THEORY OF 1 T-1 AND 1/T-2 NMRD PROFILES OF SOLUTIONS OF MAGNETIC NANOPARTICLES/

Organically coated iron oxide crystallites with diameters of 5-50 nm ( ''nanoparticles'') are potential magnetic resonance imaging contrast a gents. 1/T-1 and 1/T-2 of solvent water protons are increased dramatic ally by magnetic interactions in the ''outer sphere'' environment of t he nanoparticles; subsequent diffusive mixing distributes this relaxat ion throughout the solvent. Published theory, valid for the solute mag netic energy small compared with thermal energy, is applicable to smal l magnetic solutes (e.g,, gadolinium and manganese diethylenetriaminop entaacetic acid, and nitroxide free radicals) at generally accessible fields (less than or equal to 50 T). It fails for nanoparticles at fie lds above similar to 0.05 T, i.e., at most imaging fields. The authors have reformulated outer sphere relaxation theory to incorporate progr essive magnetic saturation of solute nanoparticles and, in addition, i ndicate how to use empirical magnetization data for realistic particle s when their magnetic properties are not ideal. It is important to han dle the effects of rapid thermally induced reorientation of the magnet ization of the nanoparticles (their ''superparamagnetism'') effectivel y, including their sensitivity to particle size. The theoretical resul ts are presented as the magnetic field dependence (NMRD profiles) of 1 /T-1 and 1/T-2, normalized to Fe content, for three sizes of particles , and then compared with the limited data extant for well-characterize d material.