Secondary photon scatter in imaging cassettes.

Secondary photon processes within a cassette image receptor have been identified in the literature for at least three decades. These secondary processes have varying effects on the contrast, resolution and noise of a resulting radiographic image. The magnitude of these effects can be quantified in terms of the percentage of scatter to total absorbed energy for each process recorded by the radiographic cassette, and also as a reduction in system MTF. Two of these processes, cassette face scatter and particularly film crossover, have been subject to considerable past study. The third, due to K-characteristic photon 'crossover', has only recently been quantified. The total magnitude of all secondary cassette effects for a radiographic image, could vary from a worst case where in excess of 40% of the total absorbed energy is due to secondary processes (for an aluminium cassette with Gd2O2S screen with high crossover film, exposed at high kVp) to a best case of less than 10% (for a carbon fibre cassette, tantilate screens with 'zero crossover' film) leading to corresponding contrast reductions of > 40% to < 10% respectively. The MTF components of all secondary effects have been determined and are shown to decrease rapidly with increasing spatial frequency. This results in the MTF of the total image receptor system being reduced for high spatial frequencies by the same percentage as the contrast reduction. Theory predicts that the affected image noise due to cassette secondary processes will be complex and vary with spatial frequency.