How does an MRI scan work?

How does an MRI scan work?

MRI stands for 'magnetic resonance imaging'. An MRI scan uses strong magnets and radio waves to create highly detailed images of the inside of your body.

Instead, it works by aligning hydrogen atoms in your body and then using radio waves to create a clear picture of your soft tissue, organs, and other internal structures.

This type of scan is particularly useful for diagnosing and assessing conditions affecting the brain, spine, joints, and soft tissues.

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The difference between an X-ray and an MRI

'X-rays' are used in many diagnostic scans including CT imaging, mammograms and standard X-rays.

An X-ray is a relatively high-energy photon or light particle. We can see everything light touches and this type of light can pass through tissue, allowing us to see inside the human body.

This exposure has to be limited because X-rays are ionising - meaning they cause atoms to acquire a positive or negative charge by losing or gaining electrons. Too much ionising radiation can damage the structure of cells.

Do MRI scans use radiation?

MRI scans don't use this type of radiation. Instead, they uses magnets and radio frequencies which are completely harmless.

The magnet used in MRI would be strong enough to pick up a car if it were used in a standard electromagnet (more than 1.5 Teslas), but MRI uses the magnetic field in a very different way.

Click here to learn more about radiation in different types of scans

The science behind it all

Hydrogen is the most abundant element in the universe. There are two hydrogen atoms in every water molecule and water makes up around 70% of your body. Hydrogen has a single positively-charged proton as its nucleus.

These protons have what's known as a 'magnetic moment' - basically a north and south pole and a tendency to spin on an axis, just like the Earth. The magnetic field generated by an MRI scan causes these protons to line up and spin at a particular frequency.

A secondary magnet turns the molecules to face new directions and when it's switched off they realign. The rate at which they realign depends on the type of tissue the molecule resides in.

The radio frequencies are detected by coils placed over your body, and this information allows a computer to visualise everything from soft tissue to bone and organs like the heart and brain.

The process has no known side effects. 

Why are MRI scanners so noisy?

The noise of an MRI scanner is caused by running an electrical current through a coiled wire to create the magnetic field. When switched on, the energy forces the coil outward, it contracts when the magnet is switched off.

During a scan, currents are switched on and off rapidly to measure the changes in the proton spins. This is what causes that series of 'loud clicks' that MRI scans are known for.

Why do I have to lie still and hold my breath?

MRI scans measure tiny sub-atomic movements in your body. For the scan to be accurate, all movement not caused by the magnetic field needs to be kept to a minimum.

During some scans you may be asked to hold your breath for a short period of time. This ensures the most accurate scan possible.

If you're worried about an upcoming scan, take a look at our article featuring 6 ways to keep calm during your MRI scan.

What to expect on the day of your scan

When you arrive for an MRI at a Nuffield Health hospital, our welcome team will greet you and address any questions you might have ahead of your scan. You’ll then change into a gown, keeping metal objects out of the scan area to make sure imaging isn't interfered with.

In the MRI room, you’ll see the scanner which resembles a large tube-like machine. Your radiographer will guide you every step of the way and have you lie down on a cushioned table that slides into the machine.

To make the experience more comfortable, they will provide headphones or earplugs to soften the tapping sounds during the scan.

You’ll need to lie still to capture the best images, and scans can take between 15 and 90 minutes depending on the area being scanned. Throughout the process, the radiographer will check to ensure you’re comfortable and informed about what's happening.

Last updated Friday 8 November 2024

First published on Monday 7 November 2016