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In this Letter, the authors present a novel imaging algorithm for multireceiver synthetic aperture sonar (SAS). The Loffeld's bistatic formula (LBF) including quasi‐monostatic (QM) and multireceiver deformation (MD) phases is first reformulated into range‐variant and range‐invariant phases. After compensating the range‐invariant phase, the subsequent steps are the extended Stolt mapping for each bistatic SAS. Based on simulations, the presented method can also provide high‐resolution result which is similar to that of back projection (BP) algorithm. [ABSTRACT FROM AUTHOR]

Laser ultrasonic synthetic aperture focusing technique (SAFT) is susceptible to multiple modes of ultrasonic generated by laser pulse. In this work, we propose an optimized synthetic aperture imaging method based on the differential technique, resulting in the effective detection of internal defects. The laser ultrasonic SAFT finite element model is established, and the models are calculated in the case of existing internal defect or not. Thereby, the original ultrasonic data containing the defect reflected waves and the differential data are obtained, and the internal defects are reconstructed by the delay superposition algorithm. The results show that the defect imaging is submerged in the high-amplitude background noise superimposed by the surface acoustic wave; when only the original data are used. However, the optimized SAFT imaging based on the differential technique can not only reduce the interference of other mode waves on the imaging area significantly, but also retain the defect reflected waves effectively. Moreover, the imaging and precise locating of multiple defects are realized, which pave the way for enhancing the internal defects detection ability of laser ultrasonic. [ABSTRACT FROM AUTHOR]

In synthetic aperture (SA) imaging reported in the ultrasound imaging literature, typically, the delay and sum (DAS) beamformer is used; however, it is computationally expensive due to the pixel-by-pixel processing performed in the time domain. Recently, the adaptation of frequency-domain beamformers for medical ultrasound SA imaging, particularly to single-element/multielement synthetic transmit aperture (STA/MSTA) schemes, has been reported. In such reports, usually, less attention is paid to reducing system complexity. Recently, a sparse-transmit sparse-receive version of diverging beam-based synthetic aperture technique (DBSAT) was shown to achieve a reduction in system complexity by using fewer parallel receive channels, yet it achieves better quality and higher frame rate than conventional focused beamforming. However, this was also demonstrated using the DAS beamformer. In this work, we aim at achieving a reduction in computational cost, in addition to a reduction in system complexity, by implementing a fast and efficient frequency–wavenumber ($\omega $ - ${k}$) algorithm for the sparse DBSAT scheme. In doing so, an additional novel step of recovering missing frame data due to sparse transmit is introduced, namely, projection onto elliptical sets (POES). The results from this novel combination of $\omega $ - ${k}$ with POES recovery showed that it is feasible to achieve several orders of magnitude faster reconstruction compared with the standard DAS beamforming, without any compromise in the image quality and, in some cases, with improved image quality. The average value of the contrast-to-noise ratio (CNR) and signal-to-noise ratio (SNR) calculated from cyst at 15-mm depth obtained using the different schemes was 4.94 and 5.73 dB better when $\omega $ - ${k}$ was employed instead of DAS, respectively. In addition, for the sparse data set acquired with a 50% overlap during transmit and 64 active receive elements, DAS reconstruction takes as long as ~647 s, whereas the $\omega $ - ${k}$ algorithm takes only ~2 s when programmed and executed in MATLAB. [ABSTRACT FROM AUTHOR]

A fast synthetic aperture (SA) focusing algorithm is proposed in this article to overcome the problem of huge computational complexity in developing high-performance SA ultrasound imaging systems. The proposed method realizes the fast focusing of SA imaging by utilizing a virtual array (VA) technique in combination with nonuniform fast Fourier transform (VA-NUFFT) method. The computational performance of the VA-NUFFT algorithm is evaluated with the computational complexity under acceptable beam errors, compared with the conventional delay-and-sum (DAS) method and the frequency-domain direct method (DM), as well as a multiple NUFFT (M-NUFFT) method. Simulation results reveal that under a beam error level less than −57.4 dB, 25 and 10 reduction ratios of entire real operations can be achieved by the VA-NUFFT at larger SA scan lines of 2048, when compared with the DAS and DM, respectively. In the key SA focusing step, i.e., the “spatial process” step, the computational load of VA-NUFFT is lower than two orders of magnitude of that of the DM. SA imaging of a phantom with four cysts is also simulated and a contrast-to-noise ratio (CNR) criterion is adopted for evaluation of imaging quality. CNR results reflect that SA images of using VA-NUFFT make almost no difference to that of DAS focusing under different signal-to-noise ratios (SNRs). [ABSTRACT FROM AUTHOR]

Abstract—Ultrasonic nondestructive testing has been widely used in the detection and evaluation of fatigue cracks and defects of high-speed railway. In order to improve the detection speed of cylindrical components such as rims, a theoretical model of the forward vector algorithm of ultrasonic immersion curved surface is established. For improving image quality, an averaging correction factor is introduced in this paper to avoid multiple irradiation of the same pixel by the same element, and the visualization of cylindrical components with internal defects is realized by simulation and experiment. The traditional synthetic aperture focusing technique (SAFT), Newton iterative synthetic aperture (NISA) and ultrasonic forward vector algorithm (UFVA) are compared and analyzed on the aspects of imaging effect, array performance index (API) and imaging efficiency. Compared with the other two algorithms, the ultrasonic forward vector algorithm can effectively overcome the iteration of calculation of the refraction point and improve the imaging efficiency. The efficiency is improved by 2 and 1 orders of magnitude respectively while imaging quality is guaranteed. At the same time, this algorithm plays an important role on the measurement of the inner and outer diameter of the rim, the defect detection, and the quantitative analysis of the surface wear, which provides an effective means for the rapid detection of internal defects of the regular curved parts. [ABSTRACT FROM AUTHOR]

Background: Ultrasound computed tomography (USCT) is usually realized with a ring array. It can provide better imaging performance and more tissue information by emitting and receiving the ultrasound signal in different directions simultaneously. However, USCT imaging is usually applied with the synthetic aperture (SA) emission method, which leads to a long scanning time with a large number of elements on the ring array. The echo image can provide the structural information, and has a higher resolution than maps of other parameters in USCT. Hence, we proposed plane wave (PW) imaging for ring array to acquire the echo wave and reduce the scanning time considerably.Results: In this paper, an emitting and receiving process was proposed to realize plane wave imaging with a ring array. With the proposed scanning method, the number of emission events can be reduced greatly. A beamforming method based on the eigenspace-based minimum variance (ESBMV) was also combined with the scanning method. With ESBMV beamformer, the resolution and contrast ratio of reconstruction result can be maintained or even improved under a fewer-emissions condition. We validated the method using both computer simulations with Field II and phantom experiments with a ring array of 512 elements. The Verasonics® system was used to transmit and receive the ultrasound signal in the phantom experiments.Conclusions: According to the results of the experiments, the imaging results will have a better contrast ratio with a higher emitting energy. Additionally, the scanning time with the proposed method can be only one-tenth of that with the SA emission method, while the echo imaging performance still remains at a similar level or even better. [ABSTRACT FROM AUTHOR]

Two main metrics are usually employed to assess the quality of medical ultrasound (US) images, namely the contrast and the spatial resolution. A number of imaging algorithms have been proposed to improve one of those metrics, often at the expense of the other one. This paper presents the application of a correlation-based ultrasound imaging method, called Excitelet , to medical US imaging applications and the inclusion of a new Phase Coherence (PC) metric within its formalism. The main idea behind this algorithm, originally developed and validated for Non-Destructive Testing (NDT) applications, is to correlate a reference signal database with the measured signals acquired from a transducer array. In this paper, it is shown that improved lateral resolutions and a reduction of imaging artifacts are obtained over the Synthetic Aperture Focusing Technique (SAFT) when using Excitelet in conjunction with a PC filter. This novel method shows potential for the imaging of specular reflectors, such as invasive surgical tools. Numerical and experimental results presented in this paper demonstrate the benefit, in terms of contrast and resolution, of using the Excitelet method combined with PC for the imaging of strong reflectors. • A new phase coherence filter is presented for improved contrasts. • The correlation-based algorithm Excitelet is applied to ultrasound medical imaging. • Excitelet outperforms the synthetic aperture method in terms of resolutions. • The correlation-based method is efficient for the imaging of surgical tools. [ABSTRACT FROM AUTHOR]

In ultrasonic nondestructive testing (NDT), the phase shift migration (PSM) technique, as a frequency-domain implementation of the synthetic aperture focusing technique (SAFT), can be adopted for imaging of regularly layered objects that are inhomogeneous only in depth but isotropic and homogeneous in the lateral direction. To deal with irregularly layered objects that are anisotropic and inhomogeneous in both the depth and lateral directions, a generalized frequency- domain SAFT, called generalized phase shift migration (GPSM), is proposed in this paper. Compared with PSM, the most significant innovation of GPSM is that the phase shift factor is generalized to handle anisotropic media with lateral velocity variations. The generalization is accomplished by computer programming techniques without modifying the PSM model. In addition, SRFFT (split-radix fast Fourier transform) input/output pruning algorithms are developed and employed in the GPSM algorithm to speed up the image reconstructions. The experiments show that the proposed imaging techniques are capable of reconstructing accurate shapes and interfaces of irregularly layered objects. The computing time of the GPSM algorithm is much less than the time-domain SAFT combined with the ray-tracing technique, which is, at present, the common method used in ultrasonic NDT industry for imaging layered objects. Furthermore, imaging regularly layered objects can be regarded as a special case of the presented technique. [ABSTRACT FROM AUTHOR]

This work provides a guide to design ultrasonic synthetic aperture systems for non-grid two-dimensional sparse arrays such as spirals or annular segmented arrays. It presents an algorithm that identifies which elements have a more significant impact on the beampattern characteristics and uses this information to reduce the number of signals, the number of emitters and the number of parallel receiver channels involved in the beamforming process. Consequently, we can optimise the 3D synthetic aperture ultrasonic imaging system for a specific sparse array, reducing the computational cost, the hardware requirements and the system complexity. Simulations using a Fermat spiral array and experimental data based on an annular segmented array with 64 elements are used to assess this algorithm. [ABSTRACT FROM AUTHOR]

Abstract: The paper presents the modified multi-element synthetic transmit aperture (MSTA) method for ultrasound imaging. It is based on coherent summation of RF echo signals with apodization weights taking into account the finite size of the transmit subaperture and of the receive element. The work presents extension of the previous study where the modified synthetic transmit aperture (STA) method was considered and verified [1]. In the case of MSTA algorithm the apodization weights were calculated for each imaging point and all combinations of the transmit subaperture and receive element using their angular directivity functions (ADFs). The ADFs were obtained from the exact solution of the corresponding mixed boundary-value problem for periodic baffle system modeling the transducer array. Performance of the developed method was tested using Field II simulated synthetic aperture data of point reflectors for 4MHz 128-element transducer array with 0.3mm pitch and 0.02mm kerf to estimate the visualization depth and lateral resolution. Also experimentally determined data of the tissue-mimicking phantom (Dansk Fantom Service, model 571) obtained using 128 elements, 4MHz, linear transducer array (model L14-5/38) and Ultrasonix SonixTOUCH Research platform were used for qualitative assessment of imaging contrast improvement. Comparison of the results obtained by the modified and conventional MSTA algorithms indicated 15dB improvement of the noise reduction in the vicinity of transducer’s surface (1mm depth), and concurrent increase in the visualization depth (86% augment of the scattered amplitude at the depth of 90mm). However, this increase was achieved at the expense of minor degradation of the lateral resolution of approximately 8% at the depth of 50mm and 5% at the depth of 90mm. [Copyright &y& Elsevier]

The conventional synthetic aperture focusing technique (SAFT) can improve the lateral resolution and increase signal-to-noise ratio (SNR) in ultrasonic imaging. However, the use of SAFT in immersed inspection is limited due to the issues related to the refraction effects at the boundary between the water and a solid object. The virtual transducer (VT) method employing a focused transducer or phased array is proposed as a solution to this problem. In the VT method, the focal point of a focused aperture is used as a virtual source of ultrasonic waves. If the virtual source is placed on or below the surface of an inspected solid object, SAFT can be applied in the same way as for contact inspection. Combining VT with the new phase-shift migration SAFT (PSM-SAFT) algorithm allows one to reconstruct images of multilayer objects with high resolution and high SNR. What is more, the computation efficiency of PSM-SAFT can significantly reduce the time needed for image reconstruction. Experimental validation of the proposed technique is presented in this paper. We present the results of experiments where the VT was created using a focused transducer and a phased array in the immersion inspection of a stainless steel block containing artificial defects. The performance of four different focused apertures used for generating a virtual transducer is evaluated in relation to the PSM-SAFT algorithm implemented in the frequency domain. [ABSTRACT FROM AUTHOR]

Abstract: The modified synthetic transmit aperture (STA) algorithm is described. The primary goal of this work was to assess the possibility to improve the image quality achievable using synthetic aperture (SA) approach and to evaluate the performance and the clinical applicability of the modified algorithm using phantoms. The modified algorithm is based on the coherent summation of back-scattered RF echo signals with weights calculated for each point in the image and for all possible combinations of the transmit–receive pairs. The weights are calculated using the angular directivity functions of the transmit–receive elements, which are approximated by a far-field radiation pattern of a narrow strip transducer element vibrating with uniform pressure amplitude over its width. In this way, the algorithm takes into account the finite aperture of each individual element in the imaging transducer array. The performance of the approach developed was tested using FIELD II simulated synthetic aperture data of the point reflectors, which allowed the visualization (penetration) depth and lateral resolution to be estimated. Also, both simulated and measured data of cyst phantom were used for qualitative assessment of the imaging contrast improvement. The experimental data were obtained using 128 elements, 4MHz, linear transducer array of the Ultrasonix research platform. The comparison of the results obtained using the modified and conventional (unweighted) STA algorithms revealed that the modified STA exhibited an increase in the penetration depth accompanied by a minor, yet discernible upon the closer examination, degradation in lateral resolution, mainly in the proximity of the transducer aperture. Overall, however, a considerable (12dB) improvement in the image quality, particularly in the immediate vicinity of the transducer’s surface was demonstrated. The modified STA method holds promise to be of clinical importance, especially in the applications where the quality of the “near-field” image, that is the image in the immediate vicinity of the scanhead is of critical importance such as for instance in skin- and breast-examinations. [Copyright &y& Elsevier]

A method for ultrasonic synthetic aperture imaging using finite-sized transducers is introduced that is based on a virtual source (VS) concept. In this setup, a focused transducer creates a VS element at its focal point that facilitates the use of synthetic aperture focusing technique (SAFT). It is shown that the performance of the VS method may be unsatisfactory due to the distortion introduced by the diffraction effects of the aperture used for creating the VS element. A solution to this problem is proposed that consists of replacing the classical SAFT by the extended synthetic aperature focusing technique (ESAFT) algorithm presented in our earlier works. In ESAFT, the full geometry of the VS is modeled, instead of applying the simplified point source approximation used when VS is combined with classical SAFT. The proposed method yields a substantial improvement in spatial resolution compared to that obtained using SAFT. Performance of the proposed algorithm is first demonstrated on simulated data, then verified on real data acquired with an array system. [ABSTRACT FROM AUTHOR]

With sheet metal hydroforming, a sheet metal is formed by a liquid medium under high pressure (up to 1000 bar) and a cavity contour (die). As the exact state of forming is of interest, an ultrasonic imaging system is under development. The task is to determine the geometry of a sheet metal contour with respect to the original (before forming) and the final (die) state of the sheet metal. For this purpose, two different contour reconstruction algorithms were designed, tested and compared. With the reconstruction results it will be possible to determine the optimal distribution of transmitters and receivers in the ultrasonic transducer matrix. Experiments were conducted with one pair of transducers (unfocussed, center frequency 2 MHz) and a three axis stepper motor set-up. For each experimental set of data, the contour was reconstructed with both SAFT reconstruction algorithms. Both algorithms incorporate a priori information such as original and final contour and maximal axial dislocation of the sheet metal. The results for both algorithms are compared and the relative mean error in axial direction is 0.30% and 0.48%. [Copyright &y& Elsevier]