This page tells you about CT scans. You can find the following information
What a CT scan is
A CT scan (or CAT scan) takes a series of X-rays of the body from different angles. A computer puts the X-rays together to form a picture. CT scans can show up where a tumour is in the body and how big it is.
How you have a CT scan
Before scans on some areas of the body you need special preparation, such as not eating for a few hours. Your doctor or nurse will tell you about this. Before some scans you have a drink or an injection into the bloodstream of a dye called contrast medium. The dye shows up body tissues more clearly on the scan.
To have the scan you lie on a couch that moves through the scanner. The radiographer controls the scan from outside the room. They can see you and talk to you all the time. You need to lie still. You won’t feel anything during the test.
The scan itself only takes about 10 to 15 minutes. But you should expect to be in the department for up to a couple of hours. Lying still during the scan can be difficult. Tell the radiographer if you get stiff and need to move. If you are likely to feel claustrophobic contact the department before your test.
CT scans use X-rays and involve radiation. But the amount you have is small and won’t make you feel unwell. Apart from in an emergency, you should not have a CT scan if you are pregnant. Rarely, people have an allergic reaction to the contrast medium. If this happens, the staff give medicines to control the reaction.
It usually takes from a few days to a couple of weeks to get the results. Contact your doctor if you have not heard anything after this time.
CT scan (or CAT scan) stands for computerised (axial) tomography scan. This means a scan that takes a series of X-rays and uses a computer to put them together. The scan is painless. The CT machine takes pictures of your body from different angles and gives a series of cross sections (slices) through the part of the body being scanned. A very detailed picture of the inside of the body can be built up in this way.
Together these cross sections give a very accurate picture of where an abnormal area such as a tumour is and how big it is. They also show how close the area that needs to be treated or operated on is to major body organs.
A CT scanning machine is large and shaped rather like a doughnut. There is a couch that you lie on.
The couch can slowly slide backwards and forwards through the hole of the doughnut. The pictures are taken as you move through the machine. Below is a CT scan of the pelvis with an area marked for radiotherapy treatment.
The scan itself only takes about 10 to 15 minutes. But you will need some preparation time beforehand. You should expect to be in the hospital for up to a couple of hours.
You usually have a CT scan in the X-ray (radiology) department. But if you are having the CT scan to plan radiotherapy, you may have the scan in the radiotherapy department. When you arrive, you check in with the receptionist so the radiographers know you are there. Then you usually take a seat in the waiting room.
View a transcript of the video showing what happens when you have a CT scan. (Opens in a new window)
When you are called, you may first go to a cubicle to take off your outer clothing. You may have to strip down to your underwear and put on a hospital gown. If you are just having a CT of your head, you may not be asked to undress. You must take off any jewellery in the area to be scanned because metal interferes with the images produced by the scanner.
When you are ready, the radiographer or an assistant will take you into the scanning room. You will probably have to lie down on the machine couch on your back. Sometimes the scan is done with you on your side or lying on your front. You need to lie as still as you can.
Once you are in the right position on the couch, the radiographer will leave the room. They will be able to see you on a TV screen or through a window, and you can talk to each other through an intercom. The radiographer controls the position of the couch from outside. The couch can move automatically through the CT scanner so that the part of the body to be scanned is in the machine. The radiographer will tell you that they are about to start the scan and will remind you to keep as still as you can. You will hear a whirring noise from the scanner. Some people describe this as similar to the noise of a washing machine. For some scans, the radiographer may ask you to hold your breath at various times. If you are getting stiff and need to move, tell the radiographers through the intercom. When the scan is over, the radiographer will come back into the room and lower the couch so that you can get up.
Although the CT scanner is fairly open, a few people may feel a bit claustrophobic or closed in when they are having a scan. If you think you are likely to feel this way, it is helpful to tell the radiographers before the day of your appointment. If your radiographers know you are nervous, they will take extra care in making sure you are comfortable and that you understand what is going on. Keeping your eyes closed during the scan sometimes helps.
Some CT scans need special preparation beforehand. This is explained below for scans of different parts of the body. For most scans, you will have a drink or an injection of contrast medium, or both. This is a dye that shows up body tissues more clearly on the scan. You have the injection through a small thin tube (cannula) in your arm. The tube is left in place until 15 to 30 minutes after your scan, in case you have any problems after having the injection.
The injection may make you feel hot and flushed for a minute or two, and you may have a metallic taste in your mouth. You may have the sensation that you are passing urine (but you are not) - this feeling is common and passes quickly. Before you have the contrast medium, the radiographer will ask you about any medical conditions or allergies, as some people can be allergic to it.
If you are having a CT scan of your abdomen, you may be asked
- To drink a liquid contrast medium some time before the scan
- To drink more of the liquid contrast or water in the X-ray department
- Not to eat or drink after midnight the night before the scan (this is for a CT scan of the inside of the large bowel, called CT colonography)
You may have the contrast medium by injection either instead of, or as well as, the drink. The contrast medium makes the digestive system (gut) show up more clearly in the scan. Below is an example of a CT scan of the abdomen.
For some brain scans, you may have an injection of the contrast medium dye beforehand to make the scan clearer.
For some chest (thoracic) scans, you may have an injection of the contrast medium dye beforehand. This is to help show up the tissues close to the area containing the cancer, for example blood vessels. It may help to show whether the cancer can be removed with surgery or not.
Below is an example of a CT scan of the chest.
If you are having a CT scan of the pelvis, you may be asked
- Not to eat or drink for some time before the scan
- To have an injection of contrast medium just beforehand
Depending on the part of your pelvis being scanned, you may have an injection of a drug to slow down the normal movement of your bowel. This movement (called peristalsis) can distort the scan and make it more difficult to read.
Occasionally, for a rectal scan, you need to have an enema of contrast medium dye. This shows up on the X-ray and makes the outline of the bowel show up more on the scan. This type of contrast isn't used very often. It may make you constipated. Your first couple of bowel motions will be white, but there are no other side effects.
There is a very detailed scan of the bowel called a CT colonography (also called virtual colonoscopy). Instead of having a tube and a camera put into your bowel to look inside (a colonoscopy), you will have CT scans. If you are having one of these, you will be asked to prepare by clearing the bowel by taking strong laxatives and drinking a special liquid with meals about 2 days before the test. You will also follow a special diet for a couple of days beforehand. Your doctor or nurse will tell you about this. You can read more about having a CT colonography.
A CT scan is a safe test for most people, but like all medical tests it has some possible risks. Your doctor and radiographer make sure the benefits of having the test outweigh these risks.
Having a CT scan involves being exposed to some radiation. The radiation is kept to the minimum necessary, and won't make you feel unwell. The risk of the radiation causing any problems in the future is very small. Apart from in an emergency, pregnant women should not have a CT scan. If you are pregnant, you should contact the radiology department for advice as soon as you can before the scan.
If you have an injection of contrast medium, this may leak outside the vein and cause swelling and pain in your arm. This is rare. If this happens the radiographer will give you further advice.
Very rarely, people have an allergic reaction to the contrast medium. The reaction most often starts with weakness, sweating and difficulty breathing. The doctors and radiographers will know what to do if you have this type of reaction and will treat you very quickly. If you feel ill during or after the test, tell the radiographer straight away.
It can take time for test results to come through. How long will depend on why you are having the scan. Usually, a specialist in radiology examines the scan and writes a report. This is then sent to your specialist, who will give you the results. If your GP has sent you for the test, the results will be sent directly to their surgery.
Understandably, waiting for results can make you anxious. It usually takes several days for the results to come through but some scans may take up to a couple of weeks. If your doctor needs the results urgently, they make a note of this on the scan request form and the results will be ready sooner. Try to remember to ask your doctor how long you should expect to wait for the results when you are first asked to go for the test. If it is not an emergency, and you have not heard after a couple of weeks, ring your doctor's secretary or GP to check if the results are back.
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