Visualization of Radial Peripapillary Capillaries Using Optical Coherence Tomography Angiography: The Effect of Image Averaging

Objectives
To assess the effect of image registration and averaging on the visualization and quantification of the radial peripapillary capillary (RPC) network on optical coherence tomography angiography (OCTA).
Methods
Twenty-two healthy controls were imaged with a commercial OCTA system (AngioVue, Optovue, Inc.). Ten 10x10° scans of the optic disc were obtained, and the most superficial layer (50-μm slab extending from the inner limiting membrane) was extracted for analysis. Rigid registration was achieved using ImageJ, and averaging of each 2 to 10 frames was performed in five ~2x2° regions of interest (ROI) located 1° from the optic disc margin. The ROI were automatically skeletonized. Signal-to-noise ratio (SNR), number of endpoints and mean capillary length from the skeleton, capillary density, and mean intercapillary distance (ICD) were measured for the reference and each averaged ROI. Repeated measures analysis of variance was used to assess statistical significance. Three patients with primary open angle glaucoma were also imaged to compare RPC density to controls.
Results
Qualitatively, vessels appeared smoother and closer to histologic descriptions with increasing number of averaged frames. Quantitatively, number of endpoints decreased by 51%, and SNR, mean capillary length, capillary density, and ICD increased by 44%, 91%, 11%, and 4.5% from single frame to 10-frame averaged, respectively. The 10-frame averaged images from the glaucomatous eyes revealed decreased density correlating to visual field defects and retinal nerve fiber layer thinning.
Conclusions
OCTA image registration and averaging is a viable and accessible method to enhance the visualization of RPCs, with significant improvements in image quality and RPC quantitative parameters. With this technique, we will be able to non-invasively and reliably study RPC involvement in diseases such as glaucoma.