Page 355 - Biomedical Engineering and Design Handbook Volume 2, Applications

NUCLEAR MEDICINE IMAGING INSTRUMENTATION 333

Detector 1 Detector 2

Amplifier Amplifier

P P
Coincidence
H H
unit ± x ns
A A

0 1 2

Scaler

FIGURE 11.11 Coincidence detection. Events are counted only if both detectors are hit simultane-
ously. Coincidence detection allows source localization without resorting to lead collimation.
in Fig. 11.11, two single detection systems are used with an additional coincidence module. Each indi-
vidual system will generate a logic pulse when they detect an event that falls in the selected energy
window. If the two logic pulses overlap in time at the coincidence module, a coincidence event is
recorded. PET systems use a large number (>10,000) of detectors arranged as multiple rings to form a
cylinder. Since any one detector can be in coincidence with other detectors in the cylinder, the result-
ing LORs provide sufficient sampling to collect the projection information required for tomography.
The important issues associated with coincidence detection are discussed below.
The intrinsic detection efficiency for a singles detector depends on the atomic number, density,
and thickness of the detector. Ideally, the intrinsic detection efficiency should be 1, but at 511 keV
that is difficult to achieve, although intrinsic efficiency for some of the detectors is greater than 0.8.
Coincidence detection requires that both detectors register an event. Since the interactions at the two
detectors are independent, the coincidence intrinsic efficiency depends on the product of the intrin-
sic efficiency at each detector. As a result, coincidence detection efficiency is always less than that
for a single detector, and that difference gets magnified for low-efficiency detectors. Because of the
need for high intrinsic efficiency, scintillators are virtually the only materials currently used as detec-
tors in PET imaging systems. A list of the scintillators that are used in PET tomographs along with
their properties is given in Table 11.5.
A coincidence event is recorded when there is an overlap of the singles logic outputs at the coin-
cidence modules. The time width of the overlap depends on the scintillation characteristics of the
detectors. For current PET scanners, that width ranges from 6 to 12 ns. Although that is a very short

TABLE 11.5 Summary of PET Scintillator Properties
Property BGO LSO, LYSO GSO
Atomic number 73 65 58
3
Density (g/cm ) 7.1 7.4 6.7
Intrinsic efficiency (20 mm) 0.85 0.82 0.75
Coincidence efficiency (20 mm) 0.72 0.67 0.56
Energy resolution 15% 10% 8.50%
Decay time (ns) 300 40 60