Canadian Cancer Society logo
You are here: 

Magnetic Resonance Imaging (MRI)

A magnetic resonance imaging (MRI) is an imaging test that uses powerful magnetic forces and radio-frequency (RF) waves to make detailed 3-dimensional pictures of organs, soft tissues, bone and most other internal body structures. Some MRI scans require a contrast mediumcontrast mediumA substance used in some diagnostic procedures to help parts of the body show up better on x-rays or other imaging tests. to provide clearer images.

Different tissues react differently to the magnetic current and this produces various images. No ionizing radiationionizing radiationA type of high-frequency radiation that can remove particles from matter it passes through, resulting in charged ions. These charged ions can cause changes to cells’ DNA that can damage or kill the cells. is used in MRI.

MRI cannot be done if the person has certain metal devices inside their body (such as a pacemaker, implanted port or pump). The magnetic force is so strong that it can damage or dislodge these devices. In most cases, MRI can be done on people who have joint replacements, surgical clips or screws.

An MRI is also called nuclear magnetic resonance imaging (NMRI).

Why an MRI is done

An MRI may be done to:

  • diagnose cancer
    • MRI can help find tumours in the brain, spinal cord, head, neck, bones, breast, muscles or other soft tissue.
    • It can be used to determine the stage (how far the cancer has spread and if it is present in other organs and tissues).
  • help plan cancer treatment

How an MRI is done

MRI is usually done as an outpatient in a hospital or a specialized MRI centre. The test takes up to 2 hours, depending on the area being scanned. If anesthetic is necessary, the test may take longer (2–3 hours).

Special preparation may be needed for an MRI with contrast medium, sedation or anesthetic.

  • If a contrast medium is used, it is usually injected into a vein in the hand or arm.
    • The contrast medium most commonly used is gadolinium.
  • Some people may need sedation to relax and lie still for the whole test.
  • If sedation or a general anesthetic is used, the person may:
    • not be allowed to eat or drink several hours before the test
    • have an IV put in
    • be on a heart monitor

The person should dress in comfortable clothing with no metal snaps or zippers. Clothing, metal jewellery and objects with a magnetic strip (such as bank cards or credit cards) that may interfere with the MRI are removed. The person may have to wear a gown, depending on the part of the body to be scanned.

Because the machine produces magnetic waves, the technologist is in a separate room. The person can see the technologist at all times and they can talk with each other through an intercom.

During the MRI:

  • The person lies on a movable exam table. Straps and pillows may be used to help the person stay in the correct position and hold still during the exam.
  • Surface coils may be placed around or near the area being scanned. These coils help improve the image quality of superficial structures, such as the neck, shoulder, knee or breast.
  • The person lies on a table, which glides into a narrow cylinder that houses the MRI scanning magnet.
    • The inside of the scanner is well lit and has a fan that gently blows fresh air.
    • The part of the body being examined is positioned in the centre of the cylinder.
    • The surface of the cylinder may be just a few inches from the person’s face.
  • The person needs to lie still inside the MRI machine while it takes pictures.
  • The person may be asked to hold their breath at times to ensure a clearer picture.
  • Loud knocking noises may be heard during the scan.

Potential side effects

MRI uses no radiation and has no known harmful effects.

  • Some people may have a mild reaction to the contrast medium. Symptoms may include:
    • nausea
    • pain at the injection site
    • headache
  • There are some risks associated with sedation or general anesthetic. People should discuss these risks with their doctor before the scan.

What the results mean

An MRI scan can show:

  • a change in the shape, size or structure of tissues or organs
    • These changes could be due to injury or disease.
  • a mass or lesion that could be a tumour or caused by another disease
    • An MRI cannot always tell the difference between a cancerous (malignant) tumour and a non-cancerous (benign) tumour.
  • the tumour’s approximate shape, size and location
  • cancer spread (metastasis)
  • the tumour’s response to treatment
    • An MRI can be used to determine if the tumour has gotten smaller, stayed the same or grown after treatment.

What happens if a change or abnormality is found

The doctor will decide whether further tests, procedures, follow-up care or additional treatment are needed.

Special considerations for children

Being prepared for a test or procedure can reduce anxiety, increase cooperation and help the child develop coping skills. Parents or caregivers can help prepare children by explaining to them what will happen, including what they will see, feel and hear during the test.

  • The child needs to lie still on the table during the MRI.
    • Foam cushions and Velcro straps may be placed on the forehead and arms to help the child remain still.
    • Children under 8 years of age are usually given general anesthesia to help them relax and lie still for the whole test.
    • For young children, it may be helpful to wake them early or keep them from napping on the day of the scan, so they fall asleep more easily when sedation is given.
  • If sedation or a general anesthetic is being used, the child may:
    • not be allowed to eat or drink several hours before the test
    • have an IV put in
    • be on a heart monitor
  • If a contrast medium is used and given intravenously (into a vein), the child will feel a slight pin prick when the needle is inserted.
  • The child will probably be alone in the room, unless it is requested that someone be with the child.
  • Lying still can be stressful for children. Some children may be frightened by the knocking sound.
    • An MRI can be explained as a camera that looks like a tunnel.
    • Tell the child that the machine is noisy but does not hurt.
    • Older children can bring in their favourite music to listen to during the scan.
    • Younger children can take a cuddly toy or blanket into the MRI machine with them, as long as it has no metal parts.

The preparation you can provide for an MRI depends on the age and experience of the child. See the following for more age-specific information on helping children cope with tests and treatment.

Stories

Dr John Dick A new understanding of blood cells

Read more

Cancer information in over a hundred languages

Illustration of question mark

The Canadian Cancer Society’s Cancer Information Service (CIS) is Canada’s only national, bilingual, toll-free service that offers personalized comprehensive cancer information in over 100 languages.

Learn more