Retinal nerve fibre layer thickness measurement reproducibility improved with spectral domain optical coherence tomography

Abstract

Background/aims: To investigate retinal nerve fibre layer (RNFL) thickness measurement reproducibility using conventional time-domain optical coherence tomography (TD-OCT) and spectral-domain OCT (SD-OCT), and to evaluate two methods defining the optic nerve head (ONH) centring: Centred Each Time (CET) vs Centred Once (CO), in terms of RNFL thickness measurement variability on SD-OCT.

Methods: Twenty-seven eyes (14 healthy subjects) had three circumpapillary scans with TD-OCT and three raster scans (three-dimensional or 3D image data) around ONH with SD-OCT. SD-OCT images were analysed in two ways: (1) CET: ONH centre was defined on each image separately and (2) CO: ONH centre was defined on one image and exported to other images after scan registration. After defining the ONH centre, a 3.4 mm diameter virtual circular OCT was resampled on SD-OCT images to mimic the conventional circumpapillary RNFL thickness measurements taken with TD-OCT.

Results: CET and CO showed statistically significantly better reproducibility than TD-OCT except for 11:00 with CET. CET and CO methods showed similar reproducibility.

Conclusions: SD-OCT 3D cube data generally showed better RNFL measurement reproducibility than TD-OCT. The choice of ONH centring methods did not affect RNFL measurement reproducibility.

Figure 1

(A) Major sources of retinal nerve fibre layer thickness measurement variability using 3.4 mm circular time-domain optical coherence tomography scan centred on the optic nerve head. Schematic presentation of sampling points scattered along 3.4 mm diameter circle due to eye motion during the scan (left). Scanning circle placement may vary from scan to scan (right; yellow, green, and blue circles). (B) Spectral-domain optical coherence tomography fundus (en face) image, generated by summing the back scattering signal at each axial scan on the retina. The pseudo-three-dimensional representation on the upper-left shows the direction of summation, and the output image (optical coherence tomography fundus image) is shown on the upper right. Eye motion is detected as disrupted retinal vessels on an optical coherence tomography fundus image (lower-left), which can be emphasised with edge-detection filtering (lower-right).

Figure 2

Two separate optical coherence tomography fundus images (A, B) from the same eye overlaid (C) to emphasise the differences induced by minor eye motion during scanning, head position, and slight shift in the subjects’ fixation point. Note that the integrity of retinal vessels within each optical coherence tomography fundus image is intact.

Figure 3

Centred Each Time method utilising an automatically detected optic nerve head centre on each of the cube data obtained using spectral domain optical coherence tomography. Slight variation in the disc margin detection leads to a displacement of the resampling circle location that can be observed relative to the vessel branches (blue arrow).

Figure 4

Centred Once method importing the automatically detected optic nerve head centre from the reference scan (A) onto the other cube data (B and C) obtained from the same eye. Retinal nerve fibre layer thickness measurements were then obtained on each image along the 3.4 mm diameter circle (white circles on (D) and (E)). (F, G) Images visualising the performance of optical coherence tomography fundus image registration by superimposing both the reference image ((A) in red) and the target image ((D) and (E) in green).

Figure 5

Square root of scan variance component (“SD”) plot from table 3. The reproducibility of both Centred Each Time (CET) and Centred Once (CO) methods was uniformly and substantially better than time-domain optical coherence tomography (TD-OCT) and the differences were statistically significant.