Ability of different enface OCT slab methods to detect RNFL reflectance defects in glaucoma

Abstract

Purpose : Optimal methods to extract enface OCT slab images to detect retinal nerve fibre bundle (RNFB) defects are undetermined. We explored the ability of several methods for slab extraction to objectively assess glaucomatous RNFB reflectance defects in enface OCT images.

Methods : Dense SD-OCT scans were performed in 16 eyes with glaucoma (median age: 70, range 61-77) and 19 age-similar controls. Enface slab images depth-averaging reflectivity below the inner limiting membrane (ILM) were generated with 6 different methods. Five methods considered single slabs of various thickness and depth (Figure 1). One novel method combined seven 16µm thick slabs from 8 to 116µm below the ILM (Figure 2), seeking to explore all depths with potential RNFB presence. In the combined slabs method, defects were defined when occurring in any slab. All methods adjusted for the individual position of the raphe, fovea and optic disc. Superpixels of glaucoma eyes were considered abnormal if reflectivity fell below the kernel density estimated 1^st percentile of control data. Ability to detect glaucoma defects was measured by proportion of abnormal superpixels. Proportion of superpixels below the 1^st and 5^th percentile in controls was used as a surrogate for false positive rate. Differences in performance between slab methods were tested with linear mixed models.

Results : Ability to detect glaucomatous defects varied significantly among slab methods (χ²₍₅₎=119.9, p<0.0001), with the combined slabs method detecting 5-9% more abnormal superpixels than others (all p<0.0001). No method found abnormal superpixels at the 1% level in controls. Proportion of abnormal superpixels in controls at the 5% level varied slightly between approaches (χ²₍₅₎=15.5, p=0.009), being similar or slightly larger (1.8-2.2%) for the combined slabs approach.

Conclusions : Slab extraction method affects ability to detect glaucoma abnormalities in enface OCT images. Our novel method evaluates all depths with potential RNFB presence by combining several thin slabs at each location, resulting in greater detection of glaucomatous reflectance abnormalities.

This is a 2021 ARVO Annual Meeting abstract.

 

Examples of images and deviation maps generated with single slab methods. Red lines demarcate regions of the slab that include different depths.

 

Example of the combined slabs method for the same glaucoma eye shown in Figure 1. The bottom-right deviation map combines abnormal superpixel (<1%) found in any slab.