Sound field calculations for an ultrasonic linear phased array with a spherical liquid lens

Lenses are often used to provide focusing in the elevation dimension of ultrasonic linear phased-array transducers. The use of a liquid lens in this application adds a variable geometric focusing capability, determined by the radius of curvature of the lens surface and speed of sound in the liquid, to the electronic focusing produced by the linear phased array. An efficient method to calculate the sound field radiated from the linear phased-array transducer through the liquid lens is presented. It treats the lens surface as a secondary source distribution according to Huygens's principle, and employs a modified form of the rectangular radiator method to calculate the field. The appropriate phases for the array elements to focus and steer the beam are calculated by considering the refraction on the lens surface. Comparisons of computer simulations and experimental measurements of the field intensity distribution of a prototype linear array transducer with a liquid lens demonstrate the accuracy of the proposed method.     

Design and optimization of an OPFC ultrasonic linear phased array transducer

In contrast to piezoceramics, orthotropic piezoelectric fibre composites (OPFC) which can be used as actuator/sensor elements in damage detection show clear advantages because of their high sensitivity along the fibre direction and directivity. The focusing acoustic field distribution of the OPFC phased array transducer is analyzed by the finite element method together with directivity analysis in metallic materials. The optimal array parameters such as spacing, width and number of elements are obtained by studying the total displacement changes as various parameters change at the focus point. The feasibility of an OPFC ultrasonic phased array transducer for damage detection is validated.     

Generation of sinc wave by a one-dimensional array for applicationsin ultrasonic imaging

A new solution to the 2-D scalar wave equation is presented which describes an ultrasonic beam maintaining the lateral field response expressed by the sinc function over a finite depth of field. This new beam is realizable with a linear array transducer, and less subject to diffraction spreading than conventional focused beams, physically, it is a superposition of plane waves having the same wavelength, but traveling at different angles. It is shown by numerical simulation that the beam can provide more uniform lateral beamwidth and smoother on-axis field magnitude over a greater depth of field than the rectangular transducers and Gaussian apodized transmitters which have been used to increase the limited depth of field of conventional focused beams. Compared with currently developed limited diffraction beams which must be generated by 2-D array transducers, the beam has a wider lateral beamwidth but with lower sidelobe levels. In ultrasonic medical imaging, the beam enables one to obtain a line focus using a 1-D array transducer and to eliminate the diffraction correction required in some applications such as tissue characterization     

Quantum theory of easy plane antiferromagnets in a magnetic field

The spin excitations of easy plane antiferromagnets with Dzyaloshinskii interactions in an external magnetic field of arbitrary direction are considered using the solution of the self-consistent quantum problem. The one-ion magnetization in the magnetic field is shown to be independent of the field only when S = 1/2; but when S = 1 the quantum equation for the spin configurations differs from the quasiclassical one. A theory of the interaction of different spin excitation branches is given without limitations on the parameters involved in the problem.     

Speckle reduction in pulse-echo ultrasonic imaging using a two-dimensional receiving array

An experimental pulse-echo imager was developed for the purpose of reducing speckle in ultrasonic images. The system utilized a 64-element spherically focused segmented annuli array receiver with a common transmitter. Compounded images were formed using subapertures of varying size, shape, and overlap, and the speckle and resolution characteristics of the final images were observed. A pointlike scatterer was imaged to determine the resolution, point spread function, and sensitivity of the system along with a new measure called the resolution cell size. The response of the system was also simulated for comparisons. It was found that lateral resolution, and resolution cell sizes only gradually increased with a decrease in subaperture size and system sensitivity was not greatly diminished. Incoherent summation of signals from small groups of elements decreased the speckle noise by a factor of four while maintaining enough resolution to improve the image quality as measured by the CSR/d by a factor of almost two.     

Minimum variance ultrasonic imaging applied to an in situ sparse guided wave array

Ultrasonic guided wave imaging with a sparse, or spatially distributed, array can detect and localize damage over large areas. Conventional delay-and-sum images from such an array typically have a relatively high noise floor, however, and contain artifacts that often cannot be discriminated from damage. Considered here is minimum variance distortionless response (MVDR) imaging, which is a variation of delay-and-sum imaging whereby weighting coefficients are adaptively computed at each pixel location. Utilization of MVDR significantly improves image quality compared with delay-and-sum imaging, and additional improvements are obtained from incorporation of a priori scattering information in the MVDR method, use of phase information, and instantaneous windowing. Simulated data from a through-hole scatterer are used to illustrate performance improvements, and a performance metric is proposed that allows for quantitative comparisons of images from a known scatterer. Experimental results from a through-hole scatterer are also provided that illustrate imaging efficacy.     

Inducing and imaging thermal strain using a single ultrasound linear array

For the first time, the feasibility of inducing and imaging thermal strain using an ultrasound imaging array is demonstrated. A commercial ultrasound scanner was used to heat and image a gelatin phantom with a cylindrical rubber inclusion. The inclusion was successfully characterized as an oil-bearing material using thermal strain imaging.     

Demonstration of a linear optical true-time delay device by use of a microelectromechanical mirror array

We present the design and proof-of-concept demonstration of an optical device capable of producing true-time delay(s) (TTD)(s) for phased array antennas. This TTD device uses a free-space approach consisting of a single microelectromechanical systems (MEMS) mirror array in a multiple reflection spherical mirror configuration based on the White cell. Divergence is avoided by periodic refocusing by the mirrors. By using the MEMS mirror to switch between paths of different lengths, time delays are generated. Six different delays in 1-ns increments were demonstrated by using the Texas Instruments Digital Micromirror Device as the switching element. Losses of 1.6 to 5.2 dB per bounce and crosstalk of -27 dB were also measured, both resulting primarily from diffraction from holes in each pixel and the inter-pixel gaps of the MEMS.     

A parametric study of ultrasonic beam profiles for a linear phased array transducer

A numerical simulation model is presented to investigate the influences of design parameters of linear phased array transducers on beam focusing and steering performance. The characteristic of ultrasonic beam profiles has been simulated on the basis of the Huygen's superposition principle. For the simulation, a linear phased array is considered as the composition of finite number of elements separated by equidistance. Individual elements are considered as two-dimensional point sources. The waves generated from piezoelectric elements are considered as simplified transient ultrasonic waves that are constructed with the cosine function enveloped with a Hanning window. The characteristic of ultrasonic wave propagation into a medium from the phased array transducer is described. The effects of the number, the interelement spacing, steering angle, the focal length, and frequency bandwidth of the piezoelectric elements on beam directivity and ultrasonic pressure field in a linear phased array transducer are systematically discussed.     

Real-time terahertz scanning imaging by use of a pyroelectric array camera and image denoising

A high-resolution large-area terahertz (THz) scanning imaging system is demonstrated based on a 124×124 pyroelectric array camera and a CO(2) pumped continuous-wave THz laser. By applying a scanning mechanism to the real-time imaging setup, images of large-area targets were accomplished. Self-made resolution charts were employed to test the resolution. In order to improve the image quality, the noise in the images was studied and modeled, and then the performance of several denoising methods was compared with real-time THz original images. The experimental results show that, with the help of anisotropic diffusion, noise can be effectively suppressed, and the results are visually pleasant even when there is great attenuation. Those results greatly confirm application potentials of THz imaging using pyroelectric cameras in the field of concealed object detection.     

Least-squares estimation of imaging parameters for an ultrasonic array using known geometric image features

Ultrasonic array images are adversely affected by errors in the assumed or measured imaging parameters. For non-destructive testing and evaluation, this can result in reduced defect detection and characterization performance. In this paper, an autofocus algorithm is presented for estimating and correcting imaging parameter errors using the collected echo data and a priori knowledge of the image geometry. Focusing is achieved by isolating a known geometric feature in the collected data and then performing a weighted leastsquares minimization of the errors between the data and a feature model, with respect to the unknown parameters. The autofocus algorithm is described for the estimation of element positions in a flexible array coupled to a specimen with an unknown surface profile. Experimental results are shown using a prototype flexible array and it is demonstrated that (for an isolated feature and a well-prescribed feature model) the algorithm is capable of generating autofocused images that are comparable in quality to benchmark images generated using accurately known imaging parameters.     

Modulation transfer function measurement of an infrared focal plane array by use of the self-imaging property of a canted periodic target

We present a new technique for measuring the modulation transfer function (MTF) of a focal plane array (FPA). The main idea is to project a periodic pattern of thin lines that are canted with respect to the sensor's columns. Practically, one aims the projection by using the self-imaging property of a periodic target. The technique, called the canted periodic target test, has been validated experimentally on a specific infrared FPA, leading to MTF evaluation to as great as five times the Nyquist frequency.     

超声相控阵全聚焦成像算法比较分析

先进的成像算法推动了超声相控阵技术的发展,全聚焦方法(Total Focusing Method,TFM)是一种基于全矩阵捕获的虚拟聚焦后处理及缺陷图像重构算法.文章基于一维线阵相控阵纵波探头全矩阵数据模式,利用Matlab软件结合Field Ⅱ自带开源函数包编写了算法程序,比较了TFM和1/2矩阵方法成像效果并对缺陷...     

Analysis of the ultrasonic field radiated by time-delay cylindrically focused linear arrays

The ultrasonic field at the scanning central plane of a linear array whose driving delays are quadratically varied for cylindrical focusing is studied. An efficient and accurate algorithm based on the classical time domain convolution-impulse response methodology is applied for calculations. The method takes into account the full size of the array elements and does not use any paraxial or far-field approximations. The cases of linear and sector scanning are studied. Patterns of array impulse responses h(A)(x ,t) at selected field zones, including the main propagation axis and the focal line, are described for different steering conditions. In order to facilitate the analysis of h(A)(.) curves, a set of temporal and spatial parameters are defined. In particular, the concepts of "elements" times of arrival vectors" and "virtual aperture" are shown to be helpful in achieving a simple interpretation of array fields. Plots of pressure under wide-band and continuous-wave radiations are also presented and discussed. In particular, several discrepancies with classical approaches are mentioned, as well as other aspects of the focused held such as the location of the point of maximum amplitude, and the lateral deterioration and asymmetries caused by steering.     

TiCxNy and TiCx-TiN films obtained by CVD in an ultrasonic field

TiC _x N _y mono- and TiC_x-TiN double-layer films with a thickness of 30 to 100 m were prepared on a carbon steel (C: 0.6 to 0.7%) substrate by CVD in an ultrasonic field (ultrasound frequency: 19kHz; power: 10 to 20Wcm^–2). The moderate deposition conditions for obtaining an adherent and thick film of TiC _x N _y were: substrate temperature: 1050° C; H₂, N₂, TiCl₄ and CH₄ flow rates: 6.2, 4.0, 0.9 and 0.26 to 2.0 ml sec^–1, respectively. The growth rate, grain size and degree of 2 2 0 preferred orientation were found to decrease with increase in CH₄ concentration. TiC _x N _y film on carbon steel had a Vickers microhardness of 1800 to 2600 and an adhesion strength to the substrate of more than 120 kg cm^–2. A TiC _x -TiN (x0.5) double-layer film was obtained at 1050° C by a controlled alternative deposition of TiC _x or TiN. Quasiepitaxial growth of crystallites in the double layers was found to prevail in both coatings of TiC _x (220)/TiN (220)/steel and TiN (200)/TiC_x (200)/steel.     

An ultrasonic Gaussian transducer and its diffraction field: theory and practice

In this paper, the authors present a novel way to describe the diffraction field for a Gaussian source, which becomes a Gaussian itself. It is described by the Rayleigh surface integral based on the Huygens' theorem. The derivation does not require the parabolic approximation used by previous authors. An improved spherical button Gaussian transducer design also is presented to verify the theory. A theoretical principle of this design based on electromagnetic theory is developed. Both megahertz and kilohertz experimental results show that the sound fields generated by Gaussian transducers of this design agree very well with the theoretical predictions.     

Shear stress induced by a gas bubble pulsating in an ultrasonic field near a wall

Some of the effects that therapeutic ultrasound has in medicine and biology may be associated with steady oscillations of gas bubbles in liquid, very close to tissue surface. The bubble oscillations induce on the surface steady shear stress attributed to microstreaming. A mathematical simulation of the problem for both free and capsulated bubbles, known as contrast agents, is presented here. The simulation is based on a solution of Laplace's equation for potential flow and existing models for microstreaming. The solution for potential flow was obtained numerically using a boundary integral method. The solution provides the evolution of the bubble shape, the distribution of the velocity potential on the surface, and the shear stress along the surface. The simulation shows that significant shear stresses develop on the surface when the bubble bounces near the tissue surface. In this case, pressure amplitude of 20 kPa generates maximal steady shear stress of several kilo Pascal. Substantial shear stress on the tissue surface takes place inside a circular zone with a radius about half of the bubble radius. The predicted shear stress is greater than stress that causes hemolysis in blood and several orders of magnitude greater than the physiological stress induced on the vessel wall by the flowing blood.     

Infrared microspectroscopic imaging using a large radius germanium internal reflection element and a focal plane array detector

Previously, we established the ability to collect infrared microspectroscopic images of large areas using a large radius hemisphere internal reflection element (IRE) with both a single point and a linear array detector. In this paper, preliminary work in applying this same method to a focal plane array (FPA) infrared imaging system is demonstrated. Mosaic tile imaging using a large radius germanium hemispherical IRE on a FPA Fourier transform infrared microscope imaging system can be used to image samples nearly 1.5 mm x 2 mm in size. A polymer film with a metal mask is imaged using this method for comparison to previous work. Images of hair and skin samples are presented, highlighting the complexity of this method. Comparisons are made between the linear array and FPA methods.     

Fabrication and analysis of spatially uniform field electrokinetic flow devices: theory and experiment

A uniform-field design approach can improve the performance of microanalytical, chip-based devices for a number of applications, including separations and sample preparation. The faceted prism paradigm allows the design of microfluidic devices possessing spatially uniform fields in electrokinetically driven flows. We present the first quantitative study of the velocity fields obtained using faceted interfaces between deep and shallow channel sections. Electrokinetic flows were generated in a series of wet-etch fabricated microfluidic channels. The resulting velocity fields were analyzed by particle image velocimetry and compared with simulations of the two-dimensional Laplace equation using both the designed channel geometry and the as-fabricated channel geometry. This analysis found localized differences between the designed and observed flow fields that were directly attributable to the limitations of isotropic substrate etching. Simulations using the as-fabricated channel geometry reproduced the experimental electrokinetic velocity field, quantitatively accounting for speed field variations due to the limits of the fabrication method. The electrokinetic speed fields were also compared to corresponding pressure-driven speed fields.