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L. M. Johnson, A. Raghuram, M. Rondon, N. J. Coletta; Foveal Pit Dimensions as a Function of Refractive Error: Effects of Transverse Magnification in a Fourier-domain OCT Instrument. Invest. Ophthalmol. Vis. Sci. 2009;50(13):358.
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© ARVO (1962-2015); The Authors (2016-present)
Lateral retinal dimensions measured with OCT are susceptible to errors resulting from the variation in transverse magnification across refraction. We investigated the effect of transverse magnification on measurements of the foveal pit width in 20 eyes with refractions ranging from plano to -10.60 D.
Retinal thickness was measured with an Optovue RTVue FD-OCT, using the MM6 radial scan pattern that consists of twelve 6-mm line scans and provides a nominal transverse resolution of 5.88 µm/pixel. Minimum foveal thickness and maximum thickness on either side of the foveal pit were determined for each of the 12 scans, using custom software. The foveal pit width, taken at half its depth (FWHM) and across the maximum thickness points (FWFM), was examined as a function of refraction. The foveal pit depth was estimated as the difference between the minimum and average maximum thickness. The OCT magnification was measured using an artificial eye with adjustable axial length. Line scans of a millimeter scale in the artificial eye’s retinal plane showed a decrease in transverse magnification as axial length, and hence myopia, increased (r=0.99; p<0.0001). This function was used to correct the foveal pit width data. Axial thickness measurements in the artificial eye were not affected by its axial length.
Without correction for transverse magnification, both the FWHM (r=0.61; p=0.004) and FWFM (r=0.52; p=0.018) values decreased significantly as myopia increased. With magnification correction, the FWHM decreased significantly with increasing myopia (r=0.54; p=0.015) while the FWFM trended lower with increasing myopia (r=0.40; p=0.09). The average magnification-corrected values for FWHM and FWFM were 883.3µm ±115.5 s.d. and 2270.4µm ±198.8 s.d., respectively. The decrease in foveal pit width in myopia may be due to the decreasing depth of the foveal pit. Foveal pit depth became shallower with increasing myopia (r=0.48; p=0.03). The magnification-corrected FWHM was significantly and positively correlated with the foveal pit depth (r=0.60; p=0.0056).
Myopes showed a decrease in foveal pit width that may be associated with the shallower foveal pit in myopia. Transverse magnification of the OCT exaggerated the correlation between foveal pit width and refraction.
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