An apparatus for processing volumetric image data to identify vessel regions having a predetermined condition, comprises a measurement unit for measuring at least one vessel parameter, a reference identification unit for obtaining data representing a branch-free vessel and identifying at least one reference section of the branch-free vessel, a calculation unit for calculating an expected value of the parameter in a further section of the branch-free vessel based on at least one measured value of the parameter in the at least one reference section, and a region identification unit for identifying a region of the vessel having the predetermined condition in dependence on both the expected value of the parameter in the further section and a measured value of the parameter in the further section.
