What is an MRI Scanner?

I ponder all sorts of useless questions when I go to bed at night, and this was one from last night. Among many other body parts, MRI scanners are used to scan the brain after a suspected stroke, so the question has some broad degree of relevance to me.

So let’s start with the individual words. MRI is Magnetic Resonance Imaging. The easiest word there is Imaging – building up a picture of something. I also know that when MRI was first introduced, it was called Nuclear MRI, so there’s another word. (Given the unpopularity of nuclear power, I can easily understand why that word was later dropped.) So we still have Nuclear, Magnetic and Resonance. Well, Nuclear must refer to something to do with the central part of every atom, the protons and neutrons. Magnetic suggests that either electricity or magnetism is involved. Electricity and magnetism are actually the same thing. And resonance implies that you throw a range of stronger/weaker things (energy?) at an object, and sooner or later you can observe some kind of effect. It’s like that famous American suspension bridge – it could withstand all sorts of windy conditions, but when wind of one particular speed hit it, it buckled and fell down!

I can’t resist this:

But really, beyond the words, I’ve never really taken the trouble to find out (or I’ve more likely forgotten!) how an MRI scanner works. However I have a background in physics so with a bit of reading I should be able to understand how it works. With a fair wind behind me, I might also be able to explain it!

Here goes (it’s actually pretty straightforward, trust me!):

So, in our tissue we have water, and the first step of the process is to excite this water. (In fact it is specifically the hydrogen nuclei, which form part of “water”, which are targeted. And when I say “excite”, I’m talking about using magnetism to get all of the hydrogen nuclei pointing is a certain direction – just like the needle on a compass.) The way in which this excitation is caused is by applying a magnetic field to the tissue. Different tissues contain different amounts of water, and the water (at the molecular level) behaves differently (magnetically-speaking) depending on its surroundings, so we can identify different types of tissue, hence the usefulness of the process.

You then turn off this field. As these excited water molecules settle back down, so this change causes changes the electric field they generate, and these changes can be measured. So a machine has two things going on. The first bit is to use a (varying) magnetic field to get everything excited, the second is to detect, essentially, this level of excitement. The overall image is built-up because the location of the “excitement” detector is known quite precisely, and so, during a scan, lots of tiny snapshots are all put together. And the heavy “clunking” noise that we hear? Is basically the just the detectors, detecting these changes.

In case you’re wondering, the magnetic fields involved here are quite strong. Certainly, stronger than we experience in everyday life. And the way they’re produced uses things like mega-low temperatures (the kind of temperatures which turn helium into a liquid) and something called superconductivity. So these are actually quite complex techniques, but to produce something that is simple enough that we all have an appreciation of it – a magnet.

Would the reader think it a cop-out if I also posted a further source of information? https://en.wikipedia.org/wiki/Physics_of_magnetic_resonance_imagin. I’ve cut a few (many) corners here both for brevity and for ease of understanding, but this article describes things more precisely.

The older I get, the more I understand the beauty of how we can use bits of physics to get hold of real-life, useful information.Wonderful.

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