What you’ll be able to do after this lesson

  • Explain the mechanism by which switched gradients stimulate peripheral nerves
  • Describe how gradient limits are set relative to a stimulation threshold rather than as a fixed number
  • Recognise PNS when a patient describes it, and distinguish it from RF heating
  • Name the sequences and patient factors that make PNS more likely
  • Respond to a patient reporting twitching or tingling during a scan

The mechanism

The imaging gradients are electromagnets that switch on and off very rapidly during a sequence. A changing magnetic field induces an electric field in any conductor within it — and the patient is a conductor.

That induced electric field, if strong enough and applied for long enough, can depolarise a peripheral nerve. The patient feels tingling, twitching, tapping, or a sensation often described as light electric shocks, usually across the shoulders, back, hip, chest wall, or the bridge of the nose.

The quantity that matters is dB/dt — the rate of change of the gradient field with time — which depends on the gradient amplitude and the slew rate, and on where in the body the induced field concentrates.

This is a completely separate hazard from RF heating. RF heating comes from the transmit coil at the Larmor frequency and causes warmth or burning. PNS comes from the gradients at audio frequencies and causes twitching and tingling. They feel different, they arise from different hardware, and they are prevented by different controls. A patient who describes “a tapping on my side” and a patient who describes “my leg is burning” have told you two different things.

How the limit is set

Here is what makes gradient limits different from SAR limits: there is no single universal number in watts or teslas per second that applies to all patients and all scanners.

The magnitude of the induced electric field depends on the gradient hardware, the coil geometry, the axis being switched, the waveform, and — substantially — on the size and shape of the patient. So IEC 60601-2-33 sets the limit relative to a peripheral nerve stimulation threshold determined for the system, using a model with rheobase and chronaxie parameters derived from human volunteer studies.

Against that threshold:

  • Normal operating mode limits gradient output to approximately 80% of the PNS threshold
  • First Level Controlled mode permits up to 100% of the threshold
  • Second Level Controlled mode goes beyond that — current MHRA guidance puts the two controlled modes at 100% and 120% of threshold respectively — and is for research under ethics approval

The MHRA guidance describes the practical consequence plainly: in Normal mode, some patients may experience PNS but uncomfortable PNS is prevented; in the controlled mode, some patients may experience uncomfortable PNS.

Two implications for practice. First, PNS in Normal mode is not necessarily a fault. Some patients will feel something, and the standard anticipates that. Second, because the limit is a percentage of a modelled threshold rather than a physiological guarantee for an individual, an individual patient can be stimulated below it. Your patient’s report is real data, whatever the console says.

Check the current edition of the standard and your local policy — the modelling and the mode definitions are revised between editions.

Note also the terminology: the ACR Manual (2026) standardised on peripheral nerve stimulation. You may still see older documents using other phrasings; they refer to the same phenomenon.

What makes it more likely

Sequences. Anything that switches gradients hard and fast: echo-planar imaging, diffusion-weighted imaging, diffusion tensor imaging, fast gradient echo with strong spoiling, some real-time and cardiac sequences, and high-resolution acquisitions with strong readout gradients. Routine spin echo rarely provokes it.

Axis. Different gradient axes stimulate different body regions. Oblique acquisitions combine axes and can produce stimulation that neither axis would alone.

Patient factors. Larger patients present a larger conducting loop and are stimulated at lower thresholds. Positioning matters for the same reason: crossed or clasped hands, crossed ankles and skin-to-skin contact close conducting loops and lower the threshold — the same positioning discipline that prevents RF burns also reduces PNS. Landmarking that places a large body cross-section at the point of maximum gradient variation increases the effect.

Cardiac stimulation is the theoretical concern beyond PNS, and it is what the limits are ultimately protecting against. The threshold for cardiac stimulation is far above the threshold for peripheral nerve stimulation, which is the point of setting the operating limits where they are: PNS acts as a warning that occurs well before anything dangerous. Reassure patients accordingly, and take the warning seriously.

Recognising and responding

Patients describe PNS in ordinary language: twitching, tapping, tingling, pins and needles, a muscle jumping, a light electric shock, sometimes on the nose or across the shoulders. It appears and disappears with the sequence — which is itself diagnostic, and worth listening for.

What to do:

Tell them beforehand. A patient warned that they may feel tapping or twitching during the fast sequences, that it is not dangerous, and that they should tell you if it becomes uncomfortable, will handle it. An unwarned patient can panic, move, and abandon the scan.

If a patient reports it during a scan, talk to them and find out whether it is tolerable. If it is uncomfortable, stop the sequence.

Then change something. Uncross ankles and hands, pad between skin surfaces, reposition arms away from the trunk. Adjust the sequence: lower the slew rate or gradient amplitude if your system exposes that, lengthen the echo spacing, reduce the acceleration, change the phase-encode direction or slice orientation, or reduce the resolution demand. Consider a different sequence altogether.

Confirm the mode. If the scan is running in First Level Controlled mode, ask why, and whether it needs to be.

Check the implants. Some device conditions cap gradient slew rate, so a PNS-provoking protocol may also be outside a device’s labelled conditions. That is a separate reason to change the sequence.

Document it if it was significant, and report it through your local system.

At the console

A slim, tall patient having a diffusion-weighted abdominal study reports “a twitching in my side, like a muscle jumping” during the DWI, which stops between acquisitions.

That is textbook PNS: a high-slew sequence, symptom onset and offset following the acquisition. It is not a burn — nothing is hot — and it is not a fault.

Talk to the patient, establish whether it is tolerable, and change the conditions rather than repeating the same acquisition and hoping. Check the position first, because it is free: are the arms against the trunk, hands clasped, ankles crossed? Pad and separate. Then, if needed, adjust the sequence — a longer echo spacing, a reduced acceleration, a different phase-encode direction, or lower resolution.

If your system is in First Level Controlled mode for this protocol and there is no clinical reason for it to be, bring it back to Normal.

Second scenario: a patient with a spinal cord stimulator whose IFU caps gradient slew rate. Here PNS management and device conditions converge. The protocol has to be built to respect the slew cap from the outset — this is a protocol-design task with your MRSE and MRSO, not an adjustment made per sequence at three o’clock on a Friday.

Check yourself

In your own words, and without looking back: how would you explain to a nervous patient, in two sentences, what the twitching is and why it is not dangerous? Then the harder one: a patient reports twitching in Normal operating mode, when the console says you are below the system’s limit. Is the console wrong, is the patient wrong, or is neither?

This is a private prompt. Nothing you write is stored or assessed.

Key takeaways

  • Switched gradients induce electric fields in the patient; strong enough, these depolarise peripheral nerves and cause twitching, tapping or tingling.
  • PNS and RF heating are separate hazards with separate hardware, sensations and controls. Warmth is RF; twitching is gradients.
  • IEC 60601-2-33 sets gradient output relative to a modelled PNS threshold — approximately 80% of threshold in Normal operating mode and up to 100% in First Level Controlled mode. Check the current edition and your local policy.
  • PNS in Normal mode is anticipated by the standard; the mode is intended to prevent uncomfortable PNS, not all sensation. An individual can still be stimulated below the limit.
  • EPI, diffusion and other high-slew sequences provoke it; larger patients, oblique axes and closed conducting loops lower the threshold.
  • Uncrossing limbs and padding between skin surfaces reduces PNS as well as burn risk. Warn patients in advance, act on their report, and change something before repeating.
  • Some implant conditions cap gradient slew rate, so PNS management and device conditions can be the same problem.

References

  1. IEC 60601-2-33:2022 (Edition 4.0) — gradient output limits expressed relative to the PNS threshold
  2. Elster AD. Questions and Answers in MRI — MRI operating modes (gradient limits, 80% and 100% of threshold) https://mriquestions.com/operating-modes.html
  3. MHRA. Safety Guidelines for MRI Equipment in Clinical Use, 5th edition (July 2026) — peripheral nerve stimulation https://www.gov.uk/government/publications/safety-guidelines-for-magnetic-resonance-imaging-equipment-in-clinical-use
  4. ACR Manual on MR Safety, 2026 edition — terminology updated to peripheral nerve stimulation; figures verified against Pedrosa I, et al. Radiology, 2025 https://www.acr.org/Clinical-Resources/Clinical-Tools-and-Reference/Radiology-Safety/MR-Safety
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