Dependence of brain-tissue R2 on MRI field strength.

Purpose: To quantify T₂ 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 T₂ 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 T₂ relaxation from static dephasing by B₀ field inhomogeneities and the effects of radiofrequency refocusing imperfections. The average relaxation rates (R₂ = 1/T₂) in multiple regions of interest in the brain were fitted with a model linear in B₀ and correlated with literature iron values.

Results: The relationship between the R₂ values at the two field strengths appeared to be linear over all regions of interest. The R₂ values (in s^-1) in the regions of interest for which both an iron and a lipid mass fraction have been documented in the literature were fitted as $R_2=9+\left(0.9+2·{10}^4\left[\mathrm{Fe}\right]+5.7\left[\text{lipid}\right]\right)·B_0$ , where $\left[\mathrm{Fe}\right]$ and $\left[\text{lipid}\right]$ indicate the putative mass fractions of iron and lipid.

Conclusion: The R₂ relaxation rate is well described by a constant plus a term linear in B₀, with both iron and lipid content contributing to the slope. This indicates that the contributions of lipid and iron to R₂ cannot be separated based solely on the field dependence of R₂ in the field range of 3-7 T.