The paralleling technique of intraoral radiography was developed by Gordon M. Fitzgerald, and is so named because the object (tooth), receptor (film packet), and end of the position indicating device (PID) are all kept on parallel planes. Its basis lies in the principle that image sharpness is primarily affected by focal-film distance (distance from the focal spot within the tube head and the film), object film distance, motion, and the effective size of the focal spot of the x-ray tube.
Successfully using the paralleling technique depends largely on maintaining certain essential conditions as illustrated in Figure 8. These are: 1) the film packet should be flat; 2) the film packet must be positioned parallel to the long axis of the teeth; and 3) the central ray of the x-ray beam must be kept perpendicular to the teeth and film.
To achieve parallelism between the film and tooth (i.e., to avoid bending or angling the film) there must be space between the object and film. However, remember that as the object-to-film distance increases, the image magnification or distortion also increases. To compensate, manufacturers are recessing the target (focal spot) into the back of the tube head. Depending on the machine’s age, and placement of the focal spot within the tube head, you may encounter long, medium, or short cones/PIDs. The goal is to have the focal spot at least 12” or 30 cm from the film to reduce image distortion. The anatomic configuration of the oral cavity determines the distance needed between film and object and varies among individuals. However, even under difficult conditions, a diagnostic quality radiograph can be obtained provided that the film packet is not more than 20 degrees out of parallel with the tooth, and that the face of the PID/cone is exactly parallel to the film packet to produce a central beam which is perpendicular to the long axis of the tooth and the film packet.
The major advantage of the paralleling technique, when done correctly, is that the image formed on the film will have both linear and dimensional accuracy. The major disadvantages are the difficulty in placing the film packet and the relative discomfort the patient must endure as a result of the film holding devices used to maintain parallelism. The latter is particularly acute in patients with small mouths and in children. In certain circumstances the film and holder may be slightly tipped toward the palate to accommodate oral space and patient comfort. Too much palatal tipping will throw off all parallel planes.