In case you were wondering whether pornoscanners are harder on the vast majority of innocent, non-terrorist fliers, or the minuscule minority of terrorists, wonder no more. From Leon Kaufman and Joseph W. Carlson's "An evaluation of airport x-ray backscatter units based
on image characteristics," published in the Journal of Transportation Security:
The penetration not only distributes exposure throughout the body (this affecting the
calculation of effective dose, which comprises a sum over all organs), but tends to diffuse
the effects caused by contraband materials. Images can be made at low entrance
exposures, but of very poor spatial resolution and S/N. The calculated signal excursions
at high kilovoltage are so small as to make it doubtful that at any reasonable exposure
levels density differences will be noticeable unless the contraband is packed thickly and
with hard edges. Although the excursions are larger at low kilovoltage, they are still
small and in the noise of the device's operational limits. The eye is a good signal
averager at certain spatial frequencies, but it is doubtful that an operator can be trained to
detect these differences unless the material is hard-edged, not too large and regular-
shaped. Anatomic features and benign objects add structured noise that interferes with
signal averaging. Figure 18 shows a widely-distributed backscatter image. On the left
is a complete view of her torso, on the right, a section has been blacked out. While the
breasts are easily recognized at right, without some prior knowledge of the subject, it
would be hard to distinguish the increase of intensity in the superior part of her breasts
from the natural gradients of the image.
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. Thus, a third of a kilo of PETN, easily picked up in a competent pat
down, would be missed by backscatter "high technology". Forty grams of PETN, a
purportedly dangerous amount, would fit in a 1.25 mm-thick pancake of the
dimensions simulated here and be virtually invisible. Packed in a compact mode,
say, a 1 cm×4 cm×5 cm brick, it would be detected.
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. 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. 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.
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