MRI of the knee: why it is done, what to expect and how results work

An MRI scanner is a large machine with a short tunnel through it and a flat bed that moves through the scanner. For a knee scan the leg being examined is positioned in the scanner and you lie still while it takes pictures. The NHS describes MRI as taking detailed pictures of the inside of your body and says it can be used to find out what is causing symptoms, to check bones and organs, and to see how well a treatment is working.

Unlike an X-ray, which shows bone well, an MRI also shows soft tissues. In the knee that means the cartilage and menisci, the ligaments, the tendons around the joint and the lining of the joint. This is why it is the usual next step when a knee problem is thought to involve these structures rather than the bone alone.

The NHS knee pain page explains that many knee problems can be treated at home and improve in a few days, and that you should see a GP if knee pain does not improve within a few weeks. A scan is not the first step for most people. The same page says a doctor might prescribe medicine or physiotherapy, or refer you to hospital for a scan or specialist treatment, depending on what is causing the pain.

It lists the kinds of problem that tend to follow an injury, including sprains and strains, a torn ligament, tendon or meniscus, cartilage damage and a dislocated kneecap. It also lists problems with no obvious injury such as osteoarthritis and bursitis. A scan is more likely to be considered where symptoms point to a structural injury, or the knee locks, gives way or will not straighten.

The NHS says to get advice from 111 now if the knee is very painful, you cannot move it or put weight on it, it is badly swollen or has changed shape, it locks or gives way, or there is a very high temperature with redness or heat around the knee, which can be a sign of infection.

The hospital will tell you if you need to do anything beforehand. The NHS says you may be asked not to wear anything that has metal in it, such as jewellery, hair clips or hearing aids, to remove coins or keys from your pockets, and to change into a hospital gown if your clothes have metal parts such as zips.

You will be asked to fill in a form that asks whether you are pregnant or may be pregnant, are breastfeeding, have had surgery before, have kidney problems, have any metal parts in the body such as surgical clips, a pacemaker, dental fillings or bridges, or have allergies, including to the dye called contrast medium. Answer fully, because some implants and devices affect whether and how a scan can be done.

If you feel anxious or have a fear of small spaces, the NHS advises talking to a healthcare professional at the hospital before the appointment. You may be able to have a medicine to help you relax, and you can ask whether you can bring someone to support you.

A radiographer performs the scan. You will be asked to lie flat on the bed and given ear plugs or headphones because the scanner makes loud bangs and clicks. The radiographer controls the scanner from another room, can hear and speak to you, and you need to lie still so the scanner can take clear images. You may be asked to hold your breath during parts of some scans.

The NHS says that at your appointment you may be given an injection of a contrast medium (dye) to show more detail, and that if you have contrast you may be asked to wait in hospital for up to 30 minutes to check for a reaction. Whether contrast is needed depends on the question being asked, and the radiologist or referring clinician decides.

After the scan the radiographer returns to lower the bed so you can get up. You can eat and drink as normal. If you had a medicine to help you relax you will need someone to take you home and, according to the NHS, you should not drive for 24 hours.

The images are looked at by a specialist doctor called a radiologist, who writes a report for the clinician who referred you. The NHS says it usually takes between 1 and 2 weeks to get results and that you may need a follow-up appointment to discuss them. If you have not heard anything after a few weeks, contact the doctor who referred you.

A knee MRI report describes what the radiologist sees in each structure: menisci, ligaments, cartilage, tendons, bone and joint fluid. Reports commonly describe changes that are also seen in people with no knee symptoms, so the result is always read alongside your symptoms and examination. Ask the referring clinician to explain what the findings mean for you and what treatment, such as physiotherapy, injections or surgery, they suggest.

The NHS describes MRI as a common procedure in which serious complications are rare. If you have contrast dye you may get a small bruise where the injection went in, and the dye may cause mild, short-lived side effects such as feeling or being sick, a skin rash, a headache, a metallic taste or feeling dizzy. Allergic reactions are rare but can include coughing, wheezing, sneezing, fainting, difficulty breathing or swallowing, or a rash. Most reactions happen within the first hour, and you should tell the radiographer if you feel unwell during or after the scan.

On the NHS, a knee MRI is usually requested by a GP, physiotherapist or hospital specialist when the result will change management, and waiting times vary by area. Some people choose to pay privately for a scan to be done sooner, either through a referral or by self-referring to an independent imaging provider.

The scanner technology and the safety checks are the same. The important practical points are to make sure the radiologist's report reaches a clinician who can act on it, to bring any previous knee imaging or reports for comparison, and to give the full safety information about implants and previous surgery.