An apparatus for analyzing coronary vessels and a corresponding method are provided in which simulated pullback data obtained from (non-invasively) acquired diagnostic images is co-registered with invasively acquired intravascular pullback data and the co-registration is used to identify disparities in the pullback data obtained using the two modalities. These disparities allow for deriving further information about the vessel geometry and/or the blood flow through the vessel. They may therefore be used to improve the physiological model.