This PDF file contains the front matter associated with SPIE Proceedings Volume 11646, including the Title Page, Copyright information, and Table of Contents.

Introduction to SPIE Photonics West BiOS conference 11646: Polarized light and Optical Angular Momentum for biomedical diagnostics

The aim of this study was to demonstrate the feasibility of methylene blue (MB) fluorescence polarization (Fpol) imaging for reliable detection of breast cancer at the cellular level. Breast cells were collected from discarded malignant, benign, and normal breast tissues following surgery. MB co-/cross-polarized fluorescence emission and Fpol images provided complimentary morphological and quantitative evaluations of the samples, respectively. Statistical analysis confirmed that MB Fpol is significantly higher (P<.0001) in cancerous versus noncancerous cells. This technique may provide a rapid and accurate tool to assist cytopathological differentiation of breast lesions.

The accurate detection of brain tumor border during neurosurgery is crucial for the safe and complete tumor resection, but it is often difficult to differentiate solid tumor tissue from infiltrated white matter. To address this problem we suggest detecting optical anisotropy of brain white matter which consists of bundles of axons (or fiber tracts). Tumor growth erases this optical anisotropy of healthy brain. We used a wide-field imaging Mueller polarimeter to measure thick fixed human and fresh animal brain sections in reflection. The maps of azimuth of fast optical axis of linear birefringent medium obtained from Lu-Chipman decomposition of the experimental Mueller matrices showed a compelling correlation with the fiber tracts directions on histology image of thin whole mount silver-stained brain tissue section.

An improved smoke removal model for surgery is introduced with transmission parameters related to a medium’s optical depth rather than scene distance. Theoretical analysis and observation of experimental data shows that cross-polarized signals generated by multiple scattering are less affected by smoke compared to co-polarized signals. We analyze the transmission process of linearly polarized light interacting with different media, and then use polarization difference imaging and color channel information to detect smoke and estimate the transmission parameters. Several further processing procedures including parameter compensation and image smoothing are implemented to recover tissue visibility from surgical images.

Biomechanical function of musculoskeletal soft tissues is dictated by their hierarchically organized collagen extracellular matrices. Our lab has used quantitative polarized light imaging (QPLI) to evaluate dynamic collagen fiber alignment in soft tissues. However, thinning of tissue for light transmission precludes the use of QPLI in more physiologically relevant scenarios. Reflectance mode QPLI could allow for in situ analysis of collagen microstructure. In this study, signal obtained via reflectance and transmission mode QPLI of engineered soft tissue analogs with prescribed collagen fiber alignment were compared in order to understand differences between modes and to provide context for further orthopedic applications.

Water content of stratum corneum has one of the most important biological effects on the physiological function of the skin. Measuring and adjusting the water content can be helpful to understand the physiological state of skin and delay skin aging. However, most existing skin water content analyzers have to contact the skin and the results may be affected by personal usage habits prominently. Mueller matrix polarimetry is sensitive to structural features of tissues. Parameters derived from Mueller matrix can provide the microstructural information quantitatively, such as the sizes of scatterers, the distribution of collagen fibers and so on. In this study, we demonstrate a novel, quantitative, non-contact and in situ technique based on Mueller matrix polarimetry for monitoring the microstructural changes of skin tissues during the process of skin water content reduction. We measure the Mueller matrices of rat skin samples and porcine abdominal skin samples, then analyze the Mueller matrix derived parameters to indicate microstructural changes during the skin water content reduction processes. Comparison between the rat skin samples applied with and without moisturizing cream show that the Mueller matrix derived parameters are potential indicators to reflect the water content of the skin quantitatively. This technique can provide a non-contact detection method and be used to evaluate the change of skin water content when different skin-care cosmetics are used on the skin.

Polarization parameters derived from Mueller matrix (MM) can describe different structural and optical properties of the media. Anisotropy of light-structure interactions are useful in many fields like biomedical research, but can be affected by the orientation relationship between samples and detection systems. In this paper, we used backscattering MM imaging systems with different backscattering angles to obtain MMs and several anisotropic parameters. The angle between backscattering and the normal of the sample surface changes from 180° to 110° while the incident light is parallel to the normal. Concentrically aligned fibers sample is used as scattering media. It has fibers of complete 360° directions. The experimental results showed the colinear backscattering MM imaging system can provide anisotropic parameters with better periodical variation than non-colinear backscattering MM imaging system does. The amplitude of signal attenuates with the scattering angle declines. The collinear backscattering MM imaging system has better detection ability measuring anisotropy of samples. However, further studies are still necessary to analyze the additional information obtained from non-collinear backscattering MM imaging system.

We combine Mueller matrix polarimetry (MMP) with spatial frequency domain imaging (SFDI) to create a technique that is sensitive to near-surface material anisotropy. We demonstrate this imaging modality with scattering and absorbing phantoms and with a fiber optic bundle. Images of depolarization show reduced depolarization under high spatial frequency illumination and demodulation and in some cases, reduced contrast to deeper features. The images of a fiber optic bundle show marked differences between illumination modulations that are aligned with the fibers versus those crossed with the fibers, demonstrating the impact of polarization on scattering direction.

Polarization systems allow creating a variety of devices with required polarization characteristics. Ordinarily, polarization systems are used to control polarization for only one wavelength. With such a system, to modify polarizations of two beams with different wavelengths requires the whole system rebuilding or reconstruction. However, in experimental optics, there is a need for devices for continuous, simultaneous and independent transformations of polarization for multiple wavelengths. We propose a new method for independent polarization control of two light beams with different wavelengths. We show here that such polarization systems should consist of not less than four phase elements, including birefringent plates with controlled axis orientation relative to each other and/or liquid crystal cells with the fixed axis orientation but controlled phase retardations. To demonstrate our approach, we have designed such a four-component system that functions as a half-wave plate for the first wavelength and a retardation plate with an arbitrary phase retardation for the second wavelength. The theoretical analysis presented here supports our approach and shows it’s validity for any two different wavelengths.

In this work we review some techniques useful for the generation of vortex beams and vector beams. We start from geometric-phase components such as q-plates or polarization diffraction gratings. Then we show that these components can be implemented and/or combined with liquid-crystal spatial light modulators in order to achieve higher control of the polarization and diffraction properties of the modulated beam. We show some examples where spiral phases and different diffraction grating designs are combined encoded with different polarizations to generate multiple vortex and vector beams. Employed in an inverted configuration, the same components can be employed to detect vector beams. Such arrangements can be used to generate vector-beam based polarimetry.

Invasive cervical cancer is a slow progressing disease, taking more than 10 years to fully develop from infection. The anatomic accessibility and possible treatment of precancerous lesions make early screening a suitable and effective management. An imager based on Mueller matrix polarimetry consisting of a polarization state generator of four input polarization states and a polarization state analyzer on board of the detector, a polarized camera, is introduced. A 4x3 reduced form of the Mueller matrix is decomposed to extract polarimetric parameters such as the retardance, depolarization and orientation. The use of this device on biological samples is tested.

We performed Mueller matrix Monte Carlo simulations of the propagation of optical radiation in diffusely scat- tering media for collimated incidence and report depolarization in the transmitted rays as a function of thickness, the angle subtended by the detector, and the area of the material sampled. This paper expands upon previous work [Germer, J. Opt. Soc. Am A 37 980 (2020)], whereby it was shown that the complex paths that rays follow serve to depolarize the light and that the measurement geometry is important for obtaining consistent results. In addition, we perform extinction theorem calculations for spheroidal particles and show that for a rea- sonable distribution of particle eccentricity, the depolarization due to the fluctuations of the diattenuation and birefringence of a solution of such spheroids is insignificant compared to the calculated depolarization induced by scattering.

Polarized light imaging can gate photons reflected from a tissue such that superficially scattered photons from the sub-diffuse regime are distinguished from multiply scattered photons from the diffuse regime. Such polarization- gated sub-diffuse images can restrict the image contrast to the superficial epithelial (or epidermal) layer of a tissue where pathology often originates, and avoid being blinded by the much larger amount of diffuse reflectance from deeper tissue. A pair of co-polarized (HH) and cross-polarized (HV) images of a tissue are acquired and the difference image HQ = HH-HV subtracts the depolarized photons from the diffuse regime and yields an image using only the partially polarized photons from the superficial sub-diffuse regime. Using polarized Monte Carlo simulations (0.5-μm-dia. microspheres in water at 633 nm wavelength), this paper addresses the question, “How many scattering events are involved in the HQ image?” The simulations show that HQ(n) decays to HQ(1)e⁻¹ or 0.37HQ(1) after 6 scatterings. Images with a polarization camera were acquired on reflectance standards and on skin of the cheek using a blue LED (405 nm) for illumination. Analysis yielded the total diffuse reflectance (Rd = 0.419), the sub-diffuse-regime reflectance (S = 0.013), and the diffuse-regime reflectance (D = 0.406).

Light interaction with material systems may introduce depolarization to the incident light. This phenomenon comes from multiple scattering processes that take place inside the media and strongly depends on the particle characteristics. In the case of botany, plant leaves can be understood as depolarizing systems. A non-contact method to analyze these samples consist of illuminating them with well-known polarized light and study the scattered light to retrieve the physical characteristics of the sample. This physical study can be done by measuring the Mueller matrix of samples, in which the physical information of samples is encoded in their 16 elements and further mathematical treatment is required to extract the information. In the case of scattering systems, the depolarization content carries very valuable information but it is usually not inspected in the botanic field. A way to study depolarized content is by determining the so-called depolarization index PΔ, which gives an overall measure of the degree of depolarization of a system but it does not measure possible anisotropic dependence of the depolarization. For instance, a depolarizer equally depolarizing any fully polarized input polarization or a depolarizer that depolarizes them in a strongly heterogenous way, may lead to the same PΔ value. In contrast, the Indices of Polarimetric Purity (IPP) are a group of metrics that further synthesize the depolarizing content, taking into account the anisotropic depolarization. In this work, we describe the main physical characteristics of samples achieved by using these IPP through plant samples. Moreover, we show how IPP highlights some structures hidden in regular intensity measurements, highlighting the potential of these metrics for botanical applications.

Mueller matrix microscopy is a promising non-invasive tool for pathological diagnosis due to its sensitivity to microstructures and its non-reliance on high spatial resolution. Such technique is sensitive to anisotropy, but the majority of such information is deeply hidden within the orientation parameters such as αq, αr, αP and αD. Analysis of them is challenging because orientation parameters varies when the sample’s spatial azimuthal angle changes relative to the imaging system, and the range boundary imposed by the arctan function prevents the parameters from forming a continuous distribution. As the result, the use of orientation parameters is generally avoided during quantitative analysis, despite the rich information they encode. In an effort to resolve these challenges, we propose a novel method for analyzing orientation parameters extracted from Mueller matrix polarimetry. The angular pixel values in the parameter images are unwrapped by assuming continuity, transforming the distorted distribution into one that is statistically viable. The unwrapped orientation parameters are then used for pathological slides analysis. Frequency distribution histograms of the orientation parameters before and after unwrapping are compared, the validity of the proposed method is demonstrated.

Exploiting correlation properties of speckles behind diffusers have shown to be a promising method for non-invasive and high-speed biomedical imaging. We propose a theoretical and numerical model based on a stack of thin diffusers to study speckle correlations inside forward scattering media such as biological tissue. Optical vortices in such media have the specificity to be created in pairs to satisfy topological-charge conservation. Statistics of optical vortices generation and propagation through and inside forward scattering media will be discussed.

The capability to increase the robustness to scattering has become a crucial request for communication protocols and imaging systems. Here we perform a complete analysis regarding the spatial features and the polarization of structured beams propagating in different scattering media. We observe different behaviors for structured light scattered by a solution of polystyrene latex beads in water and by tissue-mimicking phantom. The reported study can help in establishing a framework for the application of structured light illumination in imaging and diagnostic.

In the current report, we present the international efforts to create a unified open-access Monte Carlo (MC) based computational method and a set tools for simulation of Total Angular Momentum (TAM) of light propagating and localizing in turbid tissue-like scattering media. We specifically focus on performing the simulations with a variety of combinations of spin and orbital angular momentum and the subsequent analyses of the spin-orbit interactions. We present an open-access application and a media model which considers both spatial and volumetric variations in the media optical properties. The online application has been accelerated by parallel computations on graphics cards and is being extensively used in the ongoing studies of light's angular momentum propagation in turbid media (e.g. phantoms and cancerous lesions). Rigorous validation against data obtained during experimental studies is presented.

Complete characterization of biological samples is of potential interest in different industrial and research areas, as for instance, in biomedical applications, for the recognition of organic structures or for the early detection of some diseases. During the last decades, polarimetric methods are experiencing an increase of attention in the study of biomedical tissues, and they are nowadays used in such framework to provide qualitative (polarimetric imaging) and quantitative (data processing) information for the studied samples. Polarimetric methods are based on the analysis of polarization modifications produced by light-matter interactions which can be triggered by a number of complex internal processes but can be roughly understood as the result of the combination of three pure polarimetric features of the sample: its diattenuation, retardance and depolarization. For the analysis of the depolarization content, we propose the use of the Indices of Polarimetric Purity (IPP) to describe the sample behavior. Related with the randomness of the scattering processes, IPPs provide more information of depolarizing systems than the widely used depolarization index (p▵), which further synthetize the depolarization content of samples. Moreover, certain combinations of IPP parameters leads to p▵. As a result, IPPs allow the revelation of some structures from tissue samples hidden in regular intensity images of even in the p▵ channel, leading to better tissue classification results. In this work, we present different applications of IPPs in biomedical tissue that show its potential, which are not restricted to the biomedical framework as relevant results in plants characterization are also presented.

Polarized nonlinear scanning imaging using two photon fluorescence, harmonic generation or coherent Raman has been extensively developed to image molecular orientational organization in tissues. Current approaches using fast modulation (electro-optic) are however still limited to seconds time scales due to their required voltage shape modulation. To reach higher frequencies, we have developed an interferometric frequency shift-based method using Acousto-Optic Modulation. We present degree-precision performances in the retrieved information even at low signal to noise level and sub-second dynamics. The method is applied to real time actin structural imaging in cells as well as collagen and lipids organization imaging in tissues.

We propose a cross-modality method that translates polarimetric images into bright-field. In the lung tissue histological analysis, immunohistochemical (IHC) staining of tissues is widely used to specify particular cellular events especially in precision medicine. In this work, we measured hematoxylin and eosin (HE) stained slices by Mueller matrix (MM) microscopy and then fed polarimetric data into a well-designed generative adversarial network (GAN). The network can generate images that are equivalent to the IHC stained from bright-field microscopy. This will assist pathologists with the real IHC staining procedure and pathological diagnosis. Instead of preparing specimens from scratch, we collected already existing specimens, i.e., the adjacent HE and IHC stained slices from the same tissue volume. We adopted the CycleGAN to learn the translation between unaligned images from two domains. We used a U-Net based generator and a PixelGAN based discriminator in the model. The efficacy of this method was demonstrated on smooth muscle actin (SMA) staining in lung tissue. The results are evaluated by three image quality assessment methods by comparing the generated and real staining images.

We present a simple low-cost microscope that uses a vectorial extension of Fourier ptychography to recover the absorption, phase and polarization properties of a sample at high-NA across a wide field-of-view. Our principle is validated by experimentally imaging quantitative test targets as well as a plant root and rabbit spinal cord cross-sections, with which we demonstrate the ability to record complex specimen birefringence over 10.4 mm2 field-of-view at 0.73um resolution. Our new Fourier ptychographic approach also enables the measurement and correction of polarization-dependent pupil aberrations. We hope this simplicity helps adapt joint polarization and phase imaging to a wider array of applications.

Polarization-sensitive optical coherence tomography (PS-OCT) measures the depth-dependent polarization states of light backscattered in tissue. We have developed algorithms for the robust reconstruction of the local, depth-resolved OAx orientation, which recursively compensate for the cumulative effect of the preceding tissue layers. Local OAx imaging in healthy human skin in vivo revealed dense, weaving patterns that are imperceptible in OCT intensity tomograms and that suggest a mesh-like tissue organization, consistent with the morphology of dermal collagen. Local OAx orientation as a contrast mechanism merits further exploration for applications in dermatology and in other tissues and organs presenting intrinsic birefringence contrast.

Degenerative joint disease (DJD) is a disease that the articular cartilage changes from hyaline cartilage to fibrous cartilage. PS-OCT may provide a method to quantitatively analyze cartilage birefringence and diagnose DJD. Here, we proposed a novel PS-OCT system that uses spun fiber to construct the sample arm. In this work, phase retardation map, optical axis map, and conventional OCT images of hyaline cartilage and fibrous cartilage are presented, and the differences in the birefringence of these two types of cartilage are identified. The proposed PS-OCT system demonstrates great potential for accurate diagnosis of DJD in the clinic.

Along with second harmonic generation and two-photon excited fluorescence measured with Non-Linear Microscopy, polarization properties measured with Mueller Matrix Polarimetry Microscopy can improve our understanding of the remodeling process in preterm pregnancy. This is critical to define therapeutic targets and to develop clinical tools for early and accurate detection of preterm risks. While manual analyzing and classifying individual cervical samples is time-consuming, automated algorithms can be advantageous when the number of samples is large. To such extent, we demonstrate the use of Convolutional Neural Networks (CNN) for feature extraction and K-Nearest Neighbor (KNN) for classification as an alternative to manual assessment.

Aerosol particle classification are of great importance in many cases. But currently there is almost no universal method to deal with this task. In this study, we develop a one-dimensional convolution neural network taking multi-angle polarization time series signal of single suspended particle as input to identify its category. We train the network and reach quite high accuracy on a large dataset which contains signals of multi-kinds particles such as carbon black, PSL, dust and water-soluble salts. This method gives us a new way of looking deeper into single suspended particles in the air and the knowledge learned in this task may be able to be transferred to deal with tasks of recognizing more kinds or more complex aerosol particles such as bioaerosol or even airborne pathogen.

Early diagnosis and fast screening of cervical cancer is the key to prognosis of treatment and patient survival. Polarimetry technique with high sensitivity to microstructures and low requirement for resolution is promising at facilitating the fast screening and quantitative diagnosis. In this study, we apply the Mueller matrix microscope and multichannel convolutional neural network for the detection of human cervical intraepithelial neoplasia (CIN) samples from normal samples. The Mueller matrix polar decomposition and transformation parameters, rotation invariant parameters, and Mueller matrix symmetry-related parameters of the cervical tissues in epithelial region and at different stages are calculated and analyzed. For detection of early cervical lesions, the selection method of polarimetry parameters based on statistical features and multichannel convolutional neural network (CNN) for classification are proposed. To illustrate, we select the input parameters of CNN models from all commonly used polarimetry parameters according to the amount of information which are evaluated by the mean value, standard deviation, and information entropy of all pixels in 2D parameters images of the training samples. In multichannel CNN classification, each selected parameter is treated as an input of a channel. The proper multichannel CNN models learn deep features from the selected polarimetry parameters of training samples and show good performance for detecting CIN samples under a low-resolution system.

Current processing techniques of polarization-sensitive optical coherence tomography (PS-OCT) can recover the tissue’s local, i.e. depth-resolved scalar retardance and optic axis orientation. However, system-induced polarization mode dispersion (PMD) and the presence of speckle in the measured tomograms complicate reconstruction and result in a detrimental trade-off with spatial resolution. We speculate that a machine learning approach should work well for generating an improved reconstruction. By training the model on simulated tomograms that encode the forward model and include system PMD and noise, and by testing the algorithm on experimentally acquired PS-OCT data, we aim to demonstrate a generalized PS-OCT reconstruction tool.

Cancer progression is closely related to changes in the structure and mechanical properties of the tumor microenvironment in a complex and not well-understood manner. In many solid tumors, including pancreatic cancer, the complex interplay among the different components of tumor microenvironment leads to a desmoplastic reaction associated with fibroblasts activation and collagen overproduction. Desmoplasia is responsible for tumor stiffening, and poses a major barrier to the effective delivery of drugs and has been associated with poor prognosis. Thus, the understanding of the involved mechanisms and the identification of collagen-based signatures that characterize the state of a particular tumor can lead to the development of novel diagnostic and prognostic biomarkers. In this study, pancreatic tumor models were developed employing the human pancreatic cancer cell lines BxPc-3 and MIAPaCa-2 and tissue biopsies were obtained at different stages of cancer progression. Polarized microscopy on picrosirius red stained tumor sections was used in order to assess collagen-based optical signatures in correlation with tumor progression, while Atomic Force Microscopy (AFM) was applied for the nano-mechanical characterization of the samples. The results demonstrated that pancreatic cancer presents unique collagen-based characteristics that can be used as a novel biomarker for cancer diagnosis and prognosis.

Tissue polarimetry could be identified as a complementary optical and non-invasive technique to assist the gold standard histopathology analysis of tissue. In general, polarimetric diagnostics is based on tracing different polarimetric responses (including light depolarization) in tissue zones with structure altered by the benign and pre/cancerous formations. In this manuscript, both healthy and malignant tissue zones of a thick formalin-fixed colon specimen were used for Mueller matrix measurements. Additionally, two more Mueller matrices from Monte Carlo simulation and tissue mimicking phantom were also evaluated, in order to assess polarimetric char- acterization and modeling of turbid media. Symmetric decomposition algorithm of Mueller matrices developed in house was adopted to extract both polarization and depolarization properties, encoded in the Mueller matrix elements. The decomposition products allowed to reveal important information about the internal tissue struc- ture and morphology. The depolarization and polarization parameters were found to follow the particular trends that depend on a choice of parametric space.

Tissue optical clearing techniques have become more and more prominent in biomedical applications, since they can reduce scattering and improve the imaging depth, resolution and contrast. Polarization is sensitive to the microstructure of tissues, when apply the rapid Stokes imaging method to monitor the process of optical tissue clearing, we observe that the polarization parameters oscillate periodically during clearing. A series of experiments have been designed to verify the truth of oscillations, we first eliminate the influence of system noise by measuring air and other polarization optics, the system error is less than 1% and is stable, then we analyze the impacts from optical clearing agents, we take quartz plate, the quartz plate with saturated sucrose solution, the fact is that there are slight fluctuations in polarization parameters but no oscillations. However, when we test the porcine skin which immersed in optical clearing agents, there are significant oscillations which show periodicity in polarization parameter, the oscillation cycle is about 40s and the oscillation amplitudes become smaller with the increase of immersion time. Finally, we imaging the samples which have completed optical clearing and no oscillation is found. By these series experiments, we confirm the truth of oscillations of the polarization parameters during clearing, and we believe these are connected with the mechanisms of the clearing which need further study.