**7. OCTA introduction and the future of FFA**

Over the past years, OCT was used to assess the structural anatomy (3D) of the posterior segment, while fluorescein angiography (FA) was used to assess the retinal vasculature (2D). Then, optical coherence tomography angiography (OCTA) develops since 2014 to represent the functional extension of structural OCT that allows noninvasive visualization and qualification of the retinal and choroidal microcirculations, without the need for dye injections. Also, OCTA shows developmental progression in earlier diagnosis of glaucoma and neuro-ophthalmology diseases. OCTA decreases the use of the invasive FA especially after the diagnosis [15].

The main principle of OCTA is the detection of signal change over time emitted from intravascular blood cells motions. The scan consists of multiple individual A-scans using the laser light reflected from the surface of moving red blood cells, which integrated into a B-scan providing cross-sectional information (**Figure 4**). There are two methods used for motion detection: amplitude decorrelation (differences in amplitude between two different B-scan) or phase variance (the variation of phase of the emitted light wave when it intercepts moving objects). The same tissue area is repeatedly imaged, analyzing the differences between scans to differentiate between areas of high flow and that of slow/no flow [16]. The signals are amplified with different motion correction technologies aiming to improve the image quality.

OCTA machines are characterized by autosegmentation which referred to splitted slabs from a known anatomical layer of the retinal vasculature. In Optovue OCTA software, the four slabs represent the following: superficial capillary plexus SCP, deep capillary plexus DCP, outer retina and choriocapillaris as shown in **Figure 5**. While the SCP is seen on FA, the DCP is poorly seen on FA and hence the advantage of OCTA in diagnosis of certain pathological conditions as retinal angiomatous proliferation, paracentral acute middle maculopathy and parafoveal telangectasia. The outer retinal slab is useful in identification of type 2 (subretinal) neovascular membranes, while the choriocapillaris slab helps to detect early type 1 (sub-RPE) choroidal neovascular membrane (CNV) [15].

#### **Figure 4.**

*Basic principle of optical coherence tomography angiography; when the B-scans are sequentially taken from the same retinal location, any changes in signal amplitude or phase will represent the blood flow, and mathematical assessment of signal changes will represent the amount of the blood flow [15].*

#### **Figure 5.**

*Optovue OCTA four slabs; a: Inner retinal slab extends from 3 μm below the internal limiting membrane (ILM) to 15 μm below the inner plexiform layer (IPL) representing the superficial retinal vascular plexus. B: Middle retinal slab representing the deep capillary plexus; extends from 15 μm below the IPL to 70 μm below the IPL. C: Outer retinal slab showing no vessels in normal individuals, extends from 70 μm below the IPL to 30 μm below the retinal pigment epithelium (RPE). D: The last slab represents the choriocapillaris extending from 30 μm below the RPE to 60 μm below the RPE.*


#### **Table 1.**

*Main commercially available devices of OCT-A [16].*

Variable interscan time analysis (VISTA) allows visualization of different ranges of blood flow speeds using a color-encoded images in which high flow is represented in red color and blue represents areas of relatively low flow.

OCTA provides images of blood cell movements (perfusion) and not anatomical structure of the vessels. Areas of ischemia are represented as dark zones. There are different chorioretinal pathologies that can be diagnosed using OCTA; abnormal flow in areas with no flow (like presence of CNV in the outer retina), abnormal areas of non-perfusion in the SCP (normally there is a foveal avascular zone in the center) with an extension of the non-perfused area as in diabetic retinopathy.

There are many challenges in OCTA that are tried to be corrected by various technologies as shown in **Table 1** (as longer data acquisition times needed for repeated B-scans, corrected by SSADA, motion artifacts corrected by dual track technology). We should note that OCTA does not give us full details about retinal periphery, and also, it gives no information about blood retinal barrier (no dye to leak), an important sign in many retinal diseases. Some limitations and artifacts are also important to consider in the OCT-A images interpretation.
