FLC - Forschung mit Lepton Collidern

Brief introduction to Positron Emission Tomography

Positron Emission Tomography (PET) is a non-invasive medical imaging technique. A positron emitter is used to mark a tracer which is injected into a living organism. By e+e- annihilation inside the organism two 511 keV photons are produced (see figure 1).
A positron emitted by the tracer atom annihilates with an electron in the tissue.

Figure 1: A positron emitted by the tracer atom annihilates with an electron in the tissue.

They are detected in coincidence and their line of response is identified (see figure 2). The reconstruction of several lines of response allows to retrieve a functional image of the tissue in which the tracer is absorbed.
Two 511 keV photons which are detected in coincidence form a line of response (LOR).

Figure 2: Two 511 keV photons which are detected in coincidence form a line of response (LOR).

The energy and the time resolution of the used gamma-detectors are two important parameters which determine the overall performance of the PET scanner. A good energy resolution is crucial in order to reject photons which have undergone Compton scattering inside the organism and thus changed their direction. The time resolution of the gamma-detectors determines the minimal width of the coincidence window and thus the amount of background arising from random coincidences. Furthermore, the time information can be used to directly improve the image quality especially for heavier patients using time-of-flight techniques (see figure 3).

Figure 3: The TOF-PET principle.

Apart from the requirements on the energy and time resolution a small pixel size of the PET camera, i.e. a high granularity, is required since this mainly determines the spatial resolution.

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