MREIT experiments with 200 µA injected currents: a feasibility study using two reconstruction algorithms, SMM and harmonicB_Z

Abstract

Magnetic resonance electrical impedance tomography (MREIT) is a technique that produces images of conductivity in tissues and phantoms. In this technique electrical currents are applied to an object and the resulting magnetic flux density is measured using magnetic resonance imaging (MRI) and the conductivity distribution is reconstructed using these MRI data. Currently the technique is used in research environments, primarily studying phantoms and animals. In order to translate MREIT to clinical applications, strict safety standards need to be established, especially for safe current limits. However, there are currently no standards for safe current limits specific to MREIT. Until such standards are established, human MREIT applications need to conform to existing electrical safety standards in medical instrumentation, such as the IEC601. This protocol limits patient auxiliary currents to 100μA for low frequencies. However, published MREIT studies have utilized currents 10 to 400 times larger than this limit, bringing into question whether the clinical applications of MREIT are attainable under current standards. In this study, we investigated the feasibility of MREIT to accurately reconstruct the relative conductivity of a simple agarose phantom using 200μA total injected current and we tested the performance of two MREIT reconstruction algorithms. These reconstruction algorithms used are the iterative sensitivity matrix method (SMM) by Ider and Birgul in 1998 with Tikhonov regularization and the Harmonic BZ proposed by Oh et al in 2003. The reconstruction techniques were tested at both 200μA and 5mA injected currents to investigate their noise sensitivity at low and high current conditions. It should be noted that 200μA total injected current into a cylindrical phantom generates only 14.7μA current in imaging slice. Similarly, 5mA total injected current results in 367μA in imaging slice. Total acquisition time for 200μA and 5mA experiments were about one hour and 8.5 minutes respectively. The results demonstrate that conductivity imaging is possible at low currents using the suggested imaging parameters and reconstructing the images using iterative SMM with Tikhonov regularization, which appears to be more tolerant to noisy data than Harmonic BZ.

A feasibility study of magnetic resonance electrical impedance tomography for prostate cancer detection

“…Results showed that the J-substitution algorithm consistently gave us better results than the harmonic B z algorithm with the same electrode configuration and noise level, which is consistent with the results published by other groups (Arpinar et al , 2012 and Eyüboğlu et al , 2010). The reason for this phenomenon is that the second-order differentiation of B z is required in the harmonic B z algorithm, while only a first-order differentiation is required in the J-substitution algorithm.…”

“…However, three components of magnetic flux density are needed in the J-substitution algorithm and the requirement of 3D rotating the subject in the MRI scanner makes this less practical. Since the sensitivity matrix method (SMM) is more robust to noise (Arpinar et al 2012) and only one component of magnetic flux density measurement is needed to reconstruct the conductivity distribution, this method is more practical in the future clinical application.…”

“…A number of research results reported that a current density of 2 A/m 2 is safe for the human body below 3 kHz (Giland et al 2007, Dalziel 1972). The primary frequency component of the bipolar current pulses used in MREIT, meanwhile, is on the order of 10 Hz (Arpinar et al 2012). Following this safety standard, the injection current strength was set as 4.65 mA for external MREIT, resulting in a maximum current density of 2 A/m 2 in the pelvis.…”

Quantitative Analysis of Electrical Current Effect on Magnetic Resonance Image Tissue Intensity

“…Meanwhile, in T2-weighted images, the mean of tissue image intensity is significantly increased in the electrical current group. This result supports the theory of MREIT that can increase the signal intensity of MR images and its quality [31]. This inconsistent result is a subject for further study.…”

Magnetic-resonance-based measurement of electromagnetic fields and conductivity in vivo using single current administration—A machine learning approach

“…Because only one component of magnetic flux density, B z , the component along the longitudinal axis of an MRI system, can be measured conveniently, specialized techniques have been developed to recover conductivity information from these data [1]. These magnetic resonance electrical impedance tomography (MREIT) approaches have included J-substitution [4], sensitivity-based methods [5], and the Harmonic B z method [6,7]. More recently, additional information provided from diffusion tensor (DT) images have been combined with B z measurements to allow in vivo imaging of conductivity tensors in the human brain [8].…”