Distinct Retinal Capillary Plexuses in Normal Eyes as Observed in Optical Coherence Tomography Angiography Axial Profile Analysis

Abstract

Optical coherence tomography angiography (OCTA) allows the retinal microvasculature to be visualized at various retinal depths. Previous studies introduced OCTA axial profile analysis and showed regional variations in the number and location of axially distinct vascular retinal plexuses. OCTA acquisition and processing approaches, however, vary in terms of their resulting transverse and axial resolutions, and especially the latter could potentially influence the profile analysis results. Our study imaged normal eyes using the Spectralis OCT2 with a full-spectrum, probabilistic OCTA algorithm, that, in marked contrast to split-spectrum approaches, preserves the original high OCT axial resolution also within the resulting OCTA signal. En face OCTA images are generally created by averaging flow signals over a finite axial depth window. However, we assessed regional OCTA signal profiles at each depth position at full axial resolution. All regions had two sharp vessel density peaks near the inner and outer boundaries of the inner nuclear layer, indicating separate intermediate and deep capillary plexuses. The superficial vascular plexus (SVP) separated into two distinct peaks within the ganglion cell layer in the parafoveal zone. The nasal, superior, and inferior perifovea had a deeper SVP peak that was shifted anteriorly compared to the parafoveal zone. Axial vascular density analysis with high-resolution, full spectrum OCTA thus allows healthy retinal vasculature to be precisely reconstructed and may be useful for clinically assessing retinal pathology.

Figure 1

Parafoveal and perifoveal axial vascular density profiles (vascular density vs. scaled relative axial depth) obtained with optical coherence tomography angiography. An en face image is also shown for reference (A,F). Profiles in parafoveal region (2.5°–3.75° radial distance to the foveal center) were obtained from the superior (B), nasal (C), temporal (D), and inferior (E) quadrants (red shaded regions in A). Profiles in the perifoveal region (6.25°–7.5° radial distance to the foveal center) were obtained from the superior (G), nasal (H), temporal (I), and inferior (J) quadrants (red shaded regions in F). Individual patient profiles are shown as grey solid lines and mean density is shown as a green solid line. The green dotted lines represent one standard deviation above and below the mean. Sharp peaks, corresponding to the intermediate and deep capillary plexuses, are apparent near the inner and outer INL borders in all examined parafoveal regions. The superficial vascular plexus contained both a small peak at the NFL-GCL junction and a larger, broader peak within the GCL. In all four perifoveal regions, sharp peaks corresponding to the intermediate and deep capillary plexuses are apparent near the inner and outer INL borders in all examined regions. Unlike in the parafoveal region, the SVP was not comprised of two distinct peaks. The tall SVP peak was shifted towards the ILM in the nasal, superior, and inferior quadrants. NFL: nerve fiber layer; GCL: ganglion cell layer; IPL: inner plexiform layer; INL: inner nuclear layer; OPL: outer plexiform layer.

Figure 2

Continuous annular vascular density heat maps of the parafoveal and perifoveal rings. The average axial OCTA signal profile over all 22 eyes is plotted against the orientation within the para- or perifoveal rings. Four vessel layers, represented as white, “hot” bands, are apparent at a relatively consistent axial location in the parafovea (A). In contrast, only three vessel layers are distinguishable in the perifovea (B). INF: inferior, TEMP: temporal, SUP: superior, NAS: nasal.

Figure 3

Vascular density heat map plotted along the fovea-BMOC axis. The top en face image (A) is a montage of one foveal-centered 15° × 15° OCTA scan and two 15° × 5° OCTA scans displaced temporally and nasally by 15° along the Fovea–BMOC axis. The red shaded area in (A) corresponds to where the vascular density map (B) was obtained. The vascular density measurement band was 2.5° wide and centered on the fovea-BMOC axis. Vascular density was measured between 22.5° temporal and 12.5° nasal. Axial vascular density profiles (vascular density vs. axial depth from IPL-INL) were obtained at the left (C), middle (D) and right (E), black dotted line in (B). The SVP, ICP, and DCP are visible throughout the vascular density heat map, including in far temporal locations. A fourth plexus in the nerve fiber layer is also detectable at approximately 5°, both nasally and temporally from the fovea. BMOC: Bruch’s membrane opening center, OCTA; optical coherence tomography angiography; SVP: superficial vascular plexus; ICP: inner capillary plexus; DCP: deep capillary plexus; SVP: superficial vascular plexus; IPL: inner plexiform layer, INL: inner nuclear layer.

Figure 4

En face macular OCTA images of vascular plexuses identified with axial profile analysis in a healthy right eye. En face OCTA vascular layers are exported from the Spectralis software after application of projection artefact removal in the deeper layers, DVC, ICP, and DCP. The SVP had centripetal branches that terminated in a capillary ring around the foveal avascular zone. En face OCTA images of the DVC could be divided into en face images of the ICP and DCP, which had distinct vascular morphologies. A vortex-like configuration of vascular loops surrounded a central seed point (single point surrounded by yellow dotted lines) was apparent in the DCP, but not in the ICP. OCTA: optical coherence tomography angiography; PAR: projection artifact removal, SVC: superficial vascular complexes, DVC: deep vascular complexes, SVP: superficial vascular plexus, ICP: intermediate capillary plexuses, DCP: deep capillary plexuses.