Dental Radiography
X-rays for dental radiography are produced by high voltages of electricity
within an x-ray head and come out through a metal tube called a
collimator. This ensures the x-rays only come out of the head in one
direction. To ensure the x-rays do not spread from the tube where they are
made the tube is surrounded by a thick cover of metal. This means the x-ray
head is very heavy –about 20kg and must be handled carefully. We call the
x-rays that come out of the machine “Ionising radiation”.
The x-rays are beams of high energy that can travel some distance. They have enough energy to pass
through the body. How much x-rays come through the body depends on what they hit on the way through.
The x-rays that come through when caught on a film or sensor will give us an image (picture) of that part of
the body.
Soft tissue like lips will not absorb many x-rays and they will mostly pass stright through to appear dark on
our image we call that a radiolucent area. Amalgam fillings will stop and absorb a lot of x-rays and so will
result in a white area in our image and we call that a radiopaque area.
These high energy x-rays when they hit the small cells in our tissue can sometimes damage them. They can
damage individual cells of sensitive tissue and sometimes damage the DNA in reproductive cells. As x-rays
(ionising radiation) can cause damage we try to keep the amount patients are given x-rays to the lowest
amount possible. We must always make a good justification for taking x-ays and use the lowest dose
possible to give the result we need.
When x-rays hit tissue or objects the rays may be absorbed or scattered and so dental taff must keep a safe
distance from the x-ray tube head when x-ryas are being taken. This area of 2 metres around the tube head is
called the “controlled zone” and only the patient must be in this area so the staff are not exposed to scatter
radiation.
As ionising radiation has dangers only staff who have been properly trained are
allowed to operate the x-ray machines.
Each machine will have a control panel that allows the operator to change the
setting or power of the x-rays. For different areas of the mouth and for different
types of film or sensor there will be different lengths of exposure. This is usually
displayed in seconds. Once the machine is switched on the operator will select the
length of exposure, take the exposure switch – usually on a long cord so they are
well outside the controlled area and press the button to generate the x-rays. A light
(usually orange) and a sound will indicate that x-rays are being generated.
The image from an x-ray exposure is captured on light
sensitive film or an electronic sensor.
The films for use inside the mouth (intraoral) come in
3 sizes.
Child size
Periapical / Bitewing
Occlussal
The electonic sensor has CCD sensor similar to digital cameras and is connected to a computer so the image
is immediately available.
The radiographic film is surrounded by black paper to help exclude any
light.
A lead foil is used to absorb some of the radiation that passes through the
film and help prevent scatter of radiation to the surrounding tissue.
Radiographic films for intraoral radiographs have a plastic waterproof and
light proof outer covering. The packets have roundd edges and are soft
enough to bend easily so they do not damage the patients mouth.
Extra oral films are much larger and come in boxes of films which are then
placed into a casettee in a dark room. The casettes have intensifying screens on
either side of the film. The intensifying screens allow a much lower dose of
radiation to be used making it safer for the patient
Intra oral films also come with different
speed settings. The faster the film (a
more sensitive film) the lower the
exposure needs to be which reduces the
risk of ionising radiation. D being a
slow speed increasing up to E speed and
the Ultra a fast speed.
Film holders and aiming devices are used
to ensure that the film is held in the
correct position to receive the parallel
beam of x-rays. Each manufacturer will
have 2 or 3 different shapes to hold the
film packets in different parts of the
mouth. The dental assistant must know
which holder to prepare for different
views of the teeth.
Processing radiographic films:
Small films can be processed manually in small tanks or by automatice processors.
When a film has been exposed to x-rays or light it alters the silver bromide in the emulsion to form a latent
image. When the film is put in a developer solution the silver bromide which has been exposed changes into
silver crystals – it should not be left in the developer too long and the developer should not be too hot or all
the silver bromide will change and result in a dark image. The film is washed in water and then put in the
fixer solution – this washes away the unexposed silver bromide and fixes the silver crystals to the gelatine
film. The film must then be thoroughly washed in cold running water to clear away all the chemicals. The
film must then be left hanging to dry thoroughly before it is mounted.
In large clinics and hospitals where different sizes of film are used there will be a “darkroom” with a red or
orange safety light and a light outside the door to indicate the darkroom is in use. The films are carefully
taken out of their packets, put on a hanger then placed in developer, water, fixer, water then washed and
dryed. To save having a darkroom there are simple manual processor where the dental assistant can put their
hands through tight cuffs and look through an orange shield to be able to see and control the film. The
operator will check the temperature is 20degrees C then manually dip the films in the correct tanks of
solutions timing how long the film is in the developer and the fixer.
With the automatic
processor the operator
needs to check the
temperature then put their
hands through the light
proof cuffs and unwrap the
x-ray film packet.
Handling the film only by
the edges the film is placed
in a slot and sometimes a
button pressed to start the
electric rollers that will
take the film through the
different processing tanks.
To ensure good quality radiographic
films a test film using a step wedge
should be used at the start of each day
to ensure the processing chemicals do
not need to be replaced. The water
tanks need to be replaced each day.
The automatic rollers need to be
cleaned every week in cleaning
solution otherwise marks will appear
on the developed radiographs.
Care needs to be taken when handling the x-ray processing chemicals. PPE
of gloves, mask, eye glasses and apron should be worn and care taken not to
spill or splash the chemicals.
Bitewings are intra-oral films that show the upper and lower posterior teeth. They are usually taken as part
of a routine examination of a patient. On a good bitewing you should be able to see clearly the enamel,
dentine, pulp and alveolar bone, and there should not be any overlap of the teeth. Any existing fillings,
crowns or root treatments will also show. They are used to detect any caries either below the occlusal
surface, interproximal caries or secondary caries under existing restorations. They will also show any
defective fillings which either have overhangs (beyond the surface of the tooth) or fillings that do not reach
the outer surface of the tooth.
Sub gingival calculus will show as a small mark on the surface of the root and any bone loss will also show.
For a full periodontal assessment of pockets periapical radiographs will be taken.
Periapical radiographs are the same size as bitewings bur are placed along the length of the tooth to
provide a full picture of one or more teeth from the crown to the root and include the surrounding bone.
This periapical view of a patient with pain shows the upper
left posterior teeth. A root filling and crown are present on
the upper second premolar and large calculus deposits seen
on all teeth. There is bone loss due to an infected
periodontal pocket.
The end of the roots (apex) can be clearly seen as can the
position of the maxillary sinus. This view helps the dentist
either find the cause of the pain or be able to eliminate
several possible causes.
During endodontic (root) treatment a periapical will
be taken before starting to find the number and shape
of the roots. Another will be taken to help measure
the length of each of the roots and a final x-ray taken
to show the final root filling.
Periapicals are also used to find the level of bone supporting a tooth
or series of teeth. If the bone level will give sufficient support and
the surface of the bone is even and appears healthy then extra work
can take place to restore these teeth such as crowns or bridges.
Occlusal radiographs can be
taken of the upper or lower arch
usually to show the position of
any unerupted canines or
supernumerary (extra) teeth
The dental panoramic machine has a film in a cassette which along with the x-ray head rotates round the
patients head to give a view of the whole mouth. It will include the Temporal Manibular Joints, the mandible
completely, the maxilla and sinuses as well as all the teeth.
It is used to detect any pathology such as cysts, to see the position of unerupted teeth, the position of roots
that lie close to nerves and is used regularly by oral surgeons and orthodontists.