Image Processing Reference
In-Depth Information
Figure4.25 Visible (left) and gamma-ray (right) images of a marble statue. (Courtesyof
CEA)
imaging technique known as the PET scan does not require an external gamma-
ray source. Instead, the subject is made to emit gamma rays. PET stands for
positron emission tomography, a method of imaging chemical processes within
living tissue. It works in the following way: specialtracermolecules are ingested,
inhaled, or injected into living tissue in the form of a drug “cocktail.” They are
then selectively concentrated in tissue via processes (such as metabolism) that are
normally not directly observable. The tracers can be compounds such as simple
sugars that are specially prepared to contain one or more radioactive atoms that
spontaneously emitpositrons. The positrons are antimatter: positively charged
electrons that rapidly collide with electrons in neighboring atoms. The collision
results in the annihilation of both the positron and electron and the creation of
two gamma rays with energies of 511 keV (equal to the energy of a positron
or electron). This powerful technique allows physicians to observe biochemical
processes directly and compare them with clinical data in order to detect some
abnormality noninvasively.
The PET scanner detects gamma rays with a ring of gamma-ray detectors placed
around the subject, as shown in Fig. 4.26. The gamma-ray detectors, known as
scintillation crystals, convert the gamma rays into visible light, which is then
detected by a high-speed light detector. A computer analyzes the electrical signals
from the light detectors and generates a 3D image. The physics of the annihilation
process dictates that the two gamma rays are emitted in exactly opposite directions.
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