1. A method for improving the volume reconstruction in navigationally guided operations on a subject of investigation, making use of an X-ray diagnostic machine with 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 using the X-ray diagnostic machine to create a series of 2D projection images of the investigated region with different projection geometries, each projection containing at least some of the X-ray positive marks of the phantom; arranging the phantom on the subject in the region being explored, remaining rigidly connected to it during the entire operation and remaining fixed in space during each scan, determining the positions of the X-ray positive marks from the 2D projection images in the image processing computer, compared with the calculated nominal positions of the X-ray positive marks, and each 2D projection image is itself calibrated and transformed so that the sum of the deviation magnitudes between the positions of the marks in the recorded and in the transformed 2D data set is a minimum; and constructing the volume reconstruction with the transformed 2D data sets.