Influence of signal-to-noise ratio, glaucoma stage and segmentation algorithm on OCT usability for quantifying layer thicknesses in the peripapillary retina.

Abstract

OCT can be used for glaucoma assessment, but its usefulness may depend on image quality, disease stage and segmentation algorithm. We aimed to determine how layer thicknesses as assessed with different algorithms depend on image quality and disease stage, how reproducible the algorithms are, and if the algorithms (dis)agree. Optic disc OCT data (Canon OCT-HS100) from 20 healthy subjects and 28 early, 29 moderate, and 23 severe glaucoma patients were assessed with four different algorithms (CANON, IOWA, FWHM, DOCTRAP). We measured retinal nerve fibre layer thickness (RNFLT) and total retinal thickness (TRT) along the 1.7-mm-radius OCT measurement circle centred at the optic disc. In healthy subjects, image quality was degraded with neutral density filters (0.3-0.9 optical density [OD]); three scans were made to assess repeatability. Results were analysed with ANOVA with Bonferroni corrected t-tests for post hoc analysis and with intraclass correlation coefficient (ICC) analysis. For all algorithms, RNFLT was more sensitive to image quality than TRT. Both RNFLT and TRT showed differences between healthy and glaucoma (all algorithms p < 0.001 for both RNFLT and TRT) and between early and moderate glaucoma (RNFLT: p=0.001 to p=0.09; TRT: p < 0.001 to p=0.03); neither was able to discriminate between moderate and severe glaucoma (p=0.08 to p=1.0). Generally, repeatability was excellent (ICC >0.75); agreement between algorithms varied from moderate to excellent. OCT becomes less informative with glaucoma progression, irrespective of the algorithm. For good-quality scans, RNFLT and TRT perform similarly; TRT may be advantageous with poor image quality.