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What is a CT scan?



A CT (computerised tomography) scan takes a series of images using x-rays. The scanning machine is like a large doughnut, through which you will pass whilst lying very still on a couch. The images taken show detailed cross-sectional images or slices of your body which will help the doctors decide if there is anything concerning that needs further attention.


What are the risks of having a CT scan?


A CT scan involves radiation, however the radiation dose you receive will be as low as possible. Remember that your doctor has decided that the benefits of you having the CT scan far outweigh any of the risks involved.


Some CT scans require an injection of contrast media (X-ray dye). There is a small risk of an allergic reaction to the contrast, however before the scan begins the radiographer will ask you a number of safety questions to identify if you are at risk of an adverse reaction occurring.


If there is the possibility that you might be pregnant, kindly inform the radiographer.


How should I prepare for the CT scan?


If you are taking any medicines, please continue to take these as usual unless you have been told

otherwise by your doctor.


It is important not to eat for 4 hours before the examination. However, it is very important to drink water during these 4 hours.


You will be asked to change into a hospital gown before the procedure.


Occasionally your appointment may be delayed if we need to scan a patient urgently. We will keep you informed of any possible delays.


What happens during the CT scan?


If you need an injection of contrast media, a small cannula will be inserted into a vein in your arm or the back of your hand through which the contrast will be given. The contrast may cause a warm sensation or an altered sense of taste in your mouth for a few seconds.


The radiographers who perform the scan will not be in the scanning room during your scan but you can communicate with them through an intercom and they will be watching you through a glass window.


You will be asked to lie very still on your back or your front on the scanning table as it moves through the scanner. You may be asked to hold your breath a few seconds at a time whilst the scanner takes the pictures.


The actual examination can last up to 20 minutes depending upon the part of your body being scanned. When the examination is completed you will be asked to leave the scanning room.


What happens after the CT scan?


In the majority of cases, you can go home as soon as the scan is finished and resume your usual daily activities. You can eat and drink normally.


If you are given a contrast injection, you will be asked to wait in the waiting area for a short while. This is to ensure that you are feeling well and not showing any signs of an allergic reaction to the contrast media.  The cannula used will then be removed and you may leave the hospital.


Are there any possible complications or risks?


The use of intravenous dye is also generally very safe. Mild and self-limiting side effects may include a headache, a skin rash and dizziness.  The dye may rarely cause an allergic reaction which can present itself as a rash, difficulty breathing and swelling of the lips and mouth. It is therefore important to inform the radiographer if you feel unwell as the CT room is equipped with the necessary medications to treat these rare reactions.  

Compiled by Dr. Nathan Edwards




Guy’s and St Thomas’ NHS Foundation Trust. (2017) Having a CT scan [Online] Available at: (accessed 5 July 2018)


Oxford University Hospitals NHS Trust. (2012) Information for patients having a CT scan [Online]. Available at: (accessed 5 July 2018)


University College London Hospitals NHS Foundation Trust. (2013) Having a CT scan [Online]. Available at: (accessed 5 July 2018)

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