New frontiers in ocular imaging
Optical coherence tomography angiography (OCTA) is a recent, non-invasive, and dye-less imaging technique for evaluating the vessels by capturing the dynamic motion of the erythrocytes. The OCTA device generates images of the superficial and deep retinal vascular plexus, outer retina and choriocapillaries layer. It has been shown to be a useful imaging modality for evaluation of ophthalmologic diseases, such as age-related macular degeneration (ARMD), diabetic retinopathy (DR), arteriovenous occlusions (AVO), and glaucoma. Retinal imaging group of IBMS focuses on application of advanced image processing and mathematical modeling techniques on OCT and OCTA images of different retinal diseases to diagnose and track the health status of patients and study the epidemiologic traits in the population.
Optical coherence tomography angiography (OCTA) in normal eyes
We performed this study to evaluate a fully automated local fractal dimension method to quantify vessel density and foveal avascular zone (FAZ) area in optical coherence tomography angiography (OCTA) images. OCTA images of superficial and deep retinal vascular plexus was imaged. FAZ area and vessel density were quantified in circular and sectoral zones around the fovea. A unique contour map of vessel density and dropout zones was developed to perform regional comparisons. The vessel density was greater in the deep than the superficial retinal vascular plexus. When the image was subdivided into sectors around the FAZ, inferior sector had greater vessel density than the others (temporal, superior, and nasal) in both superficial and deep retinal vascular plexus. These observations were similar to recent studies on animal retinal vasculature map. In conclusion, we demonstrated a novel method to calculate vessel density and FAZ area. Age did not impact vessel density but sectoral analyses showed greater vessel density in the inferior zone.
Reference: Gadde SG, Anegondi N, Bhanushali D, Chidambara L, Yadav NK, Khurana A, Sinha Roy A. Quantification of Vessel Density in Retinal Optical Coherence Tomography Angiography Images Using Local Fractal Dimension. Investigative Ophthalmology and Vision Science. 2016; 57(1): 246-52.
OCTA in normal eyes
(Left to right) Contour map created using optical coherence tomography angiography (OCTA) image of a normal eye using local fractal method; Circular and sector zones analyzed in normal eyes
Optical coherence tomography angiography (OCTA) in diabetic retinopathy eyes
In this study, we correlated retinal vascular features with severity and systemic indicators of diabetic retinopathy (DR) using optical coherence tomography angiography (OCTA). The diabetic retinopathy patients were graded as having either non-proliferative diabetic retinopathy (NPDR: mild, moderate, and severe NPDR using Early Treatment Diabetic Retinopathy Study classification) or proliferative diabetic retinopathy (PDR). Local fractal analysis was applied to the superficial and deep retinal OCTA images. Foveal avascular zone (FAZ) area, vessel density, spacing between large vessels and spacing between small vessels were analyzed. Diagnostic ability of vascular parameters was assessed. Normal eyes had a significantly lower FAZ area, higher vessel density, and lower spacing between large and small vessels compared with DR grades. In conclusion, we found that "spacing between the large vessels" in the superficial and deep retinal layers had superior diagnostic performance than overall "vessel density".
Reference: Bhanushali D, Anegondi N, Gadde SGK, Srinivasan P, Chidamabara L, Yadav NK, Sinha Roy A. Linking retinal microvasculature features with severity of diabetic retinopathy using Optical Coherence tomography angiography. Investigative Ophthalmology and Vision Science. 2016; 57(9): OCT519-25. (Special Issue on OCT)
OCTA in diabetic retinopathy eyes
(Left to right) OCTA images of mild, moderate and severe non-proliferative diabetic retinopathy (NPDR) and proliferative diabetic retinopathy (PDR) eyes and their corresponding contour maps
Optical Coherence Tomography Angiography (OCTA) in glaucoma
We performed this study to determine the discriminant function of optical coherence tomography angiography (OCTA) by disease severity in glaucoma. Subjects underwent visual fields, retinal nerve fiber layer (RNFL) measurements, and OCTA imaging. Local fractal analysis was applied to OCTA images. Vessel density en face and inside the disc and spacing between large and small vessels were quantified. A novel glaucoma severity score (range, 0-1: 0 - normal; 1 - severe glaucoma) was developed by using global and regional vascular parameters. Glaucoma severity score (GSS) was compared with visual field and RNFL indices. GSS identified preperimetric glaucoma and early glaucoma better than did visual fields. Vascular parameters could be a useful adjunct tool to evaluate/diagnose glaucoma. Longitudinal studies are needed to determine their use in early detection and prognostication.
Reference: Kumar RS, Anegondi N, Chandapura RS, Sudhakaran S, Kadambi SV, Rao HL, Aung T and Sinha Roy A. Discriminant Function of Optical Coherence Tomography Angiography to Determine Disease Severity in Glaucoma. Investigative ophthalmology & visual science 2016;57:6079-88
OCTA in glaucoma
(Left to right) Retinal nerve fibre layer (RNFL) map, ganglion cell complex (GCC) map, visual fields and contour map of optical coherence tomography angiography (OCTA) image of a severe glaucoma eye