Image Processing Reference
Figure4.28 Normal brain (left) and brain with Alzheimer's disease (right) PET scans.
Figure4.29 Epileptic brain PET scan. (CourtesyofDr.MichaelE.Phelps,UCLASchool
will act as a lens for gamma rays, but a single pinhole gives very low light-gathering
efficiency. The gamma-ray imaging spectrometer has acoded-aperturemaskthat
consists of hundreds of pinholes stamped in a special pattern on a sheet of metal.
Having hundreds of pinholes greatly increases the light gathering power of the
imager over what it would be with a single pinhole. Behind the coded-aperture
mask is a position-sensitive detector that converts the gamma rays into electrical
signals on a grid of wires. The signals are then read out by a computer that uses the
coded aperture mask pattern to calculate a real image.
The measurement of gamma-ray photon energy in an image has some interesting
applications in the handling of nuclear material. As discussed earlier, radioactive
elements that emit gamma rays do so at particular energies. It is possible to identify
nuclear materials by the gamma rays they emit. For example, Fig. 4.30 shows two
false-color images of a 1/8-inch thick sheet ofdepleteduraniumwithplutonium
rods behind it. The color bars are maps of gamma-ray intensity, where white and
green correspond to the lowest intensity, up through red to blue for the highest
intensity. In the top image, the energy sensitivity is set to 100 keV, corresponding to
the energy of gamma rays from depleted uranium. In the bottom image, the energy