Polarized light propagation in turbid media

Polarimetry, which is a comparison of the polarization state of light before and after it has interacted with a material, can be used to discriminate unscattered and weakly scattered photons from multiply scattered photons. Weakly scattered photons tend to retain their incident polarization state whereas highly scattered photons become depolarized; thus, polarization-based discrimination techniques can be used to image through tissue with decreased noise and increased contrast. Many previous studies investigating polarization-based discrimination have been conducted on tissue phantoms, with the ultimate goal being noninvasive imaging of breast tumors. We demonstrate here that linearly and circularly polarized light propagate differently in common tissue phantoms than in two different tissue types. Measurements of the degree of linear and circular polarization were conducted using two independent techniques on tissue phantoms consisting of polystyrene and Intralipid microsphere suspensions, and on porcine adipose tissue and porcine myocardium. We show that contrary to expectations made from studies in the phantoms, linearly polarized light survives through more scattering events than circularly polarized light in both adipose tissue, which contains quasi-spherical scatterers, and myocardium, which contains quasi-spherical and cylindrical scatterers. Differences between spherical and biological scatterers are discussed, along with the impact of tissue birefringence on degree of polarization measurements.