MRI scan: what to expect and what it shows

What is an MRI scan?

MRI uses a strong magnet and radio waves to build images of the inside of the body. It does not use X-rays or other ionising radiation. The person operating the scanner watches from another room.

Scan length depends on the area examined, and NHS trust leaflets quote different times, from about 10 minutes to about 90 minutes. One trust says the total time in the department is likely to be about an hour.

What happens during the scan?

Staff settle the person on a couch that slides into the scanner. An intercom lets the person talk to the radiographer during the scan. The person is asked to keep still, because movement can blur the pictures.

Some scans include an injection of contrast, a dye that helps to produce a more detailed picture and improves the accuracy of the scan. Some scans use contrast dye, which NHS trust leaflets say is injected into a vein through a small tube called a cannula. NHS Inform says complications are rare, but some people have side effects or an allergic reaction to the contrast dye.

Why is the scanner so loud, and what if it feels enclosed?

The scanner makes loud beeping or hammering noises, so earplugs or headphones are provided and some departments play music. The NHS says an MRI scan is noisy and may create a feeling of claustrophobia, which means feeling anxious in enclosed spaces. A study found that a wide-bore scanner increased the success rate of MRI in people with claustrophobia. A person who has a sedative needs someone to take them home afterwards.

Is an MRI scan safe, and why is metal a concern?

Before the scan each person completes a safety questionnaire. The NHS describes MRI as painless and safe, although it is noisy. The NHS says the MRI magnet is strong, so people with a metal implant such as a pacemaker or artificial joint may not be able to have a scan. The magnet is strong enough to pull strongly on some metals. NHS trust leaflets say patients are asked to remove magnetic objects and may change into a gown before the scan.

NHS safety policies say some implants, such as aneurysm clips and programmable shunt valves, can be scanned only if specific conditions are met. Some implants, including some pacemakers, are labelled MR conditional, which means they can be scanned only under set conditions. Trust leaflets ask people to tell the imaging staff about any implants.

Do I need to prepare?

Most MRI scans need no special preparation, according to NHS trust information. Some scans of the abdomen or pelvis may need fasting for a few hours, and bowel-relaxing medicine is sometimes given.

When do the results arrive?

Results are not usually given on the day of the scan. A radiologist, a doctor who specialises in reading scans, studies the images and writes a report for the doctor who requested the scan. That doctor then discusses the findings with the person. The NHS says complications from an MRI scan are rare, but some people get side effects or an allergic reaction to the contrast dye. Reactions to contrast dye usually happen soon after the injection, so some trusts ask people to stay for about 30 minutes afterwards.