Most of the technical developments of combining emission and x-ray tomography began in the 1990s; the first multimodality scanner combining positron emission tomography (PET) and computed tomography (CT) sequentially was introduced to the medical imaging community in 1991 [1, 2].
Despite the software co-registration issues, limited soft tissue contrast, higher radiation dose (over 10 mSv), and significantly higher cost of the integrated PET/CT system, the stand-alone PET system disappeared in about a 4-year period after the first PET/CT system was introduced . Even though integrating PET and magnetic resonance imaging (MRI) was suggested earlier in the early mid-1990s , it was the successes as well as limitations of PET/CT that further motivated the integration of PET with MRI. It took about 15 years of research and development to arrive at the first commercially available whole-body simultaneous PET/MRI system [5–8]. The main reason for this slow development progress is due to a very complicated, and hence costly, integration process.