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To obtain high T1-relaxivity colloidal particles with a simultaneously high loading of amphiphilic Gd-chelates, a novel drug dosage form based on the phase organisation of amphiphilic gadolinium chelates with cholesterol was developed.In order to find a formulation, which exhibit both high T1-relaxivity and gives small particles a D-optimal mixture design (experimental design) was applied. Gadolinium 1,4,7-tris(carboxymethyl)-10-(2-hydroxyhexadecyl)-1,4,7,10-tetraazacyclododecane (Gd-HHD-DO3A) and cholesterol at approximately equimolar ratio proved to form thermodynamic stable disc-like colloidal particles as seen by cryo-electron micrographs. T1-relaxivity of these particles was typically around 20 mM−1 s−1 and the size below 100 nm (photon correlation spectroscopy (PCS)). The particles do most probably not interact with blood components as no change in T1-relaxivity was observed when the particles were mixed with whole blood. The particles were stable at room temperature for at least 6 months.