The invention relates to a method for providing quantitative measures of the flow property of a blood vessel. The method is based on analyzing cross-sectional images of a vessel by estimating the area of the lumen of the vessel. The method comprises steps of determining a point contained within the walls of the vessel, determining a closed path which approximates the inner circumference of the wall of the vessel, and determining the area of the closed path when the vessel is most expanding in order to get a measurement of the maximum lumen. This method may enable the clinical personnel to quickly evaluate the flow property e.g. of an inserted bypass vessel and, thereby, conclude if the surgical intervention is successful or if adjustments are required.