An iterative algorithmic method determines spatial distribution of attenuation values of an object based on measured projection data of the object. An algorithm generates estimated projection data based on an estimated spatial distribution of the attenuation values. Correction data is determined based on a difference between the measured projection data and the estimated projection data. Spatially extended correction data is back projected to update the estimated spatial distribution of the attenuation values. The algorithmic iterations continue until a stopping criterion is reached.