The present invention relates to paramagnetic solid lipid nanoparticles (pSLNs) comprising an amphiphilic paramagnetic metal chelating moiety selected from: a diazepine derivative of Formula I and a tetraazocyclododecane derivative of Formula (II): being said chelating moiety complexed to a paramagnetic metal ion selected from the group consisting of: Gd(III), Mn(II), Cr(III), Cu(II), Fe(III), Pr(III), Nd(III), Sm(III), Tb(III), Yb(III), Dy(III), Ho(III) and Er(III), or salts thereof. The invention further relates to the process for preparation of said solid lipid nanoparticles comprising amphiphilic complexes of paramagnetic metals (pSLNs) and to the use of pSLNs as MRI contrast agents in the diagnostic field.