Biomedical Engineering Reference
In-Depth Information
photo cathode
dynodes
photons
anode
R
R
R
power
supply
Fig. 6 Schematic of a photomultiplier tube (PMT). Photoelectrons are ejected from the photocath-
ode and accelerated onto the 1st dynode after scintillation photons have reached the photomultiplier
window. The charge multiplication occurs along the dynode chain where more electrons are
successively ejected
causing the ejection of even more electrons in the second dynode. At each dynode
the process is repeated successively, thus producing an avalanche of electrons (from
10 7 to 10 10 electrons) that are collected at the anode originating an electrical signal.
The photomultiplier output is a short duration (
microseconds) current pulse
which, after being amplified, is analyzed by a pulse height analyzer. This circuit
allows the discrimination of the photon energy because there is proportionality
between the amplitude of the generated signal and the energy of radiation incident
on the photocathode, which is also directly related to the energy of radiation incident
on the crystal. The detected pulses can then be counted and associated to a particular
spatial position that is determined using the Anger logic [ 9 ].
In PET systems, PMTs have been used in block detectors, which consist of
an array of several dozen crystals coupled to a small number of photomultipliers
(typically 4). The signals from the photomultipliers are integrated for a certain
period of time so that their amplitudes are analysed, allowing to determine the
photon energy and the instant of detection. Since this time information will be used
to determine if two 511 keV photons were detected in coincidence (called a true
coincidence), small and fast photodetectors are necessary (given the small sizes of
the crystals used in PET, which are directly related with spatial resolution). For these
reasons, alternative photodetectors based on semiconductors are replacing the use
of conventional PMTs in PET, such as Avalanche Photodiodes (APDs) and Silicon
Photomultipliers (SiPMTs).
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