Magnetic pesonance imaging of electrical conductivity in the human brain

In this study, conductivity distribution of the human brain were obtained using 1.5 magnetic resonance (MR) imaging system. MR images were obtained with motion-probing gradients (MPGs) applied in three orthogonal directions at 25 equally spaced b factors from 200 to 5000 s/mm². The b factor was defined as b = gamma²G ²delta²(Delta - delta/3), where gamma is the gyromagnetic ratio (2.7 times 10⁸ rad s^-1 T ^-1), G and delta are the respective intensity and duration of the MPGs, and Delta is the interval between the leading edges of the MPGs. Results show that the signal intensities of MR images are attenuated with an increase in the b factor. An application of the MPG in the superior-inferior direction caused the most rapid signal attenuation. The anisotropy of signal attenuations can be attributed to the fibrous structures of the tissue. The mean conductivity (MC) values of the putamen and the internal capsule were 0.066 S/m and 0.80 S/m, respectively. In the internal capsule, the conductivity exhibited high value in the directions of neuronal fibers. This region had high anisotropy index (AI) in comparison with the putamen. Low-frequency currents are mainly conducted via migrations of ions through extracellular space. The conductivity calculated by the method employed corresponds to the values at very low frequencies because the conductivity model considers only the extracellular current