Retinal nerve fiber bundle tracing and analysis in human eye by polarization sensitive OCT

Direct measurements of collagen fiber recruitment in the posterior pole of the eye

Collagen is the main load-bearing component of the peripapillary sclera (PPS) and lamina cribrosa (LC) in the eye. Whilst it has been shown that uncrimping and recruitment of the PPS and LC collagen fibers underlies the macro-scale nonlinear stiffening of both tissues with increased intraocular pressure (IOP), the uncrimping and recruitment as a function of local stretch have not been directly measured. This knowledge is crucial to understanding their functions in bearing loads and maintaining tissue integrity. In this project we measured local stretch-induced collagen fiber bundle uncrimping and recruitment curves of the PPS and LC. Thin coronal samples of PPS and LC of sheep eyes were mounted and stretched biaxially quasi-statically using a custom system. At each step, we imaged the PPS and LC with instant polarized light microscopy and quantified pixel-level (1.5 μm/pixel) collagen fiber orientations. We used digital image correlation to measure the local stretch and quantified collagen crimp by the circular standard deviation of fiber orientations, or waviness. Local stretch-recruitment curves of PPS and LC approximated sigmoid functions. PPS recruited more fibers than the LC at the low levels of stretch. At 10% stretch the curves crossed with 75% bundles recruited. The PPS had higher uncrimping rate and waviness remaining after recruitment than the LC: 0.9º vs. 0.6º and 3.1º vs. 2.7º. Altogether our findings support describing fiber recruitment of both PPS and LC with sigmoid curves, with the PPS recruiting faster and at lower stretch than the LC, consistent with a stiffer tissue. STATEMENT OF SIGNIFICANCE: Peripapillary sclera (PPS) and lamina cribrosa (LC) collagen recruitment behaviors are central to the nonlinear mechanical behavior of the posterior pole of the eye. How PPS and LC collagen fibers recruit under stretch is crucial to develop constitutive models of the tissues but remains unclear. We used image-based stretch testing to characterize PPS and LC collagen fiber bundle recruitment under local stretch. We found that fiber-level stretch-recruitment curves of PPS and LC approximated sigmoid functions. PPS recruited more fibers at a low stretch, but at 10% bundle stretch the two curves crossed with 75% bundles recruited. We also found that PPS and LC fibers had different uncrimping rates and non-zero waviness's when recruited.

Characteristics of Henle's fiber layer in healthy and glaucoma eyes assessed by polarization-sensitive optical coherence tomography

Using conventional optical coherence tomography (OCT), it is difficult to image Henle fibers (HF) due to their low backscattering potential. However, fibrous structures exhibit form birefringence, which can be exploited to visualize the presence of HF by polarization-sensitive (PS) OCT. We found a slight asymmetry in the retardation pattern of HF in the fovea region that can be associated with the asymmetric decrease of cone density with eccentricity from the fovea. We introduce a new measure based on a PS-OCT assessment of optic axis orientation to estimate the presence of HF at various eccentricities from the fovea in a large cohort of 150 healthy subjects. By comparing a healthy age-matched sub-group (N = 87) to a cohort of 64 early-stage glaucoma patients, we found no significant difference in HF extension but a slightly decreased retardation at about 2° to 7.5° eccentricity from the fovea in the glaucoma patients. This potentially indicates that glaucoma affects this neuronal tissue at an early state.

Direct measurements of collagen fiber recruitment in the posterior pole of the eye

Collagen is the main load-bearing component of the peripapillary sclera (PPS) and lamina cribrosa (LC) in the eye. Whilst it has been shown that uncrimping and recruitment of the PPS and LC collagen fibers underlies the macro-scale nonlinear stiffening of both tissues with increased intraocular pressure (IOP), the uncrimping and recruitment as a function of local stretch have not been directly measured. This knowledge is crucial for the development of constitutive models associating micro and macro scales. In this project we measured local stretch-induced collagen fiber bundle uncrimping and recruitment curves of the PPS and LC. Thin coronal samples of PPS and LC of sheep eyes were mounted and stretched biaxially quasi-statically using a custom system. At each step, we imaged the PPS and LC with instant polarized light microscopy and quantified pixel-level (1.5 μm/pixel) collagen fiber orientations. We used digital image correlation to measure the local stretch and quantified collagen crimp by the circular standard deviation of fiber orientations, or waviness. Local stretch-recruitment curves of PPS and LC approximated sigmoid functions. PPS recruited more fibers than the LC at the low levels of stretch. At 10% stretch the curves crossed with 75% bundles recruited. The PPS had higher uncrimping rate and waviness remaining after recruitment than the LC: 0.9° vs. 0.6° and 3.1° vs. 2.7°. Altogether our findings support describing fiber recruitment of both PPS and LC with sigmoid curves, with the PPS recruiting faster and at lower stretch than the LC, consistent with a stiffer tissue.

Analysis and reduction of noise-induced depolarization in catheter based polarization sensitive optical coherence tomography

In catheter based polarization sensitive optical coherence tomography (PS-OCT), a optical fiber with a rapid rotation in the catheter can cause low signal-to-noise ratio (SNR), polarization state instability, phase change of PS-OCT signals and then heavy noise-induced depolarization, which has a strong impact on the phase retardation measurement of the sample. In this paper, we analyze the noise-induced depolarization and find that the effect of depolarization can be reduced by polar decomposition after incoherent averaging in the Mueller matrix averaging (MMA) method. Namely, MMA can reduce impact of noise on phase retardation mapping. We present a Monte Carlo method based on PS-OCT to numerically describe noise-induced depolarization effect and contrast phase retardation imaging results by MMA and Jones matrix averaging (JMA) methods. The peak signal to noise ratio (PSNR) of simulated images processed by MMA is higher than about 8.9 dB than that processed by JMA. We also implement experiments of multiple biological tissues using the catheter based PS-OCT system. From the simulation and experimental results, we find the polarization contrasts processed by the MMA are better than those by JMA, especially at areas with high depolarization, because the MMA can reduce effect of noise-induced depolarization on the phase retardation measurement.

Molecular Contrast Optical Coherence Tomography and Its Applications in Medicine

The growing need to understand the molecular mechanisms of diseases has prompted the revolution in molecular imaging techniques along with nanomedicine development. Conventional optical coherence tomography (OCT) is a low-cost in vivo imaging modality that provides unique high spatial and temporal resolution anatomic images but little molecular information. However, given the widespread adoption of OCT in research and clinical practice, its robust molecular imaging extensions are strongly desired to combine with anatomical images. A range of relevant approaches has been reported already. In this article, we review the recent advances of molecular contrast OCT imaging techniques, the corresponding contrast agents, especially the nanoparticle-based ones, and their applications. We also summarize the properties, design criteria, merit, and demerit of those contrast agents. In the end, the prospects and challenges for further research and development in this field are outlined.

Automatic retinal nerve fiber bundle tracing based on large field of view polarization sensitive OCT data

A technique to accurately estimate trajectories of retinal nerve fiber bundles (RNFB) in a large field of view (FOV) image covering 45° is described. The method utilizes stitched projections of polarization-sensitive optical coherence tomography (PS-OCT) data, as well as a mathematical model of average RNFB trajectories as prior. The fully automatic process was applied to data recorded in healthy subjects and glaucoma patients and automatically detected individual RNFB trajectories are compared to manual traces.

Polarisation optics for biomedical and clinical applications: a review

Many polarisation techniques have been harnessed for decades in biological and clinical research, each based upon measurement of the vectorial properties of light or the vectorial transformations imposed on light by objects. Various advanced vector measurement/sensing techniques, physical interpretation methods, and approaches to analyse biomedically relevant information have been developed and harnessed. In this review, we focus mainly on summarising methodologies and applications related to tissue polarimetry, with an emphasis on the adoption of the Stokes-Mueller formalism. Several recent breakthroughs, development trends, and potential multimodal uses in conjunction with other techniques are also presented. The primary goal of the review is to give the reader a general overview in the use of vectorial information that can be obtained by polarisation optics for applications in biomedical and clinical research.

Identification and quantification of fibrotic areas in the human retina using polarization-sensitive OCT

Subretinal fibrosis is one of the most prevalent causes of blindness in the elderly population, but a true gold standard to objectively diagnose fibrosis is still lacking. Since fibrotic tissue is birefringent, it can be detected by polarization-sensitive optical coherence tomography (PS-OCT). We present a new algorithm to automatically detect, segment, and quantify fibrotic lesions within 3D data sets recorded by PS-OCT. The algorithm first compensates for the birefringence of anterior ocular tissues and then uses the uniformity of the birefringent optic axis as an indicator to identify fibrotic tissue, which is then segmented and quantified. The algorithm was applied to 3D volumes recorded in 57 eyes of 57 patients with neovascular age-related macular degeneration using a spectral domain PS-OCT system. The results of fibrosis detection were compared to the clinical diagnosis based on color fundus photography (CFP), and the precision of fibrotic area measurement was assessed by three repeated measurements in a sub-set of 15 eyes. The average standard deviation of the fibrotic area obtained in eyes with a lesion area > 0.7 mm2 was 15%. Fibrosis detection by CFP and PS-OCT agreed in 48 cases, discrepancies were only observed in cases of lesion area < 0.7 mm2. These remaining discrepancies are discussed, and a new method to treat ambiguous cases is presented.

Early Identification of Retinal Neuropathy in Subclinical Diabetic Eyes by Reduced Birefringence of the Peripapillary Retinal Nerve Fiber Layer

Purpose

To study birefringence of the peripapillary retinal nerve fiber layer (RNFL) of diabetic eyes with no clinical signs of diabetic retinopathy (DR) or mild to moderate DR stages using spectral-domain polarization-sensitive (PS) optical coherence tomography (OCT).

Methods

In this observational pilot study, circular PS-OCT scans centered on the optic nerve head were recorded in prospectively recruited diabetic and age-matched healthy eyes. From averaged circumpapillary intensity and retardation tomograms plots of RNFL birefringence were obtained by a linear fit of retardation versus depth within the RNFL tissue for each A-scan position and mean birefringence values for RNFL calculated. Spectral-domain OCT imaging (Heidelberg Engineering) was performed to assess peripapillary RNFL thickness and macular ganglion cell complex (GCC).

Results

Out of 70 eyes of 43 diabetic patients (mean ± SD age: 50.86 ± 15.71) 36 showed no signs of DR, 17 mild and 17 moderate nonproliferative DR with no diabetic macular edema. Thirty-four eyes of 34 healthy subjects (53.21 ± 13.88 years) served as controls. Compared with healthy controls (0.143° ± 0.014°/µm) mean total birefringence of peripapillary RNFL was significantly reduced in subclinical diabetic eyes (0.131° ± 0.014°/µm; P = 0.0033), as well as in mild to moderate DR stages (0.125° ± 0.018°/µm, P < 0.0001) with borderline statistically significant differences between diabetic patients (P = 0.0049). Mean birefringence values were significantly lower in inferior compared with superior RNFL sectors (P < 0.0001) of diabetic eyes with no such difference detected in the healthy control group.

Conclusions

We identified evidence of early neuroretinal alteration in diabetic eyes through reduced peripapillary RNFL birefringence assessed by PS-OCT occurring before appearance of clinical microvascular lesions or GCC alterations.

Generating large field of view en-face projection images from intra-acquisition motion compensated volumetric optical coherence tomography data

A technique to generate large field of view projection maps of arbitrary optical coherence tomography (OCT) data is described. The technique is divided into two stages - an image acquisition stage that features a simple to use fast and robust retinal tracker to get motion free retinal OCT volume scans - and a stitching stage where OCT data from different retinal locations is first registered against a reference image using a custom pyramid-based approach and finally stitched together into one seamless large field of view (FOV) image. The method is applied to data recorded with a polarization sensitive OCT instrument in healthy subjects and glaucoma patients. The tracking and stitching accuracies are quantified, and finally, large FOV images of retinal nerve fiber layer retardation that contain the arcuate nerve fiber bundles from the optic nerve head to the raphe are demonstrated.

Anatomical Location of the Raphe and Extended Raphe in the Human Retina: Implications for Assessment of the Optic Nerve with OCT

Purpose

To determine the location of (1) the superior–inferior watershed between the fovea and optic disc (extended raphe) at the peripapillary optical coherence tomography (OCT) measurement circle and (2) the raphe, temporal to the fovea.

Methods

We used existing data consisting of 2285 traced retinal nerve fiber bundle trajectories from 83 fundus images. For localization of the extended raphe at the 3.46-mm-diameter OCT measurement circle, trajectories were classified as belonging to the superior or inferior hemiretina, using predefined criteria. For the raphe, we localized the endings of trajectories coming from the superior and inferior arcuate bundles.

Results

At the measurement circle, the extended raphe is located 14° (range, 12°–16°) inferiorly to a horizontal line through the optic disc center. The raphe follows a horizontal line at the latitude of the fovea if the disc is assumed to be located 15° nasal to and 2° above the fovea.

Conclusions

At the measurement circle, OCT brands use either the 9 o'clock location or a straight line connecting the center of the optic disc and the fovea as a reference for separating the hemiretinas. This results, on average, in a 14° and 6° misalignment with respect to the anatomical watershed, respectively. For the macular area, the commonly used line through the center of the optic disc and the fovea fails to describe the raphe adequately.

Translational Relevance

An unbiased asymmetry assessment of the optic nerve requires a detailed knowledge of the shape and location of the (extended) raphe.

In Vivo 3D Determination of Peripapillary Scleral and Retinal Layer Architecture Using Polarization-Sensitive Optical Coherence Tomography

Purpose

The purpose of this paper was to determine the architecture of the collagen fibers of the peripapillary sclera, the retinal nerve fiber layer (RNFL), and Henle's fiber layer in vivo in 3D using polarization-sensitive optical coherence tomography (PS-OCT).

Methods

Seven healthy volunteers were imaged with our in-house built PS-OCT system. PS-OCT imaging included intensity, local phase retardation, relative optic axis, and optic axis uniformity (OAxU). Differential Mueller matrix calculus was used for the first time in ocular tissues to visualize local orientations that varied with depth, incorporating a correction method for the fiber orientation in preceding layers.

Results

Scleral collagen fiber orientation images clearly showed an inner layer with an orientation parallel to the RNFL orientation, and a deeper layer where the collagen was circularly oriented. RNFL orientation images visualized the nerve fibers leaving the optic nerve head (ONH) in a radial pattern. The phase retardation and orientation of Henle's fiber layer were visualized locally for the first time.

Conclusions

PS-OCT successfully showed the orientation of the retinal nerve fibers, sclera, and Henle's fiber layer, and is to the extent of our knowledge the only technique able to do so in 3D in vivo.

Translational Relevance

In vivo 3D imaging of scleral collagen architecture and the retinal neural fibrous structures can improve our understanding of retinal biomechanics and structural alterations in different disease stages of myopia and glaucoma.

Visible-Light Optical Coherence Tomography Fibergraphy for Quantitative Imaging of Retinal Ganglion Cell Axon Bundles

Purpose

To develop a practical technique for visualizing and quantifying retinal ganglion cell (RGC) axon bundles in vivo.

Methods

We applied visible-light optical coherence tomography (vis-OCT) to image the RGC axon bundles, referred to as vis-OCT fibergraphy, of healthy wild-type C57BL/6 mice. After vis-OCT imaging, retinas were flat-mounted, immunostained with anti-beta-III tubulin (Tuj1) antibody for RGC axons, and imaged with confocal microscopy. We quantitatively compared the RGC axon bundle networks imaged by in vivo vis-OCT and ex vivo confocal microscopy using semi-log Sholl analysis.

Results

Side-by-side comparison of ex vivo confocal microscopy and in vivo vis-OCT confirmed that vis-OCT fibergraphy captures true RGC axon bundle networks. The semi-log Sholl regression coefficients extracted from vis-OCT fibergrams (3.7 ± 0.8 mm^–1) and confocal microscopy (3.6 ± 0.3 mm^–1) images also showed good agreement with each other (n = 6).

Conclusions

We demonstrated the feasibility of using vis-OCT fibergraphy to visualize RGC axon bundles. Further applying Sholl analysis has the potential to identify biomarkers for non-invasively assessing RGC health.

Translational Relevance

Our novel technique for visualizing and quantifying RGC axon bundles in vivo provides a potential measurement tool for diagnosing and tracking the progression of optic neuropathies.

Analysis of retinal nerve fiber layer birefringence in patients with glaucoma and diabetic retinopathy by polarization sensitive OCT

The retinal nerve fiber layer (RNFL) is a fibrous tissue that shows form birefringence. This optical tissue property is related to the microstructure of the nerve fiber axons that carry electrical signals from the retina to the brain. Ocular diseases that are known to cause neurologic changes, like glaucoma or diabetic retinopathy (DR), might alter the birefringence of the RNFL, which could be used for diagnostic purposes. In this pilot study, we used a state-of-the-art polarization sensitive optical coherence tomography (PS-OCT) system with an integrated retinal tracker to analyze the RNFL birefringence in patients with glaucoma, DR, and in age-matched healthy controls. We recorded 3D PS-OCT raster scans of the optic nerve head area and high-quality averaged circumpapillary PS-OCT scans, from which RNFL thickness, retardation and birefringence were derived. The precision of birefringence measurements was 0.005°/µm. As compared to healthy controls, glaucoma patients showed a slightly reduced birefringence (0.129 vs. 0.135°/µm), although not statistically significant. The DR patients, however, showed a stronger reduction of RNFL birefringence (0.103 vs. 0.135°/µm) which was highly significant. This result might open new avenues into early diagnosis of DR and related neurologic changes.

Polarization-sensitive optical coherence tomography as a tool to visualize the fiber direction of retinal nerves and peripapillary sclera

Polarization sensitive optical coherence tomography (PS-OCT) has been used to visualize the orientation of the nerves in the retinal nerve fiber layer (RNFL) and to visualize the orientation of the collagen fibers in peripapillary sclera in retinas of healthy volunteers. Optic axis orientation images clearly visualize the nerve fibers leaving the optic nerve head (ONH) in all radial directions. Sclera orientation images show that the sclera consist of two layers, an inner layer with an orientation parallel to the RNFL orientation, and a deeper layer where the collagen is circularly oriented.

En face optical coherence tomography: a technology review [Invited]

A review on the technological development of en face optical coherence tomography (OCT) and optical coherence microscopy (OCM) is provided. The terminology originally referred to time domain OCT, where the preferential scanning was performed in the en face plane. Potentially the fastest realization of en face image recording is full-field OCT, where the full en face plane is illuminated and recorded simultaneously. The term has nowadays been adopted for high-speed Fourier domain approaches, where the en face image is reconstructed from full 3D volumes either by direct slicing or through axial projection in post processing. The success of modern en face OCT lies in its immediate and easy image interpretation, which is in particular of advantage for OCM or OCT angiography. Applications of en face OCT with a focus on ophthalmology are presented. The review concludes by outlining exciting technological prospects of en face OCT based both on time as well as on Fourier domain OCT.

Measuring polarization changes in the human outer retina with polarization-sensitive optical coherence tomography

Morphological changes in the outer retina such as drusen are established biomarkers to diagnose age-related macular degeneration. However, earlier diagnosis might be possible by taking advantage of more subtle changes that accompany tissues that bear polarization-altering properties. To test this hypothesis, we developed a method based on polarization-sensitive optical coherence tomography with which volumetric data sets of the macula were obtained from 10 young (<25 years) and 10 older (>54 years) subjects. All young subjects and 5 of the older subjects had retardance values induced by the retinal pigment epithelium and Bruch's membrane (RPE-BM) complex that were just above the noise floor measurement (5°-13° at 840 nm). In contrast, elevated retardance, up to 180°, was observed in the other 5 older subjects. Analysis of the degree of polarization uniformity (DOPU) demonstrates that reduced DOPU (<0.4) in the RPE is associated with elevated double pass phase retardation (DPPR) below the RPE-BM complex, suggesting that the observed elevated DPPR in older subjects is the result of increased scattering or polarization scrambling. Collectively, our measurements show that the outer retina can undergo dramatic change in its polarization properties with age, and in some cases still retain its clinically normal appearance.

Nerve Fiber Flux Analysis Using Wide-Field Swept-Source Optical Coherence Tomography

Purpose

To devise a method to quantify nerve fibers over their arcuate courses over an extended peripapillary area using optical coherence tomography (OCT).

Methods

Participants were imaged with 8 × 8-mm volumetric OCT scans centered at the optic disc. A new quantity, nerve fiber flux (NFF), represents the cross-sectional area transected perpendicular to the nerve fibers. The peripapillary area was divided into 64 tracks with equal flux. An iterative algorithm traced the trajectory of the tracks assuming that the relative distribution of the NFF was conserved with compensation for fiber connections to ganglion cells on the macular side. Average trajectory was averaged from normal eyes and use to calculate the NFF maps for glaucomatous eyes. The NFF maps were divided into eight sectors that correspond to visual field regions.

Results

There were 24 healthy and 10 glaucomatous eyes enrolled. The algorithm converged on similar patterns of NFL tracks for all healthy eyes. In glaucomatous eyes, NFF correlated with visual field sensitivity in the arcuate sectors (Spearman ρ = 0.53–0.62). Focal nerve fiber loss in glaucomatous eyes appeared as uniform tracks of NFF defects that followed the expected arcuate fiber trajectory.

Conclusions

Using an algorithm based on the conservation of flux, we derived nerve fiber trajectories in the peripapillary area. The NFF map is useful for the visualization of focal defects and quantification of sector nerve fiber loss from wide-area volumetric OCT scans.

Translational Relevance

NFF provides a cumulative measure of volumetric loss along nerve fiber tracks and could improve the detection of focal glaucoma damage.

Reflectance Spectrum and Birefringence of the Retinal Nerve Fiber Layer With Hypertensive Damage of Axonal Cytoskeleton

Purpose

Glaucoma damages the retinal nerve fiber layer (RNFL). This study used precise multimodal image registration to investigate the changes of the RNFL reflectance spectrum and birefringence in nerve fiber bundles with different degrees of axonal damage.

Methods

The reflectance spectrum of individual nerve fiber bundles in normal rats and rats with experimental glaucoma was measured from 400 to 830 nm and their birefringence was measured at 500 nm. Optical measurements of the same bundles were made at different distances from the optic nerve head (ONH). After the optical measurements, the axonal cytoskeleton of the RNFL was evaluated by confocal microscopy to assess the severity of cytoskeletal change.

Results

For normal bundles, the shape of the RNFL reflectance spectrum and the value of RNFL birefringence did not change along bundles. In treated retinas, damage to the cytoskeleton varied within and across retinas. The damage in retinal sectors was subjectively graded from normal-looking to severe. Change of spectral shape occurred near the ONH in all sectors studied. This change became more prominent and occurred farther from the ONH with increased damage severity. In contrast, RNFL birefringence did not show change in normal-looking sectors, but decreased in sectors with mild and moderate damage. The birefringence of severely damaged sectors was either within or below the normal range.

Conclusions

Varying degrees of cytoskeletal damage affect the RNFL reflectance spectrum and birefringence differently, supporting differences in the ultrastructural basis for the two optical properties. Both properties, however, may provide a means to detect disease and to estimate ultrastructural damage of the RNFL in glaucoma.

Conical scan pattern for enhanced visualization of the human cornea using polarization-sensitive OCT

Conventional imaging of the human cornea with optical coherence tomography (OCT) relies on telecentric scanning optics with sampling beams that are parallel to the optical axis of the eye. Because of the shape of the cornea, the beams have in some areas considerable inclination to the corneal surface which is accompanied by low signal intensities in these areas and thus an inhomogeneous appearance of corneal structures. In addition, alterations in the polarization state of the probing light depend on the angle between the imaging beam and the birefringent axis of the sample. Therefore, changes in the polarization state observed with polarization-sensitive (PS-) OCT originate mainly from the shape of the cornea. In order to minimize the effects of the corneal shape on intensity and polarization-sensitive based data, we developed a conical scanning optics design. This design provides imaging beams that are essentially orthogonal to the corneal surface. Thus, high signal intensity throughout the entire imaged volume is obtained and the influence of the corneal shape on polarization-sensitive data is greatly reduced. We demonstrate the benefit of the concept by comparing PS-OCT imaging results of the human cornea in healthy volunteers using both scanning schemes.

Visualization of neuritic plaques in Alzheimer's disease by polarization-sensitive optical coherence microscopy

One major hallmark of Alzheimer's disease (AD) and cerebral amyloid angiopathy (CAA) is the deposition of extracellular senile plaques and vessel wall deposits composed of amyloid-beta (Aβ). In AD, degeneration of neurons is preceded by the formation of Aβ plaques, which show different morphological forms. Most of them are birefringent owing to the parallel arrangement of amyloid fibrils. Here, we present polarization sensitive optical coherence microscopy (PS-OCM) for imaging mature neuritic Aβ plaques based on their birefringent properties. Formalin-fixed, post-mortem brain samples of advanced stage AD patients were investigated. In several cortical brain regions, neuritic Aβ plaques were successfully visualized in tomographic and three-dimensional (3D) images. Cortical grey matter appeared polarization preserving, whereas neuritic plaques caused increased phase retardation. Consistent with the results from PS-OCM imaging, the 3D structure of senile Aβ plaques was computationally modelled for different illumination settings and plaque sizes. Furthermore, the birefringent properties of cortical and meningeal vessel walls in CAA were investigated in selected samples. Significantly increased birefringence was found in smaller vessels. Overall, these results provide evidence that PS-OCM is able to assess amyloidosis based on intrinsic birefringent properties.

Cytoskeletal Alteration and Change of Retinal Nerve Fiber Layer Birefringence in Hypertensive Retina

PURPOSE

Glaucoma damages the retinal nerve fiber layer (RNFL). Both RNFL thickness and retardance can be used to assess the damage, but birefringence, the ratio of retardance to thickness, is a property of the tissue itself. This study investigated the relationship between axonal cytoskeleton and RNFL birefringence in retinas with hypertensive damage.

MATERIALS AND METHODS

High intraocular pressure (IOP) was induced unilaterally in rat eyes. RNFL retardance in isolated retinas was measured. Cytostructural organization and bundle thickness were evaluated by confocal imaging of immunohistochemical staining of the cytoskeletal components: microtubules (MTs), F-actin, and neurofilaments. Bundles with different appearances of MT stain were studied, and their birefringence was calculated at different radii from the optic nerve head (ONH) center.

RESULTS

Forty bundles in eight normal retinas and 37 bundles in 10 treated retinas were examined. In normal retinas, the stain of axonal cytoskeleton was approximately uniform within bundles, and RNFL birefringence did not change along bundles. In treated retinas, elevation of IOP caused non-uniform alteration of axonal cytoskeleton across the retina, and distortion of axonal MTs was associated with decreased birefringence. The study further demonstrated that change of RNFL birefringence profiles along bundles can imply altered axonal cytoskeleton, suggesting that ultrastructural change of the RNFL can be inferred from clinical measurements of RNFL birefringence. The study also demonstrated that measuring RNFL birefringence profiles along bundles, instead of at a single location, may provide a more sensitive way to detect axonal ultrastructural change.

CONCLUSIONS

Measurement of RNFL birefringence along bundles can provide estimation of cytoskeleton alteration and sensitive detection of glaucomatous damage.

Posterior rat eye during acute intraocular pressure elevation studied using polarization sensitive optical coherence tomography

Polarization sensitive optical coherence tomography (PS-OCT) operating at 840 nm with axial resolution of 3.8 µm in tissue was used for investigating the posterior rat eye during an acute intraocular pressure (IOP) increase experiment. IOP was elevated in the eyes of anesthetized Sprague Dawley rats by cannulation of the anterior chamber. Three dimensional PS-OCT data sets were acquired at IOP levels between 14 mmHg and 105 mmHg. Maps of scleral birefringence, retinal nerve fiber layer (RNFL) retardation and relative RNFL/retina reflectivity were generated in the peripapillary area and quantitatively analyzed. All investigated parameters showed a substantial correlation with IOP. In the low IOP range of 14-45 mmHg only scleral birefringence showed statistically significant correlation. The polarization changes observed in the PS-OCT imaging study presented in this work suggest that birefringence of the sclera may be a promising IOP-related parameter to investigate.

Multi-Functional OCT Enables Longitudinal Study of Retinal Changes in a VLDLR Knockout Mouse Model

We present a multi-functional optical coherence tomography (OCT) imaging approach to study retinal changes in the very-low-density-lipoprotein-receptor (VLDLR) knockout mouse model with a threefold contrast. In the retinas of VLDLR knockout mice spontaneous retinal-chorodoidal neovascularizations form, having an appearance similar to choroidal and retinal neovascularizations (CNV and RNV) in neovascular age-related macular degeneration (AMD) or retinal angiomatous proliferation (RAP). For this longitudinal study, the mice were imaged every 4 to 6 weeks starting with an age of 4 weeks and following up to the age of 11 months. Significant retinal changes were identified by the multi-functional imaging approach offering a threefold contrast: reflectivity, polarization sensitivity (PS) and motion contrast based OCT angiography (OCTA). By use of this intrinsic contrast, the long-term development of neovascularizations was studied and associated processes, such as the migration of melanin pigments or retinal-choroidal anastomosis, were assessed in vivo. Furthermore, the in vivo imaging results were validated with histological sections at the endpoint of the experiment. Multi-functional OCT proves as a powerful tool for longitudinal retinal studies in preclinical research of ophthalmic diseases. Intrinsic contrast offered by the functional extensions of OCT might help to describe regulative processes in genetic animal models and potentially deepen the understanding of the pathogenesis of retinal diseases such as wet AMD.

Polarized light interaction with tissues

This tutorial-review introduces the fundamentals of polarized light interaction with biological tissues and presents some of the recent key polarization optical methods that have made possible the quantitative studies essential for biomedical diagnostics. Tissue structures and the corresponding models showing linear and circular birefringence, dichroism, and chirality are analyzed. As the basis for a quantitative description of the interaction of polarized light with tissues, the theory of polarization transfer in a random medium is used. This theory employs the modified transfer equation for Stokes parameters to predict the polarization properties of single- and multiple-scattered optical fields. The near-order of scatterers in tissues is accounted for to provide an adequate description of tissue polarization properties. Biomedical diagnostic techniques based on polarized light detection, including polarization imaging and spectroscopy, amplitude and intensity light scattering matrix measurements, and polarization-sensitive optical coherence tomography are described. Examples of biomedical applications of these techniques for early diagnostics of cataracts, detection of precancer, and prediction of skin disease are presented. The substantial reduction of light scattering multiplicity at tissue optical clearing that leads to a lesser influence of scattering on the measured intrinsic polarization properties of the tissue and allows for more precise quantification of these properties is demonstrated.

Polarization properties of single layers in the posterior eyes of mice and rats investigated using high resolution polarization sensitive optical coherence tomography

We present a high resolution polarization sensitive optical coherence tomography (PS-OCT) system for ocular imaging in rodents. The system operates at 840 nm and uses a broadband superluminescent diode providing an axial resolution of 5.1 µm in air. PS-OCT data was acquired at 83 kHz A-scan rate by two identical custom-made spectrometers for orthogonal polarization states. Pigmented (Brown Norway, Long Evans) and non-pigmented (Sprague Dawley) rats as well as pigmented mice (C57BL/6) were imaged. Melanin pigment related depolarization was analyzed in the retinal pigment epithelium (RPE) and choroid of these animals using the degree of polarization uniformity (DOPU). For all rat strains, significant differences between RPE and choroidal depolarization were observed. In contrast, DOPU characteristics of RPE and choroid were similar for C57BL/6 mice. Moreover, the depolarization within the same tissue type varied significantly between different rodent strains. Retinal nerve fiber layer thickness, phase retardation, and birefringence were mapped and quantitatively measured in Long Evans rats in vivo for the first time. In a circumpapillary annulus, retinal nerve fiber layer birefringence amounted to 0.16°/µm ± 0.02°/µm and 0.17°/µm ± 0.01°/µm for the left and right eyes, respectively.

Retinal nerve fiber layer reflectometry must consider directional reflectance

Recent studies reveal that measurements of retinal nerve fiber layer (RNFL) reflectance provide more sensitive detection of glaucomatous damage than RNFL thickness, but most do not consider directional reflectance of the RNFL, an important source of variability. This study quantitatively compared RNFL directional reflectance, represented by an angular spread function (ASF), measured at different scattering angles, different wavelengths and different distances from the optic nerve head (ONH) and for bundles with different thicknesses (T). An ASF was characterized by its amplitude (A) and width (W). Internal reflectance of a bundle was expressed as A/T. The study found that A varied significantly with scattering angle and wavelength and that A/T was different among bundles but constant along the same bundle, indicating that the internal structure of axons may vary among bundles but does not change with distance. This study also found that W was larger near the ONH and at longer wavelengths, but did not depend on scattering angle or T. Because a 4.3° change in incident angle can change reflected intensity by a factor of 2.7, accounting for directional reflectance should improve the accuracy and reproducibility of RNFL reflectance measurements.