Review of patient safety in time-varying gradient fields.
In magnetic resonance, time-varying gradient magnetic fields (dB/dt) may stimulate nerves or muscles by inducing electric fields in patients. Models predicted mean peripheral nerve and cardiac stimulation thresholds. For gradient ramp durations of less than a few milliseconds, mean peripheral nerve stimulation is a safe indicator of high dB/dt. At sufficient amplitudes, peripheral nerve stimulation is perceptible (i.e., tingling or tapping sensations). Magnetic fields from simultaneous gradient axes combine almost as a vector sum to produce stimulation. Patients may become uncomfortable at amplitudes 50%-100% above perception thresholds. In dogs, respiratory stimulation has been induced at about 300% of mean peripheral nerve thresholds. Cardiac stimulation has been induced in dogs by small gradient coils at thresholds near Reilly's predictions. Cardiac stimulation required nearly 80 times the energy needed to produce nerve stimulation in dogs. Nerve and cardiac stimulation thresholds for dogs were unaffected by 1.5-T magnetic fields.
General Electric Medical Systems, Waukesha, Wisconsin 53188, USA. Daniel.Schaefer@med.ge.com
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Schaefer
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Bourland
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Nyenhuis
D000818:Animals; D004285:Dogs; D004574:Electromagnetic Fields / Q000009:adverse effects*; D004869:Equipment Safety; D005260:Female; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000009:adverse effects* / Q000379:methods*; D008297:Male; D023421:Models, Animal; D008962:Models, Theoretical; D005791:Patient Care; D011835:Radiation Protection / Q000379:methods*; D017751:Safety Management / Q000379:methods*