Image Processing Reference
Figure4.14 Visible and x-ray images of a painting. (CourtesyoftheJ.PaulGettyMuseum,
object from the thousands of column density measurements. By comparing x-ray
density values (known as CT numbers) of material samples to the CT number of a
hidden object in the scan, it is possible to identify the actual material of a particular
feature of the CAT scan. For example, a medical CAT scan of a human torso with
a bullet lodged within would return a CT number equivalent to that of lead for the
volume elements in the scan corresponding to the location of the bullet, effectively
locating the bullet's position. The 3D image of the bullet's location would guide a
surgeon's decisions about how best to extract it.
A beautiful example of x-ray CAT scanning technology put to novel use is in
the field of archaeology and the examination of mummified remains. 4 Figure 4.15
shows an Egyptian mummy from the22 nd dynasty (945-715 BCE) being loaded
into a CAT scanner at St. Thomas' Hospital in London. The mummy, a priestess
by the name of Tjentmutengebtiu, currently resides in the British Museum. The
curators there chose her as the subject of a CAT-scan analysis because she was
mummified at a time when the mummification process was at its technical prime.
Also, her cartonnage 5 casket is tightly sealed and cannot be opened without
damage, requiring non-invasive techniques to study the mummy inside.
Figure 4.16 shows a frontal CAT-scan image of the mummy. Several amulets
are visible: a hawk at the breastbone and a winged figure (possibly a vulture) at
the pubis. A winged goddess was placed over the front surface of the neck, and
an unidentified object was attached to the left arm. A small object is visible over
4 Baldock et al., “3-D reconstruction of an ancient Egyptian mummy using x-ray computer
tomography,” Journal of the Royal Society of Medicine87, 806-808 (1994).
5 Cartonnage is made of linen glued together in many layers with a stucco or gesso coating on top. It
is strong and has an excellent surface for painting.