Optical coherence tomography angiography in infancy

OCTA, fluorescein/indocyaningreen angiography and structural OCT of an 82-year-old female patient with neovascular age-related macular degeneration. OCTA clearly depicts microvascular details of the choroidal neovascularization. Image courtesy of  Andreas Pollreisz MD   Optical coherence tomography angiography (OCTA) is still in its infancy, and we will learn a lot more in the future,”  Andreas Pollreisz MD, Medical University of Vienna, Austria told delegates attending the 16th EURETINA Congress in Copenhagen, Denmark. OCTA rapidly generates simultaneous structural OCT images and depth-encoded angiography. It works by taking sequential B-scans separated at the same retinal location and then calculating the logarithmic intensity difference between successive B-scans. This is referred to as the decorrelation signal, which detects both amplitude changes and phase changes. “Imagine you have a static tissue and you perform repeated B-scans at the same location but separated by several milliseconds. The photos will look the same each time. But in a dynamic tissue, such as the retinal vessels, there will be noticeable changes. By detecting changes in the vasculature between these scans, blood vessels are visualised without the use of intravascular dye such as fluorescein,” explained Dr Pollreisz. Blood flow is assigned to specific retinal or choroidal layer. The result is high-resolution 3D OCT-based angiograms offering visualisation of blood flow in retinal and choroidal microvasculature. The three-dimensionality allows localisation of flow within specific tissue layers independently, a feature not available in traditional fluorescein angiography. Because both the en-face and the OCT images are generated from the same data, OCTA also allows precise lesion localisation in each axis. “Eye motion artefacts are kept to a minimum using a mechanical eye tracker and motion-reduction software,” he said. Nevertheless, one must learn to identify and interpret artefacts such as attenuation of signals from drusen and retinal vessels, he added. The advantages of OCTA seem to outweigh the potential drawbacks. “Because it is non-invasive and as risk-free as standard OCT, extensive follow-up imaging can be conducted of patients under treatment. For example, in patients receiving anti-VEGF treatment, sequential scans clearly show choroidal neovascularisation changing in size,” said Dr Pollreisz. Scanning is extremely fast when compared to fluorescein angiography, taking only seconds to complete the examination. The absence of dye leakage means that images do not get blurred by extravasation. “This allows, for example, very nice delineation, measurement and follow-up of the foveal avascular zone and detailed visualisation of microaneurysms,” he added. Andreas Pollreisz: andreas.pollreisz@meduniwien.ac.at