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CT scan

Dr Karen Martin
Reviewed by Dr Karen MartinReviewed on 19.10.2023 | 3 minutes read

CT stands for computed tomography and it is a way of scanning that uses multiple X-rays to create accurate 3D images of the inside of your body. X-rays send small amounts of radiation energy through the body and detect the amount that has passed through to the other side.

Different tissues block or absorb different amounts of the X-rays depending on how dense they are. The more dense the matter, the more radiation it will block and so the whiter it appears on the image. So, for example bone shows up as bright white as it blocks a large amount of the radiation, whereas air, in the lungs, allows more to pass through and shows up as much darker.

What is the machine like?

A CT scanner is known for looking like a giant Life Saver mint; it is a white circle with a hole in the middle where the bed (and patient) passes through. It is a short tube rather than a long tube like the MRI scanner. You are laid down on the bed for a CT scan and the bed will move as the multiple X-rays are taken. It can be used to scan any part of the body but is particularly useful in emergencies as it is quicker than MRI scanning and also for detailed images of bone or internal organ injuries. It isn’t painful, usually takes around 15 minutes and – unlike an MRI scan – doesn’t surround your whole body at one time so is less likely to make you feel claustrophobic.

Who can have a CT scan?

Sometimes a dye known as contrast is used to help make the images clearer and show better contrast between different tissues. This dye may be swallowed, injected into your bloodstream, or given rectally depending on what area needs to be scanned. The CT images are then taken.

A small number of people may have an allergic reaction to this dye and it can also cause worsening kidney function in people who already have kidney problems. If you are diabetic, have kidney problems, or have had an allergic reaction to contrast in the past your doctor will discuss this with you prior to a CT scan with contrast and help decide whether a scan with contrast is right for you.

What about the radiation?

The risk of performing CT scanning is that you get exposed to radiation. Each X-ray gives off a small dose of radiation. A CT scan can use around 400 X-rays to produce an image and in general, the amount of radiation you're exposed to during each scan is equivalent to between a few months and a few years of exposure to natural radiation from the environment. The amount of radiation can cause a very small increased risk (less than 1 in 2,000) of developing cancer in the future and so because of this, your doctor will only arrange a CT scan if the benefits of having one outweigh the risk of the dose of radiation. It is also best avoided in people who are pregnant unless it is an emergency situation.

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Dr Karen Martin
Reviewed by Dr Karen Martin
Reviewed on 19.10.2023
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