Image Processing Reference
place an x-ray tube inside a pipe and supply it with high voltage, but a gamma-ray
source can be easily pushed or dragged through even a small pipe. One common
gamma-ray source design consists of a small titanium capsule that contains the
isotope. The capsule is kept in a lead container with thick walls when not in use,
to protect people from gamma-ray exposure; unlike an x-ray tube, one cannot turn
off a radioactive source. The gamma-ray images are made with x-ray film in light-
tight packages placed on the other side of the target from the gamma-ray source.
This technique can be used to image through several inches of steel, as shown in
The examples of mummies in sealed caskets demonstrate how useful x-ray
imaging is in non-invasive applications. Gamma-ray imaging has similar uses in
archaeology and art conservation, especially when it is used on particularly thick,
dense materials. Figure 4.25 shows a marble statue named Crouching Aphrodite
imaged in visible light and with gamma rays from a radioactive source made of
an isotope of the element iridium. The gamma-ray image shows that the head
of the statue fell off and was repaired with metal rods. This shadowgram was
recorded on film and is a negative image like most medical x-ray images: the
lightest regions correspond to the greatest thickness of material, particularly the
combined thickness of the arm around the elbow and the head directly behind it.
Gamma-ray imaging has proven to be as useful in medical research as x-ray
imaging, but it is used rather differently. X-ray imaging of tissue is active, that is,
there needs to be an external x-ray source to illuminate the subject. The gamma-ray
Figure4.24 Gamma-ray imaging with radioactive source and film.