1. A method for determining an imaging parameter for registering a three-dimensional volume data record of an object comprising a vascular tree with a two-dimensional projection image of the object comprising the vascular tree being filled with a contrast agent at an acquisition time, comprising: determining a target fill state time describing which parts of the vascular tree are filled with the contrast agent in the three-dimensional volume data record at the acquisition time; and determining the imaging parameter based on the target fill state and the two-dimensional projection image for registering the volume data record with the projection image, wherein: the volume data record comprises voxels being assigned to the vascular tree, the target fill state indicates whether the voxels are filled with the contrast agent at the acquisition time, the projection image comprises pixels being filled with the contrast agent, the voxels filled with the contrast agent at the acquisition time are mapped into the projection image, a number of the voxels that are filled with the contrast agent in the volume data record and mapped into the projection image is determined, and the imaging parameter is determined to maximize the number of the voxels.