# Clinical Applications of Optical Coherence Angiography Imaging in Ocular Vascular Diseases
## 2 Diabetic Retinopathy
### Role of OCTA in Diabetic Retinopathy
#### 2.2.2 OCTA for Proliferative Diabetic Retinopathy  
The main role of OCTA in the diagnosis and monitoring of proliferative diabetic retinopathy includes the detection of neovascularization. On cross-sectional OCTA, neovascularization appears as a positive flow signal extending from the retinal surface into the vitreous (Figure 1--yellow circle) [36]. [PERSON] et al. demonstrated that segmenting flow signal at the inner boundary of the superficial plexus, which is the internal limiting membrane, and projecting the signal in cross-sectional orientation enables visualization of vertical neovascularization. The flow signals are seen as shadows as they are elevated out of the depth range of OCTA [37]. This differs from intra-retinal microvascular abnormalities, which are confined to the retinal layers [36]. Areas of neovascularization can be clearly identified on consecutive OCTA imaging as margins are not obscured by leakage. This allows \"sentinel\" neovascular lesions to be closely monitored for regression in response to therapy with pan-retinal photocoagulation or intravitreal anti-vascular growth factor injections (Figure 2--yellow and green circles) [4; 36]. A recent study used both cross-sectional OCT and OCTA to show that active neovascularization can be differentiated from fibrovascular tufts and proliferation into the vitreous by the presence of flow signals. On the en face OCTA, active neovascularization was seen as vascular tufts and regressed neovascularization in response to injections were visualized as ghost vessels or pruned vascular loops [36]. The efficiency and ease of OCTA imaging allows consecutive imaging to be performed at every follow-up visit, which was previously not possible with conventional angiography [38]. Hence, parameters such as macula vessel density and FAZ can be closely monitored. These parameters  
Figure 1: Multi-modal imaging of a case of proliferative diabetic retinopathy and vitreous hemorrhage. (**A**): Wide-field fundus photograph depicting scattered retinal hemorrhages with vitreous hemorrhage obscuring the temporal and inferior view of the retina, (**B**): Fundus fluorescein angiography (FA), (**C**): Cross-sectional optical coherence tomography angiography (OCTA) with flow indicated in red, and (**D**): Color-coded depth-resolved en face OCTA montage, which is a fusion image of the superficial (red), deep (green), and avascular (blue) en face OCTA slabs. New vessels (yellow circle) are seen on FA as areas of leakage and on cross-sectional OCTA as areas of flow extending from the retinal surface into the vitreous, which corresponds to fronds of vessels on en face OCTA. Figure 2: Regressed neovascularization on OCTA. (**A**): Regressed neovascular lesion (blue circle) seen on vitreous segmentation on en face OCTA with the corresponding cross-sectional OCTA. (**B**) Showing the regressed neovascular lesion with patchy area of flow (indicated by red color) extending from the retina surface into the vitreous; pink lines on B depict the boundaries of vitreous segmentation; (**C**): Cross-sectional OCTA image of a regressed neovascularization lesion of the disc with no flow (yellow circle) corresponding to the en-face OCTA Montage; (**D**): Showing the regressed neovascularization of the disc with no flow (yellow circle) and flow voids (white arrows) after panretinal photooagulation. have been shown to be early predictors of DR and correlate to DR severity and visual function, with the FAZ enlarging with progression of DR [4; 29; 39]. [PERSON] et al. found a significant decrease in capillary perfusion density in nearly all retina layers of diabetic patients compared with controls. This decrease in perfusion density worsened according to disease severity [40]. Looping vessels on OCTA adjacent to areas of impaired capillary perfusion have also been demonstrated to be consistent with intraretinal microvascular abnormalities. Of note, these are not visualized in areas with good capillary perfusion (Figure 3--yellow circles) [30].