Polarization-sensitive optical coherence tomography (PS-OCT) allows the visualization of biological tissue microstructure by measuring the pathlength difference, amplitude, and polarization of backscattered light. Speckle grains complicate the visualizations due to scattering structures in tissue smaller than the PS-OCT resolution. We developed an angular compounding system to reduce speckle by rotating samples and collecting tomograms at multiple imaging angles, without modifying PS-OCT hardware or optical pathways. Tomograms were acquired, aligned with affine transformations, and averaged. This method successfully reduced the speckle and improved visualization of intensity and birefringence images.
OCT speckle carries information on sub-cellular tissue structures, and speckle statistics have been shown to be potential biomarkers in tissue characterization for disease detection and monitoring. Current methods for estimating speckle parameters use simple methods in which speckle statistics are determined inside a fixed kernel, which makes them unsuitable in heterogeneous tissue and have a clear trade-off between accuracy and spatial resolution. These limitations make them unsuitable for automatically detecting spatially-resolved differences in cellular microstructure that occurs in a diseased tissue. To address this unmet need, we have developed an algorithm based on a probabilistic approach to automatically select kernels consisting of pixels that have a high probability of sharing the same speckle probability density function and use them to estimate spatially-resolved speckle parameters using likelihood-based estimation. Our proposed method enables new capabilities in producing speckle parametric images, providing information on spatial variability of speckle distribution throughout OCT volumes and additional information to structural OCT imaging.
Simulated data plays an important role in the process of developing and testing new processing methods in optical coherence tomography (OCT); they provide ground truths, and enable the generation of large amounts of data with high diversity in terms of tissue and system properties, without the burdens of experimental acquisitions. Here, we present an open-source MATLAB simulation tool that allows the generation of synthetic OCT data in an efficient and versatile way, while modeling the most relevant characteristics of the OCT signal, by combining the well-known forward model of OCT imaging with scattering and polarization properties of tissue.
For OCTA to become a widespread tool in the ophthalmic clinic, the significant increase in acquisition time must be kept at a minimum. Therefore, we developed a new OCTA processing pipeline that enables 4D coherent averaging and leverages from it to improve angiography contrast without using more repetitions.
We first present a new metric for computing angiography contrast that exploits both spatial and time coherence, g1C, which significantly reduce phase noise while using minimal repetitions for averaging. We then show, for the first time, how to perform advanced image registration to correct for motion between repetitions while maintaining phase coherency.
We present the first laser speckle imaging device for skin cancer detection that can image and distinguish shallow and deep vessels from speckle frames acquired with a single exposure time. To do this, we have developed a new signal processing technique based on the simultaneous evaluation of two metrics: one based on contrast, and one based on a normalized version of the second order autocorrelation function, to reveal deep and shallow vessels, respectively. We will present data from the pilot studies we are conducting on healthy volunteers and skin cancer patients at MGH skin cancer clinic, in which we will compare the microvasculature images obtained with our device from different skin lesions.
KEYWORDS: Optical coherence tomography, Tissues, Scattering, In vivo imaging, Endoscopy, Signal attenuation, Signal intensity, Relays, Particles, Multiple scattering
Attenuation coefficient imaging stands to enhance the value of Optical Coherence Tomography (OCT) in clinical assessment of dysplasia. We present results combining novel signal processing and hardware strategies to more accurately acquire and analyze scattering properties in tissue to extract clinically meaningful morphological information.
Speckle reduction has been an active topic of interest in the Optical Coherence Tomography (OCT) community and several techniques have been developed ranging from hardware-based methods, conventional image-processing and deep-learning based methods. The main goal of speckle reduction is to improve the diagnostic utility of OCT images by enhancing the image quality, thereby enhancing the visual interpretation of anatomical structures. We have previously introduced a probabilistic despeckling method based on non-local means for OCT—Tomographic Non-local-means despeckling (TNode). We demonstrated that this method efficiently suppresses speckle contrast while preserving tissue structures with dimensions approaching the system resolution. Despite the merits of this method, it is computationally very expensive: processing a typical retinal OCT volume takes a few hours. A much faster version of TNode with close to real-time performance, while keeping with the open source nature of TNode, could find much greater use in the OCT community. Deep learning despeckling methods have been proposed in OCT, including variants of conditional Generative Adversarial Networks (cGAN) and convolutional neural networks CNN. However, most of these methods have used B-scan compounding as a ground truth, which presents significant limitations in terms of speckle reduced tomograms with preservation of resolution. In addition, all these methods have focused on speckle suppression of individual B-scans, and their performance on volumetric tomograms is unclear: the expectation is that three-dimensional manipulations of these processed tomograms (i.e., en face projections) will contain artifacts due to the B-scan-wise processing, disrupting the continuity of tissue structures along the slow-scan axis. In addition, speckle suppression based on individual B-scans cannot provide the neural network with information on volumetric structures in the training data, and thus is expected to perform poorly on small structures. Indeed, most deep-learning despeckling works have focused on image quality metrics based on demonstrating strong speckle suppression, rather than focusing on preservation of contrast and small tissue structures. To overcome these problems, we propose an entire workflow to enable the wide-spread use of deep-learning speckle suppression in OCT: the ground-truth is generated using volumetric TNode despeckling, and the neural network uses a new cGAN that receives OCT partial volumes as inputs to utilize the three-dimensional structural information for speckle reduction. Because of its reliance on TNode for generating ground-truth data, this hybrid deep-learning–TNode (DL-TNode) framework will be made available to the OCT community to enable easy training and implementation in a multitude of OCT systems without relying on specialty-acquired training data.
Improving the sensitivity of endoscopic optical coherence tomography (OCT) to pre-cancerous esophageal lesions stands to improve patient outcomes. Although nuclei-level features relevant to dysplasia diagnoses are below conventional OCT systems’ resolution, tissue scattering properties report local particle size and concentration, indirectly accessing nuclei diameter and nuclear-cytoplasmic ratio (NCR). Recently, we have shown that acquiring co-registered images of a sample at different numerical aperture (NA) enhances scattering property measurements through angular diversity. We present preliminary results for a custom-designed, catheter-based, dual-NA endoscopic probe demonstrating improved characterization of phantoms and tissue with histological validation, and potential utility for more sensitive dysplasia diagnoses.
We present a practical laser speckle imaging device for skin cancer detection based on a 980 nm source and black silicon camera technology, with superior penetration depth and contrast.
By operating at 980 nm we obtain deeper perfusion contrast and are immune to differences in skin pigmentation. The use of a black silicon camera ensures a high quantum efficiency without significantly increasing the cost and complexity of the device.
We will show the effectiveness of the device in imaging different skin lesions and discuss the system specifications (wavelength, power, exposure time, frame rate, and processing algorithm) that led to the optimal contrast.
OCT has enabled high resolution imaging of the subsurface microstructure of tissues. However, existing systems lack the ability to robustly measure biomechanical contrast. To image stiffness, reverberant elastography uses correlations of the velocity field and curve fitting to their expected functional shape to measure shear wave number. Prior work has assumed that raster scan systems cannot achieve spatial coherence, so current methods require reproducible synchronization of the excitation with imaging. We demonstrate through simulation and phantom imaging that spatial coherence does indeed exist within a single B-scan. Furthermore, leveraging the displacement field also allows us to overcome temporal coherence limitations.
Birefringence of the retinal nerve fiber has the potential as a useful biomarker for the study of neurodegenerative diseases such as multiple sclerosis. To realize this potential and assist in the development of diagnostic tools and therapies for neurodegenerative diseases, high-resolution retinal polarimetry suitable for humans and rodent models is needed. However, conventional polarization-sensitive optical coherence tomography (PS-OCT) processing generally imposes a resolution penalty by spatially filtering incoherent Stokes vectors, or the underlying coherent Jones-based OCT measurements. Here, we demonstrate the possibility to resolve polarimetric parameters of individual axonal bundles in-vivo with our probabilistic non-local means processing for Stokes-based PS-OCT.
Functional extensions of Optical Coherence Tomography (OCT) stand to enhance the value of structural imaging for clinical applications. Polarization sensitive (PS-) OCT and attenuation coefficient imaging provide complementary information useful for interrogating complex tissue microenvironments such as atherosclerotic plaques. However, additional hardware for generating two input polarization states, and enabling angular-diverse measurements, is necessary to fully exploit these two respective techniques. We present a platform for simultaneous, depth-multiplexed acquisition of PS- and angular diverse data, expanding prior methods for fiber-based PS-OCT and aperture-based numerical aperture (NA) multiplexing, and demonstrate its utility in discriminating plaque components in human coronary arteries.
Existing conventional OCT systems lack the ability to robustly measure biomechanical contrast. While many wave-based elastography methods have been developed for imaging stiffness, they also have limitations that make adaptation to imaging in vivo infeasible. While passive elastography doesn’t require coherence, we also cannot derive quantitative mechanical properties without it. Similarly, it been assumed that reverberant elastography also requires coherence. However, through benchtop raster-scanning and balloon catheter radially-scanning OCT, we demonstrate that it is possible to adapt reverberant elastography for unsynchronized, free-running excitation to obtain cross-sectional shear wave elastography at speeds compatible with in vivo applications.
Measuring light scattering properties using OCT stands to enhance its ability to capture clinically-relevant microstructural details, but challenges remain in accurately relating these properties to underlying tissue architecture. In this work, we demonstrate the collection of depth-multiplexed data at diverse scattering angles with a glass annulus to facilitate correction of imaging system-based signal biases and identify multiply scattering areas in tissue. Based on our promising preliminary results from phantoms and healthy excised tissue samples, we hope our approach will enhance the accuracy of quantitative scattering parameter measurements, and help to realize their potential in offering detailed microstructural characterization of biological tissues.
We present the first feasibility study of a new optical device for assisted venipuncture based on partially-coherent wide-field speckle decorrelation. Using a pseudo-thermal light source, we can vary the degree of spatial coherence, in order to change the ratio of singly- to multiply-scattered light detected by the system. This leads to an improvement in the localization of the decorrelation contrast, and therefore in the delineation of deep veins, as compared to conventional laser speckle imaging (LSI) systems.
The results obtained so far make us believe that our spatial coherence-gated LSI imaging architecture can find widespread application beyond venipuncture.
We present preliminary results using our recently developed methods for quantifying scattering parameters to analyze coronary artery cross section data acquired with OCT and relate optical results to microstructural tissue features in artery walls.
Nanosatellite and CubeSat remote sensing platforms for Earth observation missions are steadily growing in number due to their great potential for cost effectiveness. For this reason, there is a significant interest in developing small hyperspectral and multispectral cameras for earth observation compatible with the constraints of these satellite platforms. These cameras offer the advantage of operating at different spectral bands simultaneously, overcoming limitations of single-wavelength cameras, and facilitating tasks such as object classification and material identification. In this context, the use of hyperspectral cameras based on liquid crystal variable retarders (LCVRs) enables the realization of more compact devices, as they can replace dispersive elements, mirrors, and rotating polarization optics, as well as reducing costs, all of which are essential for the emerging small satellite sector. We have recently implemented LCVR technology onboard the Solar Orbiter mission to perform polarization measurements of the incoming light from the Sun. This is the first time to our knowledge this technology has been implemented for space instrumentation. Based on our implementation of LCVR technology for space, we are developing new instrumentation for hyperspectral imaging based on the principles of Fourier transform infrared spectroscopy (FTIR). We will demonstrate several hardware configurations of the hyperspectral camera using LCVRs of different thicknesses. We will discuss the hardware specifications, driving schemes and trade-offs associated with the use of LCVRs in a FTIR configuration.
Classification of atherosclerotic plaques in vivo remains challenging, even with high-resolution optical techniques, such as intravascular optical coherence tomography (OCT). Plaques contain lipid-rich, fibrous, and calcified components with unique optical properties, enabling their discrimination by quantitative light scattering analysis. We present an approach for improved computation of depth-resolved attenuation coefficients in OCT capable of determining layer-resolved backscattering fractions, thus providing complementary quantitative scattering metrics descriptive of the tissue’s physical properties. We report preliminary findings showing meaningful lesion contrast in quantitative scattering parameters in clinical and cadaver heart pullbacks, which stand to provide additional tools for improved classification of atherosclerotic plaques.
Multiple scattering and angle-dependent scattering anisotropy confound interrogation of tissues with OCT and are generally considered noise. Here we characterize a new localization-diverse OCT system that measures the scattering through a pair of neighboring locations. By varying the offset and direction between the locations, we could distinguish single- from multiple-scattering in tissue-mimicking scattering phantoms. This system has the potential to detect previously unobserved tissue anisotropy by leveraging localization diversity in OCT.
Structural optical coherence tomography (OCT) images stand to benefit from increased contrast. The OCT attenuation coefficient has previously been explored as a means of additional contrast, as it is sensitive to sub-resolution physical properties of the sample, which could be advantageous in differentiating various tissue pathologies. We have developed a new method for OCT attenuation coefficient quantification which combines advantages of prior methods of exponential fitting and the depth-resolved algorithm, and additionally calculates a layer-resolved backscattering fraction. Together, these quantitative scattering parameters provide enhanced accuracy and contrast that could aid in image interpretation in clinical applications such as intravascular OCT.
Conventional optical coherence tomography (OCT) is unable to visualize the earliest microstructural changes that accompany retinal dysfunction and pathology, delaying diagnoses in diseases such as glaucoma. We present methods for analyzing light scattering behavior using OCT to report microscopic structural changes using our layer-based, depth-resolved attenuation coefficient, and layer-resolved backscattering fraction. These quantitative scattering parameters are sensitive to sub-resolution status of pathologically relevant features such as constituent cells, intracellular components, and fiber organization. Preliminary results show additional feature contrast in a healthy subject, and we look forward to exploring the future clinical potential of these methods to enable earlier diagnoses.
We present computational refocusing in polarization-sensitive optical coherence tomography (PS-OCT) to improve spatial resolution in the calculated polarimetric parameters and extending the depth-of-field in PS-OCT. To achieve this, we successfully integrated SHARP, a computational aberration correction method compatible with phase unstable systems, into a PS-OCT system with inter--A-line polarization modulation. Together with the spectral binning technique to mitigate chromatic polarization effects in system components, we show image quality enhancement in tissue polarimetry of swine eye anterior segment ex vivo, demonstrating the potential of computational refocusing in PS-OCT.
Laser therapy has been used to perform both ablation and coagulation of diseased tissue. To avoid over or under exposure, monitoring such therapies with a cost-effective method remains an issue however. We present an integrated solution based on an optical coherence tomography (OCT) system allowing simultaneous imaging, quantitative monitoring and therapy delivery in real-time.
The system exploits a double-clad fiber coupler (DCFC) to inject the OCT signal into the double-clad fiber (DCF) core and the therapy laser into the inner cladding making them co-localized. The single fiber solution permits both imaging and therapy at the same time. Furthermore, the DCFC allows the implementation of our technique in any OCT system sharing the same wavelength bandwidth.
Therapy monitoring is achieved by measuring the speckle intensity decorrelation. During coagulation, the optical properties of the tissue start to vary, thereby changing the speckle intensity pattern seen in the OCT tomograms. The proposed algorithm includes both novel motion and noise corrections, extending the usable monitoring depth. Furthermore, the code has been optimized to run during therapy providing real-time monitoring.
In a proof of concept experiment, a system was built with a 532 nm CW laser for therapy and a 1310 nm swept-source laser for OCT imaging. We present ex-vivo cross-sectional imaging and monitoring during therapy. Experimental results were validated against Monte-Carlo simulations and visual inspection.
In contrast to conventional imaging systems that map an object point by point, measurements with random sensing functions in combination with computational reconstruction may afford novel imaging architectures. Here we demonstrate imaging of axial reflectivity profiles using random temporal-spatial encoding created by modal interference in a multimode fiber (MMF). Light from a broadband source (∆λ = 60nm) centered at 1310nm is split into a sample and a reference arm. In the sample arm, light in a single spatial mode is reflected by the axial reflectivity profile of the sample and coupled back into the same spatial mode. The reference light propagates through a MMF and interferes with the sample light in an off-axis geometry on a camera for holographic recording. Since the MMF supports various guided modes with distinct propagation constants, the short-coherence sample light only interferes with the spatial modes of the reference light that have matching path length. During an initial calibration procedure, interference patterns of a mirror reflection in the sample arm are recorded for varying axial mirror positions. Once this random sensing matrix (RSM) is established, the axial reflectivity profile of an object in the sample arm can be reconstructed from a single interference pattern by the multiplication with the inverse of RSM. By using a 2m long 0.22 NA MMF and tailoring the coupling regime within the MMF, we achieved axial ranging more than a centimeter. Flexible integration of polarization sensing or multi-focus imaging in a single snapshot could be envisioned in this random imaging architecture.
Over the past decade, single-pixel imaging (SPI) has established as a viable tool in scenarios where traditional imaging techniques struggle to provide images with acceptable quality in practicable times and reasonable costs. However, SPI still has several limitations inherent to the technique, such as working with spurious light and in real time. Here we present a novel approach, using complementary measurements and a single balanced detector. By using balanced detection, we improve the frame rate of the complementary measurement architectures by a factor of two. Furthermore, the use of a balanced detector provides environmental light immunity to the method.
One appealing aspect that compressive sensing offers is the possibility of retrieving a signal’s spectral information using a bucket detector and a characterized measurement matrix. Demonstrations of CS applied to optical coherence tomography (OCT) were performed, however, in the final signal-processing instead of the acquisition end. Here we propose a novel OCT system with a broadband superluminescent excitation and a bucket photodetector where the interferogram is obtained by spectral reconstruction. In particular, this system assumes the same interferometric setup as typical swept-source OCT systems except the excitation is replaced by a broadband source. The interferogram then passes through an off-the-shelf, fast tunable Fabry-Perot filter (FPF) of modest finesse whose free spectral range is designed to be much less than the excitation bandwidth. The spectral response is characterized a priori, before the filtered output is integrated by the photodetector. The spectral sampling measurement is repeated by altering the FPF’s resonant conditions multiple times through the cavity length. Having acquired the integrated photodetector values and the corresponding spectral filter functions, we reconstruct the original interferogram whose Fourier transform generates the tomogram. The sensitivity of this OCT technique is evaluated and compared using simulations with synthetic data. Moreover, B-scan reconstruction of the interferogram due to a fingertip was simulated using our scheme and the resultant image shows excellent reconstruction fidelity compared to the original OCT B-scan. These illustrations point towards a promising future of a new class of tomographic system which combines the respective strengths of swept-source and spectral-domain OCT.
Liquid-crystal variable retarders (LCVRs) are an emergent technology for space-based polarimeters, following its
success as polarization modulators in ground-based polarimeters and ellipsometers. Wide-field double nematic
LCVRs address the high angular sensitivity of nematic LCVRs at some voltage regimes. We present a work
in which wide-field LCVRs were designed and built, which are suitable for wide-field-of-view instruments such
as polarimetric coronagraphs. A detailed model of their angular acceptance was made, and we validated this
technology for space environmental conditions, including a campaign studying the effects of gamma, proton
irradiation, vibration and shock, thermo-vacuum and ultraviolet radiation.
The use of Liquid Crystal Variable Retarders (LCVRs) as polarization modulators are envisaged as a promising novel
technique for space instrumentation due to the inherent advantage of eliminating the need for conventional rotary
polarizing optics hence the need of mechanisms. LCVRs is a mature technology for ground applications; they are wellknow,
already used in polarimeters, and during the last ten years have undergone an important development, driven by
the fast expansion of commercial Liquid Crystal Displays.
In this work a brief review of the state of the art of imaging polarimeters based on LCVRs is presented. All of them are
ground instruments, except the solar magnetograph IMaX which flew in 2009 onboard of a stratospheric balloon as part
of the SUNRISE mission payload, since we have no knowledge about other spaceborne polarimeters using liquid crystal
up to now. Also the main results of the activity, which was recently completed, with the objective to validate the LCVRs
technology for the Solar Orbiter space mission are described. In the aforementioned mission, LCVRs will be utilized in
the polarisation modulation package of the instruments SO/PHI (Polarimetric and Helioseismic Imager for Solar Orbiter)
and METIS/COR (Multi Element Telescope for Imaging and Spectroscopy, Coronagraph).
Using the electronic speckle pattern interferometry (ESPI) technique in the in-plane arrangement, the coefficient of
thermal expansion (CTE) of a composite material that will be used in a passive focusing mechanism of an aerospace
mission was measured. This measurement with ESPI was compared with another interferometric method (Differential
Interferometer), whose principal characteristic is its high accuracy, but the measurement is only local. As a final step, the
results have been used to provide feedback with the finite element analysis (FEA). Before the composite material
measurements, a quality assessment of the technique was carried out measuring the CTE of Aluminum 6061-T6. Both
techniques were compared with the datasheet delivered by the supplier. A review of the basic concepts was done,
especially with regards to ESPI, and the considerations to predict the quality in the fringes formation were explained.
Also, a review of the basic concepts for the mechanical calculation in composite materials was done. The CTE of the
composite material found was 4.69X10-6 ± 3X10-6K-1. The most important advantage between ESPI and differential interferometry is that ESPI provides more information due to its intrinsic extended area, surface deformation
reconstruction, in comparison with the strictly local measurement of differential interferometry
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