Refractive Index of the in-vivo Human Crystalline Lens Measured using Whole-Eye Optical Coherence Tomography

Abstract

Purpose: : To quantify the mean axial refractive index of the in vivo human crystalline lens using Optical Coherence Tomography (OCT) images of the whole eye acquired during lens accommodation.

Methods: : The method described by Van Der Heijde et al (Optometry and Vision Science, Vol 66, No. 12, pp 830-833 1989) to calculate the speed of sound in the lens from ultrasound A-scans acquired during accommodation was adapted to measure the mean axial group refractive index of the lens from OCT images recorded during accommodation. Whole-eye OCT images were recorded in 3 subjects (21, 24, 35 years) with a custom built spectral-domain OCT system with 7 micron axial resolution. The system has a depth range of 10.4 mm in air and the capability to capture 3 consecutive frames at different depths covering the anterior segment and the retina. The OCT is capable of simultaneously imaging the whole anterior segment and the retinal boundaries in real-time during accommodation. Seventy-five images were acquired at 20 frames per second as the subject responded to a 10D accommodation stimulus. A quantitative analysis was performed to extract the anterior chamber depth, crystalline lens thickness and vitreous depth in each image from the central A-line intensity profile. The group refractive index of the lens was determined by calculating the slope of the plot of crystalline lens thickness versus combined optical thickness of the anterior chamber and vitreous measured in each image.

Conclusions: : The study demonstrates the feasibility of determining the mean index of refraction of the crystalline lens using extended-depth OCT in young eyes. The in vivo group refractive index is comparable to previously published values measured in vitro.

Keywords: anterior segment • imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • accommodation