Relaxometry and magnetometry of the MR contrast agent MION-46L

MION-46L is an ultrasmall monocrystalline superparamagnetic (SPM) iron oxid e that is of current interest as an MR contrast agent. It is believed to co nsist primarily of small maghemite or magnetite crystals that possess a SPM moment, but the exact magnetic properties and related mechanisms of T-1 an d T-2 proton relaxation enhancement are not well understood. We have obtain ed a comprehensive data set consisting of magnetization curves, EPR spectra , and 1/T-1 and 1/T-2 nuclear magnetic relaxation dispersion (NMRD) profile s for this contrast agent. The magnetization curves show a primary curvatur e consistent with a SPM moment of 10,300 Bohr magnetons (BM) per particle. In addition, there is a secondary high-field curvature that is consistent w ith a smaller SPM moment of 1600 BM, which may be responsible for the obser ved high-field increase in 1/T-2, Finally, there appear to be a considerabl e number of paramagnetic ions present that are needed to account for the hi gh-field increase in magnetization, and that can provide an alternative exp lanation for the magnitude of the low-field T-1 plateau. This "three-phase model" appears to be successful in explaining in a self-consistent and quan titative manner the combined results of the magnetometry, relaxometry, and EPR studies. Magn Reson Med 42:379-384, 1999. Published 1999 Wiley-Liss, In c.dagger