Dependence of brain‐tissue <scp>R<sub>2</sub></scp> on <scp>MRI</scp> field strength
Peter van Gelderen, Yicun Wang, Jacco A. de Zwart, Jeff H. Duyn
Abstract
Abstract Purpose To quantify T 2 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 2 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 2 relaxation from static dephasing by B 0 field inhomogeneities and the effects of radiofrequency refocusing imperfections. The average relaxation rates (R 2 = 1/T 2 ) in multiple regions of interest in the brain were fitted with a model linear in B 0 and correlated with literature iron values. Results The relationship between the R 2 values at the two field strengths appeared to be linear over all regions of interest. The R 2 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 , where and indicate the putative mass fractions of iron and lipid. Conclusion The R 2 relaxation rate is well described by a constant plus a term linear in B 0 , with both iron and lipid content contributing to the slope. This indicates that the contributions of lipid and iron to R 2 cannot be separated based solely on the field dependence of R 2 in the field range of 3–7 T.