Polarization-sensitive Optical Coherence Tomography System Research Articles

High-Speed Retinal Imaging with Polarization-Sensitive OCT at 1040 nm

To demonstrate the ability of a new high-speed polarization-sensitive optical coherence tomography (PS-OCT) system for retinal imaging at 1040 nm. A new polarization-sensitive swept source OCT system in the 1 μm wavelength range is used to image the retina of healthy volunteers. The instrument is operated at an A-scan rate of 100 kHz which is about three times faster than previously reported PS-OCT instruments in this wavelength region. The increased imaging speed can be used to record densely sampled volumes of the retina. Moreover, it enables averaging of several B-scans recorded at the same location to obtain high-definition B-scans without the use of an eye tracker. Polarization-sensitive images of healthy volunteers clearly show the retinal pigment epithelium as a depolarizing layer. In addition, the good tissue penetration of the system allows the visualization of the sclera, which is highly birefringent and therefore shows increased image contrast with PS-OCT. PS-OCT in the 1 μm wavelength region shows similar polarization effects as in the 840 nm wavelength range. The high speed enables averaging of several B-scans to obtain high-definition polarization-sensitive images. The new system provides excellent penetration depth into the choroid and sclera.

Mapping local retardance in birefringent samples using polarization sensitive optical coherence tomography

We proposed a method to extract depth-resolved local retardance in birefringent samples from conventional polarization-sensitive optical coherence tomography (PSOCT) that uses one circularly polarized incident light. Despite the wide use of such PSOCT systems in characterizing birefringent samples, the measured cumulative retardance does not represent the true cumulative retardance when optical axis varies with depth. A Jones calculus based algorithm was designed to derive the local depth-resolved retardance from conventional cumulative PSOCT results. The algorithm was tested in samples with homogeneous optical axis as well as samples with depth-dependent optical axis.

Experimental validation of an extended Jones matrix calculus model to study the 3D structural orientation of the collagen fibers in articular cartilage using polarization-sensitive optical coherence tomography

We report results to verify a theoretical framework to analyze the 3D depth-wise structural organization of collagen fibers in articular cartilage using polarization-sensitive optical coherence tomography. Apparent birefringence data obtained from multi-angle measurements using a time domain polarization-sensitive optical coherence tomography system has been compared with simulated data based on the extended Jones matrix calculus. Experimental data has been shown to agree with the lamellar model previously proposed for the cartilage microstructure based on scanning electron microscopy data. This tool could have potential application in mapping the collagen structural orientation information of cartilage non-invasively during arthroscopy.

Automated phase retardation oriented segmentation of chorio-scleral interface by polarization sensitive optical coherence tomography

An automated chorio-scleral interface (CSI) detection algorithm based on polarization sensitive optical coherence tomography (PS-OCT) is presented. This algorithm employs a two-step scheme based on the phase retardation variation detected by PS-OCT. In the first step, a rough CSI segmentation is implemented to distinguish the choroid and sclera by using depth-oriented second derivative of the phase retardation. Second, the CSI is further finely defined as the intersection of lines fitted to the phase retardation in the choroid and sclera. This algorithm challenges the current back-scattering intensity based CSI segmentation approaches that are not fully based on anatomical and morphological evidence, and provides a rational segmentation method for the morphological investigation of the choroid. Applications of this algorithm are demonstrated on in vivo posterior images acquired by a PS-OCT system with 1-μm probe.

Polarization-sensitive optical coherence tomography using continuous polarization modulation with arbitrary phase modulation amplitude

We demonstrate theoretically and experimentally that the phase retardance and relative optic-axis orientation of a sample can be calculated without prior knowledge of the actual value of the phase modulation amplitude when using a polarization-sensitive optical coherence tomography system based on continuous polarization modulation (CPM-PS-OCT). We also demonstrate that the sample Jones matrix can be calculated at any values of the phase modulation amplitude in a reasonable range depending on the system effective signal-to-noise ratio. This has fundamental importance for the development of clinical systems by simplifying the polarization modulator drive instrumentation and eliminating its calibration procedure. This was validated on measurements of a three-quarter waveplate and an equine tendon sample by a fiber-based swept-source CPM-PS-OCT system.

Polarimetry noise in fiber-based optical coherence tomography instrumentation

High noise levels in fiber-based polarization-sensitive optical coherence tomography (PS-OCT) have broadly limited its clinical utility. In this study we investigate contribution of polarization mode dispersion (PMD) to the polarimetry noise. We develop numerical models of the PS-OCT system including PMD and validate these models with empirical data. Using these models, we provide a framework for predicting noise levels, for processing signals to reduce noise, and for designing an optimized system.

White light interferometric detection of unpolarized light for complete Stokesmetric optical coherence tomography

Optical coherence tomography (OCT) relies on interference between a polarized reference and the target reflection. Thus, it has generally been impossible to detect any unpolarized part in the signal. Here, we demonstrate a scheme that overcomes this limitation. Using a combination of heterodyning and filtering, we realize a polarization-sensitive OCT system capable of measuring the full Stokes vector, including the depolarized part. Based on such a system, we perform full Stokesmetric imaging of different layers in a porcine tendon sample. The complete 4 × 4 backscattering Muellermetric images of one layer are acquired and investigated.

Optic axis determination by fibre-based polarization-sensitive swept-source optical coherence tomography

We describe a fibre-based variable-incidence angle (VIA) polarization-sensitive swept-source optical coherence tomography (PS-SS-OCT) system to determine the 3D optical axis of birefringent biological tissues. Single-plane VIA-PS-OCT is also explored which requires measurement of the absolute fast-axis orientation. A state-of-the-art PS-SS-OCT system with some improvements both in hardware and software was used to determine the apparent optical birefringence of equine tendon for a number of different illumination directions. Polar and azimuthal angles of cut equine tendon were produced by the VIA method and compared with the nominal values. A quarter waveplate (QWP) and equine tendon were used as test targets to validate the fast-axis measurements using the system. Polar and azimuthal angles of cut equine tendon broadly agreed with the expected values within about 8% of the nominal values. A theoretical and experimental analysis of the effect of the sample arm fibre on determination of optical axis orientation using a proposed definition based on the orientation of the eigenpolarization ellipse experimentally confirms that this algorithm only works correctly for special settings of the sample arm fibre. A proposed algorithm based on the angle between Stokes vectors on the Poincaré sphere is confirmed to work for all settings of the sample arm fibre. A calibration procedure is proposed to remove the sign ambiguity of the measured orientation and was confirmed experimentally by using the QWP.

Birefringence measurement in polarization-sensitive optical coherence tomography using differential-envelope detection method

In this research, we integrated two demodulating logarithmic amplifiers with one differential amplifier for use in a Mach-Zehnder interferometer so as to obtain a two-channel polarization-sensitive optical coherence tomography system. Birefringence signals can be acquired using this system along with a differential-envelope detection method. Because the two orthogonal polarizations are common-path propagation, common noise originating from background fluctuations or multiple scattering in turbid media can be reduced to improve the detection sensitivity and accuracy of birefringence measurement. Besides, this simple and effective technique is an analog detection method and is capable of providing high temporal response; it can also help obtain a high time-bandwidth product as compared to the conventional method of using a numerical method with a limited sampling rate. The feasibility of the proposed system is supported by theory and is also shown by performing experiments involving a human vessel, which is a highly scattering medium with weak birefringence.

Imaging Natural Occlusal Caries Lesions with Optical Coherence Tomography.

Several studies have demonstrated that polarization-sensitive optical coherence tomography (PS-OCT) can be used to nondestructively measure the severity of demineralization in the important occlusal surfaces. The purpose of this study was to assess the potential of PS-OCT and OCT methods for the measurement of the depth of natural occlusal carious lesions. Teeth were screened for potential occlusal lesions using near infrared imaging (NIR). A PS-OCT system operating at 1310-nm was used to acquire polarization resolved images of the area of interest on the occlusal surface. The teeth were serial sectioned to 200 μm thickness and examined with polarized light microscopy (PLM) and Transverse Microradiography (TMR) for comparison. The lesion depth measured nondestructively with PS-OCT was compared to the lesion depth measured with PLM and TMR to assess the performance of these methods and determine if polarization sensitivity is required. The lesion depth measured using OCT correlated well with the lesion depths measured with TMR and PLM. Although polarization sensitivity provided better contrast it was not necessary to have polarization sensitivity to identify deep occlusal lesions.

Swept-source polarization-sensitive optical coherence tomography based on polarization-maintaining fiber

We present a swept-source polarization-sensitive optical coherence tomography system based on a polarization-maintaining fiber interferometer. The system produces reflectivity and birefringence information along a depth profile with a single sweep of the optical spectrum. Unlike single-mode fiber systems, retardance and relative optical axis orientation images are calculated without compensation. The source is a 45 mW polygon-based swept-source centered at 1290 nm and tuned at a rate of 28 kHz. The interferometer consists of a single polarization-maintaining coupler that utilizes balanced detection for improved performance. Characterization data shows that this system yields accurate measurements with high sensitivity (106.2 dB) comparable to conventional setups. Images of biological tissues with high dynamic range are demonstrated.

Polarization-maintaining fiber based polarization-sensitive optical coherence tomography in spectral domain

We demonstrate a polarization-sensitive spectral-domain optical coherence tomography system based on polarization-maintaining fiber technology. Using a single-line-scan camera, the system produces reflectivity and retardance information along a depth profile with a single measurement. The relative axis orientation is available as well. System design and characterization and images of a biological tissue are presented.

Polarization maintaining fiber based ultra-high resolution spectral domain polarization sensitive optical coherence tomography

We present a new ultra high resolution spectral domain polarization sensitive optical coherence tomography (PS-OCT) system based on polarization maintaining (PM) fibers. The method transfers the principles of our previous bulk optic PS-OCT systems to a fiberized setup. The phase shift between the orthogonal polarization states travelling in the two orthogonal modes of the PM fiber is compensated by software in post processing. Thereby, the main advantage of our bulk optics setups, i.e. the use of only a single input polarization state to simultaneously acquire reflectivity, retardation, optic axis orientation, and Stokes vector, is maintained. The use of a broadband light source of 110 nm bandwidth provides improved depth resolution and smaller speckle size. The latter is important for improved resolution of depolarization imaging. We demonstrate our instrument for high-resolution PS-OCT imaging of the healthy human retina.

Retinal imaging with polarization-sensitive optical coherence tomography and adaptive optics

Various layers of the retina are well known to alter the polarization state of light. Such changes in polarization may be a sensitive indicator of tissue structure and function, and as such have gained increased clinical attention. Here we demonstrate a polarization-sensitive optical coherence tomography (PS-OCT) system that incorporates adaptive optics (AO) in the sample arm and a single line scan camera in the detection arm. We quantify the benefit of AO for PS-OCT in terms of signal-to-noise, lateral resolution, and speckle size. Double pass phase retardation per unit depth values ranging from 0.25 degrees/microm to 0.65 degrees/microm were found in the birefringent nerve fiber layer at 6 degrees eccentricity, superior to the fovea, with the highest values being noticeably higher than previously reported with PS-OCT around the optic nerve head. Moreover, fast axis orientation and degree of polarization uniformity measurements made with AO-PS-OCT demonstrate polarization scrambling in the retinal pigment epithelium at the highest resolution reported to date.

Real-time polarization-sensitive optical coherence tomography data processing with parallel computing

With the increase of the A-line speed of optical coherence tomography (OCT) systems, real-time processing of acquired data has become a bottleneck. The shared-memory parallel computing technique is used to process OCT data in real time. The real-time processing power of a quad-core personal computer (PC) is analyzed. It is shown that the quad-core PC could provide real-time OCT data processing ability of more than 80 K A-lines per second. A real-time, fiber-based, swept source polarization-sensitive OCT system with 20 K A-line speed is demonstrated with this technique. The real-time 2D and 3D polarization-sensitive imaging of chicken muscle and pig tendon is also demonstrated.

Polarization-Sensitive Optical Coherence Tomography of Necrotizing Scleritis

A polarization-sensitive swept-source optical coherence tomography system (central wavelength: 1,310 nm; A-line rate: 20 kHz) was developed to evaluate the three-dimensional structure of the anterior eye segment with the phase retardation associated with the anterior segment birefringence of the eyes. Evaluation of normal eyes and an eye with necrotizing scleritis was performed. In the sclera of the normal eyes, a striking polarization change was observed in the cumulative phase retardation images and the boundary of the sclera could be readily detected. In the eye with necrotizing scleritis, phase retardation at the sclera was low in an extensive area; this implied diffuse destruction of the collagen tissue in the sclera had occurred. Polarization-sensitive optical coherence tomography is useful as a contrast engine of the anterior eye segment and for the evaluation of pathological change in the sclera.

Measurements of depolarization distribution in the healthy human macula by polarization sensitive OCT

Polarization sensitive optical coherence tomography (PS-OCT) allows for depth resolved imaging of polarization properties of retinal structures. Different layers in the retina can be distinguished not only based on their reflectivity but also due to their birefringent, depolarizing or polarization preserving character. In contrast to other structures in the posterior segment, the retinal pigment epithelium (RPE) is depolarizing, i.e., it scrambles the polarization state of light. A spectral domain (SD) PS-OCT system was used to record 3D data sets of the retina. The depolarizing effect of the RPE was analyzed by quantitative evaluation of the spatial depolarization distribution in the macula regions of healthy eyes. Depolarization in the RPE is most pronounced close to the center of the fovea and decreases with eccentricity.

Measurement of Multiple Optical Parameters of Birefrigent Sample Using Polarization-Sensitive Optical Coherence Tomography

A new polarization-sensitive optical coherence tomography (PS-OCT) system is proposed for obtaining simultaneous measurements of the thickness, mean group refractive index, apparent phase retardation, and optical axis orientation of a linear birefringent material. The proposed PS-OCT system has no need for a high-precision scanning stage or controller and utilizes a thermal light source in order to enhance the resolution in the axial direction. Utilizing the proposed PS-OCT system, a novel technique is proposed for obtaining direct measurements of the group extraordinary and ordinary refractive indexes and group birefringence properties of the linear birefringent sample, and it could be used to inspect and verify the corresponding parameters of a sample. To the best of the authors' knowledge, the system developed in this study is the first system presented in the literature capable of obtaining simultaneous measurements of the multiple optical parameters of a linear birefringent material. As such, the PS-OCT system represents an ideal solution for industrial applications in which a precise knowledge of the optical properties of a birefringent material is required.

Enhancing tissue-contrast imaging of the anterior eye

Optical coherence tomography (OCT) has become one of the most important diagnostic tools in ophthalmology. Originally used to examine the posterior eye, which includes the retina and optic nerve, it now is being applied to the anterior eye, which includes the cornea and iris.1, 2 The back of the eye has several tissue layers, each with different light-scattering properties. OCT uses these differences for image contrast that clearly distinguishes the posterior tissues. However, the scattering properties of anterior eye tissue layers are similar, so it is difficult to selectively obtain contrast with OCT. There are several ways to increase OCT exogenous contrast, such as using nanoparticles and dye molecules, but they are somewhat invasive and thus not convenient for daily clinical use. However, OCT endogenous contrasts may be useful. Doppler OCT provides selective contrast of vasculature.3 Spectroscopic OCT,4 based on tissue absorption, may also improve contrast. And polarization-sensitive OCT (PS-OCT)5 enables birefringence (double refraction), which is strongly associated with tissue composition. We are working on tissue-contrast OCT of the anterior eye using PS-OCT detection and signal processing. This method yields pseudo-color, tissue-contrast OCT images with various tissues displayed in different colors. It can be used to selectively contrast tissues of the anterior eye, including the cornea, conjunctiva, sclera, uvea, and trabecular meshwork. We used a swept-source PS-OCT system with a probe wavelength of 1.3μm. It enables 3D tomography based on standard scattering OCT and PS-OCT, which measures phase retardation.5 The depth resolution is 12μm and themeasurement speed is 20,000 depth scans/s. These specifications are superior to the corresponding quantities in ultrasound biomicroscopy, a standard modality for examining the front of the eye. Figure 1 Figure 1. Human anterior segment cross section using (a) scattering optical coherence tomography (OCT), and (b) polarization-sensitive OCT. These images were constructed from Interactive Science Publishing data sets (see Reference 6).

Analysis of the Origin of Atypical Scanning Laser Polarimetry Patterns by Polarization-Sensitive Optical Coherence Tomography

To analyze the physical origin of atypical scanning laser polarimetry (SLP) patterns. To compare polarization-sensitive optical coherence tomography (PS-OCT) scans to SLP images. To present a method to obtain pseudo-SLP images by PS-OCT that are free of atypical artifacts. Forty-one eyes of healthy subjects, subjects with suspected glaucoma, and patients with glaucoma were imaged by SLP (GDx VCC) and a prototype spectral domain PS-OCT system. The PS-OCT system acquires three-dimensional (3D) datasets of intensity, retardation, and optic axis orientation simultaneously within 3 seconds. B-scans of intensity and retardation and en face maps of retinal nerve fiber layer (RNFL) retardation were derived from the 3D PS-OCT datasets. Results were compared with those obtained by SLP. Twenty-two eyes showed atypical retardation patterns, and 19 eyes showed normal patterns. From the 22 atypical eyes, 15 showed atypical patterns in both imaging modalities, five were atypical only in SLP images, and two were atypical only in PS-OCT images. In most (15 of 22) atypical cases, an increased penetration of the probing beam into the birefringent sclera was identified as the source of atypical patterns. In such cases, the artifacts could be eliminated in PS-OCT images by depth segmentation and exclusion of scleral signals. PS-OCT provides deeper insight into the contribution of different fundus layers to SLP images. Increased light penetration into the sclera can distort SLP retardation patterns of the RNFL.