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R. Yadav, K. Lee, J. Rolland, J. Zavislan, J. Acquavella, G. Yoon; High Resolution Ocular Surface OCT to Directly Measure Tear Film Thickness in Human Eyes. Invest. Ophthalmol. Vis. Sci. 2010;51(13):1204.
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© ARVO (1962-2015); The Authors (2016-present)
To develop a spectral domain optical coherence tomography (OCT) system which allows for direct measurements of the tear film thickness of the human eye non-invasively.
The system features a supercontinuum light source having a spectrum from 350nm to 2400nm. The 600nm to 1000nm window of the spectrum was selected for imaging. The prototype system was assembled in free space. To detect such a large bandwidth, a spectrometer based on Czerny Turner configuration was designed and used. To evaluate imaging performance of the system, a 2µm thick nitrocellulose membrane was first imaged. A model cornea with 7.8 mm radius of curvature was also fabricated with Polydimethylsiloxane (PDMS) (refractive index = 1.4) and the tear film was simulated by instilling water drops on top of the artificial cornea. The tear film thickness changes due to evaporation were measured with the OCT at every 3 sec for 90 seconds.
Although theoretical axial resolution for the OCT system was calculated to be 0.7µm in water, the actual resolution measured experimentally with a mirror in the sample arm was 2µm (full width at half maximum of the measured PSF) in water. The reduction in resolution is due to imprecise k-space remapping which could be corrected by calibrating the spectrometer more accurately. A 2µm thick nitrocellulose membrane was imaged successfully and the two interfaces were clearly visualized. For the model eye, the system was able to measure the tear thinning up to 2µm. The tear thinning rate was observed to be 0.09µm/s at approximately 25% humidity.
Our prototype spectral domain OCT system has shown the capability to resolve the 2 µm artificial tear film. An improvement to the theoretically expected resolution is possible after optimizing spectrometer performance. Our ocular surface OCT has demonstrated the feasibility of imaging the human tear film non-invasively and could enhance our understanding of tear film dynamics and its role in the mechanism of tear film breakup for dry eye research.
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