Estimation of brain iron concentration in vivo using a linear relationship between regional iron and apparent transverse relaxation rate of the tissue water at 4.7T.

Maps of the apparent transverse relaxation time (T(2) were collected on a transaxial plane across the basal ganglia in 54 healthy subjects at 4.7T using a multiecho adiabatic spin-echo (MASE) imaging sequence. We attempted to quantify the nonhemin iron concentration ([Fe]) in various brain regions in vivo based on the linear relationship between the apparent relaxation rate constant (R(2) = 1/T(2) and regional [Fe], as demonstrated previously in 12 subjects. The calculated [Fe] in five gray matter (GM) regions agreed well with the previously reported regional iron distribution as well as reproduced its age-dependent change. In particular, a decrease of iron in the thalamus region in subjects over 30 years of age was demonstrated while an upward trend was shown in other regions. Furthermore, the average R(2) in each GM region in subjects over 30 years of age showed a deviation from the regression line with [Fe] in an identical manner to that obtained in the previous 12 subjects. This strongly suggests that there is a systematic regional factor affecting R(2), in addition to iron. Interregional difference in the macromolecular mass fraction (f(M)) explained this systematic deviation well. When accounting for f(M) in the analysis, the apparent transverse relaxation rate seems to give a significantly better estimation of regional [Fe].