Lutetium Oxyorthosilicate Detector
Lutetium oxyorthosilicate (Lu 2 [SiO 4 ]O(Ce) or LSO) doped with cerium is
another solid detector that is used for scintillation counting in PET imaging.
LSO has a shorter scintillation decay time (40 ns) than BGO that favors the
use of a narrow pulse window to cut down random coincidences in PET.
Also, its higher light output gives a better energy resolution than BGO.
These detectors have high efficiency for photon detection and can be fab-
ricated in the size of a few millimeters. Many commercial manufacturers
use LSO detectors in place of BGO detectors in clinical PET scanners and
in micro-PET scanners for scanning small animals.
Gadolinium Oxyorthosilicate Detector
Gadolinium oxyorthosilicate (GSO) is another detector that can be used
for coincidence counting in PET imaging. Even though it has lower light
output and stopping power than LSO, its better energy resolution has
prompted some commercial manufacturers to use it in PET scanners. GSO
crystals are fragile and great care is warranted in their fabrication.
Yttrium Oxyorthosilicate Detector
Yttrium oxyorthosilicate (YSO) is another inorganic crystal similar to LSO
introduced for scintillation counting. The scintillation decay time of YSO is
70 ns and it gives high light output. A combination detector of YSO/LSO
has been reported for potential use in simultaneous single photon and
coincidence imaging. YSO detects low-energy photons and LSO detects
511 keV photons, and the two pulses are readily separated by pulse shape
discriminators.
Semiconductor Detectors
Germanium and Silicon Detectors
Semiconductor detectors or solid-state detectors are made of germanium
or silicon materials commonly doped with lithium. These detectors are
designated as Ge(Li) or Si(Li) detectors, of which the former are commonly
used for high-energy g-ray detection and the latter for a-particle and
low-energy radiation detection. Currently, high purity germanium (HPGe)
alone is commonly used. The basic principle of operation of these detectors
involves ionization of the semiconductor atoms, as in gas detectors. Ioniza-
tions produced in the detector by radiation are collected as current and con-
verted to voltage pulses through a resistor by the application of a voltage.
The pulses are then amplified and counted. The size of the pulse is pro-
portional to the radiation energy absorbed in the detector, but does not
depend on the type of radiation.
84 8. Scintillation and Semiconductor Detectors