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Corneal hydration plays an essential role in maintaining optimal vision. During laser ablation surgery, corneal hydration varies greatly and is likely to affect the outcome. Quantitative measurements of this interaction may help improve the results of vision correction surgery. In addition, prescreening of corneal hydration could be used to correct the laser surgery procedure for hydration variation in the patient population.
We present a functional extension of Optical Coherence Tomography (OCT) to measure cornea hydration in vitro using two light sources simultaneously, one at 1294 nm (negligible water absorption loss) and another at 1410 nm (large water absorption loss). Measuring the ratio of the intensity profile at these two wavelengths allows us to separate the effect of absorptive attenuation from the reflectivity structure of the sample. We first measured the differential absorption coefficient of a calibration target: a 1 mm cuvette containing controlled mixtures of water (H2O) and heavy water (D2O). The optical properties of heavy water are almost identical with those of water, except that it has negligible absorption near 1410 nm. Next, we scanned in vitro fresh cornea bathed in Optisol. We then scraped off the epithelium and immersed the cornea into Balanced Salt Solution in order to increase the hydration through swelling. Then, the cornea was immersed in a 15% Dextran solution to reverse the swelling. After the OCT scans, the cornea hydration level was evaluated by standard weight measurement.
The result of the calibration experiment showed that a strong correlation exists between measured differential water absorption coefficient and actual water content within the cuvette. We derived the hydration level profile over corneal depth from a least squares fit of the log-intensity ratio. Average hydration from the OCT data agreed with the hydration determined by weight measurement.
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