Measurement of true Henle’s fiber layer and outer nuclear layer thickness using volumetric directional optical coherence tomography

Abstract

Purpose : Progressive photoreceptor loss and resultant thinning of the outer nuclear layer (ONL) is an important feature of age-related macular degeneration (AMD). However, ONL thickness measurements cannot be obtained clinically in standard optical coherence tomography (OCT) because of the oblique-orientation of the Henle’s fiber layer (HFL). This observational study uses a novel volumetric directional OCT (VD-OCT) prototype, and a custom spoke-circular scanning pattern to clearly differentiate the two layers over the entire macular area with a single OCT volume.

Methods : Previously, we discovered that imaging the retina at an oblique angle can provide better contrast to differentiate the HFL and ONL. However, measuring the thickness of ONL and HFL over the entire macular required acquisition of OCT volumes at several different pupil entry positions, which is only possible with high level of patient cooperation and complex image processing. In this study, we built a novel OCT prototype that incorporates two sets of optical scanners in the sample arm to synchronously scan the imaging beam on both the pupil and retina. By synchronization of the two scanners, we can precisely control the OCT beam entry positions and maintain optimum beam incident angles on the retina to generate sufficient optical contrast for the HFL over the entire macular area.

Results : Five healthy adult volunteers were imaged upon dilation with the VD-OCT prototype for about five minutes in this study. The thickness of the true ONL can be assessed objectively on the cross-sectional scans acquired from the VD-OCT prototype (Fig. 1(a-c)). The volumetric ONL thickness heat maps can be calculated and plotted as demonstrated in Fig. 1(d) (spoke-circular scanning pattern) and Fig. 1(e) (regular spoke scanning pattern). The global and local ONL thickness variations are apparent in comparison with the heat maps: HFL became hyper-reflective in the macular region with the spoke-circular scanning pattern.

Conclusions : VD-OCT was used to visualize the otherwise invisible HFL with the aid of synchronous imaging beam scanning at both the retina and pupil planes. The VD-OCT prototype provides sufficient contrast to delineate the directionally reflective HFL tissue over the macular volume. True ONL thickness can be measured for the characterization of photoreceptor loss and its consequent thinning of the tissue.

This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.