**2.1 The development of optical coherence tomography**

OCT technology was first developed at the Massachusetts Institute of Technology by David Huang in 1991 [10] (**Figure 4**). It is a non-invasive imaging methodology for acquiring high resolution images of the retina and optic nerve using low-coherence interferometry. In this technique a beam of light is shone into the eye and reflected light is captured and measured. By comparing the reflected light to an unobstructed reference path, the location and morphology of ocular structures can be determined to generate a cross sectional image of the eye [10, 25]. This imaging modality provides key structural information such as retinal thickness (μm), RNFL thickness (μm), and abnormal reflectivity on retinal layers which could indicate the presence of edema, exudates, calcifications, or atrophy [26, 27]. Thus, OCT technology introduced a novel and precise method for visualizing and measuring defects in the optic nerve, retina, and nerve fiber layer. Soon after the publication of the first images of the optic disk, Humphrey instruments introduced the first commercial OCT scanner in 1996 [11]. Ten years later, Optovue's spectral domain (SD-OCT) scanner was the first to obtain FDA approval for clinical use [12].

#### **Figure 4.**

*Milestones in the development of optical coherence tomography angiography [10–24].*
