Physics overview of screen-film radiography.

The main components of the imaging system are the x-ray source, patient, image receptor (here, a screen-film system), processor, view box, and observer. The x-ray tube produces a beam of x rays, characterized by its spatial and energy distributions. The characteristics of the patient being radiographed--that is, the differential in attenuation of the x-ray beam among various structures within the patient (or subject contrast)--should exert the greatest influence on the final image. After passing through the patient, the x-ray beam bears the imprint of the attenuation processes (ie, absorption and scatter of photons) that took place within the patient. Subject contrast may be defined in terms of the ratio of the energy carried to the image receptor in two different locations. The number of x rays needed at the image receptor to produce the image and the radiation dose to the patient can be reduced by using an intensifying screen. The response of a screen-film system to x-ray exposure may be predicted in advance by observing the characteristic curve of the system. Radiographic image descriptors include contrast, resolution, and the detectability of subtle features. Detectability is heavily influenced by the characteristics of the observer.