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Organically coated iron oxide crystallites with diameters of 5-50 nm ("nanoparticles") are potential magnetic resonance imaging contrast agents. 1/T1 and 1/T2 of solvent water protons are increased dramatically by magnetic interactions in the "outer sphere" environment of the nanoparticles; subsequent diffusive mixing distributes this relaxation throughout the solvent. Published theory, valid for the solute magnetic energy small compared with thermal energy, is applicable to small magnetic solutes (e.g., gadolinium and manganese diethylenetriaminopentaacetic acid, and nitroxide free radicals) at generally accessible fields (< or = 50 T). It fails for nanoparticles at fields above approximately 0.05 T, i.e., at most imaging fields. The authors have reformulated outer sphere relaxation theory to incorporate progressive magnetic saturation of solute nanoparticles and, in addition, indicate how to use empirical magnetization data for realistic particles when their magnetic properties are not ideal. It is important to handle the effects of rapid thermally induced reorientation of the magnetization of the nanoparticles (their "superparamagnetism") effectively, including their sensitivity to particle size. The theoretical results are presented as the magnetic field dependence (NMRD profiles) of 1/T1 and 1/T2, normalized to Fe content, for three sizes of particles, and then compared with the limited data extant for well-characterized material.
Koenig et al. (1995) studied this question.