The utility of the single-molecule magnet Fe8 as a magnetic resonance imaging contrast agent over a broad range of concentration

Recent reports have compared Fe-8(aq) versus Magnevist with seemingly conflicting conclusions: one claims a much greater efficiency of Fe-8 for magnetic resonance imaging contrast, and the other claims a much lower efficiency. Our study shows that at concentrations below 1.5 mM Fe-8(aq) had a T-1 relaxivity, r(1), of 5.4 s(-1) mM(-1) which is comparable to Magnevist. Above 1.5 mM Fe-8(aq) had an r(1) of 1.1 s(-1) mM(-1), significantly lower than Magnevist. These results agree with the previous literature over the concentrations they examined. The results for the T-2 relaxivity, r(2), were similar. Here, we show that the concentration dependence of the relaxivity accounts for these discrepancies. Further, the relaxivity data are correlated with frequency-dependent maxima in X-ac() of frozen solutions of Fe-8 dissolved in deionized water over the temperature range of 1.8-4 K and the frequency range of 200-1400 Hz. The magnetic properties of the single-molecule magnet Fe-8 in room temperature and frozen aqueous solution were found to be highly non-linear when examined over a wide concentration range. (c) 2007 Elsevier Ltd. All rights reserved.