Evaluation of retinal vessel quantity within individual retinal structural layers using optical coherence tomography angiography.

PURPOSE To evaluate retinal vessel quantity within various retinal structural layers using optical coherence tomography angiography (OCTA). METHODS In this IRB-approved study, 22 normal eyes (from 22 subjects) were imaged using the Spectralis OCT2, with a 15 × 15 degree OCTA scan centered on fovea and two additional 15 × 5 degree OCTA scans, displaced temporally and nasally by 15 degrees along the fovea-Bruch's membrane opening (BMO) axis. Following projection artifact removal (PAR), vessel quantity (i.e., amount of flow signal) within each retinal nuclear and plexiform layer was assessed across the scan and was plotted as a vessel quantity profile over this fovea-BMO axis. Vessel quantity was correlated against the retinal layer thickness at the corresponding locations using the Spearman correlation. RESULTS For the nerve fiber layer (NFL), the vessel quantity was highest nasally and declined towards the fovea and was near zero temporal to the fovea with or without PAR. For all other retinal layers, the retinal vessel quantities were greatest in the parafoveal retina, peaking approximately 5 degrees from the foveal center. Before PAR, the parafoveal vessel quantity was highest in the inner plexiform layer (IPL). Following PAR, the vessel quantity in the IPL decreased but was relatively unchanged in the other layers. The vessel quantity correlated moderately well with retinal layer thickness (r = 0.432 to 0.511; P < 0.05 among the various layers). CONCLUSIONS Retinal vessel quantity varies significantly among the various structural layers, with significant regional variability. Projection artifact can significantly impact retinal vessel quantity in the deeper layers, but the effect appears to be most pronounced in the IPL.