Spatial helicity response metric to quantify particle size and turbidity of heterogeneous media through circular polarization imaging

Abstract

Backscattered circularly polarized light from turbid media consists of helicity-flipped and helicity-preserved photon sub-populations (i.e., photons of perpendicular and parallel circular handedness). Their intensities and spatial distributions are found to be acutely sensitive to average scatterer size and modestly sensitive to the scattering coefficient (medium turbidity) through an interplay of single and multiple scattering effects. Using a highly sensitive intensified-CCD camera, helicity-based images of backscattered light are captured, which, with the aid of corroborating Monte Carlo simulation images and statistics, enable (1) investigation of subsurface photonic pathways and (2) development of the novel ‘spatial helicity response’ metric to quantify average scatterer size and turbidity of tissue-like samples. An exciting potential application of this work is noninvasive early cancer detection since malignant tissues exhibit alterations in scatterer size (larger nuclei) and turbidity (increased cell density).

Spatial analysis of polarimetric images to enhance near-surface sampling sensitivity: feasibility in demineralized teeth and other tissue-like media

“…It is observed that orthogonalization occurs in the backward direction between 135 deg and 225 deg (exact backscattering ±45 deg cone). This angular dependency suggests that linearly orthogonalized photons will exhibit shallower pathways in a polystyrene microsphere suspension (on average), as already shown previously for circularly polarized light 23 . The orthogonalized and preserved (arc-like) pathways are schematized in Fig.…”

“…The simulation parameters were configured to match the phantom experiment: ∼108 polarized photon packets (λ=635 nm) were launched into a 2.2×2.2×2.2 cm medium of monodispersed 1.04 μm spheres, each with a refractive index of 1.59, suspended in a host refractive index of 1.33. Remaining details and description of the experimentally validated and publicly available Monte Carlo simulation platform 23 , 43 – 47 can be found in Ref. 48 and Sec.…”

“…To do so, a polystyrene microsphere suspension is employed to greatly simplify the scattering dynamics through its narrow size distribution and discrete bi-phase refractive index profile. This enables convenient simulations of polarized light propagation, for example, as done in our recent studies 23 , 47 …”

“…This enables convenient simulations of polarized light propagation, for example, as done in our recent studies. 23,47 The angular distribution of singly scattered polarized light can be calculated by multiplying the Mueller matrix of a sphere with a given Stokes vector: E Q -T A R G E T ; t e m p : i n t r a l i n k -; e 0 0 1 ; 1 1 4 ; 4 1 3…”

“…23. Indeed, this approach led to an ∼20% reduction in average pathlength 23 . The latter is important since helicity-flipped photons that exist in the radially distant regions arise through randomization/depolarization, which can be deeply penetrating.…”