High-contrast handheld FFOCT system for tissue imaging.

A low contrast is a limiting factor for imaging a microstructure beneath the biological sample surface. In this work, we describe a novel, to our knowledge, full-field optical coherence tomography (FFOCT) system with a probe connected by a fiber bundle and a multimode optical fiber. The device is based on the tandem structure of the Michelson interferometer and the Fizeau interferometer. One advantage of our device is that light propagates through the fiber bundle only once, greatly improving detection sensitivity. In addition, by spatial filtering in the Fourier domain and inverse filtering, the effects of pixelation artifacts and multiple scattering in the en face images obtained by our system are suppressed. The depth-resolved en face images of the human finger skin ex vivo and the porcine esophagus ex vivo are presented to demonstrate the capability of our system.

Light-scattering-induced retardation as a high-sensitivity image contrast revealing collagen fibers.

Retardation induced by media can be used as an image contrast to depict the cumulative birefringent features and local variations of the sample, respectively. It is commonly assumed that the retardation is induced by the light propagation; however, the light scattering would generate the retardation as well. In our work, the scattering-induced retardation as a high-sensitivity image contrast for revealing collagen fibers is presented. First, it is shown that the retardation induced by fiber scattering is equal to π when modeled as cylinders. Using the data for the chicken breast and the palm measured by the polarization-sensitive optical coherence tomography system as an example, the scattering-induced retardation is calculated. The measured value of π is in complete agreement with the theory, and the corresponding retardation per unit distance is two orders of magnitude greater than the light-propagation-induced retardation, demonstrating its predominant role on the overall retardation and providing a possibility for highly sensitive displays. Compared with the accumulated retardation image and the differential retardation image, the scattering-induced retardation images could exhibit sharper fiber structures even in deeper regions. This work might be helpful for the early diagnosis of collagen-related diseases.

The wavenumber linearisation without calibration device for spectral-domain optical coherence tomography.

The wavenumber nonlinearity leads to blurred reconstructed images in spectral-domain optical coherence tomography (SDOCT). In this work, a wavenumber-linearisation method without calibration devices is presented, based on the fact that the difference between the phases of adjacent peak and valley points is equal to π $\pi $ . The theoretical model is derived, and the efficacy of the method was proven by acquiring SDOCT data from TiO2 phantom and zebrafish. The results exhibit the superior performance of our method. Compared with the linear phase-based method, the resolution could be improved at least a factor of 2. Compared with the polynomial fitting method, the resolution could also be improved by nearly half.

Self-denoising method for OCT images with single spectrogram-based deep learning.

The presence of noise in images reconstructed with optical coherence tomography (OCT) is a key issue which limits the further improvement of the image quality. In this Letter, for the first time, to the best of our knowledge, a self-denoising method for OCT images is presented with single spectrogram-based deep learning. Different noises in different images could be customized with an extremely low computation. The deep-learning model consists of two fully connected layers, two convolution layers, and one deconvolution layer, with the input being the raw interference spectrogram and the label being its reconstructed image using the Fourier transform. The denoising image could be calculated by subtracting the noise predicted by our model from the label image. The OCT images of the TiO2 phantom, the orange, and the zebrafish obtained with our spectral-domain OCT system are used as examples to demonstrate the capability of our method. The results demonstrate its effectiveness in reducing noises such as speckle patterns and horizontal and vertical stripes. Compared with the label image, the signal-to-noise ratio could be improved by 35.0 dB, and the image contrast could be improved by a factor of two. Compared with the results denoised by the average method, the mean peak signal-to-noise ratio is 26.2 dB.

Mueller matrix polarization imaging and quantitative parameters analysis method.

Mueller matrix polarization imaging is a new biomedical optical imaging method that can generate both polarization and isotropic intensity images of structures of the biological tissue sample surface. In this paper, a Mueller polarization imaging system in the reflection mode is described for obtaining the Mueller matrix of the specimens. Diattenuation, phase retardation, and depolarization of the specimens are derived by using the conventional Mueller matrix polarization decomposition method and a newly proposed direct method. The results show that the direct method is more convenient and faster than the conventional decomposition method. The polarization parameter combination method is then presented in which any two of the diattenuation, phase retardation, and depolarization parameters are combined, and three new quantitative parameters are defined in order to reveal more detailed anisotropic structures. The images of in vitro samples are presented to demonstrate the capability of the parameters introduced.

Fourier spatial transform-based method of suppressing motion noises in OCTA.

A large amount of lateral noise will be generated in blood flow imaging with optical coherence tomography angiography (OCTA) due to the presence of muscle shaking, heartbeat, and respiration, resulting in the deterioration of images. In this paper, to the best of our knowledge, for the first time, the spatial frequency information of motion noise in the blood flow signal region is used to remove the motion noise and false connections in the blood flow signal region. The effectiveness of the proposed adaptive denoising algorithm is verified by the imaging of finger blood flow. It is found that OCTA with different projection methods has improved signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) after applying our algorithm. It is also found that the visual effect of the original blood flow image based on standard deviation projection is better, but mean projection is the most sensitive to the algorithm, and the average SNR and CNR are improved by 5.7 dB and 8.9 dB, respectively.

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.

2D MEMS-based high-speed beam-shifting technique for speckle noise reduction and flow rate measurement in optical coherence tomography.

In this manuscript, a two-dimensional (2D) micro-electro-mechanical system (MEMS)-based, high-speed beam-shifting spectral domain optical coherence tomography (MHB-SDOCT) is proposed for speckle noise reduction and absolute flow rate measurement. By combining a zigzag scanning protocol, the frame rates of 45.2 Hz for speckle reduction and 25.6 Hz for flow rate measurement are achieved for in-vivo tissue imaging. Phantom experimental results have shown that by setting the incident beam angle to ϕ = 4.76° (between optical axis of objective lens and beam axis) and rotating the beam about the optical axis in 17 discrete angular positions, 91% of speckle noise in the structural images can be reduced. Furthermore, a precision of 0.0032 µl/s is achieved for flow rate measurement with the same beam angle, using three discrete angular positions around the optical axis. In-vivo experiments on human skin and chicken embryo were also implemented to further verify the performance of speckle noise reduction and flow rate measurement of MHB-SDOCT.

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 1.4 µm and 0.8 µm, respectively. The system offers a dynamic range of ∼56 dB and an imaging rate of 30 fps. 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.

Differential phase standard-deviation-based optical coherence tomographic angiography for human retinal imaging in vivo.

We present a differential phase standard-deviation (DPSD)-based optical coherence tomographic (OCT) angiography (OCTA) technique to calculate the angiography images of the human retina. The standard deviation was calculated along the depth direction on the differential phase image of two B-scans (from the same position, at different times) to contrast dynamic vascular signals. The performance of a DPSD was verified by both phantom and in vivo experiments. When compared to other OCTA algorithms such as phase variance OCT, speckle variance OCT, and optical microangiography, we showed that a DPSD achieved improved image contrast and higher sensitivity. Furthermore, we also found the improved signal-to-noise ratio and contrast-to-noise ratio of 1.6 dB and 0.5, respectively, in large scanning range images.

Quantitative depth-resolved microcirculation imaging with optical coherence tomography angiography (Part Ι): Blood flow velocity imaging.

The research goal of the microvascular network imaging with OCT angiography is to achieve depth-resolved blood flow and vessel imaging in vivo in the clinical management of patents. In this review, we review the main phenomena that have been explored in OCT to image the blood flow velocity vector and the vessels of the microcirculation within living tissues. Parameters that limit the accurate measurements of blood flow velocity are then considered. Finally, initial clinical diagnosis applications and future developments of OCT flow images are discussed.

Quantitative depth-resolved microcirculation imaging with optical coherence tomography angiography (Part ΙΙ): Microvascular network imaging.

In this work, we review the main phenomena that have been explored in OCT angiography to image the vessels of the microcirculation within living tissues with the emphasis on how the different processing algorithms were derived to circumvent specific limitations. Parameters are then discussed that can quantitatively describe the depth-resolved microvascular network for possible clinical diagnosis applications. Finally, future directions in continuing OCT development are discussed.

High speed, wide velocity dynamic range Doppler optical coherence tomography (Part V): Optimal utilization of multi-beam scanning for Doppler and speckle variance microvascular imaging.

In this paper, a multi-beam scanning technique is proposed to optimize the microvascular images of human skin obtained with Doppler effect based methods and speckle variance processing. Flow phantom experiments were performed to investigate the suitability for combining multi-beam data to achieve enhanced microvascular imaging. To our surprise, the highly variable spot sizes (ranging from 13 to 77 µm) encountered in high numerical aperture multi-beam OCT system imaging the same target provided reasonably uniform Doppler variance and speckle variance responses as functions of flow velocity, which formed the basis for combining them to obtain better microvascular imaging without scanning penalty. In vivo 2D and 3D imaging of human skin was then performed to further demonstrate the benefit of combining multi-beam scanning to obtain improved signal-to-noise ratio (SNR) in microvascular imaging. Such SNR improvement can be as high as 10 dB. To our knowledge, this is the first demonstration of combining different spot size, staggered multiple optical foci scanning, to achieve enhanced SNR for blood flow OCT imaging.

Image contrast reduction mechanism in full-field optical coherence tomography.

Correct interpretation of image contrast obtained with full-field optical coherence tomography (FFOCT) technique is required for accurate medical diagnosis applications. In this work, first, the characteristics of microscopic structures of tissue that generate the contrast in en-face tomographic image obtained with FFOCT are discussed. Then an overview is given of the parameters that affect image contrast. Finally, the contrast correction factor for correct image interpretation and the contrast limits to practical FFOCT systems are outlined.

Estimation of parameters for evaluating subsurface microcracks in glass with in-line digital holographic microscopy.

In this work, a new method for imaging subsurface damage (SSD) is proposed, which is, to the best of our knowledge, the first application of the in-line digital holographic microscopy (IDHM) to the reconstruction of the subsurface damage in glass. By combination of the in-line arrangement and an objective lens to image the hologram on the CCD surface, the method is characterized by its high resolution in both the lateral and depth directions. Then the three-dimensional reconstruction of the microcracks within the glass was realized by numerically focusing en-face images at different depths, and the sizes of SSD along the transversal and depth directions were estimated. Based on the experimental results, the cracks can be divided into two categories: one is that the cracks begin from the surface of optical elements, the other is totally within the components. To indicate the propagation or development trajectory of the cracks and predict the magnitude of the laser-induced damage threshold, the relative intensity distributions of the light scattered by the cracks compared with the ones without cracks were also reconstructed. In this case all the required parameters for evaluating SSD are obtained with our IDHM system, so that the SSD produced in the manufacturing process can be reduced or removed more easily to optimize the performance of the optical component and extend its lifetime. These results provide the guidance for the optical system design of precision measurements.

A general model for resolution of digital holographic microscopy.

For digital holographic microscopic imaging, the resolution in the reconstructed image is one of the most important parameters. To optimize the lateral resolution, a general model for the resolution of digital holographic microscopy (DHM) is proposed in this work, in which the effects of the sizes of each pixel, total area of the charge coupled device (CCD) and the microscopic objective lens are taken into account. Comparison between our model and others was carried out by calculating the point spread function (PSF) of DHM at different reconstruction distances and with different fill factors. It is shown that the effect of fill factors on the resolution of DHM becomes significant when the reconstruction distance is long. For high resolution DHM imaging the influence of fill factors must be taken into account when estimating the resolution of the reconstructed image. Furthermore, It is also demonstrated that the sidelobe of PSF can be cut effectively choosing appropriate values of the fill factors. Finally, the reconstructions of polyethylene microspheres have been implemented to demonstrate the theoretical analysis. These results obtained are helpful for estimation of the resolution and design of the DHM systems.

Determination of spatial correlation functions of refractive index of living tissue.

We present what to our knowledge the first method of determination of spatial correlation functions of refractive index fluctuations of living tissues by Fourier domain optical coherence tomography (FDOCT). Based on the second-order statistical description of the random characteristic of living tissue, a formula which clearly relates the spectral electrical power from the detector to the Fourier spectrum of the refractive index correlation function is given. The method is characterized by its capability of noninvasive measurements in vivo. It has the potential of allowing quantitative discrimination between different tissue types or the same tissue at different pathological states by determining their Fourier components of spatial correlation functions of refractive index.

Speckle properties of the logarithmically transformed signal in optical coherence tomography.

We discuss the statistical properties of speckle of the logarithmically transformed signal in optical coherence tomography (OCT) both theoretically and experimentally. OCT signals of Intralipid solution with different volume particle concentrations ρ (correspondingly, scattering coefficient µ(s) ranges from 1.25 to 25.11 mm(-1)) were measured and analyzed under two different focusing conditions [numerical apertures (NAs) of the objective lens of 0.13 and 0.25]. We found that the effect of the speckle noise can be suppressed by displaying OCT images in the logarithmic scale and by using the objective lens with a higher NA. We also found that the speckle properties are correlated with the scattering properties of the sample, which may be used to characterize the scattering properties of biological tissue. The simulated OCT image and the in vitro OCT image of a rat liver are used as examples to demonstrate the feasibility of the method.

Anisotropic dielectric susceptibility matrix of anisotropic medium.

In this work, we introduce the concept of anisotropic dielectric susceptibility matrix of anisotropic medium for both nondepolarizing and depolarizing medium. The concept provides a new way of analyzing light scattering properties of anisotropic media illuminated by polarized light. The explicit expressions for the elements of the scattering matrix are given in terms of the elements of the Fourier transform of the anisotropic dielectric susceptibility matrix of the medium. Finally, expressions for the elements of the Jones matrix of a thin layer of a deterministic anisotropic medium and the elements of the Mueller matrix of a depolarizing medium are given. The results obtained in this work is helpful for deriving information about the correlated anisotropic structures in depolarizing media from measured Mueller matrices. The findings in this work may also well prove stimulating to researchers working on new methods for analyzing light scattering properties.

Coupling effects among elementary polarization properties.

In this work, we propose that there exist coupling effects among birefringence, dichroism and off-diagonal depolarization parameters of differential Mueller matrix of random anisotropic media. An anisotropic spatial correlation function of anisotropic random medium is proposed to explain this phenomenon. The consequences of these effects are then pointed out. The idea in this work is very helpful for accurate interpretation of the measured Mueller matrices of optically anisotropic depolarizing medium. In addition, the concept of the anisotropic spatial correlation function of anisotropic random medium will open a new door and will play a central role for analyzing polarized light scattering by anisotropic random media.