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CT uses X-rays to produce images of the inside of your body and therefore is more fully known as X-ray computed tomography. These scans are sometimes known as CAT scans (computed axial tomography). As you can see from the picture, the patient lies on a scanning bed which passes through a hole in the middle of the CT machine. A beam of X-rays passes through a slice of your body and is picked up on the other side by a bank of detectors.
The X-ray source and bank of detectors rotate all around your body to get views from every angle. The computer converts the information into an image of the slice. Then the machine moves the bed slowly through the hole to obtain images of different slices through your body. The result is a very detailed 3-dimentional picture of the inside of your body.
For some types of CT scans (e.g. brain, chest) you may be given an injection of a contrast agent before the scan. This is a type of “dye” and makes the scan clearer. This contrast agent is not normally harmful, but it may make you feel hot and flushed for a few minutes.
CT scans give detailed anatomical information but do not provide functional information. For that you would need to have a nuclear medicine scan. For example, if a tumour has been treated with radiotherapy, it may still appear on the CT scan, even if it is dead tissue. A nuclear medicine scan however can show whether the tumour tissue continues to function (i.e. the tumour is still growing).
CT scans are not dangerous. Although you are exposed to a small amount of radiation, it is still a very low risk examination (see our section on radiation risks). If you are pregnant, your case will be assessed by your doctors who will decide whether or not to postpone the scan. If you would like more information, the National Radiological Protection Board (NRPB) in the UK has produced a leaflet on the safety of X-ray examinations which can be found at
http://www.nrpb.org/publications/misc_publications/x-ray_safety_leaflet.htm