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L. N. Davies, M. C. M. Dunne, J. S. Wolffsohn; Validity of Anterior Segment Biometric Measurements Made With Optical Coherence Tomography. Invest. Ophthalmol. Vis. Sci. 2007;48(13):3858.
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Physical model eyes were used to evaluate Optical Coherence Tomography (OCT; Visante, Carl Zeiss Meditec Inc., Jena, Germany) measurements of anterior and posterior corneal surface curvature, corneal thickness and anterior chamber depth.
Five previously validated physical model eyes were used. Model corneae were of constant thickness (0.80 mm) and refractive index (1.493) while the radii of curvature of the spherical anterior (6.77, 7.79 and 8.72 mm) and posterior (6.26, 7.39 and 8.06 mm) surfaces varied. Model anterior chamber depths also varied (1.88, 2.56, 2.59, 2.63, 4.05 mm) while the refractive index of the media (1.333) remained constant. OCT measurements were made using the anterior segment single image capture mode. In built OCT software was used to correct the reconstructed image using detection of the anterior and posterior corneal surface. Measurements using the instrument’s imaging tools were taken. The instrument’s calibrated images were also exported and a Liberty BASIC 4.0 (Shoptalk Systems, Framingham, MA, USA) program was written to measure coordinates of both corneal surfaces to subpixel precision. This allowed calculation of apical radii and conic constants for each surface. Surface curvatures and intraocular distances were compared (mean ± SD, range) to the known model eye values.
OCT measurements of both corneal surfaces were typically flatter centrally with greater peripheral steepening than the true values. Errors arising at chord diameters of up to 10 mm were 70 ± 74 µm (range: 0 to 210 µm) for the anterior corneal surface and 81 ± 76 µm (range: 0 to 208 µm) for the posterior corneal surface. Corneal thickness was consistently overestimated by 60 µm while anterior chamber depth was typically overestimated by 5 ± 86 µm (range: -146 to 70 µm).
These errors need to be considered in relation to the use of OCT data for ophthalmic surgery. Improvements could be made to the calibration software to minimise the detected errors.
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