Safety Information Article
                      Information on this page is limited by the terms of our Disclaimer.  Please Read!

       Bioeffects of Gradient Magnetic Fields 

During magnetic resonance (MR) procedures, gradient or time-varying magnetic fields may stimulate nerves or muscles in patients by inducing electrical fields. This topic has been reviewed over the years by various experts including Bencsik, et al. (2007), Schaefer, et al. (2000), Nyenhuis, et al. (1997), and Bourland, et al. (1999). The potential for interactions between gradient magnetic fields and biological tissues is dependent on a variety of factors including the fundamental field frequency, the maximum flux density, the average flux density, the presence of harmonic frequencies, the waveform characteristics of the signal, the polarity of the signal, the current distribution in the body, the electrical properties, and the sensitivity of the cell membrane.


Several investigations have characterized MR system-related, gradient magnetic field-induced stimulation in human subjects. At sufficient exposure levels, peripheral nerve stimulation is perceptible as “tingling” or “tapping” sensations. At gradient magnetic field exposure levels from 50% to 100% above perception thresholds, patients may become uncomfortable or experience pain. At extremely high levels, cardiac stimulation is a concern. However, the induction of cardiac stimulation requires exceedingly large gradient fields that are more than an order of magnitude greater than those used currently by commercially available MR systems.

With regard to gradient magnetic fields, the U.S. Food and Drug Administration considers that MR procedures using rates of change (dB/dt) sufficient to produce severe discomfort or painful nerve stimulation to be a significant risk. These safety standards for gradient magnetic fields associated with present-day MR systems appear to adequately protect patients from potential hazards or injuries.

Interestingly, studies performed in human subjects have indicated that anatomical sites of peripheral nerve stimulation vary depending on the activation of a specific gradient (i.e. x-, y- or, z-gradient). For example, stimulation sites for x-gradients included the bridge of the nose, left side of the thorax, iliac crest, left thigh, buttocks, and the lower back. Stimulation sites for y-gradients included the scapula, upper arms, shoulder, right side of the thorax, iliac crest, hip, hands, and upper back. Stimulation sites for z-gradients included the scapula, thorax, xyphoid, abdomen, iliac crest, and upper and lower back. Peripheral nerve stimulation sites were typically found at bony prominences.

According to Schaefer, et al. (2000), since bone is less conductive than the surrounding tissue, it may increase current densities in narrow regions of tissue between the bone and the skin, resulting in lower nerve stimulation thresholds than expected. Modifying gradient hardware and pulse sequences may be useful strategies to avoid unpleasant peripheral nerve stimulation that occurs with certain MRI techniques, according to Weinberg, et al. (2007).


Abart J, et al. Peripheral nerve stimulation by time-varying magnetic fields. Journal of Computer Assisted Tomography 1997;21:532–538.

Andreuccetti D, et al. Weighted-peak assessment of occupational exposure due to MRI gradient fields and movements in a nonhomogeneous static magnetic field. Med Phys 2013;40:011910.

Bencsik M, Bowtell R, Bowley R. Electric fields induced in the human body by time-varying magnetic field gradients in MRI: Numerical calculations and correlation analysis. Phys Med Biol 2007;52:2337-53.

Bourland JD, Nyenhuis JA, Schaefer DJ. Physiologic effects of intense MRI gradient fields. Neuroimaging Clin North Am 1999;9:363–377.

Budinger TF, et al. Physiological effects of fast oscillating magnetic field gradients. Journal of Computer Assisted Tomography 1991;15:609–614.

Cohen MS, Weisskoff R, Kantor H. Sensory stimulation by time varying magnetic fields. Magn Reson 1990;14:409–414.

de Vocht F, et al. Exposure to alternating electromagnetic fields and effects on the visual and visuomotor systems. Br J Radiol 2007;80:822-8.

Doherty J, Whitman G, Robinson M, et al. Changes in cardiac excitability and vulnerability in NMR fields. Invest Radiol 1985;20:129-135.

Ehrhardt JC, et al. Peripheral nerve stimulation in a whole-body echo-planar imaging system. J Magn Reson Imag 1997;7:405–409.

Feldman RE, et al. Experimental determination of human peripheral nerve stimulation thresholds in a 3-axis planar gradient system. Magn Reson Med 2009;62:763-70.

Fuentes MA, et al. Analysis and measurements of magnetic field exposures for healthcare workers in selected MR environments. IEEE Trans Biomed Eng 2008;55:1355-64.

Glover PM. Interaction of MRI field gradients with the human body. Phys Med Biol 2009;54:R99-R115.

Glover PM, et al. Measurement of visual evoked potential during and after periods of pulsed magnetic field exposure. J Magn Reson Imag 2007;26:1353-6.

Ham CLG, et al. Peripheral nerve stimulation during MRI: Effects of high gradient amplitudes and switching rates. J Magn Reson Imag 1997;7:933–937.

Hartwig V, et al. Biological effects and safety in magnetic resonance imaging: A review. Int J Environ Res Public Health. 2009;6:1778-98.

Irnich W, Schmitt F. Magnetostimulation in MRI. Magn Reson Med 1995;33:619–623.

International Commission on Non-Ionizing Radiation Protection (ICNIRP) statement, medical magnetic resonance procedures: Protection of patients. Health Physics 2004;87:197-216.

Kangarlu A, Tang L, Ibrahim TS. Electric field measurements and computational modeling at ultrahigh-field MRI. Magnetic Resonance Imaging 2007;25:1222-1226.

Kannala S, et al. Occupational exposure measurements of static and pulsed gradient magnetic fields in the vicinity of MRI scanners. Phys Med Biol 2009;54:2243-57.

King KF, Schaefer DJ. Spiral scan peripheral nerve stimulation. J Magn Reson Imag 2000;12:164-170.

Li Y, et al. Numerically-simulated induced electric field and current density within a human model located close to a z-gradient coil. J Magn Reson Imag 2007;26:1286-95.

Mansfield P, Harvey PR. Limits to neural stimulation in echo-planar imaging. Magn Reson Med 1993;29:746–758.

Nyenhuis JA, et al. Health effects and safety of intense gradient fields. In: Shellock FG, Editor. Magnetic Resonance Procedures: Health Effects and Safety. Boca Raton, FL: CRC Press, 2001;31-54.

Nyenhuis JA, et al. Analysis from a stimulation perspective of magnetic field patterns of MR gradient coils. J Appl Phys 1997;81:4314–4316.

Schaap K, et al. Exposure to static and time-varying magnetic fields from working in the static magnetic stray fields of MRI scanners: A comprehensive survey in the Netherlands. Ann Occup Hyg 2014;58:1094-1110.

Schaefer DJ, Bourland JD, Nyenhuis JA. Review of patient safety in time-varying gradient fields. J Magn Reson Imag 2000;12:20-29.

Schwenzer NF, et al. Do static or time-varying magnetic fields in magnetic resonance imaging (3.0 T) alter protein-gene expression? A study on human embryonic lung fibroblasts. J Magn Reson Imag 2007;26:1210-5.

Shellock FG, Crues JV. MR procedures: Biologic effects, safety, and patient care. Radiology 2004;232:635-652.

U.S. Department of Health and Human Services, Food and Drug Administration, Center for Devices and Radiological Health, Criteria for Significant Risk Investigations of Magnetic Resonance Diagnostic Devices, Guidance for Industry and Food and Drug Administration Staff. June 20, 2014.

van Nierop LE, et al. Simultaneous exposure to MRI-related static and low-frequency movement-induced time-varying magnetic fields affects neurocognitive performance: A double-blind randomized crossover study.  Magn Reson Med 2015;74:840-9.  

Vogt FM, et al. Increased time rate of change of gradient fields: Effect on peripheral nerve stimulation at clinical MR imaging. Radiology 2004;233:548-554.

Weinberg IN, et al. Increasing the oscillation frequency of strong magnetic fields above 101 kHz significantly raises peripheral nerve excitation thresholds. Med Phys 2012;39:2578-83.

Weintraub MI, et al. Biologic effects of 3 Tesla (T) MR imaging comparing traditional 1.5-T and 0.6-T in 1,023 consecutive outpatients. J Neuroimaging 2007;17:241-5.

  (c) 2017 by Shellock R & D Services, Inc. and Frank G. Shellock, Ph.D. All Rights Reserved. All copyrights and pertinent trademarks are owned by Shellock R & D Services, Inc. and Frank G. Shellock, Ph.D. No part of the MRISAFETY.COM web site may be reproduced, stored in any retrieval system, or transmitted in any form or by any means, physical, electronic or otherwise, without the prior written permission of Shellock R & D Services, Inc. or Frank G. Shellock, Ph. D. Request for permission to reproduce any information contained on the MRISAFETY.COM web site should be addressed to:
Be sure to read our Disclaimer.