spatial resolution as a trade-off. The overall sensitivity in 2-D acquisition is
2 to 3% at best.
To improve further the sensitivity of PET scanners, the three-dimensional
(3-D) acquisition is employed in which the septa are retracted, or they are
not included in the scanner (Fig. 13.9B). In this mode, all events detected
by detectors in coincidence in all rings are counted including random and
scatter events, and the sensitivity in the 3-D mode increases four- to eight-
fold over 2-D acquisition. The incidences of random and scatter can be
reduced by having a smaller angle of acceptance; that is, a detector is con-
nected to a smaller number of opposite detectors.
Data Acquisition 1952-D 3-D
A
Block
detectors
Septa
B
Direct
plane
Cross
plane
Fig. 13.9. (A) 2-D data acquisition with the septa placed between the rings so that
true coincidence counts are obtained avoiding random events and scatters. Detec-
tors connected in the same ring give direct plane events. However, detectors are
connected in adjacent rings and cross plane data are obtained as shown. (B) When
septa are removed, the 3-D data acquisition takes place, which includes random
and scatter events along with true events. (Reprinted with the permission of the
Cleveland Clinic Foundation.)