CAT Scans So what's a CAT scan, anyway? "Computed Axial Tomography" is the process of using computers to generate a three-dimensional image from flat (i.e, two-dimensional) x-ray pictures, one slice at a time... What do you mean by "slice"? Well, imagine trying to visualize the inside of something like a fruitcake. A fruitcake? There's a lot of complexity on the inside that you can't guess from the outside, bits of fruit and nuts all over the place. How could you go about exploring the inside of a whole fruitcake? Oh, I see what you mean now. If I sliced it up I could look at each slice and get an overall picture. Right! Well, CAT Scanners do the same thing, but rather than actually cut people up, they use x-rays to make pictures of the slices.   (There is an effort underway, called the Visible Human Project, where they are actually slicing up a (very dead) person and photographing each thin slice.) OK, but we just said that CAT scans are useful because x-ray machines only take flat, two dimensional pictures and we want three dimensional pictures. So why is the process of imaging the flat "slices" difficult? Isn't making flat pictures what x-ray machines do? The problem is that the flat x-ray we get is of the whole thing. It would be like crushing our fruitcake under a steam roller and saying, "There! Now we can see everything!" But we would have lost all the information about how "deep" the pieces of fruit and nuts were. So how do we take x-rays of a slice in the middle? We could just turn the fruitcake sideways, but then all the other slices would get in the way. How do we take x-rays of only one slice? Very good! That's exactly the right question to ask. Let's explore that.