1. A method for reconstructing a three-dimensional image dataset of an object during a monitoring process, comprising: continuously recording two-dimensional X-ray projection images of the object from different projection angles corresponding to different recording geometries at a recording frequency; reconstructing the three-dimensional image dataset after recording a first number of the projection images by back projection the first number of the projection images; and removing a contribution of an oldest recorded two-dimensional projection image from the three-dimensional image dataset after each recoding of a current projection image; and updating the three-dimensional image dataset by inserting a contribution of the currently recorded projection image in the three-dimensional image dataset by back projection of the currently recorded projection image so that the three-dimensional image dataset is updated at a same frequency with the recoding frequency.