1. A method for improving volume reconstruction in navigationally guided operations on a subject of investigation, making use of an X-ray diagnostic machine with an X-ray receiver, an X-ray source, an image processing computer, a position detection system, and a phantom, which contains X-ray positive marks and marks detectable by the position detection system in a fixed spatial relation, comprising: arranging the phantom on the subject in a region being explored in a manner such that the phantom remains rigidly connected to said subject during the entire operation and remains fixed in space during each scan; using the X-ray diagnostic machine to create a series of 2D projection images of the region being explored with different projection geometries, each projection containing at least some of the X-ray positive marks of the phantom; determining the positions of the X-ray positive marks on the phantom from the 2D projection images obtained from each scan in the image processing computer; and comparing the positions of the X-ray positive marks on the phantom with the calculated nominal positions of the X-ray positive marks for each scan determined by the position detection system, thus calibrating and transforming each 2D projection image to a transformed 2D data set in a manner such that the sum of deviations between the positions of the X-ray positive marks determined from the 2D projection image and the positions of the X-ray positive marks in the transformed 2D data set is minimized and any deviation of the 2D projection of the phantom from the value calculated from kinematics is divided equally between the X-ray receiver and the X-ray source; and reconstructing a volume with the transformed 2D data sets.