A computerised tomography scan (CT or CAT scan) is a special type of x-ray that gives a highly detailed picture of the organs and other structures in your body.
On this page, you can find the following information:
- What is a CT scan?
- Why are CT scans done?
- What should I expect when I have a CT scan?
- How do I prepare for my CT scan?
- What happens during my CT scan?
- What else might my CT scan involve?
- What if my child is having a CT scan?
- What happens after my CT scan?
- How will I get the results?
- Are there any risks of having a CT scan?
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What is a CT scan?
A computerised tomography scan (CT scan) combines a series of x-ray images that are taken from different angles around your body. These pictures are processed by a computer to create cross-sectional images of both the soft tissues and bones of your body. The images are like many slices of your body and can be arranged to give a 3D view of parts of your body.
Because a CT scan gives a detailed and precise picture, it is often used to give quick information about injuries caused in car accidents or other sorts of trauma. A CT scan can also provide excellent information about a range of longer-term diseases or injuries. This information can be used to guide treatment and help you and your doctor make good decisions about your care.
CT scans are sometimes called CAT scans. They are done at a hospital or a radiology service. A radiologist – a doctor who specialises in scans and imaging – will manage your scan.
Why are CT scans done?
Your doctor may ask for a CT scan to help:
- identify the cause of unexplained weight loss, pain or a lump that you or your doctor can feel
- identify muscle and bone problems, such as injuries or fractures
- monitor the progress of diseases, eg, heart disease or cancer
- identify issues with your blood vessels, gall bladder, liver or bladder
- identify the exact location of an infection or a blood clot
- guide procedures such as biopsies, surgery and radiation treatment
- detect internal bleeding or injuries.
What should I expect when I have a CT scan?
A CT scan appointment usually takes around 1 hour. Plan to get there early as the CT scan schedule is often very full. There is always a chance that an emergency CT scan for someone else will need to be done, so sometimes your scan may be delayed on the day. For safety you should allow 4 hours for your scan but it will probably be much quicker.
How do I prepare for my CT scan?
This will depend on what the scan is for. Usually you will be asked to:
- stop eating or drinking for 4 hours before your scan
- take off some or all of your clothing and wear a hospital gown for the scan
- remove all objects such as jewellery, piercings, dentures and glasses as they will interfere with the picture quality.
If you have allergies, kidney disease, asthma or diabetes ensure the CT staff are aware of this.
What happens during my CT scan?
You will be asked to lie on a motorised table. The table may include a cradle to keep your head still and there may also be straps and pillows to help you to stay still during the scan. The table will slide through the opening of the scanner which is shaped like a large ring. The ring contains an x-ray tube and detectors. The x-ray tube and detectors rotate within the ring around you. You will hear a whirring noise as they rotate. 
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The technician doing the scan can see and hear you at all times and you can communicate with them through an intercom system. It is important to stay as still as you can throughout the scan, but pictures can be repeated if you move. The technician may ask you to hold your breath sometimes to get particularly clear pictures.
What else might my CT scan involve?
Sometimes your doctor may request that you have a contrast agent to help highlight particular parts of your body. A contrast agent is a substance that appears white on x-ray images. It can outline structures such as your intestines, blood vessels, gall bladder or liver. Depending on which structure your doctor wants to highlight, the contrast agent will be given in one of the following ways:
- An injection: Contrast agents can be injected into a vein. This is usually into a vein in your arm. You may get a warm flush and an unusual taste in your mouth when this agent is injected.
- An enema: A contrast material may be inserted in your rectum to help give a picture of your rectum and your intestines. This is uncomfortable but the staff will help you manage this.
- A drink: You may need to swallow a drink containing contrast material. This will show images of your stomach and upper bowel.
What if my child is having a CT scan?
Because it is important to stay still during a CT scan the radiologist may recommend that your child has a sedative before a CT scan. This will be discussed fully with you beforehand. The staff will also help you prepare your child for the scan.
What happens after my CT scan?
After the CT scan you should be able to return to your normal routine immediately. If you have taken a contrast material you may be asked to stay for a period of time to make sure you don’t have a reaction to it. You may also be asked to drink a lot of fluid afterwards to remove the material from your body.
How will I get the results?
A radiologist will look at your scan and will send the results to your doctor. 
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Are there any risks of having a CT scan?
A CT scan uses small amounts of radiation. This is greater than the amount you would get during a simple x-ray, however it is still a small amount. The low dose of radiation you are exposed to during a CT scan has not been shown to cause harm.
If you are pregnant, or think you are pregnant: discuss this with your doctor and the CT staff. Low dose radiation is not safe for unborn babies. There may be another type of scan such as MRI that they recommend.
Reaction to contrast material: Very occasionally people can react to contrast material. Usually these reactions are mild, however in rare cases it can be serious. If you have reduced kidney function let the CT scanning team know as contrast material can make kidney function worse. Read more about contrast material.
Learn more
CT scan patient information The Royal Australian and New Zealand College of Radiologists
Contrast material – patient information RadiologyInfo.org
CT Scan vs MRI: What’s the Difference? And How Do Doctors Choose Which Imaging Method to Use? Memorial Sloan Kettering Cancer Center, US
CT (Computed Tomography) Scan Cleveland Clinic, US
CT scan Mayo Clinic, US
References
Contrast material – patient information RadiologyInfo.org
CT (Computed Tomography) Scan Cleveland Clinic, US
CT scan Mayo Clinic, US