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

Toward the development of portable light emitting diode-based polarization spectroscopy tools for breast cancer diagnosis

A robust, affordable and portable light emitting diode-based diagnostic tools (POLS-NIRDx) using a polarization-sensitive (linear as well as circular polarization) technique were designed and developed to quantify the degree of linear polarization (DOLP), degree of circular polarization (DOCP). The study was performed on malignant (invasive ductal carcinoma) and adjacent normal ex-vivo biopsy tissues excised from N = 10 patients at the operating wavelengths of 850 and 940 nm. The average DOLP and DOCP values were lower for malignant than adjacent normal while operating at 850 and 940 nm. The highest accuracy was observed for DOLP (100%) and DOCP (80%) while operating at 850 nm, which reduced (80% for DOLP and 65% for DOCP) at 940 nm. This pilot study can be utilized as a differentiating factor to delineate malignant tissues from adjacent normal tissues.

Learning feature fusion for target detection based on polarimetric imaging

We propose a polarimetric imaging processing method based on feature fusion and apply it to the task of target detection. Four images with distinct polarization orientations were used as one parallel input, and they were fused into a single feature map with richer feature information. We designed a learning feature fusion method using convolutional neural networks (CNNs). The fusion strategy was derived from training. Meanwhile, we generated a dataset involving one original image, four polarization orientation images, ground truth masks, and bounding boxes. The effectiveness of our method was compared to that of conventional deep learning methods. Experimental results revealed that our method gets a 0.80 mean average precision (mAP) and a 0.09 miss rate (MR), which are both better than the conventional deep learning method.

Applying limiting entropy to quantify the alignment of collagen fibers by polarized light imaging

Collagen alignment has shown clinical significance in a variety of diseases. For instance, vulvar lichen sclerosus (VLS) is characterized by homogenization of collagen fibers with increasing risk of malignant transformation. To date, a variety of imaging techniques have been developed to visualize collagen fibers. However, few works focused on quantifying the alignment quality of collagen fiber. To assess the level of disorder of local fiber orientation, the homogeneity index (HI) based on limiting entropy is proposed as an indicator of disorder. Our proposed methods are validated by verification experiments on Poly Lactic Acid (PLA) filament phantoms with controlled alignment quality of fibers. A case study on 20 VLS tissue biopsies and 14 normal tissue biopsies shows that HI can effectively characterize VLS tissue from normal tissue (P < 0.01). The classification results are very promising with a sensitivity of 93% and a specificity of 95%, which indicated that our method can provide quantitative assessment for the alignment quality of collagen fibers in VLS tissue and aid in improving histopathological examination of VLS.

Development of a polarization imaging method to detect paraffin-embedded pathology tissues before applying other techniques

The present article describes the development of a technique, applied to paraffin-embedded tissues, which uses three different wavelengths of monochromatic light (λ1 = 445 nm, λ2 = 540 nm and λ3 = 660 nm) for the measures of the degree of polarization, degree of linear polarization, degree of circular polarization and birefringence, all obtained from measurements of Stokes parameters by using polarized light. The goal of this study was to detect changes in developing embryonic mouse eye when pregnant mice fed diets without folic acid for variable periods compared with a healthy control group. We present a biomedical diagnostic technique based on polarized light detection applied to paraffin-embedded tissues to visualize the structural damage to aid us in the diagnosis before applying other techniques. Through this method, we can visualize and identify which parts of the tissue were altered with respect to the control group.

The depolarization performances of scattering systems based on the Indices of Polarimetric Purity (IPPs)

In this paper, the Indices of Polarimetric Purity (IPPs) [1-3] have been proposed to analyze the depolarization performances of mono-dispersion and poly-dispersion scattering systems. Here, we mainly investigate the influences of the particles' density, Refractive Index (RI) of the medium, incident wavelengths, the mixing ratio of bi-dispersion scattering particles and particle-size distributions of poly-dispersion scattering system on the depolarization performances for the backscattering detection. For the mono-dispersion scattering system under same incident wavelength, if the relative RI ratios (m) increase linearly, the depolarization performances of the system will first weaken and then strengthen, and of course, the incident wavelength and density of scattering particles will also influence the depolarization performances of the scattering system. For the bi-dispersion scattering system, the proportion of small particles will be negatively correlated with the depolarization property of the dispersion system, and meanwhile, the particle-size distributions will also affect the depolarization performances greatly in the poly-dispersion scattering system. The results demonstrate that the IPPs can be used to describe the depolarization performances of dispersion systems effectively.

Characterization of healthy and nonmelanoma-induced mouse utilizing the Stokes-Mueller decomposition

Skin cancer is one of the most common cancers, including melanoma and nonmelanoma cancer. Melanoma can be easily detected by the observation of abnormal moles, but nonmelanoma signs and symptoms are not apparent in the early stages. We use the Stokes-Mueller matrix decomposition method to detect nonmelanoma at the early stage by decomposing the characteristics of polarized light interacting with normal and cancerous tissues. With this decomposition method, we extract nine optical parameters from biological tissues, namely the LB orientation angle (α), the LB phase retardance (β), the CB optical rotation angle (γ), the LD orientation angle (θd), the linear dichroism (D), the circular dichroism (R), the degrees of linear depolarization (e1 and e2), the degree of circular depolarization (e3), and the depolarization index (Δ). The healthy skin and the induced nonmelanoma skin cancer of mice are analyzed and compared based on their optical parameters. We find distinctive ranges of values for normal skin tissue and nonmelanoma skin cancer, in which β and D in cancerous tissue are larger and nonmelanoma skin becomes less depolarized. This research creates an innovative solid foundation for the diagnosis of skin cancer in the future.

Assessment of tissue polarimetric properties using Stokes polarimetric imaging with circularly polarized illumination

Tissue-depolarization and linear-retardance are the main polarization characteristics of interest for bulk tissue characterization, and are normally interpreted from Mueller polarimetry. Stokes polarimetry can be conducted using simpler instrumentation and in a shorter time. Here, we use Stokes polarimetric imaging with circularly polarized illumination to assess the circular-depolarization and linear-retardance properties of tissue. Results obtained were compared with Mueller polarimetry in transmission and reflection geometry, respectively. It is found that circular-depolarization obtained from these 2 methods is very similar in both geometries, and that linear-retardance is highly quantitatively similar for transmission geometry and qualitatively similar for reflection geometry. The majority of tissue circular-depolarization and linear-retardance image information (represented by local image contrast features) obtained from Mueller polarimetry is well preserved from Stokes polarimetry in both geometries. These findings can be referred to for further understanding tissue Stokes polarimetric data, and for further application of Stokes polarimetry under the circumstances where short acquisition time or low optical system complexity is a priority, such as polarimetric endoscopy and microscopy.

Real time complete Stokes polarimetric imager based on a linear polarizer array camera for tissue polarimetric imaging

Tissue polarimetric imaging measures Mueller matrices of tissues or Stokes vectors of the emergent light from tissues (normally using incidence with a fixed polarization state) over a field of view, and has demonstrated utility in a number of surgical and diagnostic applications. Here we introduce a compact complete Stokes polarimetric imager that can work for multiple wavelength bands with a frame-rate suitable for real-time applications. The imager was validated with standard polarizing components, and then employed as a polarization state analyzer of a Mueller imaging polarimeter and a standalone Stokes imaging polarimeter respectively to image the process of dehydration of bovine tendon tissue. The results obtained in this work suggested that the polarization properties of the samples rich of collagen fibres can change with the degree of dehydration, and therefore, dehydration of the samples prepared for polarimetric imaging (e.g. polarimetric microscopy) should be carefully controlled.

Review of the progress toward achieving heat confinement-the holy grail of photothermal therapy

Photothermal therapy (PTT) involves the application of normally benign light wavelengths in combination with efficient photothermal (PT) agents that convert the absorbed light to heat to ablate selected cancers. The major challenge in PTT is the ability to confine heating and thus direct cellular death to precisely where PT agents are located. The dominant strategy in the field has been to create large libraries of PT agents with increased absorption capabilities and to enhance their delivery and accumulation to achieve sufficiently high concentrations in the tissue targets of interest. While the challenge of material confinement is important for achieving “heat and lethality confinement,” this review article suggests another key prospective strategy to make this goal a reality. In this approach, equal emphasis is placed on selecting parameters of light exposure, including wavelength, duration, power density, and total power supplied, based on the intrinsic properties and geometry of tissue targets that influence heat dissipation, to truly achieve heat confinement. This review highlights significant milestones researchers have achieved, as well as examples that suggest future research directions, in this promising technique, as it becomes more relevant in clinical cancer therapy and other noncancer applications.

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.

A new approach for optical assessment of directional anisotropy in turbid media

A study of polarized light transport in scattering media exhibiting directional anisotropy or linear birefringence is presented in this paper. Novel theoretical and experimental methodologies for the quantification of birefringent alignment based on out-of-plane polarized light transport are presented here. A polarized Monte Carlo model and a polarimetric imaging system were devised to predict and measure the impact of birefringence on an impinging linearly polarized light beam. Ex-vivo experiments conducted on bovine tendon, a biological sample consisting of highly packed type I collagen fibers with birefringent property, showed good agreement with the analytical results.

On the performance of the physicality-constrained maximum-likelihood estimation of Stokes vector

We address the estimation of the Stokes vectors taking into account the physical realizability constraint. We propose a fast method for computing the constrained maximum-likelihood (CML) estimator for any measurement matrix, and we compare its performance with the classical empirical physicality-constrained estimator. We show that when the measurement matrix is based on four polarization states spanning a regular tetrahedron on the Poincaré sphere, the two estimators are very similar, but the CML provides a better estimation of the intensity. For an arbitrary measurement matrix, the CML estimator does not always yield better estimation performance than the empirical one: their comparative performances depend on the measurement matrix, the actual Stokes vector and the signal-to-noise ratio.

Quantitative fluorescence and elastic scattering tissue polarimetry using an Eigenvalue calibrated spectroscopic Mueller matrix system

A novel spectroscopic Mueller matrix system has been developed and explored for both fluorescence and elastic scattering polarimetric measurements from biological tissues. The 4 × 4 Mueller matrix measurement strategy is based on sixteen spectrally resolved (λ = 400 - 800 nm) measurements performed by sequentially generating and analyzing four elliptical polarization states. Eigenvalue calibration of the system ensured high accuracy of Mueller matrix measurement over a broad wavelength range, either for forward or backscattering geometry. The system was explored for quantitative fluorescence and elastic scattering spectroscopic polarimetric studies on normal and precancerous tissue sections from human uterine cervix. The fluorescence spectroscopic Mueller matrices yielded an interesting diattenuation parameter, exhibiting differences between normal and precancerous tissues.

General state contrast imaging: an optimized polarimetric imaging modality insensitive to spatial intensity fluctuations

In active polarization imaging, one frequently needs to be insensitive to noninformative spatial intensity fluctuations. We investigate a way of solving this issue with general state contrast (GSC) imaging. It consists in acquiring two scalar polarimetric images with optimized illumination and analysis polarization states, then forming a ratio. We propose a method for maximizing the discrimination ability between a target and a background in GSC images by determining the optimal illumination and analysis states. A further advantage of this approach is to provide an objective way of quantifying the performance improvement obtained by increasing the number of degrees of freedom of a GSC imager. The efficiency of this approach is demonstrated on simulated and real-world images.

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.

When is polarimetric imaging preferable to intensity imaging for target detection?

We consider target detection in images perturbed with additive noise. We determine the conditions in which polarimetric imaging, which consists of analyzing of the polarization of the light scattered by the scene before forming the image, yields better performance than classical intensity imaging. These results give important information to assess the interest of polarimetric imaging in a given application.

In vivo characterization of human pigmented lesions by degree of linear polarization image maps using incident linearly polarized light

BACKGROUND AND OBJECTIVE

Melanoma is the most serious form of skin cancer and often appears as an evolving multicolored skin growth. It is well documented that pre-existing atypical or dysplastic nevi can evolve into a melanoma. The development of an in vivo imaging system to characterize benign and malignant nevi has been emphasized to aid in early detection of melanoma. The goal of this study is to utilize a novel Stokes polarimetry imaging (SPI) system for the characterization of pigmented lesions, and to evaluate the SPI system in comparison to dermoscopy and histology images.

STUDY DESIGN/MATERIALS AND METHODS

Linearly polarized light with varying incident polarization angles (IPA) illuminated various types of pigmented lesions. The melanocytic nesting patterns of pigmented lesions were characterized by constructing the degree-of-linear-polarization (DOLP) image map with comparison to dermoscopy and histology. The incident polarized light was filtered by visible filters for spectral imaging and incident deeply penetrating red light was used to correlate the SPI image with histopathological examination.

RESULTS

The DOLP images with varying IPA at different visible wavelengths were used to characterize various kinds of pigmented lesions by showing subsurface melanocytic nesting distribution as well as morphological information with better resolution and contrast. In correlation with dermoscopy and histology, various defining features such as compound, junctional, lentiginous, reticular, globular patterns of melanocytic nests were identified.

CONCLUSION

When imaging pigmented melanocytic lesions, the SPI system with varying IPA at the red light wavelength can better define the melanocytic nesting patterns in both the dermal epidermal junction and the dermis. The SPI system has the potential to be an effective in vivo method of detecting pre-malignant nevi and melanoma.

Rotating linear polarization imaging technique for anisotropic tissues

A novel rotating linear polarization imaging technique is developed to characterize the anisotropic properties of tissues. Differences of orthogonal linear polarization with different incident and detection polarization angles are fitted to an analytical function to retrieve a set of parameters. Experiments with different tissues and Monte Carlo simulations indicate that two of the parameters, G and phi(3)2, are correlated to the anisotropic property and the orientation angle of the fibrous structure in the media. The technique can be used for clinical diagnosis.

Linear polarization difference imaging and its potential applications

We demonstrate a novel linear polarization imaging technique and its potential application in dermatology. This technique records a series of images corresponding to different combinations of illumination and detection polarization and calculates intensity differences between orthogonal detection polarizations pixel by pixel. Fitting the polarization difference data to an analytical expression of the incident and detection polarization angles results in two new parameters, G and (phi3)/2. It is shown that G is strongly correlated to the order of alignment of the fibrous structure in the sample, and (phi3)/2 represents the angle of orientation of the fibers. Preliminary clinical testing implies that this method may be applied for medical diagnosis of skin diseases.

Target detection in active polarization images perturbed with additive noise and illumination nonuniformity

Active imaging systems that illuminate a scene with polarized light and acquire two images in two orthogonal polarizations yield information about the intensity contrast and the orthogonal state contrast (OSC) in the scene. Both contrasts are relevant for target detection. However, in real systems, the illumination is often spatially or temporally nonuniform. This creates artificial intensity contrasts that can lead to false alarms. We derive generalized likelihood ratio test (GLRT) detectors, for which intensity information is taken into account or not and determine the relevant expressions of the contrast in these two situations. These results are used to determine in which cases considering intensity information in addition to polarimetric information is relevant or not.

Polarization-sensitive optoacoustic tomography of optically diffuse tissues

Polarization is indicative of material anisotropy, a property that reveals structural orientation information of molecules inside the material. Herein we investigate whether polarization can be detected optoacoustically in scattering and absorbing tissues. Using a laboratory prototype of polarization-sensitive optoacoustic tomography, we demonstrate high-resolution reconstructions of dichroism contrast deep in optically diffusive tissue-mimicking phantoms. The technique is expected to enable highly accurate imaging of polarization contrast in tissues, far beyond the current capabilities of pure optical polarization-imaging approaches.

Mueller matrix decomposition for extraction of individual polarization parameters from complex turbid media exhibiting multiple scattering, optical activity, and linear birefringence

Linear birefringence and optical activity are two common optical polarization effects present in biological tissue, and determination of these properties has useful biomedical applications. However, measurement and unique interpretation of these parameters in tissue is hindered by strong multiple scattering effects and by the fact that these and other polarization effects are often present simultaneously. We have investigated the efficacy of a Mueller matrix decomposition methodology to extract the individual intrinsic polarimetry characteristics (linear retardance delta and optical rotation psi, in particular) from a multiply scattering medium exhibiting simultaneous linear birefringence and optical activity. In the experimental studies, a photoelastic modulation polarimeter was used to record Mueller matrices from polyacrylamide phantoms having strain-induced birefringence, sucrose-induced optical activity, and polystyrene microspheres-induced scattering. Decomposition of the Mueller matrices recorded in the forward detection geometry from these phantoms with controlled polarization properties yielded reasonable estimates for delta and psi parameters. The confounding effects of scattering, the propagation path of multiple scattered photons, and detection geometry on the estimated values for delta and psi were further investigated using polarization-sensitive Monte Carlo simulations. The results show that in the forward detection geometry, the effects of scattering induced linear retardance and diattenuation are weak, and the decomposition of the Mueller matrix can retrieve the intrinsic values for delta and psi with reasonable accuracy. The ability of this approach to extract the individual intrinsic polarimetry characteristics should prove valuable in diagnostic photomedicine, for example, in quantifying the small optical rotations due to the presence of glucose in tissue and for monitoring changes in tissue birefringence as a signature of tissue abnormality.

Improved method for calibrating a Stokes polarimeter

We present a method for calibrating a polarization state analyzer that uses a set of well- characterized reference polarization states and makes no assumptions about the optics contained in the polarimeter other than their linearity. The method requires that a matrix be constructed that contains the data acquired for each of the reference polarization states and that this matrix be pseudoinverted. Since this matrix is usually singular, we improve the method by performing the pseudoinversion by singular value decomposition, keeping only the four largest singular values. We demonstrate the calibration technique using an imaging polarimeter based upon liquid crystal variable retarders and with light emitting diode (LED) illumination centered at 472 nm, 525 nm, and 630 nm. We generate the reference polarization states by using an unpolarized source, a single polarizer, and a Fresnel rhomb. This method is particularly useful when calibrations are performed on field-grade instruments at a centrally maintained facility and when a traceability chain needs to be maintained.