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A. N. Kuo, M. Zhao, J. A. Izatt; Corneal Aberration Measurement With Three-dimensional Refraction Correction for High-speed Spectral Domain Optical Coherence Tomography. Invest. Ophthalmol. Vis. Sci. 2009;50(13):3671.
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For meaningful analysis of the cornea from spectral domain optical coherence tomography (SD-OCT) data, it is imperative to account for the effects of refraction at the epithelial and endothelial surfaces. We present an algorithm for 3D refraction correction based on a vector representation of Snell’s law which accounts for refraction of OCT light in the cornea, use this algorithm to dewarp raw SD-OCT images, and reconstruct the true position of the corneal endothelial surface. Corneal aberrations are then determined.
Implementation of Snell’s law in 3D requires knowledge of the surface normal at the point the incident light strikes the curved epithelial surface. We first calculate the partial derivatives of the epithelial surface using local fitting. Then the surface normal in 3D, irrespective of the plane of the original B scan, is calculated for each point. The refraction-corrected volumetric dataset is then generated by local application of Snell's law. From this corrected dataset, a ray-tracing method and subsequent Zernike polynomial fitting (7th order, 36 terms) are used to determine the aberrations of the anterior and posterior corneal surfaces. This method was used to process a corneal SD-OCT dataset in MATLAB (MathWorks; Natick, MA). The dataset was from a SD-OCT system using a 1310 nm source to scan a healthy volunteer’s cornea; volumetric images were 1024 pixels x 400 pixels x 60 pixels (axial x lateral x elevation) covering 14.5mm x 7.5mm x 8.5mm in 4.5 seconds.
The figure shows the z position difference between the uncorrected and 3D corrected endothelial surfaces. The minimum difference was at the apex. The spherical aberration of the anterior and posterior surfaces were calculated to be 1.1um and -0.67um respectively over a corneal aperture of 7.5mm.
Using a 3D implementation of Snell’s law, we have developed an algorithm to produce refraction corrected representations of the corneal endothelial surface in a 3D reconstruction of the cornea from SD-OCT data. This procedure will allow for a more accurate analysis of corneal surface elevations and aberrations for clinical usage.
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