Principles of Nerve and Heart Excitation by Time-varying Magnetic Fields

A time-varying magnetic field has the potential for inducing circulating electrical currents (so-called "eddy" currents) within conducting objects, including the human body. In the case of the magnetic resonance imaging (MRI) procedures, the induced current is a by-product of the pulsed gradient field rather than a requirement of the imaging process. The level of magnetically induced current that can be caused by existing MR imagers is well below levels needed to stimulate excitable tissue. However, recent developments in fast imaging techniques are pressing technology towards rapidly switched gradient fields of such an intensity that nerve or heart excitation may be possible. Consequently, the design of future MRI devices may need to include certain constraints if one wishes to avoid excitatory responses in the exposed subject. The purpose of this article is to review the biological and physical principles through which these responses may be understood.