Spiral scan peripheral nerve stimulation

Time-varying magnetic fields induce electric fields that can cause physiolo gical stimulation. Stimulation has been empirically characterized as a func tion of dB/dt and duration based on experiments using trapezoidal and sinus oidal gradient waveforms with constant ramp time, amplitude, and direction. For two-dimensional (2D) spiral scans, the readout gradient waveforms are frequency- and amplitude-modulated sinusoids on two orthogonal axes in quad rature. The readout gradient waveform therefore rotates with amplitude and angular velocity that are generally not constant. It does not automatically follow that spiral stimulation thresholds can be predicted using available stimulation models. We scanned IS normal volunteers with a 2D spiral scan and measured global thresholds for axial. sagittal, and coronal planes. We concluded that the stimulation model evaluated accurately predicts slew rat e-limited spiral mean stimulation thresholds, if the effective ramp time is chosen to be the half-period at the end of the spiral readout. (C) 2000 Wi ley-Liss, Inc.