A device for providing improved volume reconstruction in navigationally guided operations on an examination subject comprising an X-ray diagnostic apparatus with an image-processing computer and a phantom containing X-ray-positive marks and marks detectable by a position detection system in a fixed spatial relation to each other, wherein - the X-ray diagnostic apparatus is designed to generate a series of 2D projection images of the area to be examined with different projection geometries, each projection containing at least a part of the X-ray-positive marks of the phantom, - the phantom is configured so that it is positioned on the examination object in the area to be examined, is rigidly connected to it during the entire operation, and remains fixed in space during the acquisition of the series of 2D projection images, is characterized in that - the image-processing computer is configured to determine the positions of the X-ray-positive marks from the 2D projection images, to compare them with the calculated nominal positions of the X-ray-positive marks, to calibrate each 2D projection image with itself and to transform them to the extent that the sum of the deviation amounts between the two positions of the marks in the recorded and in the transformed 2D projection images is minimal, the transformation being based on the assumption that the position of the X-ray receiver entry window of the X-ray diagnostic apparatus is determined taking the kinematics correction tables obtained in a calibration run and the deviations exclusively attributed to the displacement of the focal spot of the X-ray source of the X-ray diagnostic apparatus into account, and - the image-processing computer is further configured to perform the volume reconstruction with the transformed 2D projection images. - the X-ray diagnostic apparatus is designed to generate a series of 2D projection images of the area to be examined with different projection geometries, each projection containing at least a part of the X-ray-positive marks of the phantom, - the phantom is configured so that it is positioned on the examination object in the area to be examined, is rigidly connected to it during the entire operation, and remains fixed in space during the acquisition of the series of 2D projection images, - the image-processing computer is configured to determine the positions of the X-ray-positive marks from the 2D projection images, to compare them with the calculated nominal positions of the X-ray-positive marks, to calibrate each 2D projection image with itself and to transform them to the extent that the sum of the deviation amounts between the two positions of the marks in the recorded and in the transformed 2D projection images is minimal, the transformation being based on the assumption that the position of the X-ray receiver entry window of the X-ray diagnostic apparatus is determined taking the kinematics correction tables obtained in a calibration run and the deviations exclusively attributed to the displacement of the focal spot of the X-ray source of the X-ray diagnostic apparatus into account, and - the image-processing computer is further configured to perform the volume reconstruction with the transformed 2D projection images.