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BACKGROUND Contrast reflux, which is the retrograde movement of contrast against flow direction, is commonly observed during angiography. Despite a vast body of literature on angiography, the hemodynamic factors affecting contrast reflux have not been studied. Numerous methods have been developed to extract flow from angiography, but the reliability of these methods is not yet sufficient to be of routine clinical use. PURPOSE To evaluate the effect of baseline blood flow rates and injection conditions on the extent of contrast reflux. To estimate arterial flow rates based on measurement of contrast reflux length. MATERIALS AND METHODS Iodinated contrast was injected into an idealized tube as well as a physiologically accurate model of the cervico-cerebral vasculature. A total of 194 high-speed angiograms were acquired under varying 'blood' flow rates and injection conditions (catheter size, injection rate, injection time). The length of contrast reflux was compared to the input variables and to dimensionless fluid dynamics parameters at the catheter-tip. Arterial blood flow rates were estimated using contrast reflux length as well as a traditional transit-time method and compared to measured flow rates. RESULTS Contrast reflux lengths were significantly affected by contrast injection rate (p<0.0001), baseline blood flow rate (p = 0.0004), and catheter size (p = 0.04), but not by contrast injection time (p = 0.4). Reflux lengths were found to be correlated to dimensionless fluid dynamics parameters by an exponential function (R2 = 0.6 to 0.99). When considering the entire dataset in unison, flow estimation errors with the reflux-length method (39±33%) were significantly higher (p = 0.003) than the transit-time method (33±36%). However, when sub-grouped by catheter, the error with the reflux-length method was substantially reduced and was significantly lower (14±14%, p<0.0001) than the transit-time method. CONCLUSION Results show correlations between contrast reflux length and baseline hemodynamic parameters that have not been reported previously. Clinically relevant blood flow rate estimation is feasible by simple measurement of reflux length. In vivo and clinical studies are required to confirm these correlations and to refine the methodology of estimating blood flow by reflux. This article is protected by copyright. All rights reserved.