Brain Tissue Conductivity Measurements with MR-Electrical Properties Tomography: An In Vivo Study

Abstract

First in vivo brain conductivity reconstructions using Helmholtz MR-Electrical Properties Tomography (MR-EPT) have been published. However, a large variation in the reconstructed conductivity values is reported and these values differ from ex vivo conductivity measurements. Given this lack of agreement, we performed an in vivo study on eight healthy subjects to provide reference in vivo brain conductivity values. MR-EPT reconstructions were performed at 3 T for eight healthy subjects. Mean conductivity and standard deviation values in the white matter, gray matter and cerebrospinal fluid (σ_WM, σ_GM, and σ_CSF) were computed for each subject before and after erosion of regions at tissue boundaries, which are affected by typical MR-EPT reconstruction errors. The obtained values were compared to the reported ex vivo literature values. To benchmark the accuracy of in vivo conductivity reconstructions, the same pipeline was applied to simulated data, which allow knowledge of ground truth conductivity. Provided sufficient boundary erosion, the in vivo σ_WM and σ_GM values obtained in this study agree for the first time with literature values measured ex vivo. This could not be verified for the CSF due to its limited spatial extension. Conductivity reconstructions from simulated data verified conductivity reconstructions from in vivo data and demonstrated the importance of discarding voxels at tissue boundaries. The presented σ_WM and σ_GM values can therefore be used for comparison in future studies employing different MR-EPT techniques.

Keywords: Brain; Conductivity; Electrical properties; MR-EPT.

Fig. 1

Simulation setup used for the FDTD simulation on the Duke head model and ground truth conductivity values at 128 MHz

Fig. 2

For each subject, one slice of the in vivo conductivity reconstructions is shown as example

Fig. 3

One example slice of conductivity reconstructions for the Duke simulations. Mean WM and GM conductivity values and standard deviations (inside brackets) are also reported, after the same erosion applied for the in vivo conductivity reconstructions was performed