Backscatter x-rays are useless

Researchers at U.C. San Francisco say that backscatter X-rays are basically useless for detecting explosives:

...although images can be made at the exposure levels claimed (under 100 nanoGrey per view), detection of contraband can be foiled in these systems. Because front and back views are obtained, low Z materials can only be reliable detected if they are packed outside the sides of the body or with hard edges, while high Z materials are well seen when placed in front or back of the body, but not to the sides. Even if exposure were to be increased significantly, normal anatomy would make a dangerous amount of plastic explosive with tapered edges difficult if not impossible to detect.

Of course, this is not terribly surprising when you consider that someone apparently was able to smuggle an entire dead body aboard an airplane last month.