Stimulation by Gradients

I've heard MRI may cause a patient's arm or leg to twitch during scanning. What causes this? Is it dangerous?  

What you are referring to is Peripheral Nerve Stimulation (PNS), the excitation of nerves in the extremities from electrical voltage potentials induced by rapidly changing magnetic gradients. When mild, PNS may be perceived as a tingling or tapping sensation, often surprising the patient but presenting no real discomfort or physical danger. As stimulation intensity increases, motor nerve depolarization produces progressively severe and painful muscle fasciculations/contractions. In animal models, excitation of cardiac Purkinje fibers with arrhythmias can occur at very high exposure levels (far beyond what humans would experience in the current generation of MR scanners). 

According to Faraday's Law, the intensity of the electric field (E) producing nerve or muscle depolarization is proportional to the rate of change of the magnetic gradient field (dB/dt). As shown in the diagram below, dB/dt (and hence E) are maximized during the ramp up and ramp down portions of the gradient waveform. PNS therefore typically occurs in pulse sequences using rapid gradient switching, such as echo planar, turbo-SE, or SSFP techniques. It should be recognized that the largest gradient fields may occur outside the field-of-view since only the most linear central segments are used for imaging. Additionally, one must add contributions from all three gradients (x-, y-, and z-) to determine the full effect. The strongest induced E fields are typically located in the more superficial portions of the patient where many peripheral nerves run. E fields are also concentrated where tissues of different conductivity (such as bone, fat, and muscle) are juxtaposed or in the vicinity of metallic implants.

Peak dB/dt (and hence E) values occur during gradient ramp up and ramp down periods. Circulating tissue currents reverse when dB/dt is negative. Stimulus duration (t_stim) is the sum of the times corresponding to the 3 red blocks above.

The strongest gradients may occur outside the imaged field-of-view.

Simulated distribution of x-direction magnetic gradient (B) and and gradient switching-induced electric (E) fields in a human subject. Note the gradient (B) fields are strongest near the walls of the scanner (immediately adjacent to the coils). The E fields in the patient are concentrated superficially and accentuated at the junction of bones, fat, and muscle (which have different electrical conductivities). [Images courtesy of Valerie Klein]

Depolarization of nerve and muscle fibers depends on the intensity of the stimulus and the length of time it is applied, represented graphically by the strength-duration curve. The strength and duration are inversely related, producing a hyperbolic-shaped curve whose precise dimensions depend on the tissue being stimulated (nerve vs muscle) as well as how the stimulus is applied (unipolar vs bipolar excitation, rapidity of switching, etc.) 

Representative strength-duration curve for peripheral nerve, representing electrical threshold for stimulation (E_TH). Chronaxie and rheobase definitions are illustrated.

Because of this inverse relationship, a weak stimulus must be applied for a longer period of time to achieve excitation. The rheobase represents the minimum stimulus required for excitation, while the chronaxie is the required time for a depolarizing stimulus applied at twice the rheobase value. ​​Based on studies in human volunteers scanned in whole-body cylindrical magnets using trapezoidal gradients with various ramp times (tstim), the chronaxie for peripheral nerve stimulation is approximately 0.36 ms corresponding to a rheobase of about 2.2 V/m or 20 T/s (expressed in E and dB/dt ​units respectively).

Larger axons are more easily depolarized than smaller ones, as are those with anatomic turns and kinks (where current densities may be concentrated). Cardiac muscle contraction requires levels of simulation at least 10-100 times higher than PNS. Thus a subject accidentally exposed to very high levels of dB/dt would most certainly experience warning peripheral nerve stimulation before reaching levels risking the heart.

The International Electrotechnical Commission in its standard (IEC 60601-2-23:2015) has established acceptable levels to protect patients and subjects against PNS and cardiac stimulation, described in the Advanced Discussion below. The IEC uses an exponential (rather than hyperbolic model) for cardiac limits, resulting in generally acknowledged conservative thresholds (e.g., about 2½ times the PNS limit for 1 ms stimuli).  

Advanced Discussion (show/hide)»

The International Electrotechnical Commission Standard (IEC 60601-2-23:2015) has set the prevailing limits for gradient exposure, also adopted by many other organization such as the US Food and Drug Administration (FDA). The IEC provides several methods to satisfy their requirements:

1. Default dB/dt limits for whole body systems.. Use the threshold equation [dB/dt]_TH = rheobase x (1 + 0.36/t_samp) with rheobase of 20 T/s where t_samp is the stimulus duration time in msec. Gradient output limits for Normal Operating Mode and First Level Controlled Mode are defined as 80% and 100% of these values respectively.

2. Default electric field (E) limits for all gradient systems.. Use the threshold equation [E]_TH = rheobase x (1 + 0.36/t_samp) with rheobase of 2.2 V/m where t_samp is the stimulus duration time in msec. Gradient output limits for Normal Operating Mode and First Level Controlled Mode are defined as 80% and 100% of these values respectively.

3. Direct determination in volunteers. (This can apply to both whole body and special purpose gradients.) Using different pulse durations and axes, PNS stimulation thresholds are established in at least 11 volunteers. Gradient output limits for Normal Operating Mode and First Level Controlled Mode are defined as 80% and 100% of the lowest PNS perception level recorded.

IEC Cardiac Stimulation Limits. The IEC uses an exponential model for cardiac limits. (A hyperbolic model is used for PNS). The relevant equation is E < 2 ÷ [1 − exp(−t_stim/3)]

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References
     Davids M, Guérin B, vom Endt A, et al. Prediction of peripheral nerve stimulation thresholds of MRI gradient coils using coupled electromagnetic and neurodynamic simulations. Magnetic Reson Med 2019; 81:686-701. [DOI Link] 
     Glover PM. Interaction of MRI field gradients with the human body. Phys Med Biol 2009; 54:R99-R115. [DOI Link]  (good review)
     International Electrotechnical Commission. IEC 60601-2-33:2015: Medical Electrical Equipment - Part 2-33: Particular Requirements for the Basic Safety and Essential Performance of Magnetic Resonance Equipment for Medical Diagnosis. 3rd ed. with amendments. International Electrotechnical Commission; 2015. (all the details and references here)
     Klein V, Davids M, Schad LR, et al. Investigating cardiac stimulation limits of MRI gradient coils using electromagnetic and electrophysiological simulations in human and canine body models. Magn Reson Med 2020; 0:1-12. [DOI Link] 
     Reilly JP. Peripheral nerve stimulation by induced electric currents: exposure to time-varying magnetic fields. Med & Biol Eng & Comput 1989; 27:101-110. [DOI Link]
     Schaefer DJ, Bourland JD, Nyenhuis JA. Review of patient safety in time-varying gradient fields. J Magn Reson Imaging 2000; 12:20-29. [DOI Link] 

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