Image Processing Reference
In-Depth Information
Figure4.27 Heart muscle viability indicated by PET scans: left image shows loss of
blood flow in regions, right image shows metabolic activity. (CourtesyofDr.DavidLilien,
BiomedicalResearchFoundationofNorthwestLouisiana)
The brain of a person with Alzheimer's disease exhibits decreased metabolic
activity, which can be imaged with PET techniques. Figure 4.28 shows pairs of
images of two different brains: one normal, the other of a person with Alzheimer's
disease. The two images of each brain are tomographic “slices”; i.e, images of
a thin cross-section at two different positions, which is why they differ in size
(like two slices from a pineapple, one near the end, the other in the middle). The
pseudocolor indicates levels of metabolic activity, with red being the strongest and
blue being the weakest. The arrows indicate regions of the diseased brain that show
reduced metabolic activity. PET can serve as an early indicator of the disease,
as much as several years before other clinical methods would result in a similar
diagnosis. Figure 4.29 shows two slices of an epileptic child's brain. The arrows
indicate regions of the brain in the right hemisphere where glucose metabolism is
decreased. Removing this tissue with surgery cures the seizures about 80% of the
time.
Radioactive materials that emit gamma rays through nuclear decay processes
do so at discrete energies. A gamma-ray detector that is sensitive to the energy
of the gamma-ray photons can be used to identify these radioactive materials.
One can incorporate energy-sensitive detectors into an imaging system known
as an imaging spectrometer. It is in some ways like color television, in that the
color (wavelength or equivalently, energy) information is preserved and can be
quantified, unlike a black and white television system that only displays intensity
information. Scientists at Lawrence Livermore National Laboratory devised a
gamma-ray imaging spectrometer, or GRIS, that allows one to determine not only
the intensity of gamma rays from radioactive decay, but also the precise radioactive
element that gave rise to them in the first place. 9 Imaging gamma rays onto an array
of detectors is difficult, since conventional lens designs will not work. A pinhole
9 K. Ziock, “Gamma-ray imaging spectrometry,” LLNL Science and Technology Review, (October
1995).
 
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