Segmentation of the Four-Layered Retinal Vasculature Using High-Resolution Optical Coherence Tomography Angiography Reveals the Microcirculation Unit

Purpose: To differentiate the four layers of the retinal vessel network in the human macula and examine their morphologic features using high-resolution optical coherence tomography angiography (HR-OCTA). Methods: Macular areas measuring 464 × 464 pixels of 10 right eyes of 10 healthy subjects without ocular disease were scanned 10 times using a HR-OCTA device. Averaged OCTA images were created. Based on clear decorrelation signals, four vascular slabs were segmented, comprising one each in the retinal nerve fiber layer (RNFL), ganglion cell layer (GCL), and top and bottom of the inner nuclear layer (INL). Qualitative features and quantitative measurements in each slab were compared with those in conventionally segmented slabs. Results: HR-OCTA isolated four layers of vascular plexuses in the macula that followed the corresponding anatomic layers. Segmentations for the RNFL revealed that radial peripapillary capillaries (RPCs) extended to the central macular area. The RPCs followed relatively straight and long paths, with few apparent feed points and anastomoses. The GCL slab enhanced visualization of the capillary-free zones around the arteries and arterioles and helped to differentiate arterial and venous systems. The arterioles and venules were linked by capillaries that were arranged in a mesh-like fashion, with multiple arteriolar feed points and anastomoses. Vascular plexuses in the top and bottom of the INL consisted of capillaries in a vortex arrangement. The center of these vortex arrangements was consistent with the venules in the GCL. Conclusions: HR-OCTA can differentiate the four layers of vascular plexuses in the human macula and elucidate their angiographic features.