Abstract
Earlier studies have shown that the gradient index of refraction (GRIN) of the crystalline lens can be reconstructed in vitro using Optical Coherence Tomography (OCT) images. However, the methodology cannot be extended in vivo because it requires accurate measurements of the external geometry of the lens. Specifically, the posterior surface is measured by flipping the lens so that the posterior lens surface faces the OCT beam, a method that cannot be implemented in vivo. When the posterior surface is imaged through the lens in its natural position, it appears distorted by the unknown GRIN. In this study, we demonstrate a method to reconstruct both the GRIN and the posterior surface shape without the need to flip the lens by applying optimization routines using both on-axis and off-axis OCT images of cynomolgous monkey crystalline lenses, obtained by rotating the OCT delivery probe from -45 to +45 degrees in 5 degree steps. We found that the GRIN profile parameters can be reconstructed with precisions up to 0.009, 0.004, 1.7 and 1.1 (nucleus and surface refractive indices, and axial and meridional power law, respectively), the radius of curvature within 0.089 mm and the conic constant within 0.3. While the method was applied on isolated crystalline lenses, it paves the way to in vivo lens GRIN and posterior lens surface reconstruction.
Highlights
The crystalline lens is a biconic lens with a gradient refractive index (GRIN) that is known to peak near the lens center and decrease toward the periphery
We demonstrate a method to accurately reconstruct the gradient refractive index distribution and posterior lens surface using off-axis Optical Coherence Tomography (OCT) images [29] of the lens acquired in the anterior-up orientation, without the need for posterior-up images
The simulations show that off-axis data are essential for reconstructing the posterior lens surface and the GRIN distribution using OCT imaging, when the crystalline lens is imaged from only one orientation
Summary
The crystalline lens is a biconic lens with a gradient refractive index (GRIN) that is known to peak near the lens center and decrease toward the periphery. Information on the GRIN of the lens has been derived mostly from measurements on excised lenses using methods that either solve iteratively the ray equation [11,12,13,14,15] or optimize the variables of a model to fit the experimental data [16,17,18]. To apply these methods, which rely on precise measurements of the external geometry, the lens must be extracted from the ocular globe. The distortion introduced by the GRIN prevents accurate quantification of the posterior lens surface shape, its asphericity [19,20]
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