Focal Spot Projection and the Position of the X-ray Tube in Radiography

Abstract

MODERN roentgenography is conspicuous for its inability to stabilize and standardize technical procedures. Reduced to essentials, this is due to failure to measure and control within reasonably close limits the various physical factors involved. The technical characters of an X-ray film may be described as having two essential bases: first, the geometric projection of the shadows onto the film, and secondly, the qualitative reproduction of these shadows in the emulsion. Satisfactory management of both of these factors is indispensable to high grade roentgenography. The qualitative reproduction of projected shadows is in the main a matter of securing contrast between densities. This, in turn, resolves itself into questions, first, of the control of penetration, and secondly, of the management of non-geometric radiations, i.e., the radiations scattered from the tissues, et cetera, and those from intensifying screens. Processing of films is also concerned with the maintaining of contrast. As to the geometric projection of shadows, this should be relatively simple to analyze and control, although to our knowledge such data have not been presented to the roentgenologist. Thus, the shadows projected from a given object bear simple relations to the size of the object, to the object-film and target-film distances, and to the size, shape, and position of the source of the radiations, i.e., the focal spot. It must be the aim of the roentgenologist to have these shadow projections reproduce as closely as possible the size and shape of the causative object, and with the greatest possible sharpness. To attain such ends the geometric characters of the shadows, in terms of the causative object, must be capable of simple determination and control. When these are attained, the satisfactory qualitative reproduction of the shadows is a further problem, but one which must not be confused with the first. The present study constitutes a first step in the mensuration of shadow-projections, and relates to the effects of the position of the focal spot of the X-ray tube with respect to the film. From the data so obtained, suggestions are offered for the placing of the tube in order to attain maximum fidelity of shadow-production. Nature of Focal Spot Projection It is, of course, generally recognized that a large focal spot produces under given conditions a less satisfactory degree of sharpness and radiability than does a smaller focal spot. It is less generally recognized, however, that a given focal spot, in the type of X-ray tube in current use, varies widely in its effective size over different parts of a film. It follows that those radiographic characters which are influenced by focal spot size, namely, sharpness of shadow and the radiability of fine objects, are also variable over different parts of a film. The variable effective size of a focal spot may be expressed as “variable focal spot projection.”