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What is an MRI scan?

Magnetic resonance imaging (MRI) is one of the tools doctors have to help diagnose and monitor conditions affecting internal organs, tissue, and bone.

How MRI works

Strong magnetic fields and radio waves are used to form detailed images of the body a tiny 'slice' at a time.

Magnetic waves bounce off tiny protons in the water molecules that make up the body. The signals received from millions of protons build-up to form the scanning image.

The images can then be viewed as a 3-D view of a person's insides.


What to expect during an MRI

An MRI scan is usually carried out as a day patient procedure. After the scan, you can go back to your usual daily activities.

Before the magnetic resonance scan, you will be asked to remove anything containing metal. This includes watches, jewellery, piercings, dentures, hearing aids and some wigs.

You will probably have to wear a hospital gown.

For some MRI scans, an injection of contrast dye is needed first to help show details more clearly.

A radiographer will ask a person to lie flat and still on a bed that then goes inside a tube of magnets. You may go in head or feet first.

MRI scanner

A special frame may be placed over the head for imaging of the brain and skull.

The radiographer will operate the scanner from a control room and you can talk to them over an intercom.

Scans may take as long as 90 minutes but some may be as quick as 15 minutes depending on what part of you is being scanned and how many images are required.

The machine can be noisy, sounding like loud hammering going on inside the scanner as it does its work. Headphones or earplugs will usually be provided.

You are unlikely to get the results of the scan straight away, as the results will need to be assessed by a consultant.

Uses of MRI tests

Almost any part of the body can be scanned with an MRI. MRI scans are often requested by doctors to look at:

People with conditions including the following may have an MRI scan:

MRI scans may be used to help diagnose breast cancer but are not needed in all cases following a mammogram. An MRI scan may be used to check breast implants, such as the faulty PIP implants.

Who shouldn't have an MRI

People who are claustrophobic - anxiety over being in enclosed spaces - may have concerns over going into an MRI tube. Open MRI machines where you are not closed-in on all sides may be available as an alternative for people with these concerns. 

A sedative may be offered to help calm someone's fears. If this is taken, you will not be able to drive yourself home afterwards.

Children and babies may be given a general anaesthetic to make sure they are completely still for the scan.

The use of magnets means MRI scans are not suitable for people with implants such as pacemakers for heart conditions, implanted pain devices and cochlea implant for hearing conditions.

Artificial joints, medical clips left inside the body after operation - for example to seal blood vessels - will also rule out an MRI, as will an intrauterine device (IUD or contraceptive coil).

There is no evidence that MRI scans are a risk for pregnant women. However, as a precaution, the NHS does not usually recommend MRI scans during the first trimester (3 months) of pregnancy.

WebMD Medical Reference

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