Orthopantomography (OPT or OPG) or dental panoramic tomography (DPT)

This technique requires the use of a specialised OPT unit (Fig. 22.1A,B), the tomographic principle being that which is used to produce the image of the full mouth and its dentition. The moving tube effectively blurs out the shadow of overlying structures by placing the dental arch in the axis of the tomographic movement. Structures not lying within this axis are effectively blurred, and so their detail does not overlie the image of the teeth and mandible. However, the area of interest does show some element of unsharpness compared to radiographic images of other body parts when a non-moving tube is used.

Figure 22.1 OPT unit.

Reproduced with permission from Xograph Imaging Systems.

The technique opens out the image of the dental arch to appear in a linear arrangement on the final image. It has long been employed in the dental setting and has largely replaced full mouth periapical examinations. As mentioned in Chapter 18, the OPT examination can be used to demonstrate the temporomandibular joints and mandible.

The tomographic movement of the unit attempts to follow the dental arch, which it keeps within the tomographic axis of the beam as it travels around the patient’s face. Because of this, accurate positioning aims to place the dental arch within this axis and horizontally to follow the plane of tube movement. Positioning also aims to keep unwanted structures such as the cervical vertebrae clear from the tomographic axis.

Some tomographic units use a system of slit light beams that are aligned with the incisors, median sagittal plane (MSP), anthropological baseline or alatragal line. Lights are also used to assess the patient’s position in an anteroposterior (AP) direction; these vertical slit beams are seen as superimposed when the patient is in the optimum plane but are separated if the patient is too far forward or not forward enough. Since all these light arrangements are used in varying ways for each manufacturer, it is difficult to describe the use of each different system. Suffice to say, use of these light systems will ensure that the patient is actually in the position as described in the positioning section below. It can often be difficult to align the baselines with the lights on the OPT unit once they are switched on. Visually assessing the baselines before switching the lights on often helps, and the radiographer uses the lights to check height and positioning accuracy afterwards.

The X-ray beam leaves the tube housing via a slit collimator and the thin beam moves around the dental arch and across the image receptor (IR); this arrangement reduces the inevitable penumbra that would be caused by a wider beam. However, some penumbral effect is unavoidable.

The tomographic movement travels around the head with a horizontal beam, in opposition to traditional tomographic units that move over the supine patient and use a beam which is initially vertical and moves longitudinally or in a circular, elliptical or helical course.

Owing to the nature of this horizontal movement the use of the OPT unit may be distracting for the patient during exposure. Advance preparation must include demonstration of tube movement for the patient, using the ‘test’ setting. The unit is then returned to the start position.

If used, an OPT cassette is inserted into the erect cassette holder on the unit. Digital equipment incorporates the receptor into the unit and OPT/DPT is selected on the unit.

Positioning (Fig. 22.2)

• A disposable bite rod is inserted into the chin rest, or a disposable plastic cover is applied to the permanent bite rod

• The patient is seated or standing with their chin resting on the chin support and in the correct position to facilitate the dental arch being placed in the correct tomographic plane (with the anthropological baseline and alatragal line horizontal and the head far enough forward, often indicated by slit light indicators as designed by the manufacturer)

• The patient bites with their incisors in the groove on the bite rod, to effect separation of teeth on the image

• The MSP is vertical and perpendicular to the bite rod. The height of the unit is adjusted until the occlusal plane is horizontal (assessed by checking that the alatragal line or anthropological baseline are horizontal)

• The patient holds onto the support handles and is asked to step forward slightly to bring the cervical spine vertical. If seated, their chair is pulled forward by the radiographer. Throughout this manoeuvre the head must not tilt or rotate and the chin must not lift or drop

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