Panoramic radiographs – a review

TECHNOLOGY

Film, direct and indirect digital image receptors are used in panoramic radiography, with digital receptors becoming increasingly popular. There are a few differences with regards to the film and detector technology in relation to those employed in intraoral radiography (Chapter 1):
• Film systems: an intensifying screen with rare earth elements is used to minimise radiation exposure to the patient. A dose reduction of 50-55% has been quoted in the literature. This is achieved because film is also sensitive to the fluorescent light emitted from the intensifying screen. This light will either be of a blue or green wavelength and the appropriate film must be matched to the screen. Screen film combinations with a speed of 400 or greater are recommended.
• Direct digital systems: due to the expense of large detectors, three to four CCD sensors are arranged vertically to cover the height of the panoramic X-ray beam, and the image is constructed in increments.
• Indirect digital systems: storage phosphor plates of appropriate size replace screen films in traditional cassettes.
The radiation dose levels between the three systems are comparable. Although, direct digital radiography can produce lower doses than film and storage phosphor systems, if the lowest possible setting appropriate to the patient is chosen. The effective dose has been quoted at a range of 3.85–30 μSv. However, much higher doses have been reported with older film systems. If the patient is a child or of a smaller stature, protocols should be appropriately adjusted to reduce the radiation dose delivered.

TECHNICAL ASPECTS AND LIMITATIONS

The panoramic radiograph is produced by using the principles of conventional tomography. It is a simple curvilinear form of tomography where the X-ray source and the image receptor simultaneously rotate around the patient’s head, capturing the structures within the focal trough. Structures outside of this zone are significantly distorted, blurred or magnified to the point where they are not recognisable, ideally leaving only the dentition and adjacent structures in clearest view possible.
In addition to the usual disadvantages associated with 2D imaging (geometric distortion and projection errors, overlapping of anatomical structures, image magnification and lack of 3D information), there are additional limitations unique to panoramic imaging, detailed in most radiology texts. Some are listed below:

• Lower resolution compared with intraoral radiography and computed tomography (CT).
• Clinically relevant features may be missed if outside the focal trough.
• Real, double and ghost images are always present and familiarity with their appearances is important in the evaluation of anatomical structures and pathoses.
• Overlapping of the proximal surfaces of teeth is common, usually the premolars.
• Superimposition of the cervical spine over the incisor region is usually present.
• Unequal magnification and distortion is found throughout the image, making linear measurements unreliable. Horizontal magnification is much more unpredictable than vertical magnification.
• Objects located more lingually will be projected superiorly due to the slight craniocaudal orientation of the beam. The technical aspects are well covered in many texts. Incorrect positioning on the bite-block, and/or rotation of the patient’s head are two of the most common positioning errors which can lead to significant geometric distortion and horizontal magnification/minification of the image.
• Magnification of the teeth occur when the jaw has been lingually positioned in relation to the focal trough, and is therefore closer to the X-ray source, causing the beam to pass through it more slowly. Conversely, objects more buccally placed will appear narrower.
• One method to assist in the identification of horizontal distortion is comparing the width of the mandibular first molars. The smaller side was positioned too close to the receptor, while the larger side would have been too close to the X-ray source.
• If the chin is tilted excessively high (chin up), the mandible will be distorted with a flat or inverted occlusal plane, and the hard palate will be superimposed on the roots of the maxillary teeth. Conversely, if the chin is too low (chin down), both the condyles and symphyseal region of the mandible will not be captured and there will be excessive overlapping of the dentition

REDUCING RADIATION EXPOSURE

Similar principles to intraoral radiography apply. This was discussed in Chapter 1.

APPLICATION

For a relatively low radiation dose, the panoramic radiograph remains a useful overview of the dentoalveolar and surrounding structures. However, a lack of understanding of the substantial limitations can contribute to misdiagnosis. On the other hand, even with these limitations in mind, it can be sufficient for many situations and procedures. Where clinically appropriate, it may be a useful initial test, where relevant further imaging could then be considered. It may also be useful for patients who cannot tolerate intraoral radiography, but it is not a substitute for the information that can be obtained from an intraoral radiograph.
Panoramic radiographic imaging is reported to be inadequate for the diagnosis of dental caries. The intraoral bitewing radiograph remains the imaging of choice. Some panoramic machines offer an ‘extraoral bitewing radiograph’, suggested to help with patients who are unable to tolerate intraoral radiography. Reduced specificity due to ghost artefacts, superimposition of air spaces and overlapping of premolars has been reported. Extraoral bitewing radiographs have been shown to be inferior to intraoral radiographs.
Also, the panoramic radiograph is not the optimal technique for most other common dentoalveolar diseases. Compared to intraoral radiographs, it is less sensitive in its ability to detect periapical lesions, particularly those exhibiting early periapical changes. As well, it often underestimates the extent of periodontal osseous defects. Findings on a panoramic radiograph may need to be supplemented with intraoral radiographs or 3D imaging as appropriate.
The panoramic radiograph has been considered to be appropriate for most cases of tooth extraction, including removal of third molars. Volumetric imaging should be considered for more complicated or difficult cases, and where plain film appearances suggest that the tooth is in close proximity to critical structures, such as the mandibular canal.
It has been suggested that panoramic imaging could be used for the initial radiologic assessment in evaluation for the dental implant, supplemented by 3D imaging techniques.
Given its low radiation dose, low cost and easy accessibility, some authors suggest that panoramic imaging can be used for pre-implant vertical linear measurements in the posterior mandibular region by calculation with the appropriate magnification factor and allowing a 2 mm safety margin superior to the inferior alveolar canal or other significant anatomical structures. However, the degree of magnification and minification is unpredictable, and other important 3D variables such as the prominence of the submandibular fossa or the precise morphology and proximity of the maxillary sinus floor cannot be appreciated in these 2D views.

It has been suggested that without 3D imaging, a safety margin of 6 mm from the mental foramen would be required, which may contribute to suboptimal treatment planning. There are many factors which influence the accuracy of this method, such as incorrect patient positioning, distortion, discrepancies between the shape of the dental arch and focal trough, and beam angulation. It has been reported that only 17% of measurements from the alveolar crest to the inferior alveolar canal have errors within one millimetre. Panoramic imaging is inferior to volumetric imaging for visualisation of many critical anatomical structures. In general, pre-implant volumetric imaging is considered essential.
The panoramic radiograph is considered to be particularly useful in the evaluation of the developing dentition and any anomalies. However, it is important to remember that supernumerary teeth or pathologies are likely to be missed if not located within the focal trough.
For the assessment of impacted and/or ectopic canines, panoramic radiography alone is considered to be inadequate due to its limitations including the inability to provide 3D information on the buccal or palatal position of the tooth. Cone beam computed tomography better demonstrates the relationship of the impacted canines and adjacent structures, root position and morphology, and possible associated root resorption. The use of volumetric imaging in treatment planning is considered to improve outcomes. Ultra-low dose CBCT protocols are recommended for the younger patient.
Even though the temporomandibular joints (TMJ) are seen in most panoramic radiographs, it is considered to be an insufficient test where there are specific indications for a radiologic examination of these joints. These views are unable to depict the precise morphology of these joints. The typically oblique projection of these joints together with variations in condylar angulation contribute to a distorted image. Without the ability to view the articular surface, pathologies such as erosions and osteophytes in the TMJ are detected with low sensitivity. CT and magnetic resonance imaging are the optimal techniques for the bony and soft tissue structures of the TMJ respectively. These techniques are discussed in Chapter 3. However, the TMJs are almost always depicted in the panoramic radiograph and, despite the limitations, their appearances should be thoroughly evaluated.
Panoramic imaging is inadequate and unreliable for visualisation of sinus anatomy. There is often misinterpretation of the proximity of the maxillary tooth roots to the sinus due to the 2D nature of the image and the presence of distortion. In situations where 3D information is necessary for accurate and precise planning (such as in sinus grafting procedures, or evaluating the likelihood of an oroantral communication), volumetric imaging is recommended. Sinus disease cannot be fully excluded with the panoramic radiograph. However, the sinuses are demonstrated in these views and due attention should be made as significant disease is sometimes demonstrated.

Although routine panoramic radiographic screening for extra- gnathic occult diseases cannot be recommended, the operator should be aware of these additional findings, including the clinical relevance. For instance, panoramic imaging has a low sensitivity for detecting calcified atheromas in the carotid artery or mandibular cortical erosion in the case of osteoporosis, but the presence of these findings contribute to the diagnosis and management of such conditions. Routine panoramic imaging of the patient at set arbitrary intervals is not considered to be justified.
VIEWING CONDITIONS The viewing conditions in relation to panoramic imaging are similar to that of intraoral radiographs (Chapter 1).

INTERPRETATION

The relatively broad coverage and tomographic nature of the panoramic radiograph contributes to challenges in interpretation. It has been reported that dental students and dentists face challenges in identifying the radiologic anatomy, positioning errors and pathologies/anomalies relating to panoramic images. This highlights the need for advanced and continued education in the interpretation of these radiographs. The prerequisites include a thorough understanding of the limitations, the radiologic anatomy, orofacial pathology and radiologic features of pathoses. It is obvious that a systematic approach must be taken. All structures included in these radiographs must be appropriately evaluated. The practitioner presiding over the study is responsible for the thorough interpretation of the entire image, not only the region of interest. Radiologic interpretation is discussed in Chapter 5.