188 13. Positron Emission Tomography
limit, and all other annihilation events overlapping in their time widths (Fig.
13.3C) will be counted as coincident events. Thus the timing resolution or
coincidence timing window has to be a minimum of 2t. The typical coinci-
dence timing window of conventional PET scanners is set at 6 to 20 ns.
In a PET scanner, each detector element is connected by a coincidence
circuit with a time window to a set of opposite detector elements (both in
plane and axial). The number of opposite detectors can vary from one to a
maximum of half the total number of detectors present in a ring. Each
detector element can be connected in coincidence to a maximum of half
the total number Nof opposite detector elements (N/2). Note that less than
N/2 detectors can also be connected. As shown in Figure 13.4, depending
on the number of opposite detectors connected, each detector element has
a number of projections that define an angle of acceptance. These angles of
acceptance for all detector elements in the ring form the transaxial FOV.
The angle of acceptance increases with the number of opposite detector
elements connected in coincidence and, hence, the FOV.
Center of
PET unitAcceptance angle
for a detectorField of view (FOV)Fig. 13.4. The transverse field of view determined by the acceptance angles of indi-
vidual detectors in a PET scanner. Each detector is connected in coincidence with
as many as half the total number of detectors in a ring and the data for each detec-
tor are acquired in a “fan beam” projection. All possible fan beam acquisitions are
made for all detectors, which define the FOV as shown in the figure. (Reprinted with
the permission of the Cleveland Clinic Foundation.)