1. A positron emission tomography scanner the combination comprising: detector means for detecting coincident photons produced by annihilation events occurring within an object located within a field of view of the scanner, a sorter for determining the location (R,.theta.) of each detected annihilation event and storing an indication thereof as transmission data in a two-dimensional sinogram array; input means for receiving an indication of the amount of row direction (R) filtering (r.sub.f) to be applied to the transmission data stored in the sinogram array; calculation means coupled to the input means and the sinogram array for determining the amount of column direction (.theta.) filtering to be applied to the transmission data stored in the sinogram array as a function of the amount of row direction filtering (r.sub.f) and as a function of the size of the object located within the field of view of the scanner; means for filtering the transmission data stored in the sinogram array in both the row direction (R) and column direction (.theta.) by the amounts indicated by the input means and the calculation means; and image reconstruction means for producing an image array from the filtered transmission data in the sinogram array.