Conductivity imaging of canine brain using a 3 T MREIT system: Postmortem experiments

Magnetic resonance electrical impedance tomography (MREIT) has the potential to provide conductivity images with high spatial resolution and accuracy. Recent studies using various conductivity phantoms showed that the spatial resolution could be similar to that of conventional MR images as long as enough current is injected. Before we try in vivo animal imaging studies using a small injection current of less than 5 mA, we have performed MREIT conductivity imaging of postmortem canine brains using 40 mA injection currents. The primary goals were to produce high-resolution conductivity images of white and gray matter in situ and to accumulate experimental techniques to undertake in vivo animal imaging studies in the near future. Reconstructed conductivity images of two canine brains with a pixel size of 1.4 × 1.4 mm² showed a clear conductivity contrast between gray and white matter. Considering the anisotropic conductivity of white matter, we interpreted reconstructed conductivity images as equivalent isotropic conductivity images. Estimated conductivity ratios of white to gray matter were between 1.13 and 1.20 depending on the choice of a region of interest in reconstructed images. A higher conductivity value of white matter compared with that of gray matter stems from the fact that the reconstructed equivalent isotropic conductivity value of white matter reflects a high conductivity of white matter in the direction parallel to its fibers. We expect that this kind of postmortem animal imaging can provide conductivity information on tissues in situ to be utilized in numerous modeling studies.