Full-field Optical Coherence Tomography System Research Articles

Modeling multiply scattered light in full-field optical coherence tomographic imaging of biological tissues

In this work, we have constructed a handheld full-field optical coherence tomography (FFOCT) system with high spatial resolution, and the Michelson interferometer (served as compensating optical path) and Fizeau interferometer (served as detection) were mounted in series in this system. The expressions of the multiple scattered light decomposed as the sum of incoherent and coherent contribution were derived based on the diffusion equation and mutual coherence function, and the variations of resolution with imaging depth were derived by using the model of multiple scattering. Experiments were conducted on samples composed of the USAF 1951 resolution target covered by the onion slice and human finger-skin slice, respectively, to validate and evaluate the accuracy of our model. A method of image restoration is then presented in which the relation between the resolution and measuring depth and the simulated PSF in the absence of multiple scattering are employed. With the help of this method the lateral resolution of 2.2 μm and 3.9 μm were separately achieved in the onion slice (measuring depth of 64 μm) and human finger-skin slice (measuring depth of 60 μm), and more tiny structures in the en face image of human finger-skin and moth wings were visible. In addition, a novel method for estimating the values of mean free path and anisotropy of scattering medium was proposed.

Main Factor Analysis for Imaging Performance of Hand-Held Full-Field Optical Coherence Tomography System with Dual Interferometers

Main Factor Analysis for Imaging Performance of Hand-Held Full-Field Optical Coherence Tomography System with Dual Interferometers

In vivo depth-resolved morphological analysis of compound eye dioptric system using full-field optical coherence tomography

Presence of a range of insects with different shape, size, and structural morphology of compound eye reveals the diverse pathways adopted by the nature striving to develop a flawless vision under given set of constraints. Exploration of various compound eyes allows researchers to understand better the various stages of visual system development in insects. By far, the most frequently used techniques for the investigation of structural morphology of a compound eye are scanning electron microscopy, transmission electron microscopy, microcomputed tomography, and histology. The nature of implementation of these techniques restricts in vivo studies, and also the requirements for a complicated and costly setup put limitations on its availability at small research facilities. Despite the widespread acceptance of the optical coherence tomography (OCT) as a standard tool for the various ophthalmological investigations, reports on compound eye study using OCT are uncommon. We demonstrate the application of a full-field OCT (FF-OCT) system for depth-resolved structural imaging and morphological analysis of the compound eye dioptric system. We report the application of a time-domain FF-OCT system for depth-resolved en face and three dimensional in vivo imaging of dragonfly’s prominent compound eye with resolutions of 2.8 and 7.2 μm in lateral and axial directions, respectively. The study shows the effectiveness of the FF-OCT approach for label-free and rapid structural morphology analysis of compound eyes. The results have been compared with the histology results reported in literature.

Photoaging and Sequential Function Reversal with Cellular-Resolution Optical Coherence Tomography in a Nude Mice Model.

Although nude mice are an ideal photoaging research model, skin biopsies result in inflammation and are rarely performed at baseline. Meanwhile, studies on antiphotoaging antioxidants or rejuvenation techniques often neglect the spontaneous reversal capacity. Full-field optical coherence tomography (FFOCT) can acquire cellular details noninvasively. This study aimed to establish a photoaging and sequential function reversal nude mice model assisted by an in vivo cellular resolution FFOCT system. We investigated whether a picosecond alexandrite laser (PAL) with a diffractive lens array (DLA) accelerated the reversal. In the sequential noninvasive assessment using FFOCT, a spectrophotometer, and DermaLab Combo®, the photodamage percentage recovery plot demonstrated the spontaneous recovery capacity of the affected skin by UVB-induced transepidermal water loss and UVA-induced epidermis thickening. A PAL with DLA not only accelerated skin barrier regeneration with epidermal polarity, but also increased dermal neocollagenesis, whereas the nonlasered group still had >60% collagen intensity loss and 40% erythema from photodamage. Our study demonstrated that FFOCT images accurately resemble the living tissue. The photoaging and sequential function reversal model provides a reference to assess the spontaneous recovery capacity of nude mice from photodamage. This model can be utilized to evaluate the sequential noninvasive photodamage and reversal effects after other interventions.

Methods and applications of full-field optical coherence tomography: a review.

Significance: Full-field optical coherence tomography (FF-OCT) enables en face views of scattering samples at a given depth with subcellular resolution, similar to biopsy without the need of sample slicing or other complex preparation. This noninvasive, high-resolution, three-dimensional (3D) imaging method has the potential to become a powerful tool in biomedical research, clinical applications, and other microscopic detection.Aim: Our review provides an overview of the disruptive innovations and key technologies to further improve FF-OCT performance, promoting FF-OCT technology in biomedical and other application scenarios.Approach: A comprehensive review of state-of-the-art accomplishments in OCT has been performed. Methods to improve performance of FF-OCT systems are reviewed, including advanced phase-shift approaches for imaging speed improvement, methods of denoising, artifact reduction, and aberration correction for imaging quality optimization, innovations for imaging flux expansion (field-of-view enlargement and imaging-depth-limit extension), new implementations for multimodality systems, and deep learning enhanced FF-OCT for information mining, etc. Finally, we summarize the application status and prospects of FF-OCT in the fields of biomedicine, materials science, security, and identification.Results: The most worth-expecting FF-OCT innovations include combining the technique of spatial modulation of optical field and computational optical imaging technology to obtain greater penetration depth, as well as exploiting endogenous contrast for functional imaging, e.g., dynamic FF-OCT, which enables noninvasive visualization of tissue dynamic properties or intracellular motility. Different dynamic imaging algorithms are compared using the same OCT data of the colorectal cancer organoid, which helps to understand the disadvantages and advantages of each. In addition, deep learning enhanced FF-OCT provides more valuable characteristic information, which is of great significance for auxiliary diagnosis and organoid detection.Conclusions: FF-OCT has not been completely exploited and has substantial growth potential. By elaborating the key technologies, performance optimization methods, and application status of FF-OCT, we expect to accelerate the development of FF-OCT in both academic and industry fields. This renewed perspective on FF-OCT may also serve as a road map for future development of invasive 3D super-resolution imaging techniques to solve the problems of microscopic visualization detection.

Optimization of the performance of hand‐held FFOCT scanner

In this work, we have established an improved handheld full-field optical coherence tomography (FFOCT) system based on the tandem structure of Michelson interferometer and Fizeau interferometer. We have analyzed the influence of wave aberrations of the handheld FFOCT as a function of orientation deviation of mirror M2 on the system point spread function (PSF) by using the nodal aberration theory. It is found that the range of the orientation deviation of mirror M2 must be less than 0.1° to generate stable FFOCT images. The effect of wave aberrations of defocus as a result of the refractive index difference on the system PSF has also been calculated, the measuring depth in the case of the sample with refractive index of 1.5 and the propagation medium is air (n = 1) should be less than 20 μm. Theoretical analyses have been demonstrated by the experimental results of aluminum powder, onion and porcine myocardium. By comparing the defocused FFOCT images of aluminum powder with defocused FFOCT images of onion and porcine myocardium, it can be concluded that reducing the refractive index difference between the medium and sample can increase the imaging depth of handheld FFOCT system.

Full-field optical coherence tomography using tunable-path-difference source for three-dimensional multi-layered imaging with enhanced signal-to-noise ratio

We demonstrate a novel scheme for a full-field optical coherence tomography (FF-OCT) system using a tunable-path-difference source (TPDS). Recently, the comb-spacing-swept source (CSWS) method, including TPDS, has been successfully applied to improve the stability of three-dimensional (3D) time-domain (TD)-FF-OCT imaging by eliminating the mechanical scanning of the reference path, which has been the main source of undesirable noise in general TD-OCT. However, CSWS-OCT has the drawback of a degraded signal-to-noise ratio (SNR) from a duplicated interferometer structure. In this study, we improve the configuration of the TPDS so that it can be connected directly to the measurement interferometer, including the sample and reference paths of the OCT system. In our novel TPDS-OCT, a propagating optical path through only a single interferometer was critically helpful in enhancing the SNR while retaining the benefits of CSWS-OCT, and a free-space full-field interferometer of dual-input Mach–Zehnder type also has an even higher imaging performance owing to the simultaneous exposure of optical intensity into the camera and the short image acquisition time laterally. We theoretically and experimentally demonstrated the benefits of TPDS-OCT compared with CSWS-OCT in terms of SNR. We also demonstrated 3D multi-layered imaging, which CSWS-OCT cannot achieve owing to its limited SNR.

Effects of orientation deviation of a beam splitter and a reference mirror on the stability of a two-interferometer-based handheld FFOCT imaging probe

In this work, we present a handheld full-field optical coherence tomography (FFOCT) system based on a series connection of two interferometers: the Michelson interferometer is used as a compensation part and the Fizeau interferometer is used as a detection part. Owing to the common-path arrangement of the Fizeau interferometer, this handheld FFOCT system has a compact detection arm and is insensitive to the external disturbance. A high-output halogen lamp and high NA microscope objective contribute to achieving the spatial resolution of 0.7µm×0.5µm (transverse × axial). Low imaging stability is caused by an extremely short coherence length. We found that to generate en face images with high quality and high stability using a probe with an extremely short coherence length, the range of deviation of the orientation of the beam splitter must be less than 1°, and the range of orientation deviation of the mirror in the Michelson interferometer corresponds to the displacement between the two field stop images at a distance not to exceed 10 µm.

Application of Cellular Resolution Full-Field Optical Coherence Tomography in vivo for the Diagnosis of Skin Tumours and Inflammatory Skin Diseases: A Pilot Study

Background: Optical coherence tomography (OCT) has been shown to provide non-invasive diagnosis of common skin neoplasms, especially basal cell carcinoma. OCT produces a cross-sectional view of the tissue, similar to a traditionally sectioned histopathological view, but the resolution of conventional OCT is low and thus limits clinical application. Objectives: This study aimed to investigate the application ability of a full-field (FF)OCT system which was newly developed to scan the skin at the cellular level. Methods: Patients with skin tumours or inflammatory lesions warranting biopsy were consecutively enrolled. All lesions underwent clinical, dermoscopic, and OCT assessment, followed by routine biopsy. The adjacent normal skin was scanned for comparison. OCT images were interpreted (blinded to the biopsy results) and then compared with the histopathological diagnosis. Results: A total of 111 patients with 115 lesions completed the protocol, including 80 skin tumours, 28 inflammatory diseases, and 7 other diseases. Of the OCT images, 43.5% were of good quality and show expected features. Identifiable features of actinic keratosis, Bowen’s disease, basal cell carcinoma, extramammary Paget’s disease, seborrheic keratosis, large cell acanthoma, bullous pemphigoid, interface dermatitis, lichenoid tissue reaction, and psoriasis were demonstrated. Lesions are located deeply, and so some features were out of the field of view, accounting for 40.0% (46/115). Conclusions: This study expanded the ability of FFOCT for the clinical diagnosis of various skin conditions. This new optical technique can clearly visualise skin lesions located in the epidermis and upper dermis. It provided an effective way to perform digital skin biopsy in superficial skin diseases.

Single-shot multi-depth full-field optical coherence tomography using spatial frequency division multiplexing.

Fast 3D volumetric imaging has been essential for biology, medicine and industrial inspections, and various optical coherence tomography (OCT) methods have been developed to meet such needs. Point-scanning based approaches, such as swept-source OCT and spectral domain OCT, can obtain a depth information at once, but they require lateral scan for full 3D imaging. On the contrary, full-field OCT needs the scanning of imaging depth while it records a full lateral information at once. Here, we present a full-field OCT system that can obtain multi-depth information at once by a single-shot recording. We combine a 2D diffraction grating and a custom-made echelon to prepare multiple reference beams having different pathlengths and propagating angles. By recording a single interference image between the reflected wave from a sample and these multiple reference beams, we reconstruct full-field images at multiple depths associated with the pathlengths of the individual reference beams. We demonstrated the single-shot recording of 7 different depth images at 10 µm for biological tissues. Our method can potentially be useful for applications where high-speed recording of multiple en-face images is crucial.

Submicron spatial resolution optical coherence tomography for visualising the 3D structures of cells cultivated in complex culture systems

Three-dimensional (3D) configuration of in vitro cultivated cells has been recognised as a valuable tool in developing stem cell and cancer cell therapy. However, currently available imaging approaches for live cells have drawbacks, including unsatisfactory resolution, lack of cross-sectional and 3D images, and poor penetration of multi-layered cell products, especially when cells are cultivated on semitransparent carriers. Herein, we report a prototype of a full-field optical coherence tomography (FF-OCT) system with isotropic submicron spatial resolution in en face and cross-sectional views that provides a label-free, non-invasive platform with high-resolution 3D imaging. We validated the imaging power of this prototype by examining (1) cultivated neuron cells (N2A cell line); (2) multilayered, cultivated limbal epithelial sheets (mCLESs); (3) neuron cells (N2A cell line) and mCLESs cultivated on a semitransparent amniotic membrane (stAM); and (4) directly adherent colonies of neuron-like cells (DACNs) covered by limbal epithelial cell sheets. Our FF-OCT exhibited a penetrance of up to 150 μm in a multilayered cell sheet and displayed the morphological differences of neurons and epithelial cells in complex coculture systems. This FF-OCT is expected to facilitate the visualisation of cultivated cell products in vitro and has a high potential for cell therapy and translational medicine research.

Depth-resolved imaging of intercellular structures of large intestine with polarization-sensitive FFOCT

A polarization-sensitive full-field optical coherence tomography (PS-FFOCT) system was developed, which is capable of revealing both the reflectivity and phase-retardation caused by the anisotropic samples simultaneously with the lateral resolution of 0.98 µm and the axial resolution of 2.4 µm. The system was used to generate depth-resolved en face isotropic intensity images and birefringent images of the normal and abnormal large intestine tissue samples of human in vitro. To our knowledge, for the first time, the intestine gland cells and intercellular structures of normal intestine and infiltration of metaplasia of cancerous intestine tissue were observed in the en face images obtained with PS-FFOCT device. The resolution and effects of measurement errors are discussed. The results obtained in this work demonstrated the potential applications of PS-FFOCT technique in the rapid intra-operation diagnosis of cancers.

Light-efficient beamsplitter for Fourier-domain full-field optical coherence tomography.

Any full-field optical coherence tomography (FF-OCT) system wastes almost 75% of light, including 50% of the OCT signal, because it uses a 50/50 beamsplitter (BS) in the standard implementation. Here, a design of a light-efficient BS is presented that loses almost no light when implemented in Fourier-domain FF-OCT. It is based on pupil engineering and a small highly asymmetric BS. The presented signal-to-noise ratio (SNR) analysis demonstrates almost four times improvement over the conventional design. In addition, it is shown that the light-efficient BS can be used to suppress specular reflections from a sample and, thus, further improve the SNR.

High-throughput dark-field full-field optical coherence tomography

Full-field optical coherence tomography (FF-OCT) can rapidly acquire 2D en face OCT images through a scattering medium. However, the standard interferometer configurations waste almost 75% of light. In addition, specular reflections can saturate the detector, limiting FF-OCT performance. Here, we report on a high-throughput dark-field (HTDF) FF-OCT configuration that efficiently uses the available light budget and allows suppressing specular reflections. Specifically, we demonstrate that images acquired with the HTDF FF-OCT system have 3.5 times higher signal-to-noise ratio when compared to our previously developed regular FF-OCT system.

Comparison of full-field optical coherence tomography imaging for pancreatic tissue sample obtained by EUS-fine-needle biopsy and conventional histological examination: A study protocol for a prospective trial.

For a definitive diagnosis of fine-needle aspiration (FNA)/biopsy, one of the reliable techniques to determine the adequacy and accuracy rapid on-site evaluation (ROSE) of cytological samples is preferable. Because of the lack of trained pathologists, alternatives have to be explored. This study is primarily conducted to determine the diagnostic sensitivity and specificity of full-field optical coherence tomography (FF-OCT) and secondarily to evaluate the possibility of FF-OCT differentiating different types of pancreatic diseases. The diagnostic coherence of FF-OCT by a trained assistant (endoscopist) and trained pathologist is also compared. This is a single-center, prospective, observation trial. Eighty patients would be enrolled in the study. The tissue samples acquired by endoscopic ultrasound fine-needle biopsy (EUS-FNB) would be imaged by the FF-OCT system, interpreted by a trained endoscopist and a pathologist. The results of the image interpretation would be verified with histological findings. This study determines the diagnostic capability of FF-OCT as a ROSE technique while performing EUS-FNB, and whether endoscopists can implement the assessment.

FFOCT imaging based on compressed sensing

With the demand for high-resolution images, the storage space occupied by the images of the full-field optical coherence tomography (FFOCT) system is becoming more and more intensified. How to reduce the data volume of high-resolution image systems has become a research hotspot in the image processing industry. In particular, precision medicine requires high-resolution images of organs and tissues, and this data storage system requires a large storage space. For this reason, based on the high-resolution tomographic image obtained from our organ tissue, compressed sensing theory is used to perform compression reconstruction simulation. It is verified that the amount of data stored in the three-dimensional image of the diseased tissue is effectively reduced without changing the image resolution.

Computational-imaging-based optical coherence tomography in time- and frequency-domain

A computational 3D imaging system is developed that enables polychromatic, depth-resolved, diffraction-limited imaging of semi-transparent objects. By combining coherent diffractive imaging (CDI) and optical coherence tomography (OCT), we reconstruct tomographic images of 3D objects from a set of wavelength- and phase-resolved diffraction patterns, using numerical methods to achieve image quality beyond the hardware limits of the optical systems used. We implement both time- and frequency-domain versions of full-field OCT systems, and for both versions we demonstrate fully lensless, as well as high-numerical-aperture configurations. We provide a comparison and overview of these different practical approaches to depth-resolved computational imaging. Furthermore, we demonstrate depth-resolved imaging of multilayer samples with an isotropic resolution in the μm range over a depth range that extends well beyond the depth-of-focus given by the numerical aperture of the imaging system.

Single-shot wavelength-independent phase-shifting method for full-field optical coherence tomography.

One limitation of a piezoelectric translator-based phase-shifting method in full-field optical coherence tomography (FF-OCT) is that there exist interference residuals because a light source with broadband is used. In this work, an achromatic phase-shifting method was proposed in which a linear polarizer and a quarter-wave plate were employed to generate the circularly polarized light in the reference arm of a basic Linnik interferometer. The light field reflected from the reference arm is supposed with the unpolarized light backscattered from the sample when the path difference is within the coherence length of the light source. A first phase difference of π/2 can be generated on the propagation of superposed light beams through the polarized beam splitter. An additional phase difference of π/2 can be obtained by the proposed numerical method, thus producing the similar effects as the four-frame phase-shifting way. An en face tomographic image can be obtained with a single-shot in this new FF-OCT system. The axial and lateral resolutions of the system are around 1.4μm and 0.8μm, respectively. The system offers a dynamic range of ∼56 dB and an imaging rate of 30fps. Tomographic images of Intel microchip, onion cells, and microcracks in the glass were displayed with clear substructures. This article aims at producing fringe-free OCT images in a single shot.

En Face and Cross-sectional Corneal Tomograms Using Sub-micron spatial resolution Optical Coherence Tomography

Accurate diagnosis of corneal pathology and morphological identification of different corneal layers require clear delineation of corneal three-dimensional structures and en face or cross-sectional imaging of palisade of Vogt (POV), neovascularization (NV) or corneal nerves. Here we report a prototype of full-field optical coherence tomography (FF-OCT) system with isotropic sub-micron spatial resolution in the en face and cross-sectional views. It can also provide three-dimensional reconstructed images and a large field of view (FOV) by stitching tomograms side by side. We validated the imaging power of this prototype in in vivo rat and rabbit eyes, and quantified anatomical characteristics such as corneal layer thickness, endothelial cell density and the intensity profile of different layers. This FF-OCT delineated the ridge-like structure of POV, corneal nerve bundles, and conjunctival vessels in rat eyes. It also clearly identified the vessel walls and red blood cells in rabbit model of corneal NV. The findings provided by this FF-OCT are expected to facilitate corneal disease diagnosis and treatment.

Non-destructive analysis of flake properties in automotive paints with full-field optical coherence tomography and 3D segmentation.

Automotive coating systems are designed to protect vehicle bodies from corrosion and enhance their aesthetic value. The number, size and orientation of small metallic flakes in the base coat of the paint has a significant effect on the appearance of automotive bodies. It is important for quality assurance (QA) to be able to measure the properties of these small flakes, which are approximately 10μm in radius, yet current QA techniques are limited to measuring layer thickness. We design and develop a time-domain (TD) full-field (FF) optical coherence tomography (OCT) system to scan automotive panels volumetrically, non-destructively and without contact. We develop and integrate a segmentation method to automatically distinguish flakes and allow measurement of their properties. We test our integrated system on nine sections of five panels and demonstrate that this integrated approach can characterise small flakes in automotive coating systems in 3D, calculating the number, size and orientation accurately and consistently. This has the potential to significantly impact QA testing in the automotive industry.