CHAPTER 20: Radiotracers for PET and MR-PET Imaging

Positron emission tomography (PET) combined with magnetic resonance (MR) is a rapidly expanding imaging modality. For this application, the availability of radiotracers labelled with positron emitters is a basic requirement. These short-lived neutron deficient positron emitters are produced at a cyclotron. In a cyclotron, charged particles are accelerated up to energies enabling to overcome Coulomb repulsion and induce nuclear reactions. The beam is extracted to target systems containing stable isotopes. Due to the bombardment, these stable isotopes are converted via nuclear reactions into radionuclides. The positron emitters are subsequently used for radiolabelling molecules to obtain radiotracers for various applications. Usually radiosyntheses are carried out in automated synthesis modules. The relatively short physical half-life of the positron emitters in use requires fast and efficient radiolabelling strategies that have been optimised for the most common radiopharmaceuticals in order to facilitate robust tracer production in sufficient activity amounts. Beside the PET alone approach, emerging methods deal with simultaneous PET-MR imaging to combine the high sensitivity of PET with the high resolution of MR. One possibility to exploit the synergy of both techniques is the combination of clinically established paramagnetic contrast agents and radiopharmaceuticals. This method was studied in several preclinical and clinical studies. A much more elegant possibility is the introduction of both modalities in one single, bimodal contrast and positron-emitting probe. Thus far, this technique was only tested in preclinical set-ups.