Toward understanding transverse relaxation in human brain through its field dependence

Apparent transverse-relaxation rate constants (R 2 = 1/T 2) were measured in various regions of the healthy human brain using a multiecho adiabatic spin-echo sequence at five different magnetic fields, 1.5, 1.9, 3, 4.7, and 7 T. The R 2 values showed a clear dependence on magnetic field strength (B ₀). The regional distribution of the R 2 was well explained by the sum of three components: (1) regional nonhemin iron concentration ([Fe]), (2) regional macromolecular mass fraction (f_M), and (3) a region-independent factor. Accordingly, R 2 = α[Fe] + βf_M + γ, where coefficients α, β, and γ were experimentally determined at each magnetic field by a least square fitting method using multiple regression analysis. Although the coefficient α linearly increased with B₀, β showed a quadratic dependence on top of a field-independent component. The coefficient γ also increased slightly with B₀ on top of a field-independent component. The linear dependence of α on B₀ was consistent with that observed for the transverse-relaxation rate of water protons in ferritin solutions as found previously by others. The quadratic dependence of β on B₀ was accounted for by isochronous and anisochronous exchange mechanisms using intrinsic-relaxation parameters obtained from the literature.