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We present two versatile and reliable methods of determination of the corneal limbi localization from OCT images. These methods are fully automatic and are based on the purely geometrical dependencies of corneal shape, backgrounded by anatomical structure of the corneoscleral area. The first method enables to estimate the position of the point of corneoscleral junction (CSJ) at the anterior surface of the cornea. The second method can be used for automatic retrieval of the limbal point at the posterior surface of the cornea, which plays an important role in quantification of the irido-corneal angle.

Free to read on publisher website Nowadays medicine highly relies on non-invasive optical diagnostic methods. The likewise situation is in the case of ophthalmology, especially in diagnosis of anterior segment eye diseases, where optical coherence tomography (OCT) plays inestimable role. OCT devices can do fast, painless measurements of the anterior segment of the eye. Nonetheless it should be remembered that the OCT images are burdened with an optical distortion and they doesn’t reflect the true geometry of the eye. This study presents an algorithm for correcting the distortion in OCT images and reflecting true parameters of the anterior segment of the eye.

Procedimiento de calibración y corrección de la distorsión de barrido de cualquier sistema de tomografía de coherencia óptica, mediante el uso de patrones de referencia y la obtención de relaciones matemáticas entre las posiciones de los puntos de referencia en un patrón de referencia y las coordenadas locales de dichos puntos de referencia obtenidas mediante el citado sistema de tomografía de coherencia óptica.

[EN] Method for calibrating and correcting the scanning distortion of an optical coherence tomography system. Method for calibrating and correcting the scanning distortion of any optical coherence tomography system by using reference patterns and obtaining mathematical relationships between the positions of the reference points in a reference pattern and the local coordinates of said reference points, which coordinates are obtained by means of said optical coherence tomography system., [ES] Procedimiento de calibración y corrección de la distorsión de barrido de cualquier sistema de tomografIa de coherencia óptica, mediante el uso de patrones de referencia y la obtención de relaciones matemáticas entre las posiciones de los puntos de referencia en un patrón de referencia y las coordenadas locales de dichos puntos de referencia obtenidas mediante el citado sistema de tomografIa de coherencia óptica.

Procedimiento de calibración y corrección de la distorsión de barrido de cualquier sistema de tomografía de coherencia óptica, mediante el uso de patrones de referencia y la obtención de relaciones matemáticas entre las posiciones de los puntos de referencia en un patrón de referencia y las coordenadas locales de dichos puntos de referencia obtenidas mediante el citado sistema de tomografía de coherencia óptica

We present a method to obtain accurate corneal topography from a spectral optical coherence tomography (sOCT) system. The method includes calibration of the device, compensation of the fan (or field) distortion introduced by the scanning architecture, and image processing analysis for volumetric data extraction, segmentation and fitting. We present examples of three-dimensional (3-D) surface topography measurements on spherical and aspheric lenses, as well as on 10 human corneas in vivo. Results of sOCT surface topography (with and without fan-distortion correction) were compared with non-contact profilometry (taken as reference) on a spherical lens, and with non-contact profilometry and state-of-the art commercial corneal topography instruments on aspheric lenses and on subjects. Corneal elevation maps from all instruments were fitted by quadric surfaces (as well as by tenth-order Zernike polynomials) using custom routines. We found that the discrepancy in the estimated radius of curvature from nominal values in artificial corneas decreased from 4.6% (without fan distortion correction) to 1.6% (after fan distortion correction), and the difference in the asphericity decreased from 130% to 5%. In human corneas, the estimated corneal radius of curvature was not statistically significantly different across instruments. However, a Bland-Altman analysis showed consistent differences in the estimated asphericity and corneal shape between sOCT topographies without fan distortion correction and the rest of the measurements.

An investigation was carried out with the aim of reconstructing the gradient index (GRIN) profile of human crystalline lenses ex-vivo using optical coherence tomography (OCT) imaging with an optimization technique and to study the dependence of the GRIN profile with age. Cross-sectional images of nine isolated human crystalline lenses with ages ranging from 6 to 72 (post-mortem time 1 to 4 days) were obtained using a custom-made OCT system. Lenses were extracted from whole cadaver globes and placed in a measurement chamber filled with preservation medium (DMEM). Lenses were imaged with the anterior surface up and then flipped over and imaged again, to obtain posterior lens surface profiles both undistorted and distorted by the refraction through the anterior crystalline lens and GRIN. The GRIN distribution of the lens was described with three variables by means of power function, with variables being the nucleus and surface index, and a power coefficient that describes the decay of the refractive index from the nucleus to the surface. An optimization method was used to search for the parameters that produced the best match of the distorted posterior surface. The distorted surface was simulated with accuracy around the resolution of the OCT system (under 15 mm). The reconstructed refractive index values ranged from 1.356 to 1.388 for the surface, and from 1.396 to 1.434 for the nucleus. The power coefficient ranged between 3 and 18. The power coefficient increased significantly with age, at a rate of 0.24 per year. Optical coherence tomography allowed optical, non-invasive measurement of the 2D gradient index profile of the isolated human crystalline lens ex vivo. The age-dependent variation of the changes is consistent with previous data using magnetic resonance imaging, and the progressive formation of a refractive index plateau.

The aim of this study was to simulate the retinal image of the eye with progressive addition lens (PAL), correction (without cylindrical component) and investigate its quality. The optical design and analysis software was used in simulation. The topographical data of the front surface of PAL were taken from the scanning profilometry. Also the coordinates of the points of the distant and near power measurements were taken from topographical measurements. The back surface was assumed to be spherical with a radius of curvature matched so that the optical power of PAL meets the data given by the manufacturer. In order to describe the retinal image quality, various metrics were calculated in comparison to the eye without correction. © 2009 Elsevier GmbH. All rights reserved.

We quantify the posterior surface distortions in optical coherence tomography (OCT) images of isolated crystalline lenses. The posterior radius of curvature and asphericity obtained from OCT images acquired with the beam incident first on the anterior, and then the posterior, surface were compared. The results were compared with predictions of a ray-tracing model which includes the index gradient. The results show that the error in the radius of curvature is within the measurement reproducibility and that it can be corrected by assuming a uniform refractive index. However, accurate asphericity values require a correction algorithm that takes into account the gradient.

A method for three-dimensional 3-D optical distortion (refraction) correction on anterior segment Optical Coherence Tomography (OCT) images has been developed. The method consists of 3-D ray tracing through the different surfaces, following denoising, segmentation of the surfaces, Delaunay representation of the surfaces, and application of fan distortion correction. The correction has been applied theoretically to realistic computer eye models, and experimentally to OCT images of: an artificial eye with a Polymethyl Methacrylate (PMMA) cornea and an intraocular lens (IOL), an enucleated porcine eye, and a human eye in vivo obtained from two OCT laboratory set-ups (time domain and spectral). Data are analyzed in terms of surface radii of curvature and asphericity. Comparisons are established between the reference values for the surfaces (nominal values in the computer model; non-contact profilometric measurements for the artificial eye; Scheimpflug imaging for the real eyes in vivo and vitro). The results from the OCT data were analyzed following the conventional approach of dividing the optical path by the refractive index, after application of 2-D optical correction, and 3-D optical correction (in all cases after fan distortion correction). The application of 3-D optical distortion correction increased significantly both the accuracy of the radius of curvature estimates and particularly asphericity of the surfaces, with respect to conventional methods of OCT image analysis. We found that the discrepancies of the radii of curvature estimates from 3-D optical distortion corrected OCT images are less than 1% with respect to nominal values. Optical distortion correction in 3-D is critical for quantitative analysis of OCT anterior segment imaging, and allows accurate topography of the internal surfaces of the eye.