Polarized light has wide spread advantges in-vivo skin imaging and is used to various ends. In the case of pre cancer diagnostics, polarized light has been rightly able to distinguish between benign nevi, melanocytic nevus, melanoma, and normal skin. Here we present a general overview of non-contact polarimetry that relies on certain parameters, as it is relevant to the iToBoS project.
A Mueller or Stokes polarimeter is one such method with high reliability and non-invasive. Upon illumination, the scattering from the skin carries the vital information regarding the morphology of the tissue (benign or not).
The Mueller matrix elements offer valuable insights into the microarchitecture of samples. Primarily, they allow assessment of both the anisotropy and its extent within a sample. This can be achieved through two simple metrics: the values of non-diagonal elements and a comparison of m22 and m33. In particular, Mueller matrices from anisotropic samples, like skin tissues, typically exhibit non-zero values for non-diagonal elements, and m22 differs from m33. In contrast, isotropic samples show zero values for non-diagonal elements and m22 equals m33. Additionally, an increase in the magnitude of non-diagonal elements and a widening discrepancy between m22 and m33 values indicate a rise in sample anisotropy. , a comparison of the diagonal and non-diagonal elements of the Mueller matrices demonstrate the significantly higher intensities of the diagonal elements, suggesting the high depolarization nature of skin tissue samples. he magnitudes of diagonal elements (i.e., m22, m33 and m44) are closely related to the scattering regime in the tissue, and subsequently to the depolarization capabilities of the samples. For the mean values of the magnitude, m22 ≈ m33 > m44; this is the hallmark of Rayleigh scattering regime, where the probing light is scattered by sub-wavelength small particles. The primary component of the skin substrate (i.e., collagen fibers; size ∼1–8 μm) is much smaller than the wavelengths of the probing light and, thereby, falls in the framework of Rayleigh scattering. the information of tissue anisotropy (i.e., retardance) is consolidated in m23, m24, m32, m34, m42, and m43 elements of the Mueller matrix. However, the geometrical alignment of the fibrous structures present in the tissue sample with respect to the probing optical beam has been speculated to significantly influence the measured linear retardance
The Mueller Matrix can be decomposed into three other matrices which can be used to further characterize materials. These three matrices are the depolarization, diattenuation, and retardance matrix. The optical polarimetric parameters like depolarization, diattenuation and crosstalk, in conjunction with the Mueller matrix elements, play an important role in discriminating among healthy and cancerous lung cells as well as differentiating among different types of malignanciesThe bulwark of such work relies on certain polarization parameters for the discrimination of the tissue. These parameters are, linear birefringence (LB), linear dichroism (LD), circular birefringence (CB), circular dichroism (CD), linear depolarization (L-Dep), and circular depolarization (C-Dep). Such parameters are known to This method was successfully applied to analyze various samples, such as human blood plasma, collagen, calfskin, and tissues from mice with squamous cell carcinoma alongside normal tissues.
For distinguishing the skin tissues, it has been shown that birefringence properties of the cancerous samples are much lower than those of the healthy samples. The dichroism properties (D and R) of the cancerous samples are also lower than those of the healthy samples. Another important aspect is the depolarization, and it is debatable about its significance, in the transmission set-up configuration. As such, the low changes in the depolarization values are attributed to the thickness of the sample. The weaker depolarization caused by the increasing value of size parameter of scatterer, forward scattering becomes more predominant (value of anisotropy parameter increases), resulting in weaker randomization of photon’s propagation direction in transmission mode. Compared to the back scattering configuration, where there we have strong scattering effects taking place.
The feel for an intituitive estimation of ablove said parameters is not straightforward. But understanding the human skin tissue structure definitely helps. The variations in refractive index, size, shape and density of the scattering centers as encountered by the probing light beam play critical role in defining the landscape of the scattering, and hence the depolarization. The noticeable increase in depolarization observed in normal skin tissue samples compared to pathological ones suggests alterations in tissue microarchitecture. Specifically, the intricate and well-defined fibrous structure characteristic of normal skin tissue is likely disrupted by various pathologies, such as cancers. This disruption results in a loss of inherent tissue morphology heterogeneity, leading to better preservation of light polarization and consequently reduced depolarization.
The depolarization of cancerous cells is higher compared to the healthy cells, is consistent for both linear and circular depolarization and these plots look almost like the total depolarization. The reflectance spectroscopy with polarized light can provide quantitative morphological information which could potentially be used for non-invasive detection of changes. The changes in light scatter were measured by combining both wavelength-dependent and polarization-dependent methods. The difference in scattering is attributed to the average dimension of the “scatterers”, being a few tens of nanometers smaller in the healthy cells compared with the cancerous cells. This work also highlighted the significance of developing noninvasive, optical tissue diagnostic methods based on the sensitivity of wavelength-dependent and polarization-dependent light scattering measurements to cell morphology. In short, depolarization, diattenuation and crosstalk, in conjunction with the Mueller matrix elements, are sure way to track the malignancies.