Radiography in Modern Industry - Kodak

structures that lie above and below that plane are blurred. It is fairly common in medicalradiography, and has a few specialized applications in industrial radiography.Figure 88: Basic principle of tomography. The source-specimen distance has beenunrealistically shortened and the specimen-film distance increased, in order to show theprinciples of the system more clearly.The principle can be understood by reference to Figure 88, which shows one of the simpler andmore frequently used methods. The tube and film holder are linked at the ends of a lever pivotedat the level of the plane it is desired to render. The film and tube move horizontally during thecourse of the exposure. As the focal spot moves to the right from X 1 to X 2 and the film to the leftfrom F 1 to F m2 , the image (P 1 ) of the point P remains stationary with respect to the film. However,the images of points above (A) and below (B) the point P move with respect to the film (from A 1 toA 2 and B 1 to B 1 ) and hence are blurred on the radiograph as shown in Figure 89.The excursion of tube and film determines the thickness of the layer that is sharply imaged. Smallmotions render structures within a relatively thick section of the specimen; large motions render athin section in sharp "focus."Figure 88 a simple linear motion of tube and film, for the sake of illustrative clarity. In actualpractice, the motions are often more complicated--circular, spiral, or hypocycloidal.Radiography in Modern Industry 146