Computerised tomography

Page last reviewed: 13/07/2011

CT (or CAT) scan stands for computerised (axial) tomography scan. The scan uses X-rays and a computer to create detailed images of the inside of your body.

A CT scanner is a large ring-shaped machine. The X-ray scanner inside the ring rotates in small movements around your body as you lie on a bed that moves backwards and forwards through the ring. As you move through the machine, the scanner uses a series of X-ray beams to scan parts of your body and build up detailed images.

The images produced by a CT scan are called tomograms. They are more detailed than standard X-rays, which use a single beam of radiation, and can give views of structures inside the body including internal organs, blood vessels, bones and tumours.

A CT scan can be used to diagnose or monitor many different health conditions, including cancer and bone disease. It is often used to provide views of your body before another procedure takes place, such as a biopsy or radiotherapy treatment.

A CT scan is painless and usually takes between 10 and 30 minutes, depending on which part of your body is being scanned.

Is it safe?

You will be exposed to X-ray radiation during a CT scan. The amount of radiation you are exposed to is safe and is not enough to cause any harm.

However, CT scans are not recommended for pregnant women because of the risk to the unborn baby. Children are more at risk than adults from a build-up of radiation doses and should only have a CT scan if it is justified by a serious condition that puts them at a higher risk.

If you are advised to have a CT scan to diagnose a condition or to check symptoms of a known medical condition, the benefits of having the scan will outweigh any potential risk.

Page last reviewed: 13/07/2011

Before the scan

Before you have a CT scan, you will be asked about any existing conditions, medicines you are taking and if you have any allergies. This is to make sure that there is no risk of reactions.

If you are a woman of childbearing age, you will be asked if you are pregnant. CT scans are not recommended for pregnant women unless there is an urgent medical reason, as there is a small chance that the X-rays could cause harm to an unborn child.

If you are claustrophobic and feel anxious about having a CT scan, tell the radiographer (see box, right) before your appointment. They can give you advice to help you to feel calm and, if necessary, arrange for you to have a sedative.

Preparation

Sometimes you will be asked to remove your clothing and put on a gown. You should also remove any jewellery or metal fastenings because metal interferes with the scanning machine. If you are having a head scan, you may also be asked to remove dentures, hair clips and hearing aids.

Contrast medium

Depending on what part of your body is being scanned, you may be given a contrast medium. This is a liquid that contains a dye, which improves the image of certain tissues or blood vessels. A contrast medium is usually harmless and will pass out in your urine after the scan.

A contrast medium may be swallowed as a drink, given as an enema in your back passage or injected into the blood stream, depending on the part of your body that is to be scanned.

Many CT scans require an injection of contrast medium into a vein in the hand or arm. This is to help a radiologist tell the difference between blood vessels and other structures.

Some people find that the contrast medium injection makes them feel warm and as if they want to pass urine. This sensation is normal and usually passes quickly.

The scan

The CT scanner is a large ring-shaped machine. You will be asked to lie on a motorised bed that moves in and out of the hole. Only the part of your body that is inside the ring can be scanned.

Your radiographer will position the bed so that the correct part of your body is in the scanner. They will then leave the room and operate the scanner from a control room behind a window. This is because it is dangerous for staff to be exposed to X-rays every day. You will still be able to hear and speak to the radiographer during the procedure through an intercom.

You will be asked to lie very still and breathe normally while each scan is taken, to avoid blurring the images. You may be asked to inhale, exhale or hold your breath at certain points.

The X-ray unit inside the ring will rotate around you. Each time it goes round it creates a new X-ray scan. After each X-ray is completed, the bed on which you are lying is moved forward a small amount.

Several scans will be carried out and the whole procedure may last up to 30 minutes.

You should be able to go home as soon as the scan is over.

Spiral CT scans

A commonly used scan called a spiral or helical CT scan is much faster than a standard CT scan. It can produce detailed pictures of your organs, tissue and blood vessels in a few minutes.

It is called a spiral scan because the single X-ray beam rotates around you in a spiral shape, giving a continuous picture of your body. A spiral scan produces an image of the whole body (or part of the body) that can be sliced up into different sections if needed.

Results

The results of your CT scan are not available straight away. A computer needs to process the information collected from your scan and a radiologist needs to analyse this information and write a report of your results. This report will be sent to your specialist or GP.

Ask before you leave the hospital when to expect your results. It usually takes a couple of weeks.

Glossary

Blood
Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart.
Blood vessels
Blood vessels are the tubes in which blood travels to and from parts of the body. The three main types of blood vessels are veins, arteries and capillaries.
Enema
An enema is an injection of fluid into the large intestine (colon) to empty the bowel. It can also be used to make the bowels show up more clearly in an X-ray.
Tissues
Body tissue is made up of groups of cells that perform a specific job, such as protecting the body against infection, producing movement or storing fat.
X-ray
An X-ray is a painless way of producing pictures of inside the body using radiation.

The radiology department

The radiology department, or X-ray or imaging department, is an area of a hospital where radiological examinations of patients are carried out. These include X-rays, ultrasound scans, MRI and CT scans.

Radiologists are doctors that are specially trained to carry out examinations and interpret medical images, such as X-rays.

Radiologists are supported by radiographers, who are trained to carry out X-rays and use other scanning equipment.

Page last reviewed: 13/07/2011

CT scans can give doctors information to help them diagnose a variety of conditions. They can help to confirm or rule out a suspected diagnosis, or occasionally identify a condition that was not even suspected.

Unlike other imaging methods, a CT scan can give a detailed view of lots of different tissue types in the body, including lungs, bones, soft tissues and blood vessels.

  • Head scan: an effective method of checking the brain for suspected tumours, bleeding or swelling of the arteries. It is also useful for investigating the brain following a stroke.
  • Abdominal scan: used to detect tumours and diagnose conditions that cause internal organs - including the liver, kidneys, pancreas, intestines or lungs - to be enlarged or inflamed.
  • Vascular scan: used to assess vascular (blood flow) conditions in different parts of the body.
  • Bone scan: used to assess injury and disease to bones, particularly in the spine. It can be used to assess bone density when investigating osteoporosis.
  • Accidental injury: a CT scan can be used after serious accidents to look for internal injuries, such as tears of the spleen, kidneys or liver.
  • Preparation for tests and treatments: a CT scan can identify normal and abnormal tissue. This can be useful for planning areas for radiotherapy treatment. It can also act as a guide for taking tissue samples and needle biopsies.

CT scan screening

There is concern that, in some cases, CT scans are being used unnecessarily. Some private medical providers offer CT scans as a type of screening to detect disease in patients who do not have symptoms or any significant risk factors for a disease. This can be expensive and often put you at unnecessary risk.

CT scans are so detailed that they can detect what appear to be abnormalities in healthy people. Usually, the abnormalities are not serious, but the diagnosis can lead to undue anxiety and unnecessary further tests.

It is generally not recommended that you have a CT scan for peace of mind if you do not have any symptoms. If you have symptoms of injury or illness that require an evaluation using a CT scan, you may benefit from a scan, but should only have one after medical referral.

Glossary

Arteries
Arteries are blood vessels that carry blood from the heart to the rest of the body.
Blood
Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart.
Blood vessels
Blood vessels are the tubes in which blood travels to and from parts of the body. The three main types of blood vessels are veins, arteries and capillaries.
Brain
The brain controls thought, memory and emotion. It sends messages to the body controlling movement, speech and senses.
Inflammation
Inflammation is the body's response to infection, irritation or injury, which causes redness, swelling, pain and sometimes a feeling of heat in the affected area.
Kidneys
Kidneys are a pair of bean-shaped organs located at the back of the abdomen, which remove waste and extra fluid from the blood and pass them out of the body as urine.
Liver
The liver is the largest organ in the body. Its main jobs are to secrete bile (to help digestion), detoxify the blood and change food into energy.
Lungs
Lungs are a pair of organs in the chest that control breathing. They remove carbon dioxide from the blood and replace it with oxygen.
Radiotherapy
Radiation therapy uses X-rays to treat disease, especially cancer.
Spine
The spine supports the skeleton and surrounds and protects the delicate spinal cord and nerves. It is made up of 33 bones called the vertebrae.
Tissue
Body tissue is made up of groups of cells that perform a specific job, such as protecting the body against infection, producing movement or storing fat.  

Page last reviewed: 13/07/2011

A CT scan involves exposure to radiation in the form of X-rays. In excessive amounts, X-ray radiation can increase your risk of getting cancer. However, the amount of radiation you are exposed to during a CT scan is not enough to cause any harm.

If you have a CT scan to diagnose a condition or to check symptoms of a known medical condition, the benefits of this will outweigh any potential risk. In this situation, CT scans are quick and accurate, and often eliminate the need for invasive surgery.

However, if you are asymptomatic (have no symptoms), the benefits of having a CT scan are less clear and may not outweigh the risks, especially as it may lead to further unnecessary testing and added anxiety.

CT scans are not 100% accurate. There is a chance that the test may be wrong, causing you to be needlessly alarmed or falsely reassured. Around 1 in 20 abnormal cases may be missed from a highly sensitive CT scan.

The possible benefits and risks of having a CT scan should always be weighed up before you choose to have one. It is recommended that you only have a scan on the basis of a medical referral.

Pregnant women and children

Pregnant women should not have CT scans as there is a small risk that X-rays may cause an abnormality to the unborn child. Tell your doctor if you think there is a chance that you may be pregnant before having a scan.

Children are more at risk from a build-up of radiation doses than adults and should only have a CT scan if it is justified by a serious condition that puts them at a higher risk.

Complications

In rare cases, the contrast medium used before CT scans can cause an allergic reaction. Tell the radiologist if you have had an allergic reaction to iodine or a contrast dye in the past, or if you have any other allergies.

Glossary

Allergen
An allergen is a substance that reacts with the body's immune system and causes an allergic reaction.
Dose
Dose is a measured quantity of a medicine to be taken at any one time, such as a specified amount of medication.
Kidney
Kidneys are a pair of bean-shaped organs located at the back of the abdomen, which remove waste and extra fluid from the blood and pass them out of the body as urine.

Content provided by NHS Choices www.nhs.uk and adapted for Ireland by the Health A-Z.

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