Analysis from a stimulation perspective of the field patterns of magnetic resonance imaging gradient coils

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Abstract
Gradient coils in a magnetic resonance imager are used to produce spatial variations of magnetic field. The gradients are switched at audio frequencies and for long ramp times dB/dt ≃ 60 T/s may be sufficient to induce peripheral nerve stimulation. The magnetic field pattern from the gradient coils is non-uniform, especially near the windings, and the question arises where the peak dB/dt should be specified. We propose methods for specifying the fields so that the potential for peripheral nerve stimulation for different coil geometries can be compared. The coils considered are the z gradient coil, which produces the gradient ∂B z /∂z and the transverse gradient coils which produce ∂B z /∂x and ∂B z /∂y . Our analysis for the z coil shows that taking the maximal value of the magnetic field on the axis of the bore provides a reasonable representation of the potential for stimulation. The field distribution is more complex for the transverse coils. We conclude that the maximal radial magnetic field on the axis of the bore, which occurs near the center of an opposing coil pair, provides a good estimate of the potential for stimulation. Making use of the field measurements presented here and the knowledge of coil current required to achieve the desired gradient intensity will allow accurate comparison of the potential for stimulation for different types of whole body gradient coils.

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