Interest in the use of nanostructured materials in medicine and diagnostics has grown rapidly in recent years. The dimension of so-called nanomedicine results in new medical effects and requires novel, scientifically demanding chemistry. Thus, drug delivery systems with controllable size and shape in the nm range are needed to deliver bioactive agents and drugs for specific pathological and pharmacological purposes with improved bioavailability and pharmacokinetics. Among them organic-inorganic nanoparticles, which show great potential for imaging and diagnosis as well as for clinical therapeutics, have started to gain large interest. In order to develop new high sensitivity contrast agents while minimizing synthetic complexity, we plan to use hybrid polyion complexes obtained from complexation between metal ions and anionic block copolymers. Their formation is mainly driven by electrostatic interactions and entropy gain from the release of counter-ions. Relaxivity properties and high stability in biological medium obtained from preliminary experiments have shown the great potential of these systems. Hence the main objective of this project is to develop new highly stable and versatile medical imaging nano-objects easily generated by the complexation of double hydrophilic block copolymers in presence of metal ions. Incorporation of different family of ions or polymers within these objects will enable to extent the proposed strategy to multimodal imaging (PET, MRI…) and will also enable to target specific probe.