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

Area of Science:

• Biophysics
• Neuroscience
• Medical Imaging

Background:

• Magnetic Resonance Imaging (MRI) utilizes strong magnetic fields.
• Concerns exist regarding potential biological effects of these fields on neural tissue.
• Understanding axonal stimulation thresholds is crucial for MRI safety.

Purpose of the Study:

• To analyze axonal stimulation under MRI magnetic fields.
• To determine the threshold parameters for axonal stimulation.
• To compare the efficacy of different field parameters as indicators of stimulation.

Main Methods:

• Solving the cable equation for axons under magnetic field gradients.
• Utilizing a one-dimensional compartmental cable model of mammalian myelinated fibers.
• Performing computer simulations with sinusoidal and trapezoidal magnetic field waveforms.

Main Results:

• Axonal stimulation threshold is lowest for largest diameter axons in regions of maximum magnetic field.
• Optimized trapezoidal waveforms offer superior sub-threshold resolution compared to sinusoidal waveforms.
• The induced electric field is a more reliable indicator of stimulation threshold than magnetic field or its time derivative.

Conclusions:

• Axon diameter and location within the magnetic field influence stimulation susceptibility.
• Trapezoidal magnetic field waveforms present potential advantages for MRI applications.
• Induced electric field serves as the most effective metric for predicting axonal stimulation during MRI.