Abstract
The influence of proton diffusion on nuclear magnetic resonance (NMR) relaxation was investigated in the presence of horse spleen ferritin at 7T. Binary mixtures of water and glycerol were used to control diffusion within the range of 0.6–2.0×10−9m2/s, which was confirmed by pulsed gradient techniques. The effect of chemical exchange by hydrolysis between water and glycerol on relaxation was characterized with Carr–Purcell–Meiboom–Gill (CPMG) dispersion experiments. The relaxation rate enhancement of the protons due to ferritin was found to be inversely proportional to the diffusion coefficient. The enhancement increased by a factor of 3.6 over the range of diffusion coefficients, while the hydroxyl proton concentration decreased by a factor of 1.3. This result is in disagreement with the proton exchange dephasing model, which is independent of diffusion but predicts an inverse dependence on the hydroxyl concentration. Our data indicate that the role of diffusion dominates and must be considered when relaxation rates are used to determine iron concentration in vivo.
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