- August 21, 2017
- Posted by: mag
- Category: Awareness
A CT (computerised tomography) scanner is a special kind of X-ray machine. Find out what it is used for
What is a CT scanner?
A CT (computerised tomography) scanner is a special kind of X-ray machine.
Instead of sending out a single X-ray through your body as with ordinary X-rays, several beams are sent simultaneously from different angles.
This allows more detailed images from within the body to be constructed, and these images are then interpreted by a doctor.
CT scanners may also be referred to as CAT scans (computerised axial tomography).
Unlike an MRI scanner, where you are placed within a tunnel, a CT scanner consists of a doughnut shaped machine and therefore you should not feel claustrophobic.
How does a CT scanner work?
The X-rays from the beams are detected after they have 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 passed through denser tissue such as bone will be weaker.
A computer can use this information to work out the relative density of the tissues examined. Each set of measurements made by the scanner is, in effect, a cross-section through the body.
The computer processes the results, displaying them as a two-dimensional picture shown on a monitor. The technique of CT scanning was developed by the British inventor Sir Godfrey Hounsfield, who was awarded the Nobel Prize for his work.
What are CT scans used for?
CT scans are far more detailed than ordinary X-rays.
The information from the two-dimensional computer images can be reconstructed to produce three-dimensional images by some modern CT scanners.
The CT scanner was originally designed to take pictures of the brain. Now it is much more advanced and is used for taking pictures of virtually any part of the body.
They can be used to produce virtual images that show what a surgeon would see during an operation. They are used for a variety of different reasons, mainly because CT imaging is one of the best and fastest tools for examining the chest, abdomen and pelvis, and because it can provide cross-sectional views and highly detailed images.
Some of the most common uses of CT imaging include detecting different types of cancer (for example in the lung, bowel, liver and kidney), examining patients with severe injuries and finding the cause for sudden rapid onset symptoms (such as breathlessness, abdominal pain).
CT is also used for the detection, diagnosis and treatment of a number of vascular disease, which may ultimately lead to stroke, kidney failure or blood clots in the lungs.
This also includes abdominal aortic anueryms (AAAs), which is where the large main artery running downwards in the abdomen becomes enlarged and therefore may be prone to spontaneously rupture.
CT is also used to diagnosing and analysing many spinal problems and injuries which may occur to the hands, feet and other skeletal structures.
This is because CT is good, not just for looking at soft tissue structures, but also in providing detailed images of even very small bones.
CT scans also allows doctors to inspect the inside of the body without having to operate or perform unpleasant examinations.
It allows surgeons to plan surgery prior to starting a procedure, and allows assessment of the results after a procedure has been performed.
CT scanning has also proven invaluable in pinpointing exactly where a tumour is and planning treatment with radiotherapy.
The scanner is particularly good at testing for bleeding in the brain, for aneurysms (when the wall of an artery swells up), brain tumours and brain damage. It can also find tumours and abscesses throughout the body and is used to assess types of lung disease.
In addition, the CT scanner is used to look at internal injuries such as a torn kidney, spleen or liver; or bony injury, particularly in the spine. CT scanning can also be used to guide biopsies and therapeutic pain relieving procedures.
How is a CT scan prepared for?
Patients should always be asked about any recent illnesses or medical conditions they may have, and whether there is a history of heart disease, asthma, diabetes, kidney disease or thyroid problems.
Any of these may increase the risk of the patient having an adverse reaction during the scan procedure.
Prior to starting the procedure, the patient will usually be given a gown to hospital gown to wear. It is important that metal objects, such as jewellery, eyeglasses, dentures and hairpins, are removed before starting the procedure, since these will affect the quality of the images.
The patient may also be asked to remove hearing aids and dental work, and women will be asked to remove bras containing metal underwire. Where possible, piercings should also be removed.
If the patient is receiving an abdomen scan, for example, they will be asked not to eat for six hours before the test. They will be given a drink containing gastrografin, an aniseed flavoured X-ray dye, 45 minutes before the procedure.
This makes the intestines easier to see on the pictures. Sometimes a liquid X-ray dye is injected into the veins during the test. This also makes it easier to see the organs, blood vessels or, for example, a tumour.
The injection might be a little uncomfortable, and some people also experience a feeling of warmth in their arm. It’s important the patient inform their doctor prior to the administration of a contrast agent, if they have had a known allergy to contrast material, or ‘dye’.
How is a CT scan carried out?
The scan is a non-invasive, painless medical procedure. It usually takes approximately 10 to 30 minutes to perform, depending on the part of the body that is being scanned, the number of pictures taken and the different angles required.
The scanner itself looks like a large doughnut, with a bed passing through it.
During the scan the patient lies on a bed, with the body part under examination placed in the round tunnel or opening of the scanner.
If contrast material is to be used it will either be swallowed, injected via an intravenous line (IV) or administered by enema.
How contrast is given depends on the type of examination to be carried out. The patient will usually be asked to hold their breath during the scan. This is because any kind of motion, such as breathing or body movement, can lead to degradation of the resulting image and so make it harder to analyse and interpret.
The bed then moves slowly backwards and forwards to allow the scanner to take pictures of the body, although it does not touch the patient.
During the scan, only the patient will be in the exam room during the time the scan is carried out, unless there are special circumstances, such as a parent needing to stay in the room with their child, in which case they are required to wear a lead apron to minimise the radiation exposure.
During the scan, the technician will communicate with the patient via a speaker in the scan room, and they will be able to hear and see the patient at all times.
Does a CT scan hurt?
The examination does not hurt but some people find it uncomfortable to lie within the scanner itself.
Although the scanner is not tunnel shaped like an MRI scanner, people who suffer from severe claustrophobia sometimes have problems with CT scans. Let the doctors and radiographers know if this might be a problem.
Other people get slightly nervous because of the whirring noise the machine makes while working.
If the patient feels this might be the case, then this should be discussed prior to the procedure being carried out, since the patient may be unable to lie still during the scan therefore giving pictures of such poor quality they could not be interpreted anyway.
Is a CT scan dangerous?
Far more X-rays are involved in a CT scan than in ordinary X-rays, so doctors do not recommend CT scans without a good medical reason.
However, the amount of radiation a patient is exposed to is small, and therefore are unlikely to cause any long term harm.
The risk is greatest to those who are pregnant, as radiation exposure can cause harm to the fetus, and therefore CT sans are contraindicated in pregnant women, unless the benefits of performing the scan far outweigh the risks.
Risks are also greater in children, when compared to adults, and therefore a CT is only recommended if a child has a serious condition that puts them at greater risk.
Some patients may experience side-effects due to allergic reactions to the liquid dye injected into the veins.
In very rare cases, this dye has been known to damage already weakened kidneys.
It is important to let the X-ray doctors or technicians know if you have any allergies, asthma or kidney trouble, prior to having the X-ray dye injected.
How is a CT scan read?
A CT scan can give the doctor a much clearer picture of the inside of the body than an ordinary X-ray. For example, different types of tissue such as bone, muscle and fatty tissue are easy to see on a CT scan.
When looking at the abdomen, the scan shows various organs such as the pancreas, spleen and liver.
Any pathology or abnormality with these organs can be identified, and the doctor looking after the patient will decide on the best way to follow up any findings made on the CT scan.
When it is necessary to look at the brain, the areas containing liquid – the ventricles – are also clearly defined.
Very small shadows on the lungs can also be detected using CT and there are now studies looking into using it as a screening test for lung cancer.
How will I find out the results of my scan?
The results are analysed and interpreted by a radiologist, which is a doctor who is trained to supervise and interpret radiological procedures and images.
The radiologist will write a report based on the scan results, and this will be sent either to your GP or to the doctor who originally referred you for the scan.
They will then be in contact with you to discuss the results. In some cases, it may be necessary to have a follow-up examination, for example if there was a finding on the CT san which needs further investigation before a diagnosis can be made, or if the finding warrants a more specialised imaging technique.
Follow-up scans may also be necessary if an abnormality needs monitoring over a certain amount of time, to see if it is stable, or if treatment is being given whether the abnormality is responding to treatment.
In all cases, the referring doctor will discuss why they feel it is necessary to have a follow-up examination.