Diagnosis, monitoring and follow-up in clinical oncology traditionally are based on high resolution computed tomography (CT) images. In recent years, functional positron emission tomography (PET) imaging using dedicated radiopharmaceuticals has been recognized as an important imaging modality and adjunct to CT images by providing complementary metabolic information. To overcome the challenges of aligning PET and CT images, acquired in different conditions and separate systems, hybrid PET/CT units have been designed and are commercially available since 2000. A PET/CT scanner combines premier technology from two imaging modalities, making it possible to reveal both anatomy and functional information in a single procedure. [...]