What are X-rays?

X-rays were discovered by Röntgen in 1895. Their nature was initially unknown and so they were called X-rays. We now know that they are high energy electromagnetic radiation and are part of the electromagnetic spectrum.

What are they used for?

In dentistry, radiographs are one of our most helpful diagnostic tools. We take radiographs for a variety of reasons.


  1. Bitewing – the frequency of bitewings taken is dependent on the individual and their level of oral hygiene, although usually every 2 years. Bitewings show bone levels, caries (cavities), and the integrity of existing restorations.
  2. Periapical – this is an x-ray taken of a specific tooth/teeth. It shows the crown and root(s), bone levels, caries, and periapical area(s). They can be used to determine lengths for root canal treatments and show root formations prior to extractions of teeth.
  3. Occlusal – these x-rays show the upper or lower jaws, and are often taken to check for supernumerary (extra) teeth or for jaw fractures.


  1. OPT (orthopantomogram) – these x-rays are taken by specialised equipment. They show all erupted and unerupted teeth and their supporting structures. OPT’s are often taken prior to orthodontic treatment or for more extensive treatment cases.
  2. Cephalogram – these x-rays show the relationship of the upper and lower teeth and jaws when the teeth are in occlusion. They are often taken before and after orthodontic treatment.
  3. CBCT (cone beam computed tomography) – these are specialised x-rays often taken before extensive treatment plans such as implants. They take a series of detailed images and produce one 3D image.


Changes over the years…

  1. Collimation: we have changed from a round 6” collimator to a rectangular 5” x 4” collimator – this has resulted in a decreased area radiated, and has eliminated radiation of the parotid salivary gland. This means 30% less dose is received by the patient.
  2. Film speed: faster film requires less radiation to produce a good quality image. We moved from D speed film to E speed film which halved the exposure. We then moved from E speed film to F speed film which resulted in a further decrease of 20% dose received.
  3. FSD – focal skin distance increased from 10cm to 20cm for x-ray sets operating at 70kV.
  4. Digital film – digital film is more sensitive to radiation than conventional film so less radiation is required to produce a good quality image.
  5. Higher kV – we have moved from 50kV to 65-70kV. A higher kV still produces a good quality image but with less radiation received by the patient.
  6. Aluminium filtration at x-ray window – removes low energy X-ray photons.
  7. Timer switches and visible/audible signals – the exposure cuts out when the exposure button is released, and lights and sounds indicate an exposure is taking place.

Many improvements have been made regarding the taking and processing of radiographs. However, the ones which have made the biggest impact and reduction to the dose received by the patient are: rectangular collimation, film speed, higher kV and introduction of digital imaging.

The Legal bit

The International Commission for Radiological Protection (ICRP) state that each radiograph taken has to be justified, optimised and limited.  This means that before we take a radiograph: we will have reviewed any available history and previous radiographs and justify whether a radiograph is necessary; we will optimise the dose to As Low As Reasonably Practicable (ALARP); and we will limit the exposure using techniques and equipment designed to do so.

What does it all mean?

Doses received are measured in Sieverts (Sv).  Since 1Sv is a large quantity, doses are usually expressed in millisieverts (mSv) and microsieverts (µSv).

1Sv = 1000mSv; 1mSv = 1000µSv

We receive natural background radiation all the time. In the UK, this is from various sources and is approximately 2.7mSv annually.


The Keith Dental Practice - Radiographs table

The Keith Dental Practice - Radiation Sources

This does not mean that you should stop eating bananas and brazil nuts, or travelling by plane. It simply indicates how routine dental radiographs should be viewed – they are an extremely helpful diagnostic tool which help us plan treatment, monitor potential problems and care for your teeth more effectively, among other things.

Thank goodness for Mr Röntgen!


Written by Miss Faye Law
Senior Dental Nurse at The Keith Dental Practice
© Copyright 2019 All rights reserved on copyright and intellectual property T F Dent Ltd



ADA: https://www.ada.org/en/member-center/oral-health-topics/x-rays
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