Positron emission tomography (PET) is one of the leading molecular imaging techniques for the non-invasive detection and quantitative assessment of a variety of biochemical processes associated with neoplasms and physiopathological conditions. Following injection of a positron-emitting radiotracer to the patient, the PET scanner measures the biodistribution and kinetics of the administered radiopharmaceutical. However, this modality does not provide the anatomical information necessary for localization of metabolically active lesions. This has motivated the combination of PET with anatomical imaging modalities such as x-ray computed tomography (CT) and magnetic resonance imaging (MRI). The advent of hybrid PET/CT and PET/MRI systems and their introduction in clinical practice has tremendously improved the diagnostic confidence of PET findings. Specifically, the additional information provided by CT and MRI can be exploited for PET image reconstruction and data correction. In PET/CT, CT images can be directly converted to 511-keV attenuation maps for attenuation correction of PET data,...