Induced current magnetic resonance–electrical impedance tomography

Abstract

Magnetic resonance-electrical impedance tomography (MR-EIT) is a conductivity imaging method based on injecting currents into the object. In this study, a new MR-EIT method, whereby currents are induced inside the object by using external coils, is proposed. This new method is called induced current magnetic resonance-electrical impedance tomography. In induced current MR-EIT surface electrodes are not used and thereby artifacts due to electrodes are eliminated. The reconstruction algorithm is based on the measurement of only one component of the secondary magnetic flux density. The algorithm is an iterative one, is 3D and is based on the solution of a linear matrix equation at each iteration. For the measurement of secondary magnetic flux density, a pulse sequence to be used in the MRI system is proposed. Numerical simulations are performed to test the algorithm for both noise-free and noisy cases. The singular value behavior of the matrix is monitored and it is observed that at least two current induction profiles improve the images significantly. It is shown that induced current MR-EIT can be used to reconstruct absolute conductivity images without the need for any additional peripheral voltage measurement.

Low-parametric Induced Current - Magnetic Resonance Electrical Impedance Tomography for quantitative conductivity estimation of brain tissues using a priori information: A simulation study

“…In this paper, we focus on obtaining an as accurate as possible estimation of the conductivity values by using the following methodologies: (1) eddy-current induction gradient: In [6], [8], the proposed MR pulse sequence consists of sinusoidal excitation that induce sinusoidal eddy currents. Here, we use a trapezoidal eddy-current induction gradient.…”

“…We use in this paper segmented MR images. In this way, the high number of parameters, used in [6], [8] that needs to be recovered is reduced to a low number so that the ill-posedness of the inverse problem is significantly reduced. (3) time-efficient Independent Impedance Method (IIM) as forward eddycurrent solver: Since a forward eddy-current solver needs to be evaluated many times in an iterative loop for solving the inverse problem, an efficient forward model is needed so to avoid prohibitive computational times.…”

“…This paper is a preliminary study of the proposed methodology and we therefore use a spherical head model with a coarse discretization of 1 cm, as in [6]. The calculation time of one forward model evaluation (with 10 harmonics) is approximately 30 seconds 1 .…”

“…The calculation time of one forward model evaluation (with 10 harmonics) is approximately 30 seconds 1 . In this way, the efficiency is obtained and solving the inverse problem, where this forward model is iteratively evaluated, becomes computationally feasible in time in comparison to traditional IC-MREIT [6].…”

“…However, these electrical conductors cause problems as the appearance of higher current densities near them and susceptibility artifacts [6]. To avoid these problems, Induced Current -Magnetic Resonance Electrical Impedance Tomography (IC-MREIT) was proposed byÖzparlak anḋ Ider, similar to induced current EIT [7], whereby currents are induced inside the subject by using external coils [6].…”