GDx VCC is a confocal scanning laser polarimeter (SLP) developed to assess the retinal nerve fiber layer (RNFL) of the eye based on measurement of the phase retardation in the backscattered light from the fundus. In addition to the phase retardation measurement, a depolarization measurement is readily available from the same image series. We hypothesize that the depolarized light in the GDx signal consists of backscattering from the retinal pigment epithelium (RPE) and the RPE-Bruch's membrane junction, and further, that subRPE deposits contribute to the depolarized backscattered light in proportion to their thickness. Therefore, a quantitative macular depolarization map will provide information about both spatial distribution and heterogeneity of the RPE structure and deposit thickness. Ultimately we predict that depolarization mapping will significantly increase the positive predictive power to identify early dry AMD eyes. In this paper, depolarization measurements in normal eyes and age related changes are reported. Data collection was performed at the Duke University Eye Center. A commercial GDx VCC system was modified with a central fixation target and, instead of depolarized light intensity images, normalized depolarization images were derived and saved in the database. Macular depolarization was observed to increase with age in normal eyes at a rate of 0.27%/yr.
Conventional OCT generates one or few cross-sections of the retina and requires predetermination of measurement location and geometry. Because retinal pathologies are usually irregular and 3-dimensional in nature, a retinal imaging device with both high depth resolution and high lateral resolution is desired. The lateral resolution of the conventional OCT system is limited by sampling density, which in turn is limited by the speed of the system. In this paper, we present a three-dimensional optical coherence retinal tomograph (3D-OCT) which combines the rapid transversal imaging mode of a confocal scanning laser ophthalmoscope (cSLO) with the depth resolution of optical coherence tomography (OCT) to achieve high speed 3-D imaging. In contrary to the conventional OCT which performs adjacent A-scans to form a cross-section image (B-scan) perpendicular to the retinal surface, 3D-OCT acquires section images (C-scan) parallel to the retinal surface at defined depths across the thickness of the retina. Three-dimensional distribution of light-remitting sites within the retina is recorded at a depth resolution of ~12 μm (in eye) and lateral resolution of 10μm x 20μm within 1.2 seconds. In this paper, we present the results of in vivo retinal imaging of healthy volunteers and diabetic patients, retinal thickness mapping, and macular edema detection with the 3D-OCT device. Reproducibility of retinal thickness mapping ranges from 16 μm ~ 35 μm for different study subjects. Detailed retinal thickness map allows ready identification of location and area of macular thickening. C-scan images and continuous longitudinal cross section images provide visualization of pathological changes in the retina, such as presence of cyst formation and hard exudates. The need to predetermine measurement location and geometry is eliminated in 3D-OCT, in contrast to conventional OCT.
KEYWORDS: Optical coherence tomography, Retina, 3D image processing, Head, 3D scanning, In vivo imaging, Prototyping, Optic nerve, Image resolution, Nerve
Most of the presently used OCT systems are based on A-scans, i.e., the fast scanning direction is the z-direction. We have developed a new OCT technique for retinal imaging that is based on a transversal scanning scheme and combines the imaging modes of a scanning laser ophthalmoscope with the depth sectioning capability of OCT. A stable high-frequency carrier is generated by use of an acousto optic modulator, and high frame rate is obtained by using a resonant scanning mirror for the priority scan (x-direction). Our prototype instrument records 64 transverse images consisting of 256x128 pixels in 1.2 seconds, thus providing the fastest retinal 3D OCT time domain scanning system reported so far. We demonstrate the capabilities of our system by measuring and imaging the fovea and the optic nerve head region of healthy human volunteers in vivo.
Qienyuan Zhou, Jerry Reed, Ryan Betts, Peter Trost, Pak-Wai Lo, Charles Wallace, Richard Bienias, Guoqiang Li, Ross Winnick, William Papworth, Michael Sinai
One of the earliest signs of glaucoma presence is defects in the retinal nerve fiber layer (RNFL). Scanning laser polarimetry (SLP) provides objective assessment of RNFL, a birefringent tissue, by measuring the total retardation in the reflected light. SLP provides a potential tool for early detection of glaucoma and its progression. The birefringence of the anterior segment of the eye, mainly the cornea, is a confounding variable to SLP's clinical application, if compensation cannot be achieved properly. This paper presents a new SLP system, GDx VCC (Laser Diagnostic Technologies, Inc., San Diego, CA), with a variable corneal compensator (VCC) to achieve individualized corneal compensation. Clinical application of this device in glaucoma detection is also demonstrated.
Evaluation of the retinal nerve fiber layer (RNFL) is important in the diagnosis of glaucoma and other optic nerve diseases. To aid understanding of the mechanism of RNFL reflectance, anatomical data were obtained from electron micrographs of cross- sections of amphibian RNFL and possible and possible scattering structures (axonal membranes, microtubules, and neurofilaments) were modeled as arrays of parallel thin fibers. Microtubules and neurofilaments are already thin fibers, and axonal membranes were decomposed into arrays of adjacent parallel fibers. Internal fields were assigned to each fiber by means of Born approximation or according to rules that depended on membrane orientation. Far field scattering was then calculated by field summation. Calculated scattering was much larger from axonal membranes than from microtubules or neurofilaments. Calculated spectra from 400- 700 nm showed increased scattering at shorter wavelengths for both axonal membranes and microtubules. Calculated backscattering from axonal membranes was approximately equal for incident light polarized parallel (TM mode) and perpendicular (TE mode) to the cylinder axis, but from microtubules TE mode scattering was less than TM mode.
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