Feasibility of Imaging Tissue Electrical Conductivity by Switching Field Gradients with MRI

Abstract

Tissue conductivity is a biophysical marker of tissue structure and physiology. Present methods of measuring tissue conductivity are limited. Electrical impedance tomography and magnetic resonance electrical impedance tomography rely on passing an external current through the object being imaged, which prevents its use in most human imaging. More recently, tissue conductivity has been determined noninvasively from measurements of the radiofrequency (RF) field used in magnetic resonance imaging (MRI). This technique is promising, but conductivity at higher frequencies is less sensitive to tissue structure. Measuring tissue conductivity noninvasively at low frequencies remains elusive. It has been proposed that eddy currents generated during the rise and decay of gradient pulses could act as a current source to map low-frequency conductivity. This work centers on a gradient echo pulse sequence that uses large gradients before excitation to create eddy currents. The electric and magnetic fields during a gradient pulse are simulated by a finite-difference timedomain simulation. The sequence is also tested with a phantom and animal MRI scanner equipped with gradients of high gradient strengths and slew rates. The simulation demonstrates that eddy currents in materials with a conductivity similar to biological tissue decay with a half-life on the order of nanoseconds, and any eddy currents generated before excitation decay completely before influencing the RF signal. Gradient-induced eddy currents can influence phase accumulation after excitation, but the effect is too small to image. The animal scanner images show no measurable phase accumulation. Measuring low-frequency conductivity by gradient-induced eddy currents is presently unfeasible.