Radiography in Modern Industry - Kodak

When radiation passes through a specimen at an angle, as shown in Figure 87 (left), spatialrelationships are distorted and measurements of clearances can be significantly falsified. If, onthe other hand, the whole radiograph is made with a thin "sheet" of perpendicularly directedradiation, dimensions can be measured on the radiograph with considerable accuracy (SeeFigure 87, right).Figure 87: Left: Spatial relationships can be distorted in a conventional radiograph (seealso Figure 12). Right: Spatial relationships are preserved by moving the specimen andfilm through a thin "sheet" of vertically directed radiation. (For the sake of illustrativeclarity, the source-film distance is unrealistically short in relation to the specimenthickness.)This is often most easily accomplished by moving the film and specimen beneath a narrow slit,with the x-ray tube rigidly mounted above the slit. The carriage carrying the film and specimenmust move smoothly to avoid striations in the radiograph. Occasionally it is advantageous tomove slit and tube over the stationary specimen and film. In general, however, this requiresmoving a greater weight and bulk of equipment, with the resulting increase in power required andin difficulties encountered in achieving smooth, even motion. Weight can be minimized by locatingthe slit near the focal spot, rather than close to the specimen. The disadvantage to this, however,is that the slit must be exceedingly narrow, very carefully machined, and precisely aligned withthe central ray.It should be recognized that this technique corrects for the errors of distance measurementshown in the figure above in only one direction--that parallel to the direction of movement of thespecimen (or of tube and slit). Fortunately, most specimens to which this technique is applied arequite small, or quite long in relation to their width. If, however, the specimen is so large thatmeasurements must be made in directions parallel to the slit (that is, at right angles to the motion)and remote from the center line, it may be necessary to scan the specimen twice in directions atright angles to one another. In a few cases, the specimen can provide its own slit. An examplewould be the measurement of end-to-end spacing of cylindrical uranium fuel pellets in a thinmetal container. In this application, little radiation could reach the film unless it passed betweenthe pellets almost exactly parallel to the faces of the adjacent cylinders. In such a case the slitshown in Figure 87 (right) can be comparatively wide.TomographyTomography--in medical radiography, often termed "body-section radiography"--is a techniquethat provides a relatively distinct image of a selected plane in a specimen while the images ofRadiography in Modern Industry 145