Mehal Rathore, B.Optom

Graduate Research Scholar, Elite School of Optometry, A unit of Medical Research Foundation in collaboration with SASTRA University, Thanjavur, India.

Retinal vasculature imaging and cross-sectional en face imaging of the retina have been two separate entities for the longest time. These two important pillars of retinal management have been seamlessly combined using the Optical coherence tomography angiography, popularly referred to as the OCTA.

OCTA is an upgrade to the OCT technology to evaluate the vasculature of the posterior segment of the eye ⁽¹⁾. OCTA provides in vivo images and information about the vasculature or blood flow of the human eye ⁽¹⁾ (Figure 1). This non-invasive instrument produces fine cross-sectional, high contrast images of the posterior vasculature of the eye with an acquisition speed of very few seconds per image ⁽¹⁾.

Figure 1: A 14 mm X 14 mm angiography montage scan of a human eye with proliferative diabetic retinopathy as captured by an OCTA device. (Image courtesy https://www.zeiss.com/meditec/int/product-portfolio/optical-coherence-tomography-devices/cirrus-6000-performance-oct/angioplex.html accessed on 4th July 2020.)

Prior to the OCTA, Ocular blood flow was being evaluated using classical methods such as the invasive and dye-dependent Fluorescein Fundus angiography (FFA) ⁽²⁾ and the highly variable and less-sensitive Doppler OCT and Laser speckle flowgraphy ^{(3, 4)}, to name a few. Unlike the FFA, the OCTA does not require the injection of any additional dye to visualize the vessels. It provides both qualitative and quantitative information, even at a capillary level making it a safe clinical alternative.

The OCTA finds its application in the diagnosis and management of posterior segment vascular disorders, especially the conditions which are a result of anomalous vascularity, vessel geometry and lack of blood flow ⁽⁵⁾. The convenience and quick acquisition of the device has proven to be a valuable asset in the diagnosis and management of retinal vascular occlusive diseases, diabetic retinopathy, uveitis, inherited diseases, age-related macular degeneration, and disorders of the optic nerve such as glaucoma ⁽⁶⁾.

 Similar in principle and procedure to the OCT, The OCTA employs interferometry to reflect light from the red blood cells in circulation (Diffractive particle movement of the red blood cells). It uses an intensity-based filtering technique which captures the change in contrast with time and that gives us the geography of the vessels ^{(1, 2)}.

 Commercially-available variants of the OCTA are able to segment different layers of the retina into “slabs” to visualize the vasculature ⁽⁷⁾. The slabs include the Total retina (ILM to RPE), Superficial retinal layer, deep retinal layer, choroid, choriocapillaris, avascular zone (Estimated position of OPL till the boundary between inner and outer segments of the photoreceptors) and the combined retinal and choroidal vasculature (Figure 2). The instrument individually analyses the slabs and gives quantitative information such as the blood flow index, vessel density and perfusion percentage across all the quadrants of the area being examined ⁽⁷⁾.

Figure 2: A sample 3 mm X 3 mm OCTA report providing quantitative information on vessel density and visual qualitative information on the vasculature across the different segmented layers or “slabs” .(Image Courtesy: Electronic medical records, Sankara Nethralaya, Medical Research Foundation.)

The current commercially-available OCTA devices, such as the Zeiss CIRRUS HD-OCT AngioPlex (Carl Zeiss Meditec Inc, Dublin, USA) as shown in Figure 3, RTVue XR Avanti (Optovue Inc., Fremont, USA), etc work on similar principles but on different algorithms for image acquisition. The four functioning image acquisition algorithms are Split-Spectrum amplitude-decorrelation angiography algorithm (SSADA), Optical Microangiography algorithm (OMAG), OCTA Ratio analysis (OCTARA) and Eye Track system (TruTrack) ^(1,2,8,9).  The image processing through these algorithms plays a key role to determine the quality of the image and as it’s proprietary, it is pivotal during comparison of the brands of OCTA instruments.

Figure 3: Image of a commercially available OCTA device. (Image Courtesy: https://www.zeiss.com/meditec/int/product-portfolio/optical-coherence-tomography-devices/cirrus-6000-performance-oct.html accessed on 4th July 2020)

To conclude, the role of OCTA in the diagnosis of vascular disorders is remarkable. The instrument has the potential to replace FFA and Indocyanine Green Angiography (ICG) in the management of certain retinal and choroidal conditions.  With the advancements in technology and thriving clinical usage, OCTA is on the path of becoming a mainstream diagnostic tool.

References:

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7. Rosenfeld, P. J., Durbin, M. K., Roisman, L., Zheng, F., Miller, A., Robbins, G.,  & Gregori, G. (2016). ZEISS Angioplex™ spectral domain optical coherence tomography angiography: technical aspects. In OCT angiography in retinal and macular diseases ,Vol. 56, pp. 18-29.
8. Zhang, A., Zhang, Q., Chen, C. L., & Wang, R. K. (2015). Methods and algorithms for optical coherence tomography-based angiography: a review and comparison. Journal of biomedical optics, 20(10), 100901.
9. De Carlo, T. E., Romano, A., Waheed, N. K., & Duker, J. S. (2015). A review of optical coherence tomography angiography (OCTA). International journal of retina and vitreous, 1(1), 5.