Modeling of Human Body-coupled Electric Field Interference in Unshielded Ultra-Low Field MRI

TL;DR

This paper presents a circuit model explaining how human body-coupled electric fields cause interference in unshielded ultra-low field MRI, and demonstrates a mitigation method that significantly improves image quality.

Contribution

The authors develop a lumped-parameter circuit model for body-coupled interference in portable ULF-MRI and validate it through experiments, leading to an effective noise reduction technique.

Findings

Model accurately predicts noise dependence on subject position and geometry.

Implementing a capacitive low-impedance bypass improves SNR by approximately 3.5 times.

Circuit-based analysis aids in understanding and mitigating interference in portable MRI.

Abstract

Portable ultra-low field MRI (ULF-MRI) systems operated in unshielded environments are susceptible to electromagnetic interference (EMI). Subject presence in the imaging region will lead to substantial noise increases, yet the dominant coupling mechanism remains insufficiently characterized. We develop a lumped-parameter circuit model of the coupled environment-body-receiver system. The model indicates that ambient time-varying electric fields induce a body common-mode potential, which is converted into differential-mode noise through capacitive imbalance between the head and the receive-coil terminals, yielding strong dependence on subject position and geometry. Circuit analysis, simulations, and controlled experiments support the model, with predicted imbalance consistent with measured noise variations. Guided by this mechanism, we implement a capacitive low-impedance bypass to clamp the body potential, achieving an approximately 3.5-fold SNR improvement on a 50 mT prototype. The proposed model offers a compact circuit-based tool for analyzing and mitigating human body-coupled electric-field interference in portable ULF-MRI.