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M. L. Gabriele, H. Ishikawa, B. Dilworth, G. Wollstein, R. A. Bilonick, L. Kagemann, K. Sung, K. A. Townsend, J. G. Fujimoto, J. S. Schuman; Optical Coherence Tomography Scan Circle Location Affects Mean Retinal Nerve Fiber Layer Thickness Measurements. Invest. Ophthalmol. Vis. Sci. 2007;48(13):517.
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To investigate the effect of the standard 3.4mm diameter circle location on mean retinal nerve fiber layer (RNFL) thickness measurements using high-speed ultrahigh resolution optical coherence tomography (hsUHR-OCT).
Fifteen healthy subjects had hsUHR-OCT raster scanning of the optic nerve head (ONH) region (501x180 samplings in a 6x6 mm region around the ONH; scan time 3.84 seconds). RNFL thickness was measured on this three-dimensional dataset using a software program of our own design. RNFL thickness was resampled along 3.4mm diameter circles starting at the ONH center, then along 3.4mm diameter circles shifted horizontally (x-shift) and vertically (y-shift) up to ±500µm with a 100µm interval. All measurements were normalized to mean thickness at the starting position for all subjects to permit an intersubject comparison. A range of circle placement shifts that corresponded to the variability inherent to the commercial device (2.68µm) was determined.
Mean RNFL thickness tended to decrease with both positive and negative x- and y- shifts (quadratic fit, R2=0.40 and R2=0.43, respectively, both p<0.0001) (Figure). The ranges of shifts that caused a decrease of 2.68µm from the maximum value on the fitted quadratic were ±320µm for the horizontal shift and ±280µm for the vertical shift.
Scan circle location affected mean RNFL thickness measurements. However, within a shift range of ~±300µm, the change in mean RNFL thickness was within the measurement error reported for the commercial unit.
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