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Wireless Sensor Networks (WSNs) are being used in various applications such as structural health monitoring and industrial control. Since energy efficiency is one of the major design factors, the existing WSNs primarily rely on low-power, low-rate wireless technologies such as 802.15.4 and Bluetooth. In this paper, by proposing , we strive to tackle the challenges of developing ultra-high-rate WSNs based on the 802.11 (WiFi) standard. As an illustrative structural health monitoring application, we consider spacecraft vibration test and identify system design requirements and challenges. Our main contributions are as follows. First, we propose packet encoding methods to reduce the overhead of assigning accurate timestamps to samples. Second, we propose energy efficiency methods to enhance the system’s lifetime. Third, to enhance sampling rate and mitigate sampling rate instability, we reduce the overhead of processing outgoing packets through the network stack. Fourth, we study and reduce the delay of processing time synchronization packets through the network stack. Fifth, we propose a low-power node design particularly targeting vibration monitoring. Sixth, we use our node design to empirically evaluate energy efficiency, sampling rate, and data rate. We leave large-scale evaluations as future work.

High-voltage and fast-switching silicon carbide (SiC) power modules are needed to construct high-voltage and high power-density converters. Stacking multiple low-voltage SiC devices to increase the equivalent blocking voltage shows advantages in on -state resistance, current capacity, and cost over one single high-voltage device. An indirect series-connected SiC MOSFET power module using quasi-two-level hybrid-clamp topology is proposed in this article. The voltages across the devices are automatically balanced with an open-loop modulation strategy to avoid sensors and control algorithm. Moreover, only ceramic capacitors are needed besides the MOSFETs and diodes in the topology. Thanks to this, the topology was highly integrated into a power module using SiC dies, which is equivalent to a general-purpose two-level medium-voltage SiC MOSFET power module. The switching losses of the power module show advantages over other stacking methods and even a single high-voltage switch and it can be evaluated with a two-level half-bridge (HB). To achieve this, the automatic voltage-balancing performance is analyzed in detail and the parasitic output capacitance in the topology is investigated, based on which the design considerations are presented. The simulation results from LTspice confirm the voltage-balancing and a 3600V/20A HB power module is built in the lab and the experimental results verify the design.

Multi-sensor fusion plays a key role in 2D laser-based robot location and navigation. Although it has achieved great success, there are still some challenges, e.g., being fragile when having a large angular rotation. In this paper, we present a deep learning-based approach to localizing a mobile robot using a 2D laser and an inertial measurement unit. A novel recurrent convolutional neural network (RCNN)-based architecture is developed to fuse laser and inertial data for scan-to-scan pose estimation. A scan-to-submap optimization is also introduced to optimize the poses estimated by the RCNN for enhanced robustness and accuracy. Extensive experiments have been conducted in both simulation and practice with a real mobile robot, verifying the effectiveness of the proposed deep sensor fusion system.

The real-time monitoring of the grid-connected renewable energy generations is of great significance to the operation and control of active distribution systems (ADS). Considering the detailed photovoltaic (PV) system model and ambient influencing factors, this paper develops a state estimation framework for the ADS with integrated PV power plants. Firstly, suitable models of PV arrays and power converters for steady-state state estimation are developed. Based on these models, the state variables of a PV generation system are selected, and the measurement equations are derived. Finally, combined with the state estimation model of distribution networks, a framework for state estimation and bad data processing of an ADS with PV power plants is presented. A parameter estimation approach for PV power plants based on multiple measurement scans is also developed in order to address the practical situation of unknown model parameters. Simulation results show that the proposed framework can effectively capture the operating state of the distribution network and PV power plants. In addition, it enhances the estimation accuracy and bad data processing capability compared to conventional state estimation frameworks.

With the development of renewable energy generation, medium-voltage dc collection in large-scale photovoltaic (PV) plants has become a research focus with less losses and less system complexity. Maximum power point tracking control of PV panels requires converters with a large input voltage range and a stable output, among which active-clamp isolated boost converters qualify for PV applications, due to their high and variable step-up ratio. In this paper, an improved dual-active-clamp quasi-resonant isolated boost converter is proposed to increase the efficiency and decrease the voltage pressure across switches. The proposed topology has an additional resonant branch with a small resonant capacitor on top of the conventional active-clamp isolated boost converters, where the capacitor resonates with the leakage inductor of the transformer to improve the voltage and current waveforms during switching transients. The proposed converter achieves soft switching of all the switches at both turn-on and turn-off transients, while retaining good voltage regulation ability. Additionally, the induced voltage spikes due to the parasitic inductors in the conventional topology are suppressed well. Discussions on detailed steady-state operation, zero-voltage switching (ZVS) regions, and design considerations are given to show the advantages of the proposed converter. Simulation and experimental results are presented to verify the expected performances.

We propose and demonstrate Mach-Zehnder interferometer (MZI), which is the refractive index (RI) insensitive and temperature highly sensitive based on etched multi-core fiber (MCF) structure. The MCF and Fiber Bragg Grating (FBG) are used as hybrid sensing elements. The fabrication of the interferometer is provided a new taper-like structure by etching the MCF to further expose the side cores to the surroundings. The interferometer has produced a sensitivity of 103.2pm/°C within the ambient temperature up-to 70°C. Moreover, the superior temperature sensitivity is 89.19pm/°C, 66.64pm/°C, 56.42pm/°C in the range of 24°C to 130°C, and RI-insensitive in the range of 1.34 to 1.38, for different waists of etched seven-core fiber interferometers (E7CFIs) $\sim ~84.70\mu \text{m}$ , $93.10\mu \text{m}$ , $108.67\mu \text{m}$ , respectively. Compared with the conventional FBGs, the sensitivity of the interferometer is significantly improved by 8 times. E7CFI’s novel and advantageous features can easily be distinguished other devices. Besides, the proposed sensing architecture is compact, easy to fabricate, highly sensitive, easy to reproduce, and makes it an inexpensive fiber optic device.

Calibration of an analog-to-digital converter is an essential step to compensate for static errors and ensure accurate digital output. In addition, ad-hoc deployments and operations require fault-tolerant IoT devices capable of adapting to unpredictable environments. In this paper, we present the design and implementation of Excalibur – a low-cost, accurate, and scalable calibration tool. Excalibur is a programmable platform, which provides linear current output and rational function voltage output with a dynamic range. The basic idea is to use a set of digital switches to connect with a parallel resistor network and program the digital switches to change the total resistance of the circuit. The total resistance and output frequency of Excalibur is controlled by a program communicating through the GPIO and I2C interfaces. The software provides two salient features to improve accuracy and reliability: time synchronization and self-calibration. Furthermore, Excalibur is equipped with a temperature sensor to measure the temperature before calibration, and a current sensor which enables current calibration without using a digital multimeter. We present the mathematical model and a solution to compensate for thermal and wire resistance effects and validate scalability by incorporating the concept of the Fibonacci sequence. Our extensive experimental studies show that Excalibur can significantly improve measurement accuracy. For example, for ATMega2560, the ADC error reduces from 0.2% to 0.01%, for ADS8353, the error reduces from 0.17% to 0.0014%, for INA219, the error reduces from 0.42% to 0.02%, and for MCP3208, the error reduces from 5.29% to 0.01%.

A simple, high-accuracy and non-destructive method for the measurement of diaphragm thickness and microgap width based on modulated tri-beam interference is demonstrated. With this method, a theoretical estimation error less than 0.5% for a diaphragm thickness of ~1 μm is achievable. Several fiber-tip air bubbles with different diaphragm thicknesses (6.25, 5.0, 2.5 and 1.25 μm) were fabricated to verify our proposed measurement method. Furthermore, an improved technique was introduced by immersing the measured object into a liquid environment to simplify a four-beam interference into tri-beam one. By applying this improved technique, the diaphragm thickness of a fabricated in-fiber rectangular air bubble is measured to be about 1.47 μm, and the averaged microgap width of a standard silica capillary is measured to be about 10.07 μm, giving a corresponding measurement error only 1.27% compared with actual scanning electron microscope (SEM) results.

This paper presents system-level efficiency evaluation of the suitable isolated dc/dc converters in input-series-output-parallel power electronics transformers (PET) applied for medium-voltage dc systems and provides engineering insights into the design of PETs from a system perspective. To this end, three candidates-a single-stage dual active bridge (DAB) converter, a boost converter plus a DAB with a constant boost output voltage, and a boost converter plus a DAB with a constant DAB voltage gain-are selected and evaluated from a system-level aspect. Comprehensive system-level efficiency comparisons are done with the chosen candidates, considering numbers of normal and hot standby submodules, different working conditions, rated voltages and power of the PET, switching frequencies, and commercial devices. Analysis with detailed loss breakdowns clearly shows the distribution of losses and elaborates the profound reasons behind the efficiency differences. In the comparison, the boost and DAB with a constant boost output voltage have higher or similar efficiency than the other two candidates, verified by simulation and experimental results. Considering its advantages in redundancy in the PET, the boost and DAB with constant boost output voltage are a preferable choice for submodules in input-series-output-parallel PETs.

Synthetic aperture radar imaging requires precise position information provided by position and orientation system (POS), especially sensitive to high-order errors of position information, whereas high-order errors of global positioning system (GPS) are normally ignored in POS solution procedure. For high-precision POS, high-order errors become a significant error source with decisive effects on the accuracy of POS. In this paper, we propose a GPS high-order position errors suppression and cancellation method, which includes the traditional GPS/inertia navigation system (INS) integrated method used as the high-frequency filter, and the low-frequency filter is based on least squares support vector machine. The high- and low-frequency dual filters are constructed to suppress and eliminate high-order error. In verifying our dual-rate hybrid filtering method, POS flight experiments were conducted in November and October up to December in 2015, with results showing that the high-order position accuracy of high-precision POS (0.01°/h gyro drift) is better than 0.006 m, and the proposed method has a better performance compared with other methods.

This letter extends the constrained band selection (CBS) technique to constrained band subset selection (CBSS) in a similar manner that constrained energy minimization has been extended to linearly constrained minimum variance. CBSS constrains multiple bands as a band subset as opposed to CBS constraining a single band as a singleton set. To achieve this goal, CBSS requires a strategy to search for an optimal band subset, while CBS does not. In this letter, two new sequential algorithms, referred to as sequential CBSS and successive CBSS, which do not exist in CBS are derived for CBSS to find desired band subsets and to avoid exhaustive search.

With advances in the movie industry, composite interactions and complex visual effects require us to shoot at the designed part of a scene for immersion. Traditionally, the director of photography plans a camera path by recursively reviewing and commenting path-planning rendered results. Since the adjust–render–review process is not immediate and interactive, miscommunications happen to make the process ineffective and time consuming. Therefore, this paper proposes a What-You-See-What-You-Get camera path reviewing system for the director to interactively instruct and design camera paths. Our system consists of a camera handle, a parameter control board, and a camera tracking box with mutually perpendicular marker planes. When manipulating the handle, the attached camera captures markers on visible planes with selected parameters to adjust the world rendering view. The director can directly examine results to give immediate comments and feedbacks on transformation and parameter adjustment in order to achieve effective communication and reduce the reviewing time. Finally, we conduct a set of qualitative and quantitative evaluations to show that our system is robust and efficient and can provide means to give interactive and immediate instructions for effective communication and efficiency enhancement during path design.

We investigate the electrical and optical performance of surface-plasmon-mediated light emission in electrical-driven metal-insulator-metal (MIM) tunnel junctions fabricated from a gold (Au) film on the top and a heavily doped silicon chip on the bottom. A silicon dioxide layer is used as the tunneling barrier. The experimental results show that the device's performance strongly depends on the morphology of the Au film. A negative resistance effect was observed with high Au film roughness, from which higher efficient light emission was observed compared to that of our device exhibiting less roughness. Such MIM tunneling junctions are compatible with common metal-oxide semiconductor technology and thus open up a route toward the development of novel integrated optoelectronic and plasmonic devices.

Endmember finding has become increasingly important in hyperspectral data exploitation because endmembers can be used to specify unknown particular spectral classes. Pixel purity index (PPI) and N-finder algorithm (N-FINDR) are probably the two most widely used techniques for this purpose where many currently available endmember finding algorithms are indeed derived from these two algorithms and can be considered as their variants. Among them are three well-known algorithms derived from imposing different abundance constraints, that is, abundance-unconstrained automatic target generation process (ATGP), abundance nonnegativity constrained vertex component analysis (VCA), and fully abundance constrained simplex growing algorithm (SGA). This paper explores relationships among these three algorithms and further shows that theoretically they are essentially the same algorithms in the sense of design rationale. The reason that these three algorithms perform differently is not because they are different algorithms, but rather because they use different preprocessing steps, such as initial conditions and dimensionality reduction transforms.

Graphics processing unit (GPU)-based software beamforming has advantages over hardware-based beamforming of easier programmability and a faster design cycle, since complicated imaging algorithms can be efficiently programmed and modified. However, the need for a high data rate when transferring ultrasound radio-frequency (RF) data from the hardware front end to the software back end limits the real-time performance. Data compression methods can be applied to the hardware front end to mitigate the data transfer issue. Nevertheless, most decompression processes cannot be performed efficiently on a GPU, thus becoming another bottleneck of the real-time imaging. Moreover, lossless (or nearly lossless) compression is desirable to avoid image quality degradation. In a previous study, we proposed a real-time lossless compression-decompression algorithm and demonstrated that it can reduce the overall processing time because the reduction in data transfer time is greater than the computation time required for compression/decompression. This paper analyzes the lossless compression method in order to understand the factors limiting the compression efficiency. Based on the analytical results, a nearly lossless compression is proposed to further enhance the compression efficiency. The proposed method comprises a transformation coding method involving modified lossless compression that aims at suppressing amplitude data. The simulation results indicate that the compression ratio (CR) of the proposed approach can be enhanced from nearly 1.8 to 2.5, thus allowing a higher data acquisition rate at the front end. The spatial and contrast resolutions with and without compression were almost identical, and the process of decompressing the data of a single frame on a GPU took only several milliseconds. Moreover, the proposed method has been implemented in a 64-channel system that we built in-house to demonstrate the feasibility of the proposed algorithm in a real system. It was found that channel data from a 64-channel system can be transferred using the standard USB 3.0 interface in most practical imaging applications.

Recursive band processing of orthogonal subspace projection (RBP-OSP) is developed according to the band sequential (BSQ) format acquired by a hyperspectral imaging sensor. It can be implemented band by band recursively without waiting for data being completely collected. This is particularly important for satellite communication when data download is limited by bandwidth and transmission. Unlike band selection which requires prior knowledge of how many bands are needed to be selected, RBP-OSP has capability which allows different process units to process data whenever bands are available. In addition, it also enables users to identify significant bands during data processing. Finally and most importantly, RBP can provide progressive profiles on OSP performance, which is the best advantage that RBP-OSP can offer and cannot be accomplished by any one-shot operator.

Finding a small set of representatives from an unlabeled dataset is a core problem in a broad range of applications such as dataset summarization and information extraction. Classical exemplar selection methods such as $k$-medoids work under the assumption that the data points are close to a few cluster centroids, and cannot handle the case where data lie close to a union of subspaces. This paper proposes a new exemplar selection model that searches for a subset that best reconstructs all data points as measured by the $\ell_1$ norm of the representation coefficients. Geometrically, this subset best covers all the data points as measured by the Minkowski functional of the subset. To solve our model efficiently, we introduce a farthest first search algorithm that iteratively selects the worst represented point as an exemplar. When the dataset is drawn from a union of independent subspaces, our method is able to select sufficiently many representatives from each subspace. We further develop an exemplar based subspace clustering method that is robust to imbalanced data and efficient for large scale data. Moreover, we show that a classifier trained on the selected exemplars (when they are labeled) can correctly classify the rest of the data points., Comment: In submission; conference version at ECCV'2018

Synthetic transmit aperture (STA) has been widely investigated in ultrasound system recently with characteristics of high frame rate and low hardware cost. Since the high-resolution image (HRI) of STA is formed by summation of low-resolution images (LRIs), it is susceptible to inter-firing motions. In this paper, we propose a low-complexity global motion compensation algorithm. We use the common region of interest (ROIcom) between different transmissions of STA imaging to beamform backward and forward beam vectors. Then, the magnitude and direction of motion can be evaluated by cross-correlations between specific beam vectors in STA imaging. Compared with the uncompensated image in two-dimentional (2D) motion environment, the proposed motion compensation algorithm can improve the contrast ratio (CR) and contrast-to-noise ratio (CNR) by 13.73 and 2.04 dB, respectively. Also, the proposed algorithm improves the CR and CNR about 7.84 and 1.36 dB comparing with the reference work, respectively. In the Field II breath model, the proposed method also improves the CR and CNR about 6.65 and 1.04 dB than the reference method, respectively. Moreover, we propose a low-complexity delay generator in the architecture design to further reduce the computational complexity of the whole beamforming system. Finally, we verify the proposed low-complexity motion compensation beamforming engine by using the VLSI implementation with CMOS 90 nm technology. In the post-layout result, the core size is 2.39 mm2 at 125 MHz operating frequency and the frame rate of the beamforming system is 42.23 frames per second.

An ultrasonic power/data telemetry and a neural stimulator are presented. An on-off keying with pulse modulation is adopted for ultrasonic signal transmission. The ultrasonic power is used to charge a battery. The ultrasonic data are used for neural stimulation, and an animal study is also demonstrated. The range of the stimulation current is 0-640 μA. The frequency of the stimulation current pulse is from 60 to 265 Hz. This ultrasonic power/data telemetry with a neural stimulator is fabricated in a 0.35- μm CMOS technology. It occupies the area of 4.4 mm2. The total power dissipation is 2 mW with a stimulation current of 140 μA and a stimulation frequency of 265 Hz.

The use of acoustic radiation force has been suggested for enhancing the delivery of therapeutic substances, whereas sonothrombolysis has been developed for years as treatment by itself, or in combination with thrombolytic agents or ultrasound contrast agents. We have examined the efficacy of using acoustic radiation force to enhance the targeting of microbubbles in cavitation-induced sonothrombolysis in a flow phantom system. A clot was targeted by microbubbles using avidin-biotin binding, and the process was observed using a confocal microscope. We found that the experimental group in which radiation force was combined with cavitation showed an additional 3% to 9% weight reduction of the thrombus relative to the cavitation group. We also found that the fluorescence intensity of the clot increased with the microbubble concentration at each acoustic setting. Microbubbles traveled 10 to 20 μm further than the control group after being exposed to radiation force, cavitation, or both. These observations confirm that radiation force helps microbubbles to distribute into a clot (as does cavitation). Therefore, combining radiation force with cavitation would provide additional thrombolysis effects (based on clot weight measurements) relative to cavitation alone. A local delivery method based on acoustic radiation force has the potential to improve the safety and efficacy of sonothrombolysis.

Ultrasound nonlinear contrast imaging using microbubble-based contrast agents has been widely investigated. However, the degree of contrast enhancement is often limited by overlap between the spectra of the tissue and microbubble nonlinear responses, which makes it difficult to separate them. The use of ensemble empirical mode decomposition (EEMD) in the Hilbert-Huang transform (HHT) was previously explored with the aim of alleviating this problem. The HHT is designed for analyzing nonlinear and nonstationary data, whereas EEMD is a method associated with the HHT that allows decomposition of data into a finite number of intrinsic mode functions (IMFs). It was found that the contrast can be effectively improved in certain IMFs, but manual selection of appropriate IMFs is still required. This prompted the present study to test the hypothesis that the contrast can be enhanced without requiring manual selection by summing appropriately weighted IMFs and demodulating the signal at appropriate frequencies. That is, a data-driven mechanism for determining weights and demodulation frequencies was derived and tested. Phantom results show that an overall contrast enhancement of up to 12.5 dB can be achieved. A fused-image representation that simultaneously displays the conventional B-mode image and the new contrast-mode image is also presented.

Intravascular photoacoustic (IVPA) imaging is a technique for visualizing atherosclerotic plaques with differential composition. Unlike conventional photoacoustic tomography scanning, where the scanning device rotates around the subject, the scanning aperture in IVPA imaging is enclosed within the imaged object. The display of the intravascular structure is typically obtained by converting detected photoacoustic waves into Cartesian coordinates, which can produce images with severe artifacts. Because the acquired data are highly limited, there does not exist a stable reconstruction algorithm for such imaging geometry. The purpose of this work was to apply image reconstruction concepts to explore the feasibility and efficacy of image reconstruction algorithms in IVPA imaging using traditional analytical formulas, such as a filtered back-projection (FBP) and the lambda-tomography method. Although the closed-form formulas are not exact for the IVPA system, a general picture of and interface information about objects are provided. To improve the quality of the reconstructed image, the iterative expectation maximization and penalized least-squares methods were adopted to minimize the difference between the measured signals and those generated by a reconstructed image. In this work, we considered both the ideal point detector and the acoustic transducers with finite- size aperture. The transducer effects including the spatial response of aperture and acoustoelectrical impulse responses were incorporated in the system matrix to reduce the aroused distortion in the IVPA reconstruction. Computer simulations and experiments were carried out to validate the methods. The applicability and the limitation of the reconstruction method were also discussed.

Multidimensional (MD) discrete Fourier transform (DFT) is a key kernel algorithm in many signal processing applications. In this paper we describe an MD-DFT intellectual property (IP) generator and a bandwidth-efficient MD DFT IP for high performance implementations of 2-D and 3-D DFT on field-programmable gate array (FPGA) platforms. The proposed architecture is generated automatically and is based on a decomposition algorithm that takes into account FPGA resources and the characteristics of off-chip memory access, namely, the burst access pattern of the synchronous dynamic RAM (SDRAM). The IP generator has been integrated into an in-house FPGA development platform, AlgoFLEX, for easy verification and fast integration. The corresponding 2-D and 3-D DFT architectures have been ported onto the BEE3 board and their performance measured and analyzed. The results shows that the architecture can maintain the maximum memory bandwidth throughout the whole procedure while avoiding matrix transpose operations used in most other MD DFT implementations. To further enhance the performance, the proposed architecture is being ported onto the newly released Xilinx ML605 board. The simulation results show that 2 K × 2 K images with complex 64-bit precision can be processed in less than 27 ms.

We report here an excellent field emission performance from periodic patterned graphene nanomesh (GNM) field emitter. A simulation of the relationships between external electric field and the local electric field enhancement on the GNM edges was conducted to optimize the parameters of the nanostructure. Experimental results revealed that the gate driving voltage was as low as 20 V. This kind of emitters also shows good field emission stability. It is expected that the GNM introduced here will generate impacts for the further advancement of graphene field emission devices in emerging technologies and applications.

We report a planar triode with metal-gate-free electrode based on carbon nanotubes. Excellent field emission properties with 1.4 mA/cm2 at the gate voltage of 32 V are obtained. A turn-off phenomenon is observed when the gate voltage decreases to 0 V. The field emission at 1.2 mA/cm2 is recorded for 20 h with the fluctuation of the emission current . The results would be of significance to the development of field emission triode structure in the future.

We report the fabrication of GaN-based light-emitting diodes (LEDs) with ytterbium-doped alumina-zinc-oxide (AZO:Y) upper contact. It was found that AZO and AZO:Y are both highly transparent in the visible region with good thermal stability optically. However, it was found that AZO:Y is much more thermally stable electrically, as compared with AZO. Furthermore, it was found that the output power of GaN LEDs with AZO upper contact decreased significantly from 2.80 to 2.30 mW after 700°C annealing. With the same annealing condition, it was found that output power decreased only slightly from 2.77 to 2.69 mW for the LEDs with AZO:Y upper contact.

This paper describes the development of a miniature capacitive micromachined ultrasonic transducer (CMUT) array suitable for minimally invasive medical imaging and diagnosis. In contrast to conventional laboratory-scale CMUT platforms, which are generally integrated on a silicon substrate thicker than 550 mum, this imager array is integrated on a probe shaped silicon substrate with a typical shank dimension of 60 mum(width) times 40 mum(thickness) times 4-10 mm(length) for 1-D arrays, and 0.4-2.3 mm(width) times 100 mum(thickness) times 6-12 mm(length) for 2-D arrays. Such miniature CMUT arrays are suitable for implantation into tissue through a fine incision or by being placed inside an organ for close-range imaging. In a close-range diagnosis made possible by using such miniature CMUT arrays, ultrasound of a higher frequency can be used and the conflict associated with the penetration depth and image resolution can be resolved. This imager array was fabricated using a two-layer polysilicon surface micromachining process followed by a double-sided deep silicon etching for substrate shaping. The total mask count was eight. The central frequency of ultrasound transmitted by a circular 46 mum-diameter transducer was 3.8 MHz, while its fractional bandwidth was 116% in water. A simple transducer-fluid model was used to predict the acoustic characteristics of this device in water. Preliminary B-mode imaging using a 21-element 1-D array was demonstrated.

This paper presents the first monolithic multidirection-looking ultrasonic imager for minimally invasive medical diagnosis. In contrast to the traditional planar ultrasonic imagers that can only view in one direction, this 3-D array is able to view in multiple directions by using seven planar imagers integrated on a hexagonal silicon prism. Each facet on the prism is integrated with a planar 1- or 2-D capacitive-micromachined ultrasonic-transducer imager array for viewing in a specific direction. Each facet is connected by a flexible dielectric membrane, which is monolithically fabricated with the transducers. The dielectric membranes also support the thin-film electrical interconnects between the arrays on different facets. The substrate is folded into a hexagonal prism after completion of the transducer microfabrication process. With this architecture, a one flip-chip bonded or monolithically integrated front-end electronic circuit will be able to manage all the imagers on the 3-D array. The number of bonding wires for a connection to external electronics can therefore be reduced. Imager prisms, which are ranging from 1 to 4 mm in diameter and 2 to 4 mm in length, and positioned to view in seven directions, have been prototyped. Preliminary testing shows that the imager transducers behaved consistently before and after the assembly process. Applications of this 3-D imager array include capsule ultrasound endoscope, intravascular ultrasound, and other internal imaging needs.

Photoacoustic (PA) imaging has been used mainly for anatomical and functional imaging. Although functionalized nanoparticles also have been developed for PA molecular imaging, only single targeting has been demonstrated. In this study, PA imaging of multiple targets using gold nanorods is demonstrated experimentally using HER2 and CXCR4 as target molecules. The two corresponding monoclonal antibodies were conjugated to two types of gold nanorod with different aspect ratios. Gold nanorods with mean aspect ratios of 5.9 and 3.7 exhibited peak optical absorptions at 1000 and 785 nm, respectively. Appropriate selection of laser irradiation wavelength enhances PA signals by 7-12 dB and allows signals from gold nanorods corresponding to specific bindings to be distinguished. This approach potentially allows the expression levels of different oncogenes of cancer cells to be revealed simultaneously.

This paper presents a new coded excitation scheme that efficiently synthesizes codes for arbitrary waveforms using a bipolar square wave pulser. In a coded excitation system, pulse compression is performed to restore the axial resolution. In order to maintain low range sidelobes, the system needs to transmit signals that have smooth spectra. However, such a transmitter requires the generation of arbitrary waveforms and, therefore, is more expensive. In other words, a trade-off is necessary between the compression performance and the transmitter cost. Here we propose a method that preserves the low-cost advantage of a bipolar pulser while achieving approximately the same compression performance as an arbitrary waveform generator. The key idea of the proposed method is the conversion of a nonbinary code (i.e., requiring an arbitrary waveform generator) with good compression performance into a binary code (i.e., requiring only a bipolar pulser) by code translation and code tuning. The code translation is implemented by sending the nonbinary code into a virtual one-bit, sigma-delta modulator, and the code tuning involves minimizing the root-mean-square error between the resultant binary code and the original nonbinary code by sequential and iterative tuning while taking the transducer response into account. Tukey-windowed chirps are known to have good compression performance. Such chirps of different durations (16, 20. and 24 /spl mu/s), all with a taper ratio of 0.15, a center frequency of 2.5 MHz, and an equivalent bandwidth of 1.5 MHz, were converted into binary Tukey-windowed chirps that were compared with pseudochirps (i.e., direct binary approximations of the original chirp) over the same spectral band. The bit rate was 40 MHz. Simulation results show that the use of binary Tukey-windowed chirps can reduce the code duration by 20.6% or the peak sidelobe level by 6 dB compared to the commonly used pseudochirps. Experimental results obtained under the same settings were in agreement with the simulations. Our results demonstrate that arbitrary waveform coded excitation can be realized using bipolar square wave pulsers for applications in medical ultrasound.

An approach for reconstructing the sound velocity distribution in the breast was previously proposed and verified by simulations, and the present study investigated the approach experimentally. The experimental setup comprised a 5-MHz, 128-channel linear array, a programmable digital array system, a phantom containing objects with differing physical properties, and a computer. The array system was used to collect channel data for simultaneous B-mode image formation and limited-angle tomographic sound velocity reconstruction. The phantom was constructed from materials mimicking the following tissues in the breast: glandular tissue, fat, cysts, high-attenuation tumors, and irregular tumors. The sound velocities in these materials matched those in the corresponding real tissues. The imaging setup is similar to that of x-ray mammography, in which a linear array is placed at the top of the breast and a metal plate is placed at the bottom for reflecting sound waves. Thus, both B-mode images and the sound velocity distribution can be acquired using the same setup. An algorithm based on a convex programming formulation was used to reconstruct the sound velocity images. By scanning the phantom at different positions, nine cases were evaluated. In each of the nine cases, the image object comprised a background (glandular tissue) and one or three regions of interest (fat, tumor, or cyst). The sound velocity was accurately estimated in the nine cases evaluated, with sound velocity errors being less than 5 m/s in 8 of 11 regions of interest. Thus, obtaining the sound velocity distribution is feasible with a B-mode imaging setup using linear arrays. Knowledge of the sound velocity distribution in the breast can be used to complement B-mode imaging and to enhance the detection of breast cancer.

An approach based on limited-angle transmission tomography for reconstruction of the sound velocity distribution in the breast is proposed. The imaging setup is similar to that of x-ray mammography. With this setup, the time-of-flight data are acquired by a linear array positioned at the top of the compressed breast that both transmits and receives, and a metal plate is placed at the bottom as a reflector. The setup allows acoustic data acquisition for simultaneous B-mode image formation and the tomographic sound velocity reconstruction. In order to improve the sound velocity estimation accuracy, a new reconstruction algorithm based on a convex programming formulation has been developed. Extensive simulations for both imaging and time-of-flight data based on a 5-MHz linear array were performed on tissues with different geometries and acoustic parameters. Results show that the sound velocity error was generally 1-3 m/s, with a maximum of 5.8 m/s. The radii of the objects under investigation varied from 2 to 6 mm, and all of them were detected successfully. Thus, the proposed approach has been shown to be both feasible and accurate. The approach can be used to complement conventional B-mode imaging to further enhance the detection of breast cancer.

Pulse-inversion-based fundamental imaging was experimentally investigated for the enhancement of contrast detection. The pulse-inversion technique involves two firings with inverted waveforms. When the returning echoes from the two firings are summed, the residue signal is limited to even-order harmonics for tissue. However, when the returning echoes are from microbubbles, the fundamental signal is not completely cancelled because the reaction of the bubbles under compression is different from that under rarefaction. Thus, with the application of pulse-inversion technique, the fundamental signal can be used to enhance the contrast-to-tissue ratio. In this paper, B-mode, pulse-inversion-based fundamental images were constructed with various transmit waveforms. Motion artifacts also were studied. The results indicate that the contrast-to-tissue ratio was significantly enhanced compared to that obtained using either conventional, fundamental imaging or second-harmonic imaging. Longer transmit pulses resulted in a better signal-to-noise ratio, but did not noticeably affect the nonlinear response of the bubbles. In addition, the optimal ratio of the magnitude of the positive pulse to that of the negative pulse was unity, in terms of avoiding the uncancelled, third-order response in the fundamental frequency range. It also was found that the pulse-inversion fundamental technique is highly sensitive to tissue motion because the fundamental tissue signal is not cancelled when motion is present.

With the proliferation of handheld devices, information access on mobile devices is a topic of growing relevance. This paper presents a system that allows the user to search for information on mobile devices using spoken natural-language queries. We explore several issues related to the creation of this system, which combines state-of-the-art speech-recognition and information-retrieval technologies. This is the first work that we are aware of which evaluates spoken query based information retrieval on a commonly available and well researched text database, the Chinese news corpus used in the National Institute of Standards and Technology (NIST)s TREC-5 and TREC-6 benchmarks. To compare spoken-query retrieval performance for different relevant scenarios and recognition accuracies, the benchmark queries-read verbatim by 20 speakers-were recorded simultaneously through three channels: headset microphone, PDA microphone, and cellular phone. Our results show that for mobile devices with high-quality microphones, spoken-query retrieval based on existing technologies yields retrieval precisions that come close to that for perfect text input (mean average precision 0.459 and 0.489, respectively, on TREC-6).

Motion artifacts of the pulse inversion technique were studied for finite amplitude distortion-based harmonic imaging. Motion in both the axial and the lateral directions was considered. Two performance issues were investigated. One is the harmonic signal intensity relative to the fundamental intensity and the other is the potential image quality degradation resulting from spectral leakage. A one-dimensional (1-D) correlation-based correction scheme also was used to compensate for motion artifacts. Results indicated that the tissue harmonic signal is significantly affected by tissue motion. For axial motion, the tissue harmonic intensity decreases much more rapidly than with lateral motion. The fundamental signal increases for both axial and lateral motion. Thus, filtering is still required to remove the fundamental signal, even if the pulse inversion technique is applied. The motion also potentially decreases contrast resolution because of the uncancelled spectral leakage. Also, it was indicated that 1-D motion correction is not adequate if nonaxial motion is present.

Quantitative estimation of blood velocity using Doppler techniques is fundamentally limited because only the axial component can be detected. Speckle decorrelation resulting from scatterer motion may be used to compute non-axial components and to obtain quantitative flow information. Based on both simulations and experimental results, it is shown that the decorrelation technique is feasible only for constant flows. If flow gradients are present, the correlation between two signals along the same line of observation may be significantly affected by the gradients. Therefore, the decorrelation method cannot be used for quantitative flow estimation if flow gradients are not accurately measured and effects on signal correlation are not fully compensated. Results in this paper show that accurate estimation of flow gradients is practically difficult. It is further shown that effects of signal-to-noise ratio (SNR) on the correlation must also be taken into account for quantitative flow analysis.

A new method for aberration correction is presented. Using multiple receive beams reconstructed at each transmit direction, image artifacts due to imperfect beam forming can be identified and greatly reduced. Previous studies have demonstrated the efficacy of such a compensation algorithm for blocked elements. In this study, the algorithm is extended for magnitude and phase aberrations. Experimental results show that substantial improvements are possible for small amplitude and phase errors. The compensation method is less effective for large errors, however. Based on this, a two-step procedure is proposed for general aberration correction. First, cross-correlation processing is used to reduce significant phase errors. Then, the adaptive compensation method is applied to remove remaining artifacts due to blocked elements, amplitude aberrations, and residual phase errors. This two-step procedure is shown to be effective in significantly reducing image artifacts due to amplitude and phase errors.

We have developed a method for designing sparse periodic arrays. Grating lobes in the two-way radiation pattern are avoided by using different element spacings on transmission and reception. The transmit and receive aperture functions are selected such that the convolution of the aperture functions produces a desired effective aperture. A desired effective aperture is simply an aperture with an appropriate width, element spacing, and shape such that the Fourier transform of this function gives the desired two-way radiation pattern. If a synthetic aperture approach is used, an exact solution to the problem is possible. However, for conventional imaging, often only an approximation of the desired effective aperture can be found. Different strategies for obtaining these approximate solutions are described. The radiation pattern of a sparse array designed using the effective aperture concept is compared experimentally with the radiation patterns of a dense array, and sparse arrays with periodic and random element spacing. We show that the number of elements in a 128-element linear array can be reduced by at least four times with little degradation of the beam forming properties of the array.

Multi-element synthetic aperture imaging methods suitable for applications with severe cost and size limitations are explored. Array apertures are synthesized using an active multi-element receive subaperture and a multi-element transmit subaperture defocused to emulate a single-element spatial response with high acoustic power. Echo signals are recorded independently by individual elements of the receive subaperture. Each method uses different spatial frequencies and acquisition strategies for imaging, and therefore different sets of active transmit/receive element combinations. Following acquisition, image points are reconstructed using the complete data set with full dynamic focus on both transmit and receive. Various factors affecting image quality have been evaluated and compared to conventional imagers through measurements with a 3.5 MHz, 128-element transducer array on different gel phantoms. Multielement synthetic aperture methods achieve higher electronic signal to noise ratio and better contrast resolution than conventional synthetic aperture techniques, approaching conventional phased array performance. >

Two-dimensional conformal arrays are proposed to enhance low contrast lesion detection deep in the body. The arrays conform to the body maintaining good contact over a large area. To provide full three-dimensional focusing for two-dimensional imaging, such arrays are densely sampled in the scan direction (x) and coarsely sampled in the nonscan direction (y), i.e., the arrays are anisotropic. To illustrate reduction in slice thickness with increased array length in y, a two-dimensional array is synthesized using a one-dimensional, 128 element array with a 3.5 MHz center frequency. A mask is attached confining transmission and reception of acoustic waves to 2 mm in y. Using a mechanical scan system, the one-dimensional array is moved along y covering a 28.16 mm/spl times/20.0 mm aperture. Accordingly, the synthetic array has 128 elements in x and 10 elements in y. To correct for geometric irregularities due to array movement, a gelatin based phantom containing three-dimensional point targets is used for phase aberration correction. Results show that elevational beam quality is degraded if small geometric errors are not removed. Emulated conformality at the body surface and phase aberrations induced by spatial inhomogeneities in tissue are further imposed and shown to produce severe beam-forming artifacts. Two-dimensional phase aberration correction is applied and results indicate that the method is adequate to compensate for large phase excursions across the entire array. To fully realize the potential of large, two-dimensional, conformal arrays, proper two-dimensional phase aberration correction methods are necessary. >

In clinical applications using large apertures, a significant number of phased array elements may be blocked due to discontinuous acoustic windows into the body. These blocked elements produce undesired beamforming artifacts, degrading spatial and contrast resolution. To minimize these artifacts, an algorithm using multiple receive beams and the total-least-squares method is proposed. Simulations and experimental results show that this algorithm can effectively reduce imperfections in the point spread function of the imager. Combined with first-and second-order scatterer statistics derived from multiple receive beams, the algorithm is modified for blocked element compensation on distributed scattering sources. Results also indicate that compensated images are comparable to full array images, and that even full array images can be improved by removing undesired sidelobe contributions. This method, therefore, can enhance detection of low contrast lesions using large phased-array apertures. >

A new scheme has been developed to measure the plasma density directly with the use of a short antenna driven by a sweeping frequency RF source and biased by a variable negative dc voltage.