Here is the best explanation of how a CAT scan works that I have seen.
It's illustrated by a number of Java demonstrations culminating in a great simulation of how one slice is averaged from many X-Rays.
For those generally unfamiliar with the technology, a CAT scan is Computer Assisted Tomography. The technology is used to get a 3D look inside something (a body, usually) by generating slices with X-Rays. Since X-Rays usually produce a flattened picture the way we're used to thinking about them, rather than a slice, the really fascinating thing for me was how the heck do they generate a slice from multiple X-Rays. According to the explanation I linked to above, it's not quite as complicated as I thought. Narrow X-Rays from multiple angles are stretched out and then superimposed on one another, and in the process of superimposition, an accurate picture of a slice forms. Check out the link and see for yourself.