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X. Zhang, O. Tan, D. Huang; The Effect of Signal Strength in the Measurement of Ganglion Cell Complex and Nerve Fiber Layer in Fourier-Domain Optical Coherence Tomography. Invest. Ophthalmol. Vis. Sci. 2010;51(13):235.
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The thickness of peripapillary retinal nerve fiber layer (NFL) and macular ganglion cell complex (GCC) is measured by Fourier domain optical coherence tomography (FD-OCT) for glaucoma diagnosis. The purpose of this study is to determine the effect of OCT signal strength on these measurements
The study analyzed data from participants enrolled at the Doheny site of the longitudinal Advanced Imaging for Glaucoma Study (www.AIGStudy.net). An FD-OCT system (RTVue) was used to map GCC and NFL thickness 3 times on each study visit. The overall average GCC or NFL thickness from each OCT scan was analyzed. Each measurement was associated with a signal strength index (SSI) on a scale from 0 to 100 (higher is better) generated automatically by the device. Signal levels are stratified into 20 bins by SSI intervals of 5. Within each visit, measurements of the same SSI bin were used to calculate the intravisit standard deviation (SD) for that SSI level. Repeatability from different visits and eyes were combined by pooled SD. Within each visit, thickness measurements with the largest SSI difference were selected and the thickness difference calculated. The effect of SSI on measured NFL and GCC thickness were then assessed using linear regression.
The analysis included 65 eyes (33 participants) from normal group; 47 eyes (36 participants) from glaucoma suspect or pre-perimetric glaucomatous (GSPPG) group; and 88 eyes (57 participants) from perimetric glaucoma (PG) group. Each eye was measured on 1 to 6 visits (average 3.2). Repeatability was generally better for images with higher SSI. For GCC, the repeatability is significantly better for data with SSI > 45 (SD 1.44 vs. 2.39 micron, p<0.01). For NFL, 35 appeared to be the best cutpoint but no significant difference in repeatability was observed. Measurements with SSI above the chosen cutpoints were then analyzed to assess the effect of SSI on NFL and GCC measurments. The effects were consistent among three groups. For GCC, each unit SSI increase increased measured thickness by 0.05 micron (p =0.043). For NFL, the slope was 0.11 per unit SSI.
Adequate signal strength is important for accurate and repeatable FD-OCT measurements used in glaucoma diagnosis. For the RTVue instrument, we recommend SSI > 45 for GCC measurements and SSI > 35 for NFL measurements. To track glaucoma by detecting changes in GCC and NFL thickness over time, the effect of varying signal strength at different visit could be compensated using a regression model.
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