A circularly focused and scanned acoustic scanner

This paper describes a new type of electronically scanned and focused acoustic imaging device called the cylindrical grating acoustic scanner. With this device, a new transducer called the circular-edge-bonded transducer (CEBT) is devised that is bonded on one end of a cylindrical substrate and can generate a surface acoustic wave propagating along the cylindrical surface of the substrate. An array of grooves is fabricated on the cylindrical surface of the substrate to serve as a scattering grating. The periodical grooves scatter a chirped surface-wave pulse coherently into a focused bulk-wave "ring" that scans at the surface-wave velocity. The focal length and resolution can be adjusted by changing the chirp rate and time-bandwidth product of the chirp, respectively. A 2.3-MHz circular scanner with 25-cm field of view and 2.6-mm resolution has been constructed and studied. Acoustic image of an artificial defect in an aluminum pipe is obtained.     

三维前视声呐信号处理方法

三维前视声像声呐是安装在小型水下载体上的重要声学探测设备。提出了基于波达方向估计技术的三维前视声呐信号处理方法,接收阵水平方向采用波束形成技术,垂直方向采用波达方向估计技术。在波束内采用分裂孔径相位法提高水平方向分辨率,形成声呐阵前方的三维声像。仿真结果和水池实验结果表明:在接收阵面面积相同的条件下,能够获得优于常规技术的分辨率,可以实现点目标和连续目标的探测,适合于在水下载体上安装使用。     

Orthogonal quadratic chirp signals for simultaneous multi-zone focusing in medical ultrasound imaging

In medical ultrasound imaging, multi-zone focusing on transmission is used to enhance the lateral resolution at the expense of frame rate. As an alternative, this paper proposes a simultaneous multi-zone focusing method using orthogonal quadratic chirp signals to improve lateral resolution without sacrificing frame rate. In the proposed method, two weighted quadratic chirp signals with different spectra are simultaneously transmitted with different transmit time delays for multi-zone focusing. Because the two weighted quadratic chirps can be designed to have a desired level of cross-correlation after compression, the degradation of axial resolution resulting from the division of a spectrum is minimized. Through simulation, the performances of the proposed method were evaluated and compared with those of two-cycle pulsed excitation as a gold standard and two sub-band weighted linear chirps. In the simulation, the proposed method improved -6-dB and -20-dB lateral beam widths by factors of 1.67 and 1.84, respectively, compared with the pulsed excitation. The degradation of axial resolution in the proposed method was maximally 43% less than that in the linear chirp case. The results demonstrate that the proposed method is useful in the improvement of overall ultrasound image quality because the axial resolution of conventional ultrasound images is generally a few times higher than the lateral resolution.     

Characterization of a 1/spl times/32 element metal-semiconductor-metal optoelectronic mixer array for FM/cw LADAR

We characterize a 1/spl times/32 element metal-semiconductor-metal photodetector (MSM-PD) array utilized for optoelectronic mixing in an incoherent, amplitude-modulated laser detection and ranging (LADAR) system. The MSM-PDs that make up the one-dimensional array internally detect and down-convert light signals that are amplitude modulated at ultrahigh frequency (UHF). Range information is contained in the low-frequency mixing product derived by mixing a reference UHF chirp with a detected, time-delayed UHF chirp. When utilized in the LADAR system, the MSM-PDs eliminate the need for wideband transimpedance amplifiers in the LADAR receiver. This, in turn, reduces both the cost and complexity of the system. The breadboard LADAR architecture and components are described, and fundamental measurements and imagery taken from the LADAR, using these unique MSM-PDs, are also presented.     

SAW device implementation of a weighted stepped chirp code signal for direct sequence spread spectrum communications systems

This paper introduces a new weighted stepped chirp code signal for direct sequence spread spectrum (DS/SS) communications systems. This code signal uses the truncated cosine series functions as the chip functions, and it is the result of discretizing a continuous wave (CW) chirp that results in enhanced performance versus a pseudonoise (PN) code and equivalent performance and easier implementation than a CW chirp. This code signal will be shown to have improved compression ratio (CR) and peak sidelobe level (PSL) versus a PN code with identical code length and chip length. It also will be shown to have a similar CR and PSL compared to a CW chirp with identical pulse length and frequency deviation. The code signal is implemented on surface acoustic wave (SAW) devices that will be used as the code generator at the transmitter and the correlator at the receiver. The design considerations for the SAW device implementation of the code signal are discussed, including the effects of intersymbol interference. Experimental data is presented and compared to the predicted results for 8 different SAW devices examining the effects of code length (9 or 13 chips), weighting (uniform, cosine-squared, and Hamming), and sampling on the performance of the code signal.     

Design and characterization of a real-time angular scatter ultrasound imaging system

A novel ultrasound imaging system has been implemented using two 32-element linear phased array transducers oriented at an angle of 40 degrees to one another. The system simultaneously acquires and displays, in real time, a conventional backscatter image and an angular scatter image formed using side-scattered echoes from the same region. The design of the system is shaped by the influence of the scatter angle on the spatial resolution and receive signal processing requirements of the instrument. The subtended scatter angles are restricted to values >90 degrees to ensure that the angular scatter receiver effectively tracks the transmitted pulse and that the spatial resolution in the two images is comparable. The system is sufficiently tolerant of small variations in the average acoustic velocity of the medium to guarantee reliable pulse tracking in biomedical applications. The angular scatter signal magnitude is significantly weighted by the directivity of the receive array. The imaging system will most effectively demonstrate angular variations in scattering at scatter angles between 125 and 145 degrees , where the angular response of the receiver is near its peak.     

Electric time-domain reflectometry distributed flow sensor

Liquid composite molding (LCM) has become an important processing technique to manufacture high-performance composite parts. The sensing of the process parameters, such as resin fill of the porous material, are key to improve repeatability, maximize quality and minimize cost. This paper describes a distributed flow sensor, which considerably decreases tooling integration costs and improves spatial resolution by allowing sensing of hundreds of sensing elements with a single input/output port. The transmission line sensor is virtually divided into a large number of small discrete transmission lines treated as a long array of sensing elements. Piecewise sensing is achieved by electric time-domain reflectometry and inversion of a non-uniform transmission line model. The paper describes the distributed sensing approach, experimentally validates distributed sensing in a LCM setup, and analyzes critical sensor parameters.     

On an approach for improving the range resolution of pulsed coherent Doppler lidars

As a result of an analysis of the autocovariance of the complex heterodyne lidar signal, some general-enough inverse techniques (algorithms) are derived for recovering with high range resolution, below the sensing pulse length, of Doppler-velocity profiles in the atmosphere. Unlike our preceding works, it is assumed here that the laser pulses can have arbitrary fluctuating shape. The presence is also supposed of possible regular, arbitrary in form, intrapulse frequency deviations (chirp) and random frequency, phase and radial (Doppler)-velocity fluctuations. The algorithm performance and efficiency are studied and illustrated by computer simulations, taking into account the influence of the chirp and various random factors such as additive noise, pulse-shape fluctuations and radial-velocity fluctuations. It is shown that the algorithms developed allow the Doppler-velocity profiles to be determined with a considerably shorter resolution interval compared with the pulse length, at a reasonable number of signal realizations (laser shots) and appropriate data processing to reduce the statistical error due to the random factors.     

Automatic Yarn Characterization System: Design of a Prototype

This paper presents Yarn System Quality (YSQ), an innovative, low-cost, portable and high-precision yarn evaluation tester, for quality control of yarn characteristics under laboratory conditions. It presents a modular architecture, simultaneously integrating measurements of yarn hairiness, mass, regularity and diameter. An external module to obtain the yarn production characteristics using analogue optics and image processing is also available. The quantification of yarn hairiness and diameter variation (with a sampling resolution length of 1 mm) is carried out using photodiodes; the diameter characterization, based on 0.5 mm width samples, employs a linear photodiode array; the measurements of mass variation, based on samples of 1 mm, uses a parallel plate capacitive sensor. In the YSQ measurement parameters based on optical sensors a coherent signal processing technique with Fourier analysis is used, to obtain linear output signal variations. A comparison between the results obtained using the YSQ tester and a commercial solution is also presented.      

LEAST-SQUARES SOLUTIONS OF A GENERAL NUMERICAL METHOD FOR ARBITRARY IRREGULAR GRIDS

This article describes a computational method to calculate solutions of elliptic boundary value problems using arbitrary irregular grids. The main feature of the numerical method is its ability to approximate solutions of differential equations without co-ordinate mapping or metric tensor information. For linear differential equations, the numerical method yields coupled linear systems on local computational cells. Optimal least-squares solutions of coupled linear systems are discussed and applied to the scheme. The numerical method is used to simulate potential flow for two model problems. The computational results are in very good agreement with analytical solutions. © 1997 by John Wiley & Sons, Ltd.     

HDTV transmission formats and migration path

The current proposal for terrestrial broadcasting of HDTV in the United States incorporates a flexible approach toward encoding and transmission. Specifically, the format used to encode an HDTV picture will not be fixed. Instead, a variety of transmission formats will be available, unlike the current (NTSC) system. Furthermore, although a basic set of formats will exist initially, it is possible that this set can change over time, depending on the evolution of relevant technologies. This article will consider various topics related to the current and future transmission formats associated with HDTV systems in the United States. This article discusses the usefulness of allowing multiple transmission formats and describes the formats which will be immediately available for HDTV broadcasting, as agreed upon by the Grand Alliance. The relationship between source formats, transmission formats, and display formats are discussed, with reference to some of the signal processing modules required to convert between formats. Finally, the migration of HDTV to incorporate additional formats is addressed. This report describes a method of achieving this migration in a backward-compatible manner, so that the basic HDTV receivers will not become obsolete.©1994 John Wiley & Sons Inc     

Elliptical-Tukey chirp signal for high-resolution, air-coupled ultrasonic imaging

A new signal processing method, which uses a modified chirp signal for air-coupled ultrasonic imaging, is described. A combination of the elliptical and Tukey window functions has been shown to give a better performance than the Hanning windowing used in most pulse-compression algorithms for air-coupled applications. The elliptical-Tukey chirp signal provides an improvement in both the resolution of images and signal-to-noise ratios. In addition, this type of signal also reduces the level of signal voltages required to drive the source transducer while maintaining the performance of the system. This approach, thus, may have wide interest in all forms of wide bandwidth ultrasonic imaging.     

基于重叠互相关器的合成孔径水池实验研究

在过去十几年里,为了增加拖曳线列阵系统的空间增益、提高方位分辨率,各种合成孔径技术应运而生。针对AUV舷侧阵系统水下目标远程探测的研究需要,给出了基于重叠互相关器的合成孔径处理算法(OCSAP),这种方法是在假设AUV旁侧阵匀速直线航行前提下,通过在波束域利用FFT变换合成孔径,并且在连续的时间间隔内对子孔径信号进行相关处理来实现的。在对该算法进行计算机仿真研究的基础上,在消声水池中进行了8阵元合成48阵元以及单阵元合成8阵元的逆合成孔径实验研究,两种实验结果均验证了OCSAP算法的有效性和可行性。实验结果表明合成孔径处理与常规物理孔径处理相比具有较好的鲁棒性,并且在接收信号时域相关长度大于合成孔径所需时间的水下或海洋环境里,合成阵增益与等长的物理阵增益基本相等。     

High frequency piezoelectric MEMS ultrasound transducers

High-frequency ultrasound array transducers using piezoelectric thin films on larger structures are being developed for high-resolution imaging systems. The increase in resolution is achieved by a simultaneous increase in operating frequency (30 MHz to about 1 GHz) and close coupling of the electronic circuitry. Two different processing methods were explored to fabricate array transducers. In one implementation, a xylophone bar transducer was prototyped, using thin film PbZr(0.52)Ti(0.48)O(3) (PZT) as the active piezoelectric layer. In the other, the piezoelectric transducer was prepared by mist deposition of PZT films over electroplated Ni posts. Because the PZT films are excited through the film thickness, the drive voltages of these transducers are low, and close coupling of the electronic circuitry is possible. A complementary metal-oxidesemiconductor (CMOS) transceiver chip for a 16-element array was fabricated in 0.35-microm process technology. The ultrasound front-end chip contains beam-forming electronics, receiver circuitry, and analog-to-digital converters with 3-Kbyte on-chip buffer memory.     

High frequency ultrasound imaging with optical arrays

Dynamically focused and steered high frequency ultrasound imaging systems require arrays with fine element spacing, wide bandwidths, and large apertures. However, these characteristics are difficult to achieve at frequencies greater than 30 MHz using conventional array construction methods. Optical schemes offer a solution. Focused laser beams incident on a suitable surface can generate and detect acoustic radiation. Precisely controlling the position and size of the beams defines points of transmission and detection, making it possible for pulse-echo image formation by synthetic aperture methods. An optical detection array was built, relying on a conventional piezoelectric transducer as an ultrasound source. The detection system, with near optimal resolution over a wide depth of field, demonstrates the potential for high frequency array implementation using optical techniques. A possible application is in pathology, where 2-D or 3-D fine resolution pulse-echo imaging can be performed in situ without the need for biopsies.     

A system for ultrasonic beacon-guidance of catheters and other minimally-invasive medical devices

Catheters and other interventional medical devices are presently guided by X-ray imaging, despite the advantages of ultrasound imaging over X-ray imaging in cost, safety, and availability. X-ray imaging is used because ultrasound reflects specularly from catheters and similar devices; their visibility is highly angle-dependent. With an omni-directional receiver mounted on a device, the receiver's location in the ultrasound image can be deduced from knowing which acoustic ray struck the receiver and the time from transmission of the imaging pulse to its reception by the receiver. This information is independent of specular reflection. The location of the device can then be indicated in the ultrasound image by an arrow pointing to the sensor, making possible ultrasound guidance of these devices. This paper describes the technical and practical considerations in the design and construction of the device-mounted receiver and associated electronics, and describes some clinical uses.     

A system for ultrasonic beacon-guidance of catheters and other minimally-invasive medical devices

Catheters and other interventional medical devices are presently guided by X-ray imaging, despite the advantages of ultrasound imaging over X-ray imaging in cost, safety, and availability. X-ray imaging is used because ultrasound reflects specularly from catheters and similar devices; their visibility is highly angle-dependent. With an omni-directional receiver mounted on a device, the receiver's location in the ultrasound image can be deduced from knowing which acoustic ray struck the receiver and the time from transmission of the imaging pulse to its reception by the receiver. This information is independent of specular reflection. The location of the device can then be indicated in the ultrasound image by an arrow pointing to the sensor, making possible ultrasound guidance of these devices. This paper describes the technical and practical considerations in the design and construction of the device-mounted receiver and associated electronics, and describes some clinical uses.     

Frequency division transmission imaging and synthetic aperture reconstruction

In synthetic transmit aperture imaging only a few transducer elements are used in every transmission, which limits the signal-to-noise ratio (SNR). The penetration depth can be increased by using all transmitters in every transmission. In this paper, a method for exciting all transmitters in every transmission and separating them at the receiver is proposed. The coding is done by designing narrow-band linearly frequency modulated signals, which are approximately disjointed in the frequency domain and assigning one waveform to each transmitter. By designing a filterbank consisting of the matched filters corresponding to the excitation waveforms, the different transmitters can be decoded at the receiver. The matched filter of a specific waveform will allow information only from this waveform to pass through, thereby separating it from the other waveforms. This means that all transmitters can be used in every transmission, and the information from the different transmitters can be separated instantaneously. Compared to traditional synthetic transmit aperture (STA) imaging, in which the different transmitters are excited sequentially, more energy is transmitted in every transmission, and a better signal-to-noise-ratio is attained. The method has been tested in simulation, in which the resolution and contrast was compared to a standard synthetic transmit aperture system with a single sinusoid excitation. The resolution and contrast was comparable for the two systems. The method also has been tested using the experimental ultrasound scanner RASMUS. The resolution was evaluated using a string phantom. The method was compared to a conventional STA using both sinusoidal excitation and linear frequency modulated (FM) signals as excitation. The system using the FM signals and the frequency division approach yielded the same performance concerning both axial (of approximately equal to 3 wavelengths) and lateral resolution (of approximately equal to 4.5 wavelengths). A SNR measurement showed an increase in SNR of 6.5 dB compared to the system using the conventional STA method and FM signal excitation.     

Real-time volume imaging using a crossed electrode array

This paper describes a unique crossed electrode array for real-time volume ultrasound imaging. By placing orthogonal linear array electrode patterns on the opposite sides of a hemispherically shaped composite transducer substrate, a 2D array can be fabricated using a small fraction of the elements required for a traditional 2D array. The performance of the array is investigated using a computer simulation of the radiation pattern. We show that by using a 288-element crossed electrode pattern it is possible to collect large field of view volume images (60deg times 60degsector) at real-time frame rates (>20 volume images/s), with image contrast and resolution comparable to what can be obtained using a conventional 128-element linear phased array.     

Harmonic chirp imaging method for ultrasound contrast agent

Coded excitation is currently used in medical ultrasound to increase signal-to-noise ratio (SNR) and penetration depth. We propose a chirp excitation method for contrast agents using the second harmonic component of the response. This method is based on a compression filter that selectively compresses and extracts the second harmonic component from the received echo signal. Simulations have shown a clear increase in response for chirp excitation over pulse excitation with the same peak amplitude. This was confirmed by two-dimensional (2-D) optical observations of bubble response with a fast framing camera. To evaluate the harmonic compression method, we applied it to simulated bubble echoes, to measured propagation harmonics, and to B-mode scans of a flow phantom and compared it to regular pulse excitation imaging. An increase of approximately 10 dB in SNR was found for chirp excitation. The compression method was found to perform well in terms of resolution. Axial resolution was in all cases within 10% of the axial resolution from pulse excitation. Range side-lobe levels were 30 dB below the main lobe for the simulated bubble echoes and measured propagation harmonics. However, side-lobes were visible in the B-mode contrast images.