Peripheral Nerve Stimulation
Valerie Klein1,2
1A. A. Martinos Center for Biomedical Imaging, Charlestown, MA, United States, 2Harvard Medical School, Boston, MA, United States

Synopsis

Keywords: Physics & Engineering: Gradient & B0 Safety, Physics & Engineering: Hardware

Time-varying MRI gradient fields induce electric fields in the patient (Faraday induction) that can stimulate excitable tissues such as peripheral nerve fibers. Peripheral nerve stimulation (PNS) can be experienced as a mild tingling or tapping sensation but can lead to muscle contractions and even pain at higher levels, which must be avoided. With new gradient coil designs and increasing power levels, PNS has become a major limitation to gradient performance and thus imaging speed and resolution. This talk addresses different approaches to characterize and mitigate PNS in MRI and will touch upon other gradient-patient interactions, including cardiac and retina stimulation.

Objectives

This course will talk about unwanted bio-effects of MRI gradient systems, which can limit the usable gradient performance and thus degrade imaging speed and resolution. Various numerical and experimental approaches used to characterize and mitigate these effects will be introduced.

Purpose

Gradient coils apply rapidly switched magnetic fields (in the 100 Hz to 5000 Hz range) to encode the image as well as for signal preparation for diffusion or velocity encoding. These time-varying magnetic fields induce electric fields (E-fields) in the human body via Faraday induction, which in turn can stimulate electrically excitable muscle and nerve tissue[1-4]. At onset, peripheral nerve stimulation (PNS) is usually perceived as a mild tingling or tapping sensation. At higher stimulation intensities, PNS can become uncomfortable and even intolerable[4, 5]. While PNS itself is not dangerous, painful stimulations must be avoided. Virtually all gradient systems are limited by PNS, meaning that either the gradient amplitude or the maximum slew rate (gradient amplitude over rise time) cannot be fully used in clinical practice. PNS is therefore becoming the principal limitation to gradient performance and thus imaging speed in MRI. An additional concern next to PNS is that even higher induced E-fields could stimulate the myocardium[4, 6, 7]. While it is highly unlikely that today’s gradient systems reach the threshold for cardiac stimulation (CS), regulatory guidelines such as the IEC 60601-2-33 impose conservative safety limits to avoid CS in patients[8].

Methods

Tissue stimulation is usually characterized by the strength-duration curve[9], which expresses the stimulation threshold as a function of the duration of the applied E-field stimulus with two parameters: a time constant (the “chronaxie”), and a threshold asymptote for long stimulus durations (the “rheobase”). While the strength-duration curve can be used to characterize experimental data, it is not straightforward to derive universal chronaxie and rheobase values that describe stimulation characteristics of all gradient coils (e.g., chronaxie values reported for body gradients vary from 138 µs to 810 µs[10]).
In fact, PNS thresholds depend on a multitude of factors, e.g., the coil design, the patient position in the coil, the patient’s anatomy (size, height, body shape), and the gradient waveform. In recent years, a lot of advances have been made to study the intricate PNS behavior using numerical modeling with increasing complexity. This modeling ranges from E-field simulations[11-14] in simple or more detailed computational body models to coupled electromagnetic-neurodynamic simulations that can predict the response of individual nerves to the gradient-induced E-field[15-18]. These simulations have yielded valuable insights into the underlying mechanisms of action of PNS. For example, it has been shown that the PNS threshold does not only depend on the E-field amplitude (an assumption made by the strength-duration curve), but rather on the tangential E-field projection along the nerve[19].
In practice, PNS limits of a new gradient prototype are usually measured in a healthy volunteer study. This study provides a system-specific analysis of the average PNS threshold, which forms the basis for a safety watchdog that is integrated into the scanner and stops the scan if the mean PNS level is exceeded.

Results and Discussion

Several approaches have been successfully applied to mitigate PNS, including gradient coils with a smaller region of linearity (such as head gradients)[20] or optimized gradient waveforms[21]. Recently, PNS simulations have been successfully incorporated into the coil design process, resulting in novel gradient coils with inherently higher PNS thresholds (and thus better usable gradient performance)[22-25].
Even if the PNS limit can be raised in future generations of gradient systems, the IEC cardiac safety limit will continue to restrict high-amplitude gradient systems. CS is therefore becoming an increasingly relevant safety concern for MRI gradients, even though both recent simulations[26] and measurements in animals[27, 28] have indicated that CS limits are substantially higher than PNS thresholds.

Acknowledgements

No acknowledgement found.

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22. Davids M, Vendramini L, Ferris N G, Klein V, Guérin B, and Wald L L. Design analysis and prototype construction of PNS-optimized MRI gradient coils. In: Proceedings of the Joint Annual Meeting ISMRM-ESMRMB. 2022. London, UK; p. 1369.

23. Davids M, Vendramini L, Klein V, Ferris N G, Guérin B, and Wald L L. Experimental validation of a PNS optimized body gradient coil. In: Proceedings of the Annual Meeting of the ISMRM. 2023. Toronto, Canada; p. 1369.

24. Davids M, Dietz P, Ruyters G, Roesler M, Klein V, Guérin B, Feinberg D, and Wald L L, Peripheral nerve stimulation informed design of a high-performance asymmetric head gradient coil. Magn Reson Med, 2023. 90: 784-801.

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26. Klein V, Davids M, Schad L R, Wald L L, and Guérin B, Investigating cardiac stimulation limits of MRI gradient coils using electromagnetic and electrophysiological simulations in human and canine body models. Magn. Reson. Med., 2021. 85: 1047–1061.

27. Klein V, Coll-Font J, Vendramini L, Straney D, Davids M, Ferris N G, Schad L R, Sosnovik D E, Nguyen C T, Wald L L, and Guérin B, Measurement of magnetostimulation thresholds in the porcine heart. Magn. Reson. Med., 2022. 88: 2242-2258.

28. Klein V, Davids M, Vendramini L, Ferris N G, Schad L R, Sosnovik D E, Nguyen C T, Wald L L, and Guerin B, Prediction of experimental cardiac magnetotimulation thresholds using pig-specific body models. Magn Reson Med, 2023. 90: 1594-1609.

Proc. Intl. Soc. Mag. Reson. Med. 32 (2024)