Axonal stimulation under MRI magnetic field z gradients: a modeling study.

The stimulation of axons under MRI magnetic fields is analyzed solving the cable equation along the axons in the presence of magnetic field z gradients. Axons are represented using a one-dimensional compartmental cable model including the kinetics of mammalian myelinated fibers. Computer simulations of the model were performed for sinusoidal and trapezoidal fields. Several axon and field parameters were tested to determine the threshold values for axonal stimulation of the magnetic field, induced electric field and time derivative of the magnetic field. The results indicate that 1) threshold for stimulation is lowest for the largest diameter axons terminating in the region where the magnetic field is maximum, 2) trapezoidal waveforms can be optimized to allow better sub-threshold resolution than sinusoidal waveforms, and 3) the induced electric field is better than the magnetic field and time derivative of the magnetic field as indicators of stimulation threshold.