1. A method of determining at least one geometrical parameter of a side branch of a main branch of a blood vessel, the method comprising: receiving a multitude of cross section data sets, the data sets representing geometries of cross sections of the main branch over a length of the main branch; receiving position data of a cut plane of the main branch, the cut plane at least partially intersecting the side branch and the cut plane being defined under an angle with the cross-sections; based on data in at least a part of the multitude of cross section data sets, constructing image data of a constructed cross section of the main branch in the cut plane; receiving contour data defining a contour in the constructed cross section, the contour being provided in the side branch; and determining the geometrical parameter by determining a geometrical parameter of the contour.