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EW RETINA 66 October 2015 by Maxine Lipner EyeWorld Senior Contributing Writer flow in the area, if it is fibrotic, or if there is hemorrhage. "In OCT angiography we can distinguish neovascular blood vessels and can actually quantify their flow area," Dr. Huang said. This is not possible with fluorescein angiography either. Dr. Huang also finds that it is much simpler to interpret three-dimen- sional OCT angiography, which can be segmented into different layers, allowing these to be measured indi- vidually. As part of the research, investi- gators were able to show that it was possible to get good OCT angio- grams with commercially available 70 kilohertz spectral OCT systems. "It doesn't require super high-speed prototypes," Dr. Huang said. Clinical potential Investigators determined that they could separate out the retinal circu- lation with OCT angiography, some- thing not possible on fluorescein angiography. "There is no automat- ed computer algorithm to compute capillary dropout areas because the contrast on fluorescein angiography varies so much," Dr. Huang said. However, with OCT angiography, which has an automated algorithm, it is relatively easy to standardize a threshold to measure the avascular area, Dr. Huang pointed out. In fact, upcoming research shows that with the new technology trained on avas- cular tissue, investigators were able to accurately detect diabetic retinop- athy. "We were able to distinguish 12 diabetic retinopathy patients from 12 normal controls perfectly," he said. While this would have been possible with fluorescein angiogra- phy, it would have been more diffi- cult. What's more, with OCT angi- ography there are no injections with which to contend. "People don't use fluorescein angiography to look for diabetic retinopathy because it's not worth the risk for the annual diabetic screening to inject someone with fluorescein," he said. "OCT is completely non-invasive, with no injections, no bright light, and no dilation needed." This would be an increase the signal-to-noise ratio of flow detection without additional scan time," he said. Investigators have determined that there are some classes of pa- thology that they can pick up well with the new technology, such as capillary non-perfusion and neovas- cularization. "We can pick those up in the choroid and outer retina, such as in AMD, or in the inner retina and vitreous, as in diabetic retinop- athy," Dr. Huang said, adding that these are the two leading causes of blindness. In addition, with glauco- ma it is possible to detect the loss of capillary perfusion in the retina. "Around the optic nerve head, the circulation of the nerve fiber layer can be visualized well and we can quantify the reduction," he said. Dr. Huang finds this is very different from conventional OCT in which you simply get the structure. Although conventional structural OCT can visualize large blood ves- sels, capillaries are pretty much in- visible, which makes it impossible to look for capillary non-perfusion, he explained. In addition, while neo- vascular membranes can be viewed with conventional OCT, practi- tioners can't tell if there is blood New split-spectrum approach The new split-spectrum technique works because of what is known as the speckle phenomenon, Dr. Huang explained. "When red blood cells flow through capillaries or other blood vessels, the reflection on OCT images has varying intensity," he said, adding that speckle refers to the interference between the light reflected from the red cells and surrounding structures, the capil- lary wall and retinal tissue. "Speckle pattern depends on the wavelength or frequency of light," Dr. Huang said. "When you illuminate tissue with a very narrow wavelength spec- trum light, like laser light, you see a lot of speckle interference." With white light, however, which has a very broad spectrum, the speckle variation from different wavelengths tends to cancel each other out, limit- ing the amount of speckle seen. Conventional OCT is formed by using the whole spectrum, sacri- ficing a lot of the speckle contrast as a result. That realization led Dr. Huang to an insight. "I thought that if I split up the OCT signal into different spectral channels, I could detect independent speckle infor- mation about motion or flow, and New technology for retinal disease detection I magine managing some of the leading causes of blindness with the aid of optical coherence tomography (OCT) angiography instead of using the current dye-based technique. Research published in the Proceedings of the National Academy of Sciences indi- cates that the new OCT angiography could improve the management of many retinal conditions, according to David Huang, MD, Peterson professor of ophthalmology and professor of biomedical engineering, Casey Eye Institute, Oregon Health & Science University (OHSU), Port- land. Researchers wanted to highlight all of the different types of microvas- cular pathologies that can be seen with OCT angiography. They honed in on OCT angiography, which uses the split-spectrum amplitude-decor- relation angiography (SSADA) algo- rithm, which Dr. Huang developed in conjunction with lead author Yali Jia, PhD, assistant professor of ophthalmology and biomedical engineering, OHSU. Viewing retinal disease with the aid of OCT angiography This type 1 CNV case was identified by OCT angiography, something that is not clearly defined using fluoroscein angiography (on right), according to some physicians. Source: Jia Y, et al. Quantitative optical coherence tomography angiography of vascular abnormalities in the living human eye. Proc Natl Acad Sci. 2015 May 5;112(18):E2395–402.

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