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Sergio Ortiz, Damian Siedlecki, Pablo Perez-Merino, Susana Marcos; Anterior Segment Optical Coherence Tomography (OCT): From Nice Images to Accurate Topography. Invest. Ophthalmol. Vis. Sci. 2011;52(14):3022.
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© ARVO (1962-2015); The Authors (2016-present)
To provide a methodology for quantitative corneal topography measurements from anterior segment Optical Coherence Tomography (OCT) systems.
A protocol was developed for correction of fan distortion in anterior segment OCT and calibration of the spatial axial and lateral coordinates. OCT images of a calibrating grid surface were acquired at different axial positions and integrated to generate 2-D images. Axial and lateral euclidian coordinates were obtained for the relationship between the locations of the nodes of the grid image (obtained from the intersection of the distorted lines, fitted to quadric functions) and their actual position in space. The magnitude of axial distortion is estimated from the deviation of the surface from a plane, in instrument coordinates. Images were acquired using a custom-developed spectral anterior segment OCT provided with software tools for surface denoising and segmentation. Measurements were performed on spherical and aspheric test surfaces, and on 5 normal subjects in vivo. Acquisition protocols for fast image acquisition were developed to minimize the impact of motion artifacts: 360 A-Scans x 50 B-Scans (12 x 10 mm) in 0.72 s. The resolution of the images was 0.1 x 0.2 mm x 3.5 µm. Topographic data from were compared to non-contact profilometry and Placido ring videokeratoscopy and Scheimpflug imaging in samples and real eyes, respectively. Elevation maps were fitted by conics (described by the radius of curvature R and asphericity Q) and 8th degree Zernike polynomial expansions.
Correction of the calibrated fan distortion decreased the error in the estimation of R from 19.8% to 1.1% in spherical test surfaces, and from 4.6% to 1.6% in aspheric surfaces, and Q from 130 to 5.0 %. In real eyes, the corrected R and Q were not statistically significantly different from those obtained from commercial corneal topographers, although significant differences were found in the Zernike elevation maps (particularly in astigmatisc terms Z-22 and Z22)
Quantitative corneal topography can be obtained from anterior segment OCT after calibration and correction of fan distortion. Accurate full anterior segment biometry from OCT will be of great application in the clinic.
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