According to a paper in the Journal of Transportation Security:
While carelessly placed contraband will be detected, the machines have glaring blind-spots and have difficulty distinguishing explosives from human tissue. As they write, ‘It is very likely that a large (15–20 cm in diameter), irregularly-shaped, cm-thick pancake [of PETN explosive] with beveled edges, taped to the abdomen, would be invisible to this technology. … It is also easy to see that an object such as a wire or a boxcutter blade, taped to the side of the body, or even a small gun in the same location, will be invisible.
And that doesn’t even get at what you could tape to the bottoms of your feet.
These machines are an expensive boondoggle, designed to benefit their manufacturers who lobby for the appropriations, not to actually ‘make us safe’.