## 1 Introduction  
Optical coherence tomography angiography (OCTA) is a novel imaging modality that noninvasively and rapidly images the retinal microvasculature in multiple layers with a resolution resembling histologic appearance [1, 2]. It can identify retinal vessels by the detection of intensity and/or phase property variations of optical coherence tomography (OCT) signals in multiple B-scans at the same point resulting from red blood cell movement [3]. OCTA generates angiographic images by employing motion contrast imaging to obtain high-resolution volumetric blood flow information with excellent intra-visit repeatability [4]. OCTA employs an algorithm called the split spectrum amplitude decorrelation angiography (SSADA), which detects the flow of blood in the retinal microvasculature with no use of dye [5]. SSADA allows for obtaining OCTAs of high quality using the same available speeds in a retinal commercial OCT system [6]. An advantage of using SSADA is that it enhances the flow detection's signal-to-noise ratio [7]. Normal OCT provides in-depth imaging of the retinal and choroidal vasculature. The retina consists of a three-neuron circuit that detects light and then transmits that signal to the brain via the optic nerve. The photoreceptors are located in the outer retina, which is avascular. Thus, OCTA images of the outer retina show no vessels under normal conditions. The outer retina receives its oxygen from the choriocapillaris, the richly vascular layer of the eye as deep as the retinal pigment epithelium (RPE), which is a monolayer of cells that divides the outer retina from the choroid. OCTA readily images both the choroid and choriocapillaris. After detecting light, photoreceptor cells transmit their electrical signal to bipolar cells in the middle retina, which in turn connect to retinal ganglion cells in the inner retina. These funnel into the optic nerve, which travels posteriorly through the optic chiasm to the brain. Oxygen to the middle and inner retina is supplied by the deep and superficial plexuses of the retinal vasculature, both of which are beautifully imaged by OCTA. Finally, there is a peripapillary vasculature supplied by the central retinal artery that encircles the optic nerve and supplies the pre-laminar optic nerve. Thus, under normal conditions, OCTA provides high-resolution imaging of vessels in the choroid, choriocapillaris, middle and inner retina, and peripapillary vessels. Thanks to OCTA, retinal, choroidal and optic nerve vasculature can be studied safely with no dye injection. Specific layers of the retina and choroid can be seperately visualized by en-face OCTA which helps in detecting vascular changes level in the choriocapillaris, the middle choroid, the inner retina, and the outer retina [6]. It can demonstrate vessel density (VD), shape of the foveal avascular zone (FAZ), and alteration in retinal blood flow [8]. OCTA acquisition time is short compared to indocyanine green angiography (ICGA) or fluorescein angiography (FA) because there is no dependence on the dye's flooding kinetics. Instead, acquisition time depends on parameters such as scan volume, speed of eye tracking system, scan speed, resolution, and numbers of scans averaged [9]. OCTA's ability to detect the shape and morphology of FAZ is very helpful as FAZ is an indicator of the perifroval microcirculation [10]. Important retinrovascular diseases, such as retinal vein occlusion (RVO) and Diabetic Retinopathy (DR), interfere with the retinal microvascular circulation and alter FAZ size and shape [11]. OCTA can detect capillary dropout in DR better than FA because of lack of leakage [12]. In the following sections, we will discuss the advantages of OCTA as a new imaging modality in comparison with other modalities such as FA and ICGA. We will also provide a short review of clinical findings of OCTA in several retinal and optic-nerve-related diseases.