Positron emission tomography (PET) is a non-invasive technique that can be used to image the function, i.e. metabolism, rather than the anatomy in a living system. As such, it is complementary to anatomy imaging modalities like X-ray CT and Magnetic Resonance Imaging (MRI). In PET, positron emitting radionuclides are introduced to the living system to be used as tracers. A positron emitted from the isotope nucleus annihilates with an electron at rest to give out two 511-keV γ-rays emitted at opposite directions. The resulting γ-rays can be detected by γ-ray detectors arranged in one or more circular rings that are part of the PET tomograph.