How MRI scans work

What's the science behind an MRI scan?
26 January 2021


photo of an MRI scanner


Adam Murphy explains the basics of an MRI scan, and then Katie Haylor experiences one...

Adam - An MRI is a magnetic resonance imager. It is in essence, a giant, really powerful, magnet. Hospitals are always really careful to not let anything magnetic into the room while it’s on, because it would get sucked in and damage their expensive machine (as well as anyone in it!)

MRIs are particularly good at imaging the squishier parts of us. Although they don’t always give the resolution that CT  would. They're also quite complicated in how they work.

Your body has a huge amount of water in it. We’re 65 % water. And that water has two hydrogen atoms, and those atoms contain protons, little particles of positive charge. Those protons have a property called spin. You can think of it a little bit like how the Earth spins on its axis. If you put a person in a strong magnetic field, the protons in all the water atoms of your body start to spin either with, or against the magnetic field.

When you blast radio waves of just the right energy, which depends on that magnetic field, at these protons, they flip away from the magnetic field. When you stop, they go back to their original positions. As they do, they give out some energy. We measure that energy, and that tells us where water is in the body, and that’s how we generate the image.

By precisely tuning the strength of the magnetic field in different places, we can get information about the different amount of water all around the body, in 3-D

MRI is particularly useful, because unlike X-Ray, or CT, or even PET, there’s no ionising radiation, so it doesn’t damage you to do it.  Although it can take a bit of time, and you have to sit very still in a large loud machine.

Katie - I actually got to experience an MRI scan of my brain a few years back. The resulting image has pride of place on my desk at home. And I remember taking off any metal, as Adam just mentioned, and being given some earplugs. I took my shoes off and laid down on actually quite a comfy bed, which slowly slid into a rather large donut...

Hi Katie, how are you?

Katie - I'm pretty snug to be honest.

So are you ready to start?

Katie - I am.

Okay. So I just need you to stay very still. Okay. And then you're just going to hear some sounds and then I'll speak to you after that. Okay.

[MRI sounds].

Michael - Katie Haylor, meet Katie Haylor's brain. I just want to emphasise the specialness of this, because the fraction of all humanity who has ever lived on Earth who has actually gotten a chance to see their own brain is very tiny. And you now are welcomed to that club.



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