Evaluation of magnetohydrodynamic effects in magnetic resonance electrical impedance tomography at ultra‐high magnetic fields

Abstract

Purpose Artifacts observed in experimental magnetic resonance electrical impedance tomography images were hypothesized to be because of magnetohydrodynamic (MHD) effects. Theory and Methods Simulations of MREIT acquisition in the presence of MHD and electrical current flow were performed to confirm findings. Laminar flow and (electrostatic) electrical conduction equations were bidirectionally coupled via Lorentz force equations, and finite element simulations were performed to predict flow velocity as a function of time. Gradient sequences used in spin‐echo and gradient echo acquisitions were used to calculate overall effects on MR phase images for different electrical current application or phase‐encoding directions. Results Calculated and experimental phase images agreed relatively well, both qualitatively and quantitatively, with some exceptions. Refocusing pulses in spin echo sequences did not appear to affect experimental phase images. Conclusion MHD effects were confirmed as the cause of observed experimental phase changes in MREIT images obtained at high fields. These findings may have implications for quantitative measurement of viscosity using MRI techniques. Methods developed here may be also important in studies of safety and in vivo artifacts observed in high field MRI systems.

Sensitivity and resolution improvement for in vivo magnetic resonance current‐density imaging of the human brain

“…Double‐echo planar imaging 60 might help to reduce the sensitivity of the MR data to physiological field fluctuations, but likely has lower sensitivity to the current‐induced magnetic fields. Furthermore, ultrahigh‐field MRI, for example, at 7T or 9.4T, generally offers higher SNRs, 61 but also requires careful optimization of the imaging methods to achieve robust results 62 . Another important goal would be the extension to simultaneous multislice 63 or volume acquisition to obtain more complete information of the current‐induced magnetic field changes, which might help the accurate reconstruction of the TES currents and the conductivity distribution in the brain.…”

“…Furthermore, ultrahigh-field MRI, for example, at 7T or 9.4T, generally offers higher SNRs, 61 but also requires careful optimization of the imaging methods to achieve robust results. 62 Another important goal would be the extension to simultaneous multislice 63 or volume acquisition to obtain more complete information of the current-induced magnetic field changes, which might help the accurate reconstruction of the TES currents and the conductivity distribution in the brain. However, maintaining the required sensitivity levels, image quality, and robustness to physiological noise within a larger brain volume might be challenging.…”

Doppler ultrasound cardiac gating of intracranial flow at 7T

“…Finally, Frauenrath et al suggested to use the characteristics of the increased MHD effect for gating [ 6 ] and several studies have investigated measuring and modelling of the MHD effects throughout the body [ 40 , 41 ]. These methods show promise, but are at an early stage of technical development.…”