A Study On Quantitative Polarization Characterization Of Tissues For Diagnosis Applications

Polarization imaging techniques can obtain the micro- and macro-structural and optical properties of tissues and have been proved as a potentially powerful tool for cancer diagnosis. However, due to the complex structures of biological tissues, the polarization contrast mechanisms are not clear. Therefore studies on polarization measurements are important for clinic applications.Expending a previously published sphere-cylinder scattering model (SCSM), we developed a new model with the spherical and cylindrical scatterers embedded a birefringent or optically active medium, to simulate the anisotropic biological tissues. Based on this scattering model, we developed a Monte Carlo program to simulate polarized light propagation in anisotropic media. In the Monte Carlo program, variables include the refractive indices, the scattering coefficients, and the sizes of both the spheres and cylinders, together with the orientation and angular distribution of the cylinders can be adjusted. For the ambient medium, the refractive index, the absorption coefficient, the optical activity coefficient, and the value and direction of birefringence are all variables. In this model, the spherical scatterers represent structures such as cell nuclei, mitochondria and ribosomes; the cylindrical scatterers represent the fiber structures such as collagen, elastin and actin; the birefringence effect represent the linear anisotropy of fibrous structures such as muscle, teeth and bone; the optical activity effect simulate chiral molecules such as glucose, protein and lipids.We validated the reliability of the birefringence and optical activity module by comparing the simulated results with the experimental results of standard samples. Simulations of the backscattering Mueller matrix demonstrated that the sphere-cylinder birefringence model (SCBM) is in better agreement with experimental results than SCSM. It is revealed that the spherical scatterers, cvlindrical scatterers and birefrinaent medium are responsible to the anisotropic properties of biological tissues. By varying the parameters in the tissue optics model, we examined characteristic features in the two-dimensional backscattering Mueller matrix, as well as the images of surface-illumination Mueller matrix elements, degree of polarization (DOP), Mueller matrix polar decomposition (MMPD) parameters and Mueller matrix transformation (MMT) parameters. The studies demonstrate the possibility of extracting quantitatively the structural and optical properties of tissues from different polarization sensitive parameters.Using the above tissue optics model and the polarization parameters, we interpreted and analyzed the polarization measurements of cancerous tissues. We compared three different polarization imaging methods:DOP, MMPD and MMT using three cancerous tissues of different microstructures:human basal cell carcinoma (BCC), papillary thyroid carcinoma (PTC) and cervical squamous cell carcinoma. Using proper scattering models and Monte Carlo simulations, we examined 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.