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M.L. Gabriele, H. Ishikawa, G. Wollstein, R.A. Bilonick, L. Kagemann, M. Wojtkowski, J.S. Duker, V. Srinivasan, J.G. Fujimoto, J.S. Schuman; Is a 3.4mm Diameter Appropriate for Optical Coherence Tomography Nerve Fiber Layer Analysis? . Invest. Ophthalmol. Vis. Sci. 2006;47(13):3348.
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© ARVO (1962-2015); The Authors (2016-present)
To test the hypothesis that the standard 3.4 mm diameter circumpapillary scan measures retinal nerve fiber layer (NFL) thickness in a region of low NFL thickness variability.
We obtained two–dimensional NFL thickness measurements on raster scan data (501x180 axial scans in a 6x6 mm region, acquisition time ∼4 seconds, centered on the optic nerve head (ONH)) using high–speed, ultrahigh–resolution OCT (OCT with Fourier / spectral domain detection) in twelve healthy subjects. Criteria for acceptable images were no eye movements or blinking and consistent signal quality across the scan. NFL thickness was measured on this three–dimensional data set using a software program of our own design. The disc margin was defined subjectively in each image and average NFL thickness relative to the distance from the center of ONH was computed for each quadrant. Spline curves were fit to NFL thickness profile graphs in order to find the point at which the derivative of the curve was 0, which represented the least variable location.
NFL thickness data showed consistent characteristics: a plateau of high NFL thickness was seen near the disc margin in both the superior and inferior quadrants, while NFL thickness decayed without a plateau in the temporal and nasal quadrants. The radial location of the point of most prominent NFL thickness in each quadrant showed significant correlation with disc size except for superior quadrant (r=0.66, p=0.02 (temporal); r=0.30, p=0.33 (superior); r=0.79, p=0.002 (nasal), and r=0.76, p=0.004 (inferior)). The distance from the least variable location to the 3.4 mm circle sample location was significantly smaller than half of the plateau size in both superior and inferior quadrants (0.13 ± 0.11 vs 0.29 ± 0.08 (superior) and 0.16 ± 0.13 vs 0.26 ± 0.08 mm (inferior), both p<0.008, paired t–test). The 3.4 mm circle was within the plateau for most cases.
The standard fixed 3.4 mm diameter circle for OCT NFL assessment sampled a location of low NFL thickness variability despite the fact that the radial location of the point of most prominent NFL thickness was proportional to disc size in 3 out of 4 quadrants.
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