AXONAL STIMULATION UNDER MRI MAGNETIC-FIELD Z-GRADIENT - 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 compartment al cable model including the kinetics of mammalian myelinated fibers, Computer simulations of the model were performed for sinusoidal and tr apezoidal fields, Several axon and field parameters were tested to det ermine the threshold values for axonal stimulation of the magnetic fie ld, induced electric field and time derivative of the magnetic field, The results indicate that 1) threshold for stimulation is lowest for t he 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 deri vative of the magnetic field as indicators of stimulation threshold.