Nuclear magnetic resonance micro-imaging of the human eye lens

The exact causes of presbyopia and the development of senile cataract are still unclear. In this project magnetic resonance microscopy (MRM) has been used to study the kinetics of long-range water transport, anisotropic diffusion and refractive index distribution in the human eye lens. A decline in the transpmi process, changes to the nature of anisotropic diffusion and changes to the refractive index distribution in the lens may all be related to the onset of presbyopia and cataract. The results of the long-range water transport experiments showed that as lenses age there is reduction in the rate at which water (and presumably also water soluble low molecular weight metabolites) can enter the cells of the lens nucleus via the epithelium and cortex. Since this is the mechanism by which nuclear cells can obtain nutrients and anti-oxidants to protect the crystallins from degradation, the decrease in transport rates could lead to increased damage to lenses with age, and ultimately a potential cause of presbyopia and senile cataract. Using a modified pulsed field gradient spin echo (PFGSE) sequence, diffusion tensor maps of human lenses were acquired to study the nature of anisotropic diffusion. The nature of diffusive transport as measured by this technique was found to be related to the known morphology o the human lens. A barrier to diffusion in a region surrounding the lens nuclei was observed. Changes to the permeability of this barrier with age were also found to be significant. This may be partly responsible for degeneration of lens function and contribute to presbyopia and senile cataract. A Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence was used to study the nature of transverse relaxation in lenses and lens homogenates. The rate of this relaxation (R2) was found to be linearly related to the refractive index. The refractive index distribution in intact human lenses was then acquired by obtaining spatial image maps of R2 using a modified CPMG imaging sequence. The results ofthese measurements show subtle but potentially important differences in refractive index distribution from those currently used in models of the lens refractive index gradient. The results also provide new insight into changes in the refractive index distribution with aging. These changes may contribute to presbyopia and help to explain the "lens paradox".