A CAT Scan is “Computerized Axial Tomography” (also known as a CT Scan), which is a process of using computers to make a 3-dimensional image from a 2-dimensional picture (X-ray). During the process, a series of x-ray photographs are taken from different angles and then combined by computer into a composite 3-dimensional representation. So, pictures are taken one slice at a time going through the whole object and then the slices are combined to get a great visual of the whole thing.
A large donut-shaped x-ray machine takes x-ray pictures at several different angles around the body. These images are processed by a computer to create cross-sectional pictures of the body. In each of these pictures, the body is seen as an x-ray “slice” of the body, which is recorded on a film. It’s like looking at a loaf of bread that has been sliced up–you can see the whole thing and also remove slices and see the entire slice and everything inside of it.