# En-Face Optical Coherence Tomography Angiography for Longitudinal Monitoring of Retinal Injury
## 4 Discussion
Although inner capillary dropout is a known contributing factor to retinal ischemia in glaucoma and diabetic retinopathy visible through OCTA [55], it has not previously been attributed as a feature of CNV or AMD. With the swelling and edema in the early phases of CNV, the displacement of the inner layers may cause the vasculature to be pinched, restricting or stopping blood flow altogether. The CNV lesion dynamics of the RPE and PR layers in the chronic phase correlate to the in-vivo findings of previous studies [27; 56], indicating that the resulting pathology exhibits similar features in all studies. Therefore, the combined orthogonal OCT and OCTA imaging approach allow for the study of how the swelling and edema in inflammation progression causes displacement that affects the microvasculature of the inner retina. Furthermore, the method of post-processing allows for comparison of the dynamics of acute vs. chronic injury and specifically offers a longitudinal method to non-invasively correlate the edema formation to vascular changes. The contrast of the vasculature visualized with OCTA is dependent on the spatial/temporal speckle pattern of erythrocytes in blood flow. The algorithm we developed utilizes the spatial domain component of OCT speckle patterns to highlight and extract the angiogram from an en-face OCT volume [36; 57]. The high-density volumetric scanning of en-face OCT allows for digital re-slicing and processing with a custom 3D vessel enhancement algorithm extending the principles of vessel detection strategies for fundus images [58]. Our OCTA methodology allowed the findings in this study to be confirmed across subjects by re-slicing the volumes to focus on the inner retina vasculature that was perfectly registered to the high-resolution B-scans of the lesions. Additionally, by using a single high-density scan and the spatial portion of the speckle pattern, instead of repeated scans, the algorithm covers a wider area. The tradeoff to the approach is that the flow rate cannot be calculated through this method. However, currently flow rate cannot be validated, thereby large area screening of vascular morphology was a better utilization of the data. As a variant of the speckle decorrelation algorithm, our en-face OCTA methodology is capable of resolving the microvascular morphology comparable to that which was observed in the previous rodent studies [32]. In human CNV studies using OCTA, only the choriocapillaris was compared to the FA imaging [27; 59], and OCTA is highly correlated to the leakage. While FA is used clinically to determine the presence or absence of vascular leakage or edema, lectin staining is used in pre-clinical trials for measuring vascular dysfunction indirectly as leakage area. Lectin staining on rat retinal flatmount has shown that there is no increase in lesion growth after day 10; however, such methods cannot assess the functional aspect of microvasculature perfusion under in-vivo conditions [37]. Our longitudinal analysis of the inner retina non-perfusion and reperfusion was only possible using an OCTA methodology. OCTA with blood flow velocity analysis or new oximetry techniques [60; 61] is required to verify that there is a temporary loss of the blood perfusion increasing localized hypoxia.