A method for calculating a 3D representation of a vascular malformation of a vascular structure is provided. The method comprises: providing 2D X-ray image data of a vascular structure containing a contrast agent and having a vascular malformation, providing 3D image data of the vascular structure and transforming the 3D image data into transformed 2D image data, overlaying the 2D X-ray image data and the transformed 2D image data; calculating a correspondence between the 2D X-ray image data and the transformed 2D image data to determine a co-registration between the 2D X-ray image data and the 3D image data, determining in the co-registered 2D X-ray image data the vascular malformation, and calculating the 3D representation of the vascular malformation based on the determined vascular malformation of the co-registered 2D X-ray image data.