Physical Considerations The xerographic process is based upon the electrical characteristics of certain semiconductors such as selenium which, although normally good insulators, become charge conductors under the action of light or ionizing radiation. If a metal plate is coated with such a material, the resultant product can be used in numerous applications as a substitute for conventional photographic emulsion. A xerographic plate consists of a metal surface coated with a thin layer of selenium. An electric charge may be uniformly deposited upon the selenium layer. If the plate is then placed in a light-proof box into which is introduced a cloud of powder which has been given an electrical charge of polarity opposite to that of the plate, the powder will be attracted to the plate and uniformly deposited upon it. If a freshly charged plate is exposed to x-rays, the charge will leak off the selenium to the metal plate directly behind. The amount of charge which leaks off is related to the incident x-ray exposure so that the resultant charge pattern on the xerographic plate can be considered as analogous to the latent image of a photographic exposure. Development is accomplished by placing the plate in a charged cloud of powder. The powder which may be white (CaCo3) or light blue (plastic material) will be deposited on the plate in relation to the charge remaining. The distribution of powder, when illuminated with oblique light, gives a clear, sharp image of the incident x-ray field. This can be photographed or transferred to a permanent paper base. The sensitive plate can then be cleaned and re-used. Superficially, the end products, i.e., the image of conventional radiography and that of xeroradiography, appear quite similar. There are, however, major differences and certain specific characteristics which make xeroradiography especially suitable for mammography. The most significant difference is that although the overall image is one of low contrast, very small variations in radiation absorption can be observed, since the xeroradiographic process accentuates the edges of adjacent tissues of differing absorption or of tissue discontinuities. This phenomenon, which is associated with a characteristic electrostatic repulsion of powder, results in a decrease or absence of powder on the plate where dissimilarities in charge density exist. This “edge effect” is easily seen because the powder has a tendency to bunch at boundaries of differing charge distributions. Even though the variations in tissue absorption may be slight, this exaggeration of the edges produces a bas-relief type of image which emphasizes differences in tissue structures. Another characteristic which distinguishes xerography from conventional radiography is that large differences in exposure do not result in great differences in the amount of powder deposited. Thus over- or underexposure is less likely to produce nondiagnostic plates.