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: a) receiving a multitude of cross section data sets from within the blood vessel, the data sets representing geometries of cross sections of the main branch over a length of the main branch; b) 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 non-parallel with the cross sections; c) 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; d) receiving contour data defining a contour in the constructed cross section, the contour being provided in the side branch; and e) determining the geometrical parameter by determining a geometrical parameter of the contour.