A process for the preparation of coacervate nanoparticles comprising the following steps: a) Providing a water in oil emulsion of a biocompatible polyelectrolyte polymer selected from the group consisting of poly(L-lysine), chitosan, bovine serum albumin, human serum albumin, poly lactic-co-glycolic acid (PLGA), poly lactic-co-glycolic acid-polyethyleneglycol (PLGA-PEG) di and tri-block, poly lactic acid and poly lactic acid-polyethyleneglycol (PLA-PEG) tri and di-block, cellulose derivatives selected from chitosan-carboxymethyl cellulose, n-alginate and hydroxypropyl methyl cellulose (HPMC); starch derivatives, dextran derivatives selected from Poly(N-isopropylacrylamide) (PNIPAAm) and Poly(N-vinylcaprolactam)- hydroxyethylmethacrylate (PVCL-HEMA) grafted on dextran chain, methacrylated dextran (dex-MA) and hydroxyethyl-methacrylated dextran (dex-HEMA), glycidyl methacrylated dextran(dex-GMA); poly (vinylbenzyl trialkyl ammonium), poly (4-vinyl-N-alkyl-pyridimiun), poly (acryloyl-oxyalkyl-trialkyl ammonium), poly (acryamidoalkyl-trialkyl ammonium), poly (diallydimethyl-ammonium), Poly(N-isopropylacrylamide) (PNIPAAm), poly-(hydroxyethylmethacrylate) poly HEMA and maleic acid/diallylamine copolymer; b) Providing an aqueous solution of a biocompatible polyelectrolyte polymer having opposite charges of the polyelectrolyte of step a) selected from the group consisting of: hyaluronic acid, poly(L-glutamic) acid, carraggenan, alginates, pectin, chitin, cellulose derivatives, starch derivatives, dextran derivatives, poly (styrenesulfonic acid), poly (vinylsulfonic acid), poly (acrylic or methacrylic acid), poly (itaconic acid), maleic acid/ diallyamine copolymer and chitosan; c) Adding two crosslinking agents, one to the emulsion and the other one to the aqueous solution; d) Adding a contrast agent for medical imaging either to the emulsion or the aqueous solution; e) Adding the aqueous solution to the emulsion at a constant temperature comprised between 19 and 37 °C and at a pH comprised between 3 and 7, thus obtaining the separation of the coacervate nanoparticles. f) Optionally adding a further contrast agent, and optical tracer or a radiotracer for medical imaging to the coacervate nanoparticles obtained in step e). a) Providing a water in oil emulsion of a biocompatible polyelectrolyte polymer selected from the group consisting of poly(L-lysine), chitosan, bovine serum albumin, human serum albumin, poly lactic-co-glycolic acid (PLGA), poly lactic-co-glycolic acid-polyethyleneglycol (PLGA-PEG) di and tri-block, poly lactic acid and poly lactic acid-polyethyleneglycol (PLA-PEG) tri and di-block, cellulose derivatives selected from chitosan-carboxymethyl cellulose, n-alginate and hydroxypropyl methyl cellulose (HPMC); starch derivatives, dextran derivatives selected from Poly(N-isopropylacrylamide) (PNIPAAm) and Poly(N-vinylcaprolactam)- hydroxyethylmethacrylate (PVCL-HEMA) grafted on dextran chain, methacrylated dextran (dex-MA) and hydroxyethyl-methacrylated dextran (dex-HEMA), glycidyl methacrylated dextran(dex-GMA); poly (vinylbenzyl trialkyl ammonium), poly (4-vinyl-N-alkyl-pyridimiun), poly (acryloyl-oxyalkyl-trialkyl ammonium), poly (acryamidoalkyl-trialkyl ammonium), poly (diallydimethyl-ammonium), Poly(N-isopropylacrylamide) (PNIPAAm), poly-(hydroxyethylmethacrylate) poly HEMA and maleic acid/diallylamine copolymer; b) Providing an aqueous solution of a biocompatible polyelectrolyte polymer having opposite charges of the polyelectrolyte of step a) selected from the group consisting of: hyaluronic acid, poly(L-glutamic) acid, carraggenan, alginates, pectin, chitin, cellulose derivatives, starch derivatives, dextran derivatives, poly (styrenesulfonic acid), poly (vinylsulfonic acid), poly (acrylic or methacrylic acid), poly (itaconic acid), maleic acid/ diallyamine copolymer and chitosan; c) Adding two crosslinking agents, one to the emulsion and the other one to the aqueous solution; d) Adding a contrast agent for medical imaging either to the emulsion or the aqueous solution; e) Adding the aqueous solution to the emulsion at a constant temperature comprised between 19 and 37 °C and at a pH comprised between 3 and 7, thus obtaining the separation of the coacervate nanoparticles. f) Optionally adding a further contrast agent, and optical tracer or a radiotracer for medical imaging to the coacervate nanoparticles obtained in step e).