Imaging skin pathology with polarized light

Abstract

Linearly polarized light that illuminates skin is backscattered by superficial layers and rapidly depolarized by birefringent collagen fibers. It is possible to distinguish such superficially backscattered light from the total diffusely reflected light that is dominated by light penetrating deeply into the dermis. The method involves acquisition of two images through an analyzing linear polarizer in front of the camera, one image (I(par)) acquired with the analyzer oriented parallel to the polarization of illumination and one image (I(per)) acquired with the analyzer oriented perpendicular to the illumination. An image based on the polarization ratio, Pol=(I(par)-I(per))/(I(par)+I(per)), is created. This paper compares normal light images, represented by I(per), and Pol images of various skin pathologies in a pilot clinical study using incoherent visible-spectrum light. Images include pigmented skin sites (freckle, tattoo, pigmented nevi) and unpigmented skin sites [nonpigmented intradermal nevus, neurofibroma, actinic keratosis, malignant basal cell carcinoma, squamous cell carcinoma, vascular abnormality (venous lake), burn scar]. Images of a shadow cast from a razor blade onto a forearm skin site illustrate the behavior of Pol values near the shadow edge. Near the shadow edge, Pol approximately doubles in value because no I(per) photons are superficially scattered into the shadow-edge pixels by the shadow region while I(par) photons are directly backscattered from the superficial layer of these pixels. This result suggests that the point spread function in skin for cross-talk between Pol pixels has a half-width-half-max of about 390 microm.

Turbid medium polarimetry in biomedical imaging and diagnosis

“…For fluorescence spectra recorded at larger Δθ, the deeper connective tissue layer (stroma) is probed. The corresponding spectra show strong signature of collagen and elastin (characteristic peak at ∼400 nm [50,51]) present in that layer. Note that polarized fluorescence spectra (recorded at varying Δθ) shown in the figure were normalized by the corresponding elastic scattering spectra recorded under the same condition.…”

“…Based on the gained insight into depolarization properties of multiply scattered light in tissue simulating phantoms and in actual tissues, several research groups have explored polarization gating for imaging of tissue and its underlying structures [49][50][51][52]. Various polarization gating schemes and imaging strategies have been investigated for this purpose.…”

“…Since the DOP image is formed by the ratio of the numerator that is primarily comprised of superficial subsurface reflectance and the denominator that is representative of total subsurface reflectance, it is insensitive to any spatial variation of illuminating light and variations in surface pigmentations (melanin pigmentations such as freckles or age marks, etc.). Based on this principle Jacques et al also developed a polarized light camera system, which has undergone clinical trial for imaging various types of skin pathologies [50].…”

The Polar Decomposition And Vector Parametrization Of The Mueller Matrices

“…One of the more popular is the method based on the Stokes vector parameters o f r a d i a t i o n a n d t h e M u e l l e r m a t r i x e s o f t h e investigated objects [5,6]. This method becomes very important recently due to the wide application of Stokes and Mueller polarimetry in medicine [7,8], biology [9], nondestructive testing [10,11] and remote sensing of materials and objects [12,13].…”

Comparative study of the best achievable contrast in scalar, Stokes and Mueller images

“…It can reveal contrasts that do not appear in classical intensity images or provide information about the nature of the objects present in the scene (surface state, orientation). It has many applications in remote sensing [1], imaging through turbid media [2], biomedical imaging [3], industrial control [4].…”