What it is
A (computed tomography, also called a CAT scan) uses X-rays and a computer to make detailed pictures of the inside of the body. A ring-shaped scanner moves around you while you lie on a flat bed, and a computer builds the pictures. It shows more detail than an ordinary X-ray. Some scans use contrast, a dye that makes parts show up more clearly. It is given as a drink or an injection into a vein.
Why it is done
A may be used to check bones and organs after an accident, to find or follow a cancer, to look for blood clots in the lungs, or to guide a needle. A head scan can show bleeding or the clot behind a stroke, and a chest scan can show pneumonia or fluid around the lung.
What it helps with
How to prepare
You will be told what to do, which may include fasting for some hours. If dye is planned, a kidney blood test may be done first.
Do
- Tell the team if you are or might be pregnant, or are breastfeeding
- Tell them about kidney problems, diabetes, asthma, a thyroid problem, or any earlier reaction to a scan dye
- Give a list of all your medicines, including for diabetes, and ask if any need to change that day
- Take off jewellery, zips, poppers, bras and hair clips
Do not
- Do not eat or drink during the hours you were told to fast
- Do not stop or change any medicine, including for diabetes, unless the doctor or scan team tells you to
What happens
- If dye is needed by injection, a thin needle goes into a vein in your arm or hand.
- You lie on a flat bed that slides into the ring-shaped scanner. The radiographer is in another room but can see, hear and speak to you.
- You lie very still and may be asked to hold your breath for a few seconds. The scanning takes seconds to a few minutes.
- If dye was injected, you may feel hot and flushed, taste metal, or feel you need to pass urine. This passes quickly.
Afterwards
Most people go home soon afterwards. If you had dye, you may be asked to wait up to 30 minutes to check for a reaction, and drinking fluids helps the body clear it. A radiologist writes to the doctor who asked for the scan, usually within 1 to 2 weeks. If you have heard nothing after 4 weeks, contact that doctor.
Risks and side effects
A uses more radiation than an ordinary X-ray. The chance of harm from one scan is small and rises slightly with repeated scans, so doctors weigh the benefit against it. Children are more sensitive. In pregnancy, doctors prefer or MRI for the tummy and pelvis, and use CT only if those cannot answer the question or time matters, so tell the team first.
Dye can cause itching, hives or sickness, and rarely trouble breathing, so tell staff at once if you feel unwell. It can harm kidneys that already work poorly.
Results and what comes next
The doctor who asked for the scan explains the result and may suggest another visit or more tests.
What a cannot show: some soft parts, such as the brain, spinal cord and joints, are often better seen on an . An scan, with no radiation, may be chosen for the tummy or pelvis.
Questions to ask your doctor
Tick the ones you want answered and take them with you.
Common questions
Does a CT scan hurt?
The scan does not hurt. You may feel a needle prick if dye is given by injection, and the bed can be hard. The dye can cause a warm flush, a metallic taste or a feeling of needing to pass urine, which pass quickly.
Is the radiation from a CT scan harmful?
A CT scan uses more radiation than an ordinary X-ray, but the amount is kept as low as possible and doctors weigh it against the benefit. The risk rises slightly with repeated scans, so tell your doctor about scans you have had. Children and pregnant women need special thought.
Why is a dye used, and what if I am allergic?
Dye makes blood vessels and organs show up more clearly. Tell the team about any earlier reaction, and about kidney disease, before the scan. Some people with an earlier reaction are given medicine beforehand, and staff watch for reactions and can treat them.
How is a CT scan different from an MRI scan?
A CT scan uses X-rays and is quick. An MRI uses a strong magnet and radio waves, with no radiation, and takes longer and is noisier. MRI shows soft parts in more detail, and CT is often used for bones, the chest and when time matters.