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Sebastien Besner, Giuliano Scarcelli, Roberto Pineda, Seok H Yun; Characterization of cornea biomechanics by Brillouin microscopy: Instrument and initial in vivo data. Invest. Ophthalmol. Vis. Sci. 2014;55(13):3718.
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© ARVO (1962-2015); The Authors (2016-present)
To develop and test Brillouin microscopy for mapping the biomechanical properties of the human cornea in vivo.
An off-axis confocal Brillouin microscope employing real-time eye pupil detection and beam registration was developed. The light source was a laser providing near-infrared light at 780 nm with 750 µW on the cornea. The longitudinal elastic modulus was obtained by analyzing the measured Brillouin frequency shift as the focus of the beam was scanned with an integration time of 0.4 s per point. Central elasticity profiles were acquired in 10 healthy eyes. Enlarged elasticity maps of the cornea over a half disk of 5 mm in diameter were acquired to assess the lateral variability. A set of analysis metrics, such as Brillouin modulus extrema, Brillouin anterior mean modulus and anterior and posterior axial slopes, were defined.
A high lateral and axial resolution of 5 µm and 40 µm, respectively, was achieved in the cornea. The off-axis scan reduced the spurious specular reflection from the corneal surface by 30 dB in comparison to epi-detection configuration, allowing full-depth imaging. The accuracy and repeatability of lateral beam positioning were about 45 µm and 120 µm, respectively. The frequency shift measurement error was about 22 MHz, which corresponds to a relative error of <1% in longitudinal modulus. Along the depth, the Brillouin frequency shift decreased from 5.66±0.04 GHz at the epithelium to 5.23±0.03 GHz at the cornea/aqueous humor interface. The Brillouin modulus slope was measured to be -0.035±0.044 GHz/mm in the first 400-460 µm layer but the decrease was much steeper in the last 70-120 µm part. Along the lateral dimension, Brillouin modulus was homogeneous within the measurement error over an area of 5 mm in diameter in the central cornea.
Brillouin microscopy can measure the longitudinal elastic modulus in the human cornea safely with a three-dimensional resolution. The measured data from healthy subjects will serve as the baseline in the future clinical study that aims the validation of the technology for early diagnostic of keratoconus.
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