Skin, as many other tissues is a polarization anisotropic (i. e., birefringent) medium [1]. Birefringence is the phenomenon exhibited by certain materials in which an incident ray of light is split into two rays, called an ordinary ray and an extraordinary ray, which are plane – (linear) polarized in mutually orthogonal planes, or circular-polarized in opposite directions (left and right). Skin linear birefringence results primarily from the linear anisotropy of a dermis fibrous structure. The refractive index of dermis is higher along the length of fibers than across them. A specific dermis structure can be modeled as a system composed of parallel cylinders that create a uniaxial birefringent medium with the optic axis parallel to the cylinder axes. This is called birefringence of form that arises when the relative optical phase between the orthogonal polarization components is nonzero for forward-scattered light. After multiple forward scattering events, a relative phase difference accumulates and a phase delay (doe) similar to that observed in birefringent crystalline materials is introduced between orthogonal polarization components. For organized linear structures, an increase in phase delay may be characterized by a difference ((Anoe) in the effective refractive index for light polarized along, and perpendicular to, the long axis of the linear structures. The effect of tissue birefringence on the propagation of linearly polarized light is dependent on the angle between the incident polarization orientation and the tissue axis. Phase retardation doe between orthogonal polarization components, is proportional to the distance d traveled through the birefringent medium
A medium of parallel cylinders is a positive uniaxial birefringent medium [Anoe = (ne – no) > 0]. Therefore, a case defined by an incident optical field directed parallel to the cylinder axes will be called “extraordinary,” and a case with the incident optical field perpendicular to the cylinder axes will be called “ordinary”. The difference (ne – no) between the extraordinary index and the ordinary index is a measure of the birefringence of a medium comprised of cylinders. For the Rayleigh limit (l >> cylinder diameter), the form birefringence becomes [1].
where f is the volume fraction of the cylinders; f2 is the volume fraction of the ground substance; and n1, n2 are the corresponding indices. For a given difference of indices n1 and n2 , maximal birefringence is expected for approximately equal volume fractions of thin cylinders and ground material.
Form birefringence is used as an instrument for studying tissue composition. If n1 and n2 are known, the measured phase shift doe and evaluation of the corresponding birefringence Anoe allows one to assess the volume fraction occupied by the particles [see Eqs. (3.12) and (3.13)]. The reported value of the human skin birefringence Anoe is of the order of 10-3 [1].
A new technique—polarization-sensitive optical coherence tomography (PS OCT)— allows for a high precision measurement of linear birefringence in a turbid tissue [1]. For example, the porcine skin birefringence measured by PS OCT is of Anoe = 1.5 x 10-3 – 3.5 x 10-3. Such birefringence provides up to 90% phase retardation at a depth on the order of several hundred micrometers.
The magnitude of birefringence is related to the density and refractive properties of the collagen fibers, whereas the orientation of the fast axis indicates the orientation of the collagen fibers. The amplitude and orientation of birefringence of the skin and cartilage are not as uniformly distributed as in a tendon. In other words, the densities of collagen fibers in skin and cartilage are not as uniform as in a tendon, and the orientation of the collagen fibers is not distributed in as orderly a fashion.
It was experimentally demonstrated that in a turbid tissue, laser radiation retains linear polarization on the level of PL < 0.1 within a few TMFP lt, that is, 2.5lt. Specifically, for skin irradiated in the red and NIR ranges, ц = 0.4 cm-1, ц’ = 20 cm-1, and correspondingly lt = 0.48 mm. Consequently, light propagating in skin can retain linear polarization within a length of about 1.2 mm. Such an optical path in a tissue corresponds to a time delay on the order of 5.3 ps, which provides an opportunity to produce polarization images of macro-inhomogeneities in a tissue with a spatial resolution equivalent to the spatial resolution that can be achieved by the selection of photons using more sophisticated time – resolved techniques. In addition to the selection of diffuse-scattered photons, polarization imaging makes it possible to eliminate the specular reflection from the surface of a tissue, which allows one to use this technique to image microvessels in facile skin and detect birefringence and optical activity in superficial tissue layers [1,31-34].
Polarization imaging is a new modality in tissue optics [1,31-34]. The most prospective approaches for polarization tissue imaging are: linear polarization degree mapping, twodimensional backscattering, PS OCT, and full-field polarization-speckle technique. The most robust and cheap is a linear polarization degree (PL) mapping technique which is based on registration of two-dimensional polarization patterns for the backscattering of a polarized incident narrow laser beam [33]. As an illustration in Fig. 3.10 , a scheme of experimental setup for polarization imaging and three different types of images of skin burn lesion are shown. Two images within the imaging area (x, y) are acquired: one “parallel” [іц(х, y)] and one “perpendicular” [/±(x, y)]. These images are algebraically combined to yield: PL(x, y) = (/| – /±)/(/| + /±). The numerator rejects randomly polarized diffuse
reflectance. Normalization by the denominator cancels common attenuation due to melanin pigmentation. The copolarized surface image is characteristic by a clearly seen superficial skin papillary pattern, as well as cross-polarized image gives more information about the status of subsurface skin vessels.
A similar camera system, but one that uses an incoherent white light source such as xenon lamp, is described in ref. 33, where results of a pilot clinical study of various skin pathologies using polarized light are presented. The polarization images of pigmented skin sites (freckles, tattoos, pigmented nevi) and unpigmented skin sites (nonpigmented intradermal nevi, neurofibromas, actinic keratosis, malignant basal cell carcinomas, squamous
cell carcinomas, vascular abnormalities (venous lakes), and burn scars) are analyzed to find the differences caused by various skin pathologies (see some examples in Fig. 3.11) [33]. A comparative analysis of polarization images of normal and diseased human skin has shown the ability of the aforementioned approach to emphasize image contrast based on light scattering in the superficial layers of the skin. The polarization images can visualize disruption of the normal texture of the papillary and upper reticular layers caused by skin pathology. Polarization imaging can be considered as an adequately effective tool for identifying skin cancer margins, and for guiding surgical excision of skin cancer. Various modalities of polarization imaging are also considered in ref. [35].