1. A method for geometrically correctly associating at least two 3D image data items of a patient with one another, comprising: fixing a dimensionally stable marker field in a stationary position relative to a patient, said marker field defining a reference system and being visible when imaged in an x-ray image; positioning an x-ray apparatus at a 3D selectable first acquisition position and, with said x-ray apparatus at said first position, acquiring a plurality of 2D x-ray images, which allow reconstruction of first 3D image data of the patient, respectively at a plurality of angle positions of said x-ray apparatus, and acquiring a marker field image, at at least one of said angle positions, in which at least a portion of said marker field is imaged; moving said x-ray apparatus to a 3D selectable second position and, with the x-ray apparatus at said second position, acquiring a further plurality of 2D x-ray images, which allow second 3D image data of the patient to be reconstructed therefrom, respectively at said angle positions of said x-ray apparatus, and acquiring a further marker field image, at at least one of said angle positions, in which at least a portion of the marker field is imaged; in a processor, from said reference marker image, determining an attitude of said first acquisition position and an attitude of said first 3D image data in said reference system and, from said further reference field image, determining an attitude of said second acquisition position and an attitude of said second 3D image data in said reference system; and in said processor, geometrically correctly associating said first 3D image data and said second 3D image data with one another from their respective attitudes in said reference system.