Enhanced polarization-sensitive optical coherence tomography (EPS-OCT) for characterization of tissue anisotropy
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An Enhanced Polarization-Sensitive Optical Coherence Tomography (EPS-OCT) instrument for high sensitivity cross-sectional imaging of optical anisotropy in turbid media has been designed, constructed, and verified. Enhanced sensitivity to small transformations in light polarization state is provided by a novel multi-state nonlinear fitting algorithm capable of detecting phase retardation (δ) with ±1° uncertainty. Introduction of a new polarimetric term, biattenuance, describing differential attenuation of light amplitudes is theoretically and experimentally motivated. Biattenuance (∆χ) is complimentary to birefringence (∆n), which results in a differential delay (phase retardation, δ) between modes polarized parallel and perpendicular to the anisotropic axis orientation (θ). In addition, a physical model is formulated to calculate the relative contributions of ∆χ and ∆n to polarimetric transformations in anisotropic media. Optical anisotropy properties birefringence (∆n), biattenuance (∆χ), and axis orientation (θ) convey information about the sub-microscopic structure of fibrous tissue (e.g., connective, muscle, and nervous tissue). A method for nondestructively characterizing these properties in multiple-layered fibrous tissue using EPS-OCT is demonstrated in ex vivo specimens of porcine intervertebral disc cartilage. Diseases or traumas often alter tissue on the ultrastructural level; thus, noninvasive polarimetric imaging using EPS-OCT is expected to be a valuable tool for in vivo medical research as well as for diagnosis and management of disease. For example, glaucoma affects the vitality of retinal ganglion cell axons in the retinal nerve fiber layer (RNFL) and may be clinically detected through a change in RNFL birefringence. Comprehensive peripapillary maps of healthy primate RNFL birefringence were constructed using EPS-OCT. A preliminary model relating RNFL birefringence to the area-density of intracellular neurotubules suggests that superior and inferior quadrants have a higher neurotubule density than that in nasal and temporal quadrants.