Biomedical Engineering Reference
In-Depth Information
(A)
(B)
(C)
(D)
Figure 2.4:
Types of coincidence event recorded by a full-ring PET system.
The white circle indicates the site of positron annihilation, and the solid line
represents the gamma ray, (A) true coincidence, (B) scattered coincidence, (C)
random (or accidental) coincidence, and (D) multiple coincidence. The mispo-
sitioned line of response is indicated by the dashed line.
Scattered coincidence
occurs when one or both of the emitted photons un-
dergo a Compton scatter interaction in tissue. Compton scattering causes a loss
in energy of the photon and change in direction of the photon. Since the direction
is changed, the origin where the photons were emitted cannot be located cor-
rectly and, as a result, the event is mispositioned, leading to decreased contrast
and deteriorated quantification.
A
random
(or
accidental
)
coincidence
occurs when two unrelated photons,
which have not originated from the same site of positron annihilation, strike
opposing detectors within the coincidence timing window. Since the random
events are produced by photons emitted from unrelated decays, they are spa-
tially uncorrelated with the activity distribution. The random coincidences are
