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C.J. Roberts, A.M. Mahmoud; The Impact of Spatial Resolution on Zernike Reconstruction of Spherical Aberration . Invest. Ophthalmol. Vis. Sci. 2005;46(13):4648.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To determine the impact of spatial resolution on the accuracy of representing spherical aberration using a 5th order Zernike reconstruction. Methods: A retrospective study of 120 eyes of 60 patients who received Keratron Scout corneal topography (Optikon, Rome, Italy) pre–operatively and six months post–LASIK using one of three lasers (Bausch&Lomb Technolas 217A, Autonomous LADARVision 4000, and the VISX Star S3) was conducted. At least two scans were acquired per eye at each time point. The data were then exported for processing. The multiple scans at each time point were averaged and reprocessed to ensure that the elevation, tangential, and axial data remain consistent. Zernike decompositions were performed ranging in order from 5th to 12th over a 6mm region of interest (ROI) and the surfaces were reconstructed from the resulting truncated series polynomials. The curvature of the surfaces were calculated over the ROI and compared to the reprossessed curvature in 0.5mm zones. The RMS error of each zone was calculated and then summed together. The Zernike polynomial with the smallest sum was deemed "optimized" and served as the gold standard. The 6mm optimized surfaces were then resampled in a Cartesian grid at 5 spatial resolutions: 10x10, 25x25, 50x50, 75x75, and 100x100. The surfaces were reconstructed using 5th order Zernike Polynomials. The resulting 4th order spherical aberration terms of the resampled surfaces were statistically compared to the known spherical aberration terms of the optimized surfaces, as a function of spatial resolution. Results: Using a 10x10 sampling grid, the 4th order spherical aberration terms were significantly different (p<0.0001) from the known values in a 5th order Zernike reconstruction for both pre–op and post–op maps. Only post–op maps showed a significant difference (p = 0.009) for the 25x25 sampling grid. Although statistically significant differences from the known values of spherical aberration were found for both the pre–op and post–op maps (p<0.005) using the 50x50 and the 75x75 sampling grids, the differences were not clinically meaningful (< 0.015µ). No significant differences were found using the 100x100 sampling grid. Conclusions: A 10x10 sampling grid represents inadequate spatial resolution for accurate reconstruction of spherical aberration, using 5th order Zernike polynomials. This is similar to the spatial resolution of some commercially available wavefront sensors. Resolution of 50x50 and higher is adequate for accurate reconstruction of spherical aberration.
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