Neil R. Owen

A method to synchronize high-intensity, focused ultrasound with an arbitrary ultrasound imager

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

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.

1H-4 Use of Acoustic Scattering to Monitor Kidney Stone Fragmentation During Shock Wave Lithotripsy

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

Vibro‐acoustography for targeting kidney stones during lithotripsy

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 ...

Interstitial thermal ablation with a fast rotating dual-mode transducer

Interstitial ultrasound applicators can be a minimally invasive alternative for treating targets that are unresectable or are inaccessible by extracorporeal methods. Dualmode 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° 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/ cm2 to maintain transducer temperature below 66°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/cm2 applied for a period of 240 s over a therapy aperture of 40°. 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.

Detecting Fragmentation of Kidney Stones in Lithotripsy by Means of Shock Wave Scattering

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 Helm...

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

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 scatteri...

5A-5 Identification of Kidney Stone Fragmentation in Shock Wave Lithotripsy

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.

Characterization of a vibro‐acoustography system designed to detect kidney stones during lithotripsy

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...

Multilayer transducer for nonlinear imaging with application to targeting and monitoring of therapeutic ultrasound

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.