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• The multi-mode laser ultrasonic F-SAFT and F-TFM imaging have realized. • The frequency-domain imaging results were weighted by phase circular statistical vector factor. • Achieved blind spot free images of the minimum 0.3 mm transverse hole defect. • Continuous scanning frequency-domain imaging of multiple defects verified the detection ability. Far-field laser technology has greatly promoted the progress of nondestructive ultrasonic imaging of bulk structures. However, under thermoelastic excitation, the body waves exhibit a relatively low signal-to-noise ratio, resulting in images with low resolution and contrast. Based on the motivation, this paper developed a frequency-domain phase weighted imaging method to improve the quality of laser ultrasonic defect imaging. Firstly, laser ultrasonic scanning was performed on the sample with artificial transverse hole defects. The cylindrical lens focused line source was used to improve the intensity of the body wave signals, and ensure that there was no damage on the material surface under high laser energies. Then, the frequency-domain phase shift migration (PSM) algorithm was used to perform multimode imaging of defects, achieving frequency-domain synthetic aperture focusing technique (F-SAFT) and total focused method (F-TFM) imaging based on full matrix capture. Furthermore, the phase circular statistical vector (PCSV) was proposed for weighted optimization, which improved the image quality, suppressed the background noise and multimode artifacts. Finally, the imaging quality of several algorithms were discussed. The results indicate that frequency-domain images were superior to time-domain results. After phase weighting, the imaging quality can be further improved, and the detection blind zone was significantly reduced. This work will contribute to the rapid and high-quality defect imaging of laser ultrasonic. [ABSTRACT FROM AUTHOR]

Laser ultrasound is a widely used tool for industrial quality assurance when a contactless and fast method is required. In this work, we used a laboratory setup based on a confocal Fabry–Perot interferometer to examine weld seam models. The focus was placed on small samples with curved surfaces (small in the sense that the radius of curvature is comparable to the largest ultrasonic wavelength) and on efficient ways to detect the presence and volume of process pores, with the goal to transfer this method to industrial applications. In addition to this experimental method for investigating welds, a numerical method that models the experimental setup was implemented in MATLAB. For this purpose, first the thermal effects of the excitation process were taken into account by solving the thermal diffusion equation with an explicit scheme. Then, the elastodynamic equations were solved using the Elastodynamic Finite Integration Technique, taking into account the stresses induced by the excitation process. The B-Scans generated with this numerical model were compared with experimental B-Scans for simple test cases and good agreement was found. In a next step, the additional structures in the B-Scans resulting from air inclusions were identified and investigated with both methods using flat test specimens at first. Besides the direct echoes, structures from skimming surface waves and multiple reflections were visible. These additional structures are unwanted in defect reconstruction methods like the Synthetic Aperture Focusing Technique (SAFT) as they would lead to artifacts. In samples much larger than the largest ultrasound wavelength, however, these unwanted structures are still negligible in amplitude or can be well separated temporally, but for small samples this is no longer the case. As a result, reconstruction methods based on direct echoes like SAFT are difficult to apply. For many industrial applications, the reconstruction is not decisive at all, but only the knowledge of the total volume of process pores (TVPP). It is shown with both experimental and numerical methods, that this TVPP can be estimated from the variation in the B-Scans from various small weld seam models. • Defect detection for curved surface samples aiming for industrial applicability. • Numerical model based on EFIT (Elastodynamic Finite Integration Technique). • Estimation of process pore volume in small welds from laser ultrasound B-Scans. • Applicability of SAFT (Synthetic Aperture Focusing Technique) investigated. [ABSTRACT FROM AUTHOR]

• DAS and DMAS responded differently to changes in f-number. • DAS showed a tradeoff in spatial resolution when optimizing contrast. • DMAS did not show the same tradeoff as DAS when optimizing contrast. • DAS typically outperformed DMAS for image contrast. • DMAS typically outperformed DAS for spatial resolution. The choices of transmit and receive f-numbers impact both ultrasound image contrast and spatial resolution. Although previous studies have evaluated the impact of receive f-number in delay-and-sum (DAS) plane wave imaging, there has not been a systematic study of f-numbers in DAS or delay-multiply-and-sum (DMAS) synthetic aperture (SA) imaging. In this study, we measured the impact on main lobe to side lobe energy ratio (MSER), generalized contrast-to-noise ratio (gCNR), and spatial resolution when varying receive and transmit f-numbers from 1 to 5 in 0.2 increments in DAS and DMAS reconstructed SA images. A wire target in a water tank and a standard imaging phantom were used to measure these metrics. From the water tank wire target images, higher MSER values were achieved with middle-range transmit f-numbers (2–4) and high receive f-numbers (>4) for both DAS and DMAS. From the phantom contrast target images, DAS produced images with high gCNR when using high transmit f-numbers (>4) and high receive f-numbers (>4). This came at the cost of reduced spatial resolution. DMAS produced images with high gCNR when using low transmit f-numbers (<3) and high receive f-numbers (>4). DMAS was not found to have as severe of a tradeoff in spatial resolution when seeking maximum gCNR. However, gCNR was typically lower for DMAS than DAS. For both DAS and DMAS, point target images had high spatial resolution when using low receive f-numbers (<2). Spatial resolution was typically higher for DMAS than DAS. Hanning apodization was found to produce similar trends as those found with rectangular apodization. These findings give insight on the behaviors of DAS and DMAS SA reconstruction algorithms and could be used to guide f-number selection. [ABSTRACT FROM AUTHOR]

• We compare three wide unfocused wavefront used for convex-array imaging. • We give explicit wave solutions to these wide unfocused wavefronts. • We perform simulation to analyze synthesize transmit field response, B-mode image quality is compared via simulation and in vivo experiments. Ultrafast ultrasound imaging modalities have attracted a lot of attention in the ultrasound community. It breaks the compromise between the frame rate and the region of interest by insonifying the whole medium with wide unfocused waves. Coherent compounding can be performed to enhance the image quality at a cost of frame rate. Ultrafast imaging has wide clinical applications, such as vector Doppler imaging and shear elastography. On the other hand, the use of unfocused waves is still marginal with convex-array transducers. For convex array, plane wave imaging is limited by the complicated transmission delay calculation, limited field-of-view, and inefficient coherent compounding. In this article, we study three wide unfocused wavefronts, namely, lateral virtual-source defined diverging wave imaging (latDWI), tilt virtual-source defined diverging wave imaging (tiltDWI), and Archimedean-spiral-based imaging (AMI) for convex-array imaging using the full-aperture transmission. The analytical monochromatic wave solutions to this three imaging are given. The mainlobe width and grating lobe position are given explicitly. Theoretical −6 dB beamwidth and synthetic transmit field response are studied. Simulation studies are carried on with the point targets and hypoechoic cysts. Time-of-flight formulas are given explicitly for beamforming. The conclusions are in good agreement with the theory: latDWI provides the finest lateral resolution but generates the severest axial lobe level for scatterers with large obliquities (i.e., for scatterers located at the image border) which degrades the image contrast. This effect gets worsen as the compound number increases. The tiltDWI and AMI give a very close performance on resolution and image contrast. AMI displays better contrast with a small compound number. [ABSTRACT FROM AUTHOR]

• Decorrelated compounding synthetic aperture ultrasound imaging reduces speckle. • Improved image quality can be used to monitor focused ultrasound thermal therapy. • Precision of backscattered energy measurements for thermometry are improved. • Decorrelated compounding technique is feasible for use in ultrasound thermometry. Decorrelated Compounding (DC) for synthetic aperture ultrasound can reduce speckle variation in images, suggesting enhanced detectability of low-contrast targets in tissue including thermal lesions produced by focused ultrasound (FUS). The DC imaging method has primarily been investigated in simulation and in phantom studies. This work investigates the feasibility of the DC method in monitoring thermal therapy via image guidance and non-invasive thermometry based on the change in backscattered energy (CBE). Ex vivo porcine tissue was exposed to FUS exposures at acoustic powers of 5 W and 1 W, with peak pressure amplitudes of 0.64 MPa and 0.27 MPa respectively. During FUS exposure, RF echo data frames was acquired using a 7.8 MHz linear array probe and a Verasonics VantageTM ultrasound scanner (Verasonics Inc., Redmond, WA). RF echo data was taken to produce B-mode images, as reference images. Synthetic aperture RF echo data was also acquired and processed using delay-and-sum (DAS), a combination of spatial and frequency compounding referred to as Traditional Compounding (TC), and the proposed DC imaging methods. Image quality was assessed using the contrast-to-noise ratio (CNR) at the FUS beam focus, and the speckle SNR (sSNR) of the background region as preliminary metrics. A calibrated thermocouple was placed near the FUS beam focus for temperature measurements and calibrations using the CBE method. The DC imaging method significantly improved image quality to detect low contrast thermal lesions in treated ex vivo porcine tissue in comparison to other imaging methods. In comparison to B-mode imaging, the lesion CNR measured using the DC imaging was shown to improve up to a factor of approximately 5.5. The corresponding sSNR improved by a factor of approximately 4.2 in comparison to B-mode imaging. CBE calculation using the DC imaging method yielded more precise measurements of the backscattered energy compared to other imaging methods studied. The despeckling performance of the DC imaging method significantly improves the lesion CNR in comparison to B-mode imaging. This suggests that the proposed method can detect low-contrast thermal lesions induced by FUS therapy that are not detectable using standard B-mode imaging. Furthermore, the signal change at the focal point were more precisely measured by DC imaging, and the signal change in response to FUS exposure follows the temperature profile more closely than changes measured using B-mode, as well as synthetic aperture DAS and TC images. These suggest that DC imaging can potentially be used with the CBE method to improve non-invasive thermometry. [ABSTRACT FROM AUTHOR]

The Correlation-Based (CB) imaging method is characterized by its high spatial resolution capabilities, but it is known to require heavy computational resources due to its high complexity. This paper shows that the CB imaging method can be used to estimate the phase of the complex reflection coefficients contained in the observation window. The resulting Correlation-Based Phase Imaging (CBPI) method can be used to segment and identify different features or tissue elasticity variations in a given medium. A Numerical validation is first proposed by considering a set of fifteen point-like scatterers on a Verasonics Simulator. Then, three experimental datasets are used to show the potential of CBPI on scatterers and specular reflectors. In vitro imaging results are first presented to show that CBPI allows retrieving phase information on hyperechoic reflectors, but also on weak reflectors such as elasticity targets. It is demonstrated that CBPI helps distinguishing regions of different elasticity, but of same low-contrast echogenicity, which is otherwise impossible with standard B-mode or Synthetic Aperture Focusing Techniques (SAFT). Then, CBPI of a needle in an ex vivo chicken breast is performed to show that the method works on specular reflectors. It is shown that the phase of the different interfaces associated to the first wall of the needle are well reconstructed using CBPI. The heterogeneous architecture used to enable real-time CBPI is presented. A Nvidia GeForce RTX 2080 Ti Graphics Processing Unit (GPU) is used to process the real-time acquired signals from a Verasonics Vantage 128 research echograph. Frame rates of 18 frames per second are achieved for the whole acquisition and signal processing chain on standard a 500 × 200 pixels grid. • Correlation enables phase imaging through reflection coefficients reconstruction. • Real-time CBPI is achieved on the Verasonics system using GPU programming. • CBPI can be used to visualize low grayscale contrast targets. • CBPI can distinguish features of different elasticities. • CBPI can be used to track medical tools and strong reflectors. [ABSTRACT FROM AUTHOR]

• A generalized mathematical framework for performing contrast-enhanced ultrasound (CEUS) imaging is introduced. • Pulse inversion spectral deconvolution (PISD) is a software postprocessing approach that allows universal CEUS imaging functionality. • Preclinical results demonstrate PISD-based CEUS imaging improves vascular visualization compared to traditional nonlinear ultrasound imaging. A generalized mathematical framework for performing contrast-enhanced ultrasound (CEUS) imaging is introduced. Termed pulse inversion spectral deconvolution (PISD), this CEUS approach is founded on Gaussian derivative functions (GDFs). PISD pulses are used to form two inverted pulse sequences, which are then used to filter backscattered ultrasound (US) data for isolation of the nonlinear (NL) microbubble (MB) signal component. An US scanner equipped with a linear array transducer was used for data acquisition. With a vascular flow phantom perfused with MBs, data was collected using PISD and NL-based CEUS imaging. The role of wide-beam transmit aperture size (32 or 64 elements) was also evaluated using an US pulse frequency of 6.25 MHz. Image enhancement was quantified by a contrast-to-noise ratio (CNR). Preliminary in vivo data was collected in the hindlimb and kidney of healthy rats. Overall, in vitro wide-beam CEUS imaging using an aperture size of 64 elements yielded improved CNR values. Specifically, PISD-based CEUS imaging produced CNR values of 37.3 dB. For comparison, CNR values obtained using B-mode US or NL approaches were 2.1 and 12.1 dB, respectively. In vivo results demonstrated that PISD-based CEUS imaging improved vascular visualization compared to the NL imaging strategy. [ABSTRACT FROM AUTHOR]

• Aims to develop an economic volumetric transrectal ultrasound (TRUS) imaging. • Reduces scanning-angle disorientations (SAD) during a free-hand radial scanning. • A synthetic radial aperture focusing to reduce SAD-induced image distortion. • Might help adopting volumetric TRUS in resource and budget-limited settings. In this paper, we present a volumetric transrectal ultrasound (TRUS) imaging under the presence of radial scanning angle disorientation (SAD) in a resource-limited diagnostic setting. Herein, we test our hypothesis that a synthetic radial aperture focusing (TRUS-rSAF) technique, in which a radial plane in target volume is reconstructed by coherent compounding of multiple transmittance/reception events, will reject a randomized SAD in a free-hand scanning setup based on external angular tracking. Based on an analytical model of the TRUS-rSAF technique, we first tested specific scenarios using a clinically available TRUS transducer under different SADs in a range of normal distributions (σ = 0.1°, 0.2°, 0.5°, 1°, 2°, and 5°). We found a benefit of the TRUS-rSAF technique for higher robustness when the SAD is contained within the radial synthetic aperture window, i.e., ±0.71° from a target scanning angle. However, no enhancement was found in spatial resolution because of the limited transmit beam field of the clinical TRUS transducer, limiting the synthetic aperture window. We further evaluated the TRUS-rSAF technique with a modified TRUS transducer for an extended synthetic aperture window to test whether higher spatial resolution and robustness to SAD can be obtained in the same evaluation setup. Widening of the synthetic aperture window (±3.54°, ± 5.91°, ± 8.27°, ± 10.63°, ± 12.99°, ± 15.35°) resulted in proportional enhancements of spatial resolution, but it also progressively built up sidelobe artifacts due to randomized synthesis with limited phase cancellations. The results suggest the need for careful calibration of the TRUS-rSAF technique to enable TRUS imaging with free-hand radial scanning and external angle tracking in resource-limited settings. [ABSTRACT FROM AUTHOR]

Echo imaging in ultrasound computed tomography (USCT) using the synthetic aperture technique is performed with the assumption that the speed of sound is constant in the system. However, tissue heterogeneity causes a mismatch between the predicted arrival time and the actual arrival time of the echo signal, which will result in phase aberration, leading to the quality degradation of the reconstructed B-mode image. The conventional correction methods that use the correlation of each different channel require the presence of strong point scatterers and involve the problem of local solutions due to excessive correction. In this study, we propose a novel approach to correcting the signal distortion due to sound speed heterogeneity using a deep neural network (DNN). The DNN was trained to convert the distorted radio frequency (RF) inputs for the heterogeneous medium to the distortion-free RF outputs for the homogeneous medium. The network with U-net architecture using ResNet-34 as a backbone was trained using the hetero–homo corresponding channel-domain RF data generated via numerical simulations. The trained network performed phase aberration correction in the channel-domain RF, with the B-mode images reconstructed with the corrected RF demonstrating a higher contrast and an improved resolution compared with uncorrected cases. It was also demonstrated that the DNN model is robust to both varied reflection intensities and varied sound speed heterogeneities. The successful results demonstrated that the proposed DNN-based method is effective for phase aberration correction in US imaging. • Deep neural network (DNN) for phase aberration correction in ultrasound imaging is proposed. • DNN was trained as to convert heterogeneous medium RF to homogeneous medium RF. • Image quality of B-mode images were imporved by the trained RF correction model. [ABSTRACT FROM AUTHOR]

Decreased thickness of the bone cortex due to bone loss in the course of ageing and osteoporosis is associated with reduced bone strength. Cortical thickness measurement from ultrasound images was recently demonstrated in young adults. This requires the identification of both the outer (periosteum) and inner (endosteum) surfaces of the bone cortex. However, with bone loss, the cortical porosity and the size of the vascular pores increase resulting in enhanced ultrasound scattering which may prevent the detection of the endosteum. The aim of this work was to study the influence of cortical bone microstructure variables, such as porosity and pore size, on the contrast of the endosteum in ultrasound images. We wanted to estimate the range of these variables for which ultrasound imaging of the endosteum is feasible. We generated synthetic data using a two-dimensional time-domain code to simulate the propagation of elastodynamic waves. A synthetic aperture imaging sequence with an array transducer operating at a center frequency of 2.5 MHz was used. The numerical simulations were conducted for 105 cortical microstructures obtained from high resolution X-ray computed tomography images of ex vivo bone samples with a porosity ranging from 2% to 24 %. Images were reconstructed using a delay-and-sum (DAS) algorithm with optimized f-number, correction of refraction at the periosteum, and sample-specific wave-speed. We observed a range variation of 18 dB of endosteum contrast in our data set depending on the bone microstructure. We found that as porosity increases, speckle intensity inside the bone cortex increases whereas the intensity of the signal from the endosteum decreases. Also, a microstructure with large pores (diameter > 250 μ m) was associated with poor endosteum visibility, compared with a microstructure with equal porosity but a more narrow distribution of pore sizes. These findings suggest that ultrasound imaging of the bone cortex with a probe operating at a central frequency of 2.5 MHz using refraction-corrected DAS is capable of detecting the endosteum of a cortex with moderate porosity (less than about 10%) if the largest pores remain smaller than about 200 μ m. • Cortical thickness estimation from ultrasound image requires endosteum recognition. • Scattering by large pores may prevent endosteum imaging in osteoporotic bones. • The influence of bone microstructure on the visibility of the endosteum is studied. • A refraction-corrected delay-and-sum algorithm with optimized f-number is used. • At 2.5 MHz the endosteum is detected for low porosity and small pore diameter. [ABSTRACT FROM AUTHOR]

• Two novel Beamformers called ST-DMAS and ST-D ew MAS are reported. • ST-DMAS and ST-D ew MAS were evaluated on Sparse STA with just 8 transmits. • This novelity yields better resolution and contrast compared to 128 transmits. • ST-DMAS and ST-D ew MAS can enhance both frame rate and image quality simultaneously. • ST-DMAS and ST-D ew MAS can enhance frame rate and reduce memory needed by 16 times. The development of two modified non-linear beamformers, Spatio-Temporal Delay Multiply and Sum (ST-DMAS) and Spatio-Temporal Delay Euclidian-Weighted Multiply and Sum (ST-D ew MAS) is reported in this paper. A sparse-transmit scheme (with only 8 transmits) on Synthetic Transmit Aperture technique (sparse STA) was chosen to evaluate the beamformers ability to generate the high-resolution Ultrasound image. These methods allow for obtaining superior-quality imaging at enhanced frame rates. The different beamformers of ST-D ew MAS, ST-DMAS, Filtered Delay Multiply and Sum (F-DMAS), and Delay and Sum (DAS), were compared in terms of the Axial and Lateral Resolutions, AR and LR, respectively, Contrast-to-Noise Ratio (CNR), Contrast Ratio (CR), and Generalized CNR (GCNR). Experimental results demonstrate that the developed ST-DMAS and ST-D ew MAS reconstruction on sparse STA technique resulted in better quality images compared to those obtained using DAS and F-DMAS. Specifically, the metrics of AR, LR CR, CNR, and GCNR showed improvements of more than 25% (for ST-DMAS) and 40 % (for ST-D ew MAS) over those from DAS and F-DMAS beamformed images, respectively. Thus, the results demonstrate that the frame rate and image quality of an US system can both be enhanced by ST-D ew MAS compared to the beamformers of F-DMAS and DAS. [ABSTRACT FROM AUTHOR]

Ultrasonic linear arrays have great potential to generate high-quality three-dimensional (3D) images by scanning the array. However, the generated images suffer from low resolution in the elevation plane, limiting the image quality for a reliable 3D Non-Destructive Testing (NDT) inspection. Although several ultrasonic imaging methods have been implemented to inspect different types of defects, there has been limited research to characterise surface-breaking cracks (SBCs) in 3D quantitatively. To improve the characterisation of surface-breaking cracks (SBCs), a 3D hybrid imaging method is proposed by combining the Half Skip Total Focusing Method (HSTFM) and the Synthetic Aperture Focusing Technique (SAFT) using a linear array. This paper proposed the implementation of an array with a reduced element length for full matrix capture (FMC) data acquisition. In conjunction with the hybrid imaging method, a reduced element array enables the utilisation of the information from a broad ultrasonic beam in the elevation direction to achieve improved image resolution. The imaging capability is assessed via a point spread function (PSF) as well as numerical simulations. From the PSF measurements, the image resolution is shown to improve with the smaller element length of the array, which is attributable to the combination of wide beamwidth and hybrid imaging method. Thereafter, experimental validation was performed with arrays of different elevation lengths, where an excellent match with the numerical results was observed. Furthermore, the crack sizing was performed using a 6-dB-drop rule, which assisted in accurately predicting the shape and size of the SBCs and is shown to measure the depth of SBCs with greater confidence. It is shown that a reduced array elevation with the hybrid imaging method and sizing method yields improved image resolution contrary to conventional linear arrays. This approach can offer a significant improvement in manifesting complete comprehension of the spatial defect relationship, enabling NDT engineers to analyse the inspection results quantitatively in 3D for progressive reliability. • 3D ultrasonic imaging of surface-breaking cracks. • Improved image capability with a hybrid technique combining Half Skip Total Focusing Method (HSTFM) and Synthetic Aperture Focusing Technique (SAFT). • Image resolution improved with reduced array elevation. [ABSTRACT FROM AUTHOR]

• The overall process of ultrasonics methodology of T-shaped R-zone components is proposed and verified by the theoretical and experimental results in this paper. • The defect measurement is realized by linear phased array probe and synthetic aperture total focusing method. • The stress measurement is realized by straight probe, K1 angle probe, acoustoelastic effect and Mohr circle stress theory. The T-shaped R-zone component widely exists in aerospace and satellite communication area, such as supporting structure under the wing skin and the mechanical supporting frame of the satellite large load-bearing high-precision test platform, which seriously affects the performance and the safety of the aircraft and the satellite. In this paper, the overall process of ultrasonics methodology of T-shaped R-zone components is proposed, systematically investigated and experimentally verified. The detection of the defect and the stress is respectively realized based on the synthetic aperture total focusing method and the Mohr's stress circle theory with the aid of the linear phased array probe and the longitudinal wave along the stress propagation direction. The theoretical simulations and the experimental measurements both demonstrate the effectiveness of the proposed methodology. The dimensional and the positioning errors are respectively around 0.67 mm and 1.69 mm for the 1 mm × 4 mm crack defect existed in the T-shaped R-zone component. The detecting error of the stress is less than 17.91 MPa in the entire inner of the T-shaped R-zone component, which is applied a pressure in the range of 0–400 MPa. [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]

Synthetic aperture (SA) ultrasound imaging can obtain images with high-resolution owing to its ability to dynamically focus in both directions. The signal-to-noise ratio (SNR) of SA imaging is poor because the pulse energy using one array element is quite low. Thus, the SA method with bidirectional pixel-based focusing (SA-BiPBF) was previously proposed as a solution to this challenge. However, using the nonadaptive delay-and-sum (DAS) beamforming still limits its imaging performance. This study proposes an adaptive scaled coherence factor (AscCF) for SA-BiPBF to further boost the image quality. The AscCF exploits generalized coherence factor (GCF) to measure the signal coherence to adaptively adapt the parameters in SNR estimation rather than fixed ones. Comparisons were made with several other weighting techniques by performing simulations and experiments for performance evaluation. Results confirm that AscCF applied to SA-BiPBF offers a good image contrast while reservation of the speckle pattern. AscCF achieves maximal improvements of contrast ratio (CR) by 48.5% and 47.76 % compared with scaled coherence factor (scCF), respectively in simulation and experiment. Simultaneously, the maximum of improvements in speckle signal-to-noise ratio (sSNR) of AscCF are 11.28 % and 20.01 % upon scCF in simulation and experiment, respectively. From the in vivo result, it also appears a potential for AscCF to act in clinical situations to better detect lesion and retain speckle pattern. • The weighting methods are introduced into synthetic aperture with bidirectional pixel-based focusing (SA-BiPBF) to enhance the image quality. • In this work, based on conventional scaled coherence factor (scCF), a new adaptive scaled coherence factor (AscCF) is proposed as a weighting factor for SA-BiPBF strategy. • The proposed method can further reduce noise and reject clutter, which can exhibit its abilities in improving the image contrast while preserving the speckle pattern. [ABSTRACT FROM AUTHOR]

Pixel-based beamforming generates focused data by assuming that the waveforms received on a linear transducer array are composed of spherical pulses. It does not take into account the spatiotemporal spread in the data from the length of the excitation pulse or from the transfer functions of the transducer elements. As a result, these beamformers primarily have impacts on lateral, rather than axial, resolution. This paper proposes an efficient method to improve the axial resolution for pixel-based beamforming. We extend our field pattern analysis and show that the received waveforms should be passed through a Wiener filter before being used in the coherent pixel-based beamformer. This filter is designed based on signals echoed from a single scatterer at the transmit focus. The beamformer output is then combined with a coherence factor , that is adaptive to the signal-to-noise ratio, to improve the image contrast and suppress artifacts that have arisen during the filtering process. We validate the proposed method and compare it with other beamforming strategies using a series of experiments, including simulation, phantom and in vivo studies. It is shown to offer significant improvements in axial resolution and contrast over coherent pixel-based beamforming, as well as other spatial filters derived from synthetic aperture imaging. The method also demonstrates robustness to modeling errors in the experimental data. Overall, the imaging results show that the proposed approach has the potential to be of value in clinical applications. • A novel pixel-based beamformer with Wiener filter to improve ultrasound imaging. • Design the Wiener filter with signals at the transmit focus to reduce computation. • The Wiener filter efficiently enhances the axial resolution of ultrasound images. • SNR-dependent coherence factor further boosts the contrast and suppresses artifacts. [ABSTRACT FROM AUTHOR]

• In-phase/quadrature (IQ) data were used to accelerate CS-STA. • IQ-domain CS-STA maintains high image quality of conventional RF-domain CS-STA. • CS-STA obtains similar resolution, higher contrast and higher frame rate, compared with STA. • More compatible with clinical ultrasound system than RF-domain CS-STA. Compressed sensing-based synthetic transmit aperture (CS-STA) was previously proposed to recover the full radio-frequency (RF) channel dataset of synthetic transmit aperture (STA) from that of a smaller number of randomly apodized plane wave (PW) transmissions. In this way, the imaging frame rate (FR) and contrast are improved with maintained spatial resolution, compared with those of STA. Because CS-STA reconstruction is repeated for all receive elements and RF samples (with a high sampling frequency), the recovery of STA dataset in RF domain is time-consuming. In the meantime, a large amount of RF data needs to be transferred and stored, resulting in an increase of system complexity and required memory space. In this study, CS-STA is extended to in-phase/quadrature (IQ) domain (with lower sampling frequency) for the recovery of baseband STA IQ dataset to accelerate the CS-STA reconstruction by reducing the amount of data to be processed. More importantly, CS-STA reconstruction using IQ data is of practical importance, as clinical ultrasound systems typically record baseband IQ signal instead of RF signal. Simulations, phantom and in vivo experiments verify the feasibility of CS-STA in IQ domain for the recovery of STA dataset. More specifically, CS-STA using IQ data achieves similar image quality and appreciably improves reconstruction speed (by ∼3 times) compared with that using RF data. These findings demonstrate that IQ-domain CS-STA is capable of relieving the computational and storage burdens, which may facilitate the implementation of CS-STA in practical ultrasound systems. [ABSTRACT FROM AUTHOR]

• A virtual transducer model is built for the focused transducer. • The non-uniform fast Fourier transform and deconvolution operations have remarkably improved the efficiency and accuracy. • The lateral resolution of the results can exceed the resolution limitation of the focused transducer. • It shows the potential for real time 3-D imaging. In ultrasonic non-destructive testing, image reconstruction is essential to restore the diffracted ultrasound signals to improve the lateral resolution of images. Some reconstruction methods, like DAS-based synthetic aperture imaging, are inefficient, especially for reconstructing three-dimensional (3-D) images. Other methods do not provide high-resolution results, because they neglect the distortion effect introduced by transducer geometry. To overcome these disadvantages, we propose a 3-D ultrasonic image reconstruction method based on synthetic aperture wavenumber algorithm. It considers wave diffraction and transducer geometry effects, and can refocus the reflectors even in non-focal zone, which suits for large depth range imaging. This method builds a virtual transducer model in frequency domain by treating the focused transducer as a virtual planar transducer on its focal plane. In addition, the method uses non-uniform fast Fourier transform and deconvolution operation to achieve the 3-D image reconstruction, which has remarkably improved the efficiency and accuracy. According to the experimental results, the lateral resolution of an image reconstructed by the proposed method can reach 290.2 μm, exceeding the lateral resolution limitation of the 15 MHz focused transducer (523.24 μm). Furthermore, the proposed method only takes 0.744 s to reconstruct a 3-D image with 1000 × 100 × 100 pixels, while the time domain SAFT takes about 1163.8 s. It shows the potential for real-time 3-D imaging under advanced hardware. [ABSTRACT FROM AUTHOR]

• An acoustic-resolution photoacoustic microscopy based on a compact and low-cost delta configuration actuator is introduced. • The effect of the data acquisition positioning inaccuracy on the image quality degradation is studied. • Advanced 3D synthetic aperture focusing methods mitigate the image quality degradation caused by actuation uncertainty. The state-of-the-art configurations for acoustic-resolution photoacoustic (PA) microscope (AR-PAM) are large in size and expensive, hindering their democratization. While previous research on AR-PAMs introduced a low-cost light source to reduce the cost, few studies have investigated the possibility of optimizing the sensor actuation, particularly for the AR-PAM. Additionally, there is an unmet need to evaluate the image quality deterioration associated with the actuation inaccuracy. A low-cost actuation device is introduced to reduce the system size and cost of the AR-PAM while maintaining the image quality by implementing the advanced beamformers. This work proposes an AR-RAM incorporating the delta configuration actuator adaptable from a low-cost off-the-shelf 3D printer as the sensor actuation device. The image degradation due to the data acquisition positioning inaccuracy is evaluated in the simulation. We further assess the mitigation of potential actuation precision uncertainty through advanced 3D synthetic aperture focusing algorithms represented by the Delay-and-Sum (DAS) with Coherence Factor (DAS+CF) and Delay-Multiply-and-Sum (DMAS) algorithms. The simulation study demonstrated the tolerance of image quality on actuation inaccuracy and the effect of compensating the actuator motion precision error through advanced reconstruction algorithms. With those algorithms, the image quality degradation was suppressed to within 25% with the presence of 0.2 mm motion inaccuracy. The experimental evaluation using phantoms and an ex-vivo sample presented the applicability of low-cost delta configuration actuators for AR-PAMs. The measured full width at half maximum of the 0.2 mm diameter pencil-lead phantom were 0.45 ± 0.06 mm, 0.31 ± 0.04 mm, and 0.35 ± 0.07 mm, by applying the DAS, DAS+CF, and DMAS algorithms, respectively. AR-PAMs with a compact and low-cost delta configuration provide high-quality PA imaging with better accessibility for biomedical applications. The research evaluated the image degradation contributed by the actuation inaccuracy and suggested that the advanced beamformers are capable of suppressing the actuation inaccuracy. [ABSTRACT FROM AUTHOR]

• Combination of tensor completion and STA is proposed to improve the frame rate. • In the proposed method, only a limited number of elements are excited for emission. • Tensor completion is used for optimizing image quality. • By using this technique, the STA runtime is reduced significantly. • Our results confirm a close image quality of the proposed method to the original STA. One of the most important methods in medical ultrasound imaging is the synthetic transmit aperture (STA). Despite the image quality improvement in the STA, this method suffers from several limitations, including a limited data acquisition rate and an increase in the overall time to form a single frame. Tensor completion (TC) is a powerful technique that uses rank minimization to recover missing information from a low-rank tensor. This paper provides a novel random synthetic transmit aperture (RSTA) method based on using only a randomly selected part (a fraction) of the linear array elements in the transmit mode to increase the data acquisition rate and then applying the tensor completion (TC) to improve the image quality. By the proposed method, as it is not necessary to transmit all elements sequentially, the data acquisition rate is improved and the overall time for creating an image is also significantly reduced. We investigated the proposed idea by using several simulated and experimental phantoms. Results showed that the proposed method could increase the data acquisition rate up to three times with the image quality difference of less than 6% compared to the original STA method. [ABSTRACT FROM AUTHOR]