Absorption effect is a basic optical phenomenon and an important feature in tissue imaging and characterization. Based on our Monte Carlo simulation on the anisotropic tissue model (sphere-cylinder birefringence model), combined with our experiments of tissue phantoms, we demonstrate the influence of absorption effect on Mueller matrix and particularly on depolarization, linear retardance, and diattenuation parameters. The simulation and experimental results show a good consistency on the suppressed depolarization and scatterering induced retardance, and the enhanced diattenuation caused by the absorption, and also indicate the birefringence induced retardance insensitive to the absorption. Study of the phase function of different incident polarized lights and the distribution of scattering number gives a preliminary explanation about the above results.
Both cylindrical scatterers and birefringent media may contribute to the anisotropy of tissue, where anisotropy can be characterized using polarization techniques. Our previous studies have shown that a layer of well-aligned cylindrical scatterers displays anisotropic properties similar to those of a piece of birefringent media, whose equivalent extraordinary axis is along the axial direction of the cylinders. We focused on a sample consisting of two layers of anisotropic media, with each layer having a different orientation; the characteristics of this sample were representative of the properties of multilayer fibrous tissues. Using a Mueller matrix decomposition method, we examined in detail how the total retardance and the equivalent extraordinary axis of the bilayered sample varied with changes in the retardance of the two layers and the direction of the extraordinary axis. The results of this study showed that, in such bilayer samples, a layer of well-aligned cylindrical scatterers generated a retardance that behaved exactly like the retardance generated by a piece of birefringent media. The simulated results were also confirmed by the results of experiments using aligned glass fibers.
Using Monte Carlo simulations based on previously developed scattering models consisting of spherical and cylindrical scatterers imbedded in birefringent interstitial medium, we compare the polarization parameters extracted from the 3×3 and 4×4 Mueller matrix decomposition methods in forward and backward scattering directions. The results show that the parameters derived from the 3×3 Mueller matrix decomposition are usually not the same as those from the 4×4 Mueller matrix decomposition but display similar qualitative relations to changes in the microstructure of the sample, such as the density, size, and orientation distributions of the scatterers, and birefringence of the interstitial medium. The simulations are backed up by experiments when suitable samples are available.
Most of real tissues have anisotropic microstructures or anisotropic optical features. The variation of tissue anisotropy can be an effective character to describe some abnormal conditions in tissues, so it is meaningful for the extraction and comparison of parameters for anisotropy evaluation. In this paper, based on our previously proposed sphere-cylinder scattering model, we simulate and investigate the propagation and scattering of polarized light in tissue models using polarization-sensitive Monte Carlo simulation. Focusing on anisotropic tissues, we consider two type disturbance of highly ordered cylindrical elements: cylinders with a distribution of the orientation angle and the existence of the isotropic elements like spheres. By analyzing the corresponding backscattering Mueller matrices with the changes of structural parameters in our tissue model, we extract a characteristic parameter to describe the symmetry of certain Mueller matrix elements. According to the simulation, the characteristic is less sensitive to the size of cylindrical scatterers, and is especially suitable for the case of detecting the small scale isotropic perturbation in a highly anisotropic medium. The results presented in this paper confirm the feasibility of this new anisotropy factor to measure the degree of tissue anisotropy, and imply the validity of applying it in distinguishing some pathological changes.
Polarization measurements are sensitive to the microstructure of tissues and can be used to detect pathological changes. Many tissues contain anisotropic fibrous structures. We obtain the local orientation of aligned fibrous scatterers using different groups of the backscattering Mueller matrix elements. Experiments on concentrically well-aligned silk fibers and unstained human papillary thyroid carcinoma tissues show that the m22, m33, m23, and m32 elements have better contrast but higher degeneracy for the extraction of orientation angles. The m12 and m13 elements show lower contrast, but allow us to determine the orientation angle for the fibrous scatterers along all directions. Moreover, Monte Carlo simulations based on the sphere-cylinder scattering model indicate that the oblique incidence of the illumination beam introduces some errors in the orientation angles obtained by both methods. Mapping the local orientation of anisotropic tissues may not only provide information on pathological changes, but can also give new leads to reduce the orientation dependence of polarization measurements.
This paper focuses on the absorption effect due to pathological changes. Using Monte Carlo simulation on our previously proposed sphere-cylinder-birefringence model (SCBM), and forward scattering experiments on phantoms containing polystyrene microspheres, well-aligned glass fibers and ink solution, we investigate how depolarization, retardance and diattenuation change with tissue absorption by decomposing the Mueller matrix using Mueller matrix polar decomposition method. According to the simulation and experimental results, this paper presents the influence of absorption on polarization scattering process from three aspects: firstly, the absorption depresses the depolarization contributed by the spherical and cylindrical scatterers; secondly, it decreases the retardance due to the anisotropic cylinder; thirdly, the increasing absorption enhances the diattenuation from cylindrical scatterers.
We obtained backscattering Mueller matrix images on a cancerous tissue sample through a gradient-index (GRIN) lens and observed strong distortion in all the Mueller matrix elements. By measuring the intrinsic polarization properties of the GRIN lens, which is dominated by birefringence following a radial profile, and applying a matrix inversion method to the distorted Mueller matrix, we are able to remove the artifacts because of the birefringent GRIN lens and recover the polarization features of the sample. The results demonstrate the feasibility to take Mueller matrix measurements using GRIN lenses or other optical components with strong birefringence.
Polarization measurements allow one to enhance the imaging contrast of superficial tissues and obtain new polarization sensitive parameters for better descriptions of the micro- and macro- structural and optical properties of complex tissues. Since the majority of cancers originate in the epithelial layer, probing the morphological and pathological changes in the superficial tissues using an expended parameter set with improved contrast will assist in early clinical detection of cancers. We carry out Mueller matrix imaging on different cancerous tissues to look for cancer specific features. Using proper scattering models and Monte Carlo simulations, we examine the relationship between the microstructures of the samples, which are represented by the parameters of the scattering model and the characteristic features of the Mueller matrix. This study gives new clues on the contrast mechanisms of polarization sensitive measurements for different cancers and may provide new diagnostic techniques for clinical applications.
We report an anisotropic tissue model containing well-ordered birefringent cylinders. Using simulations and experiments, we examined the different polarization features for nonbirefringent and birefringent cylinders and analyzed the influence of the birefringent cylinders on the retardance obtained from Mueller matrix polar decomposition. For the well-ordered birefringent cylinders, retardance increases linearly with the intrinsic birefringence and the scattering coefficient. Furthermore, the cylinders with a larger diameter generate more retardance. Compared with the cylinder-birefringence model, in which birefringent medium exists between the scatterers, the intrinsic birefringence on the cylinders usually contributes much less to the total retardance.
In this paper, we study the cylindrical scatterers and birefringent medium as two anisotropic sources in the turbid medium by polarimetry method. Using Monte Carlo simulations on the anisotropic sample, we examine in detail the relationship between the retardance δ, depolarization Δ and the anisotropic microscopic structure and their interactions of each other by Mueller matrix decomposition method. The well-ordered cylinder scatter system can be regarded as a birefringent medium. If the orientation of the aligned cylindrical scatterers does not coincide with the extraordinary axis of the birefringent medium, the variation of total retardance in this cylinder-birefringence model would accord to the two pieces of birefringent medium overlapped together with angle between extraordinary axes.
Mueller matrix imaging techniques can be used to detect the micro-structure variations of superficial biological tissues, including the sizes and shapes of cells, the structures in cells, and the densities of the organelles. Many tissues contain anisotropic fibrous micro-structures, such as collagen fibers, elastin fibers, and muscle fibers. Changes of these fibrous structures are potentially good indicators for some pathological variations. In this paper, we propose a quantitative analysis technique based on Mueller matrix for mapping local anisotropy axis of scattering media. By conducting both experiments on silk sample and Monte Carlo simulation based on the sphere-cylinder scattering model (SCSM), we extract anisotropy axis parameters from different backscattering Mueller matrix elements. Moreover, we testify the possible applications of these parameters for biological tissues. The preliminary experimental results of human cancerous samples show that, these parameters are capable to map the local axis of fibers. Since many pathological changes including early stage cancers affect the well aligned structures for tissues, the experimental results indicate that these parameters can be used as potential tools in clinical applications for biomedical diagnosis purposes.
Polarization measurements allow one to enhance imaging contrast of superficial tissues and obtain new polarization
sensitive parameters for better description of the micro- and macro- structural and optical properties of complex tissues.
Since the majority of cancers originate in the epithelial layer, probing the morphological and pathological changes in the
superficial tissues using an expended parameter set and with improved spatial resolution and contrasts will lead to new
clues on the early clinical detection of cancers. In this wok, we carry out polarization imaging on cancerous tissues and
look for cancer specific features. Using a scattering model, which approximates the anisotropic biological tissues to a
mixture of spherical and cylindrical scatterers imbedded in birefringent ambient media, and a Monte Carlo simulation
program, we examine the relationship between the micro-structure of the model and the characteristic polarization
features. The studies help to understand the contrast mechanism of polarization sensitive measurements for different
cancers and provide the basis for potential clinical applications.
We present both the two-dimensional backscattering point-illumination and surface-illumination Mueller matrices for the anisotropic sphere-cylinder scattering media. The experimental results of the microsphere-silk sample show that the Mueller matrix elements of an anisotropic scattering medium are different from those of an isotropic medium. Moreover, both the experiments and Monte Carlo simulations show that the directions of the fibrous scatterers have prominent effects on the Mueller matrix elements. As the fibrous samples rotate, the surface-illumination Mueller matrix measurement results for the m12, m21, m13, m31, m22, m23, m32, and m33 elements represent periodical variations. Experiments on skeletal muscle and porcine liver tissue samples confirm that the periodical changes for the surface-illumination Mueller matrix elements are closely related to the well aligned fibrous scatterers. The m22, m23, m32, and m33 elements are powerful tools for quantitative characterization of anisotropic scattering media, including biological tissues.
We present both the two-dimensional backscattering point-illumination and surface-illumination Mueller matrices for the anisotropic sphere-cylinder scattering media. The experimental results of the microsphere-silk sample show that the Mueller matrix elements of an anisotropic scattering medium are different from those of an isotropic medium. Moreover, both the experiments and Monte Carlo simulations show that the directions of the fibrous scatterers have prominent effects on the Mueller matrix elements. As the fibrous samples rotate, the surface-illumination Mueller matrix measurement results for the m12, m21, m13, m31, m22, m23, m32, and m33 elements represent periodical variations. Experiments on skeletal muscle and porcine liver tissue samples confirm that the periodical changes for the surface-illumination Mueller matrix elements are closely related to the well aligned fibrous scatterers. The m22, m23, m32, and m33 elements are powerful tools for quantitative characterization of anisotropic scattering media, including biological tissues.
We have developed a sphere-cylinder birefringence model (SCBM) for anisotropic media. The new model is based on a previously published sphere-cylinder scattering model (SCSM), but the spherical and cylindrical scatterers are embedded in a linearly birefringent medium. A Monte Carlo simulation program for SCBM was also developed by adding a new module to the SCSM program to take into account the effects of birefringence. Simulations of the backscattering Mueller matrix demonstrate that SCBM results in better agreement with experimental results than SCSM and is more suitable to characterize fibrous tissues such as skeletal muscle. Using Monte Carlo simulations, we also examined the characteristics of two-dimensional backscattering Mueller matrix of SCBM and analyzed the influence of linear birefringence.
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