14: Dental Imaging

CHAPTER 14 Dental Imaging

Chapter Points

• This chapter covers the requirements of the NEBDN syllabus Section 14: Radiography.

Introduction

Dental imaging mainly consists of taking X-rays (radiographs) and photographs. Both of these are used routinely in the dental environment as aids to diagnosis and treatment planning. More advanced imaging, such as computed tomography (CT), magnetic resonance imaging (MRI), and ultrasound (US) are also used, especially in surgical and hospital practice.

Qualified dental nurses can process radiographs and may also mount radiographic films. Under supervision of an IR(ME)R operator, as a qualified dental nurse you may press the button on the X-ray machine. However, you are only allowed to place the films in the patient’s mouth or position the X-ray tube, and to take radiographs, if you are suitably trained with a post-registration certificate in dental radiography. Nurses are not under any circumstances permitted to interpret radiographs.

Term to Learn

IR(ME)R operator: a person who is trained in dental radiography, for example a dentist, therapist, hygienist or a dental nurse who has undertaken a post-registration certificate course in dental radiography. (This involves completion of a record of experience in the workplace and passing an examination.) See also p. 333.

Definitions and basics of radiography

• Radiography – the techniques involved in producing X-ray images.
• Radiology – the interpretation of radiographic images.

X-rays, like light, are electromagnetic waves but they have more energy than light so can penetrate the body tissues to varying degrees (which affects the number of X-rays reaching the film). When the X-rays hit a radiographic film, they sensitise the silver crystals on the film, which then turn black when put in a developer. The image thus formed on the film enables the clinician to see different (distinct) structures because of the large differences in absorption of X-rays by hard and soft tissues. Metals and really hard tissues such as tooth enamel appear white, other hard tissues such as dentine and bone appear grey, and soft tissues appear almost black on the film.

Radiation hazards

X-rays are a type of ionising radiation. Thus, while radiography can be essential for diagnosis and treatment planning, it involves exposure of patients and, potentially, staff to ionising radiation. The problem with ionising radiation is that it can damage DNA, causing mutations that may possibly lead to cancer. Ionising radiations, particularly X-rays, are also a potential hazard to body organs and tissues where cells are proliferating rapidly (e.g. in the fetus or a young child, or in the gonads and bone marrow). Here ionising radiation has the capacity not only to induce malignant tumours (this is called an oncogenic effect) but also to damage reproductive tissues (teratogenic effect).

Therefore, since there is always a slight risk from excessive exposure to radiation, the benefit of it must always outweigh the risk to the patient. These advantages and disadvantages must be discussed with the patient and the patient must give informed consent. Women should always inform their clinician if there is any possibility that they are pregnant. Exposure to ionising radiation in pregnant women must be kept to the absolute minimum and X-rays taken only when absolutely essential: benefit must well exceed any possible harm.

Key point

Since no X-ray exposure can be completely free from risk, the use of radiography is accompanied by a responsibility to ensure protection.

We are all constantly exposed to normal background ionising radiation arising from the earth (especially in areas where the rocks emit radon gas – in UK this is mainly in mountainous areas). People are also exposed to radiation when travelling by air. So diagnostic X-rays produce radiation in addition to this background radiation. As such the dose of X-rays that a person receives while undergoing basic dental radiography (intra-oral X-rays and panoramic radiography) is quite low, probably equivalent to only a few days of background radiation. However, it might still increase the risk of salivary gland and thyroid tumours. Having a CT scan means much higher exposures.

In the past, ionising radiation was also a serious occupational hazard to radiographers and clinical dental staff. Some clinicians even developed radiation-induced dermatitis or cancer of their hands from holding X-ray films in the patients’ mouths during radiography. Having one’s hand in the X-ray beam gives about 4000 times the exposure compared with that received 2 m away from the X-ray tube (the recommended ‘safe’ distance). Therefore this practice is illegal now, and there have been tremendous improvements in technology and techniques resulting in greater ionising radiation safety. Specific precautions to take in dental radiography are discussed later in this chapter.

Types of dental radiograph

Dental radiographs are taken to aid:

• Detection of problems not visible on clinical examination (e.g. caries on the proximal surfaces of teeth, subgingival calculus or bony changes)
• The follow-up of disease progression
• Treatment planning (e.g. to decide implant placement)
• Assessment of prognosis.

Dental radiographs can be taken with the film held within the mouth (intra-orally) or extra-orally – when the film is outside the mouth.

Intra-oral Radiographs

For these radiographs, the X-ray films commonly used are called the periapical, bitewing and occlusal films (Figure 14.1), all of which are small enough to be partially inserted into the mouth. Intra-oral radiographs are taken to detect dental pathology including small carious lesions. They can be useful in the diagnosis of:

• Interproximal caries
• Other pathology of the tooth crown
• Pathology and assessment of the morphology of tooth roots
• Periapical pathology (abscess, granuloma, cyst, etc.)
• Pathology in the periodontium and adjacent bone.
image

FIGURE 14.1 A selection of intra-oral films, extra-oral film in the cassette, holders and radiographic monitoring badge.

Positioning the patient and the film

• Precise positioning of the patient’s head is important to ensure the correct area is radiographed. The tissues to be radiographed and the X-ray beam must be in proper relationship to produce an accurate radiographic image.
• As in photography, movement during exposure will result in a blurred image so, in adjusting the chair and headrest, it is important to ensure the patient is as comfortable as possible to minimise movement during exposure. (Blurring may also be greatly reduced through the use of fast-speed film.)
• Various film holding devices (Figure 14.1) should be used to secure the film in place. Most film holders will also have a beam aiming device which allows the shape of the X-ray beam to match the shape of the film, further decreasing the dose to the patient (rectangular collimation).

Identify and Learn

Find a couple of film holders in your workplace and ask your supervisor to explain how they help aim the X-ray beam precisely.

Extra-oral Radiographs

Extra-oral radiography means using large films to visualise the skull, jaws, temporomandibular joints and sinuses. These films are used with intensifying screens in a cassette (Figure 14.1).

Examples of the use of intra- and extra-oral radiographs in dentistry are given in Table 14.1.

TABLE 14.1 Examples of the More Common Dental Radiographs and Their Main Uses

Area to be Examined Radiographic Film Used Often Used for
Whole of the mandible and the maxilla DPT (dental panoramic tomograph; Figure 14.2) Presence and position of teeth; jaw fractures
A single tooth or three to four teeth plus the supporting bone Periapical film (Figure 14.3): size varies from 35 × 22 mm to 40.5 × 30.5 mm Assessing the periapical area, for root canal treatment and to assess root fractures
Molar/premolar region Bitewing (horizontal) (Figure 14.4): size varies from 35 × 22 mm to 54 × 27 mm Caries detection interproximally
Bitewing (vertical) Periodontitis
Maxillary incisor/canine region Anterior occlusal (Figure 14.5): size is about 57 × 76 mm Impacted canines; super- numerary teeth; palatal cysts; salivary duct stones
Third molars Oblique lateral, or DPT, or periapical Inspecting unerupted or impacted third molars
Sinuses DPT or occipito-mental radiograph Sinusitis, root in sinus

Identify and Learn

Find an occipito-mental radiograph in your workplace to see how it shows the sinuses, or look it up on the internet.

Term to Learn

Intensifying screen: a screen that permits a lower radiation exposure but good quality films; digital systems can also help in the same way.

Cephalometric radiographs

These are a type of extra-oral radiograph often used in orthodontics (see Chapter 11 for more details).

image image

FIGURE 14.3 (A, B) Examples of periapical films.

Tomographs

Tomography (Greek tomos = slice) involves taking films of sections or slices of a part of the body. Panoramic radiography is a specialised tomographic technique that is commonly used in dentistry but the radiation dose may be higher than intra-oral films to show the same areas under examination.

image

FIGURE 14.4 A bitewing film.

Dental Panoramic Tomographs

A dental panoramic tomograph (DPT or orthopantomograph (OPG)) is used mainly to assess the lower part of the face. A DPT displays both the upper and lower teeth in a long flat film (Figure 14.2). It also gives a good overview of the maxillary sinuses, mandibular rami and the temporomandibular joints. It shows the number and position of all teeth including unerupted ones. However, it does not show fine detail of the anterior part of the jaws, as the spine gets superimposed during taking the film. DPTs are also not adequate for caries diagnosis. One panoramic film gives about the same radiation dose as 18–20 bitewings (see below).

image

FIGURE 14.2 Examples of dental panoramic radiographs. (A) Mixed dentition period. (B) An adult with full dentition. (C) An edentulous mouth.

Find Out More

How does the spine get superimposed on the front teeth when taking a DPT?

Key point

When a DPT is being taken the patient needs to bite on a small plastic mouthpiece attached to the machine, to keep the arches separated, and to keep still while the arm of the machine rotates around the head (but they will not come into contact with it).

Computed Tomography (CT) Scans

Being a radiographic technique, CT scans also show the bone and teeth as ‘white’, and can be useful in diagnosis of hard tissue pathology. A fairly high radiation exposure is required to produce CT scans. Cone beam CT (CBCT) is a fairly recent development that has the advantage of a lower radiation dose than conventional CT; it is especially helpful in implant treatment planning.

image image

FIGURE 14.5 (A) Maxillary anterior occlusal film, showing a cyst. (B) Mandibular anterior occlusal film, showing a salivary stone.

Angiography

Angiography is an invasive technique with a relatively high radiation dose, but useful in diagnosis of blood vessel lesions and tumours.

Arthrography

Arthrography was used in the past for diagnosis of suspected temporomandibular joint problems but, in most centres, has been superseded by MRI (see below).

Sialography

Sialography can be useful in diagnosis of salivary duct obstruction.

Scintiscanning

Also known also as ‘gamma scanning’, this is the injection of a radio-isotope (radiopharmaceutical) such as iodine or technetium which concentrates strongly in specific parts of the body. The emitted gamma rays are collected by a gamma camera, which produces images on a computer.

• Bone scintiscanning is a high radiation dose technique useful in diagnosis of bone cancer and other bone disease.
• Salivary scintiscanning is now rarely used since ultrasound has become the imaging modality of choice for assessing salivary glands.

Other types of imaging used in dentistry

Ultrasound Scans

Ultrasound is the non-invasive use of sound waves to produce images that can be used to help diagnosis of diseases. It is the preferred method of imaging for diagnosing soft tissue swellings (e.g. lymph nodes, thyroid or salivary glands). There are no known contraindications to ultrasound.

Magnetic Resonance Images

Magnetic resonance imaging (MRI) also does not use ionising radiation. On MR images, the bone shows up as black (rather than white as in X-ray films), and soft tissue lesions can be well visualised, including malignant lesions. The disadvantages of MRI are that it is expensive and liable to produce image artefacts where ferromagnetic metal objects are present (e.g. dental restorations, orthodontic appliances, metallic foreign bodies, joint prostheses, implants etc.). Contraindications to MRI include:

• Implanted electric devices (e.g. heart pacemakers and defibrillators, nerve stimulators, cochlear implants)
• Intracranial vascular clips, if these are ferromagnetic
• Prosthetic cardiac valves containing metal
• Obesity (because of the weight limit on the gantry and size of scanner)
• Claustrophobia (unless open scanner is available).

Term to Learn

Gantry: in radiology, a gantry is a device that helps to rotate the radiation source around the patient so that images can be taken from various angles and in various planes.

Photography

Photographs are part of the clinical record and are needed especially in orthodontics, cosmetic dentistry and after assaults including child abuse (non-accidental injury). Patient consent to all imaging is required.

Processing radiographs

Digital X-rays need no processing (see below). Otherwise, the radiographic film has several components apart from the actual celluloid film coated with emulsion (Table 14.2).

TABLE 14.2 Components of Radiographic Film, their Function and Disposal

Intra-oral X-ray Film Packet Component Function Dispose into Waste Marked
Plastic envelope Protects the film from moisture and light Clinical
Black paper Protects the film from light Domestic
Celluloid film Produces the radiograph Domestic
Lead foil Prevents radiation that has not been absorbed by the film passing on into the patient Special

X-ray processing can be automated or manual. X-ray film processing takes place in the dark, using an automated processor, or a locked darkroom with light for illumination compatible with the red or orange filter. The solutions must be at normal room temperature (18–22 °C)

The manual steps are as follows:

1. The correct sequence is: developing, washing, fixing, washing.
2. Wearing protective latex or other gloves, open packet and discard lead foil into special waste. Discard case into clinical waste and black paper into domestic waste.
3. Handle the film only by its edges.
4. Start the timer.
5. Immerse film in developer for one minute.
6. Remove film from developer (replace lid) and wash film in running cold water.
7. Immerse film in fixer for one minute.
8. Remove film from fixer (replace lid) and thoroughly wash film in cold running water.
9. Dry the film in warm air.
10. Mount and label radiographic film with patient’s details (first and last name, date of birth and number).

Key point

Good developing is essential for good quality images; poor processing not only can produce a poor image but may necessitate repeating the radiography procedure – and hence unnecessary radiation exposure.

Faults in Radiograph Exposure and/or Processing

Faults in radiograph exposure and/or processing are shown in Table 14.3 and the accompanying figures.

TABLE 14.3 Radiographic Film Faults

Film Appears Reasons Comments
Faint or blank Under-developed, under-exposed, not fixed Low temperature; wrong developing time; wrong strength of developer. See Figure 14.6A
Dark Over-developed or over-exposed High temperature; wrong developing time; wrong strength of developer. See Figure 14.6B
Foggy Exposed to light before developing or old film  
Fading Under-fixed  
Black />

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Related posts:

  1. 16: Dental Emergencies
  2. 12: Dental Drugs, Materials, Instruments and Equipment
  3. 2: Medical Emergencies and First Aid in the Dental Surgery
  4. 5: Oral Disease
  5. 10: Surgical Procedures
  6. 17: Human Diseases and Health Promotion

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Jan 8, 2015 | Posted by in Dental Nursing and Assisting | Comments Off on 14: Dental Imaging

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