The various aspects relate to a method of determining a geometrical parameter of a side branch of a main branch of a blood vessel, 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 and receiving position data of a cut plane of the main branch, the cut plane intersecting the side branch and the cut plane being defined under an angle with the cross-sections. The method further comprises, based on data in at least a part of the cross section data sets, constructing image data of a constructed cross section of the main branch in the cut plane. The method comprises receiving contour data defining a contour in the constructed cross section, the contour being provided in the side branch and determining the parameter by determining a parameter of the contour.