Showing papers in "IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control in 2009"

Coherent plane-wave compounding for very high frame rate ultrasonography and transient elastography

Abstract: The emergence of ultrafast frame rates in ultrasonic imaging has been recently made possible by the development of new imaging modalities such as transient elastography. Data acquisition rates reaching more than thousands of images per second enable the real-time visualization of shear mechanical waves propagating in biological tissues, which convey information about local viscoelastic properties of tissues. The first proposed approach for reaching such ultrafast frame rates consists of transmitting plane waves into the medium. However, because the beamforming process is then restricted to the receive mode, the echographic images obtained in the ultrafast mode suffer from a low quality in terms of resolution and contrast and affect the robustness of the transient elastography mode. It is here proposed to improve the beamforming process by using a coherent recombination of compounded plane-wave transmissions to recover high-quality echographic images without degrading the high frame rate capabilities. A theoretical model is derived for the comparison between the proposed method and the conventional B-mode imaging in terms of contrast, signal-to-noise ratio, and resolution. Our model predicts that a significantly smaller number of insonifications, 10 times lower, is sufficient to reach an image quality comparable to conventional B-mode. Theoretical predictions are confirmed by in vitro experiments performed in tissue-mimicking phantoms. Such results raise the appeal of coherent compounds for use with standard imaging modes such as B-mode or color flow. Moreover, in the context of transient elastography, ultrafast frame rates can be preserved while increasing the image quality compared with flat insonifications. Improvements on the transient elastography mode are presented and discussed.

Shearwave dispersion ultrasound vibrometry (SDUV) for measuring tissue elasticity and viscosity

Abstract: Characterization of tissue elasticity (stiffness) and viscosity has important medical applications because these properties are closely related to pathological changes. Quantitative measurement is more suitable than qualitative measurement (i.e., mapping with a relative scale) of tissue viscoelasticity for diagnosis of diffuse diseases where abnormality is not confined to a local region and there is no normal background tissue to provide contrast. Shearwave dispersion ultrasound vibrometry (SDUV) uses shear wave propagation speed measured in tissue at multiple frequencies (typically in the range of hundreds of Hertz) to solve quantitatively for both tissue elasticity and viscosity. A shear wave is stimulated within the tissue by an ultrasound push beam and monitored by a separate ultrasound detect beam. The phase difference of the shear wave between 2 locations along its propagation path is used to calculate shear wave speed within the tissue. In vitro SDUV measurements along and across bovine striated muscle fibers show results of tissue elasticity and viscosity close to literature values. An intermittent pulse sequence is developed to allow one array transducer for both push and detect function. Feasibility of this pulse sequence is demonstrated by in vivo SDUV measurements in swine liver using a dual transducer prototype simulating the operation of a single array transducer.

Phase Coherence Imaging

Abstract: A new method for grating and side lobes suppression in ultrasound images is presented. It is based on an analysis of the phase diversity at the aperture data. Two coherence factors, namely the phase coherence factor (PCF) and the sign coherence factor (SCF), are proposed to weight the coherent sum output. Different from other approaches, phase rather than amplitude information is used to perform the correction action.

Benefits of minimum-variance beamforming in medical ultrasound imaging

Abstract: Recently, significant improvement in image resolution has been demonstrated by applying adaptive beamforming to medical ultrasound imaging. In this paper, we have used the minimum-variance beamformer to show how the low sidelobe levels and narrow beamwidth of adaptive methods can be used, not only to increase resolution, but also to enhance imaging in several ways. By using a minimum-variance beamformer instead of delay-and-sum on reception, reduced aperture, higher frame rates, or increased depth of penetration can be achieved without sacrificing image quality. We demonstrate comparable resolution on images of wire targets and a cyst phantom obtained with a 96-element, 18.5-mm transducer using delay-and-sum, and a 48-element, 9.25-mm transducer using minimum variance. To increase frame rate, fewer and wider transmit beams in combination with several parallel receive beams may be used. We show comparable resolution to delay-and-sum using minimum variance, 1/4th of the number of transmit beams and 4 parallel receive beams, potentially increasing the frame rate by 4. Finally, we show that by lowering the frequency of the transmitted beam and beamforming the received data with the minimum variance beamformer, increased depth of penetration is achieved without sacrificing lateral resolution.

Broadband minimum variance beamforming for ultrasound imaging

Abstract: A minimum variance (MV) approach for nearfield beamforming of broadband data is proposed. The approach is implemented in the frequency domain, and it provides a set of adapted, complex apodization weights for each frequency subband. The performance of the proposed MV beamformer is tested on simulated data obtained using Field II. The method is validated using synthetic aperture data and data obtained from a plane wave emission. Data for 13 point targets and a circular cyst with a radius of 5 mm are simulated. The performance of the MV beamformer is compared with delay-and-sum (DS) using boxcar weights and Hanning weights and is quantified by the full width at half maximum (FWHM) and the peak-side-lobe level (PSL). Single emission {DS boxcar, DS Hanning, MV} provide a PSL of {-16, -36, -49} dB and a FWHM of {0.79, 1.33, 0.08} mm. Using all 128 emissions, {DS boxcar, DS Hanning, MV} provides a PSL of {-32, -49, -65} dB, and a FWHM of {0.63, 0.97, 0.08} mm. The contrast of the beamformed single emission responses of the circular cyst was calculated as {-18, -37, -40} dB. The simulations have shown that the frequency subband MV beamformer provides a significant increase in lateral resolution compared with DS, even when using considerably fewer emissions. An increase in resolution is seen when using only one single emission. Furthermore, the effect of steering vector errors is investigated. The steering vector errors are investigated by applying an error of the sound speed estimate to the ultrasound data. As the error increases, it is seen that the MV beamformer is not as robust compared with the DS beamformer with boxcar and Hanning weights. Nevertheless, it is noted that the DS does not outperform the MV beamformer. For errors of 2% and 4% of the correct value, the FWHM are {0.81, 1.25, 0.34} mm and {0.89, 1.44, 0.46} mm, respectively.

ULA-OP: an advanced open platform for ultrasound research

Abstract: The experimental test of novel ultrasound (US) investigation methods can be made difficult by the lack of flexibility of commercial US machines. In the best options, these only provide beamformed radiofrequency or demodulated echo-signals for acquisition by an external PC. More flexibility is achieved in high-level research platforms, but these are typically characterized by high cost and large size. This paper presents a powerful but portable US system, specifically developed for research purposes. The system design has been based on high-level commercial integrated circuits to obtain the maximum flexibility and wide data access with minimum of electronics. Preliminary applications involving nonstandard imaging transmit/receive strategies and simultaneous B-mode and multigate spectral Doppler mode are discussed.

A heterogeneous nonlinear attenuating full- wave model of ultrasound

Abstract: A full-wave equation that describes nonlinear propagation in a heterogeneous attenuating medium is solved numerically with finite differences in the time domain (FDTD). Three-dimensional solutions of the equation are verified with water tank measurements of a commercial diagnostic ultrasound transducer and are shown to be in excellent agreement in terms of the fundamental and harmonic acoustic fields and the power spectrum at the focus. The linear and nonlinear components of the algorithm are also verified independently. In the linear nonattenuating regime solutions match results from Field II, a well established software package used in transducer modeling, to within 0.3 dB. Nonlinear plane wave propagation is shown to closely match results from the Galerkin method up to 4 times the fundamental frequency. In addition to thermoviscous attenuation we present a numerical solution of the relaxation attenuation laws that allows modeling of arbitrary frequency dependent attenuation, such as that observed in tissue. A perfectly matched layer (PML) is implemented at the boundaries with a numerical implementation that allows the PML to be used with high-order discretizations. A -78 dB reduction in the reflected amplitude is demonstrated. The numerical algorithm is used to simulate a diagnostic ultrasound pulse propagating through a histologically measured representation of human abdominal wall with spatial variation in the speed of sound, attenuation, nonlinearity, and density. An ultrasound image is created in silico using the same physical and algorithmic process used in an ultrasound scanner: a series of pulses are transmitted through heterogeneous scattering tissue and the received echoes are used in a delay-and-sum beam-forming algorithm to generate a images. The resulting harmonic image exhibits characteristic improvement in lesion boundary definition and contrast when compared with the fundamental image. We demonstrate a mechanism of harmonic image quality improvement by showing that the harmonic point spread function is less sensitive to reverberation clutter.

Minimum variance beamforming combined with adaptive coherence weighting applied to medical ultrasound imaging

Abstract: Currently, the nonadaptive delay-and-sum (DAS) beamformer is used in medical ultrasound imaging. However, due to its data-independent nature, DAS leads to images with limited resolution and contrast. In this paper, an adaptive minimum variance (MV)-based beamformer that combines the MV and coherence factor (CF) weighting is introduced and adapted to medical ultrasound imaging. MV-adaptive beamformers can improve the image quality in terms of resolution and sidelobes by suppressing off-axis signals, while keeping onaxis ones. In addition, CF weighting can improve contrast and sidelobes by emphasizing the in-phase signals and reducing the out-of-phase ones. Combining MV and CF weighting results in simultaneous improvement of imaging resolution and contrast, outperforming both DAS and MV beamformers. In addition, because of the power of CF in reducing the focusing errors, the proposed method presents satisfactory robustness against sound velocity inhomogeneities, outperforming the regularized MV beamformer. The excellent performance of the proposed beamforming approach is demonstrated by several simulated examples.

An integrated circuit with transmit beamforming flip-chip bonded to a 2-D CMUT array for 3-D ultrasound imaging

Abstract: State-of-the-art 3-D medical ultrasound imaging requires transmitting and receiving ultrasound using a 2-D array of ultrasound transducers with hundreds or thousands of elements. A tight combination of the transducer array with integrated circuitry eliminates bulky cables connecting the elements of the transducer array to a separate system of electronics. Furthermore, preamplifiers located close to the array can lead to improved receive sensitivity. A combined IC and transducer array can lead to a portable, high-performance, and inexpensive 3-D ultrasound imaging system. This paper presents an IC flip-chip bonded to a 16times16-element capacitive micromachined ultrasonic transducer (CMUT) array for 3-D ultrasound imaging. The IC includes a transmit beamformer that generates 25-V unipolar pulses with programmable focusing delays to 224 of the 256 transducer elements. One-shot circuits allow adjustment of the pulse widths for different ultrasound transducer center frequencies. For receiving reflected ultrasound signals, the IC uses the 32-elements along the array diagonals. The IC provides each receiving element with a low-noise 25-MHz-bandwidth transimpedance amplifier. Using a field-programmable gate array (FPGA) clocked at 100 MHz to operate the IC, the IC generated properly timed transmit pulses with 5-ns accuracy. With the IC flip-chip bonded to a CMUT array, we show that the IC can produce steered and focused ultrasound beams. We present 2-D and 3-D images of a wire phantom and 2-D orthogonal cross-sectional images (Bscans) of a latex heart phantom.

Evaluation of the damage detection capability of a sparse-array guided-wave SHM system applied to a complex structure under varying thermal conditions

Abstract: A sparse-array structural health monitoring (SHM) system based on guided waves was applied to the door of a commercial shipping container. The door comprised a corrugated steel panel approximately 2.4 m by 2.4 m surrounded by a box beam frame and testing was performed in a nonlaboratory environment. A 3-D finite element (FE) model of the corrugations was used to predict transmission coefficients for the A0 and S0 modes across the corrugations as a function of incidence angle. The S0 mode transmission across the corrugations was substantially stronger, and this mode was used in the main test series. A sparse array with 9 transducers was attached to the structure, and signals from the undamaged structure were recorded at periodic intervals over a 3-week period, and the resulting signal database was used for temperature compensation of subsequent signals. Defects in the form of holes whose diameter was increased incrementally from 1 to 10 mm were introduced at 2 different points of the structure, and signals were taken for each condition. Direct analysis of subtracted signals allowed understanding of the defect detection capability of the system. Comparison of signals transmitted between different transducer pairs before and after damage was used to give an initial indication of defect detectability. Signals from all combinations of transducers were then used in imaging algorithms, and good localization of holes with a 5-mm diameter or above was possible within the sparse array, which covered half of the area of the structure.

Comments on Origins of Enhanced Piezoelectric Properties in Ferroelectrics

Abstract: This review discusses recent advances in understanding origins of large piezoelectric properties in some ferroelectric materials. In particular, it addresses the role of polarization rotation and monoclinic phases. It is suggested that the polarization rotation is an old concept that was proposed more than 30 years ago to explain enhanced properties of Pb(Zr, Ti) O3 in the morphotropic phase boundary region. It is further demonstrated that in addition to polarization rotation, polarization extension can also lead to large electro-mechanical properties. In fact, the largest piezoelectric coefficient has been reported in KH2PO4, which exhibits structural instability involving polarization extension and no monoclinic phases. It is also shown that substantial theoretical and experimental evidence exists to show that the highest piezoelectric response is often not observed in monoclinic phases but in the phase transition regions where polarization either changes direction or appears from the nonpolar state. Finally, it is proposed that the concept of free energy instability, which emerged from phenomenological and first principle calculations, is the most general approach that can be used to consistently interpret many experimental observations and underlies many theoretical results on enhancement of piezoelectric properties.

Analyses of power output of piezoelectric energy-harvesting devices directly connected to a load resistor using a coupled piezoelectric-circuit finite element method

Abstract: This paper presents, for the first time, a coupled piezoelectric-circuit finite element model (CPC-FEM) to analyze the power output of a vibration-based piezoelectric energy-harvesting device (EHD) when it is connected to a load resistor. Special focus is given to the effect of the load resistor value on the vibrational amplitude of the piezoelectric EHD, and thus on the current, voltage, and power generated by the device, which are normally assumed to be independent of the load resistor value to reduce the complexity of modeling and simulation. The presented CPC-FEM uses a cantilever with a sandwich structure and a seismic mass attached to the tip to study the following characteristics of the EHD as a result of changing the load resistor value: 1) the electric outputs: the current through and voltage across the load resistor; 2) the power dissipated by the load resistor; 3) the displacement amplitude of the tip of the cantilever; and 4) the shift in the resonant frequency of the device. It is found that these characteristics of the EHD have a significant dependence on the load resistor value, rather than being independent of it as is assumed in most literature. The CPC-FEM is capable of predicting the generated output power of the EHD with different load resistor values while simultaneously calculating the effect of the load resistor value on the displacement amplitude of the tip of the cantilever. This makes the CPC-FEM invaluable for validating the performance of a designed EHD before it is fabricated and tested, thereby reducing the recurring costs associated with repeat fabrication and trials. In addition, the proposed CPC-FEM can also be used for producing an optimized design for maximum power output.

A fast convolution-based methodology to simulate 2-Dd/3-D cardiac ultrasound images

Abstract: This paper describes a fast convolution-based methodology for simulating ultrasound images in a 2-D/3-D sector format as typically used in cardiac ultrasound. The conventional convolution model is based on the assumption of a space-invariant point spread function (PSF) and typically results in linear images. These characteristics are not representative for cardiac data sets. The spatial impulse response method (IRM) has excellent accuracy in the linear domain; however, calculation time can become an issue when scatterer numbers become significant and when 3-D volumetric data sets need to be computed. As a solution to these problems, the current manuscript proposes a new convolution-based methodology in which the data sets are produced by reducing the conventional 2-D/3-D convolution model to multiple 1-D convolutions (one for each image line). As an example, simulated 2-D/3-D phantom images are presented along with their gray scale histogram statistics. In addition, the computation time is recorded and contrasted to a commonly used implementation of IRM (Field II). It is shown that COLE can produce anatomically plausible images with local Rayleigh statistics but at improved calculation time (1200 times faster than the reference method).

50 kHz capacitive micromachined ultrasonic transducers for generation of highly directional sound with parametric arrays

Abstract: In this study, we examine the use of capacitive micromachined ultrasonic transducers (CMUTs) with vacuum- sealed cavities for transmitting directional sound with parametric arrays. We used finite element modeling to design CMUTs with 40-mum- and 60-mum-thick membranes to have resonance frequencies of 46 kHz and 54 kHz, respectively. The wafer bonding approach used to fabricate the CMUTs provides good control over device properties and the capability to fabricate CMUTs with large diameter membranes and deep cavities. Each CMUT is 8 cm in diameter and consists of 284 circular membranes. Each membrane is 4 mm in diameter. Characterization of the fabricated CMUTs shows they have center frequencies of 46 kHz and 55 kHz and 3 dB bandwidths of 1.9 kHz and 5.3 kHz for the 40-mum- and 60-mum-thick membrane devices, respectively. With dc bias voltages of 380 V and 350 V and an ac excitation of 200 V peak-to-peak, the CMUTs generate average sound pressure levels, normalized to the device's surface, of 135 dB and 129 dB (re 20 muPa), respectively. When used to generate 5 kHz sound with a parametric array, we measured sound at 3 m with a 6 dB beamwidth of 8.7deg and a sound pressure level of 58 dB. To understand how detector nonlinearity (e.g., the nonlinearity of the microphone used to make the sound level measurements) affects the measured sound pressure level, we made measurements with and without an acoustic low-pass filter placed in front of the microphone; the measured sound levels agree with numerical simulations of the pressure field. The results presented in this paper demonstrate that large-area CMUTs, which produce high-intensity ultrasound, can be fabricated for transmitting directional sound with parametric arrays.

Phase Transition Temperatures and Piezoelectric Properties of (Bi 1/2 Na 1/2 )TiO 3 -and (Bi 1/2 K 1/2 )TiO 3 -Based Bismuth Perovskite Lead-Free Ferroelectric Ceramics

Abstract: The phase transition temperatures and the dielectric, ferroelectric, and piezoelectric properties of bismuth perovskite lead-free ferroelectric ceramics such as (Bi1/2Na1/2)TiO3 (BNT)- and (Bi1/2Na1/2)TiO3 (BKT)-based solid solutions have been reviewed According to the results obtained by our group, these ceramics can be considered as superior lead-free piezoelectric materials for reducing environmental damage Perovskite-type ceramics appear to be suitable for actuator and high-power applications that require a large piezoelectric constant d33 and a high Curie temperature TC or a high depolarization temperature Td (> 200degC) In this paper, we summarize the relationship between phase transition temperatures and piezoelectric properties In the case of the BNT-based solid solutions, the highest piezoelectric properties were obtained at the morphotropic phase boundary (MPB) between rhombohedral and tetragonal phases However, d33 and Td were shown to have a tradeoff relationship Considering the high Td and high d33, the tetragonal side of the MPB composition is suitable for piezoelectric actuator application Meanwhile, the Qm values on the rhombohedral side of the MPB composition were better than those on the tetragonal side, and excellent high-power characteristics were obtained for Mn-doped BNT-(Bi1/2Na1/2)TiO3-BKT ternary systems with rhombohedral symmetry BKT ceramics were prepared by the hot-pressing (HP) method, and their ferroelectric and piezoelectric properties were clarified BKT ceramics doped with a small amount of Bi have a relatively high remanent polarization of Pr = 276 muC/cm2 and high piezoelectric properties (k33 = 040 and d33 = 101 pC/N) In addition, it was clarified that BKT ceramics have a high Td of approximately 300degC The solid solution (1-x)BKT-xBaTiO3 (BKT-BT100x) exhibited a high Td of approximately 300degC at x > 04

Characterization of Hard Piezoelectric Lead-Free Ceramics

Abstract: K4CuNb8O23 doped K0.45Na0.55NbO3 (KNN-KCN) ferroelectric ceramics were found to exhibit asymmetrical polarization hysteresis loops, related to the development of an internal bias field. The internal bias field is believed to be the result of defect dipoles of acceptor ions and oxygen vacancies, which lead to piezoelectric "hardening" effect, by stabilizing and pinning of the domain wall motion. The dielectric loss for the hard lead-free piezoelectric ceramic was found to be 0.6%, with mechanical quality factors Q on the order of >1500. Furthermore, the piezoelectric properties were found to decrease and the coercive field increased, when compared with the undoped material, exhibiting a typical characteristic of "hard" behavior. The temperature usage range was limited by the polymorphic phase transition temperature, being 188degC. The full set of material constants was determined for the KNN-KCN materials. Compared with conventional hard PZT ceramics, the lead-free possessed lower dielectric and piezoelectric properties; however, comparable values of mechanical Q, dielectric loss, and coercive fields were obtained, making acceptor modified KNN based lead-free piezoelectric material promising for high-power applications, where leadfree materials are desirable.

A 256 x 256 2-D array transducer with row-column addressing for 3-D rectilinear imaging

Abstract: We present simulation and experimental results from a 5-MHz, 256times256 2-D (65536 elements, 38.4times38.4 mm) 2-D array transducer with row-column addressing. The main benefits of this design are a reduced number of interconnects, a modified transmit/receive switching scheme with a simple diode circuit, and an ability to perform volumetric imaging of targets near the transducer with transmit beamforming in azimuth and receive beamforming in elevation. The final dimensions of the transducer were 38.4 mm times 38.4 mm times 300 mum. After a row-column transducer was prototyped, the series resonance impedance was 104 Omega at 5.4 MHz. The measured -6 dB fractional bandwidth was 53% with a center frequency of 5.3 MHz. The SNR at the transmit focus was measured to be 30 dB. At 5 MHz, the average nearest neighbor crosstalk was -25 dB. In this paper, we present 3-D images of both 5 pairs of nylon wires embedded in a clear gelatin phantom and an 8 mm diameter cylindrical anechoic cyst phantom acquired from a 256 times 256 2-D array transducer made from a 1-3 composite. We display the azimuth and elevation B-scans as well as the C-scan for each image. The cross-section of the wires is visible in the azimuth B-scan, and the long axes can be seen in the elevation B-scan and C-scans. The pair of wires with 1-mm axial separation is discernible in the elevational B-scan. When a single wire from the wire target phantom was used, the measured lateral beamwidth was 0.68 mm and 0.70 mm at 30 mm depth in transmit beamforming and receive beamforming, respectively, compared with the simulated beamwidth of 0.55 mm. The cross-section of the cyst is visible in the azimuth B-scan whereas the long axes can be seen as a rectangle in the elevation B-scan and C-scans.

Nonlinear pyroelectric energy harvesting from relaxor single crystals

Abstract: Energy harvesting from temperature variations in a Pb(Zn1/3Nb2/3)0.955Ti0.045O3 single crystal was studied and evaluated using the Ericsson thermodynamic cycle. The efficiency of this cycle related to Carnot cycle is 100 times higher than direct pyroelectric energy harvesting, and it can be as high as 5.5% for a 10degC temperature variation and 2 kV/mm electric field. The amount of harvested energy for a 60degC temperature variation and 2 kV/mm electric field is 242.7 mJmiddotcm-3. The influence of ferroelectric phase transitions on the energy harvesting performance is discussed and illustrated with experimental results.

The application of synthetic focusing for imaging crack-like defects in pipelines using guided waves

Abstract: This paper deals with quantifying the performance of a technique for detection, location, and sizing of circumferential crack-like defects in pipelines using synthetically focused guided waves. The system employs a circumferential array of piezoelectric transducer elements. A torsional probing guided wave is excited using the array, which subsequently interacts with the reflecting features of the pipe, such as defects or weld caps. The recorded backscattered signals are synthetically focused to every point of interest in the pipe wall, to form an image of the reflecting features of the pipe. The defect image amplitude is used to estimate the defect depth, and the full width at half maximum of the defect image circumferential profile is used to estimate the circumferential extent of the defect. The imaging system is tested with data from finite element simulations and from laboratory experiments. It is found that reliable sizing of circumferential cracks in finite element simulations and experiments can be achieved if the circumferential extent of the defect is greater than 1.5 lambdaS, where lambdaS is the shear wavelength at the frequency of inspection. This result is theoretically valid for any pipe size, any axial defect location, and any inspection frequency. Amplitude gains of around 18 dB over an unfocused system have been observed experimentally in an 8-inch pipe with a 9 dB SNR improvement.

Beamspace adaptive beamforming for ultrasound imaging

Abstract: Applying the Capon adaptive beamformer in medical ultrasound imaging results in enhanced resolution by improving the interference-suppressing capabilities of the array. This improvement comes at the expense of an increased computational complexity. We have investigated the application of a beamspace adaptive beamformer for medical ultrasound imaging, which can be used to achieve reduced computational complexity with performance comparable to that of the Capon beamformer. The idea behind beamspace beamforming is that, instead of using the spatial statistics of the elements in the array to differentiate between signals and interference, we use the spatial statistics of a set of orthogonal beams, which are formed in different directions. This represents a shift from element space to beamspace. Because the majority of interference in medical ultrasound imaging is constrained to a limited spatial interval due to the focused transmit beam, this latter space can be reduced to a dimension that is lower than that of element space. We show, using simulations and experimental data, that this dimension can be selected as low as 3 while still achieving performance comparable to its element space counterpart.

Three-dimensional photoacoustic imaging using a two-dimensional CMUT array

Abstract: In this paper, we describe using a 2-D array of capacitive micromachined ultrasonic transducers (CMUTs) to perform 3-D photoacoustic and acoustic imaging. A tunable optical parametric oscillator laser system that generates nanosecond laser pulses was used to induce the photoacoustic signals. To demonstrate the feasibility of the system, 2 different phantoms were imaged. The first phantom consisted of alternating black and transparent fishing lines of 180 μm and 150 μm diameter, respectively. The second phantom comprised polyethylene tubes, embedded in chicken breast tissue, filled with liquids such as the dye indocyanine green, pig blood, and a mixture of the 2. The tubes were embedded at a depth of 0.8 cm inside the tissue and were at an overall distance of 1.8 cm from the CMUT array. Two-dimensional cross-sectional slices and 3-D volume rendered images of pulse-echo data as well as photoacoustic data are presented. The profile and beamwidths of the fishing line are analyzed and compared with a numerical simulation carried out using the Field II ultrasound simulation software. We investigated using a large aperture (64 x 64 element array) to perform photoacoustic and acoustic imaging by mechanically scanning a smaller CMUT array (16 x 16 elements). Two-dimensional transducer arrays overcome many of the limitations of a mechanically scanned system and enable volumetric imaging. Advantages of CMUT technology for photoacoustic imaging include the ease of integration with electronics, ability to fabricate large, fully populated 2-D arrays with arbitrary geometries, wide-bandwidth arrays and high-frequency arrays. A CMUT based photoacoustic system is proposed as a viable alternative to a piezoelectric transducer based photoacoustic systems.

Fabricating capacitive micromachined ultrasonic transducers with a novel silicon-nitride-Based wafer bonding process

Abstract: We report the fabrication and experimental testing of 1-D 23-element capacitive micromachined ultrasonic transducer (CMUT) arrays that have been fabricated using a novel wafer-bonding process whereby the membrane and the insulation layer are both silicon nitride. The membrane and cell cavities are deposited and patterned on separate wafers and fusion-bonded in a vacuum environment to create CMUT cells. A user-grown silicon-nitride membrane layer avoids the need for expensive silicon-on-insulator (SOI) wafers, reduces parasitic capacitance, and reduces dielectric charging. It allows more freedom in selecting the membrane thickness while also providing the benefits of wafer-bonding fabrication such as excellent fill factor, ease of vacuum sealing, and a simplified fabrication process when compared with the more standard sacrificial release process. The devices fabricated have a cell diameter of 22 mum, a membrane thickness of 400 nm, a gap depth of 150 nm, and an insulation thickness of 250 nm. The resonant frequency of the CMUT in air is 17 MHz and has an attenuation compensated center frequency of ~9 MHz in immersion with a -6 dB fractional bandwidth of 123%. This paper presents the fabrication process and some characterization results.

Short-range ultrasonic communications in air using quadrature modulation

Abstract: A study has been undertaken of ultrasonic communications methods in air, using a quadrature modulation method. Simulations were first performed to establish the likely performance of quadrature phase shift keying over the limited bandwidth available in an ultrasonic system. Quadrature phase shift keying modulation was then implemented within an experimental communication system, using capacitive ultrasonic sources and receivers. The results show that such a system is feasible in principle for communications over distances of several meters, using frequencies in the 200 to 400 kHz range.

Ice detection and classification on an aircraft wing with ultrasonic shear horizontal guided waves

Abstract: Ice accumulation on airfoils has been identified as a primary cause of many accidents in commercial and military aircraft. To improve aviation safety as well as reduce cost and environmental threats related to aircraft icing, sensitive, reliable, and aerodynamically compatible ice detection techniques are in great demand. Ultrasonic guided-wave-based techniques have been proved reliable for "go" and "no go" types of ice detection in some systems including the HALO system, in which the second author of this paper is a primary contributor. In this paper, we propose a new model that takes the ice layer into guided-wave modeling. Using this model, the thickness and type of ice formation can be determined from guided-wave signals. Five experimental schemes are also proposed in this paper based on some unique features identified from the guided- wave dispersion curves. A sample experiment is also presented in this paper, where a 1 mm thick glaze ice on a 2 mm aluminum plate is clearly detected. Quantitative match of the experiment data to theoretical prediction serves as a strong support for future implementation of other testing schemes proposed in this paper.

Ultrasound simulation of complex flow velocity fields based on computational fluid dynamics

Abstract: In this work, a simulation environment for the development of flow-related ultrasound algorithms is presented. Ultrasound simulations of realistic Doppler signals require accurate modeling of blood flow. Instead of using analytically described flow behavior, complex blood movement can be derived from velocity fields obtained with computational fluid dynamics (CFD). By further modeling blood as a collection of point scatterers, resulting RF-signals can be efficiently retrieved using an existing ultrasound simulation model. The main aim of this paper is to elaborate on creating CFD-based phantoms for ultrasound simulations. The coupling of a computed flow field with an ultrasound model offers flexible control of flow and ultrasound imaging parameters, beneficial for improving and developing imaging algorithms. The proposed method was validated in a straight tube with a stationary parabolic velocity profile and further demonstrated by an eccentrically stenosis carotid bifurcation. The estimated flow velocities are in good agreement with the CFD reference, both for color flow imaging and pulsed-wave doppler simulations. The presented method can also be extended to include wall mechanics simulations in future work.

The dynamic Allan variance

Abstract: We present and discuss the dynamic Allan variance, a measure of the time-varying stability of an atomic clock. First, the dynamic Allan variance is mathematically defined, then its behavior is extensively tested on simulated and experimental data. The results prove the validity and the effectiveness of the proposed new tool.

Improved parameter estimates based on the homodyned K distribution

Abstract: Quantitative techniques based on ultrasound backscatter are promising tools for ultrasonic tissue characterization There is a need for fast and accurate processing strategies to obtain consistent estimates An improved parameter estimation algorithm for the homodyned K distribution was developed based on SNR, skewness, and kurtosis of fractional- order moments From the homodyned K distribution, estimates of the number of scatterers per resolution cell (mu parameter) and estimates of the ratio of coherent to incoherent backscatter signal energy (k parameter) were obtained Furthermore, angular compounding was used to reduce estimate variance while maintaining spatial resolution of subsequent parameter images Estimate bias and variance from Monte Carlo simulations were used to quantify the improvement using the new estimation algorithm compared with existing techniques Improvements due to angular compounding were quantified by the decrease in estimate variance in both simulations and measurements from tissue-mimicking phantoms and by the increase in target contrast Finally, the new algorithm was used to derive estimates from 2 kinds of mouse mammary tumors for tissue characterization The new estimation algorithm yielded estimates with lower bias and variance than existing techniques For a typical pair of parameters (mu = 5 and k = 1), the bias and variance were reduced 67% and 16%, respectively, for the mu parameter estimates and 79% and 37%, respectively, for the k parameter estimates The use of angular compounding further reduced the estimate variance, eg, the variance of estimates for the mu parameter from measurements was reduced by a factor of approximately 90 when using 120 angles of view Finally, statistically significant differences were observed in parameter estimates from 2 kinds of mouse mammary tumors using the new algorithm These improvements suggest estimating parameters from the backscattered envelope can enhance the diagnostic capabilities of ultrasonic imaging

Langevin acoustic radiation force of a high-order bessel beam on a rigid sphere

Abstract: The acoustic radiation force of Langevin type resulting from the interaction of a high-order Bessel beam with a rigid immovable sphere in an ideal fluid is theoretically investigated. The analysis is based on applying the generalized Rayleigh series used in the near-field acoustic scattering problem to calculate the force. With appropriate selection of specific Bessel beam parameters, results for the rigid sphere unexpectedly reveal a negative radiation force caused by the Lagrangean energy density. Specifically, the negative force on the rigid sphere arises when the kinematic energy density is larger than the potential energy density. This condition provides an impetus for further designing acoustic tweezers operating with high-order Bessel beams of progressive waves for potential applications in particle entrapment and manipulation.

Quantitative ultrasound backscatter for pulsed cavitational ultrasound therapy-histotripsy

Abstract: Histotripsy is a well-controlled ultrasonic tissue ablation technology that mechanically and progressively fractionates tissue structures using cavitation. The fractionated tissue volume can be monitored with ultrasound imaging because a significant ultrasound backscatter reduction occurs. This paper correlates the ultrasound backscatter reduction with the degree of tissue fractionation characterized by the percentage of remaining normal-appearing cell nuclei on histology. Different degrees of tissue fractionation were generated in vitro in freshly excised porcine kidneys by varying the number of therapeutic ultrasound pulses from 100 to 2000 pulses per treatment location. All ultrasound pulses were 15 cycles at 1 MHz delivered at 100 Hz pulse repetition frequency and 19 MPa peak negative pressure. The results showed that the normalized backscatter intensity decreased exponentially with increasing number of pulses. Correspondingly, the percentage of normal appearing nuclei in the treated area decreased exponentially as well. A linear correlation existed between the normalized backscatter intensity and the percentage of normal appearing cell nuclei in the treated region. This suggests that the normalized backscatter intensity may be a potential quantitative real-time feedback parameter for histotripsy-induced tissue fractionation. This quantitative feedback may allow the prediction of local clinical outcomes, i.e., when a tissue volume has been sufficiently treated.

Manual and automated media and intima thickness measurements of the common carotid artery

Abstract: The intima-media thickness (IMT) of the common carotid artery (CCA) is widely used as an early indicator of the development of cardiovascular disease (CVD). It was proposed but not thoroughly investigated that the media layer (ML) thickness (MLT), its composition, and its texture may be indicative of cardiovascular risk and for differentiating between patients with high and low risk. In this study, we investigate an automated method for segmenting the ML and the intima layer (IL) and measurement of the MLT and the intima layer thickness (ILT) in ultrasound images of the CCA. The snakes segmentation method was used and was evaluated on 100 longitudinal ultrasound images acquired from asymptomatic subjects, against manual segmentation performed by a neurovascular expert. The mean plusmn standard deviation (sd) for the first and second sets of manual and the automated IMT, MLT, and ILT measurements were 0.71 plusmn 0.17 mm, 0.72 plusmn 0.17 mm, 0.67 plusmn 0.12 mm; 0.25 plusmn 0.12 mm, 0.27 plusmn 0.14 mm, 0.25 plusmn 0.11 mm; and 0.43 plusmn 0.10 mm, 0.44 plusmn 0.13 mm, and 0.42 plusmn 0.10 mm, respectively. There was overall no significant difference between the manual and the automated IMC, ML, and IL segmentation measurements. Therefore, the automated segmentation method proposed in this study may be used successfully in the measurement of the MLT and ILT complementing the manual measurements. MLT was also shown to increase with age (for both the manual and the automated measurements). Future research will incorporate the extraction of texture features from the segmented ML and IL bands, which may indicate the risk of future cardiovascular events. However, more work is needed for validating the proposed technique in a larger sample of subjects.