OCTA in retina practice – Has it come of age?

The practice of retina has seen many “wow” moments over the last few decades, but probably none as impactful as the advances in noninvasive, high-resolution imaging techniques. The advent of optical coherence tomography (OCT) started the revolution in the way retina clinics function, providing a rapid, noninvasive, reproducible method to evaluate and follow up with patients who now need more frequent monitoring, with dye-based fundus fluorescein angiography (FFA) and indocyanine green angiography (ICGA) still playing a role in the initial assessment of many retinal disorders. However, in the last decade or so, OCT angiography (OCTA) has become a game changer by providing high-resolution, depth-resolved images of the retinal and choroidal vasculature without the need for dye injection. While initially considered purely a research tool with limited clinical application, it is now an almost indispensable part of patient care. OCT captures a volumetric image dependent on the scattering properties of the tissue by transversely scanning a focused beam and collecting the depth-encoded scattering profile (termed A-scan) through low-coherence interferometry. Transverse scanning captures a line of A-scans, termed a B-scan, and defines the fast (x) axis. Slower scanning in the orthogonal transverse direction (slow y-axis) then completes a volume scan. The principle of OCTA involves determining the change in backscattering between consecutive B-scans and then attributing the differences to the flow of erythrocytes through retinal blood vessels, which causes the signal to fluctuate. OCTA images are extracted by analyzing the time-varying OCT signal between acquisitions, most commonly cross-sectional scans (B-scans) at the same or closely adjacent locations. The processed volume created by the stack of OCTA B-scans is usually presented as a maximum intensity projection image in the transverse plane over a specified depth range.[1] While information provided from the superficial capillary plexus closely mirrors changes seen on FFA, the visualization of the deep capillary plexus and intermediate capillary plexus, which are not visible on FFA, has improved our understanding of various diseases. Similarly, the imaging of the choriocapillaries and choroidal plexus has reduced the need for the less easily available ICGA. The bulk of the workload in most retina clinics involves the detection and management of disorders involving the retinal and choroidal vasculature, notably diabetic retinopathy, retinal vein occlusions, and neovascular AMD. It is in these areas that OCTA has provided unparalleled insights.[2–5] Noninvasive nature, safety, and reproducibility. This eliminates the risk of allergic reactions to intravenous dyes and also permits quick repeatability if an area of interest has been missed. Detection of subclinical abnormalities in the microvasculature before the disease is clinically evident: This permits earlier intervention or more frequent follow-up, e.g. asymptomatic type 1 CNVM in AMD.[2] Screening and monitoring tool in diabetic retinopathy (DR): It not only enables the detection of classical features of DR, such as microaneurysms, intraretinal microvascular abnormalities, areas of ischemia, and neovascularization, but also identifies microvascular abnormalities before DR is clinically detected.[3] Evaluation of retinal vein occlusions and[5] CNV due to various etiologies. Monitoring treatment response in patients on monthly antiVEGF injections, along with OCT.[4] As a research tool: To quantify vessel density measurements, perfusion indices, and analyze flow and signal voids. Other less common but equally important conditions involving the retinal and choroidal vasculature: This includes hereditary, degenerative, inflammatory, and neoplastic conditions. Evaluation of the optic nerve in glaucoma, hereditary optic neuropathies, and ischemic optic neuropathies. While these exciting capabilities have propelled OCTA to a frontline investigation in retina practice, challenges do remain: Limited field of view in many commercial devices: Though with ever-evolving technological advances, newer wide-field OCTA machines do provide more extensive visualization. Imaging artifacts are probably the biggest challenge. These include[1]: Segmentation artifacts: Caused by improper segmentation of retinal layers by the software Motion artifacts: Caused by eye movement and poor fixation during scan acquisition—more common in wide scans and patients with a poor attention span. Blink artifacts Shadow artifacts: Caused by signal attenuation due to large vessels, media opacities, etc. Field-dependent aberrations: More often in wide scans, e.g. double rendering of vasculature. Projection artifacts or decorrelation tails: Due to light-matter interaction with vessels, causing projection of the superficial vascular network onto the deeper networks. While many of these are patient-related, projection artifacts are unavoidable. Hence, most commercially available machines have built-in projection artifact removal algorithms.Inability to detect leakage: While the ability to detect vasculature through fluid is an advantage in OCTA, the lack of ability to detect leakage itself is a limitation and may lead to erroneous decision-making. In these situations, other clues to the presence of leakage, such as FFA, may be required. Difficulty in imaging lesions with extremes of flow:[1] Commercial instruments usually collect between two and four B-scans at the same location. The value of the interscan time determines the maximum flow velocity capable of generating speckle contrast and, therefore, which vessels are visible in the OCTA image. Low decorrelation values are proportional to flow velocity, but full decorrelation leads to a saturation velocity value. Hence, there is a constant challenge to establish the optimum interscan time. Learning curve and the need to constantly update the technology. Challenges in imaging the choroidal vasculature: These have been largely improved by swept-source technology. Has OCTA in retina practice come of age? It is probably the most exciting advance in retinal imaging in recent times, steadily transitioning from a research tool to an important part of clinical retina practice. The major advantage of viewing vasculature in vivo has increased its scope to identifying the risk of life-threating systemic diseases based on changes in the retinal vasculature. While it may not completely replace traditional invasive angiography in all cases due to some of its limitations, continuous refinement of the technique with standardized acquisition and analysis protocols,[6] faster scan times, standardized nomenclature to enhance uniformity in reporting, and large data from clinical studies have increased its acceptance in routine clinical practice. Financial support and sponsorship: Nil. Conflicts of interest: There are no conflicts of interest.About the authorMuna Bhende Dr Muna Bhende is currently a senior consultant and Director, Shri Bhagwan Mahavir Vitreoretinal service, Medical Research Foundation, Sankara Nethralaya, Chennai, India. She is a fellowship trained vitreoretinal surgeon having over 3 decades of experience in managing medical and surgical vitreoretinal disorders and training ophthalmology residents and fellows in Vitreoretinal surgery and medical retina. She has 120 peer-reviewed publications, one of which was awarded the AIOS-IJO Platinum award. She has also received the AIOS- Rema Mohan award. Her special interests are retinal imaging and inherited retinal disorders. She is a member of the Macula society, VRSI, IntRIS and AIOS.