## 1 Introduction
#### OCTA Versus FA and ICGA  
FA and ICGA are important diagnostic imaging techniques for evaluating multiple retinal diseases, in particular ones of vascular or inflammatory origin. These invasive tests use intravenous dye followed by multiple photographs over at least (10 to 30) min. They provide 2-D images with dynamic visualization of blood flow and a wide field of view [13; 14]. Their images can be interpreted depending on the dye leakage, pooling, and staining patterns. While color fundus photography, FA, and ICG are generally presented as 2D images, there historically have been modalities for generating some degree of depth to these images. Scanning laser ophthalmoscopy (SLO) is an imaging modality based on a form of confocal microscopy [15]. It suffered in the past from an inability to represent true color, but improvements in this regard have been made. More recently, SLO has been combined with adaptive optics imaging techniques [16]. Despite some notable ad vantages to SLO, the technology is not ubiquitous in most ophthalmology clinics, and most commercial FA and ICG machines present 2D images.  
Several studies in the literature have investigated using FA imaging modality diagnosing the posterior segment of the eye [17]. Despite that, FA has some limitations such as its inability to visualize different levels of major capillary networks separately. This is because FA is unable to differentiate deep capillary plexus (DCP) from superficial capillary plexus (SCP). Additionally, it is hard to use FA in obtaining enhanced images of perifoveal capillaries because it has a challenge in focusing images in case of macular edema existence [18]. Moreover, FA is an invasive, time-consuming and relatively expensive modality, which makes it not ideal for regular use in clinical settings. Fluorescein dye is known to be safe; however, its side effects include nausea, allergic reactions and anaphylaxis in some rare cases [6].  
OCTA is a technique that is used to acquire angiographic information non-invasively without the need to use dye [17]. OCTA can capture images rapidly and check if the acquired images are of sufficient quality. If the acquired images are of low quality, OCTA can repeat the capturing process until images of good quality are obtained. FA in contrast has a time frame that is limited to capturing the capillary net images that are optimal. Individual vascular plexuses with the segmentation of the choriocapillaris, the middle choroid, the inner retina, and the outer retina can be visualized in the acquired en-face images [6]. OCTA outperforms FA in detection of macular ischemia because there is no dye leakage [19]. While FA cannot visualize DCP, OCTA is able to accurately delineate microvascular changes and ischemia in both DCP and SCP [1]. This is because OCTA does not have dye leakage and tissue staining problems, in addition to the longer wavelengths used in OCTA that can better penetrate through intraretinal hemorrhage [20]. Furthermore, OCTA is able to detect neovascularization elsewhere (NVE) while providing better visibility of new vessels than that produced by FA [21]. However, while NVE in the posterior pole can be readily imaged with OCTA, it suffers from the drawback that it does not image the periphery where a significant amount of NVE occurs.