An MRI scan (magnetic resonance imaging) is a medical imaging test that uses a strong magnetic field, radio waves and a computer to build detailed cross-section pictures of the inside of the body. It uses no ionising radiation, so it is different from X-rays and CT scans.
What is an MRI scan?
Magnetic resonance imaging is one of the most useful tools in modern medicine. Doctors use it to look at organs, muscles, joints and the brain without opening the body and without using X-rays. The pictures are made from signals given off by the body itself after it has been placed in a strong magnetic field.
The big idea is simple enough for GCSE physics. Your body is about 60% water, and every water molecule contains hydrogen. The nucleus of a hydrogen atom is a single proton, and a proton behaves like a tiny magnet. In a strong magnetic field, those tiny magnets line up. If you want a refresher on protons, neutrons and electrons, our guide to atomic structure and sub-atomic particles covers the basics.
A short burst of radio waves then knocks these protons out of alignment. When the burst stops, the protons relax back and give off weak radio signals. Detectors pick up those signals, and a computer turns them into an image. Different tissues relax at different rates, and that difference is what gives the picture its contrast. Part B of this article explains each step in detail.
The result is excellent soft-tissue detail. Brain, spinal cord, muscle, ligaments, cartilage and many organs show up clearly, which is why an MRI scan is so often used when a doctor needs a close look at something that an X-ray cannot show well.
What does an MRI scanner look like?
If you have never seen one, an MRI scanner looks like a large tunnel set into a big machine. The tunnel is called the bore. During a scan you lie on a table that slides into the scanner. Staff operate the machine from a separate control room and can talk to you while you are inside.
Hidden inside the casing is the main magnet. In most hospital scanners it is a superconducting electromagnet. To make it superconduct, it is cooled by liquid helium to about −269 °C. Because the magnet is always on, even when nobody is being scanned, hospitals have strict rules about what can enter the scanner room. The magnet can pull ferromagnetic objects such as keys and oxygen cylinders towards the scanner, which is why metal checks matter so much. Our article on the strongest magnets puts this kind of field in context.
Around the part of the body being scanned, the radiographer places coils. These detect the radio signals that the body gives off. Other coils, called gradient coils, switch on and off during the scan. They help work out where each signal comes from, and they are also the reason the scanner makes loud knocking and banging sounds. That is why you are given ear plugs or headphones.
Magnetic field strength is measured in tesla, and the numbers involved are easy to underestimate. Use the calculator below to compare a scanner’s field with the magnetic field of the Earth.
How does an MRI work?
The question "how does an MRI work?" has a neat answer at GCSE level. An MRI scanner uses a very strong magnetic field, radio waves and a computer. Together they turn the tiny signals given off by atoms in your body into detailed cross-section pictures. No ionising radiation is involved at any point.
The key idea is that about 60% of the body is water, and water contains hydrogen. Each hydrogen nucleus is a single proton, and a proton behaves like a tiny magnet. The scanner is built to talk to those tiny magnets.
The five steps of magnetic resonance imaging
- Protons start out random. Before the scan, the hydrogen nuclei in your body point in all directions, so their magnetic effects cancel out.
- The big magnet lines them up. When you slide into the scanner, the strong field makes the protons line up with the field, like compass needles all swinging to point the same way.
- A radio-frequency pulse knocks them out of alignment. A short burst of radio waves, tuned to the protons, pushes them away from the field direction.
- The protons relax and give off signals. When the pulse stops, the protons relax back into line. As they do, they give off radio signals of their own.
- Coils and a computer build the picture. Receiver coils detect the signals and a computer turns them into an image of the slice being scanned.
The idea of a changing magnetic field producing a signal in a coil of wire should sound familiar. If you want to see the same principle on a bench, try our electromagnetic induction practical.
Relaxation and contrast
Here is where the clever part comes in. Different tissues let their protons relax at different rates. Some signals fade quickly, others linger. The computer measures these differences and shows them as lighter and darker areas, which is what gives an MRI image its contrast.
This is why MRI is so good at showing soft tissue. Brain, muscle, ligaments and organs all contain protons, but they relax in measurably different ways, so they look different on the scan.
Some scans also use a contrast medium, usually a gadolinium-based dye injected before the scan, to show even more detail. Gadolinium contrast is usually well tolerated, but it can cause side effects such as feeling sick, a rash or a headache, and it is used with care in severe kidney disease. Staff will go through this with you beforehand.
Why is an MRI so loud?
The loud knocking, clicking and banging comes from the gradient coils. These coils switch on and off rapidly to help the computer work out where each signal is coming from. Switching them makes parts of the scanner vibrate, and that vibration is the noise you hear.
The NHS gives ear plugs or headphones for exactly this reason. You also need to keep still while the scan is running.
Explain why MRI is good at imaging soft tissue. (3 marks)
MRI detects signals from hydrogen nuclei, which are found in the water in soft tissues (1). Different tissues let their protons relax at different rates, so they give off different signals (1). The computer shows these differences as contrast, giving detailed images of soft tissue such as the brain, muscle and organs (1).
Why does an MRI scan use hydrogen?
Hydrogen is the best choice because there is so much of it. Your body is about 60% water, and every water molecule contains two hydrogen atoms. That gives the scanner an enormous number of protons to detect.
A hydrogen nucleus is a single proton, and a proton behaves like a tiny magnet. That is what makes it respond to the scanner's magnetic field and radio pulses. If you would like to know more about the element itself, see our guide to the properties of hydrogen.
The main magnet is usually a superconducting electromagnet. To keep it superconducting it is cooled by liquid helium to about −269 °C, and it stays switched on all the time. You can read more about the coolant in our article on the chemical properties of helium.
Let's check the key terms. Tap each card to flip it and test yourself.
This article is for education only and is not medical advice. If you are due an MRI scan, follow your own hospital's instructions, and tell staff about any metal in your body or any chance you might be pregnant. If you have symptoms that worry you, speak to your GP or call NHS 111.
What can an MRI scan show?
An MRI scan is best known for its detailed pictures of soft tissue, the parts of the body that X-rays struggle to separate. Because different tissues relax at different rates after a radio pulse, an MRI scan can show the difference between healthy tissue and tissue that looks unusual. A doctor (a radiologist) then interprets those images alongside your symptoms and other tests. This article is educational and cannot tell you what any particular scan means.
The NHS lists a wide range of uses for magnetic resonance imaging. Here they are grouped by body area.
- Brain and spinal cord: looking for tumours, signs of a stroke, or multiple sclerosis (MS).
- Joints, ligaments and cartilage: a common choice for sports injuries, because soft tissue shows up clearly.
- Heart and blood vessels: studying the structure of the heart and the vessels around it.
- Liver, kidneys and other abdominal organs: detailed views of organs deep inside the body.
- Prostate and breast: extra detail when other tests need following up.
- Cancer staging: helping doctors see how far a cancer has spread.
Some scans use a contrast medium, usually a gadolinium-based dye injected before or during the scan, to make certain areas stand out more.
The brain is a good example of why soft-tissue detail matters. A brain MRI separates different tissues clearly, so the folded outer layer and the fluid-filled spaces inside the head can be told apart. If you enjoy the biology behind those images, our articles on amazing facts about the brain and neuroscience and the brain explain what all that tissue actually does.
Functional MRI (fMRI)
Standard MRI shows structure. Functional MRI (fMRI) goes a step further and shows brain activity. It does this by detecting changes in blood oxygen, because working areas of the brain use oxygen differently from resting ones. Researchers use fMRI to see which regions are involved when a person looks, listens, speaks or thinks.
MRI vs CT vs X-ray
Patients are often unsure why a doctor chooses one scan over another. The short answer is that each type of imaging has strengths. X-rays are quick, CT gives more detail, and MRI gives the best soft-tissue pictures. The choice depends on what the doctor needs to see and on the patient. The table compares the three in a GCSE-friendly way.
| Feature | X-ray | CT scan | MRI scan |
|---|---|---|---|
| How it works | A beam of X-rays passes through the body onto a detector | Many X-ray slices combined by a computer | Strong magnetic field, radio waves and a computer |
| Ionising radiation | Yes, a small dose | Yes, a higher dose | None |
| Best for | Bones and the chest | Bleeding, trauma, bone and organs | Soft-tissue detail |
| Speed | Seconds | Minutes | Slower, usually 15 minutes to 1 hour |
| Noise | Much quieter | Quieter, with some whirring | Loud knocking and banging |
| Limits | Less detail of soft tissue | Higher dose than an X-ray | Not suitable for some implants |
In short, if the question is "is this bone broken?", an X-ray is often enough. If it is "is there bleeding after an injury?", the speed of CT can matter most. If it is "what is happening in this ligament, organ or part of the brain?", MRI often gives the clearest picture. This is why the same hospital may use all three, and why the right test is always a clinical decision.
How big are the radiation doses?
Doses of ionising radiation are measured in millisieverts (mSv). To put them in context, the average person in the UK receives about 2.6 mSv a year from all sources of ionising radiation, most of it natural background (UKHSA figure). Approximate adult doses are:
- Chest X-ray: about 0.014 mSv, equal to roughly 2 days of the average UK annual dose.
- CT scan of the chest: about 6.6 mSv, equal to roughly 2.5 years of the average UK dose.
- MRI: none, because it does not use ionising radiation.
Try the calculator below to compare scans with background radiation for yourself.
Who invented MRI?
No single person invented MRI. It grew from decades of physics and medical research, and several scientists share the credit. The key steps look like this.
- 1940s: Felix Bloch and Edward Purcell describe nuclear magnetic resonance (NMR), the physics behind MRI.
- 1952: Bloch and Purcell win the Nobel Prize in Physics for this work.
- 1971: Raymond Damadian shows that tumours give different NMR signals from healthy tissue.
- 1977: Damadian and colleagues make the first full-body scan of a human.
- 2003: Paul Lauterbur (USA) and Sir Peter Mansfield of the University of Nottingham, UK, win the Nobel Prize in Physiology or Medicine for their methods of making images with magnetic resonance.
You may notice that the word "nuclear" is missing from the name MRI. It was dropped partly because patients associated it with radiation, even though the scan uses none. The "nuclear" in NMR simply refers to the nuclei of atoms.
Radiation itself has a famous history in science. Our article on Marie Curie, the physicist and Nobel Prize winner covers another pioneer of that field. MRI took a different route: it uses magnets and radio waves instead of ionising radiation.
What happens during an MRI scan?
Having an MRI scan is painless, but it helps to know the routine, because the strong magnet means the preparation is stricter than for most tests. This section follows the NHS description. Always follow the instructions in your own appointment letter, because every hospital has its own procedures.
Before the scan
You will usually be asked to remove anything metal and to tell staff about anything that might be affected by the magnetic field. Use this checklist as a guide to the questions you may be asked, and tick it off as you read.
- Remove jewellery, watches, hair clips and bank cards, or follow the staff instructions on what to leave behind
- Wear metal-free clothes, or change into a gown if you are asked to
- Tell staff about any metal in your body: surgical clips, a pacemaker, other implants, or fillings and bridges
- Tell staff if you are, or might be, pregnant
- Ask whether your scan will use a contrast medium
- Collect your ear plugs or headphones, because the scanner makes loud bangs and clicks
- Mention any worries about enclosed spaces before the day, or on arrival
Some scans use a contrast medium, a dye that is usually gadolinium-based. It is injected to show more detail in the images. Staff will explain whether you need one.
During the scan
You lie on a table that slides into the scanner. A scan usually takes 15 minutes to 1 hour, and some take longer. The most important job for you is to keep still, because movement blurs the images, rather like a photograph taken of a moving subject.
The scanner makes loud knocking and banging sounds. As you saw earlier, these come from the gradient coils switching on and off. You will be given ear plugs or headphones, and you can talk to the staff through an intercom if you feel uncomfortable.
After the scan
Once the scan is finished you can usually go home, and results normally take 1 to 2 weeks. If you were given a contrast medium, staff will tell you if there is anything you need to do afterwards. If you have symptoms that worry you while you wait, contact your GP or call NHS 111.
Claustrophobia and open MRI
A conventional scanner is a tube, and lying inside it can be difficult for people who dislike enclosed spaces. This is common, and the NHS advises you to talk to the staff beforehand rather than coping alone.
There are several things that can help:
- Tell the radiography team about your anxiety when you book or on arrival, so they can explain each step.
- Ask about relaxation medication, which may be offered.
- Ask whether you can bring someone for support.
- Ask about an open MRI scanner, if one is available.
An open MRI scanner has a more open design, which can help people with claustrophobia. The trade-off is that image quality can be lower for some scans, because many open scanners use a lower field strength. This means they are not suitable for every scan, so your doctor and the radiography team decide which scanner fits your situation.
Is MRI safe?
For most people an MRI scan is safe. The biggest advantage is that it uses no ionising radiation, unlike X-ray and CT. The safety questions are different: they concern magnets, implants, contrast dye and pregnancy.
The magnet
The magnetic field is strong, typically 1.5 or 3 tesla, about 30,000 times stronger than Earth’s field at 1.5 T. That is why ferromagnetic objects can be pulled towards the scanner, and why metal checks are strict. Some make-up and tattoos can contain metal particles that may warm slightly during a scan, so tell staff if you have them or are unsure.
Implants and metal
Metal in the body does not automatically rule out a scan. Many modern implants and pacemakers are MRI-conditional, meaning they can be scanned under set conditions, and staff check each one. This is why you must tell the team about surgical clips, pacemakers, other implants, fillings and bridges before you are scanned.
Contrast and pregnancy
Gadolinium contrast is usually well tolerated, but it can cause side effects such as feeling sick, a rash or a headache, and it is used with care in severe kidney disease. If you are, or might be, pregnant, tell staff. MRI without contrast is not known to harm a developing baby, but staff are cautious, and contrast dye is usually avoided in pregnancy unless it is essential. Decisions are made case by case.
This article is for education only and is not medical advice. If you are unsure about a scan, ask your GP or the hospital team.
Frequently asked questions about MRI scans
What is an MRI scan used for?
An MRI scan uses a strong magnetic field and radio waves to make detailed images of the inside of the body. It is used to look at the brain and spinal cord, joints and ligaments, the heart and blood vessels, abdominal organs, the prostate and breast, and to help stage cancer.
How does an MRI scanner work?
The body is about 60% water, so it is full of hydrogen nuclei that act like tiny magnets. The scanner’s strong field lines them up, a radio pulse knocks them out of alignment, and as they relax they give off radio signals. Coils detect these signals and a computer builds the image.
How long does an MRI take?
The NHS says an MRI scan usually takes 15 minutes to 1 hour, though some scans take longer. You lie on a table that slides into the scanner and need to keep still. Allow extra time for checks beforehand, and follow the instructions in your appointment letter from your hospital.
Is an MRI scan safe?
MRI uses no ionising radiation, unlike X-ray and CT. The main hazard is the strong magnet, which can pull ferromagnetic objects such as keys or oxygen cylinders towards the scanner, so staff run strict metal checks. Tell staff about implants and pregnancy beforehand. Gadolinium contrast is usually well tolerated but can cause mild side effects and is used with care in severe kidney disease.
Why is an MRI so loud?
The loud knocking and banging comes from the gradient coils, which switch on and off during the scan. The NHS says scanners make loud bangs and clicks, so you are given ear plugs or headphones. You can also talk to the staff if the noise worries you.
What is the difference between MRI and CT?
MRI gives the best soft-tissue detail and uses no radiation, but is slower, noisy and unsuitable for some implants. CT uses many X-ray slices, takes minutes and is good for bleeding, trauma and bone, but gives a higher dose: about 6.6 mSv for a chest scan, against an average UK dose of roughly 2.6 mSv a year from all sources.
Can you have an MRI with metal in your body?
Often yes, but you must tell staff first. Surgical clips, pacemakers, some implants, and fillings or bridges all need checking. Many modern implants and pacemakers are MRI-conditional, and staff check each one. Because the magnet can pull ferromagnetic objects, remove jewellery, watches and cards, and follow your hospital’s instructions.
Can I have an MRI if I’m claustrophobic?
Talk to the staff before your appointment. The NHS says relaxation medication may be offered, and you can ask to bring someone for support. An open MRI scanner has a more open design and may help, though image quality can be lower for some scans, so it is not suitable for every scan.
How long do MRI results take?
Results usually take 1 to 2 weeks, according to the NHS. The time can vary, so ask staff at your appointment how and when you will be told. If you have new or worsening symptoms while waiting, contact your GP or call NHS 111 rather than waiting for the scan result.
