Single-frame Imaging Research Articles

Impact of corner radius of the aperture on the accuracy of phase retrieval analysis in single-frame CDI for nanomaterials

Numerous efforts have been developed to advance the process of Ptychography, leading to the emergence of a phenomenon known as single-frame Coherent X-ray Diffraction Imaging (CDI). It represents an advanced form of Ptychography in which a well-defined coherent X-ray beam is directed onto the sample to generate a diffraction image that includes intensity information but lacks phase details. However, phase retrieval in single-frame CDI faces challenges like overlapping loss, inadequate samples, or phase ambiguity due to the characteristics of this method. The optimising effort has been conducted in the form of aperture implementation to address these problems, particularly in the iterative phase retrieval domain. This study explores non-point symmetrical apertures, specifically triangles, to enhance image reconstruction quality in iterative phase retrieval. We developed a simulation methodology to explore the impact of corner radius on the apertures in performing single-frame CDI. Our simulator generates diffraction images for each shape, which are processed through iterative algorithms and deep learning to reconstruct images. The effectiveness of each shape is assessed by visualising the loss function results, providing insights into optimal aperture designs for diffraction imaging, and contributing to the resolution of phase retrieval challenges in single-frame CDI.

Lensless multi-core-fiber-based imaging from speckle autocorrelation under highly coherent illumination

Flexible fiber-optic lensless endoscopic imaging plays a significant role in medical and biological applications. Lensless imaging based on far-field speckle of multi-core fibers under spatially incoherent light source illumination relied on the angular optical memory effect, resulting in limited field of view and susceptibility to interference effects. Image reconstruction from fiber speckle intensity under the highly coherent light illumination has been investigated using classical coherent diffraction theory, but it imposes limitations on imaging distance and detection positions. However, existing alternative solutions involve reducing coherence level to satisfy the angular optical memory effect, which increases system complexity and sacrifices the advantages of single-frame imaging. The reconstruction of intensity-based images remains a significant challenge under highly coherent illumination, primarily due to the interference effect. In this study, an imaging algorithm that reconstructs the original image solely from the intensity of far-field fiber speckle under coherent illumination conditions is proposed. The proposed method is based on statistical optics theory and aims to extract the relevant information within coherent speckle patterns to reconstruct the image. Our approach expands the imaging field of view, simplifies the imaging setup, and broadens the range of viable light sources for speckle correlation imaging. The effectiveness of the algorithm is verified by simulated experiments and a real imaging system.

Single-frame two-color illumination computational imaging based on Kramers–Kronig relations

Annular-illumination quantitative phase imaging based on Kramers–Kronig relations (AIKK) achieves a large space-bandwidth-time product. The reconstruction algorithm is non-iteration, parameter-free, and object-independent constitutionally. Based on the observation and analyticity of intensity spectrum characteristics and symmetry in spatial and frequency domains, we propose a single-frame two-color illumination imaging based AIKK (SFTC-AIKK). In addition, we apply the two-color mode to synthetic aperture imaging, and only one measurement is sufficient with a monochrome camera. We employ SFTC-AIKK to achieve resolution corresponding to nearly two times the objective NA at camera-limited frame rate to applications with observing dynamic target at various spatial and temporal scales.

Defect Detection Algorithm for Wing Skin with Stiffener Based on Phased-Array Ultrasonic Imaging.

In response to the real-time imaging detection requirements of structural defects in the R region of rib-stiffened wing skin, a defect detection algorithm based on phased-array ultrasonic imaging for wing skin with stiffener is proposed. We select the full-matrix-full-focusing algorithm with the best imaging quality as the prototype for the required detection algorithm. To address the problem of poor real-time performance of the algorithm, a sparsity-based full-focusing algorithm with symmetry redundancy imaging mode is proposed. To address noise artifacts, an adaptive beamforming method and an equal-acoustic-path echo dynamic removal scheme are proposed to adaptively suppress noise artifacts. Finally, within 0.5 s of imaging time, the algorithm achieves a detection sensitivity of 1 mm and a resolution of 0.5 mm within a single-frame imaging range of 30 mm × 30 mm. The defect detection algorithm proposed in this paper combines phased-array ultrasonic technology and post-processing imaging technology to improve the real-time performance and noise artifact suppression of ultrasound imaging algorithms based on engineering applications. Compared with traditional single-element ultrasonic detection technology, phased-array detection technology based on post-processing algorithms has better defect detection and imaging characterization performance and is suitable for R-region structural detection scenarios.

Chemical Imaging of Mass Transport Near the No-Slip Interface of a Microfluidic Device using Attenuated Total Reflection–Fourier Transform Infrared Spectroscopy

Mass transport in geometrically confined environments is fundamental to microfluidic applications. Measuring the distribution of chemical species on flow requires the use of spatially resolved analytical tools compatible with microfluidic materials and designs. Here, the implementation of an attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) imaging (macro-ATR) approach for chemical mapping of species in microfluidic devices is described. The imaging method is configurable between a large field of view, single-frame imaging, and the use of image stitching to build composite chemical maps. Macro-ATR is used to quantify transverse diffusion in the laminar streams of coflowing fluids in dedicated microfluidic test devices. It is demonstrated that the ATR evanescent wave, which primarily probes the fluid within ∼500 nm of the channel surface, provides accurate quantification of the spatial distribution of species in the entire microfluidic device cross section. This is the case when flow and channel conditions promote vertical concentration contours in the channel as verified by three-dimensional numeric simulations of mass transport. Furthermore, the validity of treating the mass transport problem in a simplified and faster approach using reduced dimensionality numeric simulations is described. Simplified one-dimensional simulations, for the specific parameters used herein, overestimate diffusion coefficients by a factor of approximately 2, whereas full three-dimensional simulations accurately agree with experimental results.

Pushing the colorimetry camera-based fluorescence microscopy to low light imaging by denoising and dye combination.

Colorimetry camera-based fluorescence microscopy (CCFM) is a single-frame imaging method for observing multiple biological events simultaneously. Compared with the traditional multi-color fluorescence microscopy methods based on sequential excitation or spectral splitting, the CCFM method simplifies multi-color fluorescence imaging experiments, while keeping a high spatial resolution. However, when the level of the detected fluorescence signal decreases, the image quality, the demosaicking algorithm precision, and the discrimination of fluorescence channels on the colorimetry camera will also decrease. Thus, CCFM has a poor color resolution under a low signal level. For example, the crosstalk will be higher than 10% when the signal is less than 100 photons/pixel. To solve this problem, we developed a new algorithm that combines sCMOS noise correction with demosaicking, and a dye selection method based on the spectral response characteristics of the colorimetry camera. By combining the above two strategies, low crosstalk can be obtained with 4 ∼ 6 fold fewer fluorescence photons, and low light single-frame four-color fluorescence imaging was successfully performed on fixed cos-7 cells. This study expands the power of the CCFM method, and provides a simple and efficient way for various bioimaging applications in low-light conditions.

High-Speed Micro-Particle Motion Monitoring Based on Continuous Single-Frame Multi-Exposure Technology.

The impact phenomena of solid micro-particles have gathered increasing interest across a wide range of fields, including space debris protection and cold-spray additive manufacturing of large, complicated structures. Effective motion monitoring is essential to understanding the impact behaviors of micro-particles. Consequently, a convenient and efficient micro-particle motion monitoring solution is proposed based on continuous single-frame multiple-exposure imaging technology. This method adopts a camera with excellent low-light performance coupled with high-frequency light-emitting diode (LED) flashes to generate short interval illumination. This technology can, in theory, achieve 1 million effective frames per second (fps) and monitor particles as small as 10 microns with speeds up to 12 km/s. The capabilities of the proposed method were validated by a series of micro-particle motion monitoring experiments with different particles sizes and materials under varying camera configurations. The study provides a feasible and economical solution for the velocity measurement and motion monitoring of high-speed micro-particles.

Randomized probe imaging through deep k-learning

Randomized probe imaging (RPI) is a single-frame diffractive imaging method that uses highly randomized light to reconstruct the spatial features of a scattering object. The reconstruction process, known as phase retrieval, aims to recover a unique solution for the object without measuring the far-field phase information. Typically, reconstruction is done via time-consuming iterative algorithms. In this work, we propose a fast and efficient deep learning based method to reconstruct phase objects from RPI data. The method, which we call deep k-learning, applies the physical propagation operator to generate an approximation of the object as an input to the neural network. This way, the network no longer needs to parametrize the far-field diffraction physics, dramatically improving the results. Deep k-learning is shown to be computationally efficient and robust to Poisson noise. The advantages provided by our method may enable the analysis of far larger datasets in photon starved conditions, with important applications to the study of dynamic phenomena in physical science and biological engineering.

Accessing depth-resolved high spatial frequency content from the optical coherence tomography signal

Optical coherence tomography (OCT) is a rapidly evolving technology with a broad range of applications, including biomedical imaging and diagnosis. Conventional intensity-based OCT provides depth-resolved imaging with a typical resolution and sensitivity to structural alterations of about 5–10 microns. It would be desirable for functional biological imaging to detect smaller features in tissues due to the nature of pathological processes. In this article, we perform the analysis of the spatial frequency content of the OCT signal based on scattering theory. We demonstrate that the OCT signal, even at limited spectral bandwidth, contains information about high spatial frequencies present in the object which relates to the small, sub-wavelength size structures. Experimental single frame imaging of phantoms with well-known sub-micron internal structures confirms the theory. Examples of visualization of the nanoscale structural changes within mesenchymal stem cells (MSC), which are invisible using conventional OCT, are also shown. Presented results provide a theoretical and experimental basis for the extraction of high spatial frequency information to substantially improve the sensitivity of OCT to structural alterations at clinically relevant depths.

Interfacial area concentration and bubble size distribution measurement using tomography technique

Interfacial area concentration (IAC) and bubble size distribution (BSD) are two important parameters in two-phase flow simulations. High-speed tomography techniques, mainly including wire-mesh sensor and ultrafast X-ray imaging in this stage, are advanced measurement devices, which can measure the instantaneous phase distribution in the cross-section with high spatial resolution. In previous studies, IAC and BSD can be estimated by the tomography measurement data. However, the estimated value of the IAC or BSD is not satisfied in some conditions like flows with low superficial liquid velocity. In this study, the methods of estimating IAC and BSD based on tomography measurement were developed, which is independent of the superficial liquid velocity. The new methods use the area surrounded by every four measurement pixels as the control volume, which has a small thickness. The thickness of the control volume is the product of interfacial velocity and the time of single frame imaging. The local instantaneous IAC and BSD for each control volume was calculated. The time-area-averaged parameters can be calculated by averaging the local instantaneous ones. For the time-volume-averaged parameters, it can be calculated by integrating the time-area-averaged parameters at multiple axial positions. An ideal bubbly flow data with pre-defined IAC and BSD was used to verify the new methods. The measurement results from the new methods and the old methods were also compared with each other.

Demonstration of single-frame coherent X-ray diffraction imaging using triangular aperture: Towards dynamic nanoimaging of extended objects.

Coherent diffraction imaging (CDI) is a powerful method for visualizing the structure of an object with a high spatial resolution that exceeds the performance limits of the lens. Single-frame CDI in the X-ray region has potential use for probing dynamic phenomena with a high spatiotemporal resolution. Here, we experimentally demonstrate a general method for single-frame X-ray CDI using a triangular aperture and a Fresnel zone plate. Using 5 keV synchrotron radiation X-rays, we reconstructed the object image of the locally illuminated area with a spatial resolution of higher than 50 nm and an exposure time of more than 0.1 s without prior information about the sample. After a 10 s exposure, a resolution of 17 nm was achieved. The present method opens new frontiers in the study of dynamics at the nanoscale by using next-generation synchrotron radiation X-rays/free-electron lasers as light sources.

Astigmatic traction force microscopy (aTFM)

Quantifying small, rapidly progressing three-dimensional forces generated by cells remains a major challenge towards a more complete understanding of mechanobiology. Traction force microscopy is one of the most broadly applied force probing technologies but ascertaining three-dimensional information typically necessitates slow, multi-frame z-stack acquisition with limited sensitivity. Here, by performing traction force microscopy using fast single-frame astigmatic imaging coupled with total internal reflection fluorescence microscopy we improve the temporal resolution of three-dimensional mechanical force quantification up to 10-fold compared to its related super-resolution modalities. 2.5D astigmatic traction force microscopy (aTFM) thus enables live-cell force measurements approaching physiological sensitivity.

Single-frame coherent diffraction imaging of extended objects using triangular aperture.

We propose a method of single-frame coherent diffraction imaging using a triangular aperture, which can not only reconstruct the projection image of extended objects from a single-frame coherent diffraction pattern, but also improve the image of the wavefield of the probe. In this method, a plane-wave illuminates a triangular aperture. An object is placed immediately after the aperture or in the image plane of the aperture through a lens. A far-field coherent diffraction pattern is collected by a two-dimensional detector. The object image is reconstructed from the single-frame diffraction pattern using a phase retrieval algorithm without support constraints. We simulate feasible experimental setups in the hard X-ray regime and show that this method can be practical use for single-frame coherent diffraction imaging. The present method has the potential exploring dynamic phenomena in materials science and biology with high spatiotemporal resolution using synchrotron radiation/free-electron lasers.

Single-frame far-field diffractive imaging with randomized illumination.

We introduce a single-frame diffractive imaging method called randomized probe imaging (RPI). In RPI, a sample is illuminated by a structured probe field containing speckles smaller than the sample's typical feature size. Quantitative amplitude and phase images are then reconstructed from the resulting far-field diffraction pattern. The experimental geometry of RPI is straightforward to implement, requires no near-field optics, and is applicable to extended samples. When the resulting data are analyzed with a complimentary algorithm, reliable reconstructions which are robust to missing data are achieved. To realize these benefits, a resolution limit associated with the numerical aperture of the probe-forming optics is imposed. RPI therefore offers an attractive modality for quantitative X-ray phase imaging when temporal resolution and reliability are critical but spatial resolution in the tens of nanometers is sufficient. We discuss the method, introduce a reconstruction algorithm, and present two proof-of-concept experiments: one using visible light, and one using soft X-rays.

PET Parametric Imaging: Past, Present, and Future.

Positron emission tomography (PET) is actively used in a diverse range of applications in oncology, cardiology, and neurology. The use of PET in the clinical setting focuses on static (single time frame) imaging at a specific time-point post radiotracer injection and is typically considered as semi-quantitative; e.g. standardized uptake value (SUV) measures. In contrast, dynamic PET imaging requires increased acquisition times but has the advantage that it measures the full spatiotemporal distribution of a radiotracer and, in combination with tracer kinetic modeling, enables the generation of multiparametric images that more directly quantify underlying biological parameters of interest, such as blood flow, glucose metabolism, and receptor binding. Parametric images have the potential for improved detection and for more accurate and earlier therapeutic response assessment. Parametric imaging with dynamic PET has witnessed extensive research in the past four decades. In this paper, we provide an overview of past and present activities and discuss emerging opportunities in the field of parametric imaging for the future.

Experimental study on dynamic mechanism of vortex evolution in a turbulent boundary layer of low Reynolds number

The dynamic mechanism of the vortex generation and evolution process in a fully developed turbulent boundary layer with Reθ =97–194 is experimentally investigated. In this study, a moving single-frame and long-exposure (MSFLE) imaging method and a moving particle image velocimetry/particle tracing velocimetry (M-PIV/PTV) are designed and implemented for measuring the temporal and spatial evolution of vortex cores in both qualitative and quantitative ways, respectively. On the other hand, the Liutex vector, which is a new mathematical definition and identification of the vortex core proposed by Liu’s group, is first applied in the experiment for the structural visualization and quantitative analysis of the local fluid rotation. The results show that an intuitional process of vortex evolution can be clearly observed by tracking the vortex using MSFLE and verify that the roll-up of the shear layer induced by shear instability is the origin of vortex formation in turbulence. Furthermore, a quantitative investigation in terms of the critical vortex core boundary (size) and its accurate rotation strength is carried out based on the Liutex vector field analysis by M-PIV/PTV. According to statistics of the relation between vortex core size and the rotation strength during the whole process, the phy sical mechanism of vortex generation and evolution in a turbulent boundary layer of low Reynolds number can be summarized as a four-dominant-state course consisting of the “synchronous linear segment (SL)-absolute enhancement segment (AE)-absolute diffusion segment (AD)-skewing dissipation segment (SD)”.

Modeling the Moiré fringe visibility of Talbot-Lau X-ray grating interferometry for single-frame multi-contrast imaging.

Moiré fringe method in X-ray grating interferometry is characterized by its advantage to obtain multi-contrast data through single-frame imaging. However, how the visibility of the Moiré fringe is influenced by the system parameters, such as the misalignment angle, still lacks investigation, although it closely relates to the signal-to-noise ratio of the image data. In this paper, a simplified model of the Moiré fringe visibility is presented, which reveals that the Moiré fringe visibility can be expressed as the product of a misalignment-angle-related "sinc" function and a relatively independent factor. The following experimental results further suggest that the crosstalk between the detector pixels in the direction perpendicular to the Moiré fringe orientation is another main cause for the visibility reduction of the Moiré fringes.

Single-frame three-dimensional imaging using spectral-coded patterns and multispectral snapshot cameras

We present an approach for single-frame three-dimensional (3-D) imaging using multiwavelength array projection and a stereo vision setup of two multispectral snapshot cameras. Thus a sequence of aperiodic fringe patterns at different wavelengths can be projected and detected simultaneously. For the 3-D reconstruction, a computational procedure for pattern extraction from multispectral images, denoising of multispectral image data, and stereo matching is developed. In addition, a proof-of-concept is provided with experimental measurement results, showing the validity and potential of the proposed approach.

Single-shot optical speckle imaging based on pseudothermal illumination

Scattering in medium is a serious problem that limits the imaging depth or imaging distance. According to the absorption and scattering of light in biological tissues, it is difficult for both excited light and signal light to penetrate biological tissues, and the scattering effect in biological tissues will destroy the phase information of signal light, so it is difficult to directly carry out high resolution imaging in deep biological tissues. In the recent studies it is surprisingly found that two-dimensional image information of an object can be directly recovered from the disordered speckle pattern with pseudothermal light sources based on the optical memory effect (ME) and autocorrelation (AC) method. In this paper, we study a speckle imaging method based on pseudothermal illumination, where the Gerchberg-Saxton algorithm is used to perform the phase recovery of the object. Here, the advantages and disadvantages of HIO&ER algorithm and ping-pang (PP) algorithm based on the ME and AC method for imaging through random scattering medium are compared by using numerical simulation. By comparing the recovery effects and the numbers of iterations between HIO&ER algorithm and PP algorithm, it is found that PP algorithm has a fast running speed when a higher recovery quality is maintained. In addition, a continuous He-Ne laser and rotating ground glass are used to produce a pseudothermal light source. And a single frame imaging of different shape objects, which are a few millimeters away from random scattering medium, is carried out by objective lens. Then PP algorithm is adopted to recover the actual image of micron object. Furthermore, we experimentally find that the magnification, resolution and image intensity, which are qualitatively studied, are seriously affected by the distance between the focal plane of the object lens and scattering medium. We find that with the increase of the distance, the obtained autocorrelation graph and retrieval graph have corresponding amplification and the object sampling point information collected on sCOMS increases, which improves its resolution. However, the scattered light intensity collected by objective lens decreases after passing through the scattering medium, making the intensity of recovered image weaken. The results of this study will further promote the application of ME and AC method in the study of deep tissue medical imaging.

Intensity modulation-based spectral polarization measurement method of coded aperture

This paper presents a spectral polarization measurement method for single-frame imaging. The method is based on intensity-modulated, coded aperture measurement, comprising a spectral polarization intensity modulation and a compressive sensing spectral imaging system. The incident light is modulated by two multistage phase retarders and a polarizer, and the spectrum’s Stokes parameters are shifted to different frequency channels. The spectrum of the modulated light is obtained by a spectral imaging system composed of a coded aperture and a dispersive prism. The modulated spectral information is reconstructed using the TwIST algorithm, and the Stokes elements can be demodulated from it. Taking a pixel as an example, numerical simulations of the modulation of incident light intensity modulation, spectral reconstruction, and demodulation of Stokes parameters show that the method can acquire spectral polarization information of sparse images. This process needs only one measurement of the object and is therefore capable of high-speed acquisition.