Measurement and imaging of water concentration in human cornea with differential absorption optical coherence tomography.

We present measurements of water absorption in human cornea in vitro with a differential absorption optical coherence tomography (DAOCT) technique. This technique uses two OCT images recorded simultaneously with two different light sources, one centered within (1488nm) and one centered outside (1312nm) of a water absorption band. We investigated the cornea under different conditions: First, a series of OCT images was recorded at different hydration states of the cornea, starting from a normally hydrated cornea to an almost completely dehydrated cornea. To investigate the influence of scattering on our measurements, the dehydrated cornea was re-hydrated with Deuterium oxide, which shows similar optical properties like water, but negligible absorption in the used wavelength region, and a similar series of OCT images was recorded. For a quantitative analysis, we averaged the OCT signals over adjacent A-Scans and performed a linear regression analysis of the logarithmic OCT signals versus imaging depth in the cornea for each wavelength. The difference of the slopes corresponds to the difference in the absorption coefficient, if the difference in the scattering coefficient is negligible. With the known difference in the absorption cross section it is possible to calculate the mean water concentration of the cornea.