Low-frequency conductivity imaging using MRI gradient induced currents

Abstract

Due to the switching of Magnetic Resonance Imaging (MRI) gradient fields, electric fields are induced in imaged subjects which give rise to `subject eddy currents'. The feasibility of low-frequency conductivity imaging based on measuring the magnetic field (subject eddy field) due to subject eddy currents is investigated within the frame of two main goals. First goal is to understand whether conductivity reconstruction is possible provided that subject eddy fields are accurately measured. Regarding this goal, the inverse problem of obtaining conductivity distribution from subject eddy fields is formulated as a convection-reaction equation and a conductivity reconstruction algorithm is developed. In the simulations, successful conductivity reconstructions are obtained pointing the feasibility of the proposed algorithm. The second goal is to understand the fidelity by which subject eddy fields must be measured for accurately reconstructing conductivity. For measuring subject eddy fields, a pulse sequence is developed by which the contribution of subject eddy fields to MR phase images is determined. It is found that this contribution cannot be measured with an uncertainty suffciently low for accurate conductivity reconstruction. Furthermore, some artifacts other than random noise are observed in the measured phases which are modeled by considering the e ects of magnetic fields due to MRI system imperfections during readout. For feasible subject eddy current based conductivity imaging, it is required that the span of the phase accumulated by subject eddy fields is increased, and the mentioned artifacts are eliminated from the phase measurements. For the first requirement, the state-of-the-art gradient systems are evaluated and also a multi-spin-echo pulse sequence is developed. This pulse sequence is analyzed by using the extended phase graph framework. For the second requirement, the possibility of using geometric distortion correction methods are evaluated. It is found that, if the multi-spin-echo pulse sequence is used in small-sized preclinical MRI scanners which have extremely high gradient fields, the span of accumulated phase can be suffciently increased for the feasibility. On the other hand, it is found that the geometric distortions cannot be corrected to the degree that the level of artifacts becomes suffciently low for the feasibility.