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Computed Tomography

Patient about to enter a CT scanner

Patient about to enter a CT scanner (Zinkevych, iStockphoto)

Learn about the history, function, uses, benefits and risks of computed tomography (CT) as a medical imaging technology.

X-ray Computed Tomography (x-ray CT or CT) uses x-rays and a computer to generate images that look like thin slices of tissue. The word tomography comes from the Greek words tomo, meaning ‘to slice’, and graphia, meaning ‘to write’ (writing with slices). 


The invention of CT is generally credited to Sir Godfrey Hounsfield and Allan McLeod Cormack. One day in 1967, Hounsfield came up with the idea that you might be able to determine what was inside a box by taking x-ray readings at all angles around the outside of the box. In order to test his idea, he built a prototype CT machine. It included a computer that could take input from x-rays at various angles to create an image of the object in ‘slices’ (see Figure 4). He tested his prototype first on a preserved human brain, then on a fresh cow brain from a butcher shop, and later on himself. At the time, Hounsfield was not aware of the work that Allan Cormack had done on the mathematics of such a device. Together their work led to a shared Nobel Prize in Physiology or Medicine in 1979. The first CT machine (called a CT scanner) was located in England and the first scan of a patient’s brain was on October 1, 1971. These first images took about five minutes to record and 2½ hours for the computers to process. The scanner had a single detector and used a single beam of x-rays.

The first CT machine prototype
The first CT machine prototype (Source: Gdh~commonswiki [CC BY-SA] via Wikimedia Commons).

How it Works

Modern CT scanners work by rotating an x-ray tube paired with a digital x-ray detector around a part of the body.

Modern CT scanner
Modern CT scanner (Source: liz west from Boxborough, MA [CC BY] via Wikimedia Commons).

The scanner takes many images. These are then processed using computers and mathematical calculations, creating a series of images from one end of an object to the other.

CT scans of a human from the base of the skull (top left) to the top of the head (bottom right)
<p>CT scans of a human from the base of the skull (top left) to the top of the head (bottom right) (Source: Department of Radiology, Uppsala University Hospital. Uploaded by Mikael H&auml;ggstr&ouml;m via <a href="">Wikimedia Commons)</a>.</p>

Since these images are all taken along one axis (i.e., along the of the body), this process was originally called Computed Axial Tomography or CAT. You may have heard the term CAT scan before or have known someone who has had a CAT scan. CAT scans and CT scans are two different terms for the same process. CT scan is the preferred term today. This is because images can be combined using computer software to form a 3-dimensional (3D) image. 


Like x-rays, CT scans are often used to look for injuries or pathology (abnormalities) in the body. They are most commonly used on the neck, chest, abdomen and pelvis. CT scans are often used to look for brain tumours and blood clots in the brain because their rays can go through the skull. CT scans are also used to look at tissues in the heart and lungs. In some cases, a patient may go back for a series of CT scans so that the doctor can monitor how something, such as a tumour in the lungs, grows and changes.

Benefits & Risks

The greatest benefit of CT scans is that they generate 3-dimensional images. This allows doctors to look at a body part from many different angles. Also, since doctors can look at one thin slice at a time, they can pinpoint issues in very small areas (see Figure 6). Other advantages of CT scanners over regular x-ray machines is that CT scanners can vary the intensity (energy level) of the x-rays. This allows the person looking at the images to see the various body parts with better precision. The drawback of CT scans is that they expose the patient to more radiation than regular x-rays. Over time, this can lead to higher risks of cancer, but most people do not get enough CT scans in their lifetimes for this to be an issue. 

How Does a CT Scan Work?

This video by National Institute of Biomedical Imaging and Bioengineering illustrates how CT scans work, the information they can provide, and some of the images they produce. 

CT Scan

This article from Mayo Clinic explains why you might need a CT scan, the risks, and what to expect throughout the procedure.



Harris, T. (n. d.) How CAT Scans Work. HowStuff Works. 

OpenLearn from the Open University. (2008). CT scan - Imaging in medicine. YouTube.