Peripheral Nerve Stimulation (PNS)
Valerie Klein1,2
1Computer Assisted Clinical Medicine, Medical Faculty Mannheim, Heidelberg University, Mannheim, Germany, 2A. A. Martinos Center for Biomedical Imaging, Department of Radiologoy, Massachusetts General Hospital, Charlestown, MA, United States

Synopsis

Time-varying MRI gradient fields induce electric fields in the patient that can become strong enough to stimulate peripheral nerves, muscles, and possibly even the heart. These unwanted physiological effects significantly limit the performance of modern MRI gradient systems. This course will discuss the mechanisms underlying gradient field interactions with the human body and will show methods used to investigate and to minimize their occurrence.

Target audience

MR scientists and engineers, Clinicians

Objectives

This course will introduce the possible bio-effects of MRI gradient fields, with a special focus on peripheral nerve stimulation (PNS). Participants will learn about the methods that are employed to investigate the physiological gradient-induced effects and how their occurrence can be minimized.

Purpose

Rapidly switching MRI gradient fields induce electric fields (E-fields) in patients that can stimulate muscle tissue or peripheral nerves1,2,3,4. At onset, peripheral nerve stimulation (PNS) is usually perceived as a mild tingling or tapping sensation4. If the E-field amplitude increases even further, the patient may experience pain or violent muscle contractions. An additional safety concern is that very strong E-fields may stimulate the patient’s heart, which could result in life-threatening cardiac arrhythmias such as ventricular fibrillation4,5,6. MRI safety regulations therefore impose limits on the gradient field switching rate dB/dt to minimize PNS and to avoid cardiac stimulation7.

Methods

Nerve and muscle stimulation characteristics are often described in terms of the strength-duration relationship. This curve relates the E-field amplitude required for stimulation to the duration of the E-field pulse (i.e., the time during which an E-field is induced)6,8,9.
In addition to these comparatively simple stimulation laws, electrodynamic models have been used extensively in the past to study the behavior of excitable nerve or muscle cells10,11,12,13,14. These models are based on an electrical-circuit description of the cell membrane and can predict stimulation in response to an extracellular electrical potential. In recent years, significant progress has been made in the investigation of PNS with detailed simulations. These simulations are mainly based on electromagnetic field calculations in realistic computable human body models15,16,17. Furthermore, some approaches couple the simulated E-fields to electrodynamic nerve models to predict the PNS thresholds for a specific gradient coil geometry18,19,20,21.
While PNS simulations have provided some valuable insights into the underlying mechanisms, the PNS characteristics of a new gradient coil prototype are usually evaluated with a measurement study in healthy volunteers. This course will present some of the experiments that have been used to measure both PNS and cardiac stimulation thresholds and discuss the conclusions drawn from these experiments.

Results and Discussion

Several approaches have proven successful in mitigating PNS, including among others the use of asymmetric coil designs22, and smaller gradient linearity regions23,24. Despite its impact on gradient performance, PNS is usually not directly incorporated into the gradient coil design process. The recent success of simulations might make it possible to include an additional PNS constraint in the design optimization in the near future19,20.
While cardiac stimulation has not been a limitation for most gradient systems in the past, novel powerful gradients can reach amplitudes that surpass the regulatory cardiac safety limit25. This indicates a growing significance of this potential safety hazard for future generations of MRI gradients.

Acknowledgements

No acknowledgement found.

References

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Proc. Intl. Soc. Mag. Reson. Med. 28 (2020)