How does an MRI scanner work?
An MRI scanner uses strong magnets and radio waves. The waves pass through your body and cause tiny amounts of energy to be released. The scanner picks up the signals from this energy and sends them to its computer. This can turn the signals into detailed pictures.
MRI scanners do not use X-rays or radiation.
Dye (contrast medium)
You might also have an injection containing a dye before or during your MRI scan. This helps the scan to get clearer pictures of a particular part of your body. You might hear this dye called a “contrast medium”. The dye can sometimes have side effects. These are usually mild and do not last very long. They include:
- a rash
- a headache
- feeling dizzy
- feeling or being sick
The dye can sometimes cause an allergic reaction. This is very rare. But let your medical team know before your scan if you have a history of allergic reactions.
Before the scan
Having an MRI scan is usually very safe and does not hurt. But in some cases extra checks might be needed before you have your scan.
If your doctor says you might need an MRI you must always tell them if:
You are pregnant or breastfeeding
There is no evidence that MRI scans are not safe if you are pregnant or breastfeeding. But tell your doctor so they can make sure it is the best option for you.
You have any medical devices or metal in your body
MRI machines use very strong magnets. Most types of medical and dental implants are safe in an MRI scanner but it is very important to check your device or implant before your scan.
This includes:
- metal plates, rods and screws used to mend broken bones
- artificial joints used in eg hip replacements
- cochlear implants
- pacemakers
- glucose monitors
- drug pumps including insulin pumps
- implanted defibrillators
- clips, shunts and stents implanted during surgical procedures for example TIPS
- “coil” contraceptives (IUD)
- dental fillings, implants and bridges
- bullets and shrapnel
Tell your medical team if there is any chance that you may have some metal in your body. If you are not sure they might be able to find out from your medical records. Or you might have an X-ray to check.
If you cannot have an MRI because of an implant you might be offered a CT scan instead.
You have tattoos or permanent makeup
Most tattoos and permanent makeup are safe in an MRI scanner. But the ink can sometimes contain very small amounts of metal.
Let your medical team know if you have tattoos or permanent makeup. If they start to feel warm or uncomfortable during a scan tell the radiographer straight away.
If you have ever had kidney problems
If you have an MRI for liver or gallbladder problems you often have a dye injected during the scan. In rare cases the dye can be a problem if you have a kidney condition. This does not always mean you cannot have the dye. But you might need a blood test first to check on your kidneys.
You have a history of severe allergic reactions
In very rare cases the dye used in some MRI scans can cause an allergic reaction. Tell your medical team if you have a history of severe allergic reactions so they can work out if an MRI with dye is a safe option for you.
You suffer from claustrophobia or are worried about being in a small space
The space inside the MRI scanner is quite small. If this is something you might find difficult talk to your medical team about it before you have the scan. There are a number of things that could help.
You can find out more about coping with claustrophobia in an MRI scanner here.
Questions to ask your medical team:
- Why do I need an MRI scan?
- How will having the scan change my treatment?
- Will I have dye injected as part of my scan?
On the day of the scan
What to wear
As well as making sure you have no metal in your body, you must remove any metal you are wearing. You will not be able to have any metal items in the room when you have your scan. So if possible, leave them at home.
This includes:
- All watches, jewellery and piercings
- Any clothes that could contain metal, for example belts, bras and shoes
- Wigs
- Glasses
- False teeth (dentures)
- Hearing aids
- Most types of glucose monitors and insulin pumps
Wear comfortable clothes that are easy to change in and out of. You may need to change into a hospital gown for the scan.
You might have other tests on the same day. It is easiest to wear a separate top and bottom. This means doctors can examine your belly just by lifting up your top.
Wear short or loose sleeves in case you need a blood test.
Having an MRI scan
An MRI scanner is a large machine. It looks like a short tunnel with a hole at both ends. A bed slides into it. It is operated using a computer. The computer will be in a separate room. This is to stop it being broken by the strong magnets in the scanner.
Preparation
You might have a canula put in just before your scan. This allows dye to be easily injected to help make the scan images clearer. A canula is a small tube the goes into a vein, usually in the back of your hand. The canula will be held in place with tape.
MRIs are usually done by scanning specialists called radiographers. They will help you to get ready for the scan and make sure you are comfortable on the bed.
For a liver or gall bladder scan you can go into the scanner head-first or feet-first. Some people find going in feet-first is less worrying. If you would prefer this, talk to the person doing your scan. Some hospitals always do liver scans feet-first.
It is very important that you can keep still during the MRI. So you might be given cushions to help you stay comfortable. You might have a heavy covering put over your tummy area.
MRI scanners are very noisy so you will be given headphones. As well as protecting you from the noise, the headphones will let the radiographer talk to you during the scan.
You will be given a button to press in case you need help during the scan.
During the scan
Once you are ready for the scan, the radiographer will go into the computer room. They will be able to see you through a window and on a screen using a camera inside the scanner.
The scanner will take lots of different pictures. You will need to keep still while each picture is taken. This can take from a few seconds up to about 4 minutes. You might be asked to hold your breath for a few seconds for some pictures.
When the scanner is taking pictures it will make loud banging, clicking or buzzing noises. This is totally normal. You will also hear instructions from the person doing the scan.
Taking all of the pictures usually takes between 30 minutes and 1 hour. But it can sometimes be quicker than this.
Having the dye injected
If you are also having dye injected then this might happen part way through the scan. Usually the scanner will take pictures for about 15-20 minutes. Then the dye will be injected using your canula. Then more pictures will be taken.
You might feel the dye going in. It can feel cold or warm. Some people get a warm feeling in their bladder area. This can feel a bit like you have wet yourself, but it is just the dye moving through your body.
Coping with claustrophobia in an MRI scanner
The space inside the scanner is quite small. Some people find this stressful or upsetting. If you think it could be difficult for you, talk to your medical team about what can be done to make it easier.
- You can usually go in feet-first for a liver scan, your head may not need to be in the MRI.
- Some MRI scanners have a mirror in them so that you can see out.
- The headphones can be used to play relaxing music.
- You can get help at any time by pressing the buzzer.
- You can talk to the person doing the scan usually using an intercom in the scanner.
- The person doing the scan can see you and talk to you during the whole scan.
- A friend or family member can stay in the room when you have your scan. They will need to follow all the rules about metal.
You can also ask to have a sedative. This is a medicine that will make you feel calm and sleepy.
If you have a sedative you will need to ask a friend or family member to come to the hospital with you. They will need to take you home and stay with you for 24 hours. You should not drive, drink alcohol or operate dangerous machinery during this time.
Children and MRI scans
Having an MRI scan does not hurt. But it can be a bit scary. And it can be difficult for babies and children to keep still when the pictures are being taken.
To help with this, babies and children are sometimes given a general anaesthetic. This means they will be asleep during the scan.
A parent or carer can usually stay in the room with the child while they have their scan. The person in the room will need to follow all the rules about having no metal on or in their body.
After the scan
When the scan has finished, the bed will come back out of the scanner.
If you had dye injected you will be asked to wait in the scanning department for 15 to 20 minutes so that the medical team can be sure you will not have a reaction. Allergic reactions to the dye are very rare so this is just to be on the safe side.
After your scan you can go back to all your normal activities and eat and drink as normal.
If you had a sedative, you will need a friend or family member to take you home. You must not drive yourself.
Questions to ask your medical team:
- How long is my scan likely to take?
- What should I do if I am finding it hard to cope in the scanner?
- How and when will I get my results?
Getting your results
You will not get your results straight away. After your scan a specialist doctor called a radiologist will look at the results in detail. They will then send a report to your liver medical team. This usually takes 1 to 2 weeks.
If you are having other tests as well, your medical team will usually wait until they have all the results so that they can look at them all together.
If you do not have a follow up appointment booked, contact your liver medical team to find out how and when you will get your results.
Your results might give your medical team enough information to make a diagnosis. If you have been diagnosed with a liver condition you can find out more information about it here.
Or you might need to have more tests such as a CT scan or biopsy.
Find out more about other tests for liver disease.
Questions to ask your medical team:
- What did my test results show?
- What does this mean for me and how certain are you about this?
- Do I need to have any other tests?
- What are the possible next steps?
- When will I have another appointment and who will it be with?
- Who can I contact if I have more questions or need help before my next appointment?
- What is the best way to contact them?
Liver conditions found by accident during an MRI scan
There are some liver conditions which are very common and harmless. They are often found by accident if you have a scan for something else. They do not cause any problems, and you would probably never have known you had them if you had not had the scan.
Simple liver cysts are small sacs of liquid in your liver. These are usually harmless and cause no symptoms. The safest option is to leave them alone. You can find out more about simple liver cysts here.
Liver haemangiomas are a very common type of benign tumour. It is thought that about 1 in 20 people have them and it is very rare for them to cause any symptoms or problems. They are similar to strawberry birth marks that many people have on their skin. You can find out more about liver haemangiomas here.
If your MRI scan shows you have one of these conditions, you might be asked to have other tests. This is nothing to worry about, it is just to rule out any other rarer conditions.
Special thanks
We would like to thank the patients and doctors who helped up to create this information. Including Dr Christopher Clarke, Consultant Abdominal Radiologist, Nottingham University Hospitals NHS Trust.
Published: May 2025
Support for you
Find out more
Living with a liver condition
Find out more
Liver disease tests
Find out more