The power of the eye lens, unlike a conventional glass lens, derives mainly from a gradient in refractive index through the lens, rather than from refraction at the surface. However until now it has not been possible to measure the refractive index distribution non-invasively.

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Refractive index of the eye

MR images of the eye in (a) unaccommodated and (b) accommodated states.

In human lenses, the refractive index varies from approximately n=1.37 at the surface to n=1.42 at the lens centre. The high refractive index of the lens stems from a high concentration of lens proteins (crystallins) in the lens fibre cells. The human lens also has the ability to change its power in order to focus objects at different distances. This process of 'accommodation' involves the use of ciliary muscles to change the lens shape (Fig. 1).

Distractive Index distribution graph

Figure 2. New technique to measure changes in refractive index distribution with age and state of accommodation.

We have developed a novel MRI method for measuring the refractive index through the lens without making any assumptions about the form of the distribution. The method is based on the fact that the transverse relaxation of water protons in the lens (T2) depends on protein concentration and hence can be used to measure refractive index.

We have recently applied the new technique to measure changes in refractive index distribution with age and state of accommodation (fig. 2). The results provide an improved understanding of 'presbyopia' - the loss of accommodation with age, and may assist in improving the design of artificial lenses used in cataract surgery, as well as novel lenses aimed at restoring accommodation.

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