AngioVue, Optovue inc., Freemont, CA is a 70 kHz (70.000 A scan per second) 840 nm spectral OCT which implements the SAADA algorithm to obtain high quality angiograms (304 X 304 transverse points are acquired in less then 2.9 seconds). Optical coherence tomography angiography (OCTA) is a new diagnostic non-invasive method by which the vascular structures of the retina and choroid can be visualized three-dimensionally without need for using fluorescence dyes. The technology of OCTA is an advancement of the OCT. By means of more powerful software and har dware used for OCTA not only morphological but also retinal and choroidal vascular perfusion analyses can be performed. Decorrelation is a mathematical function that quantifies variation without being affected by the average signal strength, as long as the signal is strong enough to predominate over the optical and electronic noise. Optical coherence tomography angiography (OCTA) is a new, noninvasive, nondye, depth-resolved technique, the concept of OCTA is the detection of changes in blood flow in the vessels, in a static eye, without need for dye injection.The amplitude of the signal returning from nonstatic features varies rapidly over time. When calculating the decorrelation of signal amplitude from repeated consecutive B-scans at the same cross section, a contrast between static and non static tissues is created and generates a vascular decorrelation signal that enables visualizing 3-dimensional retinal and choroidal vasculature. The split-spectrum amplitude decorrelat ion angiography (SSADA) algorithm improves the signal to noise ratio by splitting the source spectrum into 11 parts and averaging the resultant 11 signals. High-density raster scanning of a 2-dimensional area of the retina generates a volumetric rendering of blood flow from the internal limiting membrane to the choroid and allows for direct visualization of normal and abnormal blood vessels. The currently available Optovue AngioVue System uses spectral-domain technology, an 840-nmlaser, and the SSADA algorithm. The 70-kHz A-scan rate on this device allows a 3 Å~ 3-mm OCT angiography volume to be acquired in 3 seconds. The lateral and axial resolutions are both 15 μm; the axial resolution is significantly less than that for structural OCT (5 μm) owing to signal averaging. OCTA visualizes vasculature using motion contrast. Stationary tissue produces a nearly constant reflection or scattering whereas moving tissue produces OCT signals that change over time. The SSADA algori thm is used to distinguish blood flow from static tissue and is briefly reviewed below. As seen in real-time OCT structural images, the amplitude of the signal returning from nonstatic tissue varies rapidly over time. By calculating the decorrelation of signal amplitude from consecutive B-scans, a contrast between static and nonstatic tissue is created that allows for the visualization of blood flow. However, decorrelation can also be generated by bulk (non-flow) eye motion. The SSADA algorithm is developed to minimize bulk axial motion noise due to orbital pulsation by splitting the spectrum and thereby lengthening the axial resolution element. Additionally, the algorithm incorporates three steps to further remove motion artifact within each angiography scan. First, using outlier analysis, the decorrelation frames with excessive median decorrelation values (i.e., frames corrupted by saccadic and micro-saccadic eye movements) are removed at each M-B scan position and the rem aining individual frames are averaged to obtain the final average decorrelation flow image. Second, if the number of remaining individual frames is less than three for averaging, the average decorrelation image at this location is replaced by the spatial neighbors. Third, the median decorrelation (an estimate of bulk motion effect) is calculated for each average decorrelation frame and then subtracted from it. This sets the decorrelation value for bulk tissue to around zero.The inherent advantages of OCTangiography appear to be the ability to optically dissect and visualize the flow in various layers of the retina, the high resolution obtainable, and the freedom and safety of not having to use an injected dye. Fluorescein angiography provides flow information in that the speed of filling can be roughly compared in patients, physiologic information concerning the health of vessels can be assessed by looking for leakage, and the field of view is much larger. In addition, OCT a ngiography requires that the patient fixate precisely for several seconds, whereas a useful fluorescein angiographic frame can be obtained in a fraction of a second. Fluorescein angiography involves injection of a dye,which has a small probability of serious complications but a common incidence of minor adverse effects, such as nausea and hives. OCTA appears to be the future in Imaging technique for both retinal vascular diseases and age related macular degeneration.

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Email: aciardella@yahoo.com
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Antonio Ciardella