T1 and T2 of ferritin solutions: effect of loading factor

Proton magnetic relaxation times T1 and T2 were measured at field strengths from 0.05 T to 1.5 T in solutions of ferritin with loading factors from 90 to 3600 iron atoms per molecule. 1/T2 increased linearly with field strength, as previously observed, and the slope per unit iron was approximately the same in all samples. This latter finding indicates that the field dependence of T2 may be used as a measure of ferritin-bound iron, regardless of loading factor. A possible explanation is presented, based on the presumed antiferromagnetic structure of the ferritin core and the linear dependence of 1/T2 on core magnetization. A nonzero contribution to 1/T2 in the limit of low field and a contribution to 1/T1 were also found, both of which increase linearly with loading factor for constant protein concentration; these effects represent quantum mechanical dipole-dipole relaxation of water protons either by iron atoms on the surface of the core or by the iron core itself. Finally, the extrapolated intercept at LF = 0 for both 1/T1 and 1/T2 indicates a contribution from a small number of iron ions bound to the protein shell. These results may help in the use of MRI to measure brain iron and possibly even ferritin loading factor.