatter to assess fragmentation in shock wave lithotripsy

Owen, N.R., O.A. Sapozhnikov, M.R. Bailey, and L.A. Crum, "Calculation and measurement of acoustic scatter to assess fragmentation in shock wave lithotripsy," J. Acoust. Soc. Am., 120, 3110, 2006., J. Acoust. Soc. Am., 120, 3110, 2006.

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1 Nov 2006

Shock wave lithotripsy (SWL) is currently conducted with little feedback on whether kidney stones are breaking. To determine if fragmentation could be assessed, acoustic scatter from intact and fractured stone models was calculated numerically and measured in vitro. Acoustic scatter from the stones, which were modeled with glass spheres, was calculated numerically using a linear elastic model, initialized with known elastic constants, and propagated from the stone model surface using the Helmholtz–Kirchhoff integral. Experimentally, shock waves were generated with a research lithotripter and scatter was measured with a broadband, spherically focused receiver. Calculated and measured results agreed well in the time domain. In frequency, power spectra were integrated to find energy and showed that scatter from the fractured stone model had higher energy in specific frequency bands that were related to the reverberation period. High-speed photography indicated that cavitation did not adversely affect the analysis of scatter. In this work it was possible to distinguish between the intact and fractured stone models. This method could be applied to stones that fragment gradually under the application of shock waves and potentially be used to estimate fragment size, and therefore the endpoint of therapy.

Photo-elastic, high-speed images of stress induced in cylindrical model kidney stones by lithotripsy

MacConaghy, B.E., T. Ikeda, M.R. Bailey, A.D. Maxwell, and O.A. Sapozhnikov, "Photo-elastic, high-speed images of stress induced in cylindrical model kidney stones by lithotripsy," J. Acoust. Soc. Am., 120, 3066, 2006.

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1 Nov 2006

Recent experiments and calculations show that a focusing shear wave generated by the shock wave traveling along the length of a cylindrical stone creates the dominant stress and causes fracture of cylindrical model stones. A small disk placed on the proximal face of the stone suppressed the longitudinal wave responsible for spallation in calculations yet had little effect on the number of shock waves required to fracture the stone in experiment. However, a disk placed around the stone blocked the shock wave traveling along the stone in calculations and suppressed fracture in measurements. The conclusion was that so-called dynamic squeezing was a dominant mechanism to spallation. Here, photo-elastic, high-speed imaging was used to observe the suppression of various waves in cylindrical stones made of acrylic. Glycerol was used to avoid artifact from the curvature of the stone. Comparison was made to calculations using the elasticity equations for an isotropic medium. Agreement between measurement and calculation was excellent and supports dynamic squeezing. The results help validate the model, and the technique and modeling may help us understand where and how stress is created in other shock wave therapies.

Radiation force imparted on a kidney stone by a Doppler-mode diagnostic pulse

Sapozhnikov, O.A., L.A. Trusov, A.I. Gromov, N.R. Owen, M.R. Bailey, and L.A. Crum, "Radiation force imparted on a kidney stone by a Doppler-mode diagnostic pulse," J. Acoust. Soc. Am., 120, 3109, 2006.

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1 Nov 2006

Detection of kidney stones and estimation of their sizes is an important part of the lithotripsy treatment. Fluoroscopy is often used to target stones, but not every stone is radio-opaque and, in addition, fluoroscopy produces ionizing radiation. Acoustic waves offer an alternative way to visualize stones. The acoustic impedance of kidney stones typically differs significantly from that of surrounding tissue. A useful consequence of the impedance mismatch is the possibility to target stones with diagnostic mode ultrasound. Another consequence is that radiation force pushes the stone. Stone displacement may be responsible for the twinkling artifact that has been observed by several authors in color Doppler mode of ultrasound imaging. This effect can be used to detect not only renal and ureteral stones, but also calcifications in other organs (e.g., breast). In this paper we model the radiation force associated with the Doppler diagnostic pulse. The problem is divided into three parts: (1) acoustic scattering; it is solved in finite differences; (2) radiation force calculation; (3) stone velocity estimation supposing the stone sits in soft tissue.

Use of a bovine eye lens for observation of HIFU-induced lesions in real-time

Lafon, C., V.A. Khokhlova, O.A. Sapozhnikov, P.J. Kaczkowski, A.A. Brayman, M.R. Bailey, and L.A. Crum, "Use of a bovine eye lens for observation of HIFU-induced lesions in real-time," Ultrasound Med. Biol. 32, 1731-1741, 2006.

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1 Nov 2006

Study of coagulative lesion formation by high intensity focused ultrasound (HIFU) in tissue usually requires performing a sequence of experiments under different exposure conditions followed by tissue sectioning. This paper, inspired by the pioneering work of Frederic L. Lizzi, reports on the use of the bovine eye lens as a laboratory model to observe visually the development of HIFU-induced lesions. The first part of this work describes the measurement of the lens shape, density, sound speed and attenuation. The measured values were within the range of previously published values. In the second part, HIFU-induced lesion development was observed in real-time and compared with good agreement with theoretical simulation. Theoretical modeling included acoustic propagation, absorptive heating and thermal dose, as well as the experimentally measured lens characteristics. Thus, the transparent eye lens can be used as a laboratory phantom to facilitate the understanding of HIFU treatment in other tissues.

A new PVDF membrane hydrophone for accurate measurement of medical shock waves

Maxwell, A.D., O.A. Sapozhnikov, and M.R. Bailey, "A new PVDF membrane hydrophone for accurate measurement of medical shock waves," Proceedings, IEEE International Ultrasonics Symposium, 2-6 October, Vancouver, Canada, 1608-1611 (IEEE, 2006).

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2 Oct 2006

Calibration of medical shock wave sources is critical and challenging. Aside from the fiber optic probe hydrophone, there are few if any commercially available hydrophones designed for measuring medical shock waves. We have developed a new PVDF membrane hydrophone and compared it to measurements with a fiber optic probe hydrophone (FOPH) in several lithotripters. One part of the hydrophone held the 5 cm times 5 cm times 25 µm PVDF film with geometrical element size 0.5 mm. The other part housed the preamplifier. By substitution comparison to FOPH and an NTR hydrophone, the sensitivity was found to be 0.035 MPa/mV at 2 MHz. Initial spot frequency comparisons showed the response to be fairly flat from 1-20 MHz but showed an elevated sensitivity at 15-20 MHz, and lithotripsy waveforms indicated some high-pass filtering. The impulse response of a 25 µm membrane was calculated and used to de-convolve the signal after which agreement with waveforms from the other hydrophones was excellent. The hydrophone is sufficiently robust to measure 1000 s of lithotripter shock waves. It is inexpensive, sensitive, and has a lower signal to noise ratio than the FOPH.

Characterization of high intensity focused ultrasound fields with a high spatio-temporal resolution

Canney, M.S., V.A. Khokhlova, M.R. Bailey, O.A. Sapozhnikov, and L.A. Crum, "Characterization of high intensity focused ultrasound fields with a high spatio-temporal resolution," Proceedings, 2006 IEEE International Ultrasonics Symposium, Vancouver, Canada, 856-859, doi:10.1109/ULTSYM.2006.231 (IEEE, 2006).

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2 Oct 2006

The accurate characterization of high intensity focused ultrasound (HIFU) fields is important for the prediction of thermal and mechanical bio-effects in tissue, as well as for the development of standards for therapeutic systems. At HIFU intensity levels, the combined effects of nonlinearity and diffraction result in the formation of asymmetric shocked waveforms and a corresponding distortion of the spatial distributions of various acoustic parameters that are responsible for different bio-effects. Acoustic probes that are capable of withstanding high pressures and that can measure waveforms with a high spatial and temporal resolution are required to capture the shock fronts and highly localized field structures that can arise at therapeutically relevant treatment regimes. An experimentally validated numerical model can also be an effective tool when direct measurements are not possible. In this work, acoustic measurements using force balance, acoustic holography, broadband fiber optic and PVDF hydrophones, were combined with simulations based on a KZK-type model to demonstrate an effective approach for the calibration of HIFU transducers in water and for derating these results to tissue.

Transient acoustic holography for diagnostic transducer characterization

Sapozhnikov, O.A., A.E. Ponomarev, and M.A. Smagin, "Transient acoustic holography for diagnostic transducer characterization," Proceedings, IEEE International Ultrasonics Symposium, Vancouver, Canada, 552 (IEEE, 2006).

2 Oct 2006

Use of acoustic scattering to monitor kidney stone fragmentation during shock wave lithotripsy

Owen, N.R., O.A. Sapozhnikov, M.R. Bailey, and L.A. Crum, "Use of acoustic scattering to monitor kidney stone fragmentation during shock wave lithotripsy," Proceedings, IEEE International Ultrasonics Symposium, 2-6 October, Vancouver, Canada, 736-739 (IEEE, 2006).

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2 Oct 2006

It is currently difficult to assess whether a kidney stone has fractured during shock wave lithotripsy. Here we report the calculation and measurement of shock wave scattering by stone models in water. Calculations were based on linear elastic theory to find pressure in the fluid and stress in the stone models, and on scattering theory to find radiation from the stone models. Measurements were made with a spherical, broadband receiver. Calculation and measurement agree well in the time domain and through frequency analysis of detected acoustic scattering it was possible to distinguish between fractured and intact model stones. Cavitation was visualized with high speed photography and was not a dominant effect in the measurements.

Acoustic cavitation and medical ultrasound

Kreider, W., L. Crum, M. Bailey, T. Matula, V. Khokhlova, and O. Sapozhnikov, "Acoustic cavitation and medical ultrasound," Proceedings, Sixth International Conference on Cavitation, 11-15 September, Wageningen, The Netherlands (MARIN, The Netherlands, 2006)(CD-ROM).

11 Sep 2006

Use of acoustic holography for characterization of therapeutic transducers

Sapozhnikov, O.A., D. Cathignol, M.R. Bailey, A.V. Morozov, and Y.A. Pishchalnikov, "Use of acoustic holography for characterization of therapeutic transducers," Proceedings, Sixth International Symposium on Therapeutic Ultrasound, 30 August - 1 September, Oxford, England, 114 (American Institute of Physics, 2006).

30 Aug 2006

Interactions of cavitation bubbles observed by high-speed imaging in shock wave lithotripsy

Pishchalnikov, Y.A., O.A. Sapozhnikov, M.R. Bailey, J.A. McAteer, J.C. Williams Jr., A.P. Evan, R.O. Cleveland, and L.A. Crum, "Interactions of cavitation bubbles observed by high-speed imaging in shock wave lithotripsy," Proceedings, 17th International Symposium on Nonlinear Acoustics, edited by A.A. Atchley, V.W. Sparrow, and R.M. Keolian, 299-302 (American Institute of Physics Conference Proceedings Vol. 383, 2006).

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30 May 2006

A multi-frame high-speed photography was used to investigate the dynamics of cavitation bubbles induced by a passage of a lithotripter shock wave in a water tank. Solitary bubbles in the free field each radiated a shock wave upon collapse, and typically emitted a micro-jet on the rebound following initial collapse. For bubbles in clouds, emitted jets were directed toward neighboring bubbles and could break the spherical symmetry of the neighboring bubbles before they in turn collapsed. Bubbles at the periphery of a cluster underwent collapse before the bubbles at the center. Observations with high-speed imaging confirm previous predictions that bubbles in a cavitation cloud do not cycle independently of one another but instead interact as a dynamic bubble cluster.

Role of shear and longitudinal waves in stone comminution by lithotripter shock waves

Bailey, M.R., A.D. Maxwell, B. MacConaghy, O.A. Sapozhnikov, and L.A. Crum, "Role of shear and longitudinal waves in stone comminution by lithotripter shock waves," Proceedings, 17th International Symposium on Nonlinear Acoustics, edited by A.A. Atchley, V.W. Sparrow, and R.M. Keolian, AIP Conference Proceedings, 838, 323-326, doi:10.1063/1.2210370, 2006.

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30 May 2006

Mechanisms of stone fragmentation by lithotripter shock waves were studied. Numerically, an isotropic-medium, elastic-wave model was employed to isolate and assess the importance of individual mechanisms in stone comminution. Experimentally, cylindrical U-30 cement stones were treated in an HM-3-style research lithotripter. Baffles were used to block specific waves responsible for spallation, squeezing, or shear. Surface cracks were added to stones to simulate the effect of cavitation, and then tested in water and glycerol (a cavitation suppressive medium). The calculated location of maximum stress compared well with the experimental observations of where cracks naturally formed. Shear waves from the shock wave in the fluid traveling along the stone surface (a kind of dynamic squeezing) led to the largest stresses in the cylindrical stones and the fewest shock waves to fracture. Reflection of the longitudinal wave from the back of the stone — spallation — and bubble-jet impact on the proximal and distal faces of the stone produced lower stresses and required more shock waves to fracture stones, but cavitation stresses become comparable in small stone pieces. Surface cracks accelerated fragmentation when created near the location where the maximum stress was predicted.

Transient acoustic holography for reconstructing the particle velocity of the surface of an acoustic transducer

Sapozhnikov, O.A, A.E. Ponomarev, and M.A. Smagin, "Transient acoustic holography for reconstructing the particle velocity of the surface of an acoustic transducer," Acoust. Phys., 52, 324-330, doi:10.1134/S1063771006030134, 2006.

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13 May 2006

A transient acoustic holography method based on the Rayleigh integral and the time-reversal mirror principle is described. The method reconstructs the particle velocity of the surface of an acoustic source from the waveform of the signal measured over a surface lying in front of the source. The possibility of applying the transient holography to studying pulsed sources used in ultrasonic diagnostics is investigated. A rectangular source that produces a short acoustic pulse and has a nonradiating defect on its surface is considered. A numerical simulation is used to demonstrate the possibility of a holographic reconstruction of the source vibrations. The effects of the spatial sampling step and the size of the measurement region on the reconstruction quality are demonstrated.

Detecting fragmentation of kidney stones in lithotripsy by means of shock wave scattering

Sapozhnikov, O.A., L.A. Trusov, N.R. Owen, M.R. Bailey, and R.O. Cleveland, "Detecting fragmentation of kidney stones in lithotripsy by means of shock wave scattering," Procedings, Fifth International Symposium on Therapeutic Ultrasound, edited by G.T. Clement, N.J. McDannold, and K. Hynynen, 308-312(American Institute of Physics, 2006).

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8 May 2006

Although extracorporeal shock wave lithotripsy (a procedure of kidney stone comminution using focused shock waves) has been used clinically for many years, a proper monitoring of the stone fragmentation is still undeveloped. A method considered here is based on recording shock wave scattering signals with a focused receiver placed far from the stone, outside the patient body. When a fracture occurs in the stone or the stone becomes smaller, the elastic waves in the stone will propagate differently (e.g. shear waves will not cross a fracture) which, in turn, will change the scattered acoustic wave in the surrounding medium. Theoretical studies of the scattering phenomenon are based on a linear elastic model to predict shock wave scattering by a stone, with and without crack present in it. The elastic waves in the stone and the nearby liquid were modeled using a finite difference time domain approach. The subsequent acoustic propagation of the scattered waves into the far-field was calculated using the Helmholtz–Kirchhoff integral.

Experimental studies were conducted using a research electrohydraulic lithotripter that produced the same acoustic output as an unmodified Dornier HM3 clinical lithotripter. Artificial stones, made from Ultracal-30 gypsum and acrylic, were used as targets. The stones had cylindrical shape and were positioned co-axially with the lithotripter axis. The scattered wave was measured by focused broadband PVDF hydrophone. It was shown that the size of the stone noticeably changed the signature of the reflected wave.

New devices and old pitfalls in shock wave therapy

Bailey, M.R., T.J. Matula, O.A. Sapozhnikov, R.O. Cleveland, Yu A. Pishchalnikov, and J.A. McAteer, "New devices and old pitfalls in shock wave therapy," Proceedings, Fifth International Symposium on Therapeutic Ultrasound, edited by G.T. Clement, N.J. McDannold, and K. Hynynen, AIP Conference Proceedings, 829, 380-384, doi:10.1063/1.2205501, 2006.

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8 May 2006

Shock waves are now used to treat a variety of musculoskeletal indications and the worldwide demand for shock wave therapy (SWT) is growing rapidly. It is a concern that very little is known about the mechanisms of action of shock waves in SWT. The technology for SWT devices is little changed from that of shock wave lithotripters developed for the treatment of urinary stones. SWT devices are engineered on the same acoustics principles as lithotripters, but the targets of therapy for SWT and shock wave lithotripsy (SWL) are altogether different. For SWT to achieve its potential as a beneficial treatment modality it will be necessary to determine precisely how SWT shock waves interact with biological targets. In addition, for SWT to evolve, the future design of these devices should be approached with caution, and lithotripsy may serve as a useful model. Indeed, there is a great deal to be learned from the basic research that has guided the development of SWL.

Optoacoustic monitoring of HIFU therapy: Feasibility study

Khokhlova, T.D., I.M. Pelivanov, O.A. Sapozhnikov, V.S. Solomatin, and A.A. Karabutov, "Optoacoustic monitoring of HIFU therapy: Feasibility study," Proceedings, Fifth International Symposium on Therapeutic Ultrasound, edited by G.T. Clement, N.J. McDannold, and K. Hynynen, AIP Conference Proceedings, 829, 181-185, doi:10.1063/1.2205462, 2006.

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8 May 2006

The main objective of this study was to evaluate the feasibility of the optoacoustic (OA) technique for the monitoring of HIFU therapy. Optoacoustic phenomenon is the generation of wideband ultrasonic transients through absorption of laser radiation and subsequent expansion of the heated volume. The excited OA transient can be detected by a wideband piezo-electric transducer and contains information on the distribution of optical properties (absorption and scattering) within the medium. If thermal lesions have different optical properties than the untreated tissue, the lesions will be detectable on the OA waveform. We used boiled and raw porcine liver as phantoms mimicking treated and untreated tissue correspondingly. Optical attenuation, absorption and scattering coefficients of raw and boiled porcine liver were measured by the optoacoustic technique, previously developed by our group. Measured optical absorption in raw liver was at least two times lower than in boiled liver at the laser wavelength of 1064 nm. Then OA technique was employed to detect a lesion produced by a 1.1 MHz focused ultrasound in a liver sample. The lesion was about 2 mm thick located about 1 cm below tissue surface. The feasibility and high promise of the OA approach to lesion detection was demonstrated.

Cavitation-induced saturation of the negative-pressure phase of lithotripter shock pulses

Pishchalnikov, Y.A., J.A. McAteer, M.R. Bailey, O.A. Sapozhnikov, I.V. Pishchalnikov, and J.C. Williams Jr., "Cavitation-induced saturation of the negative-pressure phase of lithotripter shock pulses," J. Acoust. Soc. Am., 119, 3407, 2006.

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1 May 2006

The tensile stress imposed by the negative-pressure phase of lithotripter shock pulses can cause cavitation. Bubbles continue to grow after the passage of the acoustic pulse; thus, some of the pulse energy is transformed to the kinetic and potential energy of the liquid surrounding the cavitation bubbles and, therefore, no longer belongs to the acoustic field. One might predict that this energy loss should be more pronounced for strong pulses that produce more cavitation. To investigate this, acoustic pulses were measured at the geometric focus of a Dornier HM-3 electrohydraulic lithotripter (water 39°C, dissolved gas ~8% saturation) using a fiber--optic probe hydrophone FOPH-500. Measurements showed that, while the amplitude and duration of the leading positive-pressure phase increased dramatically as charging potential was increased from 12 to 24 kV, the trailing negative-pressure phase of the pulse remained unchanged. This stabilization of the negative-pressure phase could be due to cavitation restricting the amplitude of the negative pressure that can be transmitted through the liquid, such that further increase of the amplitude at the source would not increase the negative amplitude at the target but would only result in stronger cavitation along the acoustic path.

Measurement and modeling of nonlinear waveforms in high-intensity focused ultrasound fields

Canney, M.S., M.R. Bailey, V.A. Khokhlova, M.A. Smagin, O.A. Sapozhnikov, and L.A. Crum, "Measurement and modeling of nonlinear waveforms in high-intensity focused ultrasound fields," J. Acoust. Soc. Am., 119, 3228, 2006.

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1 May 2006

Direct measurement of HIFU fields in situ is important for the accurate prediction of thermal and mechanical bioeffects, as well as for the development of standards for medical systems. An experimentally validated numerical model can be an effective tool in both laboratory and clinical settings when direct measurements are not possible. Calculations with a KZK-type model and measurements with a fiberoptic probe hydrophone were employed together to characterize HIFU fields in water and in a tissue-mimicking gel. To determine the boundary conditions for simulations, the normal velocity distribution on the transducer surface was reconstructed using acoustic holography and combined with acoustic power measurements. At the focus, highly nonlinear waveforms ( 700 and –150 bars peak pressures) were obtained both experimentally and numerically, which differed significantly from waveforms linearly extrapolated from low-amplitude results. Strongly distorted shock waveforms were localized in an axial region much smaller than the half-maximum beamwidth of the transducer excited at low level. At the highest excitation levels, the simulations predicted frequency content higher than was measurable in our configuration. Simulations also show that if these frequencies are not included, predicted heating rates are significantly lower.

The role of cavitation in therapeutic ultrasound

Crum, L., M. Bailey, V. Khokhlova, O. Sapozhnikov, B. Rabkin, A. Evan, J. McAteer, Y. Pishchalnikov, J. Williams, and R. Cleveland, "The role of cavitation in therapeutic ultrasound," J. Acoust. Soc. Am., 119, 3405, 2006.

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1 May 2006

Ed Carstensen has made many contributions to biomedical ultrasound but among those that are becoming more and more relevant to current clinical practice are those that determine the conditions under which cavitation is induced in vivo. For many years, it was assumed that the medical ultrasound devices were unable to induce cavitation in living tissue because either the acoustic conditions were not sufficient or the nucleation sites that are required were too small. With the advent of lithotripters and high-intensity focused ultrasound (HIFU) devices, cavitation generation in vivo is commonplace. Our current research at the University of Washington has focused on the role that cavitation plays in stone comminution and tissue damage during lithotripsy, as well as the enhancement or reduction of desirable coagulative necrosis during HIFU application. During HIFU application, we find enhanced heating that results from nonlinear acoustic wave propagation (a key Carstensen contribution) leads to vapor bubble formation. This presentation will review our recent studies in this area.

Advantage of a broad focal zone in SWL: Synergism between squeezing and shear

Bailey, M.R., A.D. Maxwell, B. MacConaghy, L.A. Crum, J.A. McAteer, R.O. Cleveland, and C.A. Sapozhnikov, "Advantage of a broad focal zone in SWL: Synergism between squeezing and shear," J. Urol., 175, 538 (Suppl. S), 2006.

1 Mar 2006

Gel phantom for use in high-frequency focused ultrasound dosimetry

Lafon, C., V. Zderic, M.L. Noble, J.C. Yuen, P.J. Kaczkowski, O.A. Sapozhnikov, F. Chavrier, L.A. Crum, and S. Vaezy, "Gel phantom for use in high-frequency focused ultrasound dosimetry," Ultrasound Med. Biol., 31, 1383-1389, 2005

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11 Oct 2005

An optically transparent phantom was developed for use in high-intensity focused ultrasound (US), or HIFU, dosimetry studies. The phantom is composed of polyacrylamide hydrogel, embedded with bovine serum albumin (BSA) that becomes optically opaque when denatured. Acoustic and optical properties of the phantom were characterized as a function of BSA concentration and temperature. The speed of sound (1544 m/s) and acoustic impedance (1.6 MRayls) were similar to the values in soft tissue. The attenuation coefficient was approximately 8 times lower than that of soft tissues (0.02 Np/cm/MHz for 9% BSA). The nonlinear (B/A) coefficient was similar to the value in water. HIFU lesions were readily seen during formation in the phantom. In US B-mode images, the HIFU lesions were observed as hyperechoic regions only if the cavitation activity was present. The phantom can be used for fast characterization and calibration of US-image guided HIFU devices before animal or clinical studies.

A mechanistic analysis of stone comminution in lithotripsy

Maxwell, A.D., O.A. Sapozhnikov, M.R. Bailey, B. MacConaghy, and L.A. Crum, "A mechanistic analysis of stone comminution in lithotripsy," J. Acoust. Soc. Am., 117, 2385, 2005

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2 Apr 2005

In vitro experiments and an elastic wave model were employed to isolate and assess the importance of individual mechanisms in stone comminution in lithotripsy. Cylindrical U-30 cement stones were treated in an HM-3-style research lithotripter. Baffles were used to block specific waves responsible for spallation, squeezing, or shear. Surface cracks were added to stones to simulate the effect of cavitation, then tested in water and glycerol (a cavitation suppressive medium). Each case was simulated using the elasticity equations for an isotropic medium. The calculated location of maximum stress compared well with the experimental observations of where cracks naturally formed. Shear waves from the shock wave in the fluid traveling along the stone surface (a kind of dynamic squeezing) led to the largest stresses in the cylindrical stones and the fewest SWs to fracture. Reflection of the longitudinal wave from the back of the stone — spallation — and bubble-jet impact on the proximal and distal faces of the stone produced lower stresses and required more SWs to break stones. Surface cracks accelerated fragmentation when created near the location where the maximum stress was predicted.

Observation of cavitation during shock wave lithotripsy

Bailey, M.R., L.A. Crum, Y.A. Pishchalnikov, J.A. McAteer, I.V. Pishchalnikova, A.P. Evan, O.A. Sapozhnikov, and R.O. Cleveland, "Observation of cavitation during shock wave lithotripsy," J. Acoust. Soc. Am., 117, 2371, 2005.

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1 Apr 2005

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 (PCD), using coincident signals on confocal, orthogonal receivers, were equally sensitive and were used to interrogate the renal collecting system (urine) and the kidney parenchyma (tissue). Cavitation was detected in urine immediately upon SW administration in urine or urine plus X-ray contrast agent, but in tissue, cavitation required hundreds of SWs to initiate. Localization of cavitation was confirmed by fluoroscopy, sonography, and by thermally marking the kidney using the PCD 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. Cavitation, especially in the urine space, was observed to evolve from a sparse field to a dense field with strong acoustic collapse emissions to a very dense field that no longer produced detectable collapse. The finding that cavitation occurs in kidney tissue is a critical step toward determining the mechanisms of tissue injury in SWL.

Assessing the mechanism of kidney stone comminution by a lithotripter shock pulse

Sapozhnikov, O.A., M.R. Bailey, A.D. Maxwell, B. MacConaghy, R.O. Cleveland, and L.A. Crum, "Assessing the mechanism of kidney stone comminution by a lithotripter shock pulse," Proceedings, American Institute of Physics Conference, number 754, 164-166, doi:10.1063/1.1901627, (2005).

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28 Mar 2005

Comminution of axisymmetric stones by a lithotripter shock wave was studied experimentally and theoretically. In experiments, shock waves were generated by a research electrohydraulic lithotripter modeled after the Dornier HM-3, and stones were made from U-30 cement. Cylindrical stones of various length to diameter ratios, stones of conical shape, and stones with artificial cracks were studied. In other cases, baffles to block specific waves that contribute to spallation or squeezing were used, and glycerol was used to suppress cavitation. The theory was based on the elasticity equations for an isotropic medium. The equations were written in finite differences and integrated numerically. Maximum compression, tensile and shear stresses were predicted depending on the stone shape and side-surface condition in order to investigate the importance of the stone geometry. It is shown that the theoretical model used explains the observed position of a crack in a stone. The theory also predicts the efficiency of stone fragmentation depending on its shape and size, as well as on the presence of cracks on the stone surface and baffles near the stone.

Modeling of stresses generated by lithotripter shock wave in cylindrical kidney stone

Sapozhnikov, O.A., R.O. Cleveland, M.R. Bailey, and L.A. Crum, "Modeling of stresses generated by lithotripter shock wave in cylindrical kidney stone," Proceedings of the 3rd International Symposium on Therapeutic Ultrasound, edited by J.Y. Chapelon and C. Lafon, 323-328 (Lyon, France, INSERM, 2004).

15 Sep 2004

Cavitation in shock wave lithotripsy: the critical role of bubble activity in stone breakage and kidney trauma

Bailey, M.R., R.O. Cleveland, T. Colonius, L.A. Crum, A.P. Evan, J.E. Lingeman, J.A. McAteer, O.A. Sapozhnikov, and J.C. Williams, "Cavitation in shock wave lithotripsy: the critical role of bubble activity in stone breakage and kidney trauma," Ultrason. Symp. Proc., 1, 724-727, 10.1109/ULTSYM.2003.1293503, 2003.

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8 Oct 2003

Shock wave lithotripsy (SWL) is the use of shock waves to fragment kidney stones. We have undertaken a study of the physical mechanisms responsible for stone comminution and tissue injury in SWL. SWL was originally developed on the premise that stone fragmentation could be induced by a short duration, high amplitude positive pressure pulse. Even though the SWL waveform carries a prominent tensile component, it has long been thought that SW damage to stones could be explained entirely on the basis of mechanisms such as spallation, pressure gradients, and compressive fracture. We contend that not only is cavitation also involved in SWL, bubble activity plays a critical role in stone breakage and is a key mechanism in tissue damage.

Our evidence is based upon a series of experiments in which we have suppressed or minimized cavitation, and discovered that both stone comminution and tissue injury is similarly suppressed or minimized. Some examples of these experiments are: (1) application of overpressure, (2) time reversal of acoustic waveform, (3) acoustically-transparent, cavitation-absorbing films, and (4) dual pulses. In addition, using passive and active ultrasound, we have observed the existence of cavitation, in vivo, and at the site of tissue injury.

Numerical and experimental results showed mitigation of bubble collapse intensity by time-reversing the lithotripsy pulse and in vivo treatment showed a corresponding drop from 6.1% ± 1.7% to 0.0% in the hemorrhagic lesion. The time-reversed wave did not break stones. Stone comminution and hemolysis were reduced to levels very near sham levels with the application of hydrostatic pressure greater than the near 10-MPa amplitude of the negative pressure of the lithotripter shock wave. A Mylar sheet 3-mm from the stone surface did not inhibit erosion and internal cracking, but a sheet in contact with the stone did. In water, mass lost from stones in a dual pulse lithotripter is 8 times greater than with a single lithotripter, but in glycerol, which reduces the pressures generated in bubble implosion, the enhancement is lost.

This cavitation-inclusive mechanistic understanding of SWL is gaining acceptance and has had clinical impact. Treatment at slower SW rate give- s cavitation bubble clusters time to dissolve between pulses and increases comminution. Some SWL centers now treat patients at slower SW rate to take advantage of this effect. An elegant cavitation-aware strategy to reduce renal trauma in SWL is being tested in experimental animals. Starting treatment at low amplitude causes vessels to constrict and this interferes with cavitation-mediated vascular injury. Acceptance of the role of cavitation in SWL is beginning to be embraced by the lithotripter industry, as new dual-pulse lithotripters—based on the concept of cavitation control—have now been introduced.

Cavitation bubble cluster activity in the breakage of kidney stones by lithotripter shockwaves

Pishchalnikov, Y.A., O.A. Sapozhnikov, M.R. Bailey, J.C. Williams, R.O. Cleveland, T. Colonius, L.A. Crum, A.P. Evan, and J.A. McAteer, "Cavitation bubble cluster activity in the breakage of kidney stones by lithotripter shockwaves," J. Endourology, 17 (7), 435-446, doi: 10.1089/089277903769013568, 2003.

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1 Sep 2003

High-speed photography was used to analyze cavitation bubble activity at the surface of artificial and natural kidney stones during exposure to lithotripter shock waves in vitro. Numerous individual bubbles formed at the surface of stones, but these bubbles did not remain independent and combined with one another to form bubble clusters. Bubble clusters formed at the proximal end, the distal end, and at the sides of stones. Each cluster collapsed to a narrow point of impact. Collapse of the proximal cluster caused erosion at the leading face of the stone and the collapse of clusters at the sides of stones appeared to contribute to the growth of cracks. Collapse of the distal cluster caused minimal damage. We conclude that cavitation-mediated damage to stones was due not to the action of solitary bubbles, but to the growth and collapse of bubble clusters.

Physical mechanisms of the therapeutic effect of ultrasound

Bailey, M.R., V.A. Khokhlova, O.A. Sapozhnikov, S.G. Kargl, and L.A. Crum, "Physical mechanisms of the therapeutic effect of ultrasound," Acoust. Phys., 49, 369-388, DOI: 10.1134/1.1591291, 2003

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30 Jan 2003

Therapeutic ultrasound is an emerging field with many medical applications. High intensity focused ultrasound (HIFU) provides the ability to localize the deposition of acoustic energy within the body, which can cause tissue necrosis and hemostasis. Similarly, shock waves from a lithotripter penetrate the body to comminute kidney stones, and transcutaneous ultrasound enhances the transport of chemotherapy agents. New medical applications have required advances in transducer design and advances in numerical and experimental studies of the interaction of sound with biological tissues and fluids. The primary physical mechanism in HIFU is the conversion of acoustic energy into heat, which is often enhanced by nonlinear acoustic propagation and nonlinear scattering from bubbles. Other mechanical effects from ultrasound appear to stimulate an immune response, and bubble dynamics play an important role in lithotripsy and ultrasound-enhanced drug delivery. A dramatic shift to understand and exploit these nonlinear and mechanical mechanisms has occurred over the last few years. Specific challenges remain, such as treatment protocol planning and real-time treatment monitoring. An improved understanding of the physical mechanisms is essential to meet these challenges and to further advance therapeutic ultrasound.

Cavitation bubble cluster activity in the breakage of stones by shock wave lithotripsy

Pishchalnikov, Y.A., O.A. Sapozhnikov, J.C. Williams, Jr., A.P. Evan, J.A. McAteer, R.O. Cleveland, T. Colonius, M.R. Bailey, and L.A. Crum, "Cavitation bubble cluster activity in the breakage of stones by shock wave lithotripsy," J. Acoust. Soc. Am., 111, 2461, 2002.

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1 Oct 2002

High-speed photography was used to investigate cavitation at the surface of artificial and natural kidney stones during exposure to lithotripter shock pulses in vitro. It was observed that numerous individual bubbles formed over virtually the entire surface of the stone, but these bubbles did not remain independent and combined with one another to form larger bubbles and bubble clusters. The movement of bubble boundaries across the surface left portions of the stone bubble free. The biggest cluster grew to envelop the proximal end of the stone (6.5 mm diameter artificial stone) then collapsed to a small spot that over multiple shots formed a crater in that face of the stone. The bubble clusters that developed at the sides of stones tended to align along fractures and to collapse into these cracks. High-speed camera images demonstrated that cavitation mediated damage to stones was due not to the action of solitary, individual bubbles, but to the forceful collapse of dynamic clusters of bubbles.

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

Sapozhnikov, O.A., V.A. Khokhlova, M.R. Bailey, J.C. Williams Jr., M.A. McAteer, R.O. Cleveland, and L.A. Crum, "Effect of overpressure and pulse repetition frequency on cavitation in shock wave lithotripsy," J. Acoust. Soc. Am., 112, 1183-1195, doi:10.1121/1.1500754, 2002.

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1 Oct 2002

Cavitation appears to contribute to tissue injury in lithotripsy. Reports have shown that increasing pulse repetition frequency [(PRF) 0.5–100 Hz] increases tissue damage and increasing static pressure (1–3 bar) reduces cell damage without decreasing stone comminution. Our hypothesis is that overpressure or slow PRF causes unstabilized bubbles produced by one shock pulse to dissolve before they nucleate cavitation by subsequent shock pulses. The effects of PRF and overpressure on bubble dynamics and lifetimes were studied experimentally with passive cavitation detection, high-speed photography, and B-mode ultrasound and theoretically. Overpressure significantly reduced calculated (100–2 s) and measured (55–0.5 s) bubble lifetimes. At 1.5 bar static pressure, a dense bubble cluster was measured with clinically high PRF (2–3 Hz) and a sparse cluster with clinically low PRF (0.5–1 Hz), indicating bubble lifetimes of 0.5–1 s, consistent with calculations. In contrast to cavitation in water, high-speed photography showed that overpressure did not suppress cavitation of bubbles stabilized on a cracked surface. These results suggest that a judicious use of overpressure and PRF in lithotripsy could reduce cavitation damage of tissue while maintaining cavitation comminution of stones.

An innovative synthetic tissue-mimicking material for high-intensity focused ultrasound

Lafon, C., O.A. Sapozhnikov, P.J. Kaczkowski, S. Vaezy, M. Noble, and L.A. Crum, "An innovative synthetic tissue-mimicking material for high-intensity focused ultrasound," J. Acoust. Soc. Am., 110, 2613, 2001.

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1 Nov 2001

A dosimetry study of the high-temperature and pressure regimes involved in high-intensity focused ultrasound (HIFU) requires experiments on biological tissues because no synthetic tissue-mimicking phantom is available. Unfortunately, the development of coagulative lesions cannot be observed in real-time in opaque tissues. Furthermore, the natural heterogeneous structure of tissue complicates direct comparison with numerical models. In this study, a new optically transparent phantom is evaluated. It is principally composed of a polyacrylamide gel, and includes a thermally sensitive indicator protein that becomes optically diffusive when denatured. Various tests were undertaken to characterize the acoustical, thermal, and optical properties of this material for a range of protein concentrations. The attenuation coefficient can be usefully modified by adjusting the quantity of embedded proteins to permit some selection of acoustic regime. It is also possible to emphasize cavitation activity at lower BSA concentrations, or thermal effects at higher concentrations. This new phantom adequately matches tissue for most of the measured parameters and facilitates the study of the HIFU bioeffects.

Ultrasound-guided localized detection of cavitation during lithotripsy in pig kidney in vivo

Sapozhnikov, O.A., M.R. Bailey, L.A. Crum, et al., "Ultrasound-guided localized detection of cavitation during lithotripsy in pig kidney in vivo," Ultrason. Symp. Proc., 2, 1347-1350, 2001.

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7 Oct 2001

It is supposed that inertial cavitation plays a significant role in tissue damage during extracorporeal shock wave lithotripsy (ESWL). In this work we attempted to detect cavitation in tissue. In vivo experiments with pigs were conducted in a Dornier HM3 electrohydraulic lithotripter. Kidney alignment was made using fluoroscopy and B-mode ultrasound. Cavitation was detected by a dual passive cavitation detection (DPCD) system consisting of two confocal spherical bowl PZT transducers (1.15 MHz, focal length 10 cm, radius 10 cm). An ultrasound scanhead was placed between the transducers, an hyperechoic spots in the image indicated pockets of bubbles during ESWL. A coincidence-detection algorithm and the confocal transducers made it possible to localize cavitation to within a 4 mm diameter region. The signals from both the collecting system and kidney tissue were recorded. The targeting of the DPCD focus was confirmed by using the DPCD transducers as high intensity focused ultrasound (HIFU) sources at HIFU durations below the lesion formation threshold. In this HIFU regime, a bright spot appears in the B-mode image indicating the position of the DPCD focus. In this way we could confirm that refraction and scattering in tissue did not cause a misalignment. The tissue region interrogated was also marked with a lesion produced by HIFU. Clear cavitation signals were detected from the collecting system and from pools of blood that formed near the kidney capsule and weak signals were recorded from tissue during the ESWL treatment.

Localized cavitation detection in lithotripsy in vivo

Bailey, M.R., L.A. Crum, N. Miller, L.N. Couret, O.A. Sapozhnikov, Y.A. Pishchalnikov, J.A. McAteer, B. Connors, and A.P. Evan, "Localized cavitation detection in lithotripsy in vivo," Proceedings, Seventeenth International Congress on Acoustics, Rome Italy, 2-7 September (ICA, Rome, 2001).

2 Sep 2001

Overpressure and the role of bubbles in focused ultrasound lesion shape

Bailey, M.R., L.N. Couret, O.A. Sapozhnikov, V.A. Khokhlova, G. ter Haar, S. Vaezy, X. Shi, R. Martin, and L.A. Crum, "Overpressure and the role of bubbles in focused ultrasound lesion shape," Proceedings, First International Workshop on the Application of High Intensity Focused Ultrasound (HIFU) in Medicine, 10-12 May, Chongqing, China, edited by G. R. ter Haar and F. Wu, 22 (2001).

10 May 2001

Use of overpressure to assess the role of bubbles in focused ultrasound lesion shape in vitro

Bailey, M.R., L.N. Couret, O.A. Sapozhnikov, V.A. Khokhlova, G. ter Haar, S. Vaezy, X. Shi, R. Martin, and L.A. Crum, "Use of overpressure to assess the role of bubbles in focused ultrasound lesion shape in vitro," Ultrasound Med. Biol., 27, 695-708, 2001.

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1 May 2001

Overpressure–elevated hydrostatic pressure–was used to assess the role of gas or vapor bubbles in distorting the shape and position of a high-intensity focused ultrasound (HIFU) lesion in tissue. The shift from a cigar-shaped lesion to a tadpole-shaped lesion can mean that the wrong area is treated. Overpressure minimizes bubbles and bubble activity by dissolving gas bubbles, restricting bubble oscillation and raising the boiling temperature. Therefore, comparison with and without overpressure is a tool to assess the role of bubbles. Dissolution rates, bubble dynamics and boiling temperatures were determined as functions of pressure. Experiments were made first in a low-overpressure chamber (0.7 MPa maximum) that permitted imaging by B-mode ultrasound (US). Pieces of excised beef liver (8 cm thick) were treated in the chamber with 3.5 MHz for 1 to 7 s (50% duty cycle). In situ intensities (ISP) were 600 to 3000 W/cm2. B-mode US imaging detected a hyperechoic region at the HIFU treatment site. The dissipation of this hyperechoic region following HIFU cessation corresponded well with calculated bubble dissolution rates; thus, suggesting that bubbles were present. Lesion shape was then tested in a high-pressure chamber. Intensities were 1300 and 1750 W/cm2 ( ± 20%) at 1 MHz for 30 s. Hydrostatic pressures were 0.1 or 5.6 MPa. At 1300 W/cm2, lesions were cigar-shaped, and no difference was observed between lesions formed with or without overpressure. At 1750 W/cm2, lesions formed with no overpressure were tadpole-shaped, but lesions formed with high overpressure (5.6 MPa) remained cigar-shaped. Data support the hypothesis that bubbles contribute to the lesion distortion.

Real-time observation of inception and growth of HIFU-induced tissue lesions

Lafon, C., M.R. Bailey, L.N. Couret, P.J. Kaczkowski, A.A. Brayman, L.A. Crum, and O.A. Sapozhnikov, "Real-time observation of inception and growth of HIFU-induced tissue lesions," J. Acoust. Soc. Am., 108, 2546, 2000.

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1 Nov 2000

To study the biological effects of high-intensity focused ultrasound (HIFU), experiments are usually performed on isolated or perfused tissues. Indeed, the complex phenomena occurring in tissue during HIFU-induced coagulation necrosis is difficult to mimic with synthetic phantoms. A good phantom should first match the acoustical and thermal properties of tissues. Furthermore, heating above a thermal threshold should induce a permanent, localized and observable change corresponding to protein denaturing in tissue. Lastly, the choice of a transparent material makes possible real-time examination of the development of coagulation necroses. We have used bovine eye lenses in this aim. The density, sound speed, attenuation, and thermal threshold for irreversible damage to the bovine lens were measured and found to be similar to those for liver or muscle, common tissues for HIFU experiments, although acoustic attenuation is slightly higher in the lens. Transparency of the lens allowed us to observe HIFU-induced lesion evolution in real time. The shape and size of the lesions obtained in the lens agreed well with results obtained in liver. In conclusion, the transparent bovine eye lens is a useful model for visualization of thermal lesions.

Bubble translation due to radiation force in SWL

Sokolov, D.L., M.R. Bailey, L.A. Crum, and O.A Sapozhnikov, "Bubble translation due to radiation force in SWL," J. Acoust. Soc. Am., 107, 2838, 2000.

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1 May 2000

The clustering of cavitation bubbles may lead to enhanced stone comminution and influence the extent of tissue damage during shock wave lithotripsy (SWL) treatment. Recent research has focused on changing the SWL pulse, or timing between pulses, to intensify or mitigate collapse or localize these clusters. Such research has targeted radial, not translational motion. We investigate whether bubble translation due to radiation force is sufficiently large to influence cluster formation. The translational dynamics of a single spherical bubble were modeled according to the formulation proposed by Watanabe and Kukita [Phys. Fluids 5(11) (1993)]. After radius-time data were obtained using the Gilmore equation, translational motion was calculated by numerical integration of the Watanabe equation. Calculations were performed for a range of bubble sizes (R0=2–20 μm) and pressure rise times (10-9 – 10-7 s). The results show that, during bubble growth and collapse induced by a single pulse or two pulses with microsecond delays, bubble translations are ~0.1 mm. Although bubble translation from a single pulse may not have a noticeable effect on bubble distribution, the effect may be cumulative for the +1000 shots fired during clinical SWL treatment.

SWL stone fragmentation in vitro is improved by slowing SW delivery rate

Lifshitz, D.A., J.C. Williams Jr., A.P. Evan, D.L. Rietjens, J.A. McAteer, M.R. Bailey, L.A. Crum, and O.A. Sapozhnikov, "SWL stone fragmentation in vitro is improved by slowing SW delivery rate," J. Acoust. Soc. Am., 107, 2838, doi:10.1121/1.429162, 2000.

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1 May 2000

Fast shock wave (SW) rates in lithotripsy (SWL) generate enhanced cavitation that could promote stone fragmentation. We tested the idea that SWL at the high end of clinical SW rate (2 Hz) acts to improve stone comminution. Model stones (Ultracal-30 cement) were exposed to SWs (20 kV, 400 SWs) at 0.2, 0.5, 1, and 2 Hz in a research electrohydraulic lithotripter. Fragmentation was assessed by measuring number, size, and projected surface area of the fragments. Stones treated at 0.2 Hz exhibited significantly greater fragmentation (p<0.01) than stones at 1 or 2 Hz, while fragmentation between 0.2 and 0.5 Hz was similar. Mean ± SEM for fragment area increase was 370±53% at 0.2 Hz (n=10 stones), 280±34 at 0.5 Hz (8), 130±31 at 1 Hz (5), and 101±16 at 2 Hz (20). This pronounced enhancement of fragmentation at very slow SW rate was unexpected. High-speed camera images of cavitation at solid objects show an increased bubble cloud at faster SW rates. The bubble cloud may interfere with transmission of acoustic energy to the stone surface. These in vitro data suggest the possibility that patient treatment at fast SW delivery rates may decrease the efficiency of stone comminution.

A dual passive cavitation detector for localized detection of lithotripsy-induced cavitation in vitro

Cleveland, R.O., O.A., Sapozhnikov, J.C. Bailey, and L.A. Crum, "A dual passive cavitation detector for localized detection of lithotripsy-induced cavitation in vitro," J. Acoust. Soc. Am., 107, 1745-1758, 2000.

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1 Mar 2000

A passive cavitation detector (PCD) identifies cavitation events by sensing acoustic emissions generated by the collapse of bubbles. In this work, a dual passive cavitation detector (dual PCD), consisting of a pair of orthogonal confocal receivers, is described for use in shock wave lithotripsy. Cavitation events are detected by both receivers and can be localized to within 5 mm by the nature of the small intersecting volume of the focal areas of the two receivers in association with a coincidence detection algorithm. A calibration technique, based on the impulse response of the transducer, was employed to estimate radiated pressures at collapse near the bubble. Results are presented for the in vitro cavitation fields of both a clinical and a research electrohydraulic lithotripter. The measured lifetime of the primary growth-and-collapse of the cavitation bubbles increased from 180 to 420 μs as the power setting was increased from 12 to 24 kV. The measured lifetime compared well with calculations based on the Gilmore–Akulichev formulation for bubble dynamics. The radiated acoustic pressure 10 mm from the collapsing cavitation bubble was measured to vary from 4 to 16 MPa with increasing power setting; although the trends agreed with calculations, the predicted values were four times larger than measured values. The axial length of the cavitation field correlated well with the 6-dB region of the acoustic field. However, the width of the cavitation field (10 mm) was significantly narrower than the acoustic field (25 mm) as bubbles appeared to be drawn to the acoustic axis during the collapse. The dual PCD also detected signals from "rebounds," secondary and tertiary growth-and-collapse cycles. The measured rebound time did not agree with calculations from the single-bubble model. The rebounds could be fitted to a Rayleigh collapse model by considering the entire bubble cloud as an effective single bubble. The results from the dual PCD agreed well with images from high-speed photography. The results indicate that single-bubble theory is sufficient to model lithotripsy cavitation dynamics up to time of the main collapse, but that upon collapse bubble cloud dynamics becomes important.

Inventions

Methods to Determine Optimal Ultrasound Pulse Parameters to Fragment Urinary Calculi Using Acoustic Feedback

Record of Invention Number: 47078

Adam Maxwell, Mike Bailey, Bryan Cunitz, Oleg Sapozhnikov

Disclosure

6 Oct 2014

Ultrasound Image Feedback for Lithotripsy

Record of Invention Number: 47077

Adam Maxwell, Mike Bailey, Bryan Cunitz, Wayne Kreider, Oleg Sapozhnikov

Disclosure

6 Oct 2014

Assortment of Push Profiles for Pushing a Variety of Kidney Stones

Record of Invention Number: 47072

Mike Bailey, Bryan Cunitz, Barbrina Dunmire, Oleg Sapozhnikov

Disclosure

3 Oct 2014

More Inventions

Ultrasound Technique for Trapping and Displacing Solid Objects Using a Vortex Acoustic Beam Created by a Multi-element Sector Array Transducer

Record of Invention Number: 47037

Mike Bailey, Wayne Kreider, Adam Maxwell, Oleg Sapozhnikov

Disclosure

18 Aug 2014

Derating Method for Therapeutic Applications of High Intensity Focused Ultrasound

Patent Number: 8,668,658

Vera Khokhlova, Olga Bessonova, Michael Canney, Mike Bailey, Oleg Sapozhnikov, Larry Crum

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Patent

11 Mar 2014

Methods of derating a nonlinear ultrasound field and associated systems are disclosed herein. A method of derating a nonlinear ultrasound field in accordance with an embodiment of the present technology can include, for example, calibrating an ultrasound source to a first source voltage (Vw) and generating a nonlinear acoustic wave from the ultrasound source into water. The method can further include measuring a focal waveform of the nonlinear acoustic wave and determining a second source voltage (Vt) of the ultrasound source that generates the same focal waveform in tissue.

Methods to Induce Large Volumes of Mechanically Fractionated Lesions Using Therapeutic Phased Arrays

Record of Invention Number: 46733

Vera Khokhlova, Mike Bailey, Tanya Khokhlova, Wayne Kreider, Adam Maxwell, Oleg Sapozhnikov

Disclosure

8 Nov 2013

Low-Frequency Enhancement of Boiling Histotripsy

Record of Invention Number: 46730

Vera Khokhlova, Mike Bailey, Tanya Khokhlova, Wayne Kreider, Adam Maxwell, Oleg Sapozhnikov

Disclosure

7 Nov 2013

Method to Induce Transcostal Tissue Ablation using High Intensity Focused Ultrasound with Shocks

Record of Invention Number: 46728

Vera Khokhlova, Mike Bailey, Larry Crum, Wayne Kreider, Adam Maxwell, Oleg Sapozhnikov, Leonid R. Gavrilov, Petr Yuldashev

Disclosure

6 Nov 2013

Method and Apparatus to Detect the Fragmentation of Kidney Stones by Measuring Acoustic Scatter

Patent Number: 8,535,250

Niel Owen, Mike Bailey, Oleg Sapozhnikov

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Patent

17 Sep 2013

During shock wave therapy, a determination is made that a kidney stone has begun to fracture, and then a progress of its fragmentation is assessed. This determination can reduce the number of shock waves used to disintegrate kidney stones, and thereby reduce dose-dependent tissue damage. The identification of fracture is possible through the detection and analysis of resonant acoustic scattering, which is the radiation caused by reverberations within a stone particle that is struck by a shock wave. The scattering frequency can provide both an indication that the kidney stone has fragmented, and an indication of the relative sizes of the fragments. Such techniques can be combined with vibro-acoustography based gating that better targets the stone.

Apparatus and Method for Disrupting Oil Pipeline Plugs

Record of Invention Number: 46417

Larry Crum, Barbrina Dunmire, Wayne Kreider, Tom Matula, Oleg Sapozhnikov

Disclosure

26 Feb 2013

Method for Noninvasive Focused Ultrasound Surgery

Record of Invention Number: 46356

Vera Khokhlova, Mike Bailey, Adam Maxwell, Oleg Sapozhnikov

Disclosure

11 Jan 2013

Method of Detecting Microbubbles in Tissue and Tissue Phantoms Using "Twinkling" Artifact of Doppler Imaging

Record of Invention Number: 46179

Oleg Sapozhnikov, Mike Bailey, Joo Ha Hwang, Tatiana Khokhlova, Vera Khokhlova

Disclosure

10 Aug 2012

A New Ultrasound Imaging Regime for Improved Size Measuring of Hard Concretions Present in Soft Tissue Based on Observation of Ultrasound Shadow on a B-mode Image

Record of Invention Number: 45653

Mike Bailey, Oleg Sapozhnikov

Disclosure

8 Jun 2011

New Regime of Ultrasound Imaging of Strong Scatterers in Tissue Using Envelope-based Beamforming

Record of Invention Number: 45654

Mike Bailey, Oleg Sapozhnikov

Disclosure

8 Jun 2011

Ultrasound Technique to Separate Hard Objects from Tissue by Long Lasting Reverberation in Hard Objects

Record of Invention Number: 45655

Mike Bailey, Oleg Sapozhnikov

Disclosure

8 Jun 2011

A Method of Soft Tissue Emulsification Using a Mechanism of Ultrasonic Atomization Inside Gas or Vapor Cavities

Record of Invention Number: 45567

Mike Bailey, Vera Khokhlova, Oleg Sapozhnikov, Tatiana Khokhlova, Julianna Simon

Disclosure

28 Mar 2011

Portable Acoustic Holography System for Therapeutic Ultrasound Sources

Record of Invention Number: 45469

Mike Bailey, Peter Kaczkowski, Vera Khokhlova, Wayne Kreider, Oleg Sapozhnikov

Disclosure

21 Dec 2010

A Model of an Equivalent Focused Piston Source to Characterize Nonlinear Ultrasound Fields of 2D Therapeutic (HIFU) Arrays

Record of Invention Number: 45341

Mike Bailey, Larry Crum, Vera Khokhlova, Oleg Sapozhnikov

Disclosure

12 Aug 2010

Improved Detection of Hard Concretions Present in Soft Tissues Based on Doppler Imaging Twinkling Artifact by Means of Insonifying the Imaged Region with Additional Modulated Intense Ultrasound Beam

Record of Invention Number: 8335D

Mike Bailey, Larry Crum, Oleg Sapozhnikov

Disclosure

1 Apr 2009

Improved Detection of Hard Concretions Present in Soft Tissues Based on Ultrasound Imaging Twinkling Artifact by Means of Introducing Fluctuations in Beam Structure for Consecutive Ultrasound Pulses of Each Scan Line

Record of Invention Number: 8336D

Mike Bailey, Larry Crum, Oleg Sapozhnikov

Disclosure

1 Apr 2009

Acoustics Air-Sea Interaction & Remote Sensing Center for Environmental & Information Systems Center for Industrial & Medical Ultrasound Electronic & Photonic Systems Ocean Engineering Ocean Physics Polar Science Center
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