Human in-vivo Brain Magnetic Resonance Current Density Imaging (MRCDI)

Correction of the cable-induced magnetic stray fields for SSFP-FID measurements (TR= 120 ms, N = 24) with multi-gradient-echo readouts in four subjects (no tissue current). The experiments were repeated twice, and the figure shows the results of the first experimental run. (a) Magnitude images. (b) Uncorrected magnetic field images showing the stray field generated by the current flow in the wire loop around the head. (c) Corrected magnetic field images, in which the stray field was calculated based on the reconstructed wire path and subtracted from the measured magnetic field. (d) magnetic field images of the control measurements performed without current injection.


Magnetic resonance current density imaging (MRCDI) and MR electrical impedance tomography (MREIT) are two emerging modalities, which combine weak time-varying currents injected via surface electrodes with magnetic resonance imaging (MRI) to acquire information about the current flow and ohmic conductivity distribution at high spatial resolution. The injected current flow creates a magnetic field in the head, and the component of the induced magnetic field parallel to the main scanner field causes small shifts in the precession frequency of the magnetization. The measured MRI signal is modulated by these shifts, allowing to determine the magnetic field for the reconstruction of the current flow and ohmic conductivity. Here, we demonstrate reliable magnetic field measurements in-vivo in the human brain based on multi-echo spin echo (MESE) and steady-state free precession free induction decay (SSFP-FID) sequences. In a series of experiments, we optimize their robustness for in-vivo measurements while maintaining a good sensitivity to the current-induced fields. We validate both methods by assessing the linearity of the measured magnetic field with respect to the current strength. For the more efficient SSFP-FID measurements, we demonstrate a strong influence of magnetic stray fields on the magnetic field images, caused by non-ideal paths of the electrode cables, and validate a correction method. Finally, we perform measurements with two different current injection profiles in five subjects. We demonstrate reliable recordings of magnetic fields as weak as 1 nT, caused by currents of 1 mA strength. Comparison of the magnetic field measurements with simulated magnetic field images based on FEM calculations and individualized head models reveals significant linear correlations in all subjects, but only for the stray field-corrected data. As final step, we reconstruct current density distributions from the measured and simulated magnetic field data. Reconstructions from non-corrected magnetic field measurements systematically overestimate the current densities. Comparing the current densities reconstructed from corrected magnetic field measurements and from simulated magnetic field images reveals an average coefficient of determination R2 of 71%. In addition, it shows that the simulations underestimated the current strength on average by 24%. Our results open up the possibility of using MRI to systematically validate and optimize numerical field simulations that play an important role in several neuroscience applications, such as transcranial brain stimulation, and electro- and magnetoencephalography. 

1.
Cihan Göksu, Lars G Hanson, Hartwig R Siebner, Philipp Ehses, Klaus Scheffler, Axel Thielscher:
Human in-vivo brain magnetic resonance current density imaging (MRCDI).
Neuroimage. 2018 May 1;171:26-39.
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