3. Comparison with Fluorescein Angiography (FA) and Indocyanine Green Angiography (ICG)
The most important factor that sets OCTA apart from its counterpart vessel-imaging modalities is its non-invasive nature as it is not dependent on the use of a dye [21,22]
. This obviates the injection-associated trauma and dye-associated risks such as nausea, vomiting, and, rarely, anaphylaxis [23,24]
. By avoiding the use of an external dye, the time taken for the angiography is significantly decreased with the use of OCTA which is especially desirable in patients with painful disorders such as acute burns. The test can be repeated multiple times without any need for recurrent dye injections. Additionally, the ischemic zones may be obscured by leakage of the dye from the inflamed vessels in these eyes and this disadvantage is circumvented with OCTA 
The use of light in the infrared wavelength by the OCTA provides an added advantage as the device can image vessels beneath hemorrhages from superficial conjunctival vessels [6,26]
. This is of particular significance in eyes with acute burns as these traumatic sub-conjunctival hemorrhages (SCH) are common and may mask areas of ischemia when viewed via conventional angiographic techniques. However, shadow artifacts can also be seen in AS-OCTA images due to these hemorrhages. This contrast has been depicted in Figure 1
where in the inferior area the vasculature is obscured by the SCH. At the same time in the region abutting the inferior limbus, the vasculature is clear despite the presence of the SCH. One possible explanation for this discrepancy can be the relative position of the SCH when compared to the vessels. Very superficial and large hemorrhages may block the view of all the underlying vessels as opposed to deeper SCH which may allow the imaging of vessels overlying the hemorrhage.
Figure 1. (A–R) Serial monitoring with slit lamp photographs (without and with fluorescein staining) along with anterior segment optical coherence tomography angiography (AS-OCTA) imaging in an eye with acute chemical burns. (A–L) These images show the progressive healing of the corneal and conjunctival epithelial defects in the inferior part of the eye which is complete by day 7. (M–R) AS-OCTA images show a blocked signal in the initial three visits due to the subconjunctival hemorrhages inferiorly (blue arrow heads) with clear delineation of the vasculature following the resolution of the hemorrhages. The hemorrhages abutting the limbus, however, have not affected the vascular signal (yellow arrow heads).
The infrared wavelength of light also allows for better patient comfort and has a good safety profile as well 
. Localization of the exact depth of a vascular lesion is now feasible because OCTA allows segmentation of different layers of the imaged vessels [16,28,29]
. This is in contrast to FA and ICG which generate a composite two-dimensional image. The major drawback of OCTA when compared to the traditional angiography tools lies in its inability to identify the flow characteristics and thus, patterns of leakage, the difference between afferent and efferent systems, etc. cannot be captured with OCTA [30,31]
. In the context of ocular burns, this is relevant as images from OCTA may not distinguish vasospasm from true ischemia and thus caution must be exercised during interpretation of OCTA scans. Furthermore, the field of view is larger in FA and ICG and multiple scans would be required by OCTA to cover the same surface area [3,24]
. As OCTA images are derived from the motion of blood cells, this process can be confounded by motion originating from other sources, such as movements of the patient’s eyes or head. Although the eye tracking feature of retinal OCTA can counteract these artifacts, its incorporation in anterior segment images has not been completely established.