The Physics of MRI

MRI being performed. Via Wikimeda Commons

Phew. I know, right?!

I’ve been saying I’ll write a post about the physics of MRI for months. Never got around to it. Mostly because I knew that since magnetic resonance imaging (MRI) was my field of research, that I would want to go into a lot of detail. I loved the time I spent doing MR research, and now I’ll finally share with all of you how it works.

Let’s start at the beginning.

We are all made of atoms. Atoms consist of a nucleus (made of neutrons and protons) orbited by electrons. The most common atom in our bodies is Hydrogen, which is a single proton orbited by a single electron. It is the simplest atom in nature, and it is quite fitting that it makes up the majority of our bodies.

We are mostly made of hydrogen because we are mostly water. Water has 2 hydrogen atoms and 1 oxygen atom (H₂0). When we do an MRI, what we are actually taking an image of is the hydrogen in our bodies.

How is that done? Well protons have charge, a positive charge. Any particle that has charge also has what is called a magnetic moment. A magnetic moment is essentially a measure of the strength and direction of the magnetic field of  particle. For a proton, the magnetic moment looks like this:

A proton (red circle) and it's magnetic moment (arrow)

Usually, all the magnetic moments in your body are jumbled about in all directions. That’s why you are not magnetic. When this is the case, we say that your net magnetic moment is zero.

Normally, all the magnetic moments in your body are jumbled around. Thus, your net magnetic moment is zero.

But if we apply a magnetic field from the outside, we can get the moments in your body to line up with the field, like so:

If we apply a magnetic field in a certain direction (blue arrow) the magnetic moments in your body will tend to align with the magnetic field.

In order to do that, we need BIG MAGNETS. That’s why MRI’s are so strong; you need that strong magnetic field to get the moments in your body to line up. (The diagram above is an exaggeration; in reality, only 1 in a million of the magnetic moments will line up with the field, on average).

How strong is an MRI magnet? Magnetic fields are measured in units of Tesla (in honour of Nikola Tesla). A typical MR scanner has a field of 1.5 Tesla. For comparison, this is about 30 000 times stronger than the Earth’s magnetic field, the field that makes a compass needle point north.

Now your body has what is called a net magnetization. This means that the magnetic moments have lined up and you are a little bit magnetic.

But that’s only half the battle. Next, we have to get some kind of signal from your body to make an image. How do we do that?

Well, have you ever swiped a credit card in a machine? What would happen if you just held it still? Would it still work?

The answer is no. Your credit card only works because the black strip in the back is magnetic. When you swipe it through the machine, it creates an electric current. This is due to Faraday’s Law, which says that a changing magnetic field in a loop of wire will create an electric current in that wire.

When you swipe a credit card through a credit card machine, you induce an electric current in the machine.

We can apply this concept to your body too now that its been magnetized by the MRI machine. You see, MRIs actually have a radio transmitter/receiver inside of them as well. How does THAT help?

Unlike a credit card, we can’t swipe YOU through the MRI machine. That’s not feasible. But we can manipulate the fact that you are now magnetized. This can be done using a radio-frequency pulse. Basically this is just a short, intense burst of radio waves at your body. This will actually make your net magnetic moment spin.

If your net magnetic moment is now spinning, it creates the exact same effect as if we were to swipe you through the MRI machine: it generates an electric current in a receiver in the MRI. Cool eh?

Ok so now we have a signal from your body by magnetizing it, and then making that net magnetization vector spin. How does that make an image?

So we made your net magnetization spin by using a radio-frequency pulse. How fast it spins depends on how strong the magnetic field is. So what MR scanners do is they make the magnetic field a bit different at each point in your body. That way, the magnetization from each part of your body spins at a different rate (or frequency) and we can then determine what part of your body is giving the signal.

Confused? Think of it this way:

Let’s say you had a piano. You hit a key on the left side of the piano and what happens? It makes a deep, low-frequency sound. Now you hit a key on the other end of the piano and you get a high-pitched, high frequency sound.

Different keys on the piano make different frequencies of sound. You can get an idea of what key someone hit from the frequency of the sound, without actually seeing the piano.

Now pretend you weren’t looking at the piano at all, and someone else hit a key. If you hear a low-frequency sound, you know they hit a key on the left side of the piano. If you hear a high frequency sound, you know it came from the right side. You were able to determine the position of the key by its frequency.

Its the exact same idea in MRI. Using the frequency of the signal we get, we can determine where in your body that signal came from. We then put all those signals from all parts of your body together, and we get an MRI image.

MRI of the head (Photo: NASA)

I know, there’s a lot of steps to MRI,  so I’ll recap:

1. We magnetize your body by using a really big, strong magnet.
2. We make your net magnetization spin using a radio-frequency pulse.
3. This spinning magnetization generates an electric current in the radio receiver in the MRI machine.
4. We can tell where in your body this signal came from by its frequency.
5. Putting all the signals from all over your body together, we can make an image.

So there you go. Simple right?

If you want some more information you can check out the MRI article on HowStuffWorks.

Or if you want something a little more technical and goes a bit deeper into MR theory (e.g. relaxation, pulse sequences), check out this online book by J.P. Hornak, or the Wikipedia page on the physics of MRI.

Happy learning!