I was waiting for this paper to be published. Basically, xrays are deflected more by higher "Z" materials and this contrast is what creates the pixel values in the image. This means the heavier nuclei like that in calcium in our bones (z=20) deflect them a lot better than the carbon in our soft tissues. I'm no explosives expert but I understand that these are made from the same elements in our soft tissue like H,C,N,O (z=1,6,7,8).
edit:
Drugs too. Generally they are just organic molecules.
edit: Drugs too. Generally they are just organic molecules.