The paralleling technique of intraoral radiography was developed by Gordon M. Fitzgerald, and is so named because the tooth, receptor, 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-receptor distance (distance from the focal spot within the tube head and the receptor), object receptor distance, motion, and the effective size of the focal spot of the tube.
Successfully using the paralleling technique depends largely on maintaining certain essential conditions as illustrated in Figure 8. These are: 1) the receptor should be flat; 2) the receptor must be positioned parallel to the long axis of the teeth; and 3) the central ray of the radiographic beam must be kept perpendicular to the teeth and receptor.
|Principles of the Paralleling Technique|
To achieve parallelism between the receptor and tooth (i.e., to avoid bending or angling the receptor) there must be space between the object and receptor. However, remember that as the object-to-receptor 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 receptor to reduce image distortion.
The anatomic configuration of the oral cavity determines the distance needed between receptor and tooth and varies among individuals. However, even under difficult conditions, a diagnostic quality image can be obtained provided that the receptor 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 receptor to produce a central beam which is perpendicular to the long axis of the tooth and the receptor.
The major advantage of the paralleling technique, when done correctly, is that the image formed on the receptor will have both linear and dimensional accuracy. The major disadvantages are the difficulty in placing the receptor and the relative discomfort the patient must endure as a result of the receptor holding devices used to maintain parallelism. The latter is particularly acute in patients with small mouths and in children. In certain circumstances the receptor 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.