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Retina Advances in Optical Coherence Tomography Angiography Talisa E de Carlo and Caroline R Baumal New England Eye Center, Tufts University School of Medicine, Boston, US Abstract Optical coherence tomography angiography (OCTA) is an exciting new imaging technology that allows for non-invasive non-dye-based visualization of blood flow in the posterior pole. The technology has been used to image a variety of chorioretinal disorders including choroidal neovascularization in age-related macular degeneration, diabetic retinopathy, retinal arterial occlusion, retinal vein occlusion, and macular telangiectasia type 2. Continued advances in the software will improve the quality of OCTA and reduce image artifacts. Keywords Optical coherence tomography angiography, age-related macular degeneration, diabetic retinopathy, retinal disease, imaging Disclosure: Caroline R Baumal has nothing to declare in relation to this article. She serves on an advisory board for Allergan (2015/2016). Talisa de Carlo has nothing to declare in relation to this article. Open Access: This article is published under the Creative Commons Attribution Noncommercial License, which permits any noncommercial use, distribution, adaptation, and reproduction provided the original author(s) and source are given appropriate credit. Received: February 21, 2016 Accepted: March 7, 2016 Citation: US Ophthalmic Review, 2016;9(1):37–40 Correspondence: Caroline R Baumal, Associate Professor, Tufts University, Division of Vitreoretinal Surgery, New England Eye Center, 800 Washington St, box 450, Boston, MA 02111, US. E: Support: The publication of this article was supported in part by an unrestricted grant from Research to Prevent Blindness and Massachusetts Lions Club. The views and opinions expressed are those of the author and do not necessarily reflect those of of Research to Prevent Blindness or Massachusetts Lions Club. In 2015, a novel imaging technology called optical coherence tomography angiography (OCTA) became available for clinical use. The features of many disorders, including diabetic retinopathy (DR), choroidal neovascularization (CNV), macular telangiectasia type 2, retinal vascular occlusion, and sickle cell retinopathy have been characterized by OCTA. OCTA is a non-invasive, non-dye-based imaging technique that employs motion contrast to create high-resolution depth-resolved angiographic images of vascular flow in a matter of minutes. 1–3 The software compares the decorrelation signal or phase variance between consecutive OCT B-scans acquired at the same retinal cross-section to detect motion contrast. 4–6 Differences between OCT B-scans are assumed to represent movement due to erythrocytes in the vasculature. Using this information from multiple retinal cross-sections, a volumetric retinal map (OCT angiogram) is constructed and co-registered with the OCT B-scans used to develop the OCT angiogram. This allows for evaluation of blood flow in the OCT angiogram and structural information in the OCT B-scans in tandem. The scanning areas currently vary from 2 x 2 mm to 12 x 12 mm, and are comprised of multiple A-scans, depending on machine parameters. OCT angiograms can be segmented into en face slabs to evaluate the different retinal vascular layers (e.g. superficial inner retina, deep inner retina, outer retina, choriocapillaris) in isolation (Figure 1). 7 OCTA is one of the only in vivo techniques to visualize choroidal flow in detail. 8 Image Interpretation Current limitations of OCTA are that image resolution is reduced when its scanning area is increased, that quantitative measurement software is TOU CH MED ICA L MEDIA limited, and the occurrence of image artifacts. With the Optovue Avanti TM software (OptoVue Inc., Fremont, CA) the same number of OCT B-scans are used regardless of the scanning area, with the regions between the OCT B-scans interpolated from the data provided. Therefore, as scanning height and width each double, resolution is reduced four-fold. This can be improved by montaging multiple smaller scan areas together and software is being created to do this in an automated fashion. 9 While much of the current utility of OCTA revolves around qualitative assessment, quantitative tools are under development. The size of the foveal avascular zone (FAZ) is quantifiable using the prototype AngioVue software of the RTVue ® XR OCT device (OptoVue Inc., Fremont, CA). Carpineto et al. found highly reproducible automated FAZ measurements in 60 healthy eyes. 10 Automated software has been able to quantify decreased capillary perfusion density in eyes with DR compared with control eyes. 11,12 Multiple OCTA image artifacts may affect image quality. Motion artifact due to blinking or gross eye movement appear as horizontal or vertical black and white lines, respectively, and motion correction software can create vessel doubling, stretching of the image, quilting, or loss of detail in the attempt to compensate for these motion artifacts. 13,14 Signal attenuation or masking can occur below optically dense material such as thickened retinal pigment epithelium (RPE) or hard exudates, making the area below dark. This artifact can be mistaken for flow voids. 37