Visible-Light Optical Coherence Tomography Fibergraphy of the Tree Shrew Retina

Abstract

Purpose : Tree shrew eyes share many similar features to human eyes, including a structured lamina cribrosa, making them a preferred model for glaucoma than rodents. However, as an emerging animal model, a technique for quantifying and visualizing the retinal ganglion cell (RGC) axon bundle structure for tracking optic neuropathies has yet to be developed. We demonstrate visible-light optical coherence tomography fibergraphy (vis-OCTF) in the tree shrew retina to visualize individual RGC axon bundles and their surrounding vasculature.

Methods : We acquired vis-OCTF images from healthy 5-36-month-old tree shrews. Before imaging, tree shrews were anesthetized using 3% isoflurane with supplemental oxygen and given tropicamide and phenylephrine drops to dilate the pupils. During imaging, tree shrews were kept warm with an infrared heat lamp and given artificial tears to prevent corneal dehydration. We positioned the retina to maximize RGC axon bundle reflectance throughout the field-of-view. We acquired 5 repeated OCTA volumes consisting of 512 A-lines × 512 B-scans with each B-scan repeated twice. With an A-line rate of 75 kHz, each acquisition was completed in ~35 seconds. We registered the repeated volumes in the axial and lateral dimensions and then averaged to reduce speckle noise. Fibergram images were generated by segmenting the retinal nerve fiber layer and taking the mean intensity projection along the axial dimension.

Results : Vis-OCT images are shown in Fig. 1. The mean intensity projection fundus image in Fig. 1a and angiogram in Fig. 1b highlight the tree shrew’s unique retinal vasculature. The fibergram in Fig. 1c and resampled B-scan in Fig. 1d depict the densely packed RGC axon bundles around the optic nerve head (ONH). We observed ~16 bundles/mm at 1.2 mm from the ONH with an axial bundle thickness of 59.4±19.8 μm (n=54) and a lateral bundle width of 25.0±8.7 μm (n=96).

Conclusions : Vis-OCTF enables a more wholistic evaluation of RGC axon bundle health and provides new parameters for tracking the progression of optic neuropathies in tree shrews.

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

 

Figure 1. Example tree shrew vis-OCT (a) fundus image, (b) angiogram, (c) fibergram, and (d) resampled circular scan reconstructed from path highlighted in (a). The blue arrow in (a) represents the leftmost A-line in (d).