1. A method for determining a 2D contour of a vessel structure imaged in 3D image data for a first slice plane of the vessel structure, wherein the 3D image data was generated using a medical imaging system and has a multiplicity of image voxels which are all assigned a respective image value, the method comprising: providing 3D image data; determining a multiplicity of first initial 2D contours of the vessel structure, wherein the first initial 2D contours in the 3D image data are determined for slice planes of the vessel structure, the planes being arranged orthogonally in respect of a central line of the vessel structure and spaced apart from one another along the central line; determining a first initial 3D contour from the determined first initial 2D contours; determining a smoothed second 3D contour by applying an active 3D contour model to the first initial 3D contour; determining a second initial 2D contour as a slice through the smoothed second 3D contour including the first slice plane; and determining the 2D contour by iteratively fitting the second initial 2D contour to the vessel structure imaged in the 3D image data which results for the first slice plane.