PM Tubes and Pulse-Height Analyzers 183Detector for PET
Several types of scintillation detectors are available for PET scanners, of
which BGO and LSO are most commonly used by commercial manufac-
turers because of their properties (illustrated in Table 8.1). NaI(Tl) was ini-
tially used in earlier PET scanners, but nowadays only one vendor uses this
detector (C-PET, marketed by Philips Medical Inc.). Both BGO and LSO
are not hygroscopic, and hermetic sealing is not required. Both detectors
have high stopping power (high density and linear attenuation coefficient),
but LSO has shorter scintillation decay time (40 ns) than BGO (300 ns) and
has higher light output (29 photons vs. 6 photons). Currently, LSO has
become the preferred detector in PET scanners. However, due to the intrin-
sic property, its energy resolution is poor. It is to be noted that LSO con-
tains a naturally occurring radioisotope of its own,^176 Lu, with an abundance
of 2.6% and a half-life of 3.6 × 1010 years. This radionuclide decays by emis-
sion of b−rays and g-rays of 88 to 400 keV. However, the activity level is
low enough to ignore radiation exposure from^176 Lu, and its photon energy
lower than 511 keV does not interfere in PET imaging.
GSO has been used in PET scanners by some manufacturers, despite its
lower light output and stopping power than LSO. These crystals are fragile
and require great care in fabrication.
BaF 2 has the shortest decay time of 0.6 ns and is primarily used in time-
of-flight PET scanners that are rarely used clinically.
PM Tubes and Pulse-Height Analyzers
Like conventional cameras, PET scanners use PM tubes and pulse-height
analyzers (PHA) for determining the locations of the origin of pulses and
their amplitudes. As mentioned in Chapter 8, PM tubes convert light
Table13.1. Characteristics of PET radionuclides.
Radionuclides Half-life Mode of decay (%) Eb+max(MeV)
(^11) C 20.4 min b+(100) 0.970
(^13) N 10 min b+(100) 1.2
(^15) O 2 min b+(100) 1.74
(^18) F 110 min b+(97) 0.64
EC (3)
(^68) Ga 68 min b+(89) 1.90
EC(11)
(^82) Rb 75 s b+(95) 3.15
EC(5)
(^124) I 4.2d b+(23) 2.14
EC(77)