At the moment scanning looks at x-rays that pass straight through an object, creating a ‘shadow picture’ of what’s inside. You can tell how dense and thick something is but it doesn’t tell you what the material is. If you had a explosive cut in the shape of a shoe it would look like a shoe on an x-ray image.
By instead detecting the x-rays that are scattered either backwards, or forwards though small angles, you find out more about what the material actually is. I work with x-rays acttered forwards, which form patterns called difraction patterns, which depend on the spacing between the moleculs so are like fingerprints for each material. This makes it possible to identify materials as well as shapes.