Two-Dimensional Versus Three-Dimensional
Data Acquisition
Coincident counts detected by a detector pair are called the promptswhich
include true, random, and scatter events described later (Fig. 13.8). To elim-
inate random and scatter events, annular septa (~1 mm thick and radial
width of 7 to 10 cm) made of tungsten or lead are inserted between rings
in multiring PET scanners (Fig. 13.9A). The septa act as do parallel hole
collimators in gamma cameras. They mostly allow direct coincidence events
to be recorded from a given ring and prevent random and scatter from other
rings. This mode of data collection is called two-dimensional (2-D) acquisi-
tion. The use of septa reduces the contribution of scattered photons from
30–40% without septa to 10–15%. To improve sensitivity, detector pairs in
two adjacent or nearby rings are also connected in coincidence. Such cross-
coincidence connections can be made at most among five adjacent rings.
Coincidence events detected by the detectors connected in the same ring
are called the direct planeevents, whereas those detected by detectors
interconnected between different rings are called the cross planeevents.
Although the cross plane events increase the sensitivity, they degrade the
194 13. Positron Emission Tomography
Fig. 13.8.(A)True coincidence events.(B)Random coincidence events detected by
two detectors connected in coincidence along the dotted line. The two 511-keV
photons originated from different positron annihilations.(C)Scattered coincidence
events. One or both of the 511 keV photons from the same annihilation event may
be scattered with little loss of energy and may fall within the PHA window and also
within the coincidence time window to be detected as a coincidence event by two
detectors.