1. A method of determining volume information of a body, wherein projection-image data are available which were produced by a radiation-based projection method, the method of determining volume information comprising: specifying initial voxel data for a plurality of voxels of the body, generating synthetic projection-image data based upon the initial voxel data with a projection rule modeling a course of the radiation-based projection method, comparing the synthetic projection-image data with the real projection-image data and determining projection-error data, imaging the projection-error data on the basis of a back-projection rule dependent on the projection rule and generating voxel-error data, determining correction data from the voxel-error data by a gradient-based optimizing algorithm, and determining corrected voxel data based upon the correction data.