As OCT-A devices become faster, the algorithms advance and the instrument costs decrease, more widespread utilisation in retina practices is likely.
Optical coherence tomography angiography (OCT-A) is an evolving multimodal imaging modality with some barriers to its widespread usage and implementation in retina practices, most of which are related to its financial burden, for example, the cost and reimbursement. Another limiting factor is that many retina physicians are not trained to interpret the data and images that OCT-A produces.
Standardised criteria and guidelines should be defined in order to help retinal physicians interpret the scans. As a specialist in retina and vitreous diseases, I have had the unique opportunity to use this non-invasive imaging technique for regular diagnosis, management and follow up of retina patients in my practice. Herein, I will share my ‘real-world’ experience of using OCT-A daily in three main areas of macular disease: age-related macular degeneration (AMD), central serous retinopathy and other pachychoroidal cases.
Commercially available OCT-As are either spectral domain (SD) OCT-As—which include the Angioplex OCT-A (Carl Zeiss), the AngioVue OCT-A (Optovue) and the Spectralis OCT-A Module with TruTrack (Heidelberg Engineering)—or swept-source (SS) OCT-A, which are the Plex Elite OCT-A (Carl Zeiss) and DRI OCT Triton (Topcon). An SS-OCT-A has a centre wavelength of approximately 1,060 nm compared with SD OCT-A, which is approximately 840 nm. SS-OCTA provides a higher scanning speed of 100,000 to 200,000 A-scans/second that is coupled with a longer wavelength and less sensitivity roll-off, hence denser, wider, deeper and faster scans can be obtained.
OCT-A can demonstrate layer-specific flow and structural information that is not possible with dye-based imaging. Recent research has shown that the technique provides good visualisation of the neovascular network, choriocapillaris and choroid compared with fluorescein angiography (FA) and indocyanine green angiography (ICGA).1
OCT-A can provide information about the presence or absence of choroidal neovascularisation (CNV) when other dye-based studies may be inconclusive due to blockage from subretinal haemorrhage or non-specific diffuse staining patterns.2 OCT-A is also helpful for positively identifying a neovascular network and differentiating vascular from avascular components in subretinal hyper-reflective material or vitelliform cases seen in macular diseases.3,4
I primarily use OCT-A (AngioPlex) for tracking and diagnosing AMD, assessing choroidal neovascularisation and determining whether a patient has wet or dry AMD or central serous retinopathy. If I examine an eye that reveals the presence of fluid on standard OCT but I am not entirely sure that there is a neovascular process, I obtain an OCT-A scan to verify the presence of fluid. The results help to guide my treatment choices.
Another benefit of using OCT-A is to investigate what is traditionally called dry macular degeneration, where we do not see any exudation on OCT but we do see something on OCT-A, which Dr Philip J. Rosenfeld refers to as the double-layer sign. This means that there is a low-lying pigment epithelial detachment, which has vascularised. I like to refer to it as neovascular non-exudative AMD.
Abnormal ICGA findings, such as focal hot spots or plaques, were previously detected in these eyes but angiography could not be justified as a screening tool due to increased costs and risks. However, with access to OCT-A imaging, I can assess a neovascular process and diagnose and follow these patients accordingly. Even though I may not treat them with anti-vascular endothelial growth factor drugs, OCT-A results help me to alter my follow-up algorithm so that I monitor those patients every 8–12 weeks, which is more closely than normal because they have a higher likelihood of developing exudative macular degeneration over the course of time as demonstrated in natural history studies.5
Understanding your machine
It is very important to understand your individual machine and recognise that image interpretation is a dynamic process that may require direct interaction with the OCT-A device. It is also important to choose the correct segmentation slab to obtain the right information from the OCT-A image.
I usually obtain 3x3 and 6x6 scans of the macula. The scan segments the superficial retina, the deep retina, the choroid and the choriocapillaris, and I create a custom slab retinal pigment epithelium (RPE) to RPE-fit (Bruch’s membrane) to detect type 1 CNVs.
It is also important to critically analyse the OCT-A images and exclude the possibility that the presumed CNV flow may be a projection artifact arising from the retinal vasculature or due to hyper-transmission defects arising from an attenuated or absent RPE.6 The software algorithms continue to improve, but sometimes they make segmentation errors, such as in cases with large pigment epithelial detachments (PEDs).
In these cases, I recommend the user manually segments these layers to obtain an accurate representation of the PED. Therefore, it is important to be familiar with how to create segmentation slabs that enable the parameters that the machine provides to be manipulated in order to adequately analyse the OCT-A images.
As we look towards the future and as OCT-A and, more specifically, SS-OCTA technology evolves, we will be able to obtain wide-field and montage images that will provide us with greater insight into the pathophysiology and management of diabetic retinopathy and allow earlier identification of pathologic changes. With time, as the devices become faster, the algorithms advance and the instrument costs decrease, I expect more widespread utilisation of OCT-A in retina practices.
Dr Jorge Fortun, MD
Dr Fortun is an associate professor of clinical ophthalmology and medical director of Bascom Palmer Eye Institute at Palm Beach Gardens, University of Miami, Florida, United States. Dr Fortun is a consultant for Carl Zeiss Meditec.