PET (Positron Emission Tomography)

In contrast to MRI and CT, positron emission tomography (PET) is not used for anatomical imaging. Instead, it measures the functional distribution and pathologyassociated enrichment of a positron-emitting radionuclide within the patient or sample.

 

In general, isotopes with an excess of protons (neutron-deficient nuclei) can undergo nuclear transmutation with β + decay. In β + decay, a proton is transmuted into a neutron, and the excessive positive charge is emitted via the emission of a positron (same mass but opposite charge than an electron) and an electron neutrino ve  (Figure 1B). This positron emission is the basic principle underlying PET imaging. The emitted positron travels through the surrounding tissue until it loses its kinetic energy and annihilates with a random tissue electron (Figure 1B). The annihilation event results in the emission of two annihilation photons (alternative also referred to as or γ-rays) that travel through the sample in nearly opposite directions with the speed of light until they reach the scintillation crystals (or γ-ray detectors, (Figure 1B). The γ-ray detectors work like the X-ray detectors previously presented (see CT). Each of the annihilation photons has an energy of 511 keV (Figure 1B). A PET signal is only generated after the nearly 180 -degree opposite coincidence detection of two 511 keV photons (also known as annihilation coincidence detection or ACD) (Figure 1D). Without scattering, the opposite coincidence detection defines a straight line between two γ-ray detectors which originates at the site of annihilation (Figure 1D). This line is called line of response (LOR). The LOR is used to reconstruct the PET image with a filtered backprojection approach or via iterative techniques, as discussed in the CT.

 

From this perspective, one major constrain of PET imaging becomes clear. Since the positron-electron annihilation site is not identical to the initial positron emission site, there is a loss of resolution (Figure 1B). The positron range is determined by the positron's amount of energy and the surrounding tissue's electron density, which ranges from few hundreds of a micrometer to a few millimetres. However, the main advantage of PET in comparison to MR or CT imaging is its very high sensitivity. 

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In principle, unstable nuclei that undergo β + decay (like 18F, 13N, and 11C) are detectable in PET. The most commonly used PET tracer is 18 F-2-deoxy-D-glucose or FDG (Figure 1A). Here, the positron-emitting 18F is substituted for the normal C2 hydroxy group in glucose (Figure 1A). 18F has a half-life of 110 min and allows the assessment of the glucose metabolism within the sample. FDG is imported into cells like normal glucose, but it cannot be used through glycolysis. Hence, FDG is trapped and accumulates in cells with high glucose metabolisms like active inflammatory cells or malignant cells (Figure 2 and Video 1).