Correlation between radius and asphericity in surfaces fitted by conics

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Custom high-resolution high-speed anterior segment spectral domain Optical Coherence Tomography (OCT) was used to characterize three-dimensionally (3-D) corneal topography in keratoconus before and after implantation of intracorneal ring segments (ICRS). Previously described acquisition protocols were followed to minimize the impact of the motions of the eye. The collected set of images was corrected from distortions: fan (scanning) and optical (refraction). Custom algorithms were developed for automatic detection and classification of volumes in the anterior segment of the eye, in particular for the detection and classification of the implanted ICRS. Surfaces were automatically detected for quantitative analysis of the corneal elevation maps (fitted by biconicoids and Zernike polynomials) and pachymetry. Automatic tools were developed for the estimation of the 3-D positioning of the ICRS. The pupil center reference was estimated from the segmented iris volume. The developed algorithms are illustrated in a keratoconic eye (grade III) pre- and 30 days post-operatively after implantation of two triangular-section, 0.3-mm thick Ferrara ring segments. Quantitative corneal topographies reveal that the ICRS produced a flattening of the anterior surface, a steepening of the posterior surface, meridional differences in the changes in curvature and asphericity, and increased symmetry of the anterior topography. Optical distortion correction through the ICRS (of a different refractive index from the cornea) allowed accurate pachymetric estimates, which showed increased thickness in the ectatic area as well as in peripheral corneal areas. Automatic tools allowed estimation of the depth of the implanted ICRS ring, as well as its rotation with respect to the pupil plane. Anterior segment sOCT provided with fan and optical distortion correction and analysis tools is an excellent instrument for evaluating and monitoring keratoconic eyes and for the quantification of the changes produced by ICRS treatment.

Section: Methodsmentioning

confidence: 99%

Quantitative OCT-based corneal topography in keratoconus with intracorneal ring segments

Ortíz

Pérez-Merino

Alejandre

et al. 2012

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“…Manns et al reported, for the first time, measurements of the lens asphericity ex vivo [21]. Interaction effects between lens radius of curvature and asphericity in the lens profile fittings suggest that those values should not be interpreted separately [23]. Dubbelman et al showed that the asphericity of human lenses in vivo tends to increase with age for both anterior and posterior surface [22].…”

Section: Introductionmentioning

confidence: 99%

OCT 3-D surface topography of isolated human crystalline lenses

Sun

Birkenfeld

Castro

et al. 2014

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Quantitative 3-D Optical Coherence Tomography was used to measure surface topography of 36 isolated human lenses, and to evaluate the relationship between anterior and posterior lens surface shape and their changes with age. All lens surfaces were fitted to 6th order Zernike polynomials. Astigmatism was the predominant surface aberration in anterior and posterior lens surfaces (accounting for ~55% and ~63% of the variance respectively), followed by spherical terms, coma, trefoil and tetrafoil. The amount of anterior and posterior surface astigmatism did not vary significantly with age. The relative angle between anterior and posterior surface astigmatism axes was on average 36.5 deg, tended to decrease with age, and was >45 deg in 36.1% lenses. The anterior surface RMS spherical term, RMS coma and 3rd order RMS decreased significantly with age. In general, there was a statistically significant correlation between the 3rd and 4th order terms of the anterior and posterior surfaces. Understanding the coordination of anterior and posterior lens surface geometries and their topographical changes with age sheds light into the role of the lens in the optical properties of the eye and the lens aging mechanism.

“…The gradient index parameters were obtained using a reconstruction method based on optimization of optical path differences, described in detail elsewhere [23], which produced the following values: n c = 1.44319; n s = 1.37351; r pp = −3.52546 mm. Given the potential interactions between radius of curvature and asphericity [24], the differences between the nominal (measured) and simulated surfaces are given in terms of the RMS difference of their elevation. A summary of this simulation is illustrated in the top panel of Fig.…”

Section: Methodsmentioning

confidence: 99%

Distortions of the posterior surface in optical coherence tomography images of the isolated crystalline lens: effect of the lens index gradient

Borja¹,

Siedlecki²,

Castro³

et al. 2010

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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.