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Optical coherence tomography angiography (OCTA) is a non-invasive, high-resolution imaging technique which visualizes capillary vasculatures. However, up to now, no accurate quantitation of capillary velocity based on OCTA images has been reported. Currently study of capillary velocity was substantially performed based on the empirical correlation curves between OCTA signals and blood flow velocity in respective OCT setups and OCTA algorithms. In this study, we proposed an accurate measurement of the capillary velocity in blood vessels using microbubble localization and tracking imaging. Based on localization and tracking of microbubble, motion of microbubble in the blood vessel was identified and parameters of vasculature such as the direction and velocity of capillary flow were subsequently obtained. In this prediction study, a flow phantom using the polystyrene microsphere and a cellulose tubing with the internal diameter of 300μm was used to simulate the flow in a single blood vessel. The flow velocity was set to be 2-6μL/min. In the experiment, 5% milk solution and microsphere suspension were injected into the cellulose tubing using a precision syringe pump. In OCT performance, B-scan imaging was set at a frame rate of 500 frames per second (fps). In each acquisition, 1000 effective frames were acquired, which corresponded to an ultrafast data acquisition of 2s. Speckle Variance (SV) algorithm was employed to remove background tissue signals and extract the flowing microsphere signals in OCT data. The center points of the microsphere were determined by identifying the local maxima of the cross-correlation coefficient. Additionally, the Kuhn-Munkres (KM) assignment algorithm was employed for the purpose of tracking microsphere. Distances between pairs of consecutive frames were then calculated for all microsphere. Ultimately the velocity magnitude map was generated. preliminary results on blood vessel phantom validated the feasibility of the proposed technique, which will be further performed in quantitation of capillary velocity in living tissues.
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