A method for reconstruction of an actual three-dimensional image dataset of an object during a monitoring process is proposed. Two-dimensional. X-ray projection images which correspond to a recording geometry are continuously recorded from different projection angles. The three-dimensional image dataset are reconstructed from a first number of these projection images, especially by a back projection method. The proportion of the oldest projection image contained in the current three-dimensional image dataset is removed from the three-dimensional image dataset and the proportion of the actual projection image is inserted in the three-dimensional image dataset after each recording of the actual projection image.