Deviations of blood pressure due to stenosis caused by plaque pose a health risk. The deviation, often expressed in a fractional flow reserve, may be calculated on a per-location basis using deviations of the local cross-sectional area or local diameter from a reference value representing a healthy vessel. The reference value may be obtained by means of linear or higher order interpolation or linear regression techniques. Together with the flow velocity of a fluid through the vessel, a value of local deviation of a vessel geometry compared to a reference value allows for accurate, fast and efficient calculation of the fractional flow reserve for every location of the vessel for which geometry data is available. The reference value as well as actual data obtained by measurements may be represented or approximated by functions, allowing accurate and efficient calculation of data over a continuous region of the segment under scrutiny.