How does an MRI scanner see inside the body?

1. Noor is having an MRI scan so the doctor can see inside her knee. The scanner is a huge ring with a tunnel and a bed that slides in. Inside the ring is a very strong magnet.
2. More than half of your body is water. Each bit of water holds hydrogen atoms, and the middle of each one acts like a tiny magnet. Normally they point in every direction.
3. Noor lies on the bed and slides into the tunnel. The big magnet pulls on all the tiny magnets in her body and lines them up, like compass needles all pointing north. She cannot feel it.
4. Next, a coil sends a short burst of radio waves, tuned to exactly the right note. It tips the tiny magnets over, and they all wobble together.
5. When the pulse stops, the tiny magnets swing back into line. As they do, they give out a faint radio signal of their own, and the coil listens. Watery parts and fatty parts swing back at different speeds.
6. Extra magnets change the pull a little from place to place, so each spot sings a slightly different note. The computer sorts the notes into a picture of one thin slice, then the next, like slices of bread in a loaf.
7. The scanner knocks and buzzes loudly as its magnets switch, so Noor wears ear protection. She left her watch and keys outside, because the magnet pulls on metal. All she has to do is lie very still. It does not hurt.

The scanner is built in rings, like an onion. Tap each ring to see what it does.

• Main magnet. Coils of special wire make the giant magnet. It is strong enough to line up the tiny magnets in every drop of water in your body. It is switched on all the time, even at night.
• Cooling jacket. The magnet's wire only works when it is incredibly cold. So it sits in a bath of liquid helium, far colder than any freezer, inside a jacket like a giant vacuum flask.
• Gradient coils. These coils make the magnet a little stronger on one side and a little weaker on the other. That gives each place its own note, so the computer can tell where each signal came from. Switching them makes the loud knocking.
• Radio coil. The radio coil is an antenna. It sends the short radio pulses that tip the tiny magnets, and then it listens for the faint signals they send back.
• Sliding table. The padded table slides you into the middle of the tunnel, where the magnet is most even. The tunnel is open at both ends, and you can talk to the scanner team the whole time.
• Computer. The computer controls the pulses, collects millions of faint signals and sorts them by their notes to draw each slice.

Send a radio pulse with the big magnet on, then with it off. Then scan slices of the fruit and watch them stack up.

Experiment: You send the radio pulse with the big magnet switched off. What comes back?

1. With the big magnet on, send a radio pulse. Watch the tiny magnets and the signal.
2. Switch the magnet off and send the pulse again.
3. Switch the magnet back on and scan some slices.

What we found: Without the big magnet the tiny magnets point every which way, so their little signals cancel out. With the magnet on they line up, wobble together and send one clear signal. In a real scanner the magnet is never switched off.

Turn on JavaScript to try it yourself.

Make Noor safe for the magnet, then run the scan by pressing the four steps in the right order.

Your body is more than half water. Every tiny bit of water contains hydrogen atoms, and the center of each hydrogen atom behaves like a very small magnet. Normally these tiny magnets point in all directions, so you do not notice them at all.

An MRI scanner is mostly one enormous magnet shaped like a ring. When you lie inside it, the tiny magnets in your body line up with it, like compass needles. Then the scanner sends a short burst of radio waves. If the radio waves have exactly the right note, they tip the tiny magnets over. It works like pushing a swing: only pushes at the right moment make it go.

When the radio burst stops, the tiny magnets swing back into line, and while they do, they send out faint radio signals of their own. Watery parts, fatty parts and firm parts all swing back at different speeds. That is how the scanner can tell them apart and show them as lighter or darker.

To make a picture, smaller magnets change the strength of the big magnet a little from place to place. Now each spot sends its signal on a slightly different note, and a computer sorts the notes to draw one thin slice. Many slices stacked together show the whole part. Our swing comparison leaves one thing out: real swings do not send signals, but the tiny magnets do. The scan is noisy, and you have to keep still, but you feel nothing.

Key words

MRI

Short for magnetic resonance imaging: making pictures with a magnet and radio waves.

Magnetic field

The space around a magnet where it pulls or turns other magnets. Its strength is measured in tesla.

Hydrogen nucleus

The tiny center of a hydrogen atom. It acts like a very small magnet.

Resonance

When a push at just the right rhythm makes something swing a lot, like pushing a swing in time.

Radio pulse

A short burst of radio waves that tips the tiny magnets over.

Slice

A picture of one thin layer. Many slices stacked together show the whole thing.

Gradient

A steady change from one side to the other, like a ramp.

Check your thinking

1. What does the big magnet do to the tiny magnets in your body?

   Show the answer

   It lines them up. Like compass needles near a magnet, the tiny magnets all turn to point the same way.

2. What is it like to have an MRI scan?

   Show the answer

   Noisy, and you must lie still, but you feel nothing. The knocking is loud, so you wear ear protection. The magnet and radio waves cannot be felt.

3. Why must keys, coins and watches stay outside the scanner room?

   Show the answer

   The very strong magnet would pull on them. The magnet is strong enough to tug metal things across the room, so they are left outside.

4. The radio pulse stops. What do the tiny magnets do next?

   Show the answer

   They swing back into line and send out a faint signal. As they swing back, they give out a faint radio signal. The scanner listens to it to make the picture.

5. How does the scanner make a picture of a whole knee?

   Show the answer

   It makes many thin slice pictures and stacks them. Each slice shows one thin layer. Stacked together, like slices of bread, they show the whole part.

6. Why do different tissues look lighter or darker in an MRI image?

   Show the answer

   Their protons realign and fall out of step at different rates, giving different signal strengths. Relaxation times depend on the surroundings of the water molecules, so the signal at a chosen moment differs between tissues.