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Ashik Mohamed, Marco Ruggeri, Eduardo Martinez-Enriquez, Pablo Perez-Merino, Siobhan Williams, Yue Yao, Bianca Maceo Heilman, Alberto De Castro, Virender Singh Sangwan, Robert C Augusteyn, Jean-Marie A Parel, Arthur Ho, Susana Marcos, Fabrice Manns; Quantifying crystalline lens geometry and optical properties by combined optical coherence tomography & laser ray tracing. Invest. Ophthalmol. Vis. Sci. 2017;58(8):2715. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To quantify geometry and optical properties of crystalline lenses ex vivo by using a combined optical coherence tomography (OCT) & laser ray tracing (LRT) system.
3-D OCT images (Fig 1A) of ex vivo crystalline lenses (maximally stretched with a motorized lens stretcher in a tissue chamber filled with BSS) of a 4.2 y/o cynomolgus monkey were acquired using a commercial SD-OCT (Bioptigen Inc.) coupled to a custom delivery system designed for on- and off-axis imaging. Custom algorithms for image segmentation and distortion correction were applied to quantify lens shape (Fig 1B). Assuming a constant index of refraction for the lens (n=1.420@880nm), computational ray tracing (Zemax) was applied to the OCT-based lens model to predict on-axis and off-axis aberrations. A custom LRT system used the OCT unit for light delivery and a CMOS camera mounted on motorized stages below the lens acquired the spots at different axial positions. A program was developed to calculate ray slopes from the spot positions, which enabled to calculate Zernike wavefront coefficients using a least square curve fitting algorithm. Measured and predicted lens aberrations were compared to assess the impact of lens shape on aberrations and quantify the contribution of lens gradient refractive index (GRIN).
Fig 2 summarizes the image analysis results. LRT-based lens aberrations were dominated by astigmatism (1.64/1.74 µm, 23%/20%), spherical aberration (0.42/0.82 µm, 5%/8%) and coma (1.53/1.34 µm, 22%/19%), on-axis/off-axis (20°) respectively for 6-mm pupils. The simulated aberration terms overestimated the LRT-measured ones by a factor of 1.41/1.47 (astigmatism), 1.31/1.45 (spherical) and 1.09/1.86 (coma) on-axis/off-axis.
LRT-OCT can be utilized to characterize the geometrical and optical properties of the crystalline lens ex vivo. Shape and GRIN both contribute to the optical aberrations of the lens. GRIN has a compensatory role on the spherical aberration of the lens and attenuates the impact of high order aberrations off-axis. Combined information of lens shape and optical aberrations at different angles can be used to reconstruct GRIN.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
Fig 1: A. OCT image acquired on one of the two lenses which was oriented with posterior surface (up) facing the beam. B. OCT-based anterior (purple) and posterior (blue) lens surfaces reconstruction.
Fig 2: Biometry of stretched cyno lenses
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