A 'computerized tomography' (CT) (or 'computerized axial tomography' (CAT) scan) uses a computer that takes data from several X-ray images of structures inside a human's or animal's body and converts them into pictures on a monitor.
Tomography is the process of generating a 2-dimensional image of a slice or section through a 3-dimensional object. Similar to looking at one slice of bread within the whole loaf.
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At the end of this article we include a list of recent developments on CT scans from MNT news.
Fast facts on CT scans
Here are some key points about CT scans. More detail and supporting information is in the main article.
- A CT scan uses data from several X-ray images of structures inside the body and converts them into pictures.
- The technique utilizes digital geometry processing to generate 3D images.
- CT scans are a source of ionizing radiation and can cause cancer.
- A CT scanner emits a series of narrow beams through the human body, producing more detail than standard single beam X-rays.
- CT scanners are able to distinguish tissues inside a solid organ.
- Contrast dyes are sometimes used to improve the clarity of the image.
- CT scanning is particularly useful for getting detailed 3D images of certain parts of the body, such as soft tissues, blood vessels, and the brain.
- The images created are analyzed by radiologists.
- Unlike MRI scans, a CT scan uses X-rays.
- A CT scan is able to illustrate organ tear and organ injury quickly and so is often used for accident victims.
What is a CT scan?
A CT scanner uses digital geometry processing to generate a 3-dimensional (3D) image of the inside of an object. The 3D image is made after many 2-dimensional (2D) X-ray images are taken around a single axis of rotation - in other words, many pictures of the same area are taken from many angles and then placed together to produce a 3D image.
The Greek word tomos means "slice", and the Greek word graphein means "write".
Although CT is a useful tool for assisting diagnosis in medicine, it is a source of ionizing radiation and can cause cancer. The National Cancer Institute1 advises patients to discuss the risks and benefits of computerized tomography with their doctors.
How do CT scans work?
A CT imaging suite.
A CT scanner emits a series of narrow beams through the human body as it moves through an arc, unlike an X-ray machine which sends just one radiation beam. The final picture is far more detailed than an X-ray image.
Inside the CT scanner there is an X-ray detector which can see hundreds of different levels of density. It can see tissues inside a solid organ. This data is transmitted to a computer, which builds up a 3D cross-sectional picture of the part of the body and displays it on the screen.
Sometimes a contrast dye is used because it shows up much more clearly on the screen. If a 3D image of the abdomen is required the patient may have to drink a barium meal. The barium appears white on the scan as it travels through the digestive system. If images lower down the body are required, such as the rectum, the patient may be given a barium enema. If blood vessel images are the target, the barium will be injected.
The accuracy and speed of CT scans may be improved with the application of spiral CT. The X-ray beam takes a spiral path during the scanning - it gathers continuous data with no gaps between images. For a spiral scan of the chest, for example, the patient will be asked to hold his/her breath for a few seconds.
The video below, created by the UK's Open University, shows how a CT scan works. In this case the scan is being used to determine whether a patient has a fracture of the skull.
What are CT scans like for patients?
Most places will provide the patient with a gown. He/she will need to undress, usually down to their underwear, and put the gown on. If the place does not provide a gown the patient should wear loose-fitting clothes.
Doctors may ask the patient to fast (eat nothing) and even refrain from consuming liquids for a specific period before the CT scan.
The patient will be asked to lie down on a motorized examination table, which then goes into the giant doughnut-like CT scanner machine. The couch with the patient goes into the doughnut hole.
Some patients may be given a contrast dye or substance which is either swallowed, given as an enema, or injected. This improves the picture of some blood vessels or tissues. If a patient is allergic to contrast material he/she should tell the doctor beforehand. There are some medications that reduce allergic reactions to contrast materials.
As metal interferes with the workings of the CT scanner the patient will need to remove all jewelry and metal fastenings. In the majority of cases the patient will lie on his/her back, facing up. But sometimes it may be necessary to lie face-down or sideways.
After the machine has taken one X-ray picture, the couch will move slightly, and then another picture is taken, etc. The patient needs to lie very still for best results.
During the scan everybody except for the patient will leave the room. The radiographer will still be able to communicate with the patient, and vice-versa, through an intercom. If the patient is a child, a parent or adult might be allowed to stand or sit nearby - that person will have to wear a lead apron to prevent radiation exposure.
CT scans and pregnancy - any woman who suspects she may be pregnant should tell her doctor beforehand. The UK's National Health Service2 warns that CT scans are not recommended for pregnant mothers because of the risk that X-rays might harm the unborn baby.
CT scans and claustrophobia - patients who suffer from claustrophobia need to tell their doctor or radiographer beforehand, advises Cancer Research UK3. The patient may be given an injection or tablet to calm them down before the scan.
CT scans and breastfeeding - the State Government of Victoria4 in Australia says that mothers should avoid breastfeeding their babies for about 24 hours after a CT scan if an iodinated intravenous dye was used. The dye may pass into the breast milk. If you are breastfeeding, tell your doctor beforehand.
On the next page we look at when CT scans are used, who analyzes the image and the difference between CT scans and MRI scans.