Coherence Tomography System Research Articles

The Beltrami SAR Framework for Multichannel Despeckling

In this paper, a new framework for iterative speckle noise reduction in polarimetric synthetic aperture radar (SAR) data is introduced. Speckle is inherent to all coherent imaging systems and affects SAR imagery in the form of strong intensity variations in pixels with similar backscattering coefficients. This makes the interpretation of SAR data in several applications a difficult task. The proposed framework includes a preprocessing step capable of dealing with noise correlation usually found in single-look data. The general filtering approach is based on the Beltrami flow for denoising manifolds or images painted on manifolds. The principal contribution of this work is to adapt this approach to deal with covariance or coherency matrices instead of optical imagery. The evaluation presented suggests that this approach allows for good spatial and radiometric preservation compared to other state-of-the-art methods. Experiments are performed on the basis of synthetic and real-world experimental data. The validation of the proposed framework is accomplished using two refined error performance measures and the well-known effective number of looks measured. The source code of a parallel implementation of the proposed framework is released under the MPL 2.0 ( https://www.mozilla.org/en-US/MPL/2.0/ ) alongside this paper.

Adjustable frequency filtering and weighted feedback for iterative phase retrieval under noisy conditions

In coherent diffraction imaging (CDI) systems, the results of phase retrieval could be deteriorated by the inevitable experimental noise. To address the problem, a noise-robust algorithm is designed with weighted feedback and adjustable frequency filtering. The frequency filter serves as the smoothness constraint and has an adaptive parameter during iterations, to suppress the noise as well as avoid the convergence stagnancy of iterative phase retrieval. Numerical simulations and experimental results demonstrate that the proposed algorithm is superior in reconstruction quality under noisy conditions and increases the convergence speed. Furthermore, the minimum acceptable number of measurements of diffracting images is reduced. This work is beneficial to data synthesis in CDI systems.

OCT Image Denoising Based on Asymmetric Normal Laplace Mixture Model.

Optical Coherence Tomography (OCT) is one of the well-known imaging systems in ophthalmology that provides images with high resolution from retinal tissue. However, like other coherent imaging systems, OCT images suffer from speckle noise which decreases the image quality. Denoising can be considered as an estimation problem in a Bayesian framework. So, finding a suitable distribution for noiseless data is an important issue. We propose a statistical model for OCT data, namely Asymmetric Normal Laplace Mixture Model (ANLMM), and then convert its distribution to normal by Gaussianization Transform (GT). Finally, by applying the Spatially Constrained Gaussian Mixture Model (SC-GMM), a new OCT denoising algorithm is introduced, which significantly outperforms the other methods in terms of Contrast-to-Noise Ratio (CNR).

Speckle Suppression Based on Weighted Nuclear Norm Minimization and Grey Theory

Coherent imaging systems are greatly affected by speckle noise, which makes visual analysis and features extraction a difficult task. In this paper, we propose a speckle suppression algorithm based on weighted nuclear norm minimization (WNNM) and Grey theory. First, we use logarithmic transformation to the noisy images such that the speckle noise is transformed into additive noise. Second, by matching the local blocks based on Grey theory, we will get approximate low-rank matrices grouped by the similar blocks of the reference patches. We then estimate the noise variance of the noisy images with the wavelet transform. Finally, we use WNNM method to denoise the image. The results show that our algorithm not only effectively improves the visual effect of the denoised image and preserves the local structure of the image better but also improves the objective indexes values of the denoised image.

Deep learning-based super-resolution in coherent imaging systems

We present a deep learning framework based on a generative adversarial network (GAN) to perform super-resolution in coherent imaging systems. We demonstrate that this framework can enhance the resolution of both pixel size-limited and diffraction-limited coherent imaging systems. The capabilities of this approach are experimentally validated by super-resolving complex-valued images acquired using a lensfree on-chip holographic microscope, the resolution of which was pixel size-limited. Using the same GAN-based approach, we also improved the resolution of a lens-based holographic imaging system that was limited in resolution by the numerical aperture of its objective lens. This deep learning-based super-resolution framework can be broadly applied to enhance the space-bandwidth product of coherent imaging systems using image data and convolutional neural networks, and provides a rapid, non-iterative method for solving inverse image reconstruction or enhancement problems in optics.

Calibrated helium and carbon ion flow measurements in the DIII-D divertor plasma

Abstract To explore the influence of drifts on ion flows in the DIII-D divertor and Scrape-Off-Layer (SOL), a calibrated Coherence Imaging System (CIS) views the lower divertor. A new tuneable calibration laser and precision wavemeter provide the rest wavelength for CIII 465 nm plasma emissions resulting in ≤ 1 km/s uncertainty in ion velocities that are in the range of 30 km/s in the divertor plasma. The 2-D line-integrated velocity images have been tomographically inverted to obtain maps of ion velocity. Both He + and C 2 + ion velocity maps were measured in both directions of the toroidal magnetic field BT, and C 2 + images were obtained in attached and detached L- and H-mode plasmas. The measured C 2 + velocities are in the range of 20–30 km/s and are compared with UEDGE modeling with full drifts. While He II as a main ion is expected to be entrained in the plasma flow, the C 2 + impurity flow involves the competition between the frictional and ion temperature gradient forces. Both the CIS data and UEDGE modeling agree that the C 2 + flow is towards the divertor plates, but flips with the direction of BT. In general, the experimental C 2 + emission and flow profiles have a wider spatial extent than the UEDGE modeling.

Dual-channel inline coherent imaging.

We present a dual-channel inline coherent imaging system for laser machining monitoring using a single spectrometer. Sensitivity enhancement due to the added signal of the two input channels is demonstrated with a maximum sensitivity of 99dB at a 73kHz acquisition rate. We also treat, theoretically and experimentally, dual-channel detection in the case of signal saturation. A method to mitigate saturation artifacts while maintaining high signal-to-noise ratio is shown.

Compound denoising method of laser speckle noise in laser inherent field imaging

Noise is an important factor affecting the image quality of laser coherent field high resolution imaging system. And there exists not only background light additive noise but also laser multiplicative speckle noise in a laser coherent field imaging system. Both of the above noise affect the imaging quality of laser coherent field system. In order to improve the imaging quality from the perspective of noise suppression and settle the imaging quality degradation problem of laser multiplicative speckle noise and background additive noise in the laser coherent field imaging system, the model for the influence of multiplicative speckle noise and background additive noise on laser echo field demodulated signal is established in atmospheric downlink. Then, based on the model, a novel homomorphic filter and sparse matrix trace cascade compound de-noising algorithm is put forward. Firstly, based on the homomorphic filtering theory, the laser multiplicative speckle noise in the laser echo demodulated signal is converted into the additive noise by logarithmic transformation. Then the low-frequency laser multiplicative speckle noise is filtered by the high-pass filter, and the high-frequency demodulated signal is retained. The logarithmic inverse transform is used to obtain the laser echo demodulation signal after the multiplicative speckle noise has been filtered out. Next, the phase random disturbance of atmosphere in laser echo demodulated signal is suppressed by phase closure technology and the imaging spectrum component is reconstructed by the spectrum iterative reconstruction method. Then the high resolution image is obtained by spectrum component inverse Fourier transform. Finally, the effect of background additive noise on the image quality is suppressed by the sparse base tracking theory. The simulated and outdoor experiment result are used to verify the denoising effect and image quality enhancement effect of the composite de-noising method. Compared with the existing single denoising method, the composite denoising method is shown to be able to effectively eliminate laser multiplicative speckle noise and background additive noise at one time. The proposed method can improve image contrast and promote the Strehl ratio of imaging quality in a coherent imaging system. It provides a theoretical basis for improving imaging quality and denosing laser multiplicative speckle noise and background additive noise in coherent field imaging system.

Predictable fibroblast tension generation by measuring compaction of anchored collagen matrices using microscopy and optical coherence tomography

ABSTRACTThe anchored fibroblast-populated collagen matrix (aFPCM) is an appropriate model to study fibrocontractive disease mechanisms. Our goal was to determine if aFPCM height reduction (compaction) during development is sufficient to predict tension generation. Compaction was quantified daily by both traditional light microscopy and an optical coherence tomography (OCT) system. Contraction in aFPCM was revealed by releasing them from anchorage. We found that aFPCM contraction increase was correlated to the compaction increase. Cytochalasin D treatment reversibly inhibited compaction. Therefore, we demonstrated that aFPCM height reduction efficiently measures compaction, contraction, and relative maturity of the collagen matrix during development or treatment. In addition, we showed that OCT is suitable for effectively imaging the cross-sectional morphology of the aFPCM in culture. This study will pave the way for more efficient studies on the mechanisms of (and treatments that target) migration and contraction in wound healing and Dupuytren’s contracture in a tissue environment.

Observations of in vivo laser tissue ablation in animal models with different chromophores on the skin and modulating duration per laser exposure

Characteristics such as skin tone and pigmentation color vary among patients, but most researches on laser irradiation in laser ablation have revolved around minimizing damage to reduce pain. Chromophores are the most important factors in photon energy absorption, a key principle of laser ablation. We investigated the influences on ablation depth by different chromophores on the target and modulating duration per laser exposure using an Nd:YVO4 nanosecond 532-nm laser. We used a Fourier-domain optical coherence tomography (Fd-OCT) system combined with a 532-nm Nd:YVO4 laser to observe the ablation process. In addition, an external customized shutter and water-based color pens (red, green, blue, black) were used to determine the effects of modulating the duration per laser exposure and coloring chromophores on porcine skin and hairless mouse models. Experiments with modulating duration per laser exposure demonstrated that shorter duration generated shallower craters than longer one. Painted experimental group showed damaged region as craters in the experiments with coloring various chromophores. In this research, we investigated the effects of modulating duration per laser exposure and different chromophores on the target. Coloring chromophores with water-based dyes using pens increased tissue damage without dyeing cells or tissue.

Demonstration of Triband Multi-Focal Imaging with Optical Coherence Tomography.

We demonstrate an extended depth of focus optical coherence tomography (OCT) system based on the use of chromatic aberration to create displaced focal planes in the sample. The system uses a wavelength-swept source tuning over three spectral bands and three separate interferometers, each of which interfaces to a single illumination/collection fiber. The resulting three imaged volumes are merged in post-processing to generate an image with a larger depth of focus than is obtained from each band individually. The improvements are demonstrated in structural imaging of a porous phantom and a lipid-cleared murine brain, and by angiographic imaging of human skin. By using a coaxial approach with Gaussian beams, this approach enables an extended focus with relatively simple microscope optics and data-merging algorithms.

Enhancement of phase retrieval capability in ptychography by using strongly scattering property of the probe-generating device.

In common ptychographic coherent diffractive imaging (PCDI) systems, the probe-generating devices typically exhibit strong scattering, which is not fully used. Here, we report the reasonableness of using the diffraction pattern of the probe-generating device as the frequency-domain information of the scanning probe located in the sample plane, and we propose a method introducing this frequency-domain information into an iterative process to improve the imaging quality of PCDI. The new method was demonstrated using both a visible laser source and a synchrotron radiation X-ray source; the proposed method significantly improved the imaging quality in both demonstrations.

Partially coherent microscope imaging system in phase space: effect of defocus and phase reconstruction.

Explicit relationships between the defocused partially coherent cross-coefficient and phase space representations in the image plane are derived. Measurement of a phase space representation in the image plane in principle allows the complex image to be extracted. Implications for phase retrieval using the weak object transfer function or the transport of intensity equation are considered. The phase gradient transfer function, which determines the image for an object exhibiting a slowly varying phase gradient for a partially coherent microscope system, is derived. The effect of the effective source size and geometry on phase imaging with the transport of intensity equation is investigated. The primary consequence of source shape is a rescaling of the phase reconstruction. An annular source is found to give close to a linear response, while at the same time providing improved transverse resolution and an improved response to low spatial frequencies.

Origin of interference fringes and its countermeasures in coherent terahertz imaging

AbstractInterference fringes are quite common in coherent imaging systems. As a kind of waves, terahertz (THz) light has such a phenomenon inherently when multiple reflections happen in focal plane imaging systems, which generally includes several mirrors. In this paper, we will demonstrate the fringe pattern in a typical reflective imaging system, which is similar to the “Newton Ring” phenomenon as in optics. The good point is that it might be helpful to measure the gesture of the object, while it is quite annoying in imaging since important information is hidden behind the fringes and we could even not recognize the object. To obtain a decent image, we propose three methods via tuning the frequency and location of the objects, also we can tune the illumination angle of THz waves.

Laser calibration of the DIII-D coherence imaging system.

In this paper, we describe an in situ calibration technique for Coherence Imaging Spectroscopy (CIS) that measures 2-D images of ion flows on DIII-D. A low power CW diode laser that is tuneable in the range 464-468 nm along with a precision wavemeter (10-5 nm resolution) is used to characterize the interferometer phase as a function of wavelength in the region of C iii (465 nm) and He ii (468 nm). The interferometer is stabilized both mechanically and thermally to minimize drift during the calibration. Optical stirring and an integration sphere are used to obtain spatially uniform calibration images. The quality of the calibration data enables a measurement of phase versus wavelength over approximately 10 fringes of the interferometer. These coefficients can also be related to the geometry of the optics and the birefringent crystal of the interferometer. On DIII-D, the integration sphere with the laser light is inserted into the CIS optical system between shots and the laser image and wavelength are automatically recorded, providing a zero velocity reference.

GF-3 SAR IMAGE DESPECKLING BASED ON THE IMPROVED NON-LOCAL MEANS USING NON-SUBSAMPLED SHEARLET TRANSFORM

Abstract. GF-3 synthetic aperture radar (SAR) images are rich in information and have obvious sparse features. However, the speckle appears in the GF-3 SAR images due to the coherent imaging system and it hinders the interpretation of images seriously. Recently, Shearlet is applied to the image processing with its best sparse representation. A new Shearlet-transform-based method is proposed in this paper based on the improved non-local means. Firstly, the logarithmic operation and the non-subsampled Shearlet transformation are applied to the GF-3 SAR image. Secondly, in order to solve the problems that the image details are smoothed overly and the weight distribution is affected by the speckle, a new non-local means is used for the transformed high frequency coefficient. Thirdly, the Shearlet reconstruction is carried out. Finally, the final filtered image is obtained by an exponential operation. Experimental results demonstrate that, compared with other despeckling methods, the proposed method can suppress the speckle effectively in homogeneous regions and has better capability of edge preserving.

STRUCTURAL INFORMATION DETECTION BASED FILTER FOR GF-3 SAR IMAGES

Abstract. GF-3 satellite with high resolution, large swath, multi-imaging mode, long service life and other characteristics, can achieve allweather and all day monitoring for global land and ocean. It has become the highest resolution satellite system in the world with the C-band multi-polarized synthetic aperture radar (SAR) satellite. However, due to the coherent imaging system, speckle appears in GF-3 SAR images, and it hinders the understanding and interpretation of images seriously. Therefore, the processing of SAR images has big challenges owing to the appearance of speckle. The high-resolution SAR images produced by the GF-3 satellite are rich in information and have obvious feature structures such as points, edges, lines and so on. The traditional filters such as Lee filter and Gamma MAP filter are not appropriate for the GF-3 SAR images since they ignore the structural information of images. In this paper, the structural information detection based filter is constructed, successively including the point target detection in the smallest window, the adaptive windowing method based on regional characteristics, and the most homogeneous sub-window selection. The despeckling experiments on GF-3 SAR images demonstrate that compared with the traditional filters, the proposed structural information detection based filter can well preserve the points, edges and lines as well as smooth the speckle more sufficiently.

Resolution enhancement for fiber bundle imaging using maximum a posteriori estimation.

We propose a new framework to jointly improve spatial resolution and remove fixed structural patterns for coherent fiber bundle imaging systems, based on inverting a principled forward model. The forward model maps a high-resolution representation to multiple images modeling random probe motions. We then apply a point spread function to simulate low-resolution figure bundle image capture. Our forward model also uses a smoothing prior. We compute a maximum a posteriori (MAP) estimate of the high-resolution image from one or more low-resolution images using conjugate gradient descent. Unique aspects of our approach include (1)supporting a variety of possible applicable transformations; (2)applying principled forward modeling and MAP estimation to this domain. We test our method on data synthesized from the USAF target, data captured from a transmissive USAF target, and data from lens tissue. In the case of the USAF target and 16 low-resolution captures, spatial resolution is enhanced by a factor of 2.8.

Quantitative real-time optical imaging of the tissue metabolic rate of oxygen consumption.

.The tissue metabolic rate of oxygen consumption () is a clinically relevant marker for a number of pathologies including cancer and arterial occlusive disease. We present and validate a noncontact method for quantitatively mapping over a wide, scalable field of view at . We achieve this by developing a dual-wavelength, near-infrared coherent spatial frequency-domain imaging (cSFDI) system to calculate tissue optical properties (i.e., absorption, , and reduced scattering, , parameters) as well as the speckle flow index (SFI) at every pixel. Images of tissue oxy- and deoxyhemoglobin concentration ( and [HHb]) are calculated from optical properties and combined with SFI to calculate . We validate the system using a series of yeast-hemoglobin tissue-simulating phantoms and conduct in vivo tests in humans using arterial occlusions that demonstrate sensitivity to tissue metabolic oxygen debt and its repayment. Finally, we image the impact of cyanide exposure and toxicity reversal in an in vivo rabbit model showing clear instances of mitochondrial uncoupling and significantly diminished . We conclude that dual-wavelength cSFDI provides rapid, quantitative, wide-field mapping of that can reveal unique spatial and temporal dynamics relevant to tissue pathology and viability.

Speckle Suppression Based on Sparse Representation with Non-Local Priors

As speckle seriously restricts the applications of remote sensing images in many fields, the ability to efficiently and effectively suppress speckle in a coherent imaging system is indispensable. In order to overcome the over-smoothing problem caused by the speckle suppression algorithm based on classical sparse representation, we propose a non-local speckle suppression algorithm that combines the non-local prior knowledge of the image into the sparse representation. The proposed algorithm first applies shearlet to sparsely represent the input image. We then incorporate the non-local priors as constraints into the image sparse representation de-noising problem. The denoised image is obtained by utilizing an alternating minimization algorithm to solve the corresponding constrained de-noising problem. The experimental results show that the proposed algorithm can not only significantly remove speckle noise, but also improve the visual effect and retain the texture information of the image better.