Form-biattenuance ( chi) in biological tissue arises from anisotropic light scattering by regularly oriented cylindrical fibers and results in a differential attenuation (diattenuation) of light amplitudes polarized parallel and perpendicular to the fiber axis (eigenpolarizations). Form-biattenuance is complimentary to form-birefringence (n) which results in a differential delay (phase retardation) between eigenpolarizations. We justify the terminology and motivate the theoretical basis for form-biattenuance in depth-resolved polarimetry. A technique to noninvasively and accurately quantify form-biattenuance which employs a polarization-sensitive optical coherence tomography (PS-OCT) instrument in combination with an enhanced sensitivity algorithm is demonstrated on ex vivo rat tail tendon (mean chi = 5.3.10-4, N = 111), rat Achilles tendon ( chi = 1.3.10-4, N = 45), chicken drumstick tendon ( chi = 2.1.10-4, N = 57), and in vivo primate retinal nerve fiber layer ( chi = 0.18.10-4, N = 6). A physical model is formulated to calculate the contributions of chi and n to polarimetric transformations in anisotropic media.

Polarization-sensitive optical coherence tomography provides high-resolution cross-sectional characterization of birefringence in turbid media. Weakly birefringent biological tissues such as the retinal nerve fiber layer (RNFL) require advanced speckle noise reduction for high-sensitivity measurement of form birefringence. We present a novel method for high-sensitivity birefringence quantification by using enhanced polarization-sensitive optical coherence tomography (EPS-OCT) and introduce the polarimetric signal-to-noise ratio, a mathematical tool for analyzing speckle noise in polarimetry. Multiple incident polarization states and non-linear fitting of normalized Stokes vectors allow determination of retardation with +/-1 degrees uncertainty with invariance to unknown unitary polarization transformations. Results from a weakly birefringent turbid film and in vivo primate RNFL are presented. In addition, we discuss the potential of EPS-OCT for noninvasive quantification of intracellular filamentous nanostructures, such as neurotubules in the RNFL that are lost during the progression of glaucoma.

Trajectory of the normalized Stokes vector on the Poincaré sphere corresponding to light propagation in anisotropic tissues with birefringence and biattenuance is derived. Analytic expressions are determined from the Serret-Frenet formulas and derivatives of arc length for five quantities including the tangent, normal, and binormal vectors with curvature and torsion. Depth variation of curvature and torsion of normalized Stokes vector trajectories corresponding to light propagating in rodent tail tendon are given. Use of analytic expressions for depth variation of curvature and torsion of the normalized Stokes vector trajectories on the Poincaré sphere is discussed for analysis of polarization-sensitive optical coherence tomography data recorded from anisotropic biological tissues with birefringence and biattenuance.

Enhanced polarization-sensitive optical coherence tomography (EPS-OCT) is a noninvasive cross-sectional imaging technique capable of quantifying with high sensitivity the optically anisotropic properties of fibrous tissues. We present a method to measure the depth-resolved optic axis orientations in superficial and deep regions of multiple-layered form-birefringent tissue. Additionally, the bulk-optic EPS-OCT instrument provides anatomical fiber direction referenced absolutely to the laboratory frame, in contrast with fiber-based PS-OCT instruments which provide relative optic axis orientation measurements. Results presented on ex vivo murine tail tendon and porcine annulus fibrosis indicate that the method iscapable of characterizing depth-resolved fiber direction [ theta(z)], form-birefringence [Deltan(z)], and form-biattenuance [Delta chi(z)] for at least 10 successive lamellae and a depth of 0.52 mm into the intervertebral disc. Noninvasive assessment of optic axis orientation by EPS-OCT provides increased contrast in images of multiple-layered media and may improve the understanding of fibrous tissue ultrastructure and the diseases or traumas that affect fibrous tissues.

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