MRI scan - How it works 

How an MRI scan works 

During a magnetic resonance imaging (MRI) scan, you lie in a strong magnetic field and radio-frequency waves are directed at your body. This produces detailed images of the inside of your body.

Most of the human body is made up of water molecules, which consist of hydrogen and oxygen atoms. At the centre of each hydrogen atom there is an even smaller particle called a proton. Protons are like tiny magnets and are very sensitive to magnetic fields.

When you lie under the powerful scanner magnets, the protons in your body line up in the same direction, in the same way that a magnet can pull the needle of a compass.

Short bursts of radio waves are then sent to certain areas of the body, knocking the protons out of alignment. When the radio waves are turned off, the protons realign and in doing so send out radio signals, which are picked up by receivers.

These signals provide information about the exact location of the protons in the body. They also help to distinguish between the various types of tissue in the body, because the protons in different types of tissue realign at different speeds and produce distinct signals.

In the same way that millions of pixels on a computer screen can create complex pictures, the signals from the millions of protons in the body are combined to create a detailed image of the inside of the body.

Page last reviewed: 05/09/2013

Next review due: 05/09/2015

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The 2 comments posted are personal views. Any information they give has not been checked and may not be accurate.

The Bedfordshire Redneck said on 16 August 2014

If you are claustrophobic you might well have troubles with the scan if you are going into the machine head first. I have had two scans in my life, the first was on my right foot and was tolerable, the second was on my left shoulder and meant going into the scanner head first. I managed to last it out but wouldn't contemplate doing it again. I believe the radiographer was a little concerned at my appearance when I emerged !!

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cabtalite said on 11 December 2013

I didn’t see any machines when I entered the room but I subdued my surprise and got busy putting my bag and jackets on the chair provided, handed my completed questionnaire to the radiologist, and lay on the bed provided. That was where I had a cannula put in ready for the g- dye I’d be given later. I carried my jackets and bag to the neighbouring MRI scanner room and they were put into a locker for me.
A rest was put under my knees to keep my lower legs comfortable and, more importantly, still as I lay on the bed. I was given a bleeper to sound in an emergency; headphones to listen to music (I requested they got the CD from my bag – which they did!); and a thin blanket for which I was grateful. The bed moved backwards taking me headfirst into the scanner. I was fascinated by the whole procedure. There was a mirror in which I could see the radiographers behind their screen but, as they recommended, I kept my eyes shut for most of the procedure.
During the MRI scan, the internal part of the magnet produces repetitive and loud tapping, thumping and other noises. Short bursts of radio waves knocking protons in my brain out of line: who would have that that could be such a noisy process?
On a couple of occasions, a radiographer talked to me to find out how I was. There was an inbuilt microphone in the scanner through which I could easily respond to them.
After 25 minutes or so, they took out the cannula, took off the headphones, and removed the hand-held alarm. They cautioned me to drink lots of water to flush out the dye. I eased myself off the bed and said my thanks and goodbyes.
The novelty and fascination with the radio-frequency waves, magnetic fields, radio signal sending protons, the considerate and professional staff, and having thrived through my first MRI were the impressions that have lasted.

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Screening and testing

We all undergo various health checks throughout our lives. These fall into two categories: screening and testing