The Laws of Development of X-ray Films

Abstract

ALMOST any kind of development will bring out an image from a suitable exposure impressed upon the X-ray film. The resulting radiograph may be of some value, but it may not be the best that can be produced, and may therefore fail to provide some information of value to the roentgenologist which might otherwise have been obtained by more suitable treatment. It is, therefore, of importance to find the conditions of development which will give the best contrast and detail. The phenomena of development are known in a general way from practical experience, but in order to obtain more accurate and quantitative information, a systematic study was carried out, using special equipment built for the purpose. Figures 1a and 1b show photographs of the exposing machine, or sensitometer, of the type designed by Jones (1); it has a rectangular aperture in the lead-covered face for entrance of the X-rays; the film holder is mounted on horizontal guides on which it travels so that successive portions of the film, one centimeter in width at a time, are moved into the aperture and exposed to the X-rays. An electromagnet, acting on a plunger, pulls the film holder over the centimeter distance at the proper times, as governed by an automatic timing mechanism. The X-ray tube current is turned off automatically at the end of the exposure. The X-ray tube current and voltage are controlled by hand. A storage battery supplies this filament and is provided with suitable rheostats for fine regulation of the current. For regulation of the tube voltage, an auto-transformer is used, permitting variations by steps of one volt on the primary of the X-ray transformer; and finer adjustment is secured by a carbon compression rheostat in series with the auto-transformer. The films are developed in a handrocked tray. To control the temperature of the developer, the tray is built with a water jacket through which water circulates continuously from a thermostat bath held at 65° F. The films exposed in the sensitometer are cut into strips about 2 cm. wide, each having the same series of exposures, and these strips are developed for various times from 1 to 15 minutes. It is thus possible to obtain information on the characteristics of the film over a wide range of exposures and development times. These precautions in exposure and development are necessary where measurements are to be made on the films and accurate and reliable results are desired. Figure 2 shows a set of film strips all having the same series of exposures and developed for various times. The progress of development can easily be observed on these films; density grows continuously with development time; contrast grows for a time, reaches a maximum, and then diminishes owing to the veiling effect of fog on the lower densities. These and other characteristics can be expressed more accurately by graphs.