Imaging electric conductivity and conductivity anisotropy via eddy currents induced by pulsed field gradients

PURPOSE: The capacity to measure conductivity and conductivity anisotropy in vivo and non-invasively has practical significance in a number of biomedical applications. For example, large changes of tissue conductivity have been reported in tumors, strokes and myocardial infarction. Considerable early efforts were focused on the measurement of injected low-frequency currents (1-3). Electric properties tomography (EPT), on the other hand, maps electric properties based on the interaction of B1 field with tissues (4-6). While current-injection methods typically operate at a frequency below 0.5 MHz, EPT methods based on B1 field measure conductivity at the Larmor frequency (typically 100s MHz). Here, the potential of measuring “DC” conductivity and conductivity tensor based on eddy currents induced by the switching of imaging gradients is investigated both theoretically and experimentally. As the gradient directions can be readily controlled, various components of the conductivity tensor can be assessed. Simulation, phantom and in vivo human experiments were conducted to demonstrate the feasibility. The method described here provides a strategy for in vivo imaging of “DC” or low-frequency tissue conductivity non-invasively with MRI.