Application of a matched filter approach for finite aperture transducers for the synthetic aperture imaging of defects
07 Jun 2010-IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control (IEEE)-Vol. 57, Iss: 6, pp 1368-1382
TL;DR: These studies confirm that MFA is an alternative to SAFT with little additional computational burden and can also be applied blindly, like SAFT, to effect synthetic focusing with distinct advantages in treating finite transducer effects, and in handling steered beam inspections.
Abstract: The suitability of the synthetic aperture imaging of defects using a matched filter approach on finite aperture transducers was investigated. The first part of the study involved the use a finite-difference time-domain (FDTD) algorithm to simulate the phased array ultrasonic wave propagation in an aluminum block and its interaction with side-drilled hole-like defects. B-scans were generated using the FDTD method for three active aperture transducer configurations of the phased array (a) single element and (b) 16-element linear scan mode, and (c) 16-element steering mode. A matched filter algorithm (MFA) was developed using the delay laws and the spatial impulse response of a finite size rectangular phased array transducer. The conventional synthetic aperture focusing technique (SAFT) algorithm and the MFA were independently applied on the FDTD signals simulated with the probe operating at a center frequency of 5 MHz and the processed B-scans were compared. The second part of the study investigated the capability of the MFA approach to improve the SNR. Gaussian white noise was added to the FDTD generated defect signals. The noisy B-scans were then processed using the SAFT and the MFA and the improvements in the SNR were estimated. The third part of the study investigated the application of the MFA to image and size surface-crack-like defects in pipe specimens obtained using a 45° steered beam from a phased array probe. These studies confirm that MFA is an alternative to SAFT with little additional computational burden. It can also be applied blindly, like SAFT, to effect synthetic focusing with distinct advantages in treating finite transducer effects, and in handling steered beam inspections. Finally, limitations of the MFA in dealing with larger-sized transducers are discussed.
Citations
More filters
TL;DR: In this paper, an image reconstruction algorithm based on regularized least squares using a l 1 regularization norm was proposed to reconstruct an image of a point-like reflector, using both simulated and real data.
Abstract: Ultrasound imaging systems (UIS) are essential tools in nondestructive testing (NDT). In general, the quality of images depends on two factors: system hardware features and image reconstruction algorithms. This paper presents a new image reconstruction algorithm for ultrasonic NDT. The algorithm reconstructs images from A-scan signals acquired by an ultrasonic imaging system with a monostatic transducer in pulse-echo configuration. It is based on regularized least squares using a l1 regularization norm. The method is tested to reconstruct an image of a point-like reflector, using both simulated and real data. The resolution of reconstructed image is compared with four traditional ultrasonic imaging reconstruction algorithms: B-scan, SAFT, !-k SAFT and regularized least squares (RLS). The method demonstrates significant resolution improvement when compared with B-scan—about 91% using real data. The proposed scheme also outperforms traditional algorithms in terms of signal-to-noise ratio (SNR).
32 citations
TL;DR: A seminumerical simulation method called SIRFEM is presented, which enables the efficient prediction of high-frequency transducer outputs and is able to predict reflections at inner structures as well as multiple reflections between those structures and the specimen's surface.
Abstract: We present a seminumerical simulation method called SIRFEM, which enables the efficient prediction of high-frequency transducer outputs. In particular, this is important for acoustic microscopy where the specimen under investigation is immersed in a coupling fluid. Conventional finite-element (FE) simulations for such applications would consume too much computational power due to the required spatial and temporal discretization, especially for the coupling fluid between ultrasonic transducer and specimen. However, FE simulations are in most cases essential to consider the mode conversion at and inside the solid specimen as well as the wave propagation in its interior. SIRFEM reduces the computational effort of pure FE simulations by treating only the solid specimen and a small part of the fluid layer with FE. The propagation in the coupling fluid from transducer to specimen and back is processed by the so-called spatial impulse response (SIR). Through this hybrid approach, the number of elements as well as the number of time steps for the FE simulation can be reduced significantly, as it is presented for an axis-symmetric setup. Three B-mode images of a plane 2-D setup—computed at a transducer center frequency of 20 MHz—show that SIRFEM is, furthermore, able to predict reflections at inner structures as well as multiple reflections between those structures and the specimen’s surface. For the purpose of a pure 2-D setup, the SIR of a curved-line transducer is derived and compared to the response function of a cylindrically focused aperture of negligible extend in the third spatial dimension.
5 citations
Cites background from "Application of a matched filter app..."
...2510419 or optimal filter kernels can be derived [9]–[11]....
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01 Jan 2015
TL;DR: A new image reconstruction algorithm for ultrasonic NDT that reconstructs images from A-scan signals acquired by an ultrasonic imaging system with a monostatic transducer in pulse-echo configuration based on regularized least squares using a l1 regularization norm is presented.
Abstract: Ultrasound imaging systems (UIS) are essential tools in nondestructive testing (NDT). In general, the quality of images depends on two factors: system hardware features and image reconstruction algorithms. This paper presents a new image reconstruction algorithm for ultrasonic NDT. The algorithm reconstructs images from A-scan signals acquired by an ultrasonic imaging system with a monostatic transducer in pulse-echo configuration. It is based on regularized least squares using a l1 regularization norm. The method is tested to reconstruct an image of a point-like reflector, using both simulated and real data. The resolution of reconstructed image is compared with four traditional ultrasonic imaging reconstruction algorithms: B-scan, SAFT, !-k SAFT and regularized least squares (RLS). The method demonstrates significant resolution improvement when compared with B-scan—about 91% using real data. The proposed scheme also outperforms traditional algorithms in terms of signal-to-noise ratio (SNR).
3 citations
References
More filters
TL;DR: In this paper, a computer-aided, time-domain method is proposed to determine the spatial pulse response for an arbitrarily shaped source and for arbitrary aperture velocity and delay distributions.
Abstract: The computer‐aided, time‐domain method is a proposal to determine the spatial pulse response for an arbitrarily shaped source and for arbitrary aperture velocity and delay distributions. The computational procedure ‘‘generates’’ the pulse response function by directly repeating the physical stages that accompany the creation of this phenomenon, such as radiation from the surface element, and propagation and summation in the observation point. The method is very simple mathematically, employing a simple algorithm, and can be easily implemented—even on a small minicomputer. The mathematical work required prior to the computation has been greatly simplified; the commonly studied behavior of the derivatives (arrival time/source point), as well as the exact temporal limits of the pulse response occurrence, need not be analyzed. The Dirac‐type free‐space Green’s function, as well as other arbitrary types of the causal Green’s function, can also be considered. The obtainable results are approximate, but computational precision depends only on the number of the elementary surfaces used to model the aperture, and, therefore, can be obtained practically, as desired. The method can also be easily used for computations of the transducer coupling functions and of the transient field diffracted by an arbitrary object. The nonplanar sources and objects can also be considered.
68 citations
"Application of a matched filter app..." refers background or methods in this paper
...a prevalent numerical computational technique commonly used is the discrete representation array Modeling (dreaM) procedure, which is well suited for the computation of fields from planar and array transducers [6]–[8]....
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...saft, though a rapid, simple, and effective sizing technique, cannot be readily applied to the defect images obtained from a finite-size-aperture transducer because the transducer has a broader sir characteristic [6]–[8] and a shaped beam....
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TL;DR: The implementation of SAFT in a CAD environment allows us to present stacked 2D reconstructions dynamically and finalizes the overview of two decades of experience in applying this technique.
68 citations
"Application of a matched filter app..." refers background in this paper
...the most common implementation of synthetic aperture imaging is the synthetic aperture focusing technique (saft), which can be applied either in the time domain or in the frequency domain [1], [2]....
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TL;DR: A two-dimensional (2-D) finite-difference model for elastic waves in the ground has been developed and it is seen that the presence of an air-chamber within the mine gives rise to resonant oscillations that are clearly visible on the surface above the mine.
Abstract: A two-dimensional (2-D) finite-difference model for elastic waves in the ground has been developed. The model uses the equation of motion and the stress-strain relation, from which a first-order stress-velocity formulation is obtained. The resulting system of equations is discretized using centered finite-differences. A perfectly matched layer surrounds the discretized solution space and absorbs the outward traveling waves. The numerical model is validated by comparison to an analytical solution. The numerical model is used to study the interaction of elastic waves with a buried land mine. It is seen that the presence of an air-chamber within the mine gives rise to resonant oscillations that are clearly visible on the surface above the mine. The resonance is shown to be due to flexural waves being trapped within the thin layer between the surface of the ground and the air chamber of the mine. The numerical results are in good qualitative agreement with experimental observations.
66 citations
TL;DR: In this article, the authors used the finite-difference time domain (FDTD) method to simulate the effect of various angles of incidence on the defect definition, i.e., with respect to imaging and sizing, using the relative arrival time technique.
61 citations
TL;DR: Two model-based Bayesian image reconstruction methods based on a linear model of the array imaging system and they perform compensation in both the spatial and temporal domains using a minimum mean squared error (MMSE) criterion and a maximum a posteriori MAP) estimation approach are presented.
Abstract: This paper treats time-domain model-based Bayesian image reconstruction for ultrasonic array imaging and, in particular, two reconstruction methods are presented. These two methods arc based on a linear model of the array imaging system and they perform compensation in both the spatial and temporal domains using a minimum mean squared error (MMSE) criterion and a maximum a posteriori MAP) estimation approach, respectively. The presented estimators perform compensation for both the electrical and acoustical wave propagation effects for the ultrasonic array system at hand. The estimators also take uncertainties into account, and, by the incorporation of proper prior knowledge, high-contrast superresolution reconstruction results are obtained. The novel nonlinear MAP estimator constrains the scattering amplitudes to be positive, which applies in applications where the scatterers have higher acoustic impedance than the surrounding medium. The linear MMSE and nonlinear MAP estimators are compared to the traditional delay-and-sum (DAS) beamformer with respect to both resolution and signal-to-noise ratio. The algorithms are compared using both simulated and measured data. The results show that the model-based methods can successfully compensate for both side-lobes and grating lobes, and they have a superior temporal and lateral resolution compared to DAS beamforming. The ability of the nonlinear MAP estimator to suppress noise is also superior compared to both the linear MMSE estimator and the DAS beamformer.
34 citations
"Application of a matched filter app..." refers background or methods in this paper
...[25], [26] state that the MMse filter can be precomputed, thereby making the final image reconstruction a multiplication of the filter matrix with the input scan data....
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...the forward model [25], [26] that has been developed for the Mfa was based on the computation of the sir of the transducer under study....
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...the basis for the matched filter is a 2-d linear model that describes the wave propagation and reflection in the imaging domain [25], [26]....
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...[26] that with a similar approach it is possible to create a minimum mean square error (MMse)-based filter, which minimizes the overall reconstruction error in the image, giving rise to better spatial reconstruction [25], [26]....
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...[26] indicate that, for enhancing its efficiency, the MMse filter can be pre-computed such that the final image reconstruction results from a multiplication of the filter matrix with the input scan data....
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