Unusual longitudinal relaxation time behaviour of colloidal solutions of superparamagnetic nanoparticles

Superparamagnetic nanoparticles are very interesting objects having many applications among which MRI contrast agents are one of the more important. In this work the longitudinal relaxation times of Endorem and Lumirem, two colloidal solutions of iron oxide nanoparticles used as contrast agents for magnetic resonance imaging were measured at magnetic field intensities similar to the ones used in MRI. T-1 was seen to depend on nanoparticle concentrations as expected but, for the Lumirem, also on the time spend by the sample under the influence of the static magnetic field. The T1 evolution was measured for colloidal solutions both different concentrations and different viscosities. The strange T1 dependence is presented and discussed relating to the nanoparticles superparamagnetic properties. It is shown that one of the possible reasons for the fact is the formation of local field enhanced linear arrays of SPIO.