Biomedical Engineering Reference
In-Depth Information
511 keV
Annihilation
Nucleus
e
+
e
-
e
+
Positron scattering
from multiple electrons
in tissue
180
o
±
0.25
e
+
Positron
e
−
Electron
511 keV
Figure 2.1:
Positron emission and annihilation. A positron is emitted from a
proton-rich nucleus, losing energy by scattering from atomic electrons in tissue
before annihilating with an electron to produce two 511 keV photons (or gamma
rays) which are moving 180
◦
(
±
0
.
25
◦
FWHM) apart.
mass converts into energy in the form of two 511 keV photons, which are in-
distinguishable from gamma rays. To simultaneously conserve both momentum
and energy, the photons are emitted 180
◦
to each other. Figure 2.1 shows the
positron annihilation and the emission of two 511 keV photons. The detection
of these two 511 keV photons forms the basis of PET imaging.
2.5 Coincidence Detection
Since the probability that both 511 keV photons will escape from the body with-
out scattering is very high in general, the line along which the positron annihi-
lation occurred (i.e. the line of response, LoR) can be defined if both photons
can be detected with two detectors at opposite ends of the line, as illustrated in
Fig. 2.2. As the distance that a positron traveled before annihilation is generally
very small, this is a good approximation to the line along which the emitted pho-
tons must be located. The scheme for detection of photon emissions is called
