Effect of Local Signal Quality on Retinal Nerve Fiber Layer Thickness Using Time Domain Optical Coherence Tomography

Abstract

Purpose: : Time-domain optical coherence tomography (TD-OCT; Stratus OCT, Carl Zeiss Meditec, Dublin, CA; software version 4.0) uses a global signal strength index to quantify scan quality. The purpose of this study was to examine the relationship between global and local signal quality, and to explore how variation between these parameters might effect retinal nerve fiber layer (RNFL) thickness measurements.

Methods: : Three good quality (global signal strength ≥ 6) Fast RNFL scans were performed in a single visit for fourteen healthy subjects (28 eyes). Quality Index (QI) values and RNFL thickness were computed at global, quadrant, and clock hour levels. Linear mixed effects modeling was applied to evaluate the quality variance across the group and the relationship between RNFL thickness and QI.

Results: : The average global signal strength was 7.50 [95% confidence interval (5.99-9.01)] and the mean global QI was 41.45 (36.87-46.03). Using global QI variance as a reference, the quadrant QI variance was 2.71 (2.28-3.23) times larger and the clock hours had 3.48 (2.95-4.11) times larger variance (p<0.05 for both). A positive relationship between QI and RNFL thickness was observed in all sectors except for clock hours 2, 9 and the nasal quadrant. This relationship reached statistical significance in the inferior quadrant and clock hour 6.

Conclusions: : Image quality varies significantly along the multiple sectors, even in scans that were defined as good quality, based on the global indicator. This might affect the reliability of localized measurements and should be considered when assessing localized changes over time.

Keywords: imaging/image analysis: clinical • nerve fiber layer • imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound)