Phase Coherence Factor Research Articles

Lamb wave phased array imaging based on phase-amplitude compounding algorithm

Aimed at improving the imaging performance of Lamb wave phased array based on Delay-and-Sum (DAS) of signal amplitude, a compounding imaging algorithm integrating amplitude and phase information of signal is proposed in this paper. By applying back propagation dispersion compensation, the pulse echoes caused by defects will be in-phase with the excitation signal, implementing signal focusing. In the proposed imaging algorithm, the amplitude imaging index is calculated by classical total focusing method (TFM) and the phase imaging index is calculated by phase coherence factor (PCF), which establishes the nonlinear mapping between imaging index and phase difference by sigmoid function with variable gradient. The final imaging results are the compound of two imaging indexes. The experimental validation is carried out on plates of different materials and dimensions with multiple types of defects based on uniform linear array. Compared with the imaging results of conventional TFM, the proposed imaging algorithm can effectively improve the signal-to-noise ratio (SNR) of the images and suppress artifacts, implementing threshold-free imaging.

Virtual source total focusing method for crack detection in complex curved structure

Accurate non-destructive testing and evaluation of cracks is important for the quality assurance and life-cycle management of key components in aerospace and energy industries. This paper presents a novel imaging method for cracks detection in complex curved structure , combining virtual source technique, multi-mode total focusing method and phase coherence imaging method. The virtual source on the sample surface is employed as the emitter and receiver for the ultrasonic array imaging. In this way, the calculation complexity of imaging algorithm due to beam path search can be reduced, and the detection sensitivity in the region of interest can be improved. In addition, the phase coherence factor is used to suppress the background noise and imaging artifacts, which significantly improves the quality of ultrasonic array imaging. A complex curved sample with variable thickness and non-developable surface was used to examine the effectiveness of the virtual source total focusing method. The signal-to-noise ratio of the crack defect is increased by more than 25 dB using the novel method compared with that of the conventional total focusing method, and the quantitative evaluation error of defect is less than 2.67%.

An adaptive beamformer based on dynamic phase coherence factor for pixel-based medical ultrasound imaging.

Pixel-based beamforming realizes dynamic focusing at the pixel level with a focused beam by assuming that the received signals are composed of spherical pulses. Far-focused pixel-based (FPB) imaging was proposed to avoid artifacts around the focal depth. However, the contrast improvement is limited. We propose an adaptive weighting method based on dynamic phase coherence factor (DPCF) to improve the image contrast while preserving the speckle pattern. The phase variation is dynamically estimated based on the noise energy proportion of echo signals and it is used to calculate phase coherence weights for suppressing interference and preserving desired signals. A depth-dependent parameter is designed for DPCF to enhance the performance of noise and clutter suppression in the far-field region. We further use the subarray averaging technique to smooth the speckle texture. The proposed method was evaluated on simulated, phantom experimental, and in vivo data. Results show that, compared with the phase coherence factor (PCF) based method, DPCF respectively leads to average CR improvements by more than 60% and 24% in simulation and experiment, while obtaining an improved speckle signal-to-noise ratio. The proposed method is a potentially valuable approach to obtaining high-quality ultrasound images in clinical applications.

Improving lateral resolution and contrast by combining coherent plane-wave compounding with adaptive weighting for medical ultrasound imaging

Due to the severe lateral lobe artifact by coherent plane-wave compounding (CPWC) and the low signal-to-noise ratio of radiofrequency (RF) data collected from the plane wave, the adaptive beamforming methods based on focused wave imaging (FWI) are improper to be directly applied to CPWC. To obtain a high-quality image with high resolution and contrast, this study combined the threshold phase coherence factor (THR-PCF) with the reconstructed covariance matrix minimum variance (RCM-MV) and then proposed a novel CPWC-based adaptive beamforming algorithm, THR-PCF + RCM-MV. The simulation, phantom, and in-vivo experiments were performed to investigate the performance of the proposed methods in comparison with the CPWC and the classical adaptive methods including the minimum variance (MV), generalized coherence factor (GCF) and their combination GCF + MV. The simulation results demonstrated that the THR-PCF + RCM-MV beamformer improved contrast ratio (CR) by 28.14%, contrast noise ratio (CNR) by 22.01%, speckle signal-to-noise ratio (s_SNR) by 23.58%, generalized contrast-to-noise ratio (GCNR) by 0.3%, and the full width at half maximum (FWHM) by 43.38% on average, compared with the GCF + MV method. The phantom experimental results showed a better performance of the THR-PCF + RCM-MV beamformer with an average improvement by 21.95% in CR, 2.62% in s_SNR, and 48.64% in FWHM compared with the GCF + MV. Meanwhile, the results showed that the image quality of the near and far fields was enhanced by the THR-PCF + RCM-MV. The in-vivo imaging results showed that our new method had potential for clinical application. In conclusion, the lateral resolution and contrast of medical ultrasound imaging could be improved greatly with our proposed method.

Improving axial resolution based on the deconvolution recovery method combined with adaptive weighting techniques for ultrasound imaging.

A fundamental challenge in medical ultrasound imaging is to improve the resolution accurately. Adaptive beamforming is often used to improve lateral resolution, such as minimum variance (MV) and phase coherence factor (PCF). However, it is difficult to improve the axial resolution due to the limitation of the spatial pulse length (SPL) of the transmitted signal. A deconvolution recovery method combines two adaptive weighting techniques to improve axial resolution. A deconvolution recovery (DR) technique is used to improve axial resolution with a shorter SPL. Then, the DR is combined with MV and PCF (DR-MVPCF) to suppress the sidelobe. The influence of different transmission modes, regularization parameters, and the estimation of point spread function are discussed on the proposed algorithm. In simulation, DR-MVPCF improved axial resolution from 0.41 mm (0.98 λ) to 0.09 mm (0.21 λ) compared with MV-PCF. In the water bath experiment, DR-MVPCF provided improvement of axial resolution from 0.39 mm (0.93 λ) to 0.07 mm (0.17 λ) compared with MV-PCF. In-vivo data experiment, the DR-MVPCF method increased the speckle signal-to-noise ratio and visibility of the structure while the contrast ratio and contrast-noise ratio decreased. The proposed method can improve the axial resolution significantly.

Beamforming of 3D ultrasound signals with full or sparse probes: New perspectives for volumetric imaging

The advances of 2D arrays and 3D ultrasound systems open new perspectives for volumetric US imaging. Coupled with the ultrafast acquisitions, the quantity of acquired data to process expand and the beamforming could become a bottleneck in the processing, especially if volumes are reconstructed in real time. In this work, we propose to study the impact of the beamformer on 3D volumetric images, such as coherence factor, sign coherence factor, phase coherence factor or pDAS. Indeed, such beamformers, adaptive or not, can be easily transfer to 3D but their studies are limited in 3D volumetric situation. Moreover, the introduction of fully populated array required the used of advance and expensive systems. To this end, the utilisation of sparse probes is a promising solution to reduce the number of signals to manage and the complexity of the used hardware. However, the impact on the reconstructed volumes have to be evaluated and the presented beamforming strategies will be presented in both fully and sparse probes.

Circular statistics vector for improving coherent plane wave compounding image in Fourier domain

In this work, a circular statistics vector (CSV) weighting Fourier domain (FD) beamforming for ultrasound plane-wave images was proposed to achieve better image quality with a high frame rate. Firstly, the cosine and sine components of the instantaneous phase are extracted from undelayed RF signals. Secondly, the FD beamformed cosine and sine components are used to establish the CSV. Finally, the FD beamformed amplitude image is weighted by the CSV. The resolution, contrast, and computation complexity were used to assess the performance of the proposed method. The results revealed that FD_CSV could significantly reduce the computational load compared to the conventional DAS_CSV on the equal improvement of image quality. Besides, compared to coherence factor (CF), phase coherence factor (PCF), etc., based on variance calculation, the CSV based on mean resultant vector calculation can effectively preserve the speckle due to the more tolerant to phase errors. The proposed FD_CSV weighting method has successfully conducted high image quality and low computational load.

Weighted Full-Focus Defect Detection and Imaging Method Based on Threshold Fusion for Phase Coherence Factor

Total focus (TFM) ultrasonic inspection imaging only uses the amplitude of defect data for delayed stack imaging (DAS), ignoring the phase information in the echo signal, which easily leads to low imaging resolution and interferes with subsequent quantitative analysis of defect size. To solve these problems, this paper proposes a phase coherence factor (TF-PCF) weighted full-focus imaging method based on threshold fusion: by combining the image sound intensity matrix with mathematical statistics, a reasonable threshold is set to screen out the effective pixels, and the parameter selection rules of the PCI algorithm model are redefined. The simulation and experimental results show that the TF-PCF imaging algorithm can effectively eliminate the noise information in the imaging area, including the artifacts near the defects that are difficult to be processed by PCI. Compared with PCI, TF-PCF has higher ability to quantify defect size. When using p-wave to detect large objects with a large number of defects and complex location distribution, the lateral size quantization error of the TF-PCF imaging algorithm is reduced by 8.5%. When sampling shear wave imaging to detect defects with tilted wedges, the lateral and longitudinal size quantization errors of defects are reduced by 10% and 20%, respectively. The reliability of defect size quantitative analysis is improved. Under the same test conditions, the complexity of TF-PCF model is far less than that of PCI model, which greatly improves the practicability of the algorithm model.

Microwave 3D Imaging System Featuring the Phase Coherence Factor for Improved Beamforming.

This paper presents an improved radar-based imaging system for breast cancer detection that features p-slot ultrawideband antennae in a 32-array set-up. The improved reconstruction algorithm incorporates the phase coherence factor (PCF) into the conventional delay and sum (DAS) beamforming algorithm, thus effectively suppressing noise arising from the side- and gratinglobe interferences. The system is tested by using several breast models fabricated from chemical mixtures formulated on the basis of realistic human tissues. Each model is placed in a hemispherical breast radome that was fabricated from polylactide material and surrounded by 32 p-slot antennae mounted in four concentric layers. These antennae are connected to an 8.5 GHz vector network analyser through two 16-channel multiplexers that automatically switch different combinations of transmitter and receiver pairs in a sequential manner. The system can accurately detect 5 mm tumours in a complex and homogeneously dense 3D breast model with an average signal-to-clutter ratio and full-width half-maximum of 7.0 dB and 2.3 mm, respectively. These values are more competitive than the values of other beamforming algorithms, even with contrasts as low as 1:2. The proposed PCF-weighted DAS is the best-performing algorithm amongst the tested beamforming techniques. This research paves the way for a clinical trial involving human subjects. Our laboratory is planning such a trial as part of future work.

Comparative investigation of coherence factor weighting methods for an annular array photoacoustic microscope

To achieve fine visualization of the peripheral microvascular networks, we have developed a photoacoustic (PA) microscope equipped with a four-channel annular array transducer. The quality of PA images processed with delay-and-sum (DAS) method is degraded by off-axis signals. Thus, to achieve higher image quality for the PA microscope, this study evaluated the efficacy of the five coherence factor weighting methods: coherence factor, sign coherence factor, phase coherence factor, circular coherence factor, and vector coherence factor. Using PA signals acquired from a 100 μm microtube and the skin microvessels, we generated PA images with DAS and one of the weighting methods, and quantitatively evaluated the image quality by calculating the sharpness, contrast ratio, and contrast-to-noise ratio. The results showed the phase coherence factor and the vector coherence factor methods were more effective to clearly visualize the microvascular structure, in terms of vessel sharpening and noise suppression performances, than the other methods.

Detection of Point Scatterers in Medical Ultrasound.

We present an overview of the detection of point scatterers in ultrasound images and suggest strategies for evaluating and measuring the detection performance. We use synthetic aperture Field II simulations of a point scatterer in speckle background and evaluate how common imaging techniques affect point target detectability. We discuss how to compare different methods and calculate confidence intervals. In general, applying speckle reduction methods reduces the point detection performance. However, the results show that it is possible to smooth the speckle background and preserve relatively high performance with a suitable and optimized method. The different detection performances of the advanced beamforming methods coherence factor (CF), phase coherence factor (PCF), and Capon's minimum variance (MV) are presented and benchmarked with standard delay-and-sum (DAS). The results show that CF achieves slightly better detection performance than DAS for weak point scatterers, whereas PCF and MV perform worse than DAS. Choice of apodization window and adaptive aperture size affects the probability of detection. Results show that methods that preserve spatial resolution have better detection performance of point scatterers.

Multi-defect detection based on ultrasonic Lamb wave sign phase coherence factor imaging method

The ultrasonic Lamb wave total focusing method (TFM) only uses the amplitude of the defective scattered signal for virtual focused imaging, while ignoring the phase information of the scattered signal and the dispersion characteristics of the Lamb wave, resulting in low imaging resolution and easily produced artefacts in imaging. To solve this problem, an ultrasonic Lamb wave imaging method based on phase coherence is proposed in this paper and the sign coherence factor (SCF) in the full matrix scattering signal is extracted. Moreover, the method uses the SCF to weight the amplitude of the full matrix scattering signal, suppresses the side lobes of the defect echo signal and the Lamb wave dispersion effect, improves the ultrasonic Lamb wave imaging resolution and weakens the artefacts. Finally, single- and multiplehole defects in aluminium plates are detected for experimental validation using an ultrasonic phased array. The array performance indicator and signal-to-noise ratio are used as indicators for quantitative assessment of imaging performance. The results show that compared with the TFM imaging, the SCF imaging can effectively suppress the noise and scattered signal side lobes, improve the array performance indicator (API) by 69.1% and improve the signal-to-noise ratio (SNR) by 73.9%. In addition, the SCF imaging can effectively weaken the interference of scattered signals between multiple through-hole defects, resulting in fewer artefacts in imaging.

Coherence Factor-Like Beamforming for Ultrasound Computed Tomography

Ultrasound computed tomography (USCT) has important clinical application prospect in breast cancer screening and early diagnosis. In this paper, six kinds of coherence factor-like beamforming methods have been applied to improve the image quality for USCT, including coherence factor (CF), phase coherence factor (PCF), sign coherence factor (SCF), phasor dispersion based coherence factor (PDCF), spatial smoothed coherence factor (SSCF) and spatio-temporally smoothed coherence factor (STSCF). The mentioned methods were verified with the radio-frequency (RF) data of the breast phantom captured by the USCT system developed in the Medical Ultrasound Laboratory. The ring-type transducer has 1024 elements with a center frequency of 2.5 MHz. Experimental results show that the reconstructed images of the breast phantom by the CF gets the highest contrast to noise ratio (CNR), but overall image brightness reduces significantly. PCF gets the lowest variance and provides a more homogenous background. STSCF beamforming method can improve the robustness of the PCF and having the ability to suppress clutter while significant removal of black region artifacts. For practical application, these coherence factor-like beamforming methods can be implemented with low computational complexity.

Instantaneous Phase Coherence Imaging for Near-Field Defects by Ultrasonic Phased Array Inspection.

This paper describes an imaging method for near-field defect detection in aluminum plates based on Green’s function recovery and application of instantaneous phase coherence weighting factors. The directly acquired acoustic information of near-field defects is usually obscured by the nonlinear effects due to the physical limitation of the acquisition system. Using the diffuse field to recover the Green’s function can effectively retrieve the early time information. However, averaging operations of finite number in this process produces an imperfect imaging result. In order to improve the image quality, two kinds of instantaneous phased coherence weighting factors are used to weight the Green’s function to reduce the background noise and improve the signal-to-noise ratio: the instantaneous phase coherence factor (IPCF), and the instantaneous phase weighting factor (IPWF). Experiments are conducted on two aluminum plates with two and four near-field defects, respectively. As a result, the background noise of amplitude images weighted by IPCF and IPWF is less than that of the conventional total focusing method (TFM). In addition, the IPCF image achieves a better signal-to-noise ratio (SNR) than that of IPWF, and the phase discontinuity in an IPWF image is suppressed through the IPCF.

Ultrasonic Phased Array Sparse TFM Imaging Based on Virtual Source and Phase Coherent Weighting

Total focus method (TFM) is an ultrasonic phased array imaging method and has a larger dynamic focusing range and higher spatial resolution than traditional imaging methods. But TFM needs to process a large amount of data, resulting in a long imaging time, which greatly limits its application and development in the industrial field. The sparse method can improve the computing efficiency by reducing the amount of data, but the resolution and intensity of sparse image are very low. This paper proposes a virtual source sparse TFM (VSSTFM) method based on phase coherent weighting (PCW) to improve the spatial resolution and intensity of the sparse TFM imaging. The layout of virtual sources in the virtual source array is designed and optimized by genetic algorithm. According to the phase distribution characteristics of the ultrasonic testing signal, a phase coherence factor is constructed to weight the VSSTFM image to further improve the image quality. Experimental results show that compared with conventional TFM, the proposed method can improve the imaging efficiency by 66.56 % while providing significantly higher image quality than TFM imaging.

Particle‐filter‐based human target tracking in image domain for through‐wall imaging radar

This study deals with a tracking problem for hidden human targets using time-division multiple-input-multiple-output through-wall imaging radar (TWIR). An efficient image-domain tracking algorithm is proposed. Specifically, the authors first utilise back-projection algorithm and the phase coherence factor (PCF) to obtain multi-frame high quality images. Then a tracking algorithm via amplitude-distribution-based particle filter is proposed. Experimental data validates that this algorithm has a commendable effectiveness for hidden target tracking.

Mathematical analysis of intensity distribution of the optical image in various degrees of coherence of illumination (representation of intensity by Hermitian matrices)

This is a historical translation of the seminal paper by H. Gamo, originally published in Oyo Buturi ( Applied Physics , a journal of The Japan Society of Applied Physics) Vol. 25, pp. 431–443, 1956. English translation by Kenji Yamazoe, with further editing by the translator and Anthony Yen. Since optical systems have distinctive features as compared to electrical communication systems, some formulation should be prepared for the optical image in order to use it in information theory of optical systems. In this paper the following formula for the intensity distribution of the image by an optical system having a given aperture constant α in the absence of both aberration and defect in focusing is obtained by considering the nature of illumination, namely coherent, partially coherent, and incoherent: I ( y ) = ∑ n ∑ m a n m u n ( y ) u m * ( y ) , where u n(y) = sin 2πα/λ (y − nλ/2α) / 2πα/λ (y − nλ/2α) and a nm = (2α/λ)2 ∬ Γ12( x 1 − x 2) E( x 1) E * ( x 2) | A( x 1) || A * ( x 2) | u n( x 1) u m( x 2)d x 1 d x 2. I ( y ) is the intensity of the image at a point of coordinate y , Γ12 the phase coherence factor introduced by H. H. Hopkins et al., E ( x ) the complex transmission coefficient of the object and A ( x ) the complex amplitude of the incident waves at the object, and the integration is taken over the object plane. The above expression has some interesting features, namely the “intensity matrix” composed of the element a nm mentioned above is a positive-definite Hermitian matrix, and the diagonal elements are given by the intensities sampled at every point of the image plane separated by a distance λ / 2α, and the trace of the matrix or the sum of diagonal elements is equal to the total intensity integrated over the image plane. Since a Hermitian matrix can be reduced to diagonal form by a unitary transformation, the intensity distribution of the image can be expressed as I ( y ) = λ 1 | ∑ S <mml:mrow

Range Coherence Factor for Down Range Sidelobes Suppression in Radar Imaging Through Multilayered Dielectric Media

In radar imaging applications through layered dielectric media, an antenna array with a limited operating frequency bandwidth at the low end of the microwave band is usually employed to probe the target space of interests. As a result, the focused radar imagery is characterized by severe sidelobes in the down-range direction, which leads to the degradation of the image quality. Several techniques, such as windowing and apodization filtering combined with phase coherence factor (PCF) filtering, have been reported to suppress the down-range sidelobes. However, these approaches either expand the main lobe width or are sensitive to parameter tuning. In this paper, by investigating the distribution characteristics of the down-range sidelobes among sub-frequency radar imageries, a robust and efficient range coherence factor (rCF) technique is proposed to suppress the down-range sidelobes in the focused radar imagery. The performance of the proposed approach is assessed by the finite-difference time-domain (FDTD)-based numerical simulations as well as onsite experiments to verify its sidelobes' reduction capabilities.

Weighting the Passive Acoustic Mapping Technique With the Phase Coherence Factor for Passive Ultrasound Imaging of Ultrasound-Induced Cavitation.

Ultrasound (US) cavitation is currently being explored for low-invasive therapy techniques applied to a wide panel of pathologies. Because of the random behavior of cavitation, a real-time spatial monitoring system may be required. For this purpose, the US passive imaging techniques have been recently investigated. In particular, the passive acoustic mapping (PAM) beamforming method enables the reconstruction of cavitation activity maps by beamforming acoustic signals passively recorded by an array transducer. In this paper, an optimized version of PAM, PAM weighted with a phase coherence factor (PAM-PCF), is considered. A general validation process is developed including simulations on a point source and experiments on a wire. Furthermore, using a focused regulated US-induced cavitation generator, reproducible cavitation experiments are conducted in water and in agar gel. The spatial behavior of a bubble cavitation cloud is determined using the PAM-PCF beamforming method to localize the focal cavitation point in two perpendicular imaging planes.

Coherence Weighting Applied to FMC/TFM Data from Austenitic CRA Clad Lined Pipes

Lined pipe girth weld regions are challenging to inspect using ultrasonic techniques due to their anisotropic and inhomogeneous characteristics, which lead to noise and acoustic beam distortion. This paper is focused on the application of coherence weighting methods to decrease the background noise level and false indications on total focusing method (TFM) images, encompassing the triple boundary of mechanically lined pipes. Three coherence weighting approaches were used here; the coherence factor (CF), the sign CF (SCF) and the phase CF (PCF). All three were found capable to improve the TFM images, however, the CF and the SCF performed better due to their exponential sensitivity factors, unlike the PCF which has a linear sensitivity factor.