Abstract
Purpose: :
To retrieve a 3D gradient index (GRIN) distribution of human crystalline lenses in vitro from Optical Coherence Tomography (OCT) imaging.
Methods: :
The GRIN distribution of human donor lenses was estimated from OCT images by means of an optimization method based on genetic algorithms. The 3D input data (measured by OCT) are the optical path differences through the lens and the 3D shape of the anterior and posterior crystalline lens surfaces. The collected images of the lens were processed and segmented. The resultant points were corrected from fan distortion and converted into spatial units. The 3D cloud of points was fitted by Zernike polynomials up to 6th order. In a 6-mm pupil size the method searched for the parameters of a 4-variable GRIN model that best fits the distorted posterior surface of the lens in 18 different meridians. The GRIN model was described by the exponential variation in the refractive index from the nucleus to the surface, in both the axial and meridional directions. The center of the GRIN lies in the intersection of the optical axis and the equatorial plane. The method was demonstrated on an isolated human crystalline lens (67 years, 12h post-mortem). Measurements were performed with the lens immersed in DMEM preservation solution at room temperature. Images were collected using a custom-built spectral domain anterior segment OCT system. The collected data consisted of 1668 A-scans x 60 B-scans covering a zone of 12x12mm, and a total axial range of 14mm.
Results: :
The method allowed 3D reconstruction of the GRIN of a human crystalline lens in vitro. Due to possible astigmatism, meridional variations in lens shape were considered. The estimated radii of curvature were 11.0±0.31mm and 5.56±0.03mm, and the asphericities 3.4±2.9 and -0.34±0.25 for anterior and posterior surface, respectively. The estimated lens thickness was 4.97mm and the average refractive 1.40. The estimated GRIN distribution from 1.40 in the nucleus to 1.38 in lens, with an axial exponential decay of the GRIN profile of 6.4 and a meridional exponential decay of 5.4±1.9.
Conclusions: :
A new method for GRIN reconstruction based on OCT imaging (with high axial and lateral resolution) and a global search algorithm has been applied successfully to estimate the GRIN distribution in an isolated crystalline lens in 3D. The central plateau and relatively high surface index is consistent with earlier estimates from MRI.