High-resolution quantitative phase-contrast microscopy by digital holography

Single-shot differential phase contrast microscopy using ring-shaped polarisation multiplexing illumination

We propose a differential phase contrast microscopy that enables single-shot phase imaging for unstained biological samples. The proposed approach employs a ring-shaped LED array for polarisation multiplexing illumination and a polarisation camera for image acquisition. As such, multiple images of different polarisation angles can be simultaneously captured with a single shot. Through polarisation demultiplexing, the sample phase can therefore be recovered from the single-shot measurement. Both simulations and experiments demonstrate the effectiveness of the approach. We also demonstrate that ring-shaped illumination enables higher contrast and lower-distortion imaging results than disk-shaped illumination does. The proposed single-shot approach potentially enables phase contrast imaging for live cell samples in vitro. Lay Description: We propose a microscopy that enables imaging of transparent samples, unstained cells, etc. We demonstrate that the proposed method enables higher contrast and lower-distortion imaging results than conventional methods, and significantly improves imaging efficiency. The proposed method potentially enables dynamic imaging for live cell samples in vitro.

Coherent light scattering from cellular dynamics in living tissues

This review examines the biological physics of intracellular transport probed by the coherent optics of dynamic light scattering from optically thick living tissues. Cells and their constituents are in constant motion, composed of a broad range of speeds spanning many orders of magnitude that reflect the wide array of functions and mechanisms that maintain cellular health. From the organelle scale of tens of nanometers and upward in size, the motion inside living tissue is actively driven rather than thermal, propelled by the hydrolysis of bioenergetic molecules and the forces of molecular motors. Active transport can mimic the random walks of thermal Brownian motion, but mean-squared displacements are far from thermal equilibrium and can display anomalous diffusion through Lévy or fractional Brownian walks. Despite the average isotropic three-dimensional environment of cells and tissues, active cellular or intracellular transport of single light-scattering objects is often pseudo-one-dimensional, for instance as organelle displacement persists along cytoskeletal tracks or as membranes displace along the normal to cell surfaces, albeit isotropically oriented in three dimensions. Coherent light scattering is a natural tool to characterize such tissue dynamics because persistent directed transport induces Doppler shifts in the scattered light. The many frequency-shifted partial waves from the complex and dynamic media interfere to produce dynamic speckle that reveals tissue-scale processes through speckle contrast imaging and fluctuation spectroscopy. Low-coherence interferometry, dynamic optical coherence tomography, diffusing-wave spectroscopy, diffuse-correlation spectroscopy, differential dynamic microscopy and digital holography offer coherent detection methods that shed light on intracellular processes. In health-care applications, altered states of cellular health and disease display altered cellular motions that imprint on the statistical fluctuations of the scattered light. For instance, the efficacy of medical therapeutics can be monitored by measuring the changes they induce in the Doppler spectra of livingex vivocancer biopsies.

Detectability of unresolved particles in off-axis digital holographic microscopy

Off-axis digital holographic microscopy (DHM) provides both amplitude and phase images, and so it may be used for label-free 3D tracking of micro- and nano-sized particles of different compositions, including biological cells, strongly absorbing particles, and strongly scattering particles. Contrast is provided by differences in either the real or imaginary parts of the refractive index (phase contrast and absorption) and/or by scattering. While numerous studies have focused on phase contrast and improving resolution in DHM, particularly axial resolution, absent have been studies quantifying the limits of detection for unresolved particles. This limit has important implications for microbial detection, including in life-detection missions for space flight. Here we examine the limits of detection of nanosized particles as a function of particle optical properties, microscope optics (including camera well depth and substrate), and data processing techniques and find that DHM provides contrast in both amplitude and phase for unresolved spheres, in rough agreement with Mie theory scattering cross-sections. Amplitude reconstructions are more useful than phase for low-index spheres and should not be neglected in DHM analysis.

Automatic aberration compensation for digital holographic microscopy based on bicubic downsampling and improved minimization of global phase gradients

In digital holographic microscopy, aberrations caused by imperfect optical system settings can greatly affect the quantitative measurement of the target phase, so the compensation of aberrations in the distorted phase has become a key point of research in digital holographic microscopy. Here, we propose a fully automatic numerical phase aberration compensation method with fast computational speed and high robustness. The method uses bicubic downsampling to smooth the sample phase for reducing its disturbance to the background aberration fit, while reducing the computational effort of aberration compensation. Polynomial coefficients of the aberration fitting are iteratively optimized in the process of minimizing the global phase gradient by improving the phase gradient operator and constructing the loss function to achieve accurate fitting of the phase aberration. Simulation and experimental results show that the proposed method can achieve high aberration compensation accuracy without prior knowledge of the hologram recording settings or manual selection of the background area free of samples, and it is suitable for samples with moderate and relatively flat background area, which can be widely used in the quantitative analysis of biological tissues and micro and nano structures.

Computational refocusing in phase-resolved confocal microscopy

We demonstrate numerical refocusing in coherent confocal laser scanning microscopy based on synthetic optical holography. In this physics-based approach, computational propagation is implemented on the complex signal recovered in synthetic holography, consistent with wave physics and the parameters of the microscope. An experimental demonstration is shown to restore an in-focus image of a test object from data acquired at several focal plane off-sets. Numerical refocusing can provide focused views on samples with large height variation, with a potential application in confocal optical surface profiling.

Phase aberration adaptive compensation in digital holography based on phase imitation and metric optimization

We proposed a numerical and accurate quadratic phase aberration compensation method in digital holography. A phase imitation method based on Gaussian 1σ-criterion is used to obtain the morphological features of the object phase using partial differential, filtering and integration successively. We also propose an adaptive compensation method based on a maximum-minimum-average- α-standard deviation (MMAαSD) evaluation metric to obtain optimal compensated coefficients by minimizing the above metric of the compensation function. The effectiveness and robustness of our method are demonstrated by simulation and experiments.

Automatic elimination of phase aberrations in digital holography based on Gaussian 1σ- criterion and histogram segmentation

We propose a numerical and automatic quadratic phase aberration elimination method in digital holography for phase-contrast imaging. A histogram segmentation method based on Gaussian 1σ-criterion is used to obtain the accurate coefficients of quadratic aberrations using the weighted least-squares algorithm. This method needs no manual intervention for specimen-free zone or prior parameters of optical components. We also propose a maximum-minimum-average-standard deviation (MMASD) metric to quantitatively evaluate the effectiveness of quadratic aberration elimination. Simulation and experimental results are demonstrated to verify the efficacy of our proposed method over the traditional least-squares algorithm.

3D full-wave multi-scattering forward solver for coherent microscopes

A rigorous forward model solver for conventional coherent microscope is presented. The forward model is derived from Maxwell's equations and models the wave behaviour of light matter interaction. Vectorial waves and multiple-scattering effect are considered in this model. Scattered field can be calculated with given distribution of the refractive index of the biological sample. Bright field images can be obtained by combining the scattered field and reflected illumination, and experimental validation is included. Insights into the utility of the full-wave multi-scattering (FWMS) solver and comparison with the conventional Born approximation based solver are presented. The model is also generalizable to the other forms of label-free coherent microscopes, such as quantitative phase microscope and dark-field microscope.

Phase aberration separation for holographic microscopy by alternating direction sparse optimization

The morphology and dynamics of label-free tissues can be exploited by sample-induced changes in the optical field from quantitative phase imaging. Its sensitivity to subtle changes in the optical field makes the reconstructed phase susceptible to phase aberrations. We import variable sparse splitting framework on quantitative phase aberration extraction based on alternating direction aberration free method. The optimization and regularization in the reconstructed phase are decomposed into object terms and aberration terms. By formulating the aberration extraction as a convex quadratic problem, the background phase aberration can be fast and directly decomposed with the specific complete basis functions such as Zernike or standard polynomials. Faithful phase reconstruction can be obtained by eliminating global background phase aberration. The aberration-free two-dimensional and three-dimensional imaging experiments are demonstrated, showing the relaxation of the strict alignment requirements for the holographic microscopes.

Decorrelation and anti-correlation from defocus in digital holographic interferometry

This paper presents a theoretical modeling of the speckle noise decorrelation in digital Fresnel holographic interferometry in out-of-focus reconstructed images. The complex coherence factor is derived by taking into account the focus mismatch, which depends on both the sensor-to-object distance and the reconstruction distance. The theory is confirmed by both simulated data and experimental results. The very good agreement between data demonstrates the high relevance of the proposed modeling. The particular phenomenon of anti-correlation in phase data from holographic interferometry is highlighted and discussed.

Nontoxic Fluorescent Nanoprobes for Multiplexed Detection and 3D Imaging of Tumor Markers in Breast Cancer

Multiplexed fluorescent immunohistochemical analysis of breast cancer (BC) markers and high-resolution 3D immunofluorescence imaging of the tumor and its microenvironment not only facilitate making the disease prognosis and selecting effective anticancer therapy (including photodynamic therapy), but also provides information on signaling and metabolic mechanisms of carcinogenesis and helps in the search for new therapeutic targets and drugs. The characteristics of imaging nanoprobe efficiency, such as sensitivity, target affinity, depth of tissue penetration, and photostability, are determined by the properties of their components, fluorophores and capture molecules, and by the method of their conjugation. Regarding individual nanoprobe components, fluorescent nanocrystals (NCs) are widely used for optical imaging in vitro and in vivo, and single-domain antibodies (sdAbs) are well established as highly specific capture molecules in diagnostic and therapeutic applications. Moreover, the technologies of obtaining functionally active sdAb–NC conjugates with the highest possible avidity, with all sdAb molecules bound to the NC in a strictly oriented manner, provide 3D-imaging nanoprobes with strong comparative advantages. This review is aimed at highlighting the importance of an integrated approach to BC diagnosis, including the detection of biomarkers of the tumor and its microenvironment, as well as the need for their quantitative profiling and imaging of their mutual location, using advanced approaches to 3D detection in thick tissue sections. The existing approaches to 3D imaging of tumors and their microenvironment using fluorescent NCs are described, and the main comparative advantages and disadvantages of nontoxic fluorescent sdAb–NC conjugates as nanoprobes for multiplexed detection and 3D imaging of BC markers are discussed.

DH-GAN: a physics-driven untrained generative adversarial network for holographic imaging

Digital holography is a 3D imaging technique by emitting a laser beam with a plane wavefront to an object and measuring the intensity of the diffracted waveform, called holograms. The object's 3D shape can be obtained by numerical analysis of the captured holograms and recovering the incurred phase. Recently, deep learning (DL) methods have been used for more accurate holographic processing. However, most supervised methods require large datasets to train the model, which is rarely available in most DH applications due to the scarcity of samples or privacy concerns. A few one-shot DL-based recovery methods exist with no reliance on large datasets of paired images. Still, most of these methods often neglect the underlying physics law that governs wave propagation. These methods offer a black-box operation, which is not explainable, generalizable, and transferrable to other samples and applications. In this work, we propose a new DL architecture based on generative adversarial networks that uses a discriminative network for realizing a semantic measure for reconstruction quality while using a generative network as a function approximator to model the inverse of hologram formation. We impose smoothness on the background part of the recovered image using a progressive masking module powered by simulated annealing to enhance the reconstruction quality. The proposed method exhibits high transferability to similar samples, which facilitates its fast deployment in time-sensitive applications without the need for retraining the network from scratch. The results show a considerable improvement to competitor methods in reconstruction quality (about 5 dB PSNR gain) and robustness to noise (about 50% reduction in PSNR vs noise increase rate).

Reduction in required volume of imaging data and image reconstruction time for adaptive-illumination Fourier ptychographic microscopy

Fourier ptychographic microscopy (FPM) is a promising super-resolution computational imaging technology. It stitches a series of low-resolution (LR) images in the Fourier domain by an iterative method. Thus, it obtains a large field of view and high-resolution quantitative phase images. Owing to its capability to perform high-spatial bandwidth product imaging, FPM is widely used in the reconstruction of conventional static samples. However, the influence of the FPM imaging mechanism limits its application in high-speed dynamic imaging. To solve this problem, an adaptive-illumination FPM scheme using regional energy estimation is proposed. Starting with several captured real LR images, the energy distribution of all LR images is estimated, and select the measurement images with large information to perform FPM reconstruction. Simulation and experimental results show that the method produces efficient imaging performance and reduces the required volume of data to more than 65% while ensuring the quality of FPM reconstruction.

Interferometric imaging with ring-shaped apertures

We use a scattering model for image formation to demonstrate how to simulate images formed by ring-shaped illumination. The description assumes weakly scattering samples, such as phase objects of broad interest in live cell imaging, which also makes no angular approximations and covers forward and backward scattering, single-angle and angle-averaged illumination, as well as monochromatic and broadband light. The numerical experiments reveal the image formation behavior that is consistent with recent experiments reported in the literature, which shows this model can be applied to different imaging systems that are based on ring-shaped illumination with good performance, by considering the incident as a plane wave incident originating at the ring aperture.

Quantitative phase imaging based on polarization encoding

Most optical characterization methods rely on measuring the complex optical fields emerging from the interaction between light and material systems. Nevertheless, inherent scattering and absorption cause ambiguities in both interferometric and noninterferometric attempts to measure phase. Here we demonstrate that the complete information about a probe optical field can be encoded into the states of polarization, and develop a topography measurement method by blindly varying the ambient refractive index surrounding the sample in a wedged cuvette, which is capable of simultaneously measuring the thickness and the ambient refractive index of the sample in real time, as well as extending the measurement range of the sample thickness. With the method, we have successfully measured the topography of a 136.7 µm thick coverslip by blindly changing the ambient refractive index by 0.001246, resulting in the thickest sample characterization ever achieved by quantitative phase imaging, to the best of our knowledge. An efficient and complete characterization of optical fields is critical for any high-resolution imaging approach and the technique demonstrated here should prove attractive for applications ranging from microscopy to remote sensing. Thanks to the high precision and fast response speed, this method may pave a new way for measuring the topography of the thick samples, such as biological tissues.

Enhanced axial resolution of lattice light sheet microscopy by fluorescence differential detection

Lattice light-sheet microscopy (LLSM) is promising in long-term biological volumetric imaging due to its high spatiotemporal resolution and low phototoxicity. However, three-dimensional (3D) isotropic spatial resolution remains an unmet goal in LLSM because of its poorer axial resolution. Combing LLSM with fluorescence differential detection, namely LLSDM, has been proposed to improve the axial resolution of LLSM in simulation. It demonstrates the possibility of further enhancing the axial resolution in 3D volumetric imaging with LLSM by specifically discarding the off-focus photons captured using a complementary optical lattice (OL) profile generated with additional 0-π phase modulation at the objective pupil plane. The direct generation of the complementary lattice profile using the binary phase modulator conjugated to the sample plane for amplitude modulation, as used in LLSM, is also permittable. Nevertheless, the previously proposed configuration fails to provide a symmetric complementary lattice pattern along the axial axis, thus leading to the imbalanced off-focus photon suppression in the reconstructed images after subtraction [Opt. Lett.45, 2854 (2020)10.1364/OL.393378]. Here, we modified the LLSDM theory which can produce an ideal complementary lattice pattern with central zero intensity and symmetrically distributed sidelobes. We also analyzed the impact of numerical aperture matching between the original and complementary lattice patterns and presented the consistency between the simulated and experimental results. As demonstrated by imaging the distribution of fluorescent beads and microtubules in fixed U2OS cells, as well as the dynamics of filopodia in live U2OS cells, LLSDM provides about 1.5 times improvement in axial resolution, and higher imaging contrast compared with traditional LLSM.

Quantitative phase imaging through an ultra-thin lensless fiber endoscope

Quantitative phase imaging (QPI) is a label-free technique providing both morphology and quantitative biophysical information in biomedicine. However, applying such a powerful technique to in vivo pathological diagnosis remains challenging. Multi-core fiber bundles (MCFs) enable ultra-thin probes for in vivo imaging, but current MCF imaging techniques are limited to amplitude imaging modalities. We demonstrate a computational lensless microendoscope that uses an ultra-thin bare MCF to perform quantitative phase imaging with microscale lateral resolution and nanoscale axial sensitivity of the optical path length. The incident complex light field at the measurement side is precisely reconstructed from the far-field speckle pattern at the detection side, enabling digital refocusing in a multi-layer sample without any mechanical movement. The accuracy of the quantitative phase reconstruction is validated by imaging the phase target and hydrogel beads through the MCF. With the proposed imaging modality, three-dimensional imaging of human cancer cells is achieved through the ultra-thin fiber endoscope, promising widespread clinical applications.

Quantification of Motility in Bacillus subtilis at Temperatures Up to 84°C Using a Submersible Volumetric Microscope and Automated Tracking

We describe a system for high-temperature investigations of bacterial motility using a digital holographic microscope completely submerged in heated water. Temperatures above 90°C could be achieved, with a constant 5°C offset between the sample temperature and the surrounding water bath. Using this system, we observed active motility in Bacillus subtilis up to 66°C. As temperatures rose, most cells became immobilized on the surface, but a fraction of cells remained highly motile at distances of >100 μm above the surface. Suspended non-motile cells showed Brownian motion that scaled consistently with temperature and viscosity. A novel open-source automated tracking package was used to obtain 2D tracks of motile cells and quantify motility parameters, showing that swimming speed increased with temperature until ∼40°C, then plateaued. These findings are consistent with the observed heterogeneity of B. subtilis populations, and represent the highest reported temperature for swimming in this species. This technique is a simple, low-cost method for quantifying motility at high temperatures and could be useful for investigation of many different cell types, including thermophilic archaea.

Cross-Polarized Optical Coherence Tomography System with Unpolarized Light

Cross-polarized optical coherence tomography offers improved contrast for samples which can alter the polarization of light when it interacts with the sample. This property has been utilized to screen pathological conditions in several organs. Existing cross-polarized optical coherence tomography systems require several polarization-controlling elements to minimize the optical fiber movement-related image artifacts. In this work, we demonstrate a cross-polarized optical coherence tomography system using unpolarized light and only two quarter-wave plates, which is free from fiber-induced image artifacts. The simplicity of the approach will find many applications in clinical settings.

Modeling of speckle decorrelation in digital Fresnel holographic interferometry

This paper presents analytical modeling of the speckle decorrelation noise in digital Fresnel holographic interferometry. The theoretical analysis is carried out by considering the complex coherence factor between two speckled images from two digitally reconstructed holograms at two different instants. The expression giving the modulus of the coherence factor is established and depends on the local surface deformation and parameters from the holographic setup. The analysis is supported by realistic simulations and experiments. Both simulations and experimental results exhibit a very good agreement with the theoretical prediction.

Roadmap on digital holography [Invited]

This Roadmap article on digital holography provides an overview of a vast array of research activities in the field of digital holography. The paper consists of a series of 25 sections from the prominent experts in digital holography presenting various aspects of the field on sensing, 3D imaging and displays, virtual and augmented reality, microscopy, cell identification, tomography, label-free live cell imaging, and other applications. Each section represents the vision of its author to describe the significant progress, potential impact, important developments, and challenging issues in the field of digital holography.

Quantitative phase imaging in digital holographic microscopy based on image inpainting using a two-stage generative adversarial network

Based on the hologram inpainting via a two-stage Generative Adversarial Network (GAN), we present a precise phase aberration compensation method in digital holographic microscopy (DHM). In the proposed methodology, the interference fringes of the sample area in the hologram are firstly removed by the background segmentation via edge detection and morphological image processing. The vacancy area is then inpainted with the fringes generated by a deep learning algorithm. The image inpainting finally results in a sample-free reference hologram containing the total aberration of the system. The phase aberrations could be deleted by subtracting the unwrapped phase of the sample-free hologram from our inpainting network results, in no need of any complex spectrum centering procedure, prior knowledge of the system, or manual intervention. With a full and proper training of the two-stage GAN, our approach can robustly realize a distinct phase mapping, which overcomes the drawbacks of multiple iterations, noise interference or limited field of view in the recent methods using self-extension, Zernike polynomials fitting (ZPF) or geometrical transformations. The validity of the proposed procedure is confirmed by measuring the surface of preprocessed silicon wafer with a Michelson interferometer digital holographic inspection platform. The results of our experiment indicate the viability and accuracy of the presented method. Additionally, this work can pave the way for the evaluation of new applications of GAN in DHM.

Spatio-temporal performance in an incoherent holography lattice light-sheet microscope (IHLLS)

We propose an Incoherent holography detection technique for lattice light-sheet (IHLLS) systems for 3D imaging without moving either the sample stage or the detection microscope objective, providing intrinsic instrumental simplicity and high accuracy when compared to the original LLS schemes. The approach is based on a modified dual-lens Fresnel Incoherent Correlation Holography technique to produce a complex hologram and to provide the focal distance needed for the hologram reconstruction. We report such an IHLLS microscope, including characterization of the sensor performance, and demonstrate a significant contrast improvement on beads and neuronal structures within a biological test sample as well as quantitative phase imaging. The IHLLS has similar or better transverse performances when compared to the LLS technique. In addition, the IHLLS allows for volume reconstruction from fewer z-galvo displacements, thus facilitating faster volume acquisition.

Pursuing the Diffraction Limit with Nano-LED Scanning Transmission Optical Microscopy

Recent research into miniaturized illumination sources has prompted the development of alternative microscopy techniques. Although they are still being explored, emerging nano-light-emitting-diode (nano-LED) technologies show promise in approaching the optical resolution limit in a more feasible manner. This work presents the exploration of their capabilities with two different prototypes. In the first version, a resolution of less than 1 µm was shown thanks to a prototype based on an optically downscaled LED using an LED scanning transmission optical microscopy (STOM) technique. This research demonstrates how this technique can be used to improve STOM images by oversampling the acquisition. The second STOM-based microscope was fabricated with a 200 nm GaN LED. This demonstrates the possibilities for the miniaturization of on-chip-based microscopes.

A Novel Approach for a Chip-Sized Scanning Optical Microscope

The recent advances in chip-size microscopy based on optical scanning with spatially resolved nano-illumination light sources are presented. This new straightforward technique takes advantage of the currently achieved miniaturization of LEDs in fully addressable arrays. These nano-LEDs are used to scan the sample with a resolution comparable to the LED sizes, giving rise to chip-sized scanning optical microscopes without mechanical parts or optical accessories. The operation principle and the potential of this new kind of microscope are analyzed through three different implementations of decreasing LED dimensions from 20 µm down to 200 nm.

Displacement and strain mapping for osteocytes under fluid shear stress using digital holographic microscopy and digital image correlation

Osteocytes, as the mechano-sensors in bone, are always subjected to fluid shear stress (FSS) from the surrounding matrix. Quantification of FSS-induced cellular deformation is significant for clarifying the "perceive and transmit" process of cellular mechanotransduction. In this research, a label-free displacement and strain mapping method based on digital holographic microscopy (DHM) and digital image correlation (DIC) is introduced. The method, which is termed DHM-DIC, innovatively utilizes surface features extracted from holographic phase images instead of speckles as the metric for DIC searching. Simulation results on a hemisphere validate the feasibility of DHM-DIC. Displacement and strain maps of living osteocytes under 1.5 Pa FSS are evaluated from DHM-DIC and present good agreement with our previous finite element modeling results.

A Review on Lossless Compression Techniques for Wireless Capsule Endoscopic Data

BACKGROUND

The videos produced during wireless capsule endoscopy have larger data size causing difficulty in transmission with limited bandwidth. The constraint on wireless capsule endoscopy hinders the performance of the compression module.

OBJECTIVES

The objectives of this paper are as follows: (i) to conduct an extensive review of the lossless compression techniques and (ii) to find out the limitations of the existing system and the possibilities for improvement.

METHODS

The literature review was conducted with a focus on the compression schemes satisfying minimum computational complexity, less power dissipation and low memory requirements for hardware implementation. A thorough study of various lossless compression techniques was conducted under two perspectives, i.e., techniques applied to Bayer CFA and RGB images. The detail of the various stages of wireless capsule endoscopy compression was investigated to have a better understanding. The suitable performance metrics for evaluating the compression techniques were listed from various literature studies.

RESULTS

In addition to the Gastrolab database, WEO clinical endoscopy atlas and Gastrointestinal atlas were found to be better alternatives for experimentation. Pre-processing operations, especially new subsampling patterns need to be given more focus to exploit the redundancies in the images. Investigations showed that encoder module can be modified to bring more improvement towards compression. The real-time endoscopy still exists as a promising area for exploration.

CONCLUSION

This review presents a research update on the details of wireless capsule endoscopy compression together with the findings as an eye-opener and guidance for further research.

Heterodyne phase shifting method in scanning probe microscopy

The present paper describes a novel implementation of the continuous phase shifting method (PSM), named heterodyne holography, in a scanning probe microscope configuration, able to retrieve the complex scattered field in on-axis configuration. This can be achieved by acquiring a continuous sequence of holograms at different wavelengths in just a single scan through the combination of scanning interference microscopy and a low-coherent signal acquired in the frequency domain. This method exploits the main advantages of the phase shifting technique and avoids some limits relative to off-axis holography in providing quantitative phase imaging.

High-speed imaging of the sound field by parallel phase-shifting digital holography

Sound field imaging techniques have been found very useful for acoustic designs. Building on this idea, innovative techniques are needed and presented in this paper, where we report on developed imaging of the sound field radiated from speakers by parallel phase-shifting digital holography. We adopted an ultrasonic wave radiated from a speaker for an object. The phase distribution of the light wave was modulated by the sound field radiated from the speaker. The modulated phase distribution was recorded in the form of multiplexed phase-shifted holograms at the frame rate of 100,000 fps. A 40,000 Hz sound field radiated from a speaker is used as an observation target. Our proposed method can implement the imaging of the sound field successfully. Also, in order to demonstrate the digital refocusing capability of digital holography, we set two speakers, whose difference in depth positions was 6.6 cm, as a long-depth object. We demonstrated the digital refocusing on the two speakers along with the capability of measuring the positions of the objects. Furthermore, we succeeded in imaging of 40,000 Hz and 41,000 Hz sound fields radiated from the two speakers. The presented experimental results showed that parallel phase-shifting digital holography is very useful and suitable for sound field imaging.

Digital plasmonic holography with iterative phase retrieval for sensing

Propagating surface plasmon waves have been used for many applications including imaging and sensing. However, direct in-plane imaging of micro-objects with surface plasmon waves suffers from the lack of simple, two-dimensional lenses, mirrors, and other optical elements. In this paper, we apply lensless digital holographic techniques and leakage radiation microscopy to achieve in-plane surface imaging with propagating surface plasmon waves. As plasmons propagate in two-dimensions and scatter from various objects, a hologram is formed over the surface. Iterative phase retrieval techniques applied to this hologram remove twin image interference for high-resolution in-plane imaging and enable further applications in real-time plasmonic phase sensing.

Using the Gouy phase anomaly to localize and track bacteria in digital holographic microscopy 4D images

Described over 100 years ago, the Gouy phase anomaly refers to the additional π phase shift that is accumulated as a wave passes through focus. It is potentially useful in analyzing any type of phase-sensitive imaging; in light microscopy, digital holographic microscopy (DHM) provides phase information in the encoded hologram. One limitation of DHM is the weak contrast generated by many biological cells, especially unpigmented bacteria. We demonstrate here that the Gouy phase anomaly may be detected directly in the phase image using the z-derivative of the phase, allowing for precise localization of unlabeled, micrometer-sized bacteria. The use of dyes that increase phase contrast does not improve detectability. This approach is less computationally intensive than other procedures such as deconvolution and is relatively insensitive to reconstruction parameters. The software is implemented in an open-source FIJI plug-in.

Polarization-sensitive differential phase-contrast microscopy

We present a novel, to the best of our knowledge, form of polarization microscopy capable of producing quantitative optic-axis and phase retardation maps of transparent and anisotropic materials. The proposed method operates on differential phase-contrast (DPC) microscopy that produces a phase image of a thin specimen using multi-axis intensity measurements. For polarization-sensitive imaging, patterned illumination light is circularly polarized to illuminate a specimen. The light transmitted through a specimen is split into two orthogonal polarization states and measured by an image sensor. Subsequent DPC computation based on the illumination patterns, acquired images, and the imaging model enables the retrieval of polarization-dependent quantitative phase images, which are utilized to reconstruct the orientation and retardation of the specimen. We demonstrate the validity of the proposed method by measuring the optic-axis and phase retardation maps of calibrated and various anisotropic samples.

Holotomography: Refractive Index as an Intrinsic Imaging Contrast for 3-D Label-Free Live Cell Imaging

Live cell imaging provides essential information in the investigation of cell biology and related pathophysiology. Refractive index (RI) can serve as intrinsic optical imaging contrast for 3-D label-free and quantitative live cell imaging, and provide invaluable information to understand various dynamics of cells and tissues for the study of numerous fields. Recently significant advances have been made in imaging methods and analysis approaches utilizing RI, which are now being transferred to biological and medical research fields, providing novel approaches to investigate the pathophysiology of cells. To provide insight into how RI can be used as an imaging contrast for imaging of biological specimens, here we provide the basic principle of RI-based imaging techniques and summarize recent progress on applications, ranging from microbiology, hematology, infectious diseases, hematology, and histopathology.

Measurement of Cell Volume Using In-Line Digital Holography

The measurement of the volume of blood cells is important for clinical diagnosis and patient management. While digital holography microscopy (DHM) has been used to obtain such information, previous off-axis setups usually involve a separated reference beam and are thus not very easy to implement. Here, we use the simple in-line Gabor setup without separation of a reference beam to measure the shape and volume of cells mounted on glass slides. Inherent to the in-line holograms, the reconstructed phase of the object is affected by the virtual image noise, producing errors in the cell volume measurement. We optimized our approach to use a single hologram without phase retrieval, increasing distance between cell and hologram plane to reduce the measurement error of cell volume to less than 6% in some instances. Therefore, the in-line Gabor setup can be a useful and simple tool to obtain volumetric and morphologic cellular information.

Critical comparison of image analysis workflows for quantitative cell morphological evaluation in assessing cell response to biomaterials

Quantitative image analysis is an important tool in understanding cell fate processes through the study of cell morphological changes in terms of size, shape, number, and orientation. In this context, this work explores systematically the main challenges involved in the quantitative analysis of fluorescence microscopy images and also proposes a new protocol while comparing its outcome with the widely used Image J analysis. It is important to mention that fluorescence microscopy is by far most widely used in biocompatibility analysis (observing cell fate changes) of implantable biomaterials. In this study, we employed two different image analyses toolsets: (i) the conventionally employed ImageJ software, and (ii) a recently developed automated digital image analyses framework, called ImageMKS. While ImageJ offers a powerful toolset for image analyses, it requires sophisticated user expertise to design and iteratively refine the analyses workflow. This workflow primarily comprises a sequence of image transformations that typically involve de-noising and labelling of features. On the other hand, ImageMKS automates the image analyses protocol to a large extent, and thereby mitigates the influence of the user bias on the final results. This aspect is addressed using a case study of C2C12 mouse myoblast cells grown on Poly(vinyldiene difluoride) based polymeric substrates in the presence of an external electric field. In particular, we used a number of fluorescence microscopy images of murine myoblasts (muscle precursor cells) grown on Poly (vinylidene difluoride), PVDF based nanobiocomposites under the influence of electric field. It was observed that when compared with the findings obtained from ImageJ, ImageMKS workflows consistently produced more reliable results that correlated better with the prior studies. Furthermore, the MKS workflows required much less user time, because of their automation.

Label-free conduction velocity mapping and gap junction assessment of functional iPSC-Cardiomyocyte monolayers

Cardiac conduction is an important function of the heart. To date, accurate measurement of conduction velocity (CV) in vitro is hindered by the low spatial resolution and poor signal-to-noise ratio of microelectrode arrays (MEAs), or the cytotoxicity and end-point analysis of fluorescence optical imaging. Here, we have developed a new label-free method based on defocused brightfield imaging to quantify CV by analyzing centroid displacements and contraction trajectories of each cardiomyocyte in a monolayer of human stem cell-derived cardiomyocytes (iPSC-CMs). Our data revealed that the time delay between intracellular calcium release and the initiation of cell contraction is highly consistent across cardiomyocytes; however, the duration a cell takes to reach its maximum beating magnitude varies significantly, proving that the time delay in excitation-contraction coupling is largely constant in iPSC-CMs. Standard calcium imaging of the same iPSC-CM populations (~10⁶ cells) was conducted for comparison with our label-free method. The results confirmed that our label-free method was capable of achieving highly accurate CV mapping (17.64 ± 0.89 cm/s vs. 17.95 ± 2.29 cm/s, p-value>0.1). Additionally, our method effectively revealed various shapes in cell beating pattern. We also performed label-free CV mapping on disease-specific iPSC-CM monolayers with plakophilin-2 (PKP2) knockdown, which effectively quantified their low CV values and further validated the arrhythmogenic role of PKP2 mutation in arrhythmogenic right ventricular cardiomyopathy (ARVC) through the disruption of cardiac conduction. The label-free method offers a cytotoxic-free technique for long-term measurement of dynamic beating trajectories, beating propagation and conduction velocities of cardiomyocyte monolayers.

High-resolution quantitative phase imaging based on a spatial light modulator and incremental binary random sampling

We propose a single-beam high-resolution quantitative phase imaging method based on a spatial light modulator (SLM) and an incremental binary random sampling (IBRS) algorithm. In this method, the image of the test object presents on the image sensor through an optical microscopy system composed of an objective lens and a collimating lens. A transmittance SLM displaying a group of well-designed IBRS patterns is inserted in the optical microscopy system to modulate the object wavefront. The phase information of the object image can be quantitatively retrieved from the recorded intensities using the IBRS algorithm and the amplitude obtained directly from the diffraction intensity. The IBRS algorithm employed in our method has higher accuracy for phase retrieval compared with our previously proposed complementary random sampling algorithm, which is confirmed by simulations. Further, we demonstrate experimentally the feasibility of our method through several examples: phase imaging of immersion oil droplets with a diffraction-limited lateral resolution of 1.54 µm and a few microbiological specimens with 0.70 µm. Experimental results reveal that our proposed method provides a feasible single-beam technique for quantitative phase imaging with a high spatial resolution.

Digital lensless holographic microscopy: numerical simulation and reconstruction with ImageJ

The description and validation of an ImageJ open-source plugin to numerically simulate and reconstruct digital lensless holographic microscopy (DLHM) holograms are presented. Two modules compose the presented plugin: the simulation module implements a discrete version of the Rayleigh-Somerfield diffraction formula, which allows the user to directly build a simulated hologram from a known phase and/or amplitude object by just introducing the geometry parameters of the simulated setup; the plugin's reconstruction module implements a discrete version of the Kirchhoff-Helmholtz diffraction integral, thus allowing the user to reconstruct DLHM holograms by introducing the parameters of the acquisition setup and the desired reconstruction distance. The plugin offers the two said modules within the robust environment provided by a complete set of built-in tools for image processing available in ImageJ. While the simulation module has been validated through the evaluation of the forecasted lateral resolution of a DLHM setup in terms of the numerical aperture, the reconstruction module is tested by means of reconstructing experimental DLHM holograms of biological samples.

Holographic Imaging of Insect Cell Cultures: Online Non-Invasive Monitoring of Adeno-Associated Virus Production and Cell Concentration

The insect cell-baculovirus vector system has become one of the favorite platforms for the expression of viral vectors for vaccination and gene therapy purposes. As it is a lytic system, it is essential to balance maximum recombinant product expression with harvest time, minimizing product exposure to detrimental proteases. With this purpose, new bioprocess monitoring solutions are needed to accurately estimate culture progression. Herein, we used online digital holographic microscopy (DHM) to monitor bioreactor cultures of Sf9 insect cells. Batches of baculovirus-infected Sf9 cells producing recombinant adeno-associated virus (AAV) and non-infected cells were used to evaluate DHM prediction capabilities for viable cell concentration, culture viability and AAV titer. Over 30 cell-related optical attributes were quantified using DHM, followed by a forward stepwise regression to select the most significant (p < 0.05) parameters for each variable. We then applied multiple linear regression to obtain models which were able to predict culture variables with root mean squared errors (RMSE) of 7 × 105 cells/mL, 3% for cell viability and 2 × 103 AAV/cell for 3-fold cross-validation. Overall, this work shows that DHM can be implemented for online monitoring of Sf9 concentration and viability, also permitting to monitor product titer, namely AAV, or culture progression in lytic systems, making it a valuable tool to support the time of harvest decision and for the establishment of controlled feeding strategies.

Three-dimensional measurement of a particle field using phase retrieval digital holography

Digital inline holography (DIH) has long been used to measure the three-dimensional (3D) distribution of micrometer particles in suspensions. However, DIH experiences a virtual image problem that limits the particle density and the placement of the hologram plane relative to the sample volume. Here, we apply virtual-image-free phase retrieval digital holography (PRDH) to detect opaque particles in 3D volumes that exceed $ 2000\;{\rm particles}/{{\rm mm}^3} $2000particles/mm³. PRDH is based on recording two holograms whose planes are displaced along the optical axis, and then reconstructing the complete optical waves estimated by the iterative phase retrieval algorithm. Both numerical and experimental tests are performed, and results show that PRDH recovers the original 3D particle distributions even when the hologram planes are within the particle suspensions. Moreover, compared to single-hologram-based DIH, PRDH is proved to have better particle detection qualities. The uncertainty in the localization of particle centers is reduced to less than one particle diameter.

Cone-shaped optical fiber tip for cost-effective digital lensless holographic microscopy

In this work, the development and application of a cost-effective and robust digital lensless holographic microscopy (DLHM) system is presented. In the simple architecture of DLHM based on a point source and a digital camera, the production of the former is introduced by means of an engineered step-index optical fiber with a cone-shaped end tip. The conventional and regularly expensive point source in DLHM is produced by means of a high-numerical-aperture microscope objective and a metallic wavelength-sized pinhole. The proposed replacement renders to DLHM additional simplicity of building, in addition to mechanical stability and robustness, and further reduces the cost of the microscope. The simplified cost-effective DLHM architecture is utilized for imaging resolution test targets and samples of human blood and pond water, revealing competitive mechanical stability and trustable phase images of the imaged specimens.

Single-shot sequential projection phase retrieval and 3D localization from chromatic aberration

A phase retrieval method based on sequential projection and chromatic aberration is reported. Construction of this method includes a red, green and blue (RGB) LED, an objective and a color camera. Owing to the chromatic aberration characteristics of the objective, three color images obtained by the color camera correspond to three equivalent propagation planes. Equivalent relative distances among these planes can be obtained by defining and iteratively minimizing the convergence index. Then, sequential projection strategy is used for phase retrieval in each image plane. Based on the recovered phase information and angular spectrum propagation principle, digital refocusing and 3D localization can be achieved for each subregion of the sample. Finally, the feasibility of our method is demonstrated by simulations and experiments.

Comparative phase imaging of live cells by digital holographic microscopy and transport of intensity equation methods

We describe a microscopic setup implementing phase imaging by digital holographic microscopy (DHM) and transport of intensity equation (TIE) methods, which allows the results of both measurements to be quantitatively compared for either live cell or static samples. Digital holographic microscopy is a well-established method that provides robust phase reconstructions, but requires a sophisticated interferometric imaging system. TIE, on the other hand, is directly compatible with bright-field microscopy, but is more susceptible to noise artifacts. We present results comparing DHM and TIE on a custom-built microscope system that allows both techniques to be used on the same cells in rapid succession, thus permitting the comparison of the accuracy of both methods.

Dual wavelength digital holographic imaging of layered structures

We present a dual wavelength digital holographic technique for three-dimensional microscopic imaging of layered structures, where layers are separated from one another by the axial distances exceeding the wavelength of imaging light. Our methodology not only provides the three-dimensional structure of each layer, but also allows the height differentiation of distinct layers. We have also implemented a technique suppressing low intensity signal when no reliable phase information can be extracted, based on the quality of the interference fringe pattern. We utilize a dual wavelength setup, where the combination of two overlapping interferometers enables simultaneous acquisition of two phase profiles. We demonstrate that this imaging modality is particularly well-suited for imaging of multilayered electrode structures embedded in glass.

Wide area quantitative phase microscopy by spatial phase scanning digital holography

A new technique of digital holographic microscopy is introduced for large area quantitative phase microscopy, dubbed spatial phase scanning digital holography, where the object specimen in an interferometer is scanned across the tilted reference phase field, while acquiring camera frames at regular intervals. Both the large area scan and phase shift acquisition are achieved in one sweep, using a simple optomechanical system. The technique can be useful in diverse applications such as fast scans of blood smear, cell and tissue cultures, and microelectronic surface profiles.

Liquid-phase electron microscopy imaging of cellular and biomolecular systems

Liquid-phase electron microscopy, a new method for real-time nanoscopic imaging in liquid, makes it possible to study cells or biomolecules with a singular combination of spatial and temporal resolution. We review the state of the art in biological research in this growing and promising field.

Inline application of digital holography [Invited]

We describe the inline integration of the digital holographic sensor HoloTop in a precision turning plant. A fully automated part-handling system that fulfills the requirements for cycle time and stability was built and integrated into the production process. The inspection system has been running in multishift operation since 2015. For the first time, to the best of our knowledge, the results of one-year, long-term height measurements of 10 million parts under rough production conditions are presented to verify the suitability for industrial use.

Speckle-field digital polarization holographic microscopy

We present a new polarization holographic microscopy technique based on speckle-field illumination with enhanced spatial resolution and controlled coherent noise reduction. The proposed technique employs a spatial light modulator for the generation of a sequential speckle pattern for the illumination of the sample. The developed microscope is capable of simultaneous extraction of orthogonal polarization components of the field emanating from the sample. We demonstrate the potential features of the technique by presenting spatially resolved images of the known samples and the inhomogeneous anisotropic samples. The technique has substantial significance in biomedical imaging with digital auto-focusing and complex field imaging.

Phase curvature compensation in digital holographic microscopy based on phase gradient fitting and optimization

We propose a numerical method for phase curvature compensation in digital holographic microscopy, in which the phase curvature is compensated for by subtracting a numerical phase mask from the distorted phase. The parameters of the phase mask are obtained based on phase gradient fitting and optimization, in which the initial mask parameters are obtained by fitting the phase gradient, and then more accurate mask parameters are determined using a spectrum energy search. The compensation can be executed in a hologram without extra devices or any prior knowledge of the setup and specimen. A computer simulation and experimental results demonstrated the feasibility of the proposed method.

Simultaneous dual-wavelength digital holographic microscopy with compensation of chromatic aberration for accurate surface characterization

In this paper, a simultaneous dual-wavelength digital holographic microscope with compensation of chromatic aberration is presented. The microscope has been used to calibrate two standard step heights of 1340 and 1000 nm. The extracted results have been compared with those extracted by the single-shot, dual-wavelength digital holographic microscope, and the comparison has shown that measurements by the proposed microscope are more accurate by around 40 nm. We claim that the high accuracy in measurement is due to the fact that the proposed system works in real time with very low chromatic aberration. Judging the performance of the measurement process over time is explained by the control charts. Evidence from the control measurements sets a probable limit on precision for this process at about 3σ=3.5  nm.