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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.

Most scans take about half an hour. Lying still for a long time can be difficult. Tell the radiographer if you get stiff and need to move. If you are likely to feel claustrophobic contact the department a few days before your test. The staff can arrange to give you medicine to help you relax.

Possible risks

CT scans use X-rays and involve radiation. But the amount you have is small and won’t harm you. You should not have a CT scan if you are pregnant. Rarely, people have an allergic reaction to the contrast medium and the staff then give medicines to control the reaction.

Results

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.

 

CR PDF Icon You can view and print the quick guide for this page about CT scans.

 

 

How a CT scanner works

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 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. Below is a 360° photograph of a CT scanning room. If you can't see the photograph, you can download the Adobe Flash Player from the Adobe website. Use the arrows to move the picture and look around the room.

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 with an area marked for radiotherapy treatment.

CT scan

 

What happens

You usually have a CT scan in the X-ray 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 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 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 you can 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 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. 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 time is to set 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. 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.

Some people 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 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 that you 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 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 some time before the scan
  • To drink more of the liquid contrast or water in the X-ray department

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. It does not have any side effects. Below is an example of a CT scan of the abdomen.

A CT scan of the abdomen

 

CT scans of the head

For some brain scans, you may have a injection of the contrast medium dye beforehand to make the scan clearer.

 

CT scans of the chest

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.

A CT scan of the chest

 

Pelvic CT scans

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 '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 (also called a CT colonography). 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 or 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.

If you are diabetic, your doctor may want you to come into hospital a day or two before the scan, so that the nurses can keep an eye on you while you are following the special diet.

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 put air through this tube to inflate your bowel and make the scan clearer. This can cause wind from your back passage and some people find this very embarrassing. The staff are very used to doing this type of test though. They won't be embarrassed by it so you don't need to worry about that.

For virtual colonoscopy, you will have two scans – one on your back and one on your front.

 

Possible risks from a CT scan

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 involves being exposed to some radiation and this is why doctors are reluctant to do any scans without good reason. But it isn't a large enough amount of radiation for you to have any ill effects.

If you are having a scan to check out symptoms or for a known medical condition, it will give useful information to your doctors. 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 as low as possible. So, most doctors would not recommend CT scans routinely as part of a check up for a healthy person

Very, very rarely, someone has an allergic reaction to the contrast injection. 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.

 

The results

It can take time for test results to come through. How long will depend on why you are having the scan. Usually, the scan is examined by a specialist in radiology and a report is typed up. The report 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 needed the results urgently, it would have been noted on the scan request form and the results will be ready sooner than that. 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 a couple of weeks after your test, ring your doctor's secretary to check if they are back.

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Updated: 14 August 2013