# En-Face Optical Coherence Tomography Angiography for Longitudinal Monitoring of Retinal Injury
## 1 Introduction  
Recent advances in the fields of cellular and molecular biology have determined that the biological and pathophysiological processes involved in the onset and progression of retina neuroinflammatory and degenerative diseases are inflammation accompanied by tissue and vascular remodeling. An important aspect related to the understanding of retina neurodegeneration that can be studied in animal models is making a distinction between how an injury transitions from acute to either a resolving phase or a chronic response [1; 2; 3]. This response is defined as the pro-inflammatory acute or early phase that lasts only a few days before transitioning to the resolving phase where processes such as wound repair and scarring occur. In contrast, chronic inflammation is where the resolving phase does not occur, and instead the pro-inflammatory progression of the acute phase persists long after the initial insult is removed. Specific changes in the different capillary beds in response to injury have not previously been studied in-vivo comparing vascular alterations in acute and chronic phases. CNV is clinically diagnosed with a variety of image-based methods, but most commonly with fluorescent angiography (FA) using either fluorescein or indocyanine green (ICG) dyes delivered intravascularly as a contrast agent [4; 5]. Vascular leakage visualized by ICG or FA indicates the presence of altered vascular permeability; although, rare cases of anaphylaxis may occur with Fluorescein or ICG injections [6]. While FA has utility to be used in the clinic as the procedure is established, the correlation and interpretation of OCTA are not yet fully understood; however, OCTA can show alterations in individual vascular beds. The murine laser-induced model of CNV (L-CNV) induces an inflammatory response in the Retinal Pigment Epithelial Layer (RPE) that is known to be neurodegenerative and promotes pathological neovascularization [7]. Low-dose laser photocoagulation that is below the CNV threshold can cause photoreceptor ablation, an acute injury. In the acute injury dose range, there are two key thresholds; first is the safety threshold (subvisible [8]) in which photoreceptors are stressed, but there is no immediately observable lesion, and secondly, when the intensity is sufficient to ablate both the photoreceptors and the RPE cells inducing CNV. Currently, the CNV model is used in preclinical treatment efficacy studies, including anti-VEGF [9] and anti-TNF\(\alpha\)[10] therapies, where FA and histology are the current gold standards to measure the impact of drug treatment on limiting the severity of CNV. It is important to note that the low dose laser photocoagulation and photoablation, such as used in surgery, results in acute inflammation that may affect the contralateral eye [11], but does not develop into chronic inflammation [12]. Acute injury lesions lead to permanent photoreceptor ablation and subsequent scarring without directly affecting the vasculature [13; 14]. Accurate data models that can differentiate between acute and chronic phases are needed to be able to predict the effect of laser injury modulating inflammation [15; 16], as the subsequent retinal thickness changes may occur days or weeks later [17; 18]. OCT provides the non-invasive ability to create high-resolution depth profiles of the retinal structure without any contrast agent. OCT B-scan imaging is comparable in the resolving power to measure retinal layers via traditional histology [19; 20]. In mouse models, the histological findings are well correlated to measurements with the OCT B-scans [21; 22]. The C-scan, or en-face OCT scan [23], is a slice through the volume providing a more traditional _fundus-like_ view. The C-scans can be combined via projection methods as an OCT _slab_, or a portion of the depth volume, containing capillaries of the relative retinal layer. Clinical OCTA systems have been used to characterize Diabetic Retinopathy (DR) and CNV in human patients after diagnosis by traditional methods [24; 25; 26].