It is used in medicine as a diagnostic tool and as a guide for interventional procedures. To improve the quality of soft tissue images a contrast material such as barium (administered orally or rectally) or intravenous iodinated contrast is sometimes used. This is especially useful in scans of the abdomen where otherwise the appearance can be quite confusing.
Although most common in healthcare CT is also used for non-invasive examination in many fields.
The system was invented in 1972 by Godfrey Newbold Hounsfield of EMI Laboratories using gamma rays. Allan McLeod Cormack of Tufts University independently invented the same process and they shared a Nobel Prize in medicine in 1979. The first scanner took several hours to acquire the raw data and several days to produce the images. Modern multi-detector CT systems can complete a scan of the chest in less time than it takes for a single breath (useful if the patient cannot hold his/her breath) and display the computed images in a few seconds.
Pixels in an image obtained by CT scanning are displayed in terms of relative brightness. The pixel itself is displayed according to the mean attenuation of the tissue that it corresponds to on a scale from -1000 to +1000 on the Hounsfield scale. Water has an attenuation of 0 Hounsfield units (HU) while air is -1000HU, bone is typically +400HU or greater and metallic implants are usually +1000HU.
The radiation dose from CT scans is several times higher than conventional X-ray scans.
Improvements in CT technology have meant that the overall radiation dose has decreased, scan times have decreased and the ability to recalculate images (for example, to look at the same location from a different angle) has increased over time. In 2003, the cost of an average CT scanner was 1.3 million dollars.
The word tomography is derived from the Greek tomos (slice) and graphia (describing).
See also: Hounsfield scale