1. A method of determining volume information of a body, wherein a real projection-image data is 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, wherein a sequence of method-specific steps is performed repeatedly, wherein previously determined corrected voxel data is used as initial voxel data in a repeated step, and wherein, after the method-specific steps are performed at least once, the correction data is determined in the repeated step only for a subset of the voxel-error data having negative values, wherein a mathematical model for describing a time dependency of the voxel data is defined for at least one voxel of the body, wherein the model includes at least one model parameter, wherein an update vector data is computed by solving the mathematical model, wherein the update vector data comprises at least one correction value, and wherein the least one correction value is added to the at least one model parameter to generate the corrected voxel data for a further iteration step.