Magnetic resonance electrical impedance tomography (MREIT) for high-resolution conductivity imaging

Abstract

Cross-sectional imaging of an electrical conductivity distribution inside the human body has been an active research goal in impedance imaging. By injecting current into an electrically conducting object through surface electrodes, we induce current density and voltage distributions. Based on the fact that these are determined by the conductivity distribution as well as the geometry of the object and the adopted electrode configuration, electrical impedance tomography (EIT) reconstructs cross-sectional conductivity images using measured current-voltage data on the surface. Unfortunately, there exist inherent technical difficulties in EIT. First, the relationship between the boundary current-voltage data and the internal conductivity distribution bears a nonlinearity and low sensitivity, and hence the inverse problem of recovering the conductivity distribution is ill posed. Second, it is difficult to obtain accurate information on the boundary geometry and electrode positions in practice, and the inverse problem is sensitive to these modeling errors as well as measurement artifacts and noise. These result in EIT images with a poor spatial resolution. In order to produce high-resolution conductivity images, magnetic resonance electrical impedance tomography (MREIT) has been lately developed. Noting that injection current produces a magnetic as well as electric field inside the imaging object, we can measure the induced internal magnetic flux density data using an MRI scanner. Utilization of the internal magnetic flux density is the key idea of MREIT to overcome the technical difficulties in EIT. Following original ideas on MREIT in early 1990s, there has been a rapid progress in its theory, algorithm and experimental techniques. The technique has now advanced to the stage of human experiments. Though it is still a few steps away from routine clinical use, its potential is high as a new impedance imaging modality providing conductivity images with a spatial resolution of a few millimeters or less. This paper reviews MREIT from the basics to the most recent research outcomes. Focusing on measurement techniques and experimental methods rather than mathematical issues, we summarize what has been done and what needs to be done. Suggestions for future research directions, possible applications in biomedicine, biology, chemistry and material science are discussed.

In vivo mapping of current density distribution in brain tissues during deep brain stimulation (DBS)

“…In this feasibility study, current density mapping of living tissues was found to be possible using a recently developed MREIT method. 7,9,12 Since induced current density depends on the electrode position and current amplitude as well as the local conductivity distribution, 2,5,7 we obtained magnetic flux density (B z ) data from the MREIT scan to image the current density distribution during DBS. Instead of using Ampere's law directly, we recovered projected current density which is the best current density measured from one component of magnetic flux density.…”

“…A similar method, magnetic resonance electrical impedance tomography (MREIT) can image current density of living tissues using only one component of B without rotating the subject. [12][13][14] In addition, the conductivity distribution of living tissues can be obtained from two independent current injection data inside the imaging objects. 7,9,12 From numerical simulation and mimicked phantom experiments, 12,15,16 several studies have proposed that this method has a potential for visualizing current density distribution in living tissues.…”

Well-posedness of the conductivity reconstruction from an interior current density in terms of Schauder theory

“…It is an EIT type conductivity reconstruction method, which uses internal current data (see [21,22,24]). The internal magnetic field B is obtained using MRI technology and the internal current density data J is obtained by Ampere's Law…”

Three-dimensional multiexcitation magnetoacoustic tomography with magnetic induction

“…In order to solve the technical difficulties in EIT and to achieve high spatial resolution and high accuracy in conductivity imaging, magnetic resonance electrical impedance tomography ͑MREIT͒ was developed by combining EIT and magnetic resonance current density imaging ͑MRCDI͒ technique. 9,10 In MREIT, a MRI scanner is used to detect the induced magnetic field generated by injected current flowing in biological tissues. Till now, conductivity images with high spatial resolution have been obtained using the MREIT technique in both in vitro and in vivo experiments.…”

“…Till now, conductivity images with high spatial resolution have been obtained using the MREIT technique in both in vitro and in vivo experiments. 9 Currently, MREIT is mainly limited by its requirement of high level current injection to obtain an acceptable signal-to-noise ͑SNR͒ level. Recently, some researchers have also proposed to extract electrical properties of human body at Larmor frequency using MRI B1 mapping technique.…”