Fibre orientation contrast for depth-resolved identification of structural interfaces in birefringent tissue.

Incorporation of polarimetric sensitivity into optical coherence tomography can provide additional image contrast when structures of interest are optically anisotropic (e.g., fibrous tissue). We present a generalized technique based on polarization-sensitive optical coherence tomography to detect changes in depth-resolved fibre orientation and thus increase image contrast in multiple-layered birefringent tissues. A high contrast B-scan image of collagen fibre orientation is shown for a porcine intervertebral disc cartilage specimen that exhibited low backscattering intensity contrast. Interfaces in the annulus fibrosus identified using depth-resolved fibre orientation allowed quantification of lamellae thickness. Moreover, the technique detects changes in fibre orientation without intense processing needed to effectively quantify tissue retardation and diattenuation.

Detection of vulnerable plaque in a murine model of atherosclerosis with optical coherence tomography.

OBJECTIVES: The aim of this study was to evaluate the feasibility of optical coherence tomography (OCT) to identify the components of vulnerable plaques in a well-established murine model of human atherosclerosis. BACKGROUND: Although the pathologic features that predict plaque rupture at autopsy are well known, the development of a technology to identify these high risk features in vivo is lacking. OCT uses reflected light to provide histology-like images of plaque with higher resolution than competing imaging modalities. Whether OCT can reliably identify the features of an atherosclerotic plaque that define it as vulnerable-thin fibrous cap, large lipid core, and high percent of lipid in the artery-requires further study. METHODS: OCT images of the atherosclerotic innominate artery segments from the apolipoprotein E knockout (apoE(-/-)) mice were recorded and correlated with histology in both in vivo (n = 7) and well as in ex vivo experiments (n = 12). RESULTS: Excellent correlation between the OCT and histology measurements for fibrous cap thickness, lipid core size, and percentage lipid content was found. The fibrous cap thicknesses examined span those of human fibrous caps known to rupture (< 65 microm). Regions of greatest light reflection in OCT images were observed when calcium hydroxy-apatite was scattered in lipid, less in fibrous tissue, and least in lipid. CONCLUSIONS: These findings suggest that OCT holds promise for the identification of features defining vulnerable plaque including fibrous cap thickness, lipid core size, and the percentage of lipid content.

Differential geometry of normalized Stokes vector trajectories in anisotropic media.

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.

Birefringence of the primate retinal nerve fiber layer.

The purpose of this study was to measure the peripapillary retinal nerve fiber layer (RNFL) thickness, phase retardation (PR), and depth-resolved birefringence (Deltan) of the normal primate eye using Enhanced Polarization-Sensitivity Optical Coherence Tomography (EPS-OCT). Both eyes of two rhesus monkeys were imaged with EPS-OCT. A multiple incident polarization state nonlinear fitting algorithm was used to determine RNFL phase retardation. RNFL thickness (RNFLT) was determined from the corresponding EPS-OCT intensity image and phase retardation per unit depth (PR/UD, proportional to Deltan) was calculated by dividing PR by RNFLT. Peripapillary area maps consisting of pixels uniformly distributed along a radius from 0.8 to 1.8 mm from the center of the optic nervehead were constructed for RNFLT, PR, and PR/UD. Average PR/UD in the superior and inferior quadrants was 18 degrees /100 mivrom (Deltan=4.2 x 10(-4)) and average PR/UD in the nasal and temporal quadrants was 6.3 degrees /100 microm (Deltan=1.5 x 10(-4)). Relative magnitude of PR radial gradient is similar to that of RNFLT radial gradient and no radial gradient was observed for PR/UD. Polarization-dependent amplitude attenuation per unit depth (PDAA/UD) was 0.02 rad/100 microm in thick RNFL regions. RNFL birefringence was higher in the arcuate bundles compared to nasal and temporal fibers (P=0.001). Birefringence was nearly equal in nasal and temporal quadrants. No statistically significant (P=0.01) radial gradient of birefringence was observed in any quadrant. RNFL birefringence is believed to originate from anisotropic structures within the cytoskeleton of the parallel axons. Birefringence differences presented in this study cannot be explained by the known axon diameter distribution around the optic nervehead and suggest other sources of the birefringence signal including neurotubules and neurofilaments.

High-sensitivity determination of birefringence in turbid media with enhanced polarization-sensitive optical coherence tomography.

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.