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The image illustrates the processing principals of a positron emission tomograph (PET) commonly used during cancer diagnostics. It shows how during the annihilation process two photons are emitted in diametrically opposing directions. These photons are registered by the PET as soon as they arrive at the detector ring. After the registration, the data is forwarded to a processing unit which decides if two registered events are selected as a so-called coincidence event. All coincidences are forwarded to the image processing unit where the final image data is produced via image reconstruction procedures.

Photo credit: Public Domain