Optical coherence tomography angiography: Principles and applications

Optical coherence tomography (OCT) has become an indispensable ophthalmic imaging modality in both research and clinical practice due to its ability to noninvasively provide volumetric imaging of the retina at high resolution. However, despite achieving the necessary resolution, structural OCT cannot image microvascular features, including capillaries because they have low reflectance contrast with surrounding tissue. OCT angiography (OCTA) overcomes this limitation by acquiring consecutive images at a single location. Doing so enables the construction of motion contrast images in which blood flow serves as an intrinsic contrast agent, allowing capillary scale vascular imaging. In doing so, OCTA offers a means to noninvasively image vascular morphology and structure and effortlessly relate it to the supplementary anatomic detail acquired through structural OCT. In the following, we discuss the principles behind OCTA image generation, including importing signal generation algorithms. We then discuss the vascular anatomy of the retina as revealed by OCTA, which include the ability to image retinal circulation volumetrically. We also discuss common pathologic features identified and measured using OCTA. This discussion includes cutting-edge quantification techniques, including artificial intelligence-based image processing. Finally, we discuss some limitations of the technology in its present manifestation and indicate what directions we envision OCTA advancing in the future.