Magnetic resonance · Scans explained
Magnetic fields, not X-rays
An MRI scanner makes its pictures from magnetic fields and the signals the body returns inside them. No ionising radiation is involved, and that fact organises everything else about the examination.
Why does MRI use magnetic fields instead of ionising radiation?
An MRI scanner makes its pictures without ionising radiation. The UK Health Security Agency, in its publication "Medical imaging: what you need to know" of 22 September 2022, lists MRI alongside ultrasound among the "types of imaging that do not use X-rays or radioactive substances". That single property decides the shape of the machine and the shape of the appointment. The NHS page of 11 September 2025 describes the scanner as "a large machine with a short tunnel through it, and a flat bed that moves through the scanner", and gives the duration as "usually between 15 minutes and 1 hour, but can take longer".
The field at the centre of the machine is magnetic, and it stays on. The picture itself is built from signals the body returns while it lies inside that field, which is why the examination can run for the better part of an hour without the dose accounting that surrounds an X-ray based method. The UKHSA split between the two families runs through the whole of this site and through the modalities table.
Where do MRI fields sit on the electromagnetic spectrum?
On the non-ionising side, well below the boundary where radiation gains the energy to strip electrons. Readers who want that boundary charted from end to end can follow a guided tour published by FELDWERK, a Swiss magazine on the electromagnetic environment, which walks the electromagnetic spectrum explained from the Earth's static field through mains frequency, mobile bands and wireless networks up to the point where ionisation begins. The tour is written for a general readership and keeps each band next to the place it occupies.
Inside the examination room the practical consequence is different from anything in the X-ray family. There is no beam switched on for an exposure and switched off afterwards; the field is a standing condition of the room, and the safety rules are written around that fact rather than around dose.
What safety rules govern the scan room?
The NHS describes the checks the department runs before the examination. A questionnaire asks about metal inside the body, naming "surgical clips, a pacemaker, dental fillings or bridges" among the items covered, and the person is asked "not wear anything that has metal in it, such as jewellery, a bra, hair clips or hearing aids". The checking is done by the service before the examination begins, and it is done because the field acts on metal wherever the metal happens to be.
The professional side of the same discipline is set out in "Safety in Magnetic Resonance Imaging", a 2016 guidance document from the Society of Radiographers, which "overviews safety issues in Magnetic Resonance Imaging (MRI) and provides direction to radiographic staff on where appropriate information can be found". The Society, founded in 1920, is the trade union and professional body for the diagnostic imaging and radiotherapy workforce.
| Where the item is | Items named by the source | How the check is made |
|---|---|---|
| Inside the body | "Surgical clips, a pacemaker, dental fillings or bridges" | A questionnaire issued by the hospital before the appointment |
| On the body | "Jewellery, a bra, hair clips or hearing aids" | The person is asked not to wear anything containing metal |
| In the room itself | The field acts on any metal brought inside | Access is controlled by the department running the examination |
Who is in the room while the scanner runs?
Not necessarily the person operating it. The NHS states that a radiographer controls the equipment from a neighbouring room while keeping contact with the person being scanned. Radiographer is one of the titles the Health and Care Professions Council protects by law: "The professions we regulate have one or more designated titles that are protected by law", and using one requires HCPC registration, as the professions page updated on 12 May 2025 sets out.
The noise the machine makes is part of the same physical picture. The NHS gives the reason ear plugs or headphones are offered: "MRI scanners make loud bangs and clicks as they take pictures." Some examinations add an injection of "a contrast medium (dye) to help show more detail in the MRI scan pictures"; in British law those gadolinium-containing contrast agents are regulated medicines, "indicated for the enhancement of magnetic resonance imaging", as the MHRA Drug Safety Update of 14 December 2017 records.
On timing afterwards, the same NHS page gives results as "usually takes between 1 and 2 weeks". The report itself is produced under the recording duty the Ionising Radiation (Medical Exposure) Regulations 2017 place on the employer, which applies to the wider department even though MRI is not an ionising method.
Checked against the source
- MRI and ultrasound are "types of imaging that do not use X-rays or radioactive substances" (UKHSA, Medical imaging: what you need to know, 22 September 2022).
- The scanner is "a large machine with a short tunnel through it, and a flat bed that moves through the scanner", taking "usually between 15 minutes and 1 hour" (NHS, MRI scan, 11 September 2025).
- The metal check covers implants including "surgical clips, a pacemaker, dental fillings or bridges" (NHS, same page).
- "Safety in Magnetic Resonance Imaging" is the dedicated staff guidance published by the Society of Radiographers in 2016.
- A radiographer controls the scanner from the neighbouring room; "Radiographer" is an HCPC protected title (NHS, 11 September 2025; HCPC professions page, 12 May 2025).
Two families, one tunnel shape
MRI and CT share a shape in the room, a ring and a moving flat bed, which is why they are so often merged in conversation. The physics is not shared: UKHSA places CT inside the X-ray family, which uses ionising radiation, and MRI in the family that uses neither X-rays nor radioactive substances. The published durations differ too, 10 to 20 minutes for CT against 15 minutes to 1 hour for MRI, in the NHS figures.
A second confusion sits between the magnetic field and the injection. The field is the standing condition of the room and the reason for the metal checks, while the contrast medium described by the NHS is a separate, optional step used to show more detail. The MHRA regulates the latter as a medicine; the former is a property of the machine itself.