*Optical Coherence Tomography Angiography (OCT-A): Emerging Landscapes in Neuro… DOI: http://dx.doi.org/10.5772/intechopen.110810*

decorrelation algorithm, etc. These algorithms produce three-dimensional images of the retinal vasculature in less than 3 seconds. The images are created by taking repeated B-scans of the same cross-section of the eye and detecting changes in the phase and intensity of the OCT signal caused by the movement of red blood cells. OCT-A systems can be used to acquire images with a field of view larger than 66 mm, with varying levels of lateral resolution [5]. Images with a wider field of view and improved lateral resolution can be obtained by using a high-speed Swept Source (SS) OCT-A system or wide-field montages (**Figures 1** and **2**).

In addition to traditional OCT functions, OCT-A technology allows for the analysis of multiple retinal layers and corresponding vasculatures, including the inner retina, middle retina, outer retina, choriocapillaris, and choroid. This enables the examination of various vascular features, such as the presence of neovascularization, increased tortuosity, and areas of capillary loss. Quantitative analysis, such as measuring the area of the foveal avascular zone (FAZ) and the relative density, can also be performed using methods like fractal analysis or pixel counting. The flow rate, which is the average decorrelation value of sequential B-scans, can also be used as a surrogate for blood flow rate. The Optovue™ system includes an analysis package that automatically analyzes the FAZ and vessel density in different retinal sub-regions [5]. **Table 1** shows some common OCT-A parameters.

#### **Figure 1.**

*Wide field montage of healthy retinal microvasculature. The retinal nerve fiber layer is shown in red, the ganglion cell and inner plexiform layers are shown in green, and the inner nuclear and outer plexiform layers are shown in blue. (b), the fovea (c), and (d), the temporal region are all indicated by white boxes. (Reprinted from ref. [14]).*

#### **Figure 2.**

*General scanning protocol for optical coherence tomography angiography (OCT-A). (a) To find the relative flow signal, several B-scans are performed on the "x" fast axis at each of the "y" slow scan axis points. (b) Top view of the same basic scan pattern as in (a), with multiple B-scans performed along each "y" location of the slow axis on the fast axis. AngiovueTM OCT-A was used to obtain sample scans. (a, b). Reprinted from ref. [5].*


*\*Vessel density can be measured by area- or length-based measurements. Length-based measurements are more sensitive to changes in small capillaries; in area-based measurements, larger vessels have greater influence. Synonyms for vessel density: vascular/microvascular/capillary/flow density. Reprinted with permission from ref. [15].*

#### **Table 1.**

*Commonly used OCT-A expressions and parameters.*
