Tomography and panoramic radiography

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Chapter 15

Tomography and panoramic radiography

Introduction

Conventional tomography is a specialized radiographic technique developed originally for producing radiographs that showed only a section or slice of a patient. A useful analogy is to regard the technique as one that enables the patient to be imaged in slices – like a loaf of sliced bread (see Fig. 15.1). Each individual tomographic image (or slice) shows the tissues within that section sharply defined and in focus. The section is thus referred to as the focal plane or focal trough. Tissues and structures outside the tomographic section are not visible because they are very blurred and out of focus.

Fig. 15.1 Diagram illustrating the analogy of tomography – the patient is imaged and can be viewed in slices like a loaf of sliced bread.

Production of each conventional tomographic slice required controlled, accurate movement of both the X-ray tubehead and the film during the exposure, thereby making it different from all the routine radiographic techniques described in previous chapters. As will be described later in this chapter, by varying the size of the X-ray beam and the type of equipment movement employed it proved possible to change the shape of the tomographic layer from a straight (linear) line (see Fig. 15.2) to a curve, and ultimately to the approximate horseshoe shape of the dental arch, providing an overall panoramic image of all the teeth and their supporting structures – the so-called dental panoramic tomograph (DPT) or panoramic radiograph (see Fig. 15.3).

Fig. 15.2 A linear tomograph in the coronal plane. The slice passes through and contains part of the antra and facial skeleton and shows an antral tumour (arrowed).

Fig. 15.3 A dental panoramic tomograph depicting a curved, horseshoe-shaped slice that is approximately the shape of the dental arch and contains the teeth and their supporting structures. In both examples the tissues imaged in the tomographic slices are sharply defined and in focus, while the unwanted structures are blurred out.

While panoramic tomography remains very popular, conventional linear tomography has essentially been superseded in radiography by computed tomography, which enables computer-generated tomographic sectional images to be created. Cone beam computed tomography (CBCT) is described in Chapter 16 and medical computed tomography (CT) is described in Chapter 18. The concept of tomographic slices or sectional images also forms the basis of many other modern imaging modalities, such as magnetic resonance (MR) imaging and ultrasound, which are also described in Chapter 18.

Tomographic theory

The theory of panoramic tomography is complicated. Nevertheless, an understanding of how the resultant radiographic image is produced and which structures are in fact being imaged, is necessary for a critical evaluation and the interpretation of this type of radiograph.

As an explanatory introduction to how the approximately horseshoe-shaped, curved tomographic slice or focal trough in panoramic tomography is produced, the theory of tomographic movement and three methods for producing linear and curved tomographic slices are first described. These include:

• Linear tomography using a wide or broad X-ray beam

• Linear tomography using a narrow or slit X-ray beam

• Rotational curved tomography using a narrow slit X-ray beam.

Principle of tomographic movement

As stated, tomography requires controlled, accurate movement of both the X-ray tubehead and the film. They are therefore linked together. During the exposure, the X-ray tubehead moves in one direction while the film moves in the opposite direction, as shown in Fig. 15.4. The point at the centre of this rotating movement will appear in focus on the resultant radiograph, since its shadow will appear in the same place on the film throughout the exposure. All other points will appear blurred or out of focus.

The principle of tomography illustrated in Fig. 15.4 shows a very thin X-ray beam producing one point (O) – the centre of rotation – in focus on the film. To produce a section or slice of the patient in focus, a broad X-ray beam is used. For each part of the beam, there is a separate centre of rotation, all of which lie in the same focal plane. The resultant tomography will therefore show all these points sharply defined. The principle of broad-beam tomography is illustrated in Fig. 15.5.

Fig. 15.5 Diagram showing the principle of broad-beam tomography. Using a broad beam there will be multiple centres of rotation (three are indicated: •) all of which will lie in the shaded zone. As all the centres of rotation will be *in focus*, this zone represents the *focal plane* or section of the patient that will appear sharply defined on the resultant tomograph.

Panoramic tomography

The dental arch, though curved, is not the shape of an arc of a circle. To produce the required elliptical, horseshoe-shaped focal trough, panoramic tomographic equipment employs the principle of narrow-beam rotational tomography, but uses two or more centres of rotation.

There are several dental panoramic units available; they all work on the same principle but differ in how the rotational movement is modified to image the elliptical dental arch. Four main methods (see Fig. 15.8) have been used including:

Fig. 15.8 Diagrams showing the main methods that have been used to produce a focal trough that approximates to the elliptical shape of the dental arch using different centres of rotation. A 2 stationary, B 3 stationary, C continually moving, D combination of 3 stationary and moving centre.

• Two stationary centres of rotation, using two separate circular arcs

• Three stationary centres of rotation, using three separate circular arcs

• A continually moving centre of rotation using multiple circular arcs combined to form a final elliptical shape

• A combination of three stationary centres of rotation and a moving centre of rotation.

However, the focal troughs are produced, it should be remembered that they are three-dimensional. The focal trough is thus sometimes described as a focal corridor. All structures within the corridor, including the mandibular and maxillary teeth, will be in focus on the final radiograph. The vertical height of the corridor is determined by the shape and height of the X-ray beam and the size of the film, as shown in Fig. 15.9.

Fig. 15.9 Diagram showing how the height of the three-dimensional focal corridor is determined. The height (x) of the X-ray beam is collimated to just cover the height (f) of the film. The separation of the focal trough and the film (d), coupled with the 8° upward angulation of the X-ray beam results in the final image being slightly magnified.

As in other forms of narrow-beam tomography, a different part of the focal trough is imaged throughout the exposure. The final radiograph is thus built up of sections (see Fig. 15.10), each created separately, as the equipment orbits around the patient’s head.

Fig. 15.10 Diagram showing the gradual build-up of a panoramic tomograph over an 18-second cycle, illustrating how a different part of the patient is imaged at different stages in the cycle.

Unfortunately, although the final radiograph shows all the teeth and their supporting structures, the tomographic image quality is generally inferior to that obtained using intraoral radiographic techniques and interpretation is more complicated.

Selection criteria

In the UK, the 2013 Selection Criteria for Dental Radiography booklet suggests panoramic radiography in general practice in the following circumstances:

• Where a bony lesion or unerupted tooth is of a size or position that precludes its complete demonstration on intraoral radiographs

• In the case of a grossly neglected mouth

• As part of an assessment of periodontal bone support often supplemented with periapical radiographs

• For the assessment of wisdom teeth prior to planned surgical intervention. Routine radiography of unerupted third molars is not recommended

• As part of an orthodontic assessment where there is a clinical need to know the state of the dentition and the presence/absence of teeth. The use of clinical criteria to select patients rather than routine screening patients is essential.

In addition, in dental hospitals panoramic radiographs are also used to assess:

• Fractures of all parts of the mandible except the anterior region

• Antral disease — particularly to the floor, posterior and medial walls of the antra

• Destructive diseases of the articular surfaces of the TMJ

• Vertical alveolar bone height and position of anatomical structures as part of pre-implant planning.

The 2013 Selection Criteria booklet specifically states that ‘panoramic radiographs should only be taken in the presence of specific clinical signs and symptoms’, and goes on to say that ‘there is no justification for review panoramic radiography at arbitrary time intervals’.

Equipment

There are several different panoramic units available. Although varying in design and appearance, all consist of four main components, namely:

• An X-ray tubehead, producing a narrow fan-shaped X-ray beam, angled upwards at approximately 8° to the horizontal (see Fig. 15.9)

• Control panel (see Fig. 15.11)

• Patient-positioning apparatus, including chin and appropriate head immobilizing supports and light beam markers

• An image receptor (film or digital), with or without an associated carriage assembly.

Traditional panoramic equipment was designed to use indirect-action radiographic film in an extraoral cassette as the image receptor. With the advent of digital imaging several variations in image receptor now exist, including:

• Cassettes containing indirect-action film and rare-earth intensifying screens

• Cassettes containing a digital phosphor plate

• Flat cassette-sized solid-state sensors designed to fit into existing equipment

• Specially designed solid-state sensors — an integral part of new equipment (see Ch. 6).

Examples of two typical machines are shown in Fig. 15.12. Ideally:

Fig. 15.12 Examples of two different types of Planmeca panoramic radiography machines. A Traditional unit incorporating a cassette suitable for film-based or digital phosphor plate imaging. B Dedicated digital unit with specifically designed built-in solid-state digital sensor. The basic components including the X-ray tubehead, the control panel and the patient positioning apparatus are common to both units.

• Equipment should have a range of tube potential settings, preferably 60–90 kV.

• The beam height at the receiving slit of the cassette holder should not be greater than the film in use (normally 125 mm or 150 mm). The width of the beam should not be greater than 5 mm.

• Equipment should be provided with adequate patient-positioning aids incorporating light beam markers.

• Equipment should provide facilities for field-limitation techniques.

Control panel

The control panel design also varies from one machine to another but a typical panel is shown in Fig. 15.11. The main features usually allow the operator to:

• Select the field size

• Select a limited range of arch shapes and sizes

• Select the mA and kV exposure factors

• Adjust the anteroposterior position of the bite-peg

• Select the size of the patient to be X-rayed

• Adjust the height of the equipment

• Select a range of field limitation options.

• The collimation should be set to the size of field required.

• The appropriate exposure factors should be selected according to the size of the patient — typically in range 70–90 kV and 4–12 mA.

Patient positioning

• The patient should be positioned in the unit so that their spine is straight and instructed to hold any stabilizing supports or handles provided (see Fig. 15.14).

Fig. 15.14 Patient positioned in the Planmeca PM2002. Note the bite-peg, chin and temple supports and the three light-beam markers to facilitate accurate positioning.

• The patient should be instructed to bite their upper and lower incisors edge-to-edge on the bite-peg with their chin in good contact with the chin support.

• The head should be immobilized using the temple supports.

• The light beam markers should be used so that the mid-sagittal plane is vertical, the Frankfort plane is horizontal and the canine light lies between the upper lateral incisor and canine.

• The patient should be instructed to close their lips and press their tongue on the roof of their mouth so that it is in contact with their hard palate and not to move throughout the exposure cycle (approximately 15–18 seconds).

Important points to note

• Panoramic radiography is generally considered to be unsuitable for children under six years old, because of the length of the exposure and the need for the patient to keep still.

• A protective lead apron should not be used. In the UK the 2001 Guidance Notes confirm that there is no justification for using a protective lead apron. If used, it can interfere with the final image (see Fig. 15.27H).

After exposure

• The temple supports should release automatically to enable the patient to leave the machine.

• The equipment should be wiped down with a surface disinfectant and the bite-peg sterilized (see Ch. 8).

• Gloves should be discarded as clinical waste.

• The film or phosphor plate should be processed.

The importance of accurate patient positioning

The positioning of the patient’s head within this type of equipment is critical — it must be positioned accurately so that the teeth lie within the focal trough. The effects of placing the head too far forward, too far back or asymmetrically in relation to the focal trough are shown in Fig. 15.15. The parts of the jaws outside the focal trough will be out of focus. The fan-shaped X-ray beam causes patient malposition to be represented mainly as distortion in the horizontal plane, i.e. teeth appear too wide or too narrow rather than foreshortened or elongated. Thus, if the patient is rotated to the left (as shown in Fig. 15.15C), the left teeth are nearer the film and will be narrower, while the teeth on the right will be further away from the film and wider. These and other positioning errors are shown later (see Fig. 15.28).

Fig. 15.15 Diagrams showing the position of the mandible in relation to the focal trough when the patient is not positioned correctly. A The patient is too close to the film and in front of the focal trough. B The patient is too far away from the film and behind the focal trough. C and D The patient is placed asymmetrically within the machine.

However accurately the patient’s head is positioned, the inclination of the incisor teeth, or the underlying skeletal base pattern, may make it impossible to position both the mandibular and maxillary teeth ideally within the focal corridor (see Fig. 15.16).

Fig. 15.16 Diagrams showing the vertical walls of the focal trough in the incisor region and the relative positions of the teeth with different underlying dental or skeletal abnormalities. A Class I. B Gross class II division 1 malocclusion with large overjet. C Angle’s class II skeletal base. D Angle’s class III skeletal base. The shaded areas outside the focal trough will be blurred and out of focus.

Technique variations

There are a number of technique variations possible with modern equipment, including:

• Edentulous patient positioning — the chin support is used instead of the bite-peg and the canine positioning light beam is centred on the corner of the mouth.

• TMJ programmes — (see Ch. 30).

• Cross-sectional imaging for implant assessment.

• Collimation — (see Ch. 23) restricting the size of the beam so restricting the field of view, e.g. the height of the beam is automatically reduced when selecting the settings for children.

• Field limitation techniques — only preselected parts of the patient are exposed and imaged on the final film as illustrated in Fig. 15.17.

Fig. 15.17 A The effect of selecting collimation to reduce the height of the beam and field limitation techniques to exclude the rami and parotid glands producing the so-called *dentition only* image. This has been reported to reduce the effective dose by approximately 50%. B Diagram showing a selection of images that can be obtained by using collimation and field limitation techniques.

Normal anatomy

The normal anatomical shadows that are evident on panoramic radiographs vary from one machine to another, but in general they can be subdivided into:

• Real or actual shadows of structures in, or close to, the focal trough

• Ghost or artefactual shadows created by the tomographic movement and cast by structures on the opposite side or a long way from the focal trough. The 8° upward angulation of the X-ray beam means that these ghost shadows appear at a higher level than the structures that have caused them as shown in Figs 15.18 and 15.19.

Fig. 15.18 A panoramic radiograph taken of the *right* half of a hemisectioned mandible. The **real shadows** are clearly shown on the *right*. The ghost shadow of the mandible is shown on the *left* (arrowed). Note these **ghost** shadows are at a higher level on the film than the real shadows because of the 8° rising X-ray beam that is used.

Fig. 15.19 Hemisected cadaver head positioned in a dental panoramic machine. B Resultant radiograph showing the **real hard and soft tissue** shadows on the right and the **ghost** shadows on the left. (Reproduced from *Oral Radiology* (2nd edn, 1987) by kind permission of Paul W. Goaz and Stuart C. White and The C.V. Mosby Company.)

Real or actual shadows (see Fig. 15.20 and 15.21)

Important hard tissue shadows include:

Fig. 15.20 A panoramic radiograph showing the main **real hard tissue** shadows, including the plastic head support, drawn in on one side of the radiograph, NS – nasal septum, **MIT** – middle and inferior turbinates, O – orbital margin, HP – hard palate, A – floor of antrum, Z – zygomatic arch, **EAM** – external auditory meatus, MP – mastoid process, SP – styloid process, H – hyoid, P – plastic head support.

Fig. 15.21 A panoramic radiograph showing the main real soft tissue and air shadows drawn in on one side of the radiograph, NC – nasal cartilages, EL – ear lobe, SP – soft palate, DT – dorsum of tongue, Or – oropharnyx, NF – naso-labial fold, M – mouth.

• Teeth

• Mandible

• Maxilla, including the floor, medial and posterior walls of the antra

• Nasal septum and conchae

• Orbital rim.

Important air shadows include:

• Mouth/oral opening

• Oropharynx.

Important soft tissue shadows include:

Ghost or artefactual shadows (see Fig. 15.22)

The more important ghost shadows include:

Fig. 15.22 A panoramic radiograph showing the main anatomical **ghost** or **artefactual shadows** drawn in on one side of the radiograph, Pl – palate, Md – mandible, CV – cervical vertebrae.

• Cervical vertebrae

• Body, angle and ramus of the contralateral side of the mandible

• Palate.

Advantages and disadvantages

Advantages

• A large area is imaged and all the tissues within the focal trough are displayed, including the anterior teeth, even when the patient is unable to open the mouth.

• The image is easy for patients to understand, and is therefore a useful teaching aid.

• Patient movement in the vertical plane distorts only that part of the image being produced at that instant.

• Positioning is relatively simple and minimal expertise is required.

• The overall view of the jaws allows rapid assessment of any underlying, possibly unsuspected, disease.

• The view of both sides of the mandible on one film is useful when assessing fractures and is comfortable for the injured patient.

• The overall view is useful for evaluation of periodontal status and in orthodontic assessments.

• The antral floor, medial and posterior walls are well shown.

• Both condylar heads are shown on one film, allowing easy comparison (see Ch. 30).

• The radiation dose (effective dose) may be lower than a full mouth survey of intraoral images in some cases (see Ch. 6).

• Development of field limitation techniques which result in further dose reduction.

Disadvantages

• The tomographic image represents only a section of the patient. Structures or abnormalities not in the focal trough may not be evident (Fig. 15.23).

Fig. 15.23 A Upper standard occlusal showing unerupted and a large dentigerous cyst (arrowed) associated with B Panoramic radiograph showing the two unerupted canines out of focus (arrowed) and only a suggestion of the dentigerous cyst, because they are all outside the focal trough.

• Soft tissue and air shadows can overlie the required hard tissue structures (Fig. 15.24).

Fig. 15.24 A Right bitewing showing no evidence of mesial caries in (arrowed). B Panoramic radiograph showing an apparent lesion in this tooth (arrowed). This appearance is created by the overlying air shadow of the corner of the mouth.

• Ghost or artefactual shadows can overlie the structures in the focal trough (Fig. 15.25).

Fig. 15.25 A Periapical of region showing an area of radiolucency at the apex of (arrowed). B Panoramic radiograph showing no evidence of the lesion (arrowed) owing to superimposition of the shadow of the cervical vertebrae.

• The tomographic movement together with the distance between the focal trough and image receptor produce distortion and magnification of the final image (approx. × 1.3).

• The use of indirect-action film and intensifying screens results in some loss of image quality but image resolution can be improved by using digital image receptors.

• The technique is not suitable for children under six years or on some disabled patients because of the length of the exposure cycle.

• Some patients do not conform to the shape of the focal trough and some structures will be out of focus.

• Movement of the patient during the exposure can create difficulties in image interpretation (Fig. 15.26).

Fig. 15.26 A Panoramic radiograph of a young child, who had fallen over, showing a step deformity (arrowed) in the lower border suggesting a fracture. B Same radiograph enlarged showing the step deformity (large arrow), but also notice the step deformities in the crowns of the upper and lower first deciduous molars (small arrows). Sudden slight vertical movement of the patient makes interpretation difficult.

Assessment of image quality

As mentioned in previous chapters, in relation to all other radiographs, image quality assessment essentially involves three separate stages, namely:

• Comparison of the image against ideal quality criteria

• Subjective rating of image quality using published standards

• Detailed assessment of rejected films to determine the source of error.

Ideal quality criteria

Irrespective of the type of image receptor being used, typical quality criteria for a full field of view panoramic radiograph include the following.

• All the upper and lower teeth and their supporting alveolar bone should be clearly demonstrated.

• The whole of the mandible should be included.

• Magnification in the vertical and horizontal planes should be equal.

• The right and left molar teeth should be equal in their mesiodistal dimension.

• The density across the image should be uniform with no air shadow above the tongue creating a radiolucent (black) band over the roots of the upper teeth.

• The image of the hard palate should appear above the apices of the upper teeth.

• Only the slightest ghost shadows of the contralateral angle of the mandible and the cervical spine should be evident.

• There should be no evidence of artefactual shadows due to dentures, earrings and other jewellery.

• The patient identification label should not obscure any of the above features.

• The image should be clearly labelled with the patient’s name and date of the examination.

• The image should be clearly marked with a Right and/or Left letter.

Subjective rating of image quality

The simple three-point subjective rating scale published in the UK’s 2001 Guidance Notes was introduced in Chapter 9 and is discussed in detail, together with the errors associated with exposure factors and chemical processing, in Chapter 17. The summary is shown again in Table 15.1. Panoramic patient preparation and positioning errors are described below.

Table 15.1

Subjective quality rating criteria for film-captured images published in the 2001 Guidance Notes for Dental Practitioners on the Safe Use of X-ray Equipment

Rating	Quality	Basis
1	Excellent	No errors of patient preparation, exposure, positioning, processing or film handling
2	Diagnostically acceptable	Some errors of patient preparation, exposure, positioning, processing or film handling, but which do not detract from the diagnostic utility of the radiograph
3	Unacceptable	Errors of patient preparation, exposure, positioning, processing or film handling, which render the radiograph diagnostically unacceptable

Assessment of rejected films and determination of errors

Patient preparation errors (Fig. 15.27)

These can include:

• Failure to remove jewellery

– Earrings

– Necklaces

– Piercings

• Failure to remove dentures

• Failure to remove orthodontic appliances

• Failure to remove spectacles

• Inappropriate use of the lead apron.

Patient positioning errors (Figs 15.28 and 15.29)

These can include:

Fig. 15.28 Examples of common patient positioning errors. A Failure to position the neck correctly – extension of the neck causing excessive spinal ghosting shadows over the anterior teeth. B Anteroposterior error – patient positioned too far forwards (too close to the image receptor) and vertical error – Frankfort plane not horizontal (chin tipped down) creating narrow, out of focus anterior teeth, distorted occlusal plane (so-called *smiley face*) and excessive peripheral spinal shadowing. C Anteroposterior error – patient positioned too far back (too far away from the image receptor) creating widened, magnified and out of focus anterior teeth. D Anteroposterior error – patient positioned too far forwards (too close to the image receptor) creating narrowed incisors **and** failure to instruct patient to keep their tongue in contact with the palate creating the radiolucent band across the film. E Horizontal error – patient asymmetrical, rotated to the RIGHT. The RIGHT molars are closer to the image receptor and smaller, the LEFT molars are further away from the image receptor and larger. F Vertical error – Frankfort plane not horizontal (chin tipped down) creating out of focus lower incisors and excessive ghosting shadows of the contralateral angles of the mandible. G Vertical error – Frankfort plane not horizontal (chin tipped up) creating out-of-focus upper incisors and distorted occlusal plane (arrowed) (so-called *grumpy face*). H Vertical error – Frankfort plane not horizontal (chin tipped down) creating out of focus lower incisors and distorted occlusal plane (arrowed) (so-called *smiley face*).

Fig. 15.29 Examples of failure to instruct the patient to keep still during the full panoramic cycle. A Sudden movement in the vertical plane – distortion of the image 45 region creating a step-deformity in the lower border (see also Fig. 15.26). B Movement in the vertical plane caused by the patient opening their mouth causing distortion in the region (arrowed). C Multiple vertical movements while the anterior teeth were being imaged. D Continuous shaking movements throughout the cycle. E Sudden side-to-side horizontal movement while the anterior teeth were being imaged causing them to be blurred. F Horizontal movement towards the end of the cycle causing horizontal elongation and stretching of the shadow of the developing lower right third molar (arrowed).

• Failure to ensure that the spine is straight (ghosting shadow error)

• Failure to ensure the incisors are biting edge-to-edge on the bite-peg (anteroposterior error)

• Failure to use the light beam marker to ensure midsagittal plane is vertical and the head is not rotated (horizontal error)

• Failure to use the light beam marker to ensure the Frankfort plane is horizontal (vertical error)

• Failure to instruct the patient to press the tongue against the roof of the mouth (air shadow error)

• Failure to instruct the patient to remain still throughout the exposure (movement error).

Equipment positioning errors (Fig. 15.30)

These can include:

Fig. 15.30 Examples of equipment positioning errors.A The X-ray tubehead and image receptor carriage assembly positioned too low relative to the patient – the antra and condyles are not imaged. B Cassette positioned back-to-front in the carriage assembly. Name plate and hinge screws are evident (arrowed). C Modern cassette positioned back-to-front. D Cassette inadvertently used twice and double-exposed.

• Failure to set height adjustment correctly

• Failure to select correct exposure settings (see Ch. 17)

• Failure to use the cassette correctly.

Footnote

Panoramic radiographs should not be considered an alternative to high-resolution intraoral radiographs. However, they are commonly considered as an alternative to right and left oblique lateral radiographs or the bimolar projection (see Ch. 12) mainly because it is assumed that less operator expertise is required to produce panoramic images of adequate diagnostic quality. Unfortunately, the multiple and varied causes of error in panoramic radiography make the technique very operator-dependent, no matter how sophisticated the equipment and the image receptors become. The use of digital sensors (solid-state or phosphor plate) improves the resolution of panoramic images when compared to those captured using indirect-action film and intensifying screen combinations. In addition, digital images can be enhanced and manipulated using computer software (see Ch. 5).

The diagnostic value of all panoramic images is increased considerably if clinicians understand that the image created is a tomograph (whatever the image receptor) and are aware of the limitations that this imposes. A suggested systematic approach to interpretation of panoramic images is outlined in Chapter 19.

To access the self assessment questions for this chapter please go to www.whaitesessentialsdentalradiography.com