Super-paramagnetic particles with increased R1 relaxivity, process for producing said particles and use thereof

Super-paramagnetic single-domain particles having an increased R1 relaxivity and surface stabilizer substances, comprising particles of iron hydroxide, iron oxide hydrate, iron oxide, mixed iron oxides or iron having a particle size in the range between 1 and 10 nanometers, with an average particle diameter d50 of two to four nanometers;

• having an increased R1 relaxivity in the range of two to fifty, having a R2/R1 relaxivity ratio of less than five;
• and
• having at their surface first stabilizer substances selected from the group consisting of aliphatic dicarboxylic acids and aliphatic polycarboxylic acids, substitution products thereof and derivatives thereof, preventing aggregation and sedimentation of the stabilized particles in a gravitational field or in a magnetic field;
• and
• said aliphatic polycarboxylic acids being selected from the group consisting of citric acid, cyclohexane-tricarboxylic acid, cyclohexanehexacarboxylic acid, ethylene-diaminetetraacetic acid and diethylenetriaminepentaacetic acid.