Magnetic resonance imaging of mean values and anisotropy of electrical conductivity in the human brain.

Abstract

The conductivity distribution in the human brain is difficult to obtain by conventional impedance tomography methods, in which currents are applied via surface electrodes. In this study, we obtained images of anisotropic conductivity in the human brain using diffusion magnetic resonance imaging (MRI). Diffusion-weighted images of the brain were acquired by a 1.5 T MRI system using an echo planar imaging sequence. Motion-probing gradients (MPGs) were applied with 25 arrayed b-factors up to 5000 s/mm2. The fast and slow diffusion components were estimated by fitting a biexponential attenuation function to the measured signals. The effective conductivities in each direction were calculated from the fast diffusion components. The mean conductivities of the cortex, the corpus callosum, and the internal capsule were 0.10 +/- 0.03 S/m, 0.12 +/- 0.02 S/m, and 0.08 +/- 0.01 S/m, respectively. Tissues with highly anisotropic cellular structures, such as the corpus callosum and the internal capsule, exhibited high anisotropy in conductivity. The anisotropy indices in the cortex, the corpus callosum, and the internal capsule were 0.07 +/- 0.03, 0.60 +/- 0.07, and 0.65 +/- 0.05, respectively.