A method includes obtaining volumetric image data that includes a coronary vessel of a subject. The method further includes identifying the coronary vessel in the volumetric image data. The method further includes identifying a presence of a collateral flow for the identified coronary vessel. The method further includes determining a boundary condition of the collateral flow. The method further includes constructing a boundary condition parametric model that includes a term that represents the boundary condition of the collateral flow. The method further includes determining a fractional flow reserve index for the coronary vessel with the boundary condition parametric model.