Henle fiber layer phase retardation measured with polarization-sensitive optical coherence tomography

Normalization of Retinal Birefringence Scanning Signals

Signal amplitudes obtained from retinal scanning depend on numerous factors. Working with polarized light to interrogate the retina, large parts of which are birefringent, is even more prone to artifacts. This article demonstrates the necessity of using normalization when working with retinal birefringence scanning signals in polarization-sensitive ophthalmic instruments. After discussing the pros and cons of employing a normalization signal obtained by means of added optoelectronic hardware, the study shifts over and focuses on a numerical normalization method based on merely the s- and p-polarization components without additional optical or electronic hardware. This minimizes the adverse effects of optical asymmetries, the presence of certain instrumental noise, device-to-device variability, pupil diameter, retinal reflectivity, subject-to-subject variations, the position of the eye in the exit pupil of the device, and even signal degradation by cataracts. Results were experimentally and numerically tested on human data from 15 test subjects and clearly demonstrated the signal standardization achieved by numerical normalization. This is expected to lead to substantial improvement in algorithms and decision-making software, especially in ophthalmic screening instruments for pediatric applications, without added hardware cost. The proposed normalization method is also applicable to other polarization-sensitive optical instruments.

Evaluating fingerprint-like patterns in the healthy Henle fiber layer using enface OCT imaging

PURPOSE

Enface OCT may disclose a distinct "fingerprint-like' pattern within the HFL in various macular disorders. This study aims to investigate the frequency and characteristics of this pattern in healthy eyes and identify potential factors influencing its visibility.

METHODS

Two, independent masked reading center graders evaluated for the presence and prominence of a fingerprint pattern in the Henle fiber layer (HFL) on enface OCT images from 33 healthy subjects (66 eyes). The prominence of the pattern was rated qualitatively using a 0-3 scale, with 3 indicating the strongest prominence. Tilt angles (relative to the normal/perpendicular at the center) of the retina were measured on horizontal and vertical B-scans, and the retinal curvature was assessed using ImageJ, in order to determine the impact of the incident light angle on the visibility and prominence of the fingerprint pattern. Inter-grader agreement using Cohen's kappa and the frequency and percentage of patterns in the entire enface image and in each quadrant were calculated and compared using the Friedman test with Dunn's post-test. A generalized estimating equation (GEE) was used to analyze the association between these metrics and fingerprint prominence.

RESULTS

Substantial inter-grader agreement was observed (Cohen's kappa = 0.71) for assessing the prominence of the fingerprint pattern. Over 70% of eyes exhibited some evidence of the pattern (score ≥1). Significant difference in pattern prominence across quadrants was detected (p < 0.05), with lowest prominence in the temporal quadrant (p < 0.001 for pairwise comparisons against all other quadrants). The GEE analysis to account for the extent of the effect of scan tilt angle and RPE curvature was not able to predict the prominence of the fingerprint pattern, highlighting that angle of incidence (of the scanning laser light) alone could not explain the pattern.

CONCLUSIONS

This study confirms that a fingerprint-like pattern within the HFL can also be observed in healthy eyes, challenging the notion that this finding is only manifest in the setting of disease. In addition, the lack of correlation with angle of incident light suggests that the pattern may be related to other intrinsic characteristics of the HFL.

Study of Foveal Avascular Zone Growth in Individuals With Mild Diabetic Retinopathy by Optical Coherence Tomography

Purpose

The purpose of this study was to investigate the association between foveal vessels and retinal thickness in individuals with diabetic retinopathy (DR) and control subjects, and to reveal foveal avascular zone (FAZ) growth in early individuals with DR.

Methods

The regions with a thickness less than 60 µm were marked from the intima thickness maps and named FAZThic. The avascular zones extracted from the deep vascular plexus were designated as FAZAngi. The boundary of the two FAZ forms a ring region, which we called FAZRing. The FAZ growth rate was defined as the ratio of the FAZRing area to the FAZThic area. Thirty healthy controls and 30 individuals with mild nonproliferative DR were recruited for this study.

Results

The FAZThic area in individuals with mild DR and control subjects showed similar distribution. The FAZAngi area in individuals with mild DR are higher than that in control subjects on the whole, but there was no significant difference (P > 0.05). The FAZRing area in individuals with mild DR was significantly higher than that in control subjects (P < 0.001). However, there is still a small amount of overlap data between the two groups. For the FAZ growth rate, the individuals with mild DR were also significantly larger than the control subjects (P < 0.001). But there were no overlapping data between the two groups.

Conclusions

The growth of FAZ in individuals with mild DR can be inferred by comparing FAZAngi with FAZThic. This method minimizes the impact of individual variations and helps researchers to understand the progression mechanism of DR more deeply.

Feasibility of deep learning-based polarization-sensitive optical coherence tomography angiography for imaging cutaneous microvasculature

Polarization-sensitive optical coherence tomography (PS-OCT) measures the polarization states of the backscattered light from tissue that can improve angiography based on conventional optical coherence tomography (OCT). We present a feasibility study on PS-OCT integrated with deep learning for PS-OCT angiography (PS-OCTA) imaging of human cutaneous microvasculature. Two neural networks were assessed for PS-OCTA, including the residual dense network (RDN), which previously showed superior performance for angiography with conventional OCT and the upgraded grouped RDN (GRDN). We also investigated different protocols to process the multiple signal channels provided by the Jones matrices from the PS-OCT system to achieve optimal PS-OCTA performance. The training and testing of the deep learning-based PS-OCTA were performed using PS-OCT scans collected from 18 skin locations comprising 16,600 B-scan pairs. The results demonstrated a moderately improved performance of GRDN over RDN, and of the use of the combined signal from the Jones matrix elements over the separate use of the elements, as well as a similar image quality to that provided by speckle decorrelation angiography. GRDN-based PS-OCTA also showed ∼2-3 times faster processing and improved mitigation of tissue motion as compared to speckle decorrelation angiography, and enabled fully automatic processing. Deep learning-based PS-OCTA can be used for imaging cutaneous microvasculature, which may enable easy adoption of PS-OCTA for preclinical and clinical applications.

Posterior scleral birefringence measured by triple-input polarization-sensitive imaging as a biomarker of myopia progression

In myopic eyes, pathological remodelling of collagen in the posterior sclera has mostly been observed ex vivo. Here we report the development of triple-input polarization-sensitive optical coherence tomography (OCT) for measuring posterior scleral birefringence. In guinea pigs and humans, the technique offers superior imaging sensitivities and accuracies than dual-input polarization-sensitive OCT. In 8-week-long studies with young guinea pigs, scleral birefringence was positively correlated with spherical equivalent refractive errors and predicted the onset of myopia. In a cross-sectional study involving adult individuals, scleral birefringence was associated with myopia status and negatively correlated with refractive errors. Triple-input polarization-sensitive OCT may help establish posterior scleral birefringence as a non-invasive biomarker for assessing the progression of myopia.

2022 Prentice Award Lecture: Advancing Retinal Imaging and Visual Function in Patient Management and Disease Mechanisms

SIGNIFICANCE

Patient-based research plays a key role in probing basic visual mechanisms. Less-well recognized is the role of patient-based retinal imaging and visual function studies in elucidating disease mechanisms, which are accelerated by advances in imaging and function techniques and are most powerful when combined with the results from histology and animal models.A patient's visual complaints can be one key to patient management, but human data are also key to understanding disease mechanisms. Unfortunately, pathological changes can be difficult to detect. Before advanced retinal imaging, the measurement of visual function indicated the presence of pathological changes that were undetectable with existing clinical examination. Over the past few decades, advances in retinal imaging have increasingly revealed the unseen. This has led to great strides in the management of many diseases, particularly diabetic retinopathy and macular edema, and age-related macular degeneration. It is likely widely accepted that patient-based research, as in clinical trials, led to such positive outcomes. Both visual function measures and advanced retinal imaging have clearly demonstrated differences among retinal diseases. Contrary to initial thinking, sight-threatening damage in diabetes occurs to the outer retina and not only to the inner retina. This has been clearly indicated in patient results but has only gradually entered the clinical classifications and understanding of disease etiology. There is strikingly different pathophysiology for age-related macular degeneration compared with photoreceptor and retinal pigment epithelial genetic defects, yet research models and even some treatments confuse these. It is important to recognize the role that patient-based research plays in probing basic visual mechanisms and elucidating disease mechanisms, combining these findings with the concepts from histology and animal models. Thus, this article combines sample instrumentation from my laboratory and progress in the fields of retinal imaging and visual function.

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.

Structure and function of retinal ganglion cells in subjects with a history of repeated traumatic brain injury

This study tested whether repeated traumatic brain injuries (TBIs) alter the objective structure or the objective function of retinal ganglion cells (RGCs) in human subjects recruited from an optometry clinic. Case subjects (n = 25) with a history of repeated TBIs (4.12 ± 2.76 TBIs over 0-41 years) and healthy pair-matched control subjects (n = 30) were prospectively recruited. Retinal nerve fiber layer (RNFL) thickness was quantified with spectral-domain optical coherence tomography, and scanning laser polarimetry measured RNFL phase retardation. Measurements of the photopic negative response were made using full-field flash electroretinography. There was no statistically significant difference (p = 0.42) in global RNFL thickness between the case cohort (96.6 ± 9.4 microns) and the control cohort (94.9 ± 7.0 microns). There was no statistically significant difference (p = 0.80) in global RNFL phase retardation between the case cohort (57.9 ± 5.7 nm) and the control cohort (58.2 ± 4.6 nm). There were no statistically significant differences in the peak time (p = 0.95) of the PhNR or in the amplitude (p = 0.11) of the PhNR between the case cohort (69.9 ± 6.9 ms and 24.1 ± 5.1 μV, respectively) and the control cohort (70.1 ± 8.9 ms and 27.8 ± 9.1 μV, respectively). However, PhNR amplitude was more variable (p < 0.025) in the control cohort than in the case cohort. Within the case cohort, there was a strong positive (r = 0.53), but not statistically significant (p = 0.02), association between time since last TBI and PhNR amplitude. There was also a modest positive (r = 0.45), but not statistically significant (p = 0.04), association between time since first TBI and PhNR amplitude. Our results suggest that there were no statistically significant differences in the objective structure or in the objective function of RGCs between the case cohort and the control cohort. Future large, longitudinal studies will be necessary to confirm our negative results and to more fully investigate the potential interaction between PhNR amplitude and time since first or last TBI.

Two concepts for ultra-high-resolution polarization-sensitive optical coherence tomography with a single camera

Two designs with a multiplexed superluminescent diode for ultra-high-resolution spectral-domain polarization-sensitive optical coherence tomography (UHR-PS-OCT) are introduced. In the first design, a Wollaston prism separates orthogonal polarization states next to each other on one linescan camera; the other design uses a beam displacer to separate orthogonal states onto two lines of a linescan camera with multiple rows of detectors. The coherence lengths measured with the two systems were 3.6 µm and 2.9 µm (n=1.38), respectively. Measurements were collected from the fovea of a healthy subject, a healthy subject with astigmatism, and a patient with central serous retinopathy (CSR). A single volumetric scan provides double pass retardance induced by the retinal nerve fiber layer birefringence (RNFL) and its birefringence, the cumulative double pass retardance induced by the Henle fiber layer, and the retardance that is induced by the retinal pigment epithelium-Bruch's membrane complex. The high axial resolution in UHR-PS-OCT is particularly helpful for the measurements of thin retinal tissue, such as the RNFL in the fovea, where birefringence values of around 1°/µm were found. Tilting of the retina due to a CSR or by off centering the imaging beam in the pupil causes an artificial increase in the double pass retardance induced by the RNFL and Henle fiber layer.

Foveal Phase Retardation Correlates With Optically Measured Henle Fiber Layer Thickness

This study quantified and compared phase retardation distribution in the central macula with the thickness of the Henle fiber layer (HFL). A scanning laser polarimeter (SLP) was used to acquire 20° × 40° macular-centered images, either with fixed corneal compensation or with variable corneal compensation, in two cohorts of clinically normal subjects (N = 36). Phase retardation maps from SLP imaging were used to generate a macular cross pattern (fixed compensation) or an annulus pattern (variable compensation) centered on the macula. Intensity profiles in the phase retardation maps were produced using annular regions of interest at eccentricities from 0.25° to 3°. Pixel intensity was averaged at each eccentricity, acting as a surrogate for macular phase retardation. Directional OCT images were acquired in the horizontal and vertical meridians in all subjects, allowing visualization of the HFL thickness. HFL thickness was manually segmented in each meridian and averaged. In both cohorts, phase retardation and HFL thickness were highly correlated in the central 3° assessed, providing further evidence that the source of the phase retardation signal in the central macula is dominated by the HFL and that the center of the macula on cross sectional imaging corresponds closely with the center of the macular cross on SLP imaging.

Pathognomonic macular ripples are revealed by polarized infrared retinal imaging

A pathognomonic macular ripple sign has been reported with scanning laser ophthalmoscopy images in patients with foveal hypoplasia, though the optical basis of this sign is presently unknown. Here we present a case series of seven individuals with foveal hypoplasia (based on spectral domain optical coherence tomography). Each patient underwent infrared scanning laser ophthalmoscopy retinal imaging in both eyes, acquired with and without a polarization filter and assessment for a ripple-like effect in the fovea. On imaging, macular ripples were present in all eyes with foveal hypoplasia when using a polarization filter, but not when imaged without the filter. We conclude that the macular ripple sign is an imaging artifact attributable to the unique pattern of phase retardation of the Henle fiber layer in the setting of foveal hypoplasia. By utilizing a polarization filter with retinal photography, this feature can be exploited to promptly identify foveal hypoplasia in settings where OCT is not possible due to nystagmus.

Polarization properties of retinal blood vessel walls measured with polarization sensitive optical coherence tomography

A new method based on polarization-sensitive optical coherence tomography (PS-OCT) is introduced to determine the polarization properties of human retinal vessel walls, in vivo. Measurements were obtained near the optic nerve head of three healthy human subjects. The double pass phase retardation per unit depth (DPPR/UD), which is proportional to the birefringence, is higher in artery walls, presumably because of the presence of muscle tissue. Measurements in surrounding retinal nerve fiber layer tissue yielded lower DPPR/UD values, suggesting that the retinal vessel wall tissue near the optic nerve is not covered by retinal nerve fiber layer tissue (0.43°/µm vs. 0.77°/µm, respectively). Measurements were obtained from multiple artery-vein pairs, to quantify the different polarization properties. Measurements were taken along a section of the vessel wall, with changes in DPPR/UD up to 15%, while the vessel wall thickness remained relatively constant. A stationary scan pattern was applied to determine the influence of involuntary eye motion on the measurement, which was significant. Measurements were also analyzed by two examiners, with high inter-observer agreement. The measurement repeatability was determined with measurements that were acquired during multiple visits. An improvement in accuracy can be achieved with an ultra-broad-bandwidth PS-OCT system since it will provide more data points in-depth, which reduces the influence of discretization and helps to facilitate better fitting of the birefringence data.

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.

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.

Clinical prototype of pigment and flow imaging optical coherence tomography for posterior eye investigation

Measurements of the randomness of polarization (RP) obtained using polarization-sensitive optical coherence tomography (PS-OCT) are applied in several applications, and RP is attractive for posterior eye imaging. The addition of RP without retardation requires a minimal extension to standard OCT; therefore, we developed a prototype OCT system with a simplified scheme for RP measurement. A compact polarization-diversity receiver module is the only required hardware extension to a standard OCT system. All components were packed into the retinal scanning head. The degree-of-polarization uniformity and complex-decorrelation based OCT angiography were calculated using noise-corrected algorithms that accounted for the depth-dependent noise power. The structure, melanin, and blood flow distribution imaging of in vivo human eyes were demonstrated. Pathological eye imaging shows potential applications for combinations of these contrasts.

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.

Multi-directional optical coherence tomography for retinal imaging

We introduce multi-directional optical coherence tomography (OCT), a technique for investigation of the scattering properties of directionally reflective tissue samples. By combining the concepts of multi-channel and directional OCT, this approach enables simultaneous acquisition of multiple reflectivity depth-scans probing a mutual sample location from differing angular orientations. The application of multi-directional OCT in retinal imaging allows for in-depth investigations on the directional reflectivity of the retinal nerve fiber layer, Henle's fiber layer and the photoreceptor layer. Major ophthalmic diseases (such as glaucoma or age-related macular degeneration) have been reported to alter the directional reflectivity properties of these retinal layers. Hence, the concept of multi-directional OCT might help to gain improved understanding of pathology development and progression. As a first step, we demonstrate the capabilities of multi-directional OCT in the eyes of healthy human volunteers.

Foveal Curvature and Asymmetry Assessed Using Optical Coherence Tomography

PURPOSE

The aims of this study were to use cross-sectional optical coherence tomography imaging and custom curve fitting software to evaluate and model the foveal curvature as a spherical surface and to compare the radius of curvature in the horizontal and vertical meridians and test the sensitivity of this technique to anticipated meridional differences.

METHODS

Six 30-degree foveal-centered radial optical coherence tomography cross-section scans were acquired in the right eye of 20 clinically normal subjects. Cross sections were manually segmented, and custom curve fitting software was used to determine foveal pit radius of curvature using the central 500, 1000, and 1500 μm of the foveal contour. Radius of curvature was compared across different fitting distances. Root mean square error was used to determine goodness of fit. The radius of curvature was compared between the horizontal and vertical meridians for each fitting distance.

RESULTS

There radius of curvature was significantly different when comparing each of the three fitting distances (P < .01 for each comparison). The average radii of curvature were 970 μm (95% confidence interval [CI], 913 to 1028 μm), 1386 μm (95% CI, 1339 to 1439 μm), and 2121 μm (95% CI, 2066 to 2183) for the 500-, 1000-, and 1500-μm fitting distances, respectively. Root mean square error was also significantly different when comparing each fitting distance (P < .01 for each comparison). The average root mean square errors were 2.48 μm (95% CI, 2.41 to 2.53 μm), 6.22 μm (95% CI, 5.77 to 6.60 μm), and 13.82 μm (95% CI, 12.93 to 14.58 μm) for the 500-, 1000-, and 1500-μm fitting distances, respectively. The radius of curvature between the horizontal and vertical meridian radii was statistically different only in the 1000- and 1500-μm fitting distances (P < .01 for each), with the horizontal meridian being flatter than the vertical.

CONCLUSIONS

The foveal contour can be modeled as a sphere with low curve fitting error over a limited distance and capable of detecting subtle foveal contour differences between meridians.

Polarization sensitive optical coherence tomography - a review [Invited]

Optical coherence tomography (OCT) is now a well-established modality for high-resolution cross-sectional and three-dimensional imaging of transparent and translucent samples and tissues. Conventional, intensity based OCT, however, does not provide a tissue-specific contrast, causing an ambiguity with image interpretation in several cases. Polarization sensitive (PS) OCT draws advantage from the fact that several materials and tissues can change the light's polarization state, adding an additional contrast channel and providing quantitative information. In this paper, we review basic and advanced methods of PS-OCT and demonstrate its use in selected biomedical applications.

Noise stochastic corrected maximum a posteriori estimator for birefringence imaging using polarization-sensitive optical coherence tomography

This paper presents a noise-stochastic corrected maximum a posteriori estimator for birefringence imaging using Jones matrix optical coherence tomography. The estimator described in this paper is based on the relationship between probability distribution functions of the measured birefringence and the effective signal to noise ratio (ESNR) as well as the true birefringence and the true ESNR. The Monte Carlo method is used to numerically describe this relationship and adaptive 2D kernel density estimation provides the likelihood for a posteriori estimation of the true birefringence. Improved estimation is shown for the new estimator with stochastic model of ESNR in comparison to the old estimator, both based on the Jones matrix noise model. A comparison with the mean estimator is also done. Numerical simulation validates the superiority of the new estimator. The superior performance of the new estimator was also shown by in vivo measurement of optic nerve head.

Birefringence imaging of posterior eye by multi-functional Jones matrix optical coherence tomography

A clinical grade prototype of posterior multifunctional Jones matrix optical coherence tomography (JM-OCT) is presented. This JM-OCT visualized depth-localized birefringence in addition to conventional cumulative phase retardation imaging through local Jones matrix analysis. In addition, it simultaneously provides a sensitivity enhanced scattering OCT, a quantitative polarization uniformity contrast, and OCT-based angiography. The probe beam is at 1-μm wavelength band. The measurement speed and the depth-resolution were 100,000 A-lines/s, and 6.6 μm in tissue, respectively. Normal and pathologic eyes are examined and several clinical features are revealed, which includes high birefringence in the choroid and lamina cribrosa, and birefringent layered structure of the sclera. The theoretical details of the depth-localized birefringence imaging and conventional phase retardation imaging are formulated. This formulation indicates that the birefringence imaging correctly measures a depth-localized single-trip phase retardation of a tissue, while the conventional phase retardation can provide correct single-trip phase retardation only for some specific types of samples.

Pupil tracking optical coherence tomography for precise control of pupil entry position

To maximize the collection efficiency of back-scattered light, and to minimize aberrations and vignetting, the lateral position of the scan pivot of an optical coherence tomography (OCT) retinal scanner should be imaged to the center of the ocular pupil. Additionally, several retinal structures including Henle's Fiber Layer (HFL) exhibit reflectivities that depend on illumination angle, which can be controlled by varying the pupil entry position of the OCT beam. In this work, we describe an automated method for controlling the lateral pupil entry position in retinal OCT by utilizing pupil tracking in conjunction with a 2D fast steering mirror placed conjugate to the retinal plane. We demonstrate that pupil tracking prevents lateral motion artifacts from impeding desired pupil entry locations, and enables precise pupil entry positioning and therefore control of the illumination angle of incidence at the retinal plane. We use our prototype pupil tracking OCT system to directly visualize the obliquely oriented HFL.

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

We present a new semi-automatic processing method for retinal nerve fiber bundle tracing based on polarization sensitive optical coherence tomography (PS-OCT) data sets. The method for tracing is based on a nerve fiber orientation map that covers the fovea and optic nerve head (ONH) regions. In order to generate the orientation map, two types of information are used: optic axis orientation based on polarization data, and complementary information obtained from nerve fiber layer (NFL) local thickness variation to reveal fiber bundle structures around the fovea. The corresponding two orientation maps are fused into a combined fiber orientation map. En face maps of NFL retardation, thickness, and unit-depth-retardation (UDR, equivalent to birefringence) are transformed into "along-trace" maps by using the obtained traces of the nerve fiber bundles. The method is demonstrated in the eyes of healthy volunteers, and as an example of further analyses utilizing this method, maps illustrating the gradients of NFL retardation, thickness, and UDR are demonstrated.

Henle fiber layer phase retardation changes associated with age-related macular degeneration

PURPOSE

To quantify and compare phase retardation amplitude and regularity associated with the Henle fiber layer (HFL) between nonexudative AMD patients and age-matched controls using scanning laser polarimetry (SLP) imaging.

METHODS

A scanning laser polarimeter was used to collect 15 × 15° macular-centered images in 25 patients with nonexudative AMD and 25 age-matched controls. Raw image data were used to compute macular phase retardation maps associated with the HFL. Consecutive, annular regions of interest from 0.5 to 3.0° eccentricity, centered on the fovea, were used to generate intensity profiles from phase retardation data and analyzed with two complementary techniques: a normalized second harmonic frequency (2f) of the fast Fourier Transform (FFT) analysis and a curve fitting analysis using a 2f sine function. Paired t-tests were used to compare the normalized 2f FFT magnitude at each eccentricity between the two groups, the eccentricity that yielded the maximum normalized 2f FFT between paired individuals across the two groups, and curve fitting RMS error at each eccentricity between the two groups.

RESULTS

Normalized 2f FFT components were lower in the AMD group at each eccentricity, with no difference between the two groups in the maximum normalized 2f FFT component eccentricity. The root-mean-square (RMS) error from curve fitting was significantly higher in the AMD group.

CONCLUSIONS

Phase retardation changes in the central macula indicate loss and/or structural alterations to central cone photoreceptors in nonexudative AMD patients. Scanning laser polarimetry imaging is a noninvasive method for quantifying cone photoreceptor changes associated with central macular disease.

Single input state polarization sensitive swept source optical coherence tomography based on an all single mode fiber interferometer

We present a newly developed single mode fiber based swept source polarization sensitive optical coherence tomography system using a single input state at 1040 nm. Two non-polarizing fiber based beam splitters are combined to form a Mach-Zehnder interferometer, while two polarizing beam splitters are used to obtain a polarization sensitive detection. Both types of beam splitters solely feature conventional single mode fibers. Polarization control paddles are used to set and maintain the polarization states in the fibers of the interferometer and detection unit. By use of a special paddle alignment scheme we are able to eliminate any bulk optic wave plates and polarization maintaining fibers in the interferometer and detection paths while preserving the advantages of a single input state system that illuminates the sample with circularly polarized light. To demonstrate the capabilities of our system, we performed retinal measurements on healthy human volunteers.

Fiber-based polarization-sensitive OCT of the human retina with correction of system polarization distortions

In polarization-sensitive optical coherence tomography (PS-OCT) the use of single-mode fibers causes unpredictable polarization distortions which can result in increased noise levels and erroneous changes in calculated polarization parameters. In the current paper this problem is addressed by a new Jones matrix analysis method that measures and corrects system polarization distortions as a function of wavenumber by spectral analysis of the sample surface polarization state and deeper located birefringent tissue structures. This method was implemented on a passive-component depth-multiplexed swept-source PS-OCT system at 1040 nm which was theoretically modeled using Jones matrix calculus. High-resolution B-scan images are presented of the double-pass phase retardation, diattenuation, and relative optic axis orientation to show the benefits of the new analysis method for in vivo imaging of the human retina. The correction of system polarization distortions yielded reduced phase retardation noise, and better estimates of the diattenuation and the relative optic axis orientation in weakly birefringent tissues. The clinical potential of the system is shown by en face visualization of the phase retardation and optic axis orientation of the retinal nerve fiber layer in a healthy volunteer and a glaucoma patient with nerve fiber loss.

Modern technologies for retinal scanning and imaging: an introduction for the biomedical engineer

This review article is meant to help biomedical engineers and nonphysical scientists better understand the principles of, and the main trends in modern scanning and imaging modalities used in ophthalmology. It is intended to ease the communication between physicists, medical doctors and engineers, and hopefully encourage “classical” biomedical engineers to generate new ideas and to initiate projects in an area which has traditionally been dominated by optical physics. Most of the methods involved are applicable to other areas of biomedical optics and optoelectronics, such as microscopic imaging, spectroscopy, spectral imaging, opto-acoustic tomography, fluorescence imaging etc., all of which are with potential biomedical application. Although all described methods are novel and important, the emphasis of this review has been placed on three technologies introduced in the 1990’s and still undergoing vigorous development: Confocal Scanning Laser Ophthalmoscopy, Optical Coherence Tomography, and polarization-sensitive retinal scanning.

Motion artifact and speckle noise reduction in polarization sensitive optical coherence tomography by retinal tracking

We present a novel polarization sensitive optical coherence tomography (PS-OCT) system with an integrated retinal tracker. The tracking operates at up to 60 Hz, correcting PS-OCT scanning positions during the acquisition to avoid artifacts caused by eye motion. To demonstrate the practical performance of the system, we imaged several healthy volunteers and patients with AMD both with B-scan repetitions for frame averaging and with 3D raster scans. Under large retinal motions with up to 1 mm amplitude at 0.5 ~a few Hz frequency range, motion artifact suppression in the PS-OCT images as well as standard deviation noise reduction in the frame averaged retardation images are presented.