Effects of polarization state and scatterer concentration on optical imaging through scattering media

Exploiting Dual-Wavelength Depolarization of Skin-Tissues for Camera-Based Perfusion Monitoring

Perfusion index (PI), the ratio between variable pulsatile (AC) and non-pulsatile (DC) components in a photoplethysmographic (PPG) signal, is an indirect and non-invasive measure of peripheral perfusion. PI has been widely used in assessing sympathetic block success, and monitoring hemodynamics in anesthesia and intensive care. Based on the principle of dual-wavelength depolarization (DWD) of skin tissues, we propose to investigate its opportunity in quantifying the skin perfusion contactlessly. The proposed method exploits the characteristic changes in chromaticity caused by skin depolarization and chromophore absorption. The experimental results of DWD, obtained with the post occlusive reactive hyperemia test and the local cooling and heating test, were compared to the PI values obtained from the patient monitor and photoplethysmography imaging (PPGI). The comparison demonstrated the feasibility of using DWD for PI measurement. Clinical trials conducted in the anesthesia recovery room and operating theatre further showed that DWD is potentially a new metric for camera-based non-contact skin perfusion monitoring during clinical operations, such as the guidance in anesthetic surgery.

Relevance and utility of the in-vivo and ex-vivo optical properties of the skin reported in the literature: a review [Invited]

Imaging non-invasively into the human body is currently limited by cost (MRI and CT scan), image resolution (ultrasound), exposure to ionising radiation (CT scan and X-ray), and the requirement for exogenous contrast agents (CT scan and PET scan). Optical imaging has the potential to overcome all these issues but is currently limited by imaging depth due to the scattering and absorption properties of human tissue. Skin is the first barrier encountered by light when imaging non-invasively, and therefore a clear understanding of the way that light interacts with skin is required for progress on optical medical imaging to be made. Here we present a thorough review of the optical properties of human skin measured in-vivo and compare these to the previously collated ex-vivo measurements. Both in-vivo and ex-vivo published data show high inter- and intra-publication variability making definitive answers regarding optical properties at given wavelengths challenging. Overall, variability is highest for ex-vivo absorption measurements with differences of up to 77-fold compared with 9.6-fold for the in-vivo absorption case. The impact of this variation on optical penetration depth and transport mean free path is presented and potential causes of these inconsistencies are discussed. We propose a set of experimental controls and reporting requirements for future measurements. We conclude that a robust in-vivo dataset, measured across a broad spectrum of wavelengths, is required for the development of future technologies that significantly increase the depth of optical imaging.

Single-shot decoherence polarization gated imaging through turbid media

We propose a method for imaging through a turbid medium by using a single-shot decoherence polarization gate (DPG). The DPG is made up of a polarizer, an analyzer, and a weakly scattering medium. Contrary to intuition, we discover that the preferential utilization of sparsely scattered photons by introducing weakly scattering mediums can lead to better image quality. The experimental results show that the visibilities of the images acquired from the DPG imaging method are obviously improved. The contrast of the bar can be increased by 50% by the DPG imaging technique. Furthermore, we study the effect of the volume concentration of the weakly scattering medium on the speckle suppression and the enhancement of the visibilities of the images. The variances of the contrasts of the image show that there exists an optimum optical depth (∼0.8) of the weakly scattering medium for DPG imaging through a specific turbid medium.

Synthesizing the degree of polarization uniformity from non-polarization-sensitive optical coherence tomography signals using a neural network

Degree of polarization uniformity (DOPU) imaging obtained by polarization-sensitive optical coherence tomography (PS-OCT) has the potential to provide biomarkers for retinal diseases. It highlights abnormalities in the retinal pigment epithelium that are not always clear in the OCT intensity images. However, a PS-OCT system is more complicated than conventional OCT. We present a neural-network-based approach to estimate the DOPU from standard OCT images. DOPU images were used to train a neural network to synthesize the DOPU from single-polarization-component OCT intensity images. DOPU images were then synthesized by the neural network, and the clinical findings from ground truth DOPU and synthesized DOPU were compared. There is a good agreement in the findings for RPE abnormalities: recall was 0.869 and precision was 0.920 for 20 cases with retinal diseases. In five cases of healthy volunteers, no abnormalities were found in either the synthesized or ground truth DOPU images. The proposed neural-network-based DOPU synthesis method demonstrates the potential of extending the features of retinal non-PS OCT.

Reconstruction algorithm of haze image based on blind separation model of polarized orthogonal airlight

Polarization-based dehazing methods can enhance the quality of haze images. However, existing methods tend to a manual selection of sky area and bias coefficient to estimate the degree of polarization (DoP) of the airlight, which leads to inaccurate estimation of the airlight. Aiming at the problem, a reconstruction algorithm based on the blind separation model of polarized orthogonal airlight is proposed. Importantly, the depth-dependent DoP of the airlight is automatically estimated without manual selection of sky area and bias coefficient. To reduce the interference of white objects on the estimation of airlight at infinity, an adaptive estimation method using the deviation between the DoP of the airlight and incident light is proposed. In order to accurate estimate the airlight from the airlight at infinity, a blind separation model of the airlight with multi-regularization constraints is established based on the decomposition of the airlight at infinity into a pair of polarized components with orthogonal angles. The experimental results show that the method effectively improves the visibility of scenes under different haze concentrations, especially in dense or heavy haze weather.

Optical transfer function of time-gated coherent imaging in the presence of a scattering medium

Optical imaging of objects embedded within scattering media such as biological tissues suffers from the loss of resolving power. In our previous work, we proposed an approach called collective accumulation of single scattering (CASS) microscopy that attenuates this detrimental effect of multiple light scattering by combining the time-gated detection and spatial input-output correlation. In the present work, we perform a rigorous theoretical analysis on the effect of multiple light scattering to the optical transfer function of CASS microscopy. In particular, the spatial frequency-dependent signal to noise ratio (SNR) is derived depending on the intensity ratio of the single- and multiple-scattered waves. This allows us to determine the depth-dependent resolving power. We conducted experiments using a Siemens star-like target having various spatial frequency components and supported the theoretical derived SNR spectra. Our study provides a theoretical framework for understanding the effect of multiple light scattering in high-resolution and deep-tissue optical imaging.

Study of polarization memory's impact on detection range in natural water fogs

The influence of the initial polarization state of a source on the detection range of a system probing through natural dense water fog is analyzed. Information about the source is conveyed by ballistic, snake, and highly scattered photons. During propagation, the polarization state of ballistic and snake photons is not altered. It is shown that though circular polarization is not altered by simple direction changes during scattering, and has thus a tendency to be preserved longer in the highly scattered photons, it does not necessarily convey more useful information about the source than linear polarization or even an unpolarized beam. It is also shown that in any forward propagating system that can be described by the small-angle approximation the impact of polarization memory can be neglected.

Deep tissue space-gated microscopy via acousto-optic interaction

To extend the imaging depth of high-resolution optical microscopy, various gating operations-confocal, coherence, and polarization gating-have been devised to filter out the multiply scattered wave. However, the imaging depth is still limited by the multiply scattered wave that bypasses the existing gating operations. Here, we present a space gating method, whose mechanism is independent of the existing methods and yet effective enough to complement them. Specifically, we reconstruct an image only using the ballistic wave that is acousto-optically modulated at the object plane. The space gating suppresses the multiply scattered wave by 10-100 times in a highly scattering medium, and thus enables visualization of the skeletal muscle fibers in whole-body zebrafish at 30 days post fertilization. The space gating will be an important addition to optical-resolution microscopy for achieving the ultimate imaging depth set by the detection limit of ballistic wave.

Assessment of anisotropy of collagen structures through spatial frequencies of Mueller matrix images for cervical pre-cancer detection

Analysis of spatial frequency of Mueller matrix (MM) images in the Fourier domain yields quantifying parameters of anisotropy in the stromal region in normal and precancerous tissue sections of human uterine cervix. The spatial frequencies of MM elements reveal reliable information of microscopic structural organization arising from the different orientations of collagen fibers in the connective tissue and their randomization with disease progression. Specifically, the local disorder generated in the normal periodic and regular structure of collagen during the growth of the cervical cancer finds characteristic manifestation in the Fourier spectrum of the selected Mueller matrix elements encoding the anisotropy effects through retardance and birefringence. In contrast, Fourier spectra of differential polarization gated images are limited to only one orientation of collagen. Fourier spectra of first row elements M11, M12, M13, and M14 and first column elements M11, M21, M31, and M41 discriminates cervical inter-epithelial neoplasia (CIN)-I from normal cervical tissue samples with 95%-100% sensitivity and specificity. FFT spectra of first and fourth row elements classify CIN-I and CIN-II grades of cervical cancerous tissues with 90%-100% sensitivity and 87%-100% specificity. Normal and CIN-II grade samples are successfully discriminated through Fourier spectra of every MM element while that of M31 element arises as the key classifier among normal, CIN-I, and CIN-II grades of cervical cancer with 100% sensitivity and specificity. These results demonstrate the promise of spatial frequency analysis of Mueller matrix images as a novel, to the best of our knowledge, approach for cancer/precancer detection.

Fluorescence anisotropy imaging in drug discovery

Non-invasive measurement of drug-target engagement can provide critical insights in the molecular pharmacology of small molecule drugs. Fluorescence polarization/fluorescence anisotropy measurements are commonly employed in protein/cell screening assays. However, the expansion of such measurements to the in vivo setting has proven difficult until recently. With the advent of high-resolution fluorescence anisotropy microscopy it is now possible to perform kinetic measurements of intracellular drug distribution and target engagement in commonly used mouse models. In this review we discuss the background, current advances and future perspectives in intravital fluorescence anisotropy measurements to derive pharmacokinetic and pharmacodynamic measurements in single cells and whole organs.

Bio-Inspired Polarization Vision Techniques for Robotics Applications

Researchers have been inspired by nature to build the next generation of smart robots. Based on the mechanisms adopted by the animal kingdom, research teams have developed solutions to common problems that autonomous robots faced while performing basic tasks. Polarization-based behaviour is one of the most distinctive features of some species of the animal kingdom. Light polarization parameters significantly expand visual capabilities of autonomous robots. Polarization vision can be used for most tasks of color vision, like object recognition, contrast enhancement, camouflage breaking, and signal detection and discrimination. In this chapter, the authors briefly cover polarization-based visual behavior in the animal kingdom. Then, they go in depth with bio-inspired applications based on polarization in computer vision and robotics. The aim is to have a comprehensive survey highlighting the key principles of polarization-based techniques and how they are biologically inspired.

Lensless wide-field single-shot imaging through turbid media based on object-modulated speckles

The need to image objects through light-scattering materials is common in a range of applications. Different methods have been investigated to acquire the image of the object when diffusers are presented. In this paper, we demonstrate the object reconstruction with single-shot imaging based on the correlography principle and phase retrieval algorithm with coherent illumination. We prove the possibility of reconstructing positive and negative objects in both transmission and reflection modes with collimated and scattered light. Formulas for calculating the size of the object from the reconstructed image are presented. We also prove that the object can be retrieved from a small section of the raw speckle image. These interesting features will have broad potential applications in many areas (such as biomedicine, security and sensing).

Polarized light interaction with tissues

This tutorial-review introduces the fundamentals of polarized light interaction with biological tissues and presents some of the recent key polarization optical methods that have made possible the quantitative studies essential for biomedical diagnostics. Tissue structures and the corresponding models showing linear and circular birefringence, dichroism, and chirality are analyzed. As the basis for a quantitative description of the interaction of polarized light with tissues, the theory of polarization transfer in a random medium is used. This theory employs the modified transfer equation for Stokes parameters to predict the polarization properties of single- and multiple-scattered optical fields. The near-order of scatterers in tissues is accounted for to provide an adequate description of tissue polarization properties. Biomedical diagnostic techniques based on polarized light detection, including polarization imaging and spectroscopy, amplitude and intensity light scattering matrix measurements, and polarization-sensitive optical coherence tomography are described. Examples of biomedical applications of these techniques for early diagnostics of cataracts, detection of precancer, and prediction of skin disease are presented. The substantial reduction of light scattering multiplicity at tissue optical clearing that leads to a lesser influence of scattering on the measured intrinsic polarization properties of the tissue and allows for more precise quantification of these properties is demonstrated.

Quantitative analysis of light scattering in polarization-resolved nonlinear microscopy

Polarization resolved nonlinear microscopy (PRNM) is a powerful technique to gain microscopic structural information in biological media. However, deep imaging in a variety of biological specimens is hindered by light scattering phenomena, which not only degrades the image quality but also affects the polarization state purity. In order to quantify this phenomenon and give a framework for polarization resolved microscopy in thick scattering tissues, we develop a characterization methodology based on four wave mixing (FWM) process. More specifically, we take advantage of two unique features of FWM, meaning its ability to produce an intrinsic in-depth local coherent source and its capacity to quantify the presence of light depolarization in isotropic regions inside a sample. By exploring diverse experimental layouts in phantoms with different scattering properties, we study systematically the influence of scattering on the nonlinear excitation and emission processes. The results show that depolarization mechanisms for the nonlinearly generated photons are highly dependent on the scattering center size, the geometry used (epi/forward) and, most importantly, on the thickness of the sample. We show that the use of an un-analyzed detection makes the polarization-dependence read-out highly robust to scattering effects, even in regimes where imaging might be degraded. The effects are illustrated in polarization resolved imaging of myelin lipid organization in mouse spinal cords.

Bio-Inspired Polarization Vision Techniques for Robotics Applications

Researchers have been inspired by nature to build the next generation of smart robots. Based on the mechanisms adopted by the animal kingdom, research teams have developed solutions to common problems that autonomous robots faced while performing basic tasks. Polarization-based behaviour is one of the most distinctive features of some species of the animal kingdom. Light polarization parameters significantly expand visual capabilities of autonomous robots. Polarization vision can be used for most tasks of color vision, like object recognition, contrast enhancement, camouflage breaking, and signal detection and discrimination. In this chapter, the authors briefly cover polarization-based visual behavior in the animal kingdom. Then, they go in depth with bio-inspired applications based on polarization in computer vision and robotics. The aim is to have a comprehensive survey highlighting the key principles of polarization-based techniques and how they are biologically inspired.

Visual Behavior Based Bio-Inspired Polarization Techniques in Computer Vision and Robotics

For long time, it was thought that the sensing of polarization by animals is invariably related to their behavior, such as navigation and orientation. Recently, it was found that polarization can be part of a high-level visual perception, permitting a wide area of vision applications. Polarization vision can be used for most tasks of color vision including object recognition, contrast enhancement, camouflage breaking, and signal detection and discrimination. The polarization based visual behavior found in the animal kingdom is briefly covered. Then, the authors go in depth with the bio-inspired applications based on polarization in computer vision and robotics. The aim is to have a comprehensive survey highlighting the key principles of polarization based techniques and how they are biologically inspired.

Visual Behavior Based Bio-Inspired Polarization Techniques in Computer Vision and Robotics

For long time, it was thought that the sensing of polarization by animals is invariably related to their behavior, such as navigation and orientation. Recently, it was found that polarization can be part of a high-level visual perception, permitting a wide area of vision applications. Polarization vision can be used for most tasks of color vision including object recognition, contrast enhancement, camouflage breaking, and signal detection and discrimination. The polarization based visual behavior found in the animal kingdom is briefly covered. Then, the authors go in depth with the bio-inspired applications based on polarization in computer vision and robotics. The aim is to have a comprehensive survey highlighting the key principles of polarization based techniques and how they are biologically inspired.

Tissue polarimetry: concepts, challenges, applications, and outlook

Polarimetry has a long and successful history in various forms of clear media. Driven by their biomedical potential, the use of the polarimetric approaches for biological tissue assessment has also recently received considerable attention. Specifically, polarization can be used as an effective tool to discriminate against multiply scattered light (acting as a gating mechanism) in order to enhance contrast and to improve tissue imaging resolution. Moreover, the intrinsic tissue polarimetry characteristics contain a wealth of morphological and functional information of potential biomedical importance. However, in a complex random medium-like tissue, numerous complexities due to multiple scattering and simultaneous occurrences of many scattering and polarization events present formidable challenges both in terms of accurate measurements and in terms of analysis of the tissue polarimetry signal. In order to realize the potential of the polarimetric approaches for tissue imaging and characterization/diagnosis, a number of researchers are thus pursuing innovative solutions to these challenges. In this review paper, we summarize these and other issues pertinent to the polarized light methodologies in tissues. Specifically, we discuss polarized light basics, Stokes-Muller formalism, methods of polarization measurements, polarized light modeling in turbid media, applications to tissue imaging, inverse analysis for polarimetric results quantification, applications to quantitative tissue assessment, etc.

Polarization sensitivity as a contrast enhancer in pelagic predators: lessons from in situ polarization imaging of transparent zooplankton

Because light in the pelagic environment is partially polarized, it has been suggested that the polarization sensitivity found in certain pelagic species may serve to enhance the contrast of their transparent zooplankton prey. We examined its potential during cruises in the Gulf of Mexico and Atlantic Ocean and at a field station on the Great Barrier Reef. First, we collected various species of transparent zooplankton and micronekton and photographed them between crossed polarizers. Many groups, particularly the cephalopods, pelagic snails, salps and ctenophores, were found to have ciliary, muscular or connective tissues with striking birefringence. In situ polarization imagery of the same species showed that, while the degree of underwater polarization was fairly high (approx. 30% in horizontal lines of sight), tissue birefringence played little to no role in increasing visibility. This is most likely due to the low radiance of the horizontal background light when compared with the downwelling irradiance. In fact, the dominant radiance and polarization contrasts are due to unpolarized downwelling light that has been scattered from the animal viewed against the darker and polarized horizontal background light. We show that relatively simple algorithms can use this negative polarization contrast to increase visibility substantially.

Polarization-gated imaging in tissue phantoms: effect of size distribution

We have investigated the effect of size distribution of aqueous solutions of monodisperse and a mixture of polydisperse scatterers of two different sizes on the image quality using linear and circularly polarized light. The contrast and resolution are affected by the size distribution present in the mixture of a polydisperse medium, while they are affected by the refractive index in a monodisperse medium. Circularly polarized light improves image quality of polydisperse scatterers. Images in the polydisperse medium are retrieved for values of optical thickness less than those of the large-sized monodisperse medium. We offer plausible explanations for all the experimental observations.

CW transillumination imaging by extracting weakly scattered light from strongly diffused light

Transmitted light through a diffuse scattering medium includes strongly diffused light (SDL) and weakly scattered light (WSL). To realize clear transillumination imaging through thick body tissue, which is typically more than 10 mm, we developed a technique to extract the WSL component from diffused light. In experiments using a 15-mm-thick scattering medium (mu(s)' = 1.0/mm), the cross-section of the light propagation area at the center of the medium was confined to a 50% area. This method's usefulness was demonstrated by transillumination imaging through a 40-mm-thick piece of chicken meat. The possibility of depth evaluation was also verified.

Influence of size parameter and refractive index of the scatterer on polarization-gated optical imaging through turbid media

The influence of incident polarized light, refractive index, and size parameter of the scatterer on achievable resolution and contrast (image quality) of polarization-gated transillumination imaging in turbid media is reported here. Differential polarization detection led to significant improvement of image quality of an object embedded in a medium of small-sized scatterers (diameter D<<lambda, isotropic scattering medium, anisotropy parameter g<or=0.2), especially using circular polarization. In contrast, for anisotropic scattering media composed of larger-sized scatterers (D>or=lambda,g>or=0.7), the improvement in image quality was less pronounced using either linear or circular polarization gating when the refractive index of the scatterer was high (ns=1.59), but for a lower value of refractive index (ns=1.37), image quality improved with the differential circular polarization gating. We offer a plausible explanation for these observations.

Polarized temporal impulse response for scattering media from third-order frequency correlations of speckle intensity patterns

Second- and third-order frequency correlations of speckle intensity patterns are used to characterize scattering media for multiple polarization states. The polarized temporal responses thus obtained are sensitive to the degree of scatter, with results being predictable by a diffusion model with sufficiently strong scatter. Experimental data are used to reconstruct various transfer functions.

Stokes polarimetry imaging of rat-tail tissue in a turbid medium using incident circularly polarized light

BACKGROUND AND OBJECTIVES

We describe a Stokes polarimetry imaging technique that quantifies the polarization properties of remitted light backscattered from a sample.

STUDY DESIGN/MATERIALS AND METHODS

Right- and left-circularly polarized near-infrared light was used to illuminate rat-tail tissue embedded in turbid gelatin.

RESULTS

The degree of linear polarization (DoLP) and degree of circular polarization (DoCP) image-maps indicate that increasing the depth of the rat tail within the turbid medium and varying the rat-tail geometry and orientation relative to the light source affected the contrast between structures and adjacent tissue layers.

CONCLUSION

Stokes polarimetry imaging shows that the intervertebral discs and soft tissue regions of rat tails strongly depolarize incident circularly polarized light. Tendon regions remit light with a more linear form due to birefringence. Both DoLP and DoCP image-maps provide contrast between tissue structures. When differentiating between unpolarized light and light with low DoCP or DoLP, the polarization of backscattered light from the turbid medium must to be taken into consideration.

Stokes polarimetry imaging of rat tail tissue in a turbid medium: degree of linear polarization image maps using incident linearly polarized light

Illumination with incident linearly polarized light on tissue and polarization state measurements of the remitted light provide a means by which various tissue structures can be differentiated. A rat tail is embedded within a turbid gelatin such that there is a variable depth of medium above it. By varying the incident polarization angle (IPA) of the illuminating linearly polarized light, the geometry, and the orientation angle of the tissue, a series of 2-D degree of linear polarization image maps are created using our Stokes polarimetry imaging technique. The image maps show locations of the polarization-sensitive structures in the rat tail, including soft tissue, intervertebral disks, and tendons. The observed morphologies in the image maps indicate locations where the depolarization of light differs according to the tissue type and underlying layers. The data indicate the importance of varying the IPA, and that tissue dichroism and birefringence affect the degree of linear polarization image maps. Diagnostic information regarding subsurface tissue structures is obtained.

Polarization-correlation mapping of biological tissue coherent images

We investigate the statistical polarization parameters of biological tissue histological section images with different morphological structure. First we outline the results of polarization coordinate mapping and analysis of the statistics of the first to fourth orders of biological tissue image polarization azimuth and ellipticities. Second, we study the statistics of the first to fourth orders of coordinate distributions of the complex degree of mutual polarization (CDMP) of biological tissue images. Finally, we consider the diagnostic possibilities of investigating 2-D distributions of CDMP of images that correspond to physiologically normal and degeneratively and/or dystrophycally changed biological tissues that are being analyzed.

Polarization visualization and selection of biotissue image two-layer scattering medium

We analyze and experimentally test the concept of laser polarization biotissue probing. The methods of increasing the SNR in coherent images of the optically anisotropic architectonics of the morphological biotissue structure are considered. The possibilities of polarization selection and contrasting of such images screened by other biotissues are examined. The influence of the depolarization degree of the scattered background on the SNR is investigated. The possibilities of polarization correction of the probing beam for contrasting biotissue images are analyzed.

Polarized light propagation through scattering media: time-resolved Monte Carlo simulations and experiments

A study of polarized light transmitted through randomly scattering media of a polystyrene-microsphere solution is described. Temporal profiles of the Stokes vectors and the degree of polarization are measured experimentally and calculated theoretically based on a Monte Carlo technique. The experimental results match the theoretical results well, which demonstrates that the time-resolved Monte Carlo technique is a powerful tool that can contribute to the understanding of polarization propagation in biological tissue. Analysis based on the Stokes-Mueller formalism and the Mie theory shows that the first scattering event determines the major spatial patterns of the transmitted Stokes vectors. When an area detected at the output surface of a turbid medium is circularly symmetrical about the incident beam, the temporal profile of the transmitted light is independent of the incident polarization state. A linear relationship between the average order of the scatters and the light propagation time can be used to explain the exponential decay of the degree of polarization of transmitted light.

Monte Carlo simulation of multiphoton fluorescence microscopic imaging through inhomogeneous tissuelike turbid media

Image resolution and signal level in fluorescence microscopy through inhomogeneous turbid media consisting of scatterers of multiple sizes under single- (1p), two- (2p), and three-photon (3p) excitation have been investigated based on a modified Monte Carlo model. The effects of the size distribution and the concentration distribution of scattering particles are explored. Simulation results reveal that the size and the concentration distribution both have an impact on image formation in media consisting of small particles and that 3p excitation has the most significant impact. In media with scatterers of a large size, both size and concentration distributions lead to a slight effect. Image formation in a mixed medium containing small and large scattering particles is more affected by the large particles.

Characterization of layered scattering media using polarized light measurements and neural networks

Measurements of the spatial distributions of polarized light backscattered from a two-layer scattering medium are used to train a neural network. We investigated whether the absorption coefficients and thickness of the layer can be determined when the scattering properties are known. When determining the absorption of the upper layer or the layer's thickness, polarized light measurements provide better performance than unpolarized measurements, demonstrating the sensitivity of polarized light to superficial tissue. Determination of the lower layer's absorption coefficient is not improved by polarized light measurements. Prior knowledge of the tissue under investigation is also beneficial because errors are reduced if the range of absorption or thickness is restricted.

Time-resolved polarization to extract coded information from early ballistic and snake signals through turbid media

Time-resolved polarization is used to extract coded information buried within the multiple scattering profiles from the early ballistic and snake components as they pass through turbid media. By polarization analysis the depolarized diffusive component and the natural-light background are significantly reduced to enhance the signal-to-noise ratio of a coded pulse train. This procedure has the potential to improve optical wireless communication in cloudy environments.

Optical imaging based on time-resolved Stokes vectors in filamentous tissues

Time-resolved Stokes vector components of light transmitted through filamentous tissues were measured with a view to improving the imaging quality of optical images in such tissues. Temporal profiles of the Stokes vectors and the time-resolved degree of polarization (DOP) were calibrated to produce higher image quality than that of images based on time gating, polarization discrimination, or both. A thin chicken bone inserted into chicken breast tissue with filament orientation in different directions with respect to the direction of input linear polarization was scanned to demonstrate images of higher spatial resolution and contrast based on the measurement of time-resolved DOP.

Surface-reflection elimination in polarization imaging of superficial tissue

A major drawback in polarization gating of light backscattered from tissue is that surface reflections dominate the image. An optically flat plate and matching fluid applied to the tissue surface, combined with off-axis detection, were previously used to address this problem. This approach is often inappropriate or inconvenient for practical use and more importantly can affect the tissue's optical properties. A method is demonstrated that combines images obtained with linearly and circularly polarized light to produce a polarization-gated image that is free from surface reflections and does not require optically flat plates or matching fluid.

Determination of the depth of a scattering target in a turbid medium with polarization discrimination of transmitted signals

We demonstrate the feasibility of a novel method of determining target depth in a turbid medium through Monte Carlo simulations and experiments. The method is based on the strong and weak dependencies of the copolarized component and the degree of polarization (DOP), respectively, of the transmitted intensity on the target depth. The two-way measurements of the copolarized intensity can be used for determination of target depth, whereas the transversely scanned DOP results are used for estimating the two-dimensional image in a turbid system. The combination of these two sets of data could provide useful results for estimating three-dimensional images.

Comparative study of polarized light propagation in biologic tissues

We report the depolarization of light scattered by a variety of birefringent and nonbirefringent tissues. We used Stokes polarimetry to investigate how scatterer structures in each tissue contribute to the depolarization of linearly versus circularly polarized light propagating through that tissue. Experiments were performed on porcine blood, fat, tendon, artery, and myocardium. The results indicate that the two incident polarization states are depolarized differently depending on the structure of the sample. As seen in sphere suspensions, for tissues containing dilute Mie scatterers, circularly polarized light is maintained preferentially over linearly polarized light. For more dense tissues, however, the reverse is true. The results illustrate situations where polarized light will provide an improvement over unpolarized light imaging, information that is crucial to optimizing existing polarimetric imaging techniques.

Image reconstruction through turbid media under a transmission-mode microscope

In this paper, image enhancement and reconstruction through a turbid medium by utilizing polarization gating and mathematical image reconstruction methods in a microscopic imaging system are investigated. A Monte Carlo simulation model based on Mie theory and the concept of the effective point spread function (EPSF) is adopted to study image formation under a transmission-mode microscope. The results show that polarization gating methods, and particularly the differential polarization gating method, can be efficient in suppressing highly scattered light, which leads to a significant enhancement of image quality. An image reconstruction method based on the concept of the EPSF is demonstrated to be efficient in further improving image quality.

Analysis of the spatial distribution of polarized light backscattered from layered scattering media

The scattering of polarized light from a two layer scattering medium is investigated using Monte Carlo simulations. First order and normalized second order moments are used to analyze the spatial properties of the emerging light in different polarization states. Linearly and circularly polarized illumination is used to probe different depths. Absorption and layer thickness are varied and it is demonstrated that the determination of these values is aided by the inclusion of polarization information. The lateral and depth localization of light by polarization subtraction is also quantified. Potential applications of these techniques are burn depth and melanoma thickness measurements.

Analysis of the picosecond magneto-optical phenomena in scattering media of biological interest

The behaviour of a magneto-optically active biological-like medium under picosecond optical excitation is analysed. The new technique is based on the fact that photons trapped in multiple scattering events inside the magneto-optical medium leave the medium with larger induced rotation angles, as they travel longer distances. Two- and three-dimensional displacements of the photons in the medium are separately analysed. The dependence of this effect on the applied magnetic field strength, the value of the magneto-optical constant of the medium and the standard deviation of the statistical distribution of the photons scattered inside the turbid medium are studied. The best values for the magnetic field and optical parameters of the biological medium are proposed for the experimental observation of the picosecond magneto-optical phenomena in scattering media of biological origin. We also make some prospective studies to evaluate the potential application of the magneto-optical effect as a tool for optical tissue biopsy. Values for the optimum magnetic field intensities and for the expected experimental sensitivity in diverse conditions are reported.

Fluorescence depolarization in a scattering medium: effect of size parameter of a scatterer

For a monodisperse scattering medium, we investigate the dependence on scatterer size parameter for the change in anisotropy of fluorescence due to single scattering at excitation or emission wavelength. The value for the ratio of the anisotropy of fluorescence after one scattering at excitation or emission wavelength to the initial value was observed to increase with increasing value of scatterer size parameter. The effect of multiple scattering on anisotropy of fluorescence from fluorophores embedded in a scattering medium was incorporated using a photon migration model. The model was validated by experiments carried out on samples with known concentration of polystyrene microspheres as scatterers and riboflavins or reduced form of nicotinamide adenine dinucleotide as fluorophores.

Zeeman laser-scanning confocal microscopy in turbid media

A novel Zeeman laser-scanning confocal microscope (ZLSCM) is proposed. It has the same configuration as the conventional laser-scanning confocal microscope (LSCM) in which a Zeeman laser in conjunction with a Glan-Thompson analyzer is used. In our system, the analyzer with the bandpass filter, which act simultaneously as a polarization gate and a coherence gate, enhance the collection efficiency of the weak-scattering photons and simultaneously suppress the multiple-scattering photons. The improvement in depth resolution of a ZLSCM in a scattering medium compared with that of a conventional LSCM is discussed and demonstrated experimentally.

Polarized optical coherence imaging in turbid media by use of a Zeeman laser

A method that uses a Zeeman laser in conjunction with a Glan-Thompson analyzer to image an object in a turbid medium is proposed. A heterodyne signal is generated only when the scattering photons are partially polarized, and the spatial coherence is not seriously degraded after the signal propagates in the turbid medium. A system combining polarization discrimination with optical coherence detection to image the object in a scattering medium is successfully demonstrated. The medium is a solution of polystyrene microspheres measuring 1.072 mum in diameter suspended in distilled water contained in a 10-mm-thick quartz cuvette. The advantages of this optical system, including better selectivity of the weak partially polarized scattering photons and better imaging ability in higher-scattering media, are discussed.

Propagation of polarized light in turbid media: simulated animation sequences

A time-resolved Monte Carlo technique was used to simulate the propagation of polarized light in turbid media. Calculated quantities include the reflection Mueller matrices, the transmission Mueller matrices, and the degree of polarization (DOP). The effects of the polarization state of the incident light and of the size of scatterers on the propagation of DOP were studied. Results are shown in animation sequences.

Polarized light propagation through tissue phantoms containing densely packed scatterers

We demonstrate that polarized light is maintained differently in densely packed versus dilute suspensions of polystyrene microspheres. The degrees of linear and circular polarization were measured versus scatterer concentration in aqueous suspensions of 0.48-, 0.99-, 2.092-, and 9.14-mum-diameter polystyrene microspheres. The results indicate that, for dilute suspensions of microspheres where independent scattering is assumed, the degrees of linear and circular polarization decrease as the scatterer concentration increases. For dense suspensions, however, the degree of polarization begins to increase as the scatterer concentration increases. The preferential propagation of linear over circular polarization states in dense suspensions is similar to results seen in biological tissue.

Comparison of polarized-light propagation in biological tissue and phantoms

We demonstrate significant differences in the propagation of polarized light through biological tissue compared with two common tissue phantoms. Depolarization of linearly and circularly polarized light was measured versus propagation distance by use of two independent measurement techniques. The measurements were performed on adipose and myocardial tissues and on tissue phantoms that consisted of polystyrene microsphere suspensions and Intralipid. The results indicate that, in contrast with results obtained in tissue phantoms, linearly polarized light survives through longer propagation distances than circularly polarized light in biological tissue.

Polarization discrimination of coherently propagating light in turbid media

We describe the use of degree of polarization to discriminate unscattered and weakly scattered light from multiply scattered light in an optically turbid material. We use spatially resolved measurements of the degree of polarization to compare how well linearly and circularly polarized light survives in a sample. Experiments were performed on common tissue phantoms consisting of polystyrene and Intralipid microsphere suspensions and on adipose and arterial tissue. The results indicate that polarization is maintained even after unpolarized irradiance through each sample has been extinguished by several orders of magnitude. The results also show that polarized light propagation in common tissue phantoms is distinctly different from polarized light propagation in the two tissues investigated. Further, these experiments illustrate when polarization is an effective discrimination criterion and when it is not. The potential of a polarization-based discrimination scheme to image through the biological and nonbiological samples investigated here is also discussed.

Backscattering target detection in a turbid medium by polarization discrimination

We describe a method for increasing target contrast within a turbid medium by means of the polarization state of the scattered light. The backscattered Mueller matrices for various concentrations of 0.1-microm spherical scatterers were measured with and without a painted metal target. Simple discrimination based on detecting cross-polarized intensities is shown to be more effective than the use of total intensity information. As a result, the choice of polarization state is dictated primarily by the requirement to maximize depolarization at the target. This in general means that circularly polarized light is the optimum choice.

Coherence and polarization of light propagating through scattering media and biological tissues

The degree of polarization of light propagating through scattering media was measured as a function of the sample thickness in a Mach-Zehnder interferometer at a wavelength of lambda = 633 nm. For polystyrene microspheres of diameters 200, 430, and 940 nm, depolarization began to appear for thicknesses larger than 23, 19, and 15 scattering mean free paths (SMFP's), respectively, where the coherently detected scattered component dominates the ballistic component. For large particles (940 nm) the initial polarization survived partially in the scattering regime and progressively vanished up to the detection limit of our setup. This phenomenon was similarly observed in diluted blood from 12.5 to 280 SMFP's. Beyond this thickness the fluctuating parallel and crossed components of polarization became random. A dual-channel interferometer allowed us to detect simultaneously the low-frequency fluctuations of both polarized components through a few millimeters in liver tissue.

Microscopic imaging through a turbid medium by use of annular objectives for angle gating

We report a new method for microscopic imaging of an object embedded in a turbid medium. The new method is based on the angle-gating mechanism achieved by the use of polarized annular objectives in the illumination and collection paths of a microscopic imaging system. A detailed experimental study is presented of the effects of the size of annular obstructions on image quality when turbid media, including polystyrene microspheres and milk suspensions, are imaged. Images of 22-mum polystyrene microspheres embedded in the turbid media show that misinterpretation can occur when circular objectives are used, because of the detection of mainly multiply scattered photons (i.e., diffusing photons). However, when annular objectives are employed, diffusing photons from a turbid medium can be efficiently suppressed; thus image contrast appears correctly, and image resolution is increased.

Resolution improvement in microscopic imaging through turbid media based on differential polarization gating

We report a new method for microscopic imaging of an object embedded in a turbid medium, based on the differential polarization-gating mechanism. It is demonstrated that with this method, image resolution through optically thick milk suspensions can be improved by as much as 30% compared with no-gating methods. An image resolution of tens of micrometers is achieved in an optically thick turbid medium, which is approximately 10 times better than that achieved in transillumination imaging in a similar medium.

Treatment of polarization in laser remote sensing of ocean water

We review existing experimental data and methods for calculating the Mueller matrix of ocean water for use as input in a simulation model applicable to laser remote sensing. Calculations of the Mueller matrix are made for scattering media of different refractive indices, shapes, and size distributions. Dependencies of the backscattering depolarization ratio as a function of the particle refractive index are presented, and we demonstrate the potential importance of polarization in bathymetric sensing.