Chapter 23- X rays

X rays should be more correctly called radiographs. Photographs are pictures made with light rays and so radiographs are pictures made with x rays.

X rays are a type of electromagnetic radiation [others are visible light, ultra violet light and microwaves] that possess energy. X rays have high energy and so can pass through human tissue to a greater or lesser extent depending on the density of the particular tissue concerned.

This property is useful in medicine as it allows us to see ‘below the surface’.

If you think of it in terms of a photographic plate if the ray goes through you it will hit the film and the film will be dark when developed, if the ray can’t go through [say where you have metal restorations] it will not hit the film and so this bit of the film will not develop. Different body tissues have different radiopacities meaning the rays go through depending on the density of that particular tissue. This property produces an image that is ‘reverse from real life’, the denser bits of you are white and the not so dense bits darker to a greater or lesser extent.

X rays can also cause us damage because as they pass through us if they hit an atom of matter directly they release energy [either by being scattered or absorbed] and can cause cell damage, cell death or a mutation in the DNA of a cell.

It is important therefor that the lowest dose of radiation, for the shortest period of time is used when taking radiographs for diagnostic purposes. There is no ‘safe level’ of exposure to x rays, so there are very strict guidelines and regulations in place to protect patients. Your dentist has to register their equipment with the health and safety executive [HSE], the Care Quality Commission [CQC], a medical physics department at a hospital or other similar body and there has to be a radiation protection advisor, a medical physics expert, a radiation protection supervisor and a ‘legal person’ that is someone in the surgery who makes sure all this is done and all the guidelines and recommendations are followed. So if you are having x rays at the dentist you can rest assured that it is done in the safest way possible.

If your dentist is taking radiographs of you there has to be a reason [this is called clinical justification]. The dose has to be as low as possible, for the shortest period of time, to the smallest area of you to produce a ‘functional image’. This means a picture that is good enough for the clinician to see what they need to see. It’s no good exposing someone to x rays if the picture obtained is rubbish and doesn’t tell you anything.

It is also important that only the patient receives the radiation, that everyone else is out of the measured area, that the equipment is safe, regularly serviced and inspected, that the quality of the images is regularly audited, emergency procedures are in place should it malfunction, that any member of staff involved in any way is properly trained and last but not least that the images produced are ‘reported on’ ie that they are read and interpreted by someone who is trained to do so. It’s no good exposing someone to xrays if you are not going to write down what you see.

All dentists and DCPs who are allowed to take radiographs have to regularly update their knowledge of xray use by doing mandatory Continuing Professional Development [CPD] in the physics, regulations and operating skills required.

There should be information displayed in the waiting room at your dental surgery and if you have any questions what so ever about having xrays you have every right to ask your dentist to explain what you wish to know and to provide you with any further information you may require.

You also have every right to refuse to have them [or your children under 16 to have them] BUT you must be aware that without radiographs your dentist will not be able to completely inspect your mouth and diagnose any problems you may have. Some treatments may not be possible without radiographs such as extraction of wisdom teeth and root fillings.

There are 2 types of radiographs your dentist may use-

Film based [sometimes known as analogue]-  this is the type we are all used to. The xrays come out of the machine on the wall, go through you and hit a film. Just like in an old fashioned camera. This film is then developed in a dark room or in an automatic processor to produce an image [picture].

Digital- the xrays come out of the machine on the wall, go through you and hit a digital sensor. The information goes down a wire to a computer which by some magic then turns it into an image on the screen [there is another type where the sensor is not connected by a wire but is put into a machine which reads it and sends the information to the computer].


The advantage of digital radiographs is they can be enlarged, the contrast altered and they can be sent to someone else [for example if you need to see a specialist].

Radiograph Views.

So if your dentist wishes to take a radiograph/s they will want to take a particular ‘shot’ to get the best view of what they want to see.

Radiographs are either intra oral [with the film/ sensor inside the mouth] or extra oral [with the film/ sensor outside the mouth].

The commonest intra oral views are;-

Bite wings these are taken usually of either side of your mouth to look for cavities in between your teeth which are not visible on the surface.

Periapicals these show your tooth from top to bottom, the crown and the root. These might be taken perhaps to look for an abscess at the end of your root.

Both these types are usually taken with a film holder/ aiming device to hold the film/ sensor in the right place which also means that the same view can be exactly replicated in the future.

The commonest extra oral view is;-

Dental Panoramic Tomograph [DPT] which looks a bit looks like a school photo with all your teeth on it, this is not so good for looking for early decay but is excellent for looking for things such as buried wisdom teeth, cysts and  un erupted teeth. It is also the commonest view an orthodontist might take at your initial visit.

How much radiation do you receive?

This is a difficult question to answer because of the way radiation is measured and because different views need different amounts of radiation to produce an image.

We receive radiation all the time in the form of ‘background radiation’, cosmic radiation and if you live somewhere like Cornwall from the ground beneath us [Cornwall is built on granite which emits radon]. It’s thought that a little bit is actually good for us [I mean a tiny little bit].

Of course people are concerned to receive more than is desirable or necessary and no clinician would wish to subject a patient to unnecessary radiation.

 X ray doses in perspective.

1 dental x ray is the same dose as 8 hours background radiation/ eating a handful of brazil nuts/ eating 20 bananas.

2 dental xrays is the same dose as a flight to Paris [from cosmic radiation].

15 dental xrays is the same dose as a flight to Malaga [from cosmic radiation].

A dental xray has a 1 in 10 million chance of producing a malignancy [cancer].

A chest xray has a 1 in a million chance of producing a malignancy.

You have a 1 in a 100 chance of a malignancy if you smoke 10 cigs a day and a 1 in 10,000 chance of dying in a road accident.

3D imaging.

So the x rays you would normally have at the dentist produce a 2D image and most of the time that’s fine, that’s all you need but a 2D image of a 3D object will in some cases not be enough.

You will have seen on the TV people having whole body scans, where the patient goes through a long tunnel and the machine takes lots of little ‘slice’ radiographs. These images are then fed into a computer and by magic produce a 3D image of you. This can be turned around, seen at different levels all sorts of wonderful things that have completely revolutionized diagnosis and treatment in medicine.

In dentistry specialists will also use scans BUT you don’t need a scan of the whole of someone’s head [irradiating their ears, eyes not to mention their brain] to just look at a tiny area. So in dentistry we have what’s called a ‘cone beam scan’. It’s the same thing but it just produces a 3D image of a little cylinder of you, like a little can of beans measuring 5 x 5cm [this is the usual size]. The clinician might want this information to for example see how big a cyst is, see if the roots of a wisdom tooth are wrapped round a nerve and a host of other reasons.

Reading radiographs.

Dentists are unique amongst medical professionals in that we take our own radiographs [I think vets do as well].

A person who takes the radiographs/ scans is called a radiographer.

But it’s not just a question of taking the radiographs you have to be able to read them and interpret them. The person who does this in a hospital is a doctor who is a specialist in radiology- a radiologist.

In dentistry in practice we are our own radiographer and radiologist. We are trained to take and interpret [read] our own radiographs as far as our field of expertise extends. That is I would not be allowed to report on a radiograph of your leg or your brain. I would have a pretty good idea of what I am looking at because I am used to reading radiographs but I don’t have an in depth knowledge of the anatomy of other bits of you because it is beyond my field of expertise.

‘X rays are only an aid to diagnosis’ this is a mantra in dentistry and no doubt medicine too. It means you have to take all the information;- the history [the story], the symptoms [what the patient tells you about what’s wrong], the signs [what the clinician sees] and come to a provisional diagnosis. You might then decide radiographs are required to confirm your diagnosis.

X rays and pregnancy.

Although the dose of x-rays used in dentistry is very low and would probably be quite safe in pregnancy, no one would knowingly irradiate a pregnant woman because of the [very small] possibility of damaging the fetus. In an absolute emergency in the 3rd trimester [the last 3 months] it is probably quite safe as the baby is just growing by this stage not developing.

If you are trying to get pregnant and you need radiographs you need to time your visits to the start of your period and the time before you ovulate in the middle of the month.

If you are at all in doubt ASK THE DENTIST FOR ADVICE.

I would always err on the side of safety.

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