PET/CT systems are so-called multimodal imaging devices. The main motivation for uniting the two different imaging processes is the possibility of gaining anatomical (CT) and functional (PET) images of the patient at the same time (during the same examination).
The radiation exposure of a PET/CT examination is around 7-8 mSv (when the CT is only used to localize the PET image in low resolution), which corresponds to being exposed to background radiation for about 3 years, having 350 X-ray images taken or spending ~1 year working on an airplane.
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