CHAPTER 14 Dental Imaging
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.
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.
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.
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.
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.
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:
|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|
|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|
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.
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).
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.
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.
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 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:
|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)
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 are shown in Table 14.3 and the accompanying figures.
|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|