**3. Measure parameters in OCT-A**

With OCT-A it is possible to quantify the macular vascular density (MVD), defined as the proportion of vessels related to the total area studied over the base of a binarized image [9]. It is necessary to interpret the images with caution as the values vary according to sex and age, and they are related with image quality. In general, MVD values are progressively minor from healthy people to diabetic patients without DR and PDR [9, 17].

It is especially clear that the visualization of the foveal avascular zone (FAZ) has improved with the introduction of OCT-A. In healthy eyes, FAZ has very well-defined edges without interruptions [8]. FAZ enlargement can result from capillary occlusion with perifoveal arteriole loss easily identified by OCT-A [17]. In addition, there is a clear association between FAZ enlargement and vision loss in patients with DR [18]. OCTA is better than FA for FAZ detection because there is no interference caused by colorant leakage, it is noninvasive and fast, allowing a better imaging comparison over *Optic Coherence Tomography Angiography in Diabetic Retinopathy DOI: http://dx.doi.org/10.5772/intechopen.108077*

the time. Several studies show that FAZ enlargement already occurs in patients with DM1 and DM2 before any sign of DR can be detected [19–22]. These findings could be useful to identify patients without DR but with higher risk to develop the disease. However, there is a great variability in FAZ size in healthy people (0.071–0.527 mm2 ), with higher values in eyes with short axial length [9]. For this reason, FAZ circularity more than size may be useful to identify pathological changes. Some of the quantitative parameters about the ZAF include: ZAF area, acircularity index (AI), axis ratio (AR), and perimeter. AI is the ratio between the ZAF perimeter and the perimeter of a circle with equal area. The AR is the ratio between major and minor axis [13]. AI and AR are greater in patients with DR and greater in patients with PDR versus NPDR [23].

## **3.1 New parameters**

Some of the new parameters that are being studied include: vascular diameter index (VDI), fractal dimension (FD), vascular tortuosity (VT), perfusion density (PD), and skeletonized vascular density (SVD) [9, 13]. VDI is an index of the mean vessel caliper [9]. Increased values are related to rapid increases in blood glucose [9]. FD measures the complexity of the vessels pattern. FD values are lower in diabetic eyes in the SCP and DCP, but it is not related to DR severity [24]. VT is higher in diabetic patients and could be useful for early detection of DR [9]. PD is an index of vessel perfusion in a determinate area, and lower values are related with DR severity and BCVA [25]. SVD vessels longitude eliminating potential confusing factors being more sensible in identifying non-perfusion. SVD in SCP and DCP is inversely related with DR severity [26].

OCTA is also useful to identify and quantify macular ischemia (MI) in the different retinal plexus.

Diabetic macular edema (DME) is the leading cause of vision loss in patients with DR and can be visualized in the OCTA en face as non-flow areas with smooth edges that do not follow surrounding vessels [27].

## **3.2 The role of OCTA in DR early detection**

MVD is reduced in DM1 and DM2 patients in the SCP and DCP in the foveal and perifoveal area without detectable DR [28–31].

FAZ is also affected, being greater in patients without DR [22, 32].

Sun et al. have found an association between FAZ, MVD and FD values in the DCP with the risk of develop DR [33]. Greater FAZ area was related with a risk ratio (RR) of 1.829 for increase of standard deviation (SD), low MVD with a RR of 1.908 for decrease of SD and low FD with a RR of 4.464 for decrease of SD [33]. Furthermore, low values of MDV in the SCP are related with the development of DME [33].

## **3.3 DR severity**

Increased FAZ, reduced FAZ circularity, low MVD, increased VDI, reduced FD, and increased VT are associated with DR severity [25, 34–38]. Basing on these findings, it has been investigated the possibility to automatically detect and grade the severity of DR with the aid of the artificial intelligence and using the MVD, vascular caliper, and FAZ in the SCP and DCP with an accuracy of 94.3% (area under the curve (AUC) = 0.92) and reaching an accuracy of 96% (AUC = 0.96) when OCTA is combined with OCT [39].
