A recently published report confirms that full body "backscatter" scanners employed by the TSA are incapable of detecting certain types of PETN-based explosives taped to the human body.
"It is very likely that a large (15-20 cm in diameter), irregularly-shaped, cm-thick pancake with beveled edges, taped to the abdomen, would be invisible to this technology, ironically, because of its large volume, since it is easily confused with normal anatomy," explained security experts Leon Kaufman & Joseph W. Carlson.
"Thus, a third of a kilo of PETN, easily picked up in a competent pat down, would be missed by backscatter 'high technology.'"
According to the researchers, 40 grams of PETN - a purportedly dangerous amount - could easily fit into a virtually invisible 1.25 mm-thick "pancake."
"The images are very sensitive to the presence of large pieces of high Z material, e. g., iron, but unless the spatial resolution is good, thin wires will be missed because of partial volume effects.
"[Of course], it is also easy to see that an object such as a wire or a box- cutter blade, taped to the side of the body, or even a small gun in the same location, will be invisible.
"[So], while there are technical means to mildly increase the conspicuity of a thick object in air, they are ineffective for thin objects such as blades when they are aligned close to the beam direction.”