Commentary on Lavia et al: Progress of Optical Coherence Tomography Angiography for Visualizing Human Retinal Vasculature


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Optical coherence tomography angiography (OCTA) provides three-dimensional, depth-resolved images of retinal and choroidal vasculature in vivo by detecting motion contrast from blood cells moving in vessel lumens.1 The technology has emerged speedily in just a few years since its commercial introduction, allowing new insights into retinal vascular diseases that were incompletely understood from dye-based angiography.2 An article by Lavia et al3 takes further steps toward making OCTA a quantitative tool in the clinic by documenting a large normative database with new software to remove projection artifact.There are several capillary beds and a continuous parafoveal capillary ring in the macula of humans4 and nonhuman primates.5 Apart from the peripapillary area, these capillary beds are known as superficial (in the nerve fiber layer and inner aspect of the ganglion cell layer), intermediate (on inner and outer aspects of the inner nuclear layer), and deep (between the inner nuclear layer and outer plexiform layer) plexuses.1 The deep capillary plexus is planar, whereas the others are three-dimensional because of connecting vascular loops and vertical segments. A capillary-free zone immediately adjoins arteries, in the superficial plexus only.6 The direction of blood flow in these capillary beds is actively debated.7,8 A common metric to many OCTA studies is vessel density (VD), that is, the percentage of retinal area that is occupied by vessels, in a projection image of either the entire retinal thickness or in one anatomical layer isolated by automatic or manual segmentation. A second common metric is the area or equivalent diameter of the foveal avascular zone (FAZ) within the bounds of the parafoveal capillary ring. The FAZ exhibits high interindividual variability in diameter and area that is inversely related to the volume and shape of the foveal pit, attributed to glial–vascular relationships that become established during late fetal development.9,10In their new article,3 Lavia et al developed a normative database for macular OCTA using an RTVue XR Avanti (Optovue, Fremont, CA) and new software for projection artifact removal, as well as motion artifact removal and sharpness enhancement. In 148 eyes of 84 patients aged 22.2 to 75.8 and with maculas carefully screened to be normal, the authors measured VD in capillary beds and area of the FAZ. Images were selected with a stringent standard of 70 for signal strength index. Individual capillary beds were isolated by generously inclusive boundaries (9 µm from anatomical layers indicated by reflective bands on structural OCT). The authors found that VD was higher in the superficial and intermediate capillary plexuses (47.8% and 45.4%, respectively) than in the deep capillary plexus (31.6%). Furthermore, they found that with age, VD decreased in all three plexuses (r2 of 0.37–0.46), especially in the deep plexus, and FAZ area increased. Observed VD increased as signal strength index increased, i.e., more vessels were seen in better images. Vessel density was reported in two macular subregions, i.e., within 300 µm of the perifoveal capillary ring and in the perifovea, as well as superior and inferior hemifields, and four quadrants. Lavia et al suggest that deep capillary plexus is vulnerable to disease processes; lower anatomical density and greater diminution with age contributes.Strengths of this study include a large cohort with healthy maculas, wide age range, current software including projection artifact removal and improved segmentation (including around the foveal singularity11), and comprehensive reporting of tabulated data for ease of reference.

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