# Clinical Applications of Optical Coherence Angiography Imaging in Ocular Vascular Diseases
## 7 General Limitations of OCTA  
While OCTA may seem comparable to FFA and ICG in studies, there are still many limitations that need to be considered, especially in imaging of the retina and posterior segment. Firstly, current commercial OCTA platforms have a limited field of view. At present, en-face OCTA scanning area ranges from 2 mm \(\times\) 2 mm to 12 mm \(\times\) 12 mm. Due to limitations in scanning speed and image resolution, increasing the field of view worsens the degree of resolution and increases the time taken to capture the image, thus also potentially increasing the risk of more artefacts. Recent studies have described an extended field technique, which uses optical aid attachments on current OCTA machines to increase the field of view [59; 86]. Creating a montage of multiple separate OCTA images to obtain a high resolution wide-field image is also possible and is an automated function available on some machines. However taking multiple OCTA images of different retina areas is time consuming and will require good fixation by the patient for good quality images [3; 4; 78].  
As OCTA relies on change between consecutive B-scans, the slowest detectable flow is limited by the time between consecutive scans. Therefore, areas of slow blood flow such as in MAs, thrombi and fibrotic choroidal neovascularization can be missed [4]. Studies have also demonstrated that fewer MAs are visualized on OCTA compared to FA [4; 24]. In addition, as flow information is only acquired at a fixed point in time, leakage is not appreciable on OCTA [3].  
OCTA also relies on auto segmentation technology to delineate different retina layers. En face segmentation defects are common in edema or retina thinning where the normal retina anatomy has been disrupted. In addition, layer segmentation methods and definitions are not standardized, which may create discrepancies between studies. Inaccurate automated segmentation may have to be adjusted manually to better capture accurately the various venous plexi [4; 7]. Thus, it is important to look at OCTA images and structural OCT images in tandem in order to overcome this.  
OCTA may also be prone to artefact. The validity and reproducibility automated quantification of various parameters measured in the SCP and DCP are dependent on not only accurate segmentation but also the lack of projection artefact. Projection artefacts occur when more superficial flow signals are projected onto deeper retina layers [87]. One recent study showed that projection-resolved OCTA parameters significantly improved the vascular density analysis of the DCP [33]. Other artefacts include shadow artefacts from larger retinal vessels and artefacts created by fine movement from pigment epithelial defects and fine tissue [3]. Like other optical imaging systems, OCTA images can also be confounded by speckle artefact. Development of speckle noise reduction software may allow for clearer image acquisition and segmentation [88; 89; 90]. In addition, the OCTA system also requires good fixation from the patient and is sensitive to motion and blink artefact [3].  
Multiple vendors currently produce OCTA technology so variations in hardware and software algorithms need to be taken into account [6; 10; 78; 80]. With the increasing availability of automated quantitative parameters, it is mindful to note that the images and quantification are not comparable between different OCTA systems [91]. Lastly, although a trained technician can easily obtain OCTA images, there is still a significant learning curve to overcome when it comes to image interpretation [3].