The development of hybrid probes for multimodal molecular imaging is changing the diagnosis and characterisation of complex pathologies. The fusion of the outstanding sensitivity of positron emission tomography (PET) and the excellent anatomical resolution of magnetic resonance imaging (MRI), provides an ideal combination of structural and anatomical information, which is key to expand the application of molecular imaging for the diagnosis of complex and multifactorial diseases. Iron oxide nanoparticles (IONPs) have been traditionally used as negative (T2) contrast agents, darkening tissues or areas in which they accumulate. However, in recent years there has been a great amount of research focusing on the production of IONPs for positive (T1) contrast MRI. One of the most appealing properties that nanoparticles usually offer is the possibility of tailoring their synthesis and functionalisation to obtain probes that generate or enhance the signal in more than one imaging modality, via incorporation of moieties in the core or the surface of the nanoparticle. In this thesis, we developed a novel microwave-assisted method for the synthesis of IONPs coredoped with the positron emitter 68Ga and with relaxometric properties suitable for hybrid T1 MRI/PET imaging. Full characterisation revealed optimal radiochemical properties for PET imaging and excellent relaxometric properties for T1 MR imaging, demonstrating that the combined use of nanotechnology and radiochemistry can render an innovative tool for the dual imaging of biological processes in vivo…