Optical Coherence Tomography Angiography in Retinal Diseases

K V Chalam, Kumar Sambhav, K V Chalam, Kumar Sambhav

Abstract

Optical coherence tomography angiography (OCTA) is a new, non-invasive imaging system that generates volumetric data of retinal and choroidal layers. It has the ability to show both structural and blood flow information. Split-spectrum amplitude-decorrelation angiography (SSADA) algorithm (a vital component of OCTA software) helps to decrease the signal to noise ratio of flow detection thus enhancing visualization of retinal vasculature using motion contrast. Published studies describe potential efficacy for OCTA in the evaluation of common ophthalmologic diseases such as diabetic retinopathy, age related macular degeneration (AMD), retinal vascular occlusions and sickle cell disease. OCTA provides a detailed view of the retinal vasculature, which allows accurate delineation of microvascular abnormalities in diabetic eyes and vascular occlusions. It helps quantify vascular compromise depending upon the severity of diabetic retinopathy. OCTA can also elucidate the presence of choroidal neovascularization (CNV) in wet AMD. In this paper, we review the knowledge, available in English language publications regarding OCTA, and compare it with the conventional angiographic standard, fluorescein angiography (FA). Finally, we summarize its potential applications to retinal vascular diseases. Its current limitations include a relatively small field of view, inability to show leakage, and tendency for image artifacts. Further larger studies will define OCTA's utility in clinical settings and establish if the technology may offer a non-invasive option of visualizing the retinal vasculature, enabling us to decrease morbidity through early detection and intervention in retinal diseases.

Keywords: Age Related Macular Degeneration; Choroidal Neovascularization; Diabetic Retinopathy; Optical Coherence Tomography Angiography (OCTA); Split-spectrum Amplitude Decorrelation Angiography (SSADA).

Figures

Figure 1
Figure 1
Comparison of images of fundus florescence angiography (a) to OCTA images of superficial plexus (b and c) in a normal subject. Early images of FA (15.2 sec) shows normal fillings of arteries and veins. OCTA (6 × 6 mm) of superficial plexus (b) showing normal capillary network with better resolution of smaller capillary network. OCTA (3 × 3 mm) of superficial plexus (c) showing clear delineation of foveal avascular zone and well defined capillary network.
Figure 2
Figure 2
The location of different en-face zones in relation to histology of the human retina. The four en-face zones include, (i) the superficial plexus, the capillary network in ganglion cell layer and nerve fiber layer; (ii) the deep plexus, a network of capillaries in the inner plexiform layer with offshoot of 55 micron; (iii) the outer retina (photoreceptors), and (iv) the choriocapillaries (choroid) with offshoot of 30 microns.
Figure 3
Figure 3
Graphical representation of 4 en-face zone. Each enface zone is divided into inner parafoveal (1.3 mm) and outer perifoveal (3.6 mm) regions. Perfusion indices (vessel density and flow index) are obtained separately for perifoveal and parafoveal regions sparing central foveal avascular zone of 1 mm.
Figure 4
Figure 4
Optical coherence tomography angiography (OCTA) of the foveal avascular zone (FAZ) obtained using inbuilt software (mm2). (a) FAZ area in the superficial plexus of a normal subject; (b) the FAZ area obtained in the deep plexus of a normal subject.
Figure 5
Figure 5
Images of a patient with non-proliferative diabetic retinopathy but no macular edema. (a) Macular thickness map using spectral domain optical coherence tomography (SD-OCT). (b) Conventional fluorescein angiography (FA) with leaking micro aneurysm and ischemia temporal to the fovea. (c) optical coherence tomography angiography (OCTA) image of the superficial plexus of the same patient shows better delineation of the ischemic area as compared to FA.
Figure 6
Figure 6
Images of a patient with mild non-proliferative diabetic retinopathy (a and b) in comparison to a patient with severe non-proliferative diabetic retinopathy (c and d). There is decrease in perfusion indices (vessel density and flow index) as the severity of diabetic retinopathy increases.
Figure 7
Figure 7
Image of a patient with mild non-proliferative diabetic retinopathy and diabetic macular edema. (a and b) Showing the en-face image of the superficial plexus with distortion of foveal avascular zone (FAZ). (c and d) Showing the en-face image of the deep plexus with distortion of FAZ, more profound than in superficial plexus. This happens due do accumulation of fluid in retinal layers.
Figure 8
Figure 8
Optical coherence tomography angiography (OCTA) of a patient with CNV (mixed type) with corresponding spectral domain OCT. (a) The OCTA image of superficial plexus, with associated foveal cystic changes. (b) “Medusa head” appearance of neo-vascular (NV) membrane in the photoreceptor zone (white arrows). (c) NV membrane extends in the deeper choroid (red arrows) with extensive arborization.
Figure 9
Figure 9
Optical coherence tomography angiography (OCTA) images of a patient with choroidal neovascularization (CNV) before (a.c) and 2 weeks after treatment with anti VEGF (d.f). (a) OCTA image of the deep plexus, with cystic changes causing obliteration of vascular reflection. (b) NV membrane in the photoreceptor zone. (c) NV membrane extending in the deeper choroid. (d) OCTA of the deep retinal plexus after treatment with anti-VEGF. Vessels in the deeper retinal plexus are better visualized due to regression of cystic changes. (e) Regression of the NV membrane in the photoreceptor zone. Obliteration of smaller vessels and decrease in the size of main feeder trunk is noted. (f) Regression in the size of NV membrane is also noted in the choriocapillary layer.
Figure 10
Figure 10
Comparison of fluorescein angiography image (a) in branch retinal vein occlusion with that of optical coherence tomography angiography (OCTA) images obtained at the superficial (b) and deep (c) retinal plexus. Ischemic area due to capillary drop out is better delineated using OCTA. However, leaking vessels due to neovascularization are not identified using OCTA.
Figure 11
Figure 11
Conventional fluorescein angiography (FA, a) in comparison to optical coherence tomography angiography (OCTA) images of the superficial (b) and deep (c) plexus in a patient with sickle cell disease. Temporal area of thinnin (confirmed using spectral domain OCT) showed capillary dropout on OCTA, which is not as clearly visible on FA. (b) Temporal area of avascularity in superficial plexus, calculated using in built software was 2.204 mm2. (c) Temporal area of avascularity calculated using in built software in deep plexus was 1.972 mm2.

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