Effect of Improper Scan Alignment on Retinal Nerve Fiber Layer Thickness Measurements Using Stratus Optical Coherence Tomograph

Abstract

Objective Misalignment of the Stratus optical coherence tomograph scan circle placed by the operator around the optic nerve head (ONH) during each retinal nerve fiber layer (RNFL) examination can affect the instrument reproducibility and its theoretical ability to detect true structural changes in the RNFL thickness over time. We evaluated the effect of scan circle placement on RNFL measurements. Design Observational clinical study. Methods Sixteen eyes of 8 normal participants were examined using the Stratus optical coherence tomograph Fast RNFL thickness acquisition protocol (software version 4.0.7; Carl Zeiss Meditec, Dublin, CA). Four consecutive images were taken by the same operator with the circular scan centered on the optic nerve head. Four images each with the scan displaced superiorly, inferiorly, temporally, and nasally were also acquired. Differences in average and sectoral RNFL thicknesses were determined. For the centered scans, the coefficients of variation (CV) and the intraclass correlation coefficient for the average RNFL thickness measured were calculated. Results When the average RNFL thickness of the centered scans was compared with the average RNFL thickness of the displaced scans individually using analysis of variance with post-hoc analysis, no difference was found between the average RNFL thickness of the nasally (105.2 μm), superiorly (106.2 μm), or inferiorly (104.1 μm) displaced scans and the centered scans (106.4 μm). However, a significant difference (analysis of variance with Dunnett's test: F = 8.82, P<0.0001) was found between temporally displaced scans (115.8 μm) and centered scans. Significant differences in sectoral RNFL thickness measurements were found between centered and each displaced scan. The coefficient of variation for average RNFL thickness was 1.75% and intraclass correlation coefficient was 0.95. Conclusions In normal eyes, average RNFL thickness measurements are robust and similar with significant superior, inferior, and nasal scan displacement, but average RNFL thickness is greater when scans are displaced temporally. Parapapillary scan misalignment produces significant changes in RNFL assessment characterized by an increase in measured RNFL thickness in the quadrant in which the scan is closer to the disc, and a significant decrease in RNFL thickness in the quadrant in which the scan is displaced further from the optic disc.

Effect of pupil dilation on retinal nerve fibre layer thickness measurements and their repeatability with Cirrus HD-OCT

“…In contrast, the Stratus OCT requires the operator to choose the location of the circle, and previous studies have already shown that this procedure may affect RNFL thickness measurements. 32,33 Interestingly, we also confirmed that RNFL thickness measurements are slightly less repeatable in the nasal quadrant. This finding has already been reported with both TD-OCT and SD-OCT, 14 and, as previously suggested, may be related to the fact that the angle of incidence of the illuminating beam is such that the RNFL is dimmer nasally, so that the edges of the RNFL are less reliably identified.…”

Assessment of rates of structural change in glaucoma using imaging technologies

“…Although this wide range can be explained by individual characteristics, such as stage of disease and response to treatment, it is possible that non-centered scans and signal strength affected the ability to detect change. 63,64 Nonetheless, the wide individual variation should be considered when evaluating patients on a case-by-case basis. …”

Optical Coherence Tomography for the Radiologist

“…[20][21][22] The effect of suboptimal scan centration, and alignment to the foveal axis is mostly on the distribution of thickness along the TSNIT plot, resulting in measures outside normal limits on sector and quadrant analysis. [23][24][25] A-scans are acquired through the optics of the eye; ocular magnification needs to be taken into account for scans in which thickness measures change significantly with distance from the optic nerve head rim margin. Specifically, in longer and myopic eyes, the scan path falls further from the rim margin where the nerve fiber thickness measures are less.…”