## 4 Current OCTA Devices  
OCTA represents an emerging non-invasive imaging technology that provides detailed visualization of the retinal and choroidal vascular networks. This technique employs the principle of motion contrast in order to generate blood flow and thereby images the vasculature without the need for a contrast agent [23]. Since its introduction, OCTA has allowed a deep characterization of several retinal pathologies, including AMD, diabetic retinopathy, vascular diseases, and also different inherited retinal diseases such as Stargardt macular dystrophy, Best disease, and choroideremia [24,25]. OCTA is able to compare the signals of sequential OCT B-scans from the same cross-section and distinguish the moving scatters from the tissue in the background in order to provide an image of retinal and choroidal networks [23]. In this way, it allows acquiring angiograms in a short time (<5 min), without the use of dye. OCTA technology is constantly evolving to improve image quality, acquisition time, and the automatic interpretation of scans. Table 1 shows the comparison of different commercially available OCTA devices: Zeiss AngioPlex, Cirrus HD-OCT 6000; Optovue AngioVue, RTVue XR AVANTI; Topcon Triton, DRI Triton; Heidelberg Spectralis, OCT2, Angiography; Nidek AngioScan RS-3000 Advance 2 and Canon Angio Xepphilio OCT-S1. Table 1 compares the main characteristics provided by the manufacturer, including central wavelength, scanning speed, resolution, imaging depth, and imaging size. The OCTA devices available at the present can do 70,000 to 100,000 scans per seconds. The scanning speed is related to the presence of motion artifact and to the image resolution. In particular, the higher the scanning speed, the lower the motion artifacts that would be present in the final image. This feature plays a fundamental role in the clinical practice, where it is often necessary to optimize exam times. Recently, Topcon, Zeiss, and Canon developed novel systems that allow a scanning speed of 100,000 scans per second. In addition, imaging depth has been improved compared to the first models. In particular, in current available devices, the imaging depth ranges from 2.0 to 5.3 mm. The software for the visualization of volumetric data, and segmentation algorithms, is different between the devices and hardly comparable. Different approaches, such as the amplitude decorrelation algorithm and the combined intensity and phase decorrelation, have been developed to improve image quality and to reduce the motion artifacts [26; 27; 28]. Finally, the image size has been significantly increased over the years. In fact, in the first models, only 3 x 3 or 6 x 6 scans were possible. Recently, 12 x 12 by Zeiss and even 20 x 23 by Canon have also been introduced. Moreover, two main Fourier domain detection systems of OCTA are available: the spectral domain (SD-OCTA) and the swept source OCTA (SS-OCTA) [16]. The SD-OCT employs a broadband near-infrared superluminescent diode that has a wavelength of 840 nm, with a spectrometer to measure wavelengths of light. The SS-OCT instead employs a tunable swept laser that has a wavelength of 1050 nm and uses a single photodiode detector [16; 29; 30]. The main advantages of SS-OCTA imaging over SD-OCTA are (i) the faster scanning speed, which allows for denser scan patterns and larger scan areas compared with SD-OCT scans for a given acquisition time, it presents a faster scanning speed, resulting in a higher number of scans for a given