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Quantities and Units in Radiography

Learn more about the quantities and units used when discussing x-rays.
© UK Health Security Agency

In this step, we will look at the different quantities and units used to measure radiation dose. Being able to measure the amount of radiation helps us to ensure that X-ray equipment is working correctly and delivering the right quantity of radiation to the patient to get the best image whilst minimising their exposure. We can also estimate or measure how much radiation those who work with X-ray equipment might be exposed to and help design a safe dental practice.

Radiation Dose Quantities and the Units That They Are Measured In

Air Kerma

This is a simple measure of how much radiation is present in the air and is a quantity that is easily measured using an appropriate instrument. It is measured using the unit ‘Gray’ which is abbreviated to Gy. The output from your X-ray set is usually measured in air kerma. A typical intra-oral measurement is shown below. Note the units on the meter are ‘mGy’ or milligray (1/1000th of a Gray).

photo of the equipment used in a typical intra-oral measurement of air kerma

Absorbed Dose

This is a measure of the energy deposited in a specific material (such as body tissue) by the incident radiation. It is also measured using the unit ‘Gray’ (Gy). It can be directly measured using an appropriately calibrated radiation dosemeter or derived from a measurement of air kerma and knowledge of the material absorbing the radiation. In practice, for dental X-rays, the absorbed dose in body tissue is approximately the same as the air kerma value.

Equivalent and Effective Dose

The quantities used to evaluate radiation doses to workers and patients, and the relative health risks that result, are equivalent doses and effective doses. These are not directly measured but are calculated using the absorbed dose and applying weighting factors.

The quantity equivalent dose is used to describe the dose to a single organ such as the skin, or to extremities (such as the hands). Equivalent dose is the absorbed dose multiplied by a ‘radiation weighting factor’, which considers how damaging the type of radiation is. The radiation weighting factor for X-rays is 1, so the absorbed dose and equivalent dose has the same value.

Different organs and body tissues are sensitive to radiation to varying degrees, for example, bone marrow is about 10 times more sensitive to radiation than the skin. To quantify this we use tissue weighting factors. The quantity effective dose is the sum of the tissue-weighted equivalent doses and is a measure of the radiation dose to the whole body. Using the quantity effective dose allows a comparison between X-ray doses to different parts of the body with different radiation sensitivities, and gives an indication of the overall risk from exposure to radiation.

Effective dose and equivalent dose are both measured using the unit Sievert which is abbreviated to Sv. A dose of 1 Sv is a very high dose (high enough to lead to radiation injuries). Routine annual doses to workers will be very much less, perhaps thousandths or millionths of a Sievert. When reporting doses abbreviations are used to represent fractions of a Sievert or fractions of a Gray, as summarised in table 1 below. These prefixes are commonly used alongside other everyday units of measurement such as length (micrometre, i.e. 1 millionth of a metre) or volume (millilitre, i.e. one thousandth of a litre).

<img src=”” alt=”table of prefixes: 1/1000th = 10-3 = milli = m e.g. mSv 1/1000000th = 10-6 = micro = µ e.g. µSv 1/1000000000th = 10-9 = nano = n e.g. nSv”>


The table below provides a brief summary of the different quantities and units used to measure radiation dose.

table summarising the radiation dose quantities and units - 'air kerma', 'absorbed dose', 'equivalent dose', and 'effective dose'.

Further information on the typical doses received by dental practice staff and patients as a result of dental radiography is provided in step 1.8.

© UK Health Security Agency
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Dental Radiography: Radiation Protection in Dental Practice

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