Corneal hydration imaging using dual-wavelength optical coherence tomography

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 during surgery 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 depth profile at these two wavelengths allows us to separate the effect of absorptive attenuation from the reflectivity structure of the sample. We measured the differential absorption coefficient of a calibration target: a 1 mm cuvette containing controlled mixtures of water (H ₂O) and heavy water (D ₂O). Next, we scanned in vitro fresh cornea under simulated different physiologic conditions. The result of the calibration experiment showed that a linear 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.