Computed Tomography (CT) is a sensitive diagnostic tool that uses X-rays to take a series of two-dimensional cross-sectional images ('slices') around an axis. It has been in widespread use since the 1970s and  is used to image many diseases and injuries.

Using X-rays does carry a small risk. However, to put this into context, a CT scan will deliver approximately the same amount of radiation you would absorb from your normal surroundings in about three years. Another way of looking at this is to say that the benefits of having a scan outweigh the risks associated with it.

Having a CT scan

If you’ve never had a CT scan you might not know what to expect and this brief guide is designed to answer the questions that might be in your mind.

Important things to tell us

It’s important that you tell us before the scan if you:

  • are diabetic and taking metformin.
  • have any allergies or asthma.
  • have impaired renal function.
  • are pregnant or are breast-feeding.

Before you arrive

  • We will make sure that it's appropriate for you to have a CT scan. We'll ask you some basic questions when you book and you'll be required to complete a questionnaire before your appointment.
  • If we’re scanning your abdomen or pelvis, we might ask you to drink a certain amount water or contrast agent. You will be given details if this is necessary.
  • It might be necessary for you to stop eating or drinking for a specified time before your scan. You will be given the details if this is necessary.
  • Please wear loose, comfortable clothing and leave jewellery and watches at home if possible.
  • Please confirm your appointment by telephone 24 hours before your scan and arrive in plenty of time.
  • Please let us know if you have any disabilities so that we can ensure we are able to offer you the highest quality service.
  • You're welcome to bring a friend or a relative with you, but for safety reasons we won't normally allow them into the examination room. 
  • Don't forget to bring your appointment letter with you.

Preparation for the scan

  • Once you’ve checked in at reception, a member of the radiography team will meet you, explain the procedure and ask you to sign a consent form and complete a safety questionnaire. You’ll have the opportunity to ask any questions about the scanning process.
  • We might ask you to change into a hospital gown. We’ll provide somewhere to store your personal possessions.
  • Throughout the procedure, you will be looked after by the radiography team. They will explain what’s happening and will be there if you experience any discomfort.
  • We might need to give you an injection (known as a contrast medium) to increase the amount of information we can get from the scan. Depending on the area we're scanning, we might ask you drink the contrast medium instead of injecting it. In this case you might have to wait for anything up to an hour before you can be scanned.
  • You need to let the Radiographer know if you've ever reacted to an injection given for a kidney X-ray (IVP or IVU) or a previous CT scan.

During the scan

  • The radiographer operating the scanner will be able to see and hear you throughout the procedure.
  • We’ll ask you to lie down on the scanner bed and we'll make sure you're comfortable so you can stay as still as possible. We might have to ask you to hold your breath or not to swallow when the scanner is taking the images.
  • Most scan sessions last between 15 and 30 minutes.
  • You’re welcome to bring a friend or a relative with you, but for safety reasons we won’t normally allow them into the examination room. 

After the scan

  • There are no restrictions on normal activity, you can eat and drink normally, drive and return to work immediately after the scan.
  • If we’ve given you a contrast medium injection there is a very small risk of an allergic reaction so we’ll ask you to stay with us for half an hour after the scan.
  • A radiologist will examine the images shortly after your visit and send a report to your doctor or consultant, normally within a few days.
  • For ethical and professional reasons, we cannot discuss results with you. Only your doctor or consultant can do this

More facts about CT

CT – a brief technical explanation

A CT (computerised tomography) scanner is a special kind of X-ray machine. Instead of producing an image (radiograph) from a single direction, the X-ray source is rotated around the patient - acquiring a cross-sectional image (tomogram) from many angles.

The X-rays from the beams are detected after they've passed through the body and their strength is measured. Beams that have passed through less dense tissue such as the lungs will be stronger, whereas beams that have been absorbed by denser tissue such as bone will be weaker.

A computer can use this information to work out the relative density of the tissues examined. The computer processes the results, displaying them as a two-dimensional picture shown on a monitor.

CT uses X-rays – but the dose is low and so is the risk

Using X-rays does carry a small risk. To put this into context, a CT scan will deliver approximately the same amount of radiation you would absorb from your normal surroundings in about three years. Another way of looking at this is to say that the benefits of having a scan outweigh the risks associated with it.

Contrast injection

If you have an injection of contrast you may experience a sensation of warmth and a metallic taste in the mouth and very occasionally, temporary side effects such as nausea or an itchy rash may occur, but other reactions are extremely rare. If there's any chance that you may be pregnant you should tell us before the examination.

CT scans – what’s involved

For an abdominal CT you may be asked to drink a liquid to outline the bowel before commencing the examination or you may need a small injection of contrast medium to assist the examination.

During the scan you'll lie on a bed, with the body part under examination placed in opening of the scanner.

The bed then moves slowly backwards and forwards to allow the scanner to take pictures of your body, although it won't actually touch you. The length of the examination depends on the number of pictures and the different angles taken.

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