Dependence of brain-tissue R on MRI field strength.

Purpose: To quantify T relaxation in the brain at 3 T and 7 T to study its field dependence and correlation with iron content, and to investigate whether iron can be separated from other sources of T relaxation based on this field dependence.

Methods: Nine subjects were scanned at both field strengths with the same acquisition technique, which used multiple gradient-echo sampling of a spin echo. This allowed for separation of T relaxation from static dephasing by B field inhomogeneities and the effects of radiofrequency refocusing imperfections. The average relaxation rates (R = 1/T) in multiple regions of interest in the brain were fitted with a model linear in B and correlated with literature iron values.

Results: The relationship between the R values at the two field strengths appeared to be linear over all regions of interest. The R values (in s) in the regions of interest for which both an iron and a lipid mass fraction have been documented in the literature were fitted as , where and indicate the putative mass fractions of iron and lipid.

Conclusion: The R relaxation rate is well described by a constant plus a term linear in B, with both iron and lipid content contributing to the slope. This indicates that the contributions of lipid and iron to R cannot be separated based solely on the field dependence of R in the field range of 3-7 T.