Improvement In Lateral Resolution Research Articles

Lamb Wave TDTE Super-Resolution Imaging Assisted by Deep Learning

Abstract Ultrasonic Lamb waves undergo complex mode conversion and diffraction at non-penetrating defects, such as plate corrosion and cracks. Lamb wave imaging has a resolution limit due to the guided wave dispersion characteristics and Rayleigh criterion limitations. In this paper, a full convolutional network is designed to segment and reconstruct the received signals, enabling the automatic identification of target modalities. This approach eliminates clutter and mode conversion interference when calculating direct and accompanying acoustic fields in time-domain topological energy imaging. Subsequently, the measured accompanying acoustic field is reversed for adaptive focusing on defects and enhance the imaging quality. To circumvent the limitations of the Rayleigh criterion, the direct acoustic field and the accompanying acoustic field were fused to characterize the pixel distribution in the imaging region, achieving Lamb wave super-resolution imaging. Experimental results indicate that compared to the sign coherence factor-total focusing method (SCF-TFM), the proposed method achieves a 31.41% improvement in lateral resolution and a 29.53% increase in signal-to-noise ratio for single-blind-hole defects. In the case of multiple-blind-hole defects with spacings greater than the Rayleigh criterion resolution limit, it exhibits a 27.23% enhancement in signal-to-noise ratio. On the contrary, when the defect spacings are relatively smaller than the limit, this method has a higher resolution limit than SCF-TFM in super-resolution imaging.

A comprehensive review on the analysis of adhesion strength of cold spray deposits

Cold spray (CS) is a new kind of coating that has many potential uses, including functionalizing surfaces, doing localized repairs, and mass production. Its fast deposition rate and diverse feedstocks have led to its growing popularity as an additive manufacturing method, which has been driven by improvements in lateral resolution. CS materials have several uses in the transportation industry, and some of the most frequent ones include nickel super-alloys, titanium alloys, copper, aluminum and metal-ceramic composites. Nevertheless, the mechanical properties depend upon number of parameters involved during spraying process. Therefore, there is an immediate need for researchers and businesses to understand the various parameters affecting the adhesion strength of CS deposits. The current research presents an analysis of the adhesion behavior of coatings that have been deposited by CS methodology.

Tissue ultrasound imaging based on wavelet correlation analysis and pulse-inversion technique.

Pulse-inversion-based tissue harmonic imaging has been utilized for many years because it can effectively eliminate the harmonic leakage and produce low side-lobe. However, the pulse inversion method is sensitive to imaging object movements, which may result in motion artifacts. Spatial resolution and contrast were limited. To improve ultrasound image quality by a new pulse-inversion-based tissue harmonic imaging technique. Continuous wavelet transform is applied to investigate the correlation between mother wavelet and the received echoes from two opposite pulses. To get a better correlation, a novel mother wavelet named 'tissue wavelet' is designed based on the Khokhlov-Zabolotskaya- Kuznetsov (KZK) wave equation. Radio frequency data were obtained from open Ultrasonix SonixTouch imaging system. Experiments were carried on ultrasonic tissue phantom, human carotid artery and human liver. The average improvement of lateral spatial resolution is 49.52% compared to pulse-inversion-based tissue second-harmonic Imaging (PIHI). Contrast ratio (CR) and contrast-to-noise ratio (CNR) increased by 5.55 dB and 1.40 dB over PIHI. Tissue wavelet performs better than Mexh and Morl wavelet in lateral spatial resolution, CR, and CNR. The proposed technique effectively improves the imaging quality in lateral spatial resolution, CR, and CNR.

Vertical Seismic-Profile Data Local Full-Waveform Inversion Based on Marchenko Redatuming

Local full-waveform inversion (FWI) methods use redatumed seismic responses of virtual receivers within the subsurface to build the local objective function based on the convolution-type representation theorem. The Marchenko method is widely used to obtain the redatumed data. The method only requires a smoothed velocity model with correct kinematic characteristics of seismic responses for redatuming of the single-sided reflection data. However, the standard Marchenko method is insensitive to lateral propagation of the wavefield. By injecting the standard Marchenko redatumed wavefield along the boundary of the target, the local wavefield propagation modeling produces errors, which affects the accuracy of the local FWI. In this paper, a method to obtain more accurate Green’s functions is proposed by incorporating vertical seismic profile data (VSP) into the calculation process of the Marchenko source-receiver redatuming. This method allows one to obtain the accurate laterally propagating waveform, resulting in a significant improvement of lateral resolution. The proposed method is applied to a benchmark model dataset and compared with the local FWI based on standard Marchenko redatuming.

High-Resolution Quantitative Phase Microscopy with Second-Harmonic Generation and Structured Illumination from a k -Space Formulation

We propose a high-resolution label-free two-dimensional imaging method by introducing second-harmonic generation (SHG) and structured illumination in quantitative phase microscopy (QPM), termed ${\mathrm{S}}^{2}\mathrm{QPM}$, which can achieve a fourfold lateral resolution improvement compared with normal incident illumination for noncentrosymmetric structures. Unlike traditional QPM only obtaining information about the optical path difference, ${\mathrm{S}}^{2}\mathrm{QPM}$ can additionally recover structural information from the ${\ensuremath{\chi}}^{(2)}$ distribution. Analytical solutions for SHG fields are obtained by solving the inhomogeneous and nonlinear Helmholtz equation in the $\mathit{k}$-space in a both concise and accurate way. With the $\mathit{k}$-space SHG field solutions for different structured illumination patterns, an object reconstruction model is developed to retrieve the high-resolution normalized ${\ensuremath{\chi}}^{(2)}$ distributions and quantitative phase maps. We simulate high-resolution reconstructions of the normalized ${\ensuremath{\chi}}^{(2)}$ distributions and quantitative phase maps using various resolution targets. The lateral resolution improvement is quantified by synthesizing the object spectrum function and profiling resolution targets.

Photoacoustic/Ultrasound Endoscopic Imaging Reconstruction Algorithm Based on the Approximate Gaussian Acoustic Field.

This paper aims to propose a new photoacoustic/ultrasound endoscopic imaging reconstruction algorithm based on the approximate Gaussian acoustic field which significantly improves the resolution and signal-to-noise ratio (SNR) of the out-of-focus region. We demonstrated the method by numerical calculations and investigated the applicability of the algorithm in a chicken breast phantom. The validation was finally performed by the rabbit rectal endoscopy experiment. Simulation results show that the lateral resolution of the target point in the out-of-focus region can be well optimized with this new algorithm. Phantom experimental results show that the lateral resolution of the indocyanine green (ICG) tube in the photoacoustic image is reduced from 3.975 mm to 1.857 mm by using our new algorithm, which is a 52.3% improvement. Ultrasound images also show a significant improvement in lateral resolution. The results of the rabbit rectal endoscopy experiment prove that the algorithm we proposed is capable of providing higher-quality photoacoustic/ultrasound images. In conclusion, the algorithm enables fast acoustic resolution photoacoustic/ ultrasonic dynamic focusing and effectively improves the imaging quality of the system, which has significant guidance for the design of acoustic resolution photoacoustic/ultrasound endoscopy systems.

Lateral image reconstruction of optical coherence tomography using one-dimensional deep deconvolution network.

Optical coherence tomography (OCT) is a cross-sectional imaging method utilizing a low coherence interferometry. The lateral resolution of the OCT is limited by the numerical aperture (NA) of the imaging lens. Using a high NA lens improves the lateral resolution but reduces the depth of focus (DOF). In this study, we propose a method to improve the lateral resolution of OCT images by end-to-end training of a deep 1-D deconvolution network without use of high-resolution images. To improve the lateral resolution of the OCT, we trained the 1-D deconvolution network using lateral profiles of OCT images and the beam spot size. We used our image-guided laparoscopic surgical tool (IGLaST) to acquire OCT images of nonbiological and biological samples ex vivo. The OCT images were then blurred by applying Gaussian functions with various full width half maximums ranging from 40to 160 µm. The network was trained using the blurred OCT images as input and the non-blurred original OCT images as output. We quantitatively evaluated the developed network in terms of similarity and signal-to-ratio (SNR), using in-vivo images of mesenteric tissue from a porcine model that was not used for training. In addition, we performed knife-edge tests and qualitative evaluation of the network to show the lateral resolution improvement of ex-vivo and in-vivo OCT images. The proposed method showed an improvement of image quality on both blurred images and non-blurred images. When the proposed deconvolution network was applied, the similarity to the non-blurred image was improved by 1.29 times, and the SNR was improved by 1.76 dB compared to the artificially blurred images, which was superior to the conventional deconvolution method. The knife-edge tests at distances at 200 to 1000 µm from the imaging probe showed an approximately 1.2 times improvement in lateral resolution. In addition, through qualitative evaluation, it was found that the image quality of both ex-vivo and in-vivo tissue images was improved with clear structure and less noise. This study showed the ability of the 1-D deconvolution network to improve the image quality of OCT images with variable lateral resolution. We were able to train the network with a small amount of data by constraining the network in 1-D. The quantitative evaluation showed better results than conventional deconvolution methods for various amount of blurring. Qualitative evaluation showed analogous results with quantitative results. This simple yet powerful image restoration method provides improved lateral resolution and suppresses background noise, making it applicable to a variety of OCT imaging applications.

Generalized sidelobe canceler beamforming combined with Eigenspace-Wiener postfilter for medical ultrasound imaging.

BACKGROUND: The beamforming algorithm is key to the image quality of the medical ultrasound system. The generalized sidelobe canceler (GSC) beamforming can improve the image quality in lateral resolution, but the contrast is not improved correspondingly.OBJECTIVE: In our research, we try to optimize the generalized sidelobe canceler to obtain images that achieve an improvement in both lateral resolution and contrast.METHODS: We put forward a new beamforming algorithm which combines the generalized sidelobe canceler and Eigenspace-Wiener postfilter. According to eigenspace decomposition of the covariance matrix of the received data, the components of the Wiener postfilter can be calculated from the signal matrix and the noise matrix. Then, the adaptive weight vector of GSC is further constrained by the Eigenspace-Wiener postfilter, which make the output energy of the receiving array closer to the desired signal than the conventional GSC output.RESULTS: We compare the new beamforming algorithm with delay-and-sum (DS) beamforming, synthetic aperture (SA) beamforming, and GSC beamforming using the simulated and experimental data sets. The quantitative results show that our method reduces the FWHM by 85.5%, 80.5%, and 38.9% while improving the CR by 123.6%, 47.7%, 84.4% on basis of DS, SA, and GSC beamforming, respectively.CONCLUSIONS: The new beamforming algorithm can obviously improve the imaging quality of medical ultrasound imaging systems in both lateral resolution and contrast.

Fourier photoacoustic microscope improved resolution on single-pixel imaging.

A new single-pixel Fourier photoacoustic microscopy (PAM), to the best of our knowledge, is proposed to improve the resolution and region of interest (ROI) of an acquired image. In the previous structure of single-pixel Fourier PAM, called spatially invariant resolution PAM (SIR-PAM), the lateral resolution and ROI are limited by the digital micromirror device (DMD) pixel size and the number of pixels. This limitation is overcome here through illuminating fixed angle interfering plane waves, changing the fringe frequency via varying the frequency of the laser source. Given that the fringe sinusoidal patterns here can be produced by two mirrors, the DMD usage can be omitted. In this way, the fringe frequency can be changed in a wider spectrum, making it possible to capture a wider spectral bandwidth and thus a higher-resolution image. Also, the removal of the ROI limitation results in a high-resolution frequency-swept PAM structure. Monte Carlo simulations show 1.7 times improvement in lateral resolution compared to SIR-PAM based on the point-spread function and full-width-at-half-maximum.

High-resolution and high-contrast ultrafast ultrasound imaging using coherent plane wave adaptive compounding

Plane-wave imaging (PWI) is an interesting technique for increasing frame rate in medical ultrasound imaging, but it suffers from poor image quality. To overcome this problem, the coherent plane-wave compounding (CPWC) approach has been introduced, which provides the possibility of achieving the desired resolution and contrast by combining images obtained from different angles. In this paper, we proposed a new technique based on adaptive compounding of resulted radio frequency (RF) image data at different angles to obtain a high-resolution image. We called this method coherent plane wave adaptive compounding (CPWAC). The adaptive method used here is the minimum variance (MV) approach. An important benefit of the CPWAC method is that the calculation and inversion of the covariance matrix is done only once. We applied the proposed method on simulated, experimental, and in vivo datasets of the plane wave imaging challenge in medical ultrasound (PICMUS) data by considering 75 and 25 plane waves (PW). The results showed that the proposed CPWAC improves the lateral full width at half maximum (FWHM) by 35% for both simulated and experimental datasets, even by using 33% of the total angles compared to CPWC, which indicates simultaneous improvement of lateral resolution and frame rate. In addition, for the experimental dataset, this method has improved the {CR, CNR} about {103.8%, 100%} and {30.28%, 28%} in the near-field and far-field, respectively. Therefore, while the proposed method does not add much time overhead compared to the original CPWC, it improves the image quality of the CPWC significantly.

Approximate back‐projection method for improving lateral resolution in circular‐scanning‐based photoacoustic tomography

In circular-scanning-based photoacoustic tomography (PAT), the effect of finite transducer aperture has not been effectively resolved. The goal of this paper is to propose a practical reconstruction method that accounts for the finite transducer aperture to improve the lateral resolution. We for the first time propose to calculate the spatial-temporal response (STR) of the employed finite-sized transducer in a forward model, and then compensate the time delay and the directional sensitivity of the transducer in the framework of the back-projection method. Both simulation and phantom experiments were carried out to evaluate the lateral resolution improvement with the proposed method. The performance of this new method for imaging complicated targets was also assessed by calculating the mean image gradient. Simulation results showed that with this new method the lateral resolution for off-center targets can be as good as that for the center targets. Phantom experimental results showed that this new method can improve the lateral resolution more than two times for a point target about 5mm far from the rotation center. Phantom experimental results also showed that many blurred fine structures of a piece of leaf veins at the off-center regions were well restored with the new method, and the mean image gradient improved about 1.3 times. The proposed new method can effectively account for the effect of finite transducer aperture for circular-scanning-based PAT in homogenous acoustic media. This new method also features its robustness and computational efficiency, so that it is a worthy replacement to the conventional back-projection algorithm in circular-scanning-based PAT. This new method can be of great importance to the design of circular-scanning or spherical-scanning-based PAT systems.

CSPARCOM: Multi-detector reconstruction by confocal super-resolution correlation microscopy.

Image scanning microscopy (ISM), an upgraded successor of the ubiquitous confocal microscope, facilitates up to two-fold improvement in lateral resolution, and has become an indispensable element in the toolbox of the bio-imaging community. Recently, super-resolution optical fluctuation image scanning microscopy (SOFISM) integrated the analysis of intensity-fluctuations information into the basic ISM architecture, to enhance its resolving power. Both of these techniques typically rely on pixel-reassignment as a fundamental processing step, in which the parallax of different detector elements to the sample is compensated by laterally shifting the point spread function (PSF). Here, we propose an alternative analysis approach, based on the recent high-performing sparsity-based super-resolution correlation microscopy (SPARCOM) method. Through measurements of DNA origami nano-rulers and fixed cells labeled with organic dye, we experimentally show that confocal SPARCOM (cSPARCOM), which circumvents pixel-reassignment altogether, provides enhanced resolution compared to pixel-reassigned based analysis. Thus, cSPARCOM further promotes the effectiveness of ISM, and particularly that of correlation based ISM implementations such as SOFISM, where the PSF deviates significantly from spatial invariance.

Three-Dimension Resolution Enhanced Microscopy Based on Parallel Detection

Pixel reassignment image scanning microscopy (PRISM) is a useful tool to improve the resolution of confocal laser scanning microscopy (CLSM) only equipped with a detector array. However, while it can improve the lateral resolution, it has little effect on the axial resolution. Here, new microscopy has been proposed which combines three-dimension fluorescence emission difference microscopy (3D FED) with PRISM to further improve three-dimension resolution. We call this method three-dimension pixel reassignment fluorescence emission difference microscopy (3D-PRFED). Detailed theoretical analysis and simulation are presented in this paper. Additionally, the performance of lateral and axial resolution improvement of this method has been demonstrated by imaging 100 nm fluorescent beads and nuclear pore complexes samples. Experiment results show that this method in our system can improve lateral resolution by a factor of 1.85 and axial resolution by a factor of 1.48 compared with CLSM.

3Pb5-3 A Study on Lateral Resolution Improvement of a Wide Pitch Probe by Compressed Sensing

3Pb5-3 A Study on Lateral Resolution Improvement of a Wide Pitch Probe by Compressed Sensing

Enhanced multifocal structured illumination microscopy with desired optical sectioning capability and lateral resolution improvement.

Multifocal structured illumination microscopy (MSIM) can rapidly retrieve 3D structures of thick samples by using multi-spot excitation and detection. Although numerous super-resolution (SR) and optical sectioning (OS) methods have been introduced in this field, the existing OS-SR method in MSIM still has the difficulty in rejecting deep defocused light, which may lead to strong background signal in the retrieved results. To this end, an enhanced OS-SR method is proposed to simultaneously achieve the desired OS capability and significant resolution improvement in MSIM. The enhanced OS-SR image is obtained by combining the standard deviation image with the conventional OS-SR image in the frequency domain. The validity of the proposed method is demonstrated by simulation and experimental results.

Temporal averaging introduced in linear regression beamforming for improvement of contrast-to-noise ratio

Coherence-based adaptive imaging has been investigated for the improvement of image quality in medical ultrasound imaging. The coherence factor (CF) was developed for the improvement of lateral resolution and contrast. We enhanced the effect of the CF by converting it into the signal-to-noise (SNR) ratio factor, and the effect of the SNR factor was further enhanced by introducing a linear regression (LR) beamformer. Although lateral resolution and contrast were improved significantly by the SNR factor and LR beamformer, the contrast-to-noise ratio (CNR) was degraded. In the present study, a temporal averaging procedure was introduced in LR beamforming for the improvement of the CNR. The phantom experimental results showed that the CNR was improved by 40% without significant degradation in lateral resolution and contrast.

Large-Pitch Synthetic Transmit Aperture Imaging: A Feasibility Study.

A 3-D or large-aperture 2-D synthetic transmit aperture (STA) ultrasound imaging system with a fully sampled array usually leads to high hardware complexity and cost since each element in the array is individually controlled. To reduce the hardware complexity, we propose a large-pitch method for STA (LPSTA) imaging integrated with a spatial response function (SRF) in the image reconstruction to improve image quality. To achieve this, we decreased the total number of measurement channels M (the product of the number of transmissions IT and the number of the receive channels in each transmission IR ). We combined L adjacent elements in transmission and K adjacent elements in receive into subapertures (SAPs), where L and K were coprime (no common factors) integers to suppress the grating lobes. We denoted it as an ( N/L, N /K ) system, where N is the number of transducer elements. In this article, first, we derived the beam pattern of the LPSTA using a far-field approximation. We demonstrated that the coprime selection and SRF can significantly reduce the grating lobes level (GLL) by using a beam pattern analysis. We also found that the LPSTA can have a similar beam pattern as that of a full array when the target is located along the steering direction. Second, the imaging performance of LPSTA was evaluated and validated with Field II simulations and experiments. The simulation results demonstrated that the proposed LPSTA with ( N /3, N /5) can achieve on average ~25% improvement in lateral resolution, ~24.6% and ~42.3% improvement in contrast-to-noise ratio (CNR) and contrast ratio (CR), respectively, over B-mode with a large-pitch receiver with ( N , N /5). LPSTA achieved comparable image contrast to the standard STA with the full array ( N , N ) at the cost of a reduced field of view. The experiment results were consistent with the simulation results. Finally, in addition to reducing the system (hardware) complexity, the LPSTA was more computationally efficient than the standard STA with a full array. The proposed method may help in realizing clinical applications of real-time 2-D or 3-D ultrasound imaging using large arrays.

High-Throughput, High-Resolution Interferometric Light Microscopy of Biological Nanoparticles.

Label-free, visible light microscopy is an indispensable tool for studying biological nanoparticles (BNPs). However, conventional imaging techniques have two major challenges: (i) weak contrast due to low-refractive-index difference with the surrounding medium and exceptionally small size and (ii) limited spatial resolution. Advances in interferometric microscopy have overcome the weak contrast limitation and enabled direct detection of BNPs, yet lateral resolution remains as a challenge in studying BNP morphology. Here, we introduce a wide-field interferometric microscopy technique augmented by computational imaging to demonstrate a 2-fold lateral resolution improvement over a large field-of-view (>100 × 100 μm2), enabling simultaneous imaging of more than 104 BNPs at a resolution of ∼150 nm without any labels or sample preparation. We present a rigorous vectorial-optics-based forward model establishing the relationship between the intensity images captured under partially coherent asymmetric illumination and the complex permittivity distribution of nanoparticles. We demonstrate high-throughput morphological visualization of a diverse population of Ebola virus-like particles and a structurally distinct Ebola vaccine candidate. Our approach offers a low-cost and robust label-free imaging platform for high-throughput and high-resolution characterization of a broad size range of BNPs.

Coherent Plane-Wave Compounding Based on United Coherence Factor

We present a united coherence factor beamformer for coherent plane-wave compounding (CPWC). CPWC is capable to reach an image quality comparable to the conventional B-mode imaging with a much higher frame rate and could be further applied to Doppler imaging. Conventional coherence factor (CF) based beamformers for CPWC use only either the spatial coherence or angular coherence information, which limits the image quality. To take full advantage of the radio frequency (RF) data for better image quality, this paper proposes a united coherence factor (uCF) using both spatial and angular information for CPWC. The proposed uCF is applied to simulated, phantom and in vivo imaging data. We evaluate the performance of the proposed method by lateral resolution and contrast. For simulated images, the contrast ratio (CR) improvements are over 80%. In the phantom images, the maximum improvements of the lateral full width at half maximum (FWHM) and CR are 10% and 105%, respectively. To compare the most core differences, the spatial and angular methods are also presented in this paper. uCF is superior in lateral resolution improvement and contrast among the beamformers. In addition, the proposed uCF performance also shows good performance in the power Doppler experiment with improved image quality, which is further verified by the spatial similarity matrix of the Doppler image. Thus, the proposed method (uCF) has a great potential to be applied in CPWC for image quality improvement.

Fourier focusing in integral imaging with optimum visualization pixels

In this paper, we propose Fourier Volumetric Computational Reconstruction (FVCR), which is a new computational integral imaging reconstruction (CIIR) algorithm. It handles physical pixel overlapping by refocusing in the Fourier domain. Thus, the longitudinal resolution of the reconstructed 3D image is the same as optical reconstruction. It also considers the lateral resolution limit of integral imaging and minimizes the computational load in CIIR. To verify the improvements in both lateral and longitudinal resolution of the reconstructed 3D image, in this paper, we implemented optical experiments. To the best of our knowledge, this is the first report of the efficient CIIR algorithm that considers the longitudinal and lateral resolution limits of integral imaging.