Your turn
Each lab is one step from the video. Change things and see what happens.
Each lab isolates one step of an MRI scan. The models are simplified to show the idea.
Tap the buttons and see what happens.
Tap the head, the back or the knee. What kind of picture does an MRI make?
Tap a body part: MRI excels at soft tissue such as brain, spinal cord, ligaments and tendons.
Tap the head, the back or the knee.
Zoom from a glass of water down to one spinning proton.
Step from a glass of water to the hydrogen nucleus, whose spin makes it a tiny magnet.
Tap to zoom in, smaller and smaller.
Switch the magnet on, then turn up its strength. What do the tiny magnets do?
Toggle the field and change its strength: the proton axes align along it.
Switch it on. Watch the tiny magnets line up!
The toy shows far more lining up than really do (about 9 in 2 million extra at 1.5 T).
Send a radio pulse, then choose when to listen. Which looks bright, fat or water?
Tip the protons with an RF pulse and watch fat recover faster than water.
Send a radio pulse. Watch the tiny magnets wobble.
Simplified: the recovery speeds are made up to show the idea.
Drag the slice along the body. Each slice answers at its own note.
Move the selected slice: with a gradient on, each position resonates at its own frequency.
Slide the slice from feet to head.
Run a scan line by line, then flip between fat bright and water bright.
Acquire the slice line by line, then switch between T1-like (fat bright) and T2-like (water bright) contrast.
Press run. Watch the picture appear!
A simple grey sketch of a head slice, not a real scan.
An MRI machine is a big ring with a soft bed. You lie down and the bed slides in. Then doctors can see a picture of the inside of your body. There is no camera inside, and it doesn't hurt at all.
Your body has lots of water in it. Water is made of teeny tiny bits, and some of them act like tiny magnets. Most of the time they point every which way.
Inside the ring is a giant magnet. It makes a few more of the tiny magnets point along it. You can't feel it at all.
The machine sends radio waves. The tiny magnets wobble, then settle back and send a quiet radio "hello". The machine listens, and a computer turns all the hellos into a picture.
This page explains how the machine works; it isn't medical advice. If you're having a scan, the hospital team will explain what to expect.
An MRI scanner makes pictures of the inside of your body using a very strong magnet, radio waves and a computer. It uses no X-rays.12 Here's how it works, in four steps:
MRI stands for magnetic resonance imaging. "Magnetic" is the big magnet, "resonance" is the radio pulse matching the protons' natural frequency, and "imaging" is the picture the computer makes.2
Up to 60% of an adult's body is water.9 Each water molecule is made of one oxygen atom and two hydrogen atoms, and about two thirds of all the atoms in your body are hydrogen.8 The centre (nucleus) of a hydrogen atom is just one proton. Protons spin and have a tiny magnet built in, with a north and a south pole, so you can picture each one as a microscopic compass needle.26
Normally those tiny magnets point every which way, so they cancel each other out and your body isn't magnetic.3 The MRI's signal comes mainly from the protons in the water and fat in your body.12
Magnet strength is measured in tesla (T). Hospital MRI scanners are usually 1.5 T or 3 T.314 Earth's magnetic field is about 0.00005 T (0.5 gauss), so a 3-tesla scanner is about 60,000 times stronger than the field that turns a compass needle.3
| Magnet | Strength |
|---|---|
| Earth's magnetic field | about 0.5 gauss (0.00005 T) |
| A fridge magnet | about 10 gauss (0.001 T) |
| A typical hospital MRI | 1.5 T or 3 T (15,000–30,000 gauss) |
In that field, more of the protons line up with the magnet, like compass needles swinging to point north.6 In fact only a very small extra share line up: about 9 out of every 2 million protons at 1.5 T.3 But there are so many protons in your body that this tiny surplus is enough to measure. (The toy on this page exaggerates the share so you can see it.)
The main magnet is an electromagnet: electricity flowing through miles of coiled wire. Ordinary wire resists the current and heats up. So the coils are made of special wire and chilled with liquid helium to about −269 °C (−452 °F). At that temperature the wire becomes a superconductor and the current flows with no resistance, making a powerful, steady field.313 The helium is sealed inside an insulated tank, so you don't feel cold.3 Curious where that helium comes from? See where helium comes from, and how magnets work in general.
For most hospital scanners the main magnetic field is always on, even between patients.18 It reaches beyond the machine and pulls hard on iron and some steels. NIBIB says it's strong enough to fling a wheelchair across the room.1 That's why staff keep objects like screwdrivers and oxygen tanks out of the room, and why everyone fills in a safety screening form and takes off jewellery, watches, phones and clothes with metal parts first.4 Some implants, such as some pacemakers and aneurysm clips, can mean a person can't have an MRI, while dental fillings usually aren't a problem.16
Once the protons are lined up, a coil near your body sends a short pulse of radio waves. Radio waves are a kind of light our eyes can't see.2 (Read more about how light travels.) The pulse only works if it's tuned to exactly the right frequency, which depends on the strength of the magnet. That frequency match is the resonance.26 The frequencies used are close to those of FM radio stations, which is why the scanner room is shielded from outside radio signals.3
The pulse tips the protons out of line. When it stops, they swing back into line, a bit like spinning tops settling down. Scientists call this relaxation. As the protons relax, they give off faint radio signals of their own, and the scanner's coils pick them up.21
The time protons take to relax, and how much signal they give off, depends on their surroundings.1 Protons in fat relax faster than protons in water. The operator picks the timing of the pulses and when to listen, which changes which tissues look bright:3
| Scan type (simple version) | Fat | Water / fluid |
|---|---|---|
| T1-weighted (listen "early": short timings) | bright | dark |
| T2-weighted (listen "late": longer timings) | (varies) | bright |
On T1-weighted images, fat, with its short relaxation time, is bright and water is dark.15 T2-weighted images show fluid as bright.19 Bone has little hydrogen, so it looks dark.17 Sometimes a contrast agent (often containing the element gadolinium) is used to help protons realign faster, which makes some areas brighter.1 The speeds in this page's toy are made up to show the idea, not real measurements.
All the protons in the scanner would answer at once, so how does the machine know where each signal came from? The answer is three extra, weaker magnets called gradient coils, one for each direction (x, y and z).3 They make the field slightly stronger at one end of your body than the other. Because the resonance frequency depends on the field strength, protons in each spot now answer at a slightly different "note", like the keys along a piano.6
By choosing the note, the scanner can pick out one thin slice of the body, like a slice from a loaf of bread, and the slices can point in any direction.3 Paul Lauterbur first showed that gradients could turn the signals into a two-dimensional picture.6 The machine repeats the pulse-and-listen cycle many times, and a computer combines all the signals into images, even 3D ones.2
The knocking, tapping and buzzing you hear comes from the gradient coils being switched on and off very quickly.3 In some scanners the sound can reach up to 120 decibels, so you'll be given earplugs or headphones, and some headphones play music.1416 The noises are normal and don't mean anything is wrong.4
No ionizing radiation. X-rays and CT scans use ionizing radiation. MRI uses a magnetic field and radio waves, so there's no exposure to ionizing radiation.15 The older name for the physics, nuclear magnetic resonance, refers to the nucleus of the atom (the proton), not to anything radioactive. The medical name dropped "nuclear" partly to avoid that confusion.8
| MRI | CT scan | |
|---|---|---|
| Uses | A magnet, radio waves and a computer | X-rays and a computer |
| Ionizing radiation | No | Yes |
| Best at | Soft tissues: brain, spinal cord, nerves, muscles, ligaments, tendons | Faster scans |
| Scan time | Longer | Shorter |
NIBIB notes that the brain, spinal cord, nerves, muscles, ligaments and tendons are seen much more clearly with MRI than with X-rays and CT, which is why MRI is often used for knee and shoulder injuries.1 MRI scans take longer than CT scans.17 MRI is less good than X-rays at spotting broken bones, so the tools complement each other.8
An MRI exam can take anywhere from about 15 minutes to more than an hour, depending on what's being scanned.10 You need to lie still, because moving blurs the pictures.1 For people who find the tunnel uncomfortable, some hospitals have "open" MRI machines that are open at the sides.1
The physics behind MRI, nuclear magnetic resonance, was shown for protons in 1946 by Felix Bloch and Edward Mills Purcell, who shared the 1952 Nobel Prize in Physics.76 For decades it was used mainly to study chemicals. In the early 1970s, Paul Lauterbur discovered that gradients in the magnetic field could be used to make two-dimensional pictures, and Peter Mansfield showed how to analyse the signals mathematically and how imaging could be made very fast. They shared the 2003 Nobel Prize in Physiology or Medicine.6 Today an estimated 40 million MRI scans happen each year in the U.S. alone.2
Your body is mostly water, and the hydrogen in water has tiny magnetic protons. The MRI's huge magnet lines them up. A radio pulse tips them over, and when it stops they settle back and give off faint radio signals. Signals from different tissues fade and recover at different rates, and a computer turns all those signals into a detailed picture.
Not the kind X-rays and CT scans use. An MRI uses a strong magnetic field and radio waves, which are non-ionizing. There is no exposure to ionizing radiation during an MRI.
The knocking, tapping and buzzing comes from the gradient coils. These coils switch on and off very quickly to tell the machine where each signal comes from, and the switching makes the coils vibrate. Some scanners reach up to 120 decibels, so you get earplugs or headphones. The noise is normal.
The main magnet stays on all the time and pulls very hard on iron and some steels. Loose metal objects can fly toward the magnet, and some metal on or in the body can heat up, move or blur the pictures. That's why everyone fills in a screening form and removes jewellery, phones and other metal items first.
An MRI exam can take from about 15 minutes to more than an hour, depending on what is being scanned. You need to lie still, because moving blurs the pictures.
A CT scan uses X-rays and is quick and good for bones. An MRI uses a magnet and radio waves instead of X-rays, takes longer, and is especially good at showing soft tissues like the brain, spinal cord, muscles, ligaments and tendons.
MRI is generally safe and painless when the hospital's screening and safety steps are followed: staff check for metal and implants, give ear protection against the loud noise, and watch for warming. The main risks come from metal objects and some implanted devices, which is why the team asks detailed questions before a scan. If you're having one, the hospital team will explain what to expect.