Showing papers in "IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control in 2012"

Progress in atomic fountains at LNE-SYRTE

Abstract: We give an overview of the work done with the Laboratoire National de Metrologie et d'Essais-Systemes de Reference Temps-Espace (LNE-SYRTE) fountain ensemble during the last five years. After a description of the clock ensemble, comprising three fountains, FO1, FO2, and FOM, and the newest developments, we review recent studies of several systematic frequency shifts. This includes the distributed cavity phase shift, which we evaluate for the FO1 and FOM fountains, applying the techniques of our recent work on FO2. We also report calculations of the microwave lensing frequency shift for the three fountains, review the status of the blackbody radiation shift, and summarize recent experimental work to control microwave leakage and spurious phase perturbations. We give current accuracy budgets. We also describe several applications in time and frequency metrology: fountain comparisons, calibrations of the international atomic time, secondary representation of the SI second based on the 87Rb hyperfine frequency, absolute measurements of optical frequencies, tests of the T2L2 satellite laser link, and review fundamental physics applications of the LNE-SYRTE fountain ensemble. Finally, we give a summary of the tests of the PHARAO cold atom space clock performed using the FOM transportable fountain.

Ultrasound contrast plane wave imaging

Abstract: Background: Monitoring the accumulation of microbubbles within tissue vasculature with ultrasound allows both molecular and perfusion imaging. Unfortunately, conventional imaging with focused pulses can destroy a large fraction of the microbubbles it is trying to follow. Using coherent synthetic summation, ultrafast plane wave imaging could attain similar image quality, while reducing the peak acoustic pressure and bubble disruption. Method: In these experiments, microbubbles were flowed in a wall-less vessel phantom. Images were obtained on a programmable clinical scanner with a set of line-per-line focused pulses for conventional contrast imaging and with compounded plane wave transmission adapted for nonlinear imaging. Imaging was performed between 14 and 650 kPa peak negative pressure at 7.5 MHz. The disruption of the microbubbles was evaluated by comparing the microbubble intensity before and after acquisition of a set of 100 images at various pressures. Results: The acoustic intensity required to disrupt 50% of the microbubbles was 24 times higher with plane-wave imaging compared with conventional focused pulses. Although both imaging approaches yield similar resolution, at the same disruption level, plane-wave imaging showed better contrast. In particular, at similar disruption ratio (50% after 100 images), contrast-pulse sequencing (CPS) performed with plane waves displayed an improvement of 11 dB compared with conventional nonlinear imaging. Conclusion: In each resolution cell of the image, plane-wave imaging spread the spatial peak acoustic intensity over more pulses, reducing the peak pressure and, hence, preserving the microbubbles. This method could contribute to molecular imaging by allowing the continuous monitoring of the accumulation of microbubbles with improved contrast.

A CMUT probe for medical ultrasonography: from microfabrication to system integration

Abstract: Medical ultrasonography is a powerful and costeffective diagnostic technique. To date, high-end medical imaging systems are able to efficiently implement real-time image formation techniques that can dramatically improve the diagnostic capabilities of ultrasound. Highly performing and thermally efficient ultrasound probes are then required to successfully enable the most advanced techniques. In this context, ultrasound transducer technology is the current limiting factor. Capacitive micromachined ultrasonic transducers (CMUTs) are micro-electro-mechanical systems (MEMS)-based devices that have been widely recognized as a valuable alternative to piezoelectric transducer technology in a variety of medical imaging applications. Wideband operation, good thermal efficiency, and low fabrication cost, especially for those applications requiring high-volume production of small-area dice, are strength factors that may justify the adoption of this MEMS technology in the medical ultrasound imaging field. This paper presents the design, development, fabrication, and characterization of a 12-MHz ultrasound probe for medical imaging, based on a CMUT array. The CMUT array is microfabricated and packed using a novel fabrication concept specifically conceived for imaging transducer arrays. The performance of the developed probe is optimized by including analog front-end reception electronics. Characterization and imaging results are used to assess the performance of CMUTs with respect to conventional piezoelectric transducers.

Pulse wave imaging of the human carotid artery: an in vivo feasibility study

Abstract: Noninvasive quantification of regional arterial stiffness, such as measurement of the pulse wave velocity (PWV), has been shown to be of high clinical importance. Pulse wave imaging (PWI) has been previously developed by our group to visualize the propagation of the pulse wave along the aorta and to estimate the regional PWV. The objective of this paper is to determine the feasibility of PWI in the human carotid artery in vivo. The left common carotid arteries of eight (n = 8) healthy volunteers (male, age 27 + 4 years old) were scanned in a long-axis view, with a 10-MHz linear-array transducer. The beam density of the scan was reduced to 16 beams within an imaging width of 38 mm. The frame rate of ultrasound imaging was therefore increased to 1127 Hz at an image depth of 25 mm. The RF ultrasound signals were then acquired at a sampling rate of 40 MHz and used to estimate the velocity of the arterial wall using a 1-D cross-correlation-based speckle tracking method. The sequence of the wall velocity images at different times depicts the propagation of the pulse wave in the carotid artery from the proximal to distal sides. The regional PWV was estimated from the spatiotemporal variation of the wall velocities and ranged from 4.0 to 5.2 m/s in eight (n = 8) normal subjects, in agreement with findings reported in the literature. PWI was thus proven feasible in the human carotid artery, and may be proven useful for detecting vascular disease through mapping the pulse wave and estimating the regional PWV in the carotid artery.

A single FPGA-based portable ultrasound imaging system for point-of-care applications

Abstract: We present a cost-effective portable ultrasound system based on a single field-programmable gate array (FPGA) for point-of-care applications. In the portable ultrasound system developed, all the ultrasound signal and image processing modules, including an effective 32-channel receive beamformer with pseudo-dynamic focusing, are embedded in an FPGA chip. For overall system control, a mobile processor running Linux at 667 MHz is used. The scan-converted ultrasound image data from the FPGA are directly transferred to the system controller via external direct memory access without a video processing unit. The potable ultrasound system developed can provide real-time B-mode imaging with a maximum frame rate of 30, and it has a battery life of approximately 1.5 h. These results indicate that the single FPGA-based portable ultrasound system developed is able to meet the processing requirements in medical ultrasound imaging while providing improved flexibility for adapting to emerging POC applications.

Parameters affecting the resolution and accuracy of 2-D quantitative shear wave images

Abstract: Time-of-flight methods allow quantitative measurement of shear wave speed (SWS) from ultrasonically tracked displacements following impulsive acoustic radiation force excitation in tissue. In heterogeneous materials, reflections at boundaries can distort the wave shape and confound determination of the wave arrival time. The magnitude of these effects depends on the shear wavelength of the excitation, the kernel size used to calculate the SWS, and the method used to determine the wave arrival time. In this study, we perform a parametric analysis of these factors using finite element modeling of the tissue response and simulated ultrasonic tracking. Two geometries are used, a stiff vertical layer and a stiff spherical inclusion, each in a uniform background. Wave arrival times are estimated using the peak displacement, peak slope of the leading edge, and cross-correlation methods. Results are evaluated in terms of reconstruction accuracy, resolution, contrast, and contrast-to-noise ratio of reconstructed SWS images. Superior results are obtained using narrower excitation widths and arrival time estimators which identify the leading edge of the propagating wave. The optimal kernel size is determined by a tradeoff between improved accuracy for larger kernels at the expense of spatial resolution.

A reconfigurable and programmable FPGA-based system for nonstandard ultrasound methods

Abstract: The availability of programmable and reconfigurable ultrasound (US) research platforms may have a considerable impact on the advancement of ultrasound systems technology; indeed, they allow novel transmission strategies or challenging processing methods to be tested and experimentally refined. In this paper, the ULtrasound Advanced Open Platform (ULA-OP), recently developed in our University laboratory, is shown to be a flexible tool that can be easily adapted to a wide range of applications. Five nonstandard working modalities are illustrated. Vector Doppler and quasi-static elastography applications emphasize the real-time potential and versatility of the system. Flow-mediated dilation, pulse compression, and high-frame-rate imaging highlight the flexibility of data access at different points in the reception chain. For each modality, the role played by the onboard programmable devices is discussed. Experimental results are reported, indicating the relative performance of the system for each application.

Low-frequency meandering piezoelectric vibration energy harvester

Abstract: The design, fabrication, and characterization of a novel low-frequency meandering piezoelectric vibration energy harvester is presented. The energy harvester is designed for sensor node applications where the node targets a width-tolength aspect ratio close to 1:1 while simultaneously achieving a low resonant frequency. The measured power output and normalized power density are 118 μW and 5.02 μW/mm3/g2, respectively, when excited by an acceleration magnitude of 0.2 g at 49.7 Hz. The energy harvester consists of a laser-machined meandering PZT bimorph. Two methods, strain-matched electrode (SME) and strain-matched polarization (SMP), are utilized to mitigate the voltage cancellation caused by having both positive and negative strains in the piezoelectric layer during operation at the meander's first resonant frequency. We have performed finite element analysis and experimentally demonstrated a prototype harvester with a footprint of 27 x 23 mm and a height of 6.5 mm including the tip mass. The device achieves a low resonant frequency while maintaining a form factor suitable for sensor node applications. The meandering design enables energy harvesters to harvest energy from vibration sources with frequencies less than 100 Hz within a compact footprint.

Scale transform signal processing for optimal ultrasonic temperature compensation

Abstract: In structural health monitoring, temperature compensation is an important step to reduce systemic errors and avoid false-positive results. Several methods have been developed to accomplish temperature compensation in guided wave systems, but these techniques are often limited in computational speed. In this paper, we present a new methodology for optimal, stretch-based temperature compensation that operates on signals in the stretch factor and scale-transform domains. Using these tools, we demonstrate three algorithms for temperature compensation that show improved computational speed relative to other optimal methods. We test the performance of these algorithms using experimental guided wave data.

A low-complexity adaptive beamformer for ultrasound imaging using structured covariance matrix

Abstract: In recent years, adaptive beamforming methods have been successfully applied to medical ultrasound imaging, resulting in simultaneous improvement in imaging resolution and contrast. These improvements have been achieved at the expense of higher computational complexity, with respect to the conventional non-adaptive delay-and-sum (DAS) beamformer, in which computational complexity is proportional to the number of elements, O(M). The computational overhead results from the covariance matrix inversion needed for computation of the adaptive weights, the complexity of which is cubic with the subarray size, O(L3). This is a computationally intensive procedure, which makes the implementation of adaptive beamformers less attractive in spite of their advantages. Considering that, in medical ultrasound applications, most of the energy is scattered from angles close to the steering angle, assuming spatial stationarity is a good approximation, allowing us to assume the Toeplitz structure for the estimated covariance matrix. Based on this idea, in this paper, we have applied the Toeplitz structure to the spatially smoothed covariance matrix by averaging the entries along all subdiagonals. Because the inverse of the resulting Toeplitz covariance matrix can be computed in O(L2) operations, this technique results in a greatly reduced computational complexity. By using simulated and experimental RF data-point targets as well as cyst phantoms-we show that the proposed low-complexity adaptive beamformer significantly outperforms the DAS and its performance is comparable to that of the minimum variance beamformer, with reduced computational complexity.

Ultrasound beam simulations in inhomogeneous tissue geometries using the hybrid angular spectrum method

Abstract: The angular spectrum method is a fast, accurate and computationally efficient method for modeling wave propagation. However, the traditional angular spectrum method assumes that the region of propagation has homogenous properties. In this paper, the angular spectrum method is extended to calculate ultrasound wave propagation in inhomogeneous tissue geometries, important for clinical efficacy, patient safety, and treatment reliability in MR-guided focused ultrasound surgery. The inhomogeneous tissue region to be modeled is segmented into voxels, each voxel having a unique speed of sound, attenuation coefficient, and density. The pressure pattern in the 3-D model is calculated by alternating between the space domain and the spatial-frequency domain for each plane of voxels in the model. The new technique was compared with the finite-difference time-domain technique for a model containing attenuation, refraction, and reflection and for a segmented human breast model; although yielding essentially the same pattern, it results in a reduction in calculation times of at least two orders of magnitude.

Array-controlled ultrasonic manipulation of particles in planar acoustic resonator

Abstract: Ultrasonic particle manipulation tools have many promising applications in life sciences, expanding on the capabilities of current manipulation technologies. In this paper, the ultrasonic manipulation of particles and cells along a microfluidic channel with a piezoelectric array is demonstrated. An array integrated into a planar multilayer resonator structure drives particles toward the pressure nodal plane along the centerline of the channel, then toward the acoustic velocity maximum centered above the subset of elements that are active. Switching the active elements along the array moves trapped particles along the microfluidic channel. A 12-element 1-D array coupled to a rectangular capillary has been modeled and fabricated for experimental testing. The device has a 300-μm-thick channel for a half-wavelength resonance near 2.5 MHz, with 500 μm element pitch. Simulation and experiment confirm the expected trapping of particles at the center of the channel and above the set of active elements. Experiments demonstrated the feasibility of controlling the position of particles along the length of the channel by switching the active array elements.

A multifunctional, reconfigurable pulse generator for high-frequency ultrasound imaging

Abstract: High-frequency (>;20 MHz) ultrasound (HFUS) imaging systems have made it possible to image small structures with fine spatial resolution. They find a variety of biomedical applications in dermatology, ophthalmology, intravascular imaging, and small-animal imaging. One critical technical challenge of HFUS is to generate high-voltage, high-frequency pulsed signals to effectively excite the transducer for a high SNR. This paper presents the development of a multifunctional, reconfigurable pulse generator for HFUS imaging. The pulse generator can produce a high-voltage unipolar pulse, a bipolar pulse, or arbitrary pulses for B-mode imaging, Doppler measurement, and modulated excitation imaging. The characteristics of the pulses, such as timing, waveform, and frequency are reconfigurable by a high-speed field-programmable gate array (FPGA). Customized software was developed to interface with the FPGA through a USB connector for pulse selection, and easy, flexible, real-time pulse management. The hardware was implemented in a compact, printed circuit board (PCB)-based scheme using state-of-the-art electronics for cost-effectiveness and fully digital control. Testing results show that the unipolar pulse can reach over 165 Vpp with a 6-dB bandwidth of 70 MHz, and the bipolar pulse and arbitrary pulses can reach 150 and 60 Vpp with central frequencies of 60 and 120 MHz, respectively.

An improved lumped element nonlinear circuit model for a circular CMUT cell

Abstract: This paper describes a correction and an extension in the previously published large signal equivalent circuit model for a circular capacitive micromachined ultrasonic transducer (CMUT) cell The force model is rederived so that the energy and power is preserved in the equivalent circuit model The model is able to predict the entire behavior of CMUT until the membrane touches the substrate Many intrinsic properties of the CMUT cell, such as the collapse condition, collapse voltage, the voltage–displacement interrelation and the force equilibrium before and after collapse voltage in the presence of external static force, are obtained as a direct consequence of the model The small signal equivalent circuit for any bias condition is obtained from the large signal model The model can be implemented in circuit simulation tools and model predictions are in excellent agreement with finite element method simulations

Phase noise in RF and microwave amplifiers

Abstract: Understanding amplifier phase noise is a critical issue in many fields of engineering and physics, such as oscillators, frequency synthesis, telecommunication, radar, and spectroscopy; in the emerging domain of microwave photonics; and in exotic fields, such as radio astronomy, particle accelerators, etc. Focusing on the two main types of base noise in amplifiers, white and flicker, the power spectral density of the random phase φ(t) is Sφ(f) = b0 + b-1/f. White phase noise results from adding white noise to the RF spectrum in the carrier region. For a given RF noise level, b0 is proportional to the reciprocal of the carrier power P0. By contrast, flicker results from a near-dc 1/f noise-present in all electronic devices-which modulates the carrier through some parametric effect in the semiconductor. Thus, b-1 is a parameter of the amplifier, constant in a wide range of P0. The consequences are the following: Connecting m equal amplifiers in parallel, b-1 is 1/m times that of one device. Cascading m equal amplifiers, b-1 is m times that of one amplifier. Recirculating the signal in an amplifier so that the gain increases by a power of m (a factor of m in decibels) as a result of positive feedback (regeneration), we find that b-1 is m2 times that of the amplifier alone. The feedforward amplifier exhibits extremely low b-1 because the carrier is ideally nulled at the input of its internal error amplifier. Starting with an extensive review of the literature, this article introduces a system-oriented model which describes the phase flickering. Several amplifier architectures (cascaded, parallel, etc.) are analyzed systematically, deriving the phase noise from the general model. There follow numerous measurements of amplifiers using different technologies, including some old samples, and in a wide frequency range (HF to microwaves), which validate the theory. In turn, theory and results provide design guidelines and give suggestions for CAD and simulation. To conclude, this article is intended as a tutorial, a review, and a systematic treatise on the subject, supported by extensive experiments.

Angiogenesis imaging by spatiotemporal analysis of ultrasound contrast agent dispersion kinetics

Abstract: The key role of angiogenesis in cancer growth has motivated extensive research with the goal of noninvasive cancer detection by blood perfusion imaging. However, the results are still limited and the diagnosis of major forms of cancer, such as prostate cancer, are currently based on systematic biopsies. The difficulty in the detection of angiogenesis partly resides in a complex relationship between angiogenesis and perfusion. This may be overcome by analysis of the dispersion kinetics of ultrasound contrast agents. Determined by multipath trajectories through the microvasculature, dispersion permits a better characterization of the microvascular architecture and, therefore, more accurate detection of angiogenesis. In this paper, a novel dispersion analysis method is proposed for prostate cancer localization. An ultrasound contrast agent bolus is injected intravenously. Spatiotemporal analysis of the concentration evolution measured at different pixels in the prostate is used to assess the local dispersion kinetics of the injected agent. In particular, based on simulations of the convective diffusion equation, the similarity between the concentration evolutions at neighbor pixels is the adopted dispersion measure. Six measurements in patients, compared with the histology, provided a receiver operating characteristic curve integral equal to 0.87. This result was superior to that obtained by the previous approaches reported in the literature.

A blind deconvolution approach to ultrasound imaging

Abstract: In this paper, a single-input multiple-output (SIMO) channel model is introduced for the deconvolution process of ultrasound imaging; the ultrasound pulse is the single system input and tissue reflectivity functions are the channel impulse responses. A sparse regularized blind deconvolution model is developed by projecting the tissue reflectivity functions onto the null space of a cross-relation matrix and projecting the ultrasound pulse onto a low-resolution space. In this way, the computational load is greatly reduced and the estimation accuracy can be improved because the proposed deconvolution model contains fewer variables. Subsequently, an alternating direction method of multipliers (ADMM) algorithm is introduced to efficiently solve the proposed blind de convolution problem. Finally, the performance of the proposed blind deconvolution method is examined using both computer simulated data and practical in vitro and in vivo data. The results show a great improvement in the quality of ultrasound images in terms of signal-to-noise ratio and spatial resolution gain.

Design and implementation of a smartphone-based portable ultrasound pulsed-wave doppler device for blood flow measurement

Abstract: Blood flow measurement using Doppler ultrasound has become a useful tool for diagnosing cardiovascular diseases and as a physiological monitor. Recently, pocket-sized ultrasound scanners have been introduced for portable diagnosis. The present paper reports the implementation of a portable ultrasound pulsed-wave (PW) Doppler flowmeter using a smartphone. A 10-MHz ultrasonic surface transducer was designed for the dynamic monitoring of blood flow velocity. The directional baseband Doppler shift signals were obtained using a portable analog circuit system. After hardware processing, the Doppler signals were fed directly to a smartphone for Doppler spectrogram analysis and display in real time. To the best of our knowledge, this is the first report of the use of this system for medical ultrasound Doppler signal processing. A Couette flow phantom, consisting of two parallel disks with a 2-mm gap, was used to evaluate and calibrate the device. Doppler spectrograms of porcine blood flow were measured using this stand-alone portable device under the pulsatile condition. Subsequently, in vivo portable system verification was performed by measuring the arterial blood flow of a rat and comparing the results with the measurement from a commercial ultrasound duplex scanner. All of the results demonstrated the potential for using a smartphone as a novel embedded system for portable medical ultrasound applications.

Positioning FBAR technology in the frequency and timing domain

Abstract: This paper focuses on the technical differentiation of film bulk acoustic resonator (FBAR) technology from other mechanical resonator technologies for timing applications. The paper will touch on a recent modification of FBARs, the zero-drift resonator (ZDR), that is temperature compensated. One technology differentiator is the size of the chip-scale packaged resonator. Another is that the silicon lid is perfectly suitable for placement of integrated circuits and this is currently being done. Many factors (wide tuning range, high Q, high frequency, small size, integrated circuitry) are being used to differentiate potential products for the time and frequency markets.

Multi-channel pre-beamformed data acquisition system for research on advanced ultrasound imaging methods

Abstract: The lack of open access to the pre-beamformed data of an ultrasound scanner has limited the research of novel imaging methods to a few privileged laboratories. To address this need, we have developed a pre-beamformed data acquisition (DAQ) system that can collect data over 128 array elements in parallel from the Ultrasonix series of research-purpose ultrasound scanners. Our DAQ system comprises three system-level blocks: 1) a connector board that interfaces with the array probe and the scanner through a probe connector port; 2) a main board that triggers DAQ and controls data transfer to a computer; and 3) four receiver boards that are each responsible for acquiring 32 channels of digitized raw data and storing them to the on-board memory. This system can acquire pre-beamformed data with 12-bit resolution when using a 40-MHz sampling rate. It houses a 16 GB RAM buffer that is sufficient to store 128 channels of pre-beamformed data for 8000 to 25000 transmit firings, depending on imaging depth; corresponding to nearly a 2-s period in typical imaging setups. Following the acquisition, the data can be transferred through a USB 2.0 link to a computer for offline processing and analysis. To evaluate the feasibility of using the DAQ system for advanced imaging research, two proof-of-concept investigations have been conducted on beamforming and plane-wave B-flow imaging. Results show that adaptive beamforming algorithms such as the minimum variance approach can generate sharper images of a wire cross-section whose diameter is equal to the imaging wavelength (150 μm in our example). Also, plane wave B-flow imaging can provide more consistent visualization of blood speckle movement given the higher temporal resolution of this imaging approach (2500 fps in our example).

Plane-wave ultrasound beamforming using a nonuniform fast fourier transform

Abstract: Beamforming of plane-wave ultrasound echo signals in the Fourier domain provides fast and accurate image reconstruction. Conventional implementations perform a k-space interpolation from the uniform sampled grid to a nonuniform acoustic dispersion grid. In this paper, we demonstrate that this step can be replaced by a nonuniform Fourier transform. We study the performance of the nonuniform fast Fourier transform (NUFFT) in terms of signal-to-noise ratio and computational cost, and show that the NUFFT offers an advantage in the trade-off between speed and accuracy, compared with other frequency-domain beamforming strategies.

Observation and cancellation of a perturbing dc stark shift in strontium optical lattice clocks

Abstract: We report on the observation of a dc Stark frequency shift at the 10-13 level by comparing two strontium optical lattice clocks. This frequency shift arises from the presence of electric charges trapped on dielectric surfaces placed under vacuum close to the atomic sample. We show that these charges can be eliminated by shining UV light on the dielectric surfaces, and characterize the residual dc Stark frequency shift on the clock transition at the 10-18 level by applying an external electric field. This study shows that the dc Stark shift can play an important role in the accuracy budget of lattice clocks, and should be duly taken into account.

Comparison of conventional parallel beamforming with plane wave and diverging wave imaging for cardiac applications: a simulation study

Abstract: When imaging the heart, good temporal resolution is beneficial for capturing the information of short-lived cardiac phases (in particular, the isovolumetric phases). To increase the frame rate, parallel beamforming is a commonly used technique for fast cardiac imaging. Conventionally, a 4 multiple-line-acquisition (4MLA) system increases the frame rate by a factor of 4, making use of a broadened transmit beam to reduce block-like artifacts. As an alternative, it has been proposed to transmit an unfocused beam (i.e., plane wave or diverging wave) for which a large number of parallel receive beams (i.e., 16) can be formed for each transmit. However, to keep the spatial resolution acceptable in these approaches, spatial compounding of overlapping successive transmits is required. As a result, the effective gain in frame rate is similar to that of a 4MLA system. To date, it remains unclear how conventional 4MLA compares to plane-wave or diverging-wave imaging when operating at similar frame rate. The goal of this study was therefore to directly contrast the performance of these beamforming methods by computer simulation. In this study, the performance of 4 different imaging systems was investigated by quantitatively evaluating the characteristics of their beam profiles. The results showed that the conventional 4MLA and plane wave imaging were very competitive imaging strategies when operating at a similar frame rate. 4MLA performed better in the near field (i.e., 10 to 50 mm), whereas plane-wave imaging had better beam profiles in the far field (i.e., 50 to 90 mm). Although diverging-wave imaging had the poorest performance in the present study, it could potentially be improved by optimizing the settings.

The variance of quantitative estimates in shear wave imaging: Theory and experiments

Abstract: In this paper, we investigate the relationship between the estimated shear modulus produced in shear wave imaging and the acquisition parameters. Using the framework of estimation theory and the Cramer-Rao lower bound applied both to the estimation of the velocity field variance and to the estimation of the shear wave travel time, we can derive the analytical formulation of the shear modulus variance σμ2 using relevant physical parameters such as the shear wave frequency, bandwidth, and ultrasonic parameters. This variance corresponds to the reproducibility of shear modulus reconstruction for a deterministic, quasi-homogeneous, and purely elastic medium. We thus consider the shear wave propagation as a deterministic process which is then corrupted during its observation by electronic noise and speckle decorrelation caused by shearing. A good correlation was found between analytical, numerical, and experimental results, which indicates that this formulation is well suited to understand the parameters' influence in those cases. The analytical formula stresses the importance of high-frequency and wideband shear waves for good estimation. Stiffer media are more difficult to assess reliably with identical acquisition signal-to-noise ratios, and a tradeoff between the reconstruction resolution of the shear modulus maps and the shear modulus variance is demonstrated. We then propose to use this formulation as a physical ground for a pixel-based quality measure that could be helpful for improving the reconstruction of real-time shear modulus maps for clinical applications.

Temperature-stable high-energy-density capacitors using complex perovskite thin films

Abstract: Chemical-solution-derived thin film synthesis and dielectric characterizations of (1 - x)BaTiO3-xBi(Mg,Ti)O3 complex perovskites with compositions x <; 0.15 have been explored for temperature-stable high-energy-density capacitor applications. Solution chemistry has been optimized to synthesize and stabilize the precursor solution. Solution-derived (1- x)BaTiO3-xBi(Mg,Ti)O3 thin film samples in thicknesses ranging from 250 to 500 nm were fabricated by repeated spinning and subsequent crystallization processes. These thin films showed nearly linear polarization response with high relative dielectric permittivity exceeding 900, which is beneficial for high-capacitance-density and high-energy-density capacitors. Average dielectric breakdown strengths of the dense films were as high as 2.08 MV/cm. The BaTiO3-Bi(Mg,Ti)O3 samples showed very low leakage current densities of the order of 10-8 A/cm2 even at temperatures of 200°C. Based on the structural stability at high temperatures of the pseudocubic perovskite, the high dielectric permittivity and the typical P-E behaviors of the BaTiO3-Bi(Mg,Ti)O3 thin films were also maintained at such high temperatures. Resulting energy density of the 500-nm-thick 0.88BaTiO3-0.12Bi(Mg,Ti)O3 thin film was as high as 37 J/cm3 at 1.9 MV/cm. The high energy density and excellent temperature stability of the BaTiO3-Bi(Mg,Ti)O3 complex perovskite show promise for use in high-temperature pulse power capacitor applications.

FPGA-based reconfigurable processor for ultrafast interlaced ultrasound and photoacoustic imaging

Abstract: In this paper, we report, to the best of our knowledge, a unique field-programmable gate array (FPGA)-based reconfigurable processor for real-time interlaced co-registered ultrasound and photoacoustic imaging and its application in imaging tumor dynamic response. The FPGA is used to control, acquire, store, delay-and-sum, and transfer the data for real-time co-registered imaging. The FPGA controls the ultrasound transmission and ultrasound and photoacoustic data acquisition process of a customized 16-channel module that contains all of the necessary analog and digital circuits. The 16-channel module is one of multiple modules plugged into a motherboard; their beamformed outputs are made available for a digital signal processor (DSP) to access using an external memory interface (EMIF). The FPGA performs a key role through ultrafast reconfiguration and adaptation of its structure to allow real-time switching between the two imaging modes, including transmission control, laser synchronization, internal memory structure, beamforming, and EMIF structure and memory size. It performs another role by parallel accessing of internal memories and multi-thread processing to reduce the transfer of data and the processing load on the DSP. Furthermore, because the laser will be pulsing even during ultrasound pulse-echo acquisition, the FPGA ensures that the laser pulses are far enough from the pulse-echo acquisitions by appropriate time-division multiplexing (TDM). A co-registered ultrasound and photoacoustic imaging system consisting of four FPGA modules (64-channels) is constructed, and its performance is demonstrated using phantom targets and in vivo mouse tumor models.

Sound-speed image reconstruction in sparse-aperture 3-D ultrasound transmission tomography

Abstract: The paper is focused on sound-speed image reconstruction in 3-D ultrasound transmission tomography. Along with ultrasound reflectivity and the attenuation coefficient, sound speed is an important parameter which is related to the type and pathological state of the imaged tissue. This is important in the intended application, breast cancer diagnosis. In contrast to 2-D ultrasound transmission tomography systems, a 3-D system can provide an isotropic spatial resolution in the x-, y-, and z-directions in reconstructed 3-D images of ultrasound parameters. Several challenges must, however, be addressed for 3-D systems-namely, a sparse transducer distribution, low signal-to-noise ratio, and higher computational complexity. These issues are addressed in terms of sound-speed image reconstruction, using edge-preserving regularized algebraic reconstruction in combination with synthetic aperture focusing. The critical points of the implementation are also discussed, because they are crucial to enable a complete 3-D image reconstruction. The methods were tested on a synthetic data set and on data sets measured with the Karlsruhe 3-D ultrasound computer tomography (USCT) I prototype using phantoms. The sound-speed estimates in the reconstructed volumes agreed with the reference values. The breast-phantom outlines and the lesion-mimicking objects were also detectable in the resulting sound-speed volumes.

Investigations on AlN/sapphire piezoelectric bilayer structure for high-temperature SAW applications

Abstract: This paper explores the possibility of using AlN/sapphire piezoelectric bilayer structures for high-temperature SAW applications. To determine the temperature stability of AlN, homemade AlN/sapphire samples are annealed in air atmosphere for 2 to 20 h at temperatures from 700 to 1000°C. Ex situ X-ray diffraction measurements reveal that the microstructure of the thin film is not affected by temperatures below 1000°C. Ellipsometry and secondary ion mass spectroscopy investigations attest that AlN/sapphire is reliable up to 700°C. Beyond this temperature, both methods indicate ongoing surface oxidation of AlN. Additionally, Pt/Ta and Al interdigital transducers are patterned on the surface of the AlN film. The resulting SAW devices are characterized up to 500°C and 300°C, respectively, showing reliable frequency response and a large, quasi-constant temperature sensitivity, with a first-order temperature coefficient of frequency around -75 ppm/°C. Between room temperature and 300°C, both electromechanical coupling coefficient K2 and propagation losses increase, so the evolution of delay lines' insertion losses with temperature strongly depends on the length of the propagation path.

Characterizing a fiber-based frequency comb with electro-optic modulator

Abstract: We report on the characterization of a commercial core fiber-based frequency comb equipped with an intracavity free-space electro-optic modulator (EOM). We investigate the relationship between the noise of the pump diode and the laser relative intensity noise (RIN) and demonstrate the use of a low-noise current supply to substantially reduce the laser RIN. By measuring several critical transfer functions, we evaluate the potential of the EOM for comb repetition rate stabilization. We also evaluate the coupling to other relevant parameters of the comb. From these measurements, we infer the capabilities of the femtosecond laser comb to generate very-low-phase-noise microwave signals when phase-locked to a high-spectral-purity ultra-stable laser.

Ultrasound array transmitter architecture with high timing resolution using embedded phase-locked loops

Abstract: Coarse time quantization of delay profiles within ultrasound array systems can produce undesirable side lobes in the radiated beam profile. The severity of these side lobes is dependent upon the magnitude of phase quantization error? the deviation from ideal delay profiles to the achievable quantized case. This paper describes a method to improve interchannel delay accuracy without increasing system clock frequency by utilizing embedded phase-locked loop (PLL) components within commercial field-programmable gate arrays (FPGAs). Precise delays are achieved by shifting the relative phases of embedded PLL output clocks in 208-ps steps. The described architecture can achieve the necessary interelement timing resolution required for driving ultrasound arrays up to 50 MHz. The applicability of the proposed method at higher frequencies is demonstrated by extrapolating experimental results obtained using a 5-MHz array transducer. Results indicate an increase in transmit dynamic range (TDR) when using accurate delay profiles generated by the embedded-PLL method described, as opposed to using delay profiles quantized to the system clock.