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Computed tomography (CT) scan

A computed tomography (CT) scan is an imaging test that uses a computer to put a series of special x-ray images together to create detailed 3-dimensional images of organs, tissues, bones and blood vessels in the body. Some CT scans require a contrast medium to show organs and abnormalities more clearly.

A CT scan is also called computerized axial tomography (CAT) scan.

A CT scan can be done on almost any body part. It can show detailed views of many different types of tissue, such as the:

  • brain
  • airways
  • lungs
  • bones
  • soft tissues
  • blood vessels

Why a CT scan is done

A CT scan can help detect a wide range of abnormalities or disease in any part of the body. A CT scan may be done to:

  • study the chest, abdomen or pelvis
  • diagnose cancer
    • find out the size and location of tumours
    • determine the stage (how far cancer has spread and if it is present in nearby organs and tissues)
    • guide the doctor during a needle aspiration or biopsy
  • help plan cancer treatment
    • find out if cancer treatment is working by comparing the size of the tumour before, during and after treatment
    • check if cancer has come back (has recurred) after treatment

How a CT scan is done

CT scans are usually done as an outpatient procedure in the radiology department of a hospital or a specialized CT centre. The test takes up to 1 hour, depending on the size of the area being scanned. If the person is sedated, the test may take longer (2–3 hours).

Special preparation may be needed, depending on the organ or structures being studied. Preparation may include:

  • not eating or drinking anything for a certain number of hours before the test
  • taking a laxative
  • having an enema
  • removing all metal objects, including glasses, braces or jewellery
  • taking a contrast medium
    • Depending on the part of the body being scanned, a contrast medium may be given:
      • orally
      • intravenously (injected into a vein in the hand or arm)
      • by enema (a procedure used to inject a liquid into the colon and rectum through the anus)

Because the machine produces x-rays, the technologist is in a separate room but can talk with the person through an intercom.

  • It is important that women tell the x-ray technologist or radiologist if they are pregnant, or think they may be pregnant, before having any type of x-ray.
  • If a contrast medium will be used, it is important that women tell the x-ray technologist or radiologist if they are breast-feeding.

During the CT scan:

  • The person lies on a narrow table. Straps and pillows may be used to help the person stay in the correct position and hold still during the exam.
  • The table slides into the CT scanner. The CT scanner looks like a large rectangular unit with a hole in its centre, like a giant doughnut or lifesaver.
  • The camera moves around in the scanner, taking many cross-sectional images or image slices. Computer software then stacks these image slices together to make a 3-dimensional image of the body.
  • The person may be asked to hold their breath at times to ensure a clear image.
  • Clicking or whirring noises may be heard during the scan.
  • A moving light may be seen as the scanner takes images.

Potential side effects

The amount of radiation used in a CT scan is higher than a regular x-ray. CT scans create low levels of ionizing radiation, which has the potential to cause cancer and other defects. However, the risk associated with any individual scan is small. CT scans and other x-rays are strictly monitored and controlled to make sure they use the least possible amount of radiation. The expected benefits of the x-rays must always outweigh any possible risk for the x-rays to be done.

  • About 5% of people react to the contrast medium. Symptoms may include:
    • nausea
    • wheezing
    • shortness of breath
    • metallic or bitter taste in mouth
    • feeling flushed
    • itching or facial swelling
  • On rare occasions, the contrast medium may cause a severe allergic reaction.

What the results mean

A CT scan can show:

  • a change in the shape, size or structure of tissues and organs
    • These changes could be due to injury or disease.
  • a mass or lesion that could be a tumour or caused by another disease
    • A CT scan cannot always tell the difference between a cancerous (malignant) tumour and a non-cancerous (benign) tumour.
  • the tumour’s approximate shape, size and location
  • if the tumour is pressing on other organs or tissues
  • enlarged lymph nodes
  • blood vessels that feed a tumour
  • cancer spread (metastasis)
  • a tumour’s response to cancer treatment
    • A CT scan 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 will probably be alone in the room, unless it is requested that someone be with the child. That person would need to wear a lead vest as protection against the x-rays.
  • The child needs to lie still on the table while the scanner takes images.
    • Foam cushions and Velcro straps may be placed on the forehead and arms to prevent the child from moving.
    • Children 4 months to 5 years old will usually need sedation or general anesthesia so they will relax and lie still for the whole test.
  • If sedation or a general anesthetic is used, children may:
    • not be allowed to eat or drink several hours before the test
    • have an IV put in
    • be on a heart monitor
  • Babies under 2 months old may be asleep for the test.
    • Keep babies awake and do not feed them for 3 hours before the appointment.
    • Bring a bottle (unless breast-feeding), security toy or blanket and pacifier if the child uses one.
    • If babies are tired and feed just before the appointment, they will usually fall asleep for the scan.
  • 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.
  • A CT scan does not hurt, but it may be uncomfortable to lie still for up to 1 hour.

The preparation you can provide for a CT scan 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.

contrast medium

A substance used in some diagnostic procedures to help parts of the body show up better on x-rays or other imaging tests.

In most cases, contrast medium is injected into or around the structure to be examined.

Also called contrast dye or contrast agent.


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