The invention relates to a method and an apparatus for reconstructing a three-dimensional image volume from two-dimensional projection images of a subject which have been taken from different projection directions by rotating the recording system around the subject, wherein the grayscale values of the voxels of the image volume are calculated by back projection of the projection images. The invention is characterized in that prior to back projection at least one projection image is modified in such a way that it corresponds to a projection image taken with a virtual detector whose axes are aligned parallel to the rotational axis of the recording system.