Having an MRI scan

Magnetic resonance imaging, usually shortened to MRI, is one of the imaging methods listed by the UK Health Security Agency in its patient guide Medical imaging: what you need to know, published on 22 September 2022. The guide describes medical imaging as "one of the biggest advances in modern medicine" and places MRI in a distinctive category: alongside ultrasound, it belongs to the "types of imaging that do not use X-rays or radioactive substances". This article explains what the scan measures, what the appointment looks like, and why departments carry out safety checks before the examination begins.

What does an MRI scanner actually measure?

An MRI scanner does not use X-rays or radioactive substances. The UK Health Security Agency lists it, alongside ultrasound, among the "types of imaging that do not use X-rays or radioactive substances". The UK Health Security Agency guide of 22 September 2022 groups imaging into two families: X-ray based methods, which include general radiography, fluoroscopy, interventional radiology and CT, together with nuclear medicine, all of which use ionising radiation, and the non-ionising family, which contains ultrasound and MRI. The same guide notes that nuclear medicine "uses a small amount of a radioactive substance that is usually injected into a vein", a technique that includes PET-CT and PET-MRI for functional imaging. MRI sits outside both of those radiation-based approaches, which is why questions of radiation and safety are handled differently for this examination than for the CT scan, a cross-sectional technique that relies on X-rays and "produces 3D images of your body" according to the UK Health Security Agency.

What does the scan look like?

The NHS page MRI scan, dated 11 September 2025, describes the machine plainly: "An MRI scanner is a large machine with a short tunnel through it, and a flat bed that moves through the scanner." The person having the scan lies on the flat bed, which moves through the scanner. A radiographer operates the scanner from a neighbouring room while keeping in contact with the person throughout the examination.

Two features of the appointment are described by the NHS. First, ear plugs or headphones are provided "because MRI scanners make loud bangs and clicks as they take pictures". Second, the scan "usually takes between 15 minutes and 1 hour, but can take longer". Regarding results, the same NHS page states that they "usually take between 1 and 2 weeks to arrive.

A contrast medium may also be part of the appointment. The NHS page on MRI, dated 11 September 2025, states that "you may be given an injection with a contrast medium (dye) to help show more detail in the MRI scan pictures". In British law these products are regulated medicines: a Drug Safety Update published by the MHRA on 14 December 2017 states that "gadolinium-containing contrast agents (GdCAs) are indicated for the enhancement of magnetic resonance imaging (MRI)". The vocabulary of sequences and contrast agents is set out in the glossary.

Why does the department ask about metal before the scan?

The safety questions asked before an MRI examination are carried out by the service, not by the person having the scan. The NHS page of 11 September 2025 describes two distinct checks. The first concerns clothing and personal items: the department asks people "not wear anything that has metal in it, such as jewellery, a bra, hair clips or hearing aids". The second concerns implants: hospitals use a questionnaire that asks about metal implants inside the body, citing "surgical clips, a pacemaker, dental fillings or bridges" as examples. These verifications are carried out by the service before the examination.

This checking practice is not an informal habit. The Society of Radiographers published professional guidance in 2016 titled Safety in Magnetic Resonance Imaging, which "overviews safety issues in Magnetic Resonance Imaging (MRI) and provides direction to radiographic staff on where appropriate information can be found". A further systematic enquiry is required by regulation across imaging services: IR(ME)R 2017, Schedule 2, paragraph 1(c), obliges the employer to maintain a written procedure "for making enquiries of individuals of childbearing potential to establish whether the individual is or may be pregnant or breastfeeding". All of these checks are described from the department's side: the service asks, records and verifies before the scan begins.

How do durations and result times compare across scans?

The NHS publishes appointment lengths and result times for each modality, and the differences are notable. The table below gathers the figures published on the relevant NHS pages. Where a range is given, it is reproduced as published; where the NHS describes variable practice, that is what the table shows. The source does not publish an average or a single figure, so none is calculated here.

The pattern is visible in the source material: the NHS gives the MRI appointment as usually between 15 minutes and 1 hour, but it can take longer, while its result time matches the CT and PET windows. For PET, the NHS page of 04 March 2025 also mentions that a radiologist reviews the images before the result is sent, and for ultrasound the NHS page of 25 February 2025 states that the doctor who requested the scan receives the conclusions, with a follow-up appointment sometimes arranged to discuss them.