MRI Introduction For High School
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).
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
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
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
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
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
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
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!
- Page 13 of 24 -