MRI scan (magnetic resonance imaging)

What is it?

According to NIBIB: magnetic resonance imaging is a "non-invasive imaging technique that produces detailed three-dimensional anatomical images", used to detect disease, diagnose it and monitor treatment. It works by detecting the change in direction of protons in the body's tissues under the influence of a magnetic field and a radio pulse.

According to the NHS it is a test that takes detailed pictures of the inside of the body. NIBIB and MedlinePlus stress that it does not use ionising radiation.

Why is it done?

According to NIBIB: "the brain, spinal cord and nerves, as well as muscles, ligaments and tendons are seen much more clearly with MRI than with regular X-rays and CT". It is used to diagnose things such as aneurysms, tumours, and knee and shoulder injuries. As it does not use ionising radiation it is suitable for repeated imaging especially of the brain.

According to the NHS it is carried out to find the cause of certain symptoms, to examine structures of the skeleton and organs, and to see how effective a treatment is, such as measuring the size of a tumour.

Preparation

According to the NHS: do not wear anything containing metal such as jewellery, a bra, hair clips and medical hearing aids, take out coins and keys, and you may change your clothes into a gown without metal fastenings. You will fill in a form listing any metals in your body such as surgical clips, a pacemaker or dental fillings.

MedlinePlus states that you must tell the team before the scan if you have: artificial heart valves, brain aneurysm clips, a pacemaker or defibrillator, a cochlear implant, stents, or a recent artificial joint; and tell them about any previous work in a job involving metals, and about claustrophobia. You may be asked not to eat or drink for 4 to 6 hours.

According to NIBIB people with devices such as pacemakers, cochlear implants and deep brain stimulators cannot have an MRI as stated on the page, since the magnetic field is very strong. So do not decide for yourself: tell the team about every implanted device so they can decide what suits you.

Pregnancy: tell the team if you are pregnant (NHS).

How is it done?

According to the NHS, you lie on a flat bed and are given earplugs or headphones to protect your ears because the scanner makes loud knocking noises; the radiographer operates the scanner from another room while staying in contact with you. According to MedlinePlus, the table slides into the tunnel of the scanner, and small coils may be placed around the head, arm or leg to improve the image.

According to NIBIB, you must stay very still so the images are not blurred. A contrast agent containing gadolinium may be injected to make the images clearer. According to the NHS, the scan takes 15 minutes to an hour and may take longer; according to MedlinePlus, it usually takes 30 to 60 minutes.

NIBIB notes that the noise can reach 120 decibels in some scanners, which is why ear protection is used, and that tingling or twitching may occasionally occur because of the rapidly changing magnetic field.

Understanding the result

According to the NHS, a radiologist reviews the images and the result usually arrives within one to two weeks; this describes the UK hospital system, and the timing at your centre may differ.

MedlinePlus says: a normal result means the area studied looks normal; an abnormal result depends on the area examined and the condition. Your doctor will explain what the report means in your case.

Risks and side effects

According to MedlinePlus, MRI does not use ionising radiation, but strong magnetic fields can stop pacemakers and other implants from working properly. All implants and metal must therefore be disclosed before entering the scanner room.

Contrast agent: NIBIB notes that dialysis patients are at risk of nephrogenic systemic fibrosis with gadolinium, and that guidelines recommend using it only when necessary. MedlinePlus says the agent is thought to be generally safe for most people, but it is retained in the brain and other organs and may harm the organs of people with kidney failure. The NHS describes reactions as rare; nausea or a mild headache may occur.

Pregnancy: NIBIB notes that there are no documented effects on the foetus, but says that avoiding MRI as a precaution is advised, especially in the first trimester; tell the team so the doctor can decide what suits you.

Claustrophobia: NIBIB, the NHS and MedlinePlus note that it may be difficult for some people, and that options exist such as a sedative, music or open MRI scanners; discuss this with the team before the appointment.

Limits of the test

The source pages did not list detailed limits for MRI beyond the above: difficulty staying still for a long time, contraindications because of some implanted devices, and the need to use contrast cautiously in kidney disease.

Common questions