MRI Introduction For High School Students

For advanced high school students, MRI could be introduced through a discussion on magnetic fields. This introduction is by B. M. Damon.

What is MRI?
MRI stands for Magnetic Resonance Imaging. It is a way of using magnetic fields to create internal anatomical images of people, chicken eggs, fruit - just about anything. It is commonly used by medical people to examine people who have been injured or are sick. It is also used by many scientists to study anatomy and physiology. In conjunction with scientists at the University of Illinois, you will be using it to study chicken embryology.

MRI imagers use very strong magnetic fields. The magnetic fields used are actually similar in strength to those used to pick up cars at junkyards. Making images also requires fairly powerful computers and some rather sophisticated mathematics.

How does MRI work?
Atoms are made up of protons, neutrons, and electrons. Protons and neutrons live in the nucleus and electrons travel around the nucleus in orbitals. As individual particles, all of them possess a property called spin. Spin is the tendency to act like a ball of spinning charge, sort of like the earth rotating about its own axis or a toy top that is spinning (see Figure 8).

Spin Illustration
Figure 8. Spin illustration (Image courtesy of Dr. Andrew Webb.)

In a nucleus, there may be more than one proton or neutron, and so the spins of protons and neutrons add together. In some nuclei, they add to zero. In others, they add to a number other than zero. We call this having a net spin. Any nucleus with either an odd atomic number or an odd atomic weight has a net spin. If a nucleus has net spin, it too acts like a ball of spinning charge. For us, the most important example of a nucleus with spin is the hydrogen nucleus. It is so important because hydrogen is a component of water, and there is a lot of water in things like chicken eggs.

Any time an electric charge moves, it creates a magnetic field. So the spinning motion of these nuclei creates a tiny magnetic field. This magnetic field is called the magnetic moment. You can think of a magnetic nucleus as being like a tiny compass.

Compasses are magnetized pieces of metal floating in a liquid. They are actually very sensitive magnetic field detectors, having a strong tendency to point in the same direction as the earth's magnetic field - North. In case you have never seen this, find a pocket compass and spin it around a few times. Then place it down on a table and watch the compass needle. Eventually, it will settle, pointing toward North. It will stay still until you move the compass again.

The little nuclear magnets act similarly. If you take some water and put it inside of a very strong magnetic field, like the one inside of a magnetic resonance imager, the nuclear magnets in the hydrogen atoms tend to pointin the same direction as the magnetic field. The nuclear magnetic fields then add together to create a large amount of net nuclear magnetization. So now the water molecules will act in unison as a big compass, rather than individually as millions of little compasses.

But nuclear magnets do something that compasses don't do. Instead of staying still, like the compass needle, they precess. Precession is a wobbling motion that occurs when a spinning object is subjected to an external force. Precession is very important in MRI. There is another example of precession that may be more familiar to you. If you take a toy top and spin it, and then watch it as it slows down, it starts to wobble (precess). In MRI, we are very interested in how rapidly precession occurs. We call this value the precessional frequency.

So to summarize what we have discussed so far, some nuclei possess a property called spin. Spin causes these nuclei to behave like tiny magnets. When these nuclear magnets experience a strong magnetic field, net nuclear magnetization develops in the same direction as the magnetic field. This magnetization is caused by the nuclear magnets pointing towards the magnetic field and precessing. It will then remain stable - in a steady state. This steady state doesn't happen right away, though. It happens gradually. We call this process T1.

So what good is all of this? Unfortunately, none - at least, not yet. Our real goal is to observe this nuclear magnetization. In order to do this, we have to disturb this steady state. We do this by turning on a second magnetic field. It is like holding a small magnet next to the pocket compass. The compass responds to the second magnetic field by pointing at it, instead of the earth's magnetic field. (Important: if you try this, don't get the magnet too close to any computer disks).

When we turn on this second magnetic field inside of a magnetic resonance imager, we call this exciting the nuclear magnets. Just like the compass has to respond to the magnet, the nuclear magnets have to respond to the second magnetic field, too. They respond by precessing in a wider arc (actually, they precess in the plane perpendicular to the magnetic field). This creates a signal that we can record. Eventually, though, the signal decays away. We call this decay T2.

There are several interesting differences between those two signals. First of all, one of the signals is big and the other one is small. As you may have guessed, one of the thing that determines how big of a signal we record is how much water is present in the sample. Another important factor is whether the signal decays rapidly or slowly (that is, how long it takes T1 and T2 to occur).

We have now created a nuclear magnetic signal! But we can't yet use it to make an image, though. There are a few more things left to do. First, we can't try to make an image of the entire object (in your case, an egg). We have to only image part of the egg. We call that part of the egg the slice. The slice can be very thick or it can be very thin. The slice can be made lengthwise, sideways, or across the egg. It can be placed anywhere inside of the egg. You will have control over all of these things in your imaging sessions.

So let's say that we make a slice right through the middle of the egg, where the yolk is. We also need to figure out how much signal came from the yolk and how much came from the white. The way we do this is to divide the egg into a matrix. Then we assign a particular amount of the total signal to each component of the matrix. We do this by making the precessional frequency vary as a function of spatial position. The result is lots of signals, with varying sizes.

Remember that the size of a nuclear magnetic signal depends on how T1 and T2 occur and how much water is in the sample. Because the rates of T1 and T2 and the amount of water differ throughout the egg, different spots in the egg will give different amounts of signal. If there is a lot of signal, there will be a white spot in the image; if there is no signal, there will be a black spot in the image. Intermediate amounts of signal appear in the image as some shade of gray. We call this shading and the ability to tell different structures within the egg apart contrast.

Since the strength of the signal in each part of the egg depends on how much water is there and T1 and T2, we can also base contrast on how much water is there or on T1 or T2 differences. In your experiments, you will be able to provide contrast based on either T1 or T2 differences. While this may sound very complicated, it really isn't. It is really just like looking at an object through red cellophane, and then looking at the same object through blue cellophane. You are really just looking at two different aspects of the same thing.

Another control that you will have is called signal averaging. In signal averaging, we make the same image over and over again. Assuming that the bird has not moved too much, this will result in a better quality image. But it will take more time.

The sides of the slice can be made very large, or they can be made very small. We call the dimensions of the slice the field of view. Field of view is another control that you will have in your experiments.

What should you do in your experiments?
You experiment as much as possible with the MRI controls. Moving the position of the slice will help you to find different structures inside of the egg (and eventually, different parts of the bird's internal anatomy). Zooming in will help you to see things in more detail (but zooming in too much will create some rather strange looking images - try it!). Using a thinner slice will also help you to see more detail, but may make the images look grainy. Changing the contrast may help you to see some structures more clearly than others.

Doing all of this will help you learn about MRI, computers, the Web, and about chicken embryology. These are all important things for you to learn about. You will also learn about the scientific process, and will be one of the first people in the world ever to look at the inside of a chicken egg with MRI. Have fun and enjoy the process of discovery!

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