Reference

Glossary of imaging

Imaging has a working vocabulary that is rarely explained to the people it is used on. Each entry here is defined once, plainly, and carries the source it was read from.

6 September 2026
The Inside the Scanner desk

A long hospital corridor at dusk, the ceiling lights receding towards a far corner and every door along it closed

Imaging has a working vocabulary that appears on a request form, in a report and in a departmental leaflet, and which is rarely explained to the people it is used on. Each entry below is defined once, and each carries the source it was read from on 6 September 2026. Where no accepted source publishes a definition, the entry says so instead of supplying one.

The methods

Ionising radiation: The kind of radiation used by X-ray imaging and by nuclear medicine. The UK Health Security Agency classifies ultrasound and magnetic resonance imaging as types of imaging that do not use X-rays or radioactive substances (UKHSA, 22 September 2022).
X-ray: As an examination, a test that takes pictures of the inside of your body using a small amount of radiation (NHS, reviewed 4 February 2026).
General radiography: X-ray imaging producing two dimensional images, used for example where an infection or a fracture is in question (UKHSA, 22 September 2022).
Radiograph: The unit of a radiographic examination. The UK Health Security Agency expresses dental reference levels as dose area product per radiograph (UKHSA, 11 December 2025).
Fluoroscopy: An X-ray method that produces moving images of parts of the body (UKHSA, 22 September 2022).
Interventional radiology: The use of X-rays to produce moving images that guide and deliver treatment (UKHSA, 22 September 2022). The Royal College of Radiologists describes interventional radiologists as clinical radiologists who specialise in image guided procedures.
Computed tomography: An X-ray method that produces three dimensional images of the body (UKHSA, 22 September 2022), described to patients as a test that takes detailed pictures of the inside of the body (NHS, reviewed 8 November 2023).
Magnetic resonance imaging: Imaging without X-rays or radioactive substances, carried out in a large machine with a short tunnel through it and a flat bed that moves through the scanner (UKHSA, 22 September 2022; NHS, reviewed 11 September 2025).
Ultrasound: Imaging by sound waves, used to see images of the inside of the body such as muscles, organs, or a baby in the womb (NHS, reviewed 25 February 2025).
Probe: The device moved across the skin during an external ultrasound scan. The NHS describes external, internal and endoscopic forms (NHS, reviewed 25 February 2025).
Nuclear medicine: Imaging that uses a small amount of a radioactive substance, usually injected into a vein, and which includes PET-CT and PET-MRI (UKHSA, 22 September 2022).
DEXA: Dual energy X-ray absorptiometry, a type of X-ray passed through the body to measure bone density, using a much lower level of radiation than a standard X-ray (NHS, reviewed 5 October 2022).
Angiogram: An image produced during angiography while a contrast agent is in the blood vessels (NHS, reviewed 30 January 2023).

Contrast and tracers

Contrast medium: A substance given to make part of the picture clearer. In magnetic resonance it may be given as an injection to show more detail; in angiography a special dye is injected into the blood first (NHS, reviewed 11 September 2025 and 30 January 2023).
Gadolinium containing contrast agent: A regulated medicine indicated for the enhancement of magnetic resonance imaging. The Medicines and Healthcare products Regulatory Agency distinguishes linear from macrocyclic agents in its Drug Safety Update of 14 December 2017.
Tracer: The small amount of radioactive liquid injected into a vein before a PET scan (NHS, reviewed 4 March 2025).
Radiopharmaceutical: The subject of the ARSAC notes for guidance on the clinical administration of radiopharmaceuticals and the use of sealed radioactive sources (ARSAC, published 17 April 2014, updated 14 July 2026).

Dose and its units

Millisievert: The unit the UK Health Security Agency uses to compare doses. Its 2011 table gives a dental X-ray as 0.005 and a chest X-ray as 0.014, against a UK average annual dose of 2.7 (UKHSA, 18 March 2011).
Justification: The requirement that an exposure has been justified by the practitioner as showing a sufficient net benefit (regulation 11, Ionising Radiation (Medical Exposure) Regulations 2017). It is the first of the three principles set out by the ICRP in Publication 103, 2007.
Optimisation: The requirement that doses be kept as low as reasonably practicable consistent with the intended purpose (regulation 12 of the same regulations).
Dose limits: The third ICRP principle. In UK law the Ionising Radiations Regulations 2017 set 20 mSv a calendar year for employees and trainees aged 18 or over, 6 mSv for trainees under 18, and 1 mSv for any other person (Schedule 3).
Diagnostic reference level: Dose levels for typical examinations on standard sized individuals or standard phantoms, defined in regulation 2 of the 2017 regulations.
National diagnostic reference level: The national value published by the UK Health Security Agency, generally the third quartile of a national survey (UKHSA, 11 December 2025).
DAP, CTDIvol, DLP, CBDIw: The four quantities used in the UK reference levels: dose area product in Gy cm squared, CTDIvol in mGy, dose length product in mGy cm, CBDIw in mGy (UKHSA, 11 December 2025). None of them is a millisievert.

Roles and oversight

Referrer, practitioner, operator, employer: The four duty holders defined in regulation 2 of the Ionising Radiation (Medical Exposure) Regulations 2017. They are roles allocated in accordance with the employer's written procedures (Schedule 2, paragraph 1(b)), not job titles.
Medical physics expert: An individual or group with the knowledge, training and experience to act or advise on radiation physics applied to exposure, whose competence is recognised by the Secretary of State (regulation 2).
Clinical audit: A systematic examination or review of medical radiological procedures against agreed standards, required by regulation 7.
Comforters and carers: People who knowingly support someone undergoing an exposure. The 2017 regulations require dose constraints and guidance for them, and the Ionising Radiations Regulations 2017 apply a separate limit of 5 mSv over five consecutive calendar years.
Significant accidental or unintended exposure: A category of statutory notification. The Care Quality Commission enforces the regulations in England partly by reviewing such notifications from healthcare services (CQC, page updated 10 August 2022).
Radiographer: A title protected by law, along with diagnostic radiographer and therapeutic radiographer. Using one requires registration with the Health and Care Professions Council (HCPC, page updated 12 May 2025).
Sonographer: Not a protected title. The Society of Radiographers states that the sonography workforce is currently unregulated and that the title is not a legally protected one.

Words this site will not define

Two terms that circulate widely could not be found on any accepted source and are therefore absent from this glossary rather than guessed at: the Hounsfield unit, and slice as a name for one image of a series. The UK reference levels use CTDIvol and dose length product, and the same UK Health Security Agency table uses the column heading all sequences for the acquisitions of a CT examination. Why an entry is left out rather than filled in is explained in sources and method.