The wavenumber algorithm for full-matrix imaging using an ultrasonic array

Ultrasonic imaging using full-matrix capture, e.g., via the total focusing method (TFM), has been shown to increase angular inspection coverage and improve sensitivity to small defects in nondestructive evaluation. In this paper, we develop a Fourier-domain approach to full-matrix imaging based on the wavenumber algorithm used in synthetic aperture radar and sonar. The extension to the wavenumber algorithm for full-matrix data is described and the performance of the new algorithm compared with the TFM, which we use as a representative benchmark for the time-domain algorithms. The wavenumber algorithm provides a mathematically rigorous solution to the inverse problem for the assumed forward wave propagation model, whereas the TFM employs heuristic delay-and-sum beamforming. Consequently, the wavenumber algorithm has an improved point-spread function and provides better imagery. However, the major advantage of the wavenumber algorithm is its superior computational performance. For large arrays and images, the wavenumber algorithm is several orders of magnitude faster than the TFM. On the other hand, the key advantage of the TFM is its flexibility. The wavenumber algorithm requires a regularly sampled linear array, while the TFM can handle arbitrary imaging geometries. The TFM and the wavenumber algorithm are compared using simulated and experimental data.     

Backscatter coefficient estimation using array transducers

This paper describes an extension of our broadband method for estimating backscatter coefficients from random media to include data from array transducers. Four different transducer designs have now been considered: oneand two-dimensional linear arrays, annular arrays, and single-element focused pistons commonly used in mechanical sector scanners. The analysis shows that if the backscatter echo spectrum is properly normalized, the shape of the piezoelectric elements affects only the magnitude and not the frequency dependence of the backscatter coefficient estimates. Experimental data were acquired using laboratory and clinical imaging instrumentation to verify the analysis. We compared backscatter coefficients measured as a function of frequency from well-defined scattering media that were obtained using a 1-D linear array and focused piston transducers. Instrument-independent results were found that matched theoretical predictions within the measurement error between 2 and 12 MHz. We conclude from this study that accurate backscatter coefficient estimates may be easily obtained using current clinical imaging instrumentation.     

Measuring the acoustic wave velocity and sample thickness using an ultrasonic transducer array

We suggest a new method for determining the longitudinal and transverse acoustic wave velocities and sample thicknesses, which is based on the measurement and analysis of pulsed echo signals by an array of ultrasonic transducers. Analytical expressions relating the delay of signals detected by the array and the values of parameters to be determined are obtained within the framework of a ray model of the measuring system. Measurements on a reference sample have been performed. The values of ultrasonic wave velocities and sample thickness obtained using the proposed technique agree with the results of measurements using independent methods.     

Use of a laser beam with an oblique angle of incidence to measure the reduced scattering coefficient of a turbid medium

A simple and quick approach is used to measure the reduced scattering coefficient (μ(s)') of a semi-infinite turbid medium having a much smaller absorption coefficient than μ(s)'. A laser beam with an oblique angle of incidence to the medium causes the center of the diffuse reflectance that is several transport mean-free paths away from the incident point to shift away from the point of incidence by an amount Δx. This amount is used to compute μ(s)' by μ(s)' = sin(α(i))/(nΔx) where n is the refractive index of the turbid medium divided by that of the incident medium and α(i) is the angle of incidence measured from the surface normal. For a turbid medium having an absorption coefficient comparable with μ(s)', a revision to the above formula is made. This method is tested theoretically by Monte Carlo simulations and experimentally by a video reflectometer.     

由混响衰减反演海底散射系数

1随机反演理论(高斯-马尔可夫原理) 设随机变量d与m,n有线性关系: d=Gm+n, (1) 其中d为数据矢量,m为未知参量,n为随机噪声,G为未知参量与数据矢量之间的关系. 假设m,n为零均值,其协方差矩阵分别为:     

基于部分散射系数矩阵的缺陷识别方法研究

为了在相控阵超声成像检测时实现缺陷的定性分析,采用部分散射系数矩阵表征传感器采集到的部分散射场信息,并利用部分散射系数矩阵图进行缺陷识别。以相控阵超声全阵列采集获取到的全矩阵数据为基础,结合声波传播过程的衰减性,采用逆向补偿的方法将全矩阵数据补偿到以缺陷为中心的同一圆周上,以此获取缺陷的部分散射信息,并采用部分散射系数矩阵来表征。通过分析缺陷的部分散射系数矩阵图特征,对缺陷进行定性分析和判别。采用此方法对两种典型的缺陷进行仿真实验,结果显示,部分散射系数矩阵图不仅对圆孔和裂纹两类不同缺陷表现出差异性,对同类型不同尺寸和不同角度的裂纹也有明显的不同之处,表明利用该方法可实现缺陷的定性识别。     

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.     

A thermoelastic method to measure the thermal expansion coefficient

In two different laboratories, measurements are performed of the thermal expansion coefficient of several mortar specimens, based on the thermoelastic effect. The temperature of the sample surface is recorded while the specimens undergo a uniaxial adiabatic compression. The percentage change of temperature is proportional to the longitudinal strain, the constant of proportionality being the longitudinal Grüneisen parameter. Through the equation of state, this parameter allows access to the thermal expansion coefficient once the Young modulus and the specific heat at constant volume are independently available. Results obtained for groups of equally prepared specimens, using two slightly different experemental setups, are found to be identical within the experimental errors. These results based on the thermoelastic method, are also consistent with those independently obtained by direct extensometric methods.

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Résumé On présente une méthode de mesure du coefficient de dilatation thermique des matériaux à base de ciment. La méthode est fondée sur l'équation d'état et se base sur la mesure du coefficient de couplage thermoélastique, le paramètre de Grüneisen. Quand un échantillon est soumis à une charge compressive uniaxiale, en régime élastique et en conditions adiabatiques, sa température augmente par l'effet thermoélastique. La variation relative de température est proportionnelle à la déformation longitudinale, la constante de proportionnalité étant le paramètre de Grüneisen longitudinal. L'équation d'état permet d'obtenir le coefficient de dilatation thermique comme le produit du paramètre de Grüneisen longitudinal, de l'inverse du modulus de Young, et de la capacité thermique par unité de volume, à volume constant. On présente une série de mesures comparées, effectuées dans deux laboratoires sur deux séries d'échantillons de mortier. Les échantillons sont soumis à un essai de compression, pendant lequel on enregistre la charge appliquée et la température de la surface des échantillons mêmes. Le paramètre de Grüneisen longitudinal et le modulus de Young sont déterminés par les caractéristiques température-déformtion et charge-déformation. La capacité thermique par unité de volume est déterminée, dans un essai indépendant, comme le rapport de la diffusivité thermique et de la conductivité thermique, qui font souvent partie d'une qualification adéquate du matériau. Le coefficient de dilatation thermique est enfin déduit comme indiqué auparavant. Les valeurs expérimentales obtenues dans les deux laboratoires diffèrent par moins d'une déviation standard. Les valeurs dites sont comparées avec celles obtenues par une méthode extensométrique, indépendante et plus directe. Les résultats des deux méthodes se révèlent concordants. On conclut que la méthode proposée, basée sur l'effet thermoélastique, bien qu'indirecte, conduit à une mesure fine et précise du coefficient de dilatation thermique.

     

Relationship of light scattering at an angle in the backward direction to the backscattering coefficient

We revisit the problem of computing the backscattering coefficient based on the measurement of scattering at one angle in the back direction. Our approach uses theory and new observations of the volume scattering function (VSF) to evaluate the choice of angle used to estimate b(b). We add to previous studies by explicitly treating the molecular backscattering of water (b(bw)) and its contribution to the VSF shape and to b(b). We find that there are two reasons for the tight correlation between observed scattering near 120 degrees and the backscattering coefficient reported by Oishi [Appl. Opt. 29, 4658, (1990)], namely, that (1) the shape of the VSF of particles (normalized to the backscattering) does not vary much near that angle for particle assemblages of differing optical properties and size, and (2) the ratio of the VSF to the backscattering is not sensitive to the contribution by water near this angle. We provide a method to correct for the water contribution to backscattering when single-angle measurements are used in the back direction (for angles spanning from near 90 degrees to 160 degrees ) that should provide improved estimates of the backscattering coefficient.     

Plasmonic nanogap-enhanced Raman scattering using a resonant nanodome array

The optical properties and surface-enhanced Raman scattering (SERS) of plasmonic nanodome array (PNA) substrates in air and aqueous solution are investigated. PNA substrates are inexpensively and uniformly fabricated with a hot spot density of 6.25 × 10(6) mm(-2) using a large-area nanoreplica moulding technique on a flexible plastic substrate. Both experimental measurement and numerical simulation results show that PNAs exhibit a radiative localized surface plasmon resonance (LSPR) due to dipolar coupling between neighboring nanodomes and a non-radiative surface plasmon resonance (SPR) resulting from the periodic array structure. The high spatial localization of electromagnetic field within the ～10 nm nanogap together with the spectral alignment between the LSPR and excited and scattered light results in a reliable and reproducible spatially averaged SERS enhancement factor (EF) of 8.51 × 10(7) for Au-coated PNAs. The SERS enhancement is sufficient for a wide variety of biological and chemical sensing applications, including detection of common metabolites at physiologically relevant concentrations.     

Ultrasonic computed tomography reconstruction of the attenuation coefficient using a linear array

The attenuation coefficient distribution and sound velocity distribution in the breast can be used to complement B-mode ultrasound imaging in the detection of breast cancer. This study investigated an approach for reconstructing the attenuation coefficient distribution in the breast using a linear array. The imaging setup was identical to that for conventional B-mode breast imaging, and the same setup has been used for reconstruction of sound velocity distributions in previous studies. In this study, we further developed a reconstruction method for the attenuation coefficient distribution. In particular, the proposed method incorporates the segmentation information from B-mode images and uses the sound velocity distribution to compensate for refraction effects. Experiments were conducted with a setup consisting of a 5-MHz, 128-channel linear array, a programmable digital array system, a phantom, and a computer. The constructed phantom contained materials mimicking the following breast tissues: glandular tissue, fat, cysts, high-attenuation tumors, and irregular tumors. Application of the proposed technique resulted in all the cysts and tumors (including high-attenuation and irregular tumors) being distinguished by thresholding the reconstructed attenuation coefficients. We have demonstrated that it is possible to use the same imaging setup to acquire data for B-mode image, sound velocity distribution, and attenuation coefficient distribution simultaneously. Moreover, the experimental data indicate its potential in improving the detection of breast cancer.     

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.     

A 100-MHz ultrasonic transducer array using ZnO thin films

A 100-MHz ultrasonic linear transducer array made from a piezoelectric zinc oxide thin film on a sapphire substrate was developed and evaluated. Epitaxial, high-acoustic quality 10-μm-thick ZnO film layers were produced by rf-magnetron sputter deposition onto a (111)-oriented gold film (with a chromium adhesion layer) that was vacuum-evaporated onto a (0001) sapphire surface. We found that, in well-oriented growth of gold, it is important to control the chromium sublayer thickness (less than 5 nm). An array was constructed by photolithography with an appropriate etch. V-shaped grooves between adjacent elements were formed by using an anisotropic etchant (HCl and HNO₃-based) that preferentially etched the c-plane of ZnO. Typical array elements were 90 μm wide, 3.2 mm long, and 10 μm thick, and the pitch of an array was typically 100 μm. Our fine uniform array resulted in uniform ultrasonic response of individual elements throughout the array. For a 32-element array, the ultrasound beam in the azimuth plane in water could be electronically focused in the 100 MHz range to obtain a half-amplitude width of 60 μm at the focal depth, agreeing well with theoretical predictions. Besides the use demonstrated with this present transducer, piezoelectric thin films should also lead to fabrication of various other kinds of ultrasonic transducers that can operate at high frequencies and should provide opportunities for miniaturizing transducers and making integrated ultrasonic devices     

Intrinsic fiber-optic ultrasonic sensor array using multiplexed two-wave mixing interferometry

An intrinsic multiplexed laser interferometer is presented that allows for the simultaneous detection of acoustic waves by an array of fiber-optic sensors. The phase-modulated signals from each sensor are demodulated by use of an adaptive two-wave mixing setup. The light from each sensing fiber in the array is mixed with a reference beam in a single photorefractive crystal (PRC), and the output beams from the PRC are imaged onto separate photodetectors to create a multiplexed two-wave mixing (MTWM) system. The sensing fibers are embedded in graphite-epoxy composite panels, and detection of both acoustic emission and ultrasonic signals in these materials is demonstrated. The intrinsic MTWM system is an effective tool for the simultaneous demodulation of signals from a large fiber sensor array. Also, the adaptive nature of the MTWM setup obviates the need for active stabilization against ambient noise.     

相控阵超声绝对声时法测量底面开口裂纹

针对工件内表面开口裂纹单面单侧定量检测困难的问题,采用相控阵超声绝对声时法(Absolute Arrival Time Technique,AATT)从单面单侧对不同厚度试块中的底面人工开口裂纹自身高度测量进行了数值仿真和实验,并对焊缝中的自然裂纹进行了AATT测量。对比AATT测量结果与常规衍射时差法超声检测(Time of Flight Diffraction,TOFD)测量结果并分析,发现AATT从单面单侧对底面开口裂纹进行定量测量可以达到与常规TOFD同样的测量精度。AATT操作简便,可单面单侧定量测量2 mm以上底面开口裂纹的自身高度。     

Numerical analysis of low-frequency electromagnetic scattering from axially symmetric bodies using an inductance matrix

We have proposed a numerical method for calculating low-frequency electromagnetic scattering from axially symmetric conducting bodies with and without apertures. The surface of the perfectly conducting scatterer is modeled by a set of inductively coupled coil elements, and the current in each coil element is computed by solving an inductance matrix equation. A disadvantage of a conventional method for a scatterer with apertures is discussed. Scattering from various axially symmetric conducting bodies with or without apertures is calculated and the resulting fields are in good agreement with those obtained by finite-element method.     

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.     

Void characterization using ultrasonic backscatter from void clusters

A technique to extract the deconvolved response of a single cavity from the backscattered ultrasonic signal received from a cluster of cavities is presented. First, a standard deconvolution removes noise and distortions of the signal caused either by the measuring system or the specimen. This is followed by homorphic deconvolution using the complex cepstrum to extract the response of a single cavity when signals from adjacent cavities in the cluster are superimposed on each other. The technique is applied to backscatter from a cluster of spherical cavities located in an epoxy specimen. The recovered signal compares favorably with the analytically calculated, backscattered time-domain response of a spherical cavity due to the incidence of a plane longitudinal wave on the single cavity. The radius of the cavity can be obtained from the extracted response using a simple formula derived from elastodynamic ray theory.     

Using microwave multimeters to measure the complex reflection coefficient of waveguides

The application of a microwave multimeter in precision measurements of complex reflection coefficients is described. Experimental results are cited.Translated from Izmeritel'naya Tekhnika, No. 6, pp. 47–49, June, 1995.