Image Processing Reference
Figure4.17 CAT-scan cross section of a mummy. (CourtesyofProf.CliveBaldock)
A more recent CAT scan of a mummy is shown in Fig. 4.18. This mummy, a
man named Nesperennub, is also on display at the British Museum.
X-ray inspection of cargo is commonplace in airports and seaports today, and
large-scale x-ray machines can scan trucks, trains, containers, and palettes, looking
for contraband. Occasionally, these scans reveal hidden items inside of trucks, as
shown in Fig. 4.21, an image made withbackscatterx-raytechnology. 6 This type
of x-ray imaging is based on a scanned beam of x rays that scatter off of atoms in
the object back to an imaging detector, rather than on a detector placed on the far
side of the object that records transmitted x rays. The system preferentially detects
the presence of light chemical elements, such as carbon, oxygen and nitrogen.
Heavier atoms also backscatter x rays, but they then absorb them, reducing the
signal from metals, for example. Light atoms are found in explosives, drugs and
people, all of which are routinely smuggled across borders. The brightest objects
in Fig. 4.21 are those that backscatter x-ray light with the least absorption, and
they are all made of low atomic number atoms: the marijuana concealed behind
a false wall at the front of the trailer, the wooden support structure for the secret
compartment, the tires, and the fuel in the tank.
X-ray searches of living people are problematic on many levels, though
the health risk is essentially nil. Years of negative press and misinformation
has generated a strong public perception that ionizing radiation at any dose is
potentially lethal, even though people are constantly exposed to radiation in their
daily lives, whether they like it or not. People will not easily consent to having their
bodies x rayed, even though the dose administered by modern x-ray equipment
is generally only a fraction of the natural radiation from outer space that one is
exposed to during an airplane flight at 30,000 feet, for example. The backscatter
x-ray imaging technology used to make the truck image also offers a way to image
through clothing to detect contraband. Only minimal radiation is deposited deep
within living tissue, since the x rays only have to penetrate clothing and not the
6 W. Sapp et al., “A mobile x-ray system for non-intrusive inspection of vehicles,” Proceedings:
Harnessing Technology to Support the National Drug Control Strategy, ONDCP International
Technology Symposium, 14-19-14-25 (Chicago, 1997).