CT scan
This page tells you about CT scans. CT scan (or CAT scan as it is sometimes called) stands for Computerised (Axial) Tomography scan. This just means a scan that takes pictures from all around your body and uses a computer to put them together. This page has information on
How a CT scanner works
A CT scanner uses X-rays. Like an X-ray, it is painless. The CT machine takes a lot of pictures of your body from different angles. These pictures are fed into a computer. The computer puts them together to give a series of cross sections or '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 a tumour is and how big it is. They also show how close major body organs are to the area that needs to be treated or operated on.
A CT scanning machine is a large machine that is shaped rather like a doughnut. There is a couch that you lie on.
The couch can 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 marked for radiotherapy treatment.
What will happen
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 until someone calls you for your scan.
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 that is in the area to be scanned because metal interferes with the machine.
When you are ready, the radiographer or helper 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, but breathe normally.
Once you are in the right position on the couch, the radiographer will leave the room. This is because there will be X-rays in the room and it would be dangerous for the staff to be exposed to these. They see patients having X-rays and CT scans all day, every day and if they stayed in the room, would be exposed to far more X-rays than any patient.
The radiographer will be able to see you on a TV screen and you can talk to each other through an intercom. The radiographer will control 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 he or she is about to start the scan and remind you to keep as still as you can. When the scan is over, the radiographer will come back into the room and help you down from the couch.
Most scans take about half an hour. A lot of that is for setting up the scan, rather than actually taking it. Lying still for that long can be uncomfortable. If you are getting stiff and need to move, tell the radiographers through the intercom. During the actual scan, you have to try to keep as still as possible, and not cough or swallow, particularly if your head is being scanned. Mostly, you can breathe quietly but normally throughout the scan. For some scans, your radiographer may ask you to hold your breath at various times during the scan. If this is going to happen, they will tell you beforehand. You should be able to go home as soon as the scan is over.
Suggestion
Some people feel a bit claustrophobic ('closed in') when they
are having a scan. If you think you are likely to feel
this way, tell the radiographers before the
day of your appointment.
If necessary, you can have a tablet or injection to calm you down before the scan. If you need a sedative, it needs to be organised in advance so do ring the department if you think you'll need one. If your radiographers know you are nervous, they will take extra care in making sure you are comfortable and understand what is going on. Keeping your eyes closed sometimes helps.
Preparation for the scan
Some CT scans need special preparation beforehand. This is explained below for scans of different parts of the body. For some scans, you will have a drink or injection of 'contrast medium'. This is a sort of dye that shows up body tissues more clearly on the scan. Before you have this, the nurse or radiographer will ask you about allergies or asthma as some people can be allergic to it. The injection may make you feel hot and flushed for a minute or two.
Abdominal CT scans
If you are having a CT scan of your abdomen, you may be asked
- Not to eat or drink after midnight the night before the scan
- To drink a liquid 'contrast medium' 24 hours before the scan
- To drink more of the liquid in the X-ray department
CT scans of the head
For some brain scans, you may be given an injection of the 'contrast medium' dye beforehand to make the scan clearer.
CT scans of the chest
For some chest (thoracic) scans, you may be given an injection of the 'contrast medium' dye beforehand. This is to help show up the tissues in the area containing the cancer, for example blood vessels. It may help to show whether the cancer can be removed with surgery or not.Pelvic CT scans
If you are having a CT scan of the pelvis, you may be asked
- Not to eat or drink after midnight the night before the scan
- Have an injection of 'contrast medium' just before the scan
Occasionally, for a rectal scan, you need to have an enema of the 'contrast medium' dye. This shows up on the X- ray and makes the outline of the bowel stand out more on the scan. This 'rectal 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 'virtual colonoscopy'. If you are having one of these, you will be asked
- Not to eat or drink for 36 hours before the scan
- Take 2 doses of a strong laxative the day before the scan
Just before the scan, you'll have two injections. One is the 'contrast medium' dye to show up the body tissues more clearly. The other is a drug to slow down the normal movement of your bowel which can make the scan less clear. You will also have a tube put into your back passage. The radiographers will put air through this tube to inflate your bowel and make the scan clearer. Apart from the obvious and slightly embarrassing after effects of having air pumped into your bowel, there are no other side effects of this. Try to take it all in your stride. The staff are professionals and are used to doing this type of test. They won't be embarrassed by it so there is no need for you to be.
For virtual colonoscopy, you will have two scans - one on your back and one on your front.
Spiral CT scans
This is a newer type of CT scan, also known as a 'helical' CT scan. It is faster than a standard CT scan and gives an even more detailed picture of your organs and tissues, including blood vessels. This can make it useful for picking up very small tumours. It's called a spiral scan because the X-ray beam rotates around you during the scan in a spiral shape. This gives the radiographers a continuous picture, with no possible gaps between the 'slices' of the scan.Can a CT scan be dangerous?
Like any X-ray, you should not have a CT scan if you are pregnant as it could be dangerous for the baby. Other than this situation, a CT scan is not dangerous. It does involve being exposed to some radiation. This is why doctors are reluctant to undertake any scans without good reason. But it isn't a large enough amount of radiation for you to have any ill effects. The scan provides useful information to your doctors about your condition, or about how it has responded to treatment. The benefits of this outweigh any potential risk. Doctors like to keep the number of scans you have to a minimum because this also keeps your radiation exposure down to a minimum.
Very, very rarely, someone has an allergic reaction to the contrast injection. The reaction most often starts with weakness, sweating and difficulty breathing. It is possible to react to any injection in this way, and the doctors and radiographers will know what to do if you do have this type of reaction.
