T-1 and T-2 of ferritin solutions: Effect of loading factor

Proton magnetic relaxation times T-1 and T-2 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/T-2 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 T-2 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/T-2 on core magnetization, A nonzero contribution to 1/T-2 in the limit of tow field and a contribution to 1/T-1 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/T-1, and 1/T-2 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.