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.