NMR relaxation in systems with magnetic nanoparticles: a temperature study.

PURPOSE: To measure and model nuclear magnetic resonance (NMR) relaxation enhancement due to the presence of gadolinium (Gd)-substituted Zn-Mn ferrite magnetic nanoparticles (MNP) at different temperatures. MATERIALS AND METHODS: Relaxation rates were measured at 1.5 T using fast spin echo (FSE) sequences in samples of agarose gel doped with uncoated and polyethylene glycol (PEG)-coated Mn0.5 Zn0.5 Gd0.02 Fe1.98 O4 nanoparticles over the temperature range 8-58°C. Physical characterization of the MNPs synthesized using chemical coprecipitation included scanning (SEM) and transmission (TEM) electron microscopy, inductively coupled plasma (ICP), dynamic light scattering (DLS), and magnetometry. RESULTS: Relaxivity (in s(-1) mM(-1) Fe) for the uncoated and coated particles, respectively, increased as follows: from 2.5 to 3.2 and 0.4 to 0.7 for T1, while for T2 it increased from 162.3 to 253.7 and 59.7 to 82.2 over the temperature range 8-58°C. T2 data were fitted to the echo limited motional regime using one fitting parameter that reflects the degree of agglomeration of particles into a cluster. This parameter was found to increase linearly with temperature and was larger for the PEG-coated particles than the uncoated ones. CONCLUSION: The increase of 1/T2 with temperature is modeled successfully using echo limited motional regime where both diffusion of the protons and nanoparticle cluster size increase with temperature. Both transverse and longitudinal relaxation efficiencies are reduced by PEG coating at all temperatures. If prediction of relaxation rates under different particle concentrations and operating temperatures is possible then the use of MNP in temperature monitoring and hyperthermia applications may be achieved.