The invention relates to a system for generating an estimate of a photon attenuation map for an object on the basis of single-scattered coincidences measured in a PET scanner. The system comprises a simulation module configured to calculate single-scattered coincidences by a numerical model calculation based on a preliminary attenuation map, where the estimate of the photon attenuation map is generated by adapting at least some attenuation values. The model calculation is made on the basis of a grid covering the object, the grid comprising grid elements (61) to which attenuation values are assigned, and the simulation module is further configured to determine at least one set of adjacent grid elements (61) in order to form a merged image element (63) including the set of adjacent grid elements (61) and to assign a single attenuation value to the merged image element in the model calculation.
