1. A method for determining a position of an instrument in a structure of an object with a x-ray system, comprising: providing a first 3D image data record of at least one area relevant for determining the position, the x-ray system is registered with the first 3D image data record; recording at least one 2D x-ray image of the relevant area after introduction of the instrument into the structure and from at least one projection direction with a known projection geometry with the x-ray system; recording a 2D position of a first location of the instrument in the 2D x-ray image with a projection line running in accordance with the known projection geometry being placed at the 2D position through the first 3D image data record or second 3D data record derived from it; using a processor to determine a 3D position of the first location of the instrument in the first 3D image data record or second 3D data record from an intersection of the projection line with the structure; and, wherein the intersection of the projection line with the structure is determined to locate the structure in which the instrument is positioned, and wherein the projection line intersects with a number of different structures providing a number of maxima and/or minima of grey level values along the projection line in the first 3D image data record or second 3D image data record, said determining the 3D position of the first location of the instrument including analyzing the grey level values.