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2.6 PET and PET/CT 39

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of endogenous component. The main drawback of using C is related to its short
half-life (around 20.4 min). However, it may be a good candidate for developing
cost-effective cyclotrons [46]. Recently, drug-based tracers have absorbed great deal
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of attention such as [N-methyl- C]Vorozole and F-RGD peptide.
2.6.1 Emission detection in PET
As already mentioned, a positron (also called positive electron) annihilates with elec-
tron causing the release of two photons moving in opposite directions having the
energy of 511 keV. The photons then will be detected using scintillation crystals con-
nected to photomultiplier tubes. Since the radionuclides typically use in PET imag-
ing emit photons with higher energy than the radioisotopes use in nuclear medicine,
utilizing detectors with higher stopping power is needed. Crystals which are mainly
used for PET imaging have high atomic numbers and densities. The efficiency of
coincidence detection for 25-mm bismuth germinate and lutetium oxyorthosilicate
is around 80%. It should be noted that the absorption efficiency of bismuth germi-
nate crystal is higher than lutetium oxyorthosilicate due to its higher atomic number,
while lutetium oxyorthosilicate crystals emit higher light than bismuth germinate.
Gadolinium silicate possess a lower atomic number than lutetium oxyorthosilicate
and bismuth germinate. Gadolinium silicate crystals can detect photon energy in the
wide range and emit slightly higher light than bismuth germinate.
The crystals are placed into block detectors. Block detectors compose of some
channels fill with opaque materials and contain many small crystals. The advantages
of using block detector over single crystal is that each small crystal acts as an inde-
pendent system with lower detector dead time.
Due to the kinetic energy, proton travel a small distance before annihilation
called mean positron range (as shown with “a” in Fig. 2.6). Based on the density of
the environment, mean positron range is different, for example, it is smaller for bon

FIGURE 2.6 Principle of PET production. P, proton; N, neutron.