How is MRI (magnetic resonance imaging) done?

Magnetic resonance imaging (MRI) is a type of medical imaging (radiology) test, that uses magnetic fields and radio waves to produce detailed, 3-dimensional images (pictures). Health professionals can use MRI to help diagnose and manage many health conditions.

MRI scanners and a computer produce images of the internal body structures, including the brain and spinal cord, bones and joints, the heart and blood vessels, breast tissue, and other internal organs. MRIs are usually performed by radiographers, who are health professionals trained in the use of imaging technology.

How to prepare for an MRI

Safety questionnaire

Before your MRI you will be asked some questions to check that it’s safe for you to have the scan. Some radiology practices may send you a safety questionnaire, which you should fill out before the scan. This is because the strong magnets in MRI scanners can interfere with certain medical devices and implants, such as heart pacemakers, which could make the test unsafe. If you are unsure about anything in the questionnaire, call the radiology practice where you are having the scan done to ask for further information.

You need to be relaxed

While most people find the MRI procedure tolerable, if you get anxious or uncomfortable in enclosed spaces you may find it more difficult, especially if you are having an MRI scan of the chest or brain as your head will be inside the scanner. If you are worried, you may wish to discuss this with your doctor or the radiology practice. A mild sedative may be prescribed for you to take before the scan, to help you relax.

How to help your child during an MRI

Because MRI scans do not use radiation, they are often recommended for children. There are things you can do to help your child relax on the day of the scan. Some younger children, however, may have trouble keeping still long enough to obtain good-quality pictures. As a result, they may need to be given a light general anaesthetic to do the MRI. If your child is likely to need an anaesthetic before their MRI, the radiology practice will discuss this with you.

Contrast medium

Some MRI scans require you to have an injection of contrast medium beforehand. A contrast medium is a special dye that needs to be injected before the scan is taken, to improve the detail of the images. If you have a history of kidney problems, your doctor may wish to do a blood test before the scan to ensure the contrast medium (called gadolinium chelate) is safe to use.

What to expect before an MRI

In most cases you should be able to eat, drink and take your medicines as usual on the day of the MRI scan. If this is not the case, your doctor and the radiology clinic will inform you.

Remember to take your request form or referral letter with you when you go for your MRI, along with any X-rays and other scans that may be relevant.

Wear loose, comfortable clothes. You will be asked to remove all metal objects prior to the scan, including watches, jewellery, coins, and hearing aids; and clothes with metal zips or fasteners. In some situations you may be required to change into a gown.

What happens during an MRI?

During magnetic resonance imaging, you lie on a flat bed, which moves into the MRI scanner. A strong magnetic field and radio frequency waves are directed at your body, producing detailed images of internal organs and tissues.

When the powerful magnets that are used in MRI are switched on, the resulting magnetic field causes tiny particles called protons, which are present in the body's tissues, to align in a particular direction. Radio waves then cause these protons to produce signals that are picked up by a receiver within the MRI scanner. These signals are then processed by a computer and converted into images.

The MRI scanner, which is shaped like a doughnut or tunnel, is well-lit and open at both ends.

To prevent the images being blurred, it is important you keep still throughout the scan. If the image it blurred, the radiographer can see straight away and will repeat the scan.

The radiographer will control the MRI scanner using a computer, which is located in a different room. This is to keep the computer away from the magnetic field given out by the scanner. You will be able to talk to the radiographer through an intercom and he or she will be watching you on a monitor throughout the scan.

If you are feeling particularly anxious, a friend or family member may be allowed to stay in the room with you. Children often have a parent with them.

The MRI scanner will make loud thumping or knocking noises at certain times throughout the scan. These noises are 'normal' and should not cause you alarm. You will be given earplugs or headphones with music to help you relax and not notice the noise so much.

MRI scans are painless. You should not feel anything during the scan apart from possibly a slight feeling of warmth.

Including the time it takes to get ready, an MRI scan can take anywhere between 30 minutes to over an hour to complete, depending on the part of the body being scanned and the medical problem being investigated.

What happens after an MRI?

Radiologists are specialist doctors who are trained to interpret medical imaging, such as X-rays, MRIs and CT scans. A radiologist will assess your MRI images and produce a written report on the findings. Occasionally the radiologist may speak to your doctor directly if the test was urgent or they need more information about your medical problem before writing the report. Usually you will need to make an appointment with the doctor who gave you the referral to discuss the scan results.

In general, you should be able to resume your normal activities immediately after the MRI. The exception is when you’ve been given a sedative before the scan, in which case you will need to arrange for a relative or friend to take you home after the procedure.

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