Kilovoltage and Radiographic Effect

There is a fundamental analogy between photography and radiography in respect to determining the correct exposure factors. In photography the exposure on a given “material”2 depends upon the following three factors: (a) the shutter speed expressed in seconds or fractions thereof; (b) the lens opening expressed in F/stops; (c) the brightness of the scene, i.e., the amount of light reflected by the object to be photographed. Factor (c) has to be measured or estimated by the photographer, whereas the setting of factors (a) and (b) are calculated or determined from tables or memory. Similarly in radiography the factors for a given material are: (a) timer setting in milliampere seconds; (b) the kilovoltage; (c) the thickness of the patient, which is a useful though somewhat inadequate expression of the amount of radiation which is transmitted by the subject to be radiographed. Like his photographic colleague, the radiographer measures or estimates factor (c) and from it determines factors (a) and (b) by rote, formula, or table. None of the existing tables has been universal because they neither span the entire range of available kilovoltages from 40 to 125 kv or more, nor cover all thicknesses which are encountered in medical practice. In photography, a major recent advance in simplifying the calculation of the correct exposure factors is the international acceptance of the Exposure Value Scale system for the design of cameras. In the United States most new adjustable cameras now marketed are designed for this new “E.V.S.” system, whereas in Germany and Japan the identical system has been adopted for all cameras under the designation “L.V.S.” (Light Value Scale). This system consists of a set of simple index numbers assigned to the shutter settings and a similar set of numbers assigned to the lens-opening settings. A given sum of any two settings labeled by this new system will always effect the identical practical exposure regardless of the individual settings. The internationally accepted figures are as given in Table I. Under this new system the photographer needs to obtain only a single number from his light meter reading in order to use—without major calculation—a variety of combinations of timer settings and lens openings which will produce identical exposures. For example, if his meter shows an exposure value of 10, he can set his camera on 5 + 5 (F/5.6 and 1/30 sec.) or on 8 + 2 (F/16 and 1/4 sec.) or on 2 + 8 (F/2 and 1/250 sec.) and obtain a correctly exposed film each time.