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A Practical Guide to Optical Coherence Tomography Angiography Interpretation Eugenia Custo Greig1,2, Jay S

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A Practical Guide to Optical Coherence Tomography Angiography Interpretation Eugenia Custo Greig1,2, Jay S Greig et al. Int J Retin Vitr (2020) 6:55 https://doi.org/10.1186/s40942-020-00262-9 International Journal of Retina and Vitreous REVIEW Open Access A practical guide to optical coherence tomography angiography interpretation Eugenia Custo Greig1,2, Jay S. Duker1 and Nadia K. Waheed1* Abstract Background: Optical coherence tomography angiography (OCTA) can image the retinal vasculature in vivo, without the need for contrast dye. This technology has been commercially available since 2014, however, much of its use has been limited to the research setting. Over time, more clinical practices have adopted OCTA imaging. While countless publications detail OCTA’s use for the study of retinal microvasculature, few studies outline OCTA’s clinical utility. Body: This review provides an overview of OCTA imaging and details tips for successful interpretation. The review begins with a summary of OCTA technology and artifacts that arise from image acquisition. New methods and best practices to prevent image artifacts are discussed. OCTA has the unique ability among retinovascular imaging modali- ties to individually visualize each retinal plexus. Slabs ofered in standard OCTA devices are reviewed, and clinical uses for each slab are outlined. Lastly, the use of OCTA for the clinical interpretation of retinal pathology, such as diabetic retinopathy and age-related macular degeneration, is discussed. Conclusion: OCTA is evolving from a scientifc tool to a clinical imaging device. This review provides a toolkit for suc- cessful image interpretation in a clinical setting. Keywords: Optical coherence tomography angiography, Interpretation, Artifacts Background Prior to its commercial release, OCTA was available as Optical coherence tomography angiography (OCTA) frst a research tool. Investigational groups explored the utility became commercially available in 2014 [1]. Tis technol- of OCTA in a wide range of ocular pathologies including ogy allowed for the visualization of retinal microvascu- age related macular degeneration (AMD), diabetic retin- lature in vivo. Prior to OCTA, fuorescein angiography opathy (DR), and uveitis [2–8]. A large body of literature (FA) and indocyanine green angiography (ICGA) were emerged to describe vascular fndings on OCTA. Tough the mainstay modalities for retinovascular visualization. scientifcally novel and relevant to the understanding of Tese imaging modalities generated a 2-dimensional en ocular pathophysiology, these studies often reference face view of the retinal vasculature and did not allow for language and metrics that remain inaccessible to clinical the individual visualization of retinal capillary plexuses. ophthalmologists. Tis review will ofer a basic toolkit for OCTA ofered in vivo visualization of the retinal micro- OCTA interpretation in clinical practice and explore its vasculature in a depth-resolved fashion, without the need utility in assessing retinal pathology. for time-consuming dye administration. Functioning principles In 1991 optical coherence tomography (OCT) was described in the literature [9]. Tis novel imaging modal- *Correspondence: [email protected] ity allowed for detailed visualization of the posterior 1 New England Eye Center, Tufts Medical Center, 800 Washington Street, Box 450, Boston, MA 02111, USA retina in a cross sectional fashion. Tis non-invasive Full list of author information is available at the end of the article © The Author(s) 2020. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://crea- tivecommons.org/licenses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdo- main/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Greig et al. Int J Retin Vitr (2020) 6:55 Page 2 of 17 imaging technique changed the clinical management of manufacturers utilize preset interscan times to deliver an many retinal diseases. image that displays fow and minimizes artifact. In sum- Tough original clinical OCT units were based on time mary, OCTA is not a representation of retinal vessels, but domain detection, spectral domain OCT (SD-OCT) of retinal blood fow within a predetermined range [12]. is now the clinical standard. Tis technology utilizes a light beam that is split into an exploring arm and a ref- Spectral domain and swept source OCTA erence arm to generate an image. Light from the explor- Te two most commonly available OCTA devices are ing arm is refected, absorbed or scattered as it passes spectral domain and swept source devices. Tese devices through the structures of the posterior retina. Te light vary in their light source, as well as their light detectors. from the exploratory arm is then compared to the undis- Spectral domain OCTA (SD-OCTA) utilizes a near- turbed light path of the reference arm. Te diference infrared super luminescent diode with a wavelength of between the two light paths yields information regarding ~ 840 nm as a light source, and a spectrometer as a detec- the depth and brightness of structures within the retina. tor. Swept Source OCTA (SS-OCTA) utilizes a tunable Each data point generated is referred to as an A-scan. As swept laser with a wavelength of ~ 1050 nm as a light the device travels transversely through tissue it collects a source and a single photodiode as a detector [1]. Te series of A-scans that are compiled together into a retinal increased wavelength of swept source devices is safer for cross-section known as a B-scan. A collection of B-scans the eye and permits the use of a high power laser. Te produces a three-dimensional representation of the ret- faster scanning speed of swept source devices allows for ina that can be used to interpret depth-resolved struc- larger scan areas that visualize a greater portion of the tural anatomy [10]. posterior retina, as well as dense scanning patterns that OCTA was developed as an extension of OCT imag- increase detail visualization [13]. Overall the most clini- ing. OCTA technology utilizes motion contrast to detect cally signifcant diference between the two devices is blood fow. When two successive images of a scene are the longer wavelength of the swept source device. Te taken, stationary objects will not change, while moving increased wavelength of the swept source device reduces objects will become apparent. OCTA captures succes- its axial resolution when compared to SD-OCTA [14]. sive A-scans at the same retinal location, and each scan However, longer wavelengths allow for improved pen- capture is separated by a brief lapse in time. As light is etration through the retinal pigment epithelium (RPE) refected back, a diference in signal will be detected and better visualize the choroid [15]. Regardless of this between the two scans. Tis diference is due to motion added beneft, swept source devices are costly, making between the scans and is termed decorrelation signal. SD-OCTA technology more widespread [10]. Because the retina is a static tissue, the decorrelation sig- nal is attributed to the movement of blood through the Strengths of OCTA retinal vasculature. Tus, a decorrelation map is gener- OCTA displays in vivo retinal vasculature in a depth- ated that mirrors the fow of blood in the back of the eye, resolved fashion. Segmentation of volumetric data leads rendering a representation of its vascular networks. to identifcation of retinal capillary plexuses individually, Te time delay between each B-scan captured is providing detail and resolution similar to that of histo- termed interscan time. Diferent interscan times allow for logic studies [16]. Further, OCTA provides improved the detection of specifc blood fow speeds. For OCTA visualization of the deep capillary plexus and choroid technology to detect blood fow within a vessel, eryth- when compared to FA and ICGA [17]. OCTA does not rocytes must move within the length of the OCT beam utilize dye to visualize vessels which confers it with mul- between successive scans. A short interscan time cap- tiple advantages. First, OCTA generates high-contrast tures the movement of a fast-moving erythrocyte before images that are not obscured by dye leakage from vessels, it exits the length of the OCT beam. Short interscan leading to more pronounced defnition of retinal vascu- times, therefore, improve the visualization of fast-moving lature. Second, this dye–free imaging modality does not vascular beds. Increasing the interscan time allows slow- expose patients to the risks associated with contrast dye, moving erythrocytes a greater chance to move between which range from mild allergic reactions to anaphylaxis successive scans, increasing the sensitivity to slow-mov- [18]. Patients that have relative contraindications to dye ing vascular beds [11]. However, this increased sensitivity imaging,
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