8) The Paralleling Technique

P a g e | 47 ● The X-ray tube head should be positioned so that the beam meets the tooth and the image receptor at right angles in both the vertical & the horizontal planes.

● The positioning should be reproducible.

PERIAPICAL RADIOGRAPHIC TECHNIQUES:

The anatomy of the oral cavity does not always allow all these ideal positioning requirements to be satisfied.

In an attempt to overcome the problems, two techniques for periapical radiography have been developed:

● The paralleling technique

● The bisected angle technique.

Theory:

1. The image receptor is placed in a holder & positioned in the mouth parallel to the long axis of the tooth under investigation.

2. The X-ray tube head is then aimed at right angles (vertically & horizontally) to both the tooth & the image receptor.

3. By using a film/sensor holder with fixed image receptor & X-ray tube head positions, the technique is reproducible.

• This positioning has the potential to satisfy four of the five ideal requirements mentioned earlier. • However, the anatomy of the palate & the shape of the arches mean that the tooth & the image receptor cannot be both parallel & in contact. • As shown in Figure, the image receptor has to be positioned some distance from the tooth.

Film packet/sensor holders:

A variety of holders has been developed for this technique. The different holders vary in cost and design, but essentially consist of three basic components:

● A mechanism for holding the image receptor parallel to the teeth that also prevents bending of the receptor

● A bite block or platform

● An X-ray beam--aiming device. This may or may not provide additional collimation of the beam.

The different components of the various holders usually need to be assembled together before the holder can be used clinically.

The holder design used depends upon whether the tooth under investigation is:

● Anterior or posterior

● In the mandible or maxilla

● On the right or the left-hand side of the jaw

- These variables mean that assembling the holder can be confusing, but it must be done correctly.

- To facilitate this assembly some manufacturers now color--code the various components.

- Once assembled correctly the entire image receptor should be visible when viewed through the beam-aiming device.

- The choice of holder is a matter of personal preference and dependent upon the type of image receptor – film packet or digital sensor (solid--state or phosphor plate) – being used.

- Typically, the same anterior holder can be used for right and left maxillary and mandibular incisors and canines utilizing a small image receptor (22 × 35 mm) with its long axis vertical.

- Four images in the maxilla and three images in the mandible are usually required to cover the right and left incisors and canines.

- Typically, different holders are required for the right & left premolar + molar maxillary & mandibular posterior teeth.

- The different designs allow the holders to hook around the cheek & corner of the mouth.

- A large image receptor (31×41 mm) is ideally utilized with its long axis horizontal.

- Two images are usually required to cover the premolar & molar teeth in each quadrant.

Positioning techniques

The radiographic techniques for the permanent dentition can be summarized as follows:

1. The patient is positioned with the head supported and with the occlusal plane horizontal.

2. The holder and image receptor are placed in the mouth as follows:

a. Maxillary incisors & canines – the image receptor is positioned sufficiently posteriorly to enable its height to be accommodated in the vault of the palate

b. Mandibular incisors & canines – the image receptor is positioned in the floor of the mouth, approximately in line with the lower canines or first premolars

c. Maxillary premolars & molars – the image receptor is placed in the midline of the palate, again to accommodate its height in the vault of the palate

P a g e | 53 d. Mandibular premolars & molars – the image receptor is placed in the lingual sulcus next to the appropriate teeth.

3. The holder is rotated so that the teeth under investigation are touching the bite block.

4. A cottonwool roll is placed on the reverse side of the bite block. This often helps to keep the tooth and image receptor parallel and may make the holder less uncomfortable.

5. The patient is requested to bite gently together, to stabilize the holder in position.

6. The locator ring is moved down the indicator rod until it is just in contact with the patient’s face. This ensures the correct focal spot to film distance (FSD).

7. The spacer cone is aligned with the locator ring. This automatically sets the vertical and horizontal angles and centers the X-ray beam on the image receptor.

8. The exposure is made.