Magnetic flux density measurement in magnetic resonance electrical impedance tomography using a low-noise current source

Abstract

Current injected into an electrically conducting object induces distributions of magnetic flux density as well as voltage and current density. In magnetic resonance electrical impedance tomography (MREIT), we measure the induced magnetic flux density using an MRI scanner to reconstruct cross-sectional conductivity images of the object. The current injection must be synchronized with a chosen MRI pulse sequence and the current source needs to be programmable to accommodate various pulse sequences. For an injected current of 1 mA, for example, we may expect an induced magnetic flux density of a few nanoteslas, and noise reduction in measured magnetic flux density data is important for better conductivity image quality. In this paper, we describe how to reduce noise in measured magnetic flux density data based on analyses of noise sources in MREIT. Given an MRI scanner and an imaging object, we found that the current source including lead wires becomes a major noise source. We designed a low-noise programmable current source with optical links, batteries and electromagnetic shields. Equipped with lead switching capability, it automates MREIT imaging experiments using multiple pairs of electrodes. We found that the new current source improves the output current SNR by about 10 dB and the MR magnitude image SNR by about 30%. Placing it near an imaging object inside the shielded room, we could reduce the noise standard deviation in measured magnetic flux density data by 40%. We propose use of this low-noise current source for in vivo animal and human MREIT imaging studies.