Receptor saturation in roentgen films

Roentgen-film recording of small object details of low attenuation differences (e.g. pulmonary vessels) is regularly seen to be impaired when the film is exposed to yield high values of optical density (D). The high-density failure is due to receptor saturation, which implies that at high exposure values most silver halide grains of the film are made developable, leaving few grains available to receive additional informative photons. The receptor saturation is analysed by means of a mathematical model of a non-screen film yielding Dmax = 2.0. Optimum recording, defined by maximum signal-to-noise ratio in the image, is found at D = 0.64, corresponding to, on an average, 1.6 photons absorbed per grain. On the other hand, maximum contrast occurs at D approximately 1.4, where, on the average, 3.6 photons are absorbed per grain. The detective quantum efficiency of the film, i.e. the fraction of the photons actually contributing to the information content of the image, drops from 41 per cent at maximum signal-to-noise ratio to a mere 10 per cent at maximum contrast. Receptor saturation does not manifest itself as 'mottle' in the image--it is therefore easily overlooked. The importance of adequate removal of secondary radiation is emphasised by these results.