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Ultrasound imaging is useful for monitoring high-intensity, focused ultrasound (HIFU) therapy; however, interference on the ultrasound image, caused by HIFU excitation, must be avoided. A method to synchronize HIFU excitation with ultrasound imaging is described here. Synchronization was tested with two unmodified, commercial imagers and two tissue phantoms.

The properties of nonlinear acoustic wave propagation are known to be able to improve the resolution of ultrasound imaging, and could be used to dynamically estimate the physical properties of tissue. However, transducers capable of launching a wave that becomes nonlinear through propagation do not typically have the necessary bandwidth to detect the higher harmonics. Here we present the design and characterization of a novel multilayer transducer for high intensity transmit and broadband receive. The transmit layer was made from a narrow‐band, high‐power piezoceramic (PZT), with nominal frequency of 2.0 MHz, that was diced into an array of 32 elements. Each element was 0.300 mm wide and 6.3 mm in elevation, and with a pitch of 0.400 mm the overall aperture width was 12.7 mm. A quarter‐wave matching layer was attached to the PZT substrate to improve transmit efficiency and bandwidth. The overlaid receive layer was made from polyvinylidene fluoride (PVDF) that had gold metalization on one side. A custom two‐sided flex circuit routed electrical connections to the PZT elements and patterned the PVDF elements; the PZT and PVDF elements had identical apertures. A low viscosity and electrically nonconductive epoxy was used for all adhesion layers. Characterization of electrical parameters and acoustic output were performed per standard methods, where transmit and receive events were driven by a software‐controlled ultrasound engine. Echo data, collected from ex vivo tissue and digitized at 45 MS/s, exhibited frequency content up to the 4th harmonic of the 2 MHz transmit frequency.

The success of surgical management of lower pole stones is principally dependent on stone fragmentation and residual stone clearance. Choice of surgical method depends on stone size, yet all methods are subjected to post-surgical complications resulting from residual stone fragments. Here we present a novel method and device to reposition kidney stones using ultrasound radiation force delivered by focused ultrasound and guided by ultrasound imaging. The device couples a commercial imaging array with a focused annular array transducer. Feasibility of repositioning stones was investigated by implanting artificial and human stones into a kidney-mimicking phantom that simulated a lower pole and collecting system. During experiment, stones were located by ultrasound imaging and repositioned by delivering short bursts of focused ultrasound. Stone motion was concurrently monitored by fluoroscopy, ultrasound imaging, and video photography, from which displacement and velocity were estimated. Stones were seen to move immediately after delivering focused ultrasound and successfully repositioned from the lower pole to the collecting system. Estimated velocities were on the order of 1 cm/s. This in vitro study demonstrates a promising modality to facilitate spontaneous clearance of kidney stones and increased clearance of residual stone fragments after surgical management.

Nonlinear acoustic wave propagation can improve the resolution of ultrasound imaging, and could be used to dynamically estimate the physical properties of tissue. However, transducers capable of launching a wave that becomes nonlinear through propagation can typically detect only the fundamental and second harmonic. Here we present the design and characterization of a multilayer transducer with a high power transmit layer to generate nonlinear waves and a broadband receive layer to detect nonlinear scatter. The transmit array was made from a narrow-band PZT, with nominal frequency of 2.0 MHz, that was diced into an array of 32 elements. Elements had 0.300 mm width and 6.3 mm elevation, and the pitch was 0.400 mm. The receive array, placed directly above the transmit array, was made from PVDF elements that were patterned by flex circuit pads that replicated the PZT element dimensions. The PZT and PVDF elements had identical apertures. Simulations were performed to guide the selection of the transducer materials and thicknesses. Characterization of electrical parameters and acoustic output were performed per standard methods, in which transmit and receive events were driven by a software-controlled ultrasound system. Nonlinear waveforms with peak positive pressure up to 2.14 MPa were measured by a calibrated hydrophone. Echo data, collected from ex vivo tissue and digitized at 45 MS/s, exhibited frequency content up to the 4th harmonic of the 2 MHz transmit frequency.

Interstitial ultrasound applicators can be a minimally invasive alternative for treating targets that are unresectable or are inaccessible by extracorporeal methods. Dual-mode transducers for ultrasound imaging and therapy were developed to address the constraints of a miniaturized applicator and real-time treatment monitoring. We propose an original treatment strategy that combines ultrasound imaging and therapy using a dual-mode transducer rotating at 8 revolutions per second. Real-time B-mode imaging was interrupted to emit high-intensity ultrasound over a selected therapy aperture. A full 360 degrees image was taken every 8th rotation to image the therapy aperture. Numerical simulations were performed to study the effect of rotation on tissue heating, and to study the effect of the treatment sequence on transducer temperature. With the time-averaged transducer surface intensity held at 12 W/cm(2) to maintain transducer temperature below 66 degrees C, higher field intensities and deeper lesions were produced by narrower therapy apertures. A prototype system was built and tested using in vitro samples of porcine liver. Lesions up to 8 mm were produced using a time-averaged transducer surface intensity of 12 W/cm(2) applied for a period of 240 s over a therapy aperture of 40 degrees. Apparent strain imaging of the therapy aperture improved the contrast between treated and spared tissues, which could not be differentiated on B-mode images. With appropriate limits on the transducer output, real-time imaging and deep thermal ablation are feasible and sustainable using a rotating dual-mode transducer.

Miniature flat ultrasound transducers have shown to be effective for a large variety of thermal therapies, but the associated superficial heating implicates developing original strategies in order to extend therapeutic depth. The goal of the present paper is to use ultrasound contrast agents (UCA) to increase remote attenuation and heating. Theoretical simulations demonstrated that increasing attenuation from 0.27 to 0.8 Np/cm at 10 MHz beyond a distance of 18 mm from the transducer should result in longer thermal damages due to protein coagulation in a tissue mimicking phantom. Contrast agents (BR14, Bracco, Plan-les-Ouates, Switzerland) were embedded in thermo-sensitive gel and attenuations ranging from 0.27 to 1.33 Np/cm were measured at 10 MHz for concentrations of BR14 between 0 and 4.8%. Thermal damages were then induced in several gels, which had different layering configurations. Thermal damages, 12.8mm in length, were obtained in homogeneous gels. When mixing contrast agents at a concentration of 3.2% beyond a first 18 mm-thick layer of homogeneous gel, the thermal damages reached 21.5mm in length. This work demonstrated that contrast agents can be used for increasing attenuation remotely and extending therapeutic depth induced by a non-focused transducer. Additional work must be done in vivo in order to verify the remote-only distribution of bubbles and associated increase in attenuation.

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.

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.

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...

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. © 2006 American Institute of Physics.

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

Transcutaneous surgical procedures performed with a high intensity focused ultrasound (HIFU) therapy system can be monitored in real-time with an ultrasound imaging system if the HIFU is gated appropriately. Without synchronization, gated or continuous HIFU saturates the imaging system and interference occludes the image. If a gating signal is synchronized with the imaging cycle from any commercial imager, the location of any interference can be controlled and the HIFU treatment region can he visualized in real-time. Synchronization typically requires that an imaging system be customized for HIFU therapy, which is expensive and time consuming. We have developed a low-cost prototype system that synchronizes a HIFU therapy system with an arbitrary unmodified imaging system by using the HIFU transducer as a focused receiver that can detect scattered acoustic signals transmitted by the imaging probe. The receive signal is processed into a trigger that is used to control the gating and phasing of the HIFU relative to the imaging cycle. The technique is tested using a B-mode imager to monitor the formation of a lesion in a transparent tissue phantom; exposure time is 60 seconds with 40 W (time-averaged) electrical power delivered to the transducer. Performance is evaluated by recording the position of interference on the B-mode images and by comparing the B-mode images with CCD images that provide an optical view of lesion formation.

Vibro‐acoustography (VA) is an ultrasound interrogation and imaging technique with a variety of applications. Here it was used to identify optimal parameters for detecting and imaging kidney stones in phantoms. The parameters varied included the difference frequency and the position in time of the analysis window used for image construction. Experiments in a water tank were conducted using a focused PVDF membrane hydrophone (receiver) placed in a central opening of an annular, dual element transducer (source), itself mounted on a translation stage. Our source consisted of 90‐ms pulses with a center frequency of 2.0 MHz and difference frequencies between 50 and 350 kHz, applied both on and off stone. Variations in the amplitude of the measured ultrasound backscatter and acoustic emissions as a function of difference frequency, between signals from stone and phantom, guided the choice of imaging parameters. The results were detailed images of renal stones measuring 10 dB above the background tissue. These f...

A new method to address the challenging tasks of image‐guidance, targeting, and treatment monitoring during HIFU treatments is presented. The approach, enabled by the use of a novel multi‐layer PZT‐PVDF array with broad receive bandwidth in conjunction with a programmable ultrasound engine, uses the passive‐mode received echoes of the imaging array with a custom pixel‐based beamforming for HIFU focal tracking and targeting, allowing real‐time two‐dimensional (2D) B‐mode visualization of the HIFU beam. Temperature monitoring during treatment is based on acoustic nonlinear propagation theory and the physical relationship of sound speed and attenuation to frequency and temperature. The harmonics‐rich received echoes are processed differentially, encoded into an RGB additive color channel, beamformed, and overlayed in color over regular B‐mode images. Dynamic local temperature changes in the region of interest become visible as the 2D color image changes from frame to frame. Preliminary results on beam visual...

The incidence of kidney stones within the US population is now 10%, and rising. Many patients present with small stones, primary or recurrent, do not indicate interventional stone removal. We previously described a new stone removal method employing selective application of acoustic radiation force, at diagnostic output levels, to reposition stones for passive clearance. In this method, an imaging array transducer transmits pulses for image guidance and focused pulses to reposition the stone. Here we propose a new flash imaging modality to visualize the focused pulse to confirm targeting on the stone. To visualize the focused beam, short pulses were phase‐delayed across the transducer aperture to transmit a focused wave, from which echo data were collected, beamformed, and overlaid on a B‐mode image. The beam profile is visible because echo amplitude is higher within the convergent, focal, and divergent regions. During experiment, a stone was placed within a tissue phantom simulating the kidney lower pole and ureter. Once identified with B‐mode imaging, focal delays were calculated, targeting was confirmed by the beam visualization modality, and the stone was repositioned. Flash imaging visualization of the focused beam could be similarly applied to high‐intensity focused ultrasound therapy. [Work supported by NIH DK43881 NSBRI‐SMST01601.]

Interference in ultrasound imaging when used in connection with high intensity focused ultrasound (HIFU) is avoided by employing a synchronization signal to control the HIFU signal. Unless the timing of the HIFU transducer is controlled, its output will substantially overwhelm the signal produced by ultrasound imaging system and obscure the image it produces. The synchronization signal employed to control the HIFU transducer is obtained without requiring modification of the ultrasound imaging system. Signals corresponding to scattered ultrasound imaging waves are collected using either the HIFU transducer or a dedicated receiver. A synchronization processor manipulates the scattered ultrasound imaging signals to achieve the synchronization signal, which is then used to control the HIFU bursts so as to substantially reduce or eliminate HIFU interference in the ultrasound image. The synchronization processor can alternatively be implemented using a computing device or an application-specific circuit.

Miniature dual‐mode transducers can be used for minimally invasive treatment of deep‐seated tumors. While in contact with the tissue, the transducer guides and monitors localized necrosis. Here, an oscillating 5‐element piezo‐composite transducer was characterized, and then evaluated in vitro using porcine liver. Each element was 3.0 x 3.8 mm2 with a geometric cylindrical focus of 14 mm. The transmit frequency was determined by the maximal electro‐acoustic efficiency, 65%, which was found at 5.6 MHz. The transmit‐receive impulse response was 400 ns long at ‐6 dB, and the ‐6 dB fractional bandwidth, centered at 5.6 MHz, was 30%. Axial and lateral resolution measured with a 0.1 mm diameter wire was 0.5 mm and 2.0 mm, respectively. For therapy, all elements radiated simultaneously, and for imaging, independently. Treatment was performed at increments of 20° to form a composite volume of necrosis. At each angle, ultrasound was applied for 60 s at a transducer surface intensity of 15 W/cm2. Pulse‐echo data wer...

We present a summary of Inserm's experience with high power sources, which are necessary for ultrasound thermal therapy and lithotripsy. Moreover, generating high intensity pseudo‐continuous waveforms or high pressure pulses imposes different constraints on the transducer materials, specifically heat and mechanical stress. For thermal therapy, miniature piezoceramic transducers were used for interstitial, intratumoral, and endoluminal applicators. These probes operated at surface intensities up to 50 W/cm2 and generated elementary lesions in vivo within tens of seconds. Piezocomposite transducers were developed for large‐aperture, highly‐focused beams used in extracorporeal or intraoperative treatments. Focal intensities were 1000 W/cm2 or higher and up to 256 elements were utilized. Miniaturized piezocomposite transducers are currently being developed for dual‐mode imaging and therapy. For lithotripsy, piezoelectric shock wave generators were developed as alternatives to electrohydraulic or electromagnet...

Aggressive treatment of localized hepatic metastases, by surgery or other means, was proven to be a viable strategy for improving the prognoses of many patients. In that context, thermal ablation by high intensity ultrasound was proposed and used in clinics. However, for treating deep‐seated tumors and in most cases, radiofrequency and cryotherapy probes are applied interstitially. Interstitial ultrasound applicators were proposed as an intermediate solution. The treatment can be focused, deeper than with other physical agents, and the transducer can eventually both treat and image tissues. In our experience, two approaches were investigated: percutaneous and intratissular, or endo vascular. The active element was a miniature flat transducer operating at a frequency of 5 MHz, for a satisfactory tradeoff between beam penetration and energy absorption. In vivo trials on a porcine model demonstrated that both procedures are minimally invasive and that large thermal lesions, up to 20 mm deep, can be obtained....

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. [Work supported by NIH DK43881, DK55674, NSBRI SMS00402, and RFBR.]

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. [Work supported by NSBRI SMS00402 and NIH DK43881.]

Acoustic properties of a vibro‐acoustography system designed to detect kidney stones were measured. Our system was formed with two spherical transducers (10 cm diameter, 20 cm curvature) in degassed water that were confocal and separated by an angle of 30 degrees. They were driven at 1.1 MHz and 1.125 MHz to generate a difference frequency of 25 kHz. The acoustic field was characterized by scattering from a known target, the curved surface of a steel cylinder with 6.4 mm diameter. Waveforms of both the low and high frequency scattered signals were measured for different target locations, different hydrophone locations encircling the target, and different acoustic pressures. Focal dimensions of the −6 db pressure profile measured at 25 kHz and the fundamental were both 3×10 mm, in an elliptical shape, which is highly localized. Scatter amplitude was rather insensitive to hydrophone position when the target was in the focus, quite sensitive to hydrophone position when the target was out of the focus, and in...

Vibro‐acoustography can be used to measure material properties and detect calcifications within the body. Two transducers (diameter 10 cm, curvature 20 cm, frequency 1.1 MHz) are placed with overlapping foci in degassed water and driven at different frequencies to produce a dynamic radiation force in the range of 5–50 kHz. A LabVIEW program instructs the transducers to sweep through this frequency range at 500‐Hz increments while a synthetic cylindrical kidney stone is held in the focus in one of three ways: with a rubber band, within an acrylamide gel, or within a finger cot. A low‐frequency hydrophone, 10 cm from the focus and 90 deg from the direction of propagation, detects the radiated acoustic emission from the stone. The average amplitude of five signals is recorded to measure the frequency response of the stone. Unbroken stones exhibited higher amplitude response at frequencies near 10, 25, and 35 kHz. Stones are moved to simulate patient breathing and different in‐focus and out‐of‐focus acoustic ...

Synchronization for image guided therapy using high intensity focused ultrasound (HIFU) and imaging ultrasound is achieved with a new technique that uses the focused transducer as a receiver that can detect the acoustic pulses created by the imaging probe. Without synchronization, interference from the high intensity source occludes the imager’s display unpredictably, degrading the quality of the system. An imaging probe (Sonosite 180) is registered with a HIFU transducer (d=33 mm, roc=55 mm, f=3.5 MHz) such that the scan line bisects the single element focus. When acoustically coupled through a scattering medium, imaging pulses are passively detected with the HIFU transducer and electronically conditioned into a TTL level trigger. A LabVIEW program uses the trigger to create a pulse width modulated signal that controls the timing of HIFU excitation during treatment. Detection takes less than 1% of the time between displayed images when the imager is running at 20 frames per second. HIFU excitations are programmed to occur such that the single element focus is free of interference when viewed with the imager during treatment. With no electrical connections for this new, simple technique, an arbitrary imager can be selected for synchronized image guided therapy. [Work supported by NSBRI.]

High intensity focused ultrasound (HIFU) can necrose tumors or cauterize tissue bleeds at intensities on the order of 1000 w/cm2. A synchronized HIFU and B‐mode ultrasound system reveals a hyperechoic region at the treatment site, which grows with treatment duration and intensity. Our goal was to segment the hyperechoic region representing the lesion via image analysis and measure the ratio of its major and minor axes. Our algorithm uses the RF data as input, processes it, and outputs a binary image that represents the lesions cross‐sectional profile. With depth settings from the clinical ultrasound imager, it is possible to calculate lesion dimensions from the binary image. The algorithm was tested with lesions made in a transparent polyacrylamide tissue phantom that became opaque in response to focal heating during HIFU exposure. Lesion size was recorded simultaneously with ultrasound and a CCD camera, and both measurements agreed well. Additionally, computer segmentation agreed well with segmentation by HIFU users blinded to the experimental conditions. The average difference of the determined ratio was 13% for lesions less than 0.5 cm in length. Thus, it is possible to localize precisely the treated tissue region. [Work supported by NSBRI, NSF, and NIH‐SBIR.]