Apparent transverse relaxation rate in human brain varies linearly with tissue iron concentration at 4.7 T

Multiple pairs of adiabatic passage pulses were implemented in a spin-echo sequence to achieve accurate measurements of the apparent transverse relaxation time (T(2)(dagger)) in a short scan time. In experiments on agarose gel phantoms with T(2) values ranging from 30 to 105 ms, the measured T(2)(dagger) values were in good agreement with transverse relaxation times measured with a nonselective Carr-Purcell-Meiboom-Gill sequence. In experiments on normal human brain at 4.7 T, T(2) (dagger) values in five different gray matter regions were found to range from 38 +/- 2 ms (globus pallidus) to 64 +/- 2 ms (frontal cortex). The apparent relaxation rate (1/T(2)(dagger)) in these five regions showed strong correlation (r = 0.97) with published levels of iron (Fe) in those regions. The linear coefficient relating 1/T(2)(dagger) and [Fe] at 4.7 T was measured to be 0.551 (s x mg Fe/100 g f.w.)(-1). When compared with the values obtained in a previous report for six different static fields (B(0)) up to 1.5 T, the current measurement confirms the linear dependence of the linear coefficient on B(0) up to 4.7 T (r = 0.99). These results suggest that the T(2)(dagger) value in the human brain is predominantly affected by the nonhemin iron distribution. The strong correlation between the obtained T(2)(dagger) values and the regional iron concentrations suggests a role for this pulse sequence in quantifying in vivo brain iron at high magnetic field.