Evaluation of fractional anisotropy in differently aged human lenses measured by UHF-MRI at 7 Tesla

Abstract

Purpose : Presbyopia is predominantly caused by an increase in lens stiffness and clinically affects every human individual older than 45. The aim of this study was to gain insights into age dependent changes in human lens biomechanics, accompanying accommodation loss in presbyopia. For this purpose, diffusion rates and directions in human lenses were analyzed by means of ultra high-field magnetic resonance imaging (UHF-MRI).

Methods : Intracapsular lens extractions were performed directly after enucleation. Optically clear lenses (n=43) were weighed, photographed, and embedded in cooled culture medium, supplemented with 0.5% agarose, followed by UHF-MRI (7 Tesla, BioSpec 70/30, Bruker, Germany). T2-weighted Turbo-RARE sequences (Resolution: 75x75x800 µm³) and histological sections were used to identify anatomical characteristics. Spin-echo based diffusion sequences (6 directions, 4 b-values between 100-1000 s/mm²) were used to obtain apparent diffusion coefficient (ADC) and fractional anisotropy (FA) values.

Results : Lens weight increased with age from 0.19 g ± 0.016 g (31-40 years) to 0.29 g ± 0.030 g (81-90 years). Diffusion rates (ADC values) of individual lenses decreased with age in the cortex region, whereas they remained constant in the nucleus (~0.5*10^-3 mm²/s). FA maps revealed restricted radial orientations of diffusion in the anterior and posterior lens poles converting to strictly perpendicular directions in the equatorial region of the lens. This indicates that diffusion of water follows the anatomical characteristics in the human lens and is substantially constrained by the outer membranes of the lens fibre cells. In the cortex region FA values declined with age. In the nucleus, where FA was age independent, lowest values were observed in the center, demonstrating a more isotropic diffusion.

Conclusions : Lifelong lens growth has influences on spatial ADC and FA values, which were age dependent in the cortex region and age independent in the nucleus. Thus, the clinically evident age dependent hardening of the nucleus is not correlated with changes in the ADC and FA values. Tendency to a more isotropic diffusion in the cortex region with age may explain mechanisms to overcome age dependent nucleus hardening by increased diffusion based nutrition of the nucleus.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.