Refractive index measurement of the isolated crystalline lens using optical coherence tomography

Abstract

An optical coherence tomography system has been developed that was designed specifically for imaging the isolated crystalline lens. Cross-sectional OCT images were recorded on 40 lenses from 32 human donors with an age range of 6 – 82 years. A method has been developed to measure the axial thickness and average refractive index of the lens from a single recorded image. The measured average group refractive index at the measurement wavelength of 825 nm was converted to the average phase refractive index at 589 nm using lens dispersion data from the literature. The average refractive index for all lenses measured was 1.408 ± 0.005 which agrees well with recent MRI measurements of the lens index gradient. A linear regression of the data resulted in a statistically significant decrease in the average refractive index with age, but a simple linear model was insufficient to explain the age dependence. The results presented here suggest that the peak refractive index in the nucleus is closer to 1.420, rather than the previously accepted value of 1.406.

Quantitative anterior segment optical coherence tomography

“…We provided the first report of lens surface shapes in vivo from OCT upon correction of optical (refraction) distortion, although (as previous reports based on Purkinje or Scheimpflug) we did not considered a GRIN in the lens. Borja et al [26] suggested that the optimal refractive index for the correction of OCT images was closer to the average refractive index [22] than to the equivalent refractive index. Our lens radii of curvature estimates in vivo (11.90±1.34 mm for the anterior lens and 6.86±0.63 mm for the posterior lens) fall within the ranges reported in the literature.…”

“…(7) Geometrical distances calculation and further denoising: The registration of the 3-D volumetric data sets involved shift, rotation and resampling using a Delaunay surface description for interpolation [21]. Optical distances were calculated by direct subtraction of the coordinates of the different surfaces, and geometrical distances were obtained by dividing the optical distance by the corresponding group refractive index (1.376 for the cornea, 1.336 for the aqueous humor, agedependant homogeneous refractive index for the crystalline lens of the subject in the study as per a previously published age-related expression [22], 1.430 for the Crystalens IOL, 1.486 for the PMMA ICRS). (8) Distortion corrections: In order to correct the interface layers for the effect of distortion in general (fan distortion and refraction), it is required to propagate the geometrical distances obtained from the previous point along the directional cosines of the ray, taking into account the refraction that occurs at each interface between two media of different refractive indices.…”

Reconstruction of the gradient refractive index of the crystalline lens with optimization methods

“…Uno de ellos utiliza la tecnología time-domain y fue desarrollado en el laboratorio de Jean Marie Parel y Fabrice Manns [46] en el Bascom Palmer Eye Institute (University of Miami, Estados Unidos), donde fue combinado con un stretcher para simular acomodación (no usado para los datos presentados aquí). Un segundo sistema desarrollado en el Laboratorio de Óptica Visual y Biofotónica en el Instituto de Óptica del CSIC (Madrid, España) en colaboración con la Copernicus University (Toruń, Polonia) [47] permite obtener imágenes a alta velocidad ya que utiliza la tecnología Fourier-domain y es usado para medidas in vivo e in vitro de córnea y cristalino.…”

“…Las superficies observadas en las imágenes de OCT contienen la información de la diferencia de camino óptico recorrido por los rayos al pasar por el cristalino. El espesor se puede calcular estudiando la información de la distorsión de la cubeta presente en ambas imágenes [46].…”

“…De esta forma, se pueden calcular la forma de las superficies anterior y posterior de cada cristalino sin distorsión y con la distorsión debido a la refracción en la primera superficie y en el GRIN del cristalino. El espesor se puede calcular estudiando la deformación de la cubeta en las imágenes [46]. Una vez la forma es conocida y se tienen datos de diferencia de camino óptico, el algoritmo de optimización puede encontrar la combinación de variables que mejor simulan la diferencia de camino óptico medida.…”