Dec 16, 2022

A Study of Flex Miniaturized Coils for Focal Nerve Magnetic Stimulation

Micol Colella, Daniel Z. Press, Rebecca M. Lahrer, Courtney E. McIlduff, Seward B. Rutkove, Antonino M. Cassarà, Francesca Apollonio, Alvaro Pascual Leone, Micaela Liberti, and Giorgion Bonmassar, Medical Physics 2022, online 11 December 2022, doi: 10.1112/mp.16148

Peripheral magnetic stimulation (PMS) is an emerging neurostimulation modality that is complementary to the standard treatment, i.e., electrical stimulation of the peripheral nervous system (PNS). Compared to electric stimulation, PMS has the advantage that it can be used to stimulate sensory and motor nerve fibers without the uncomfortable sensations associated with electrical stimulation caused by contact with the electrode. However, stimulation with PMS coils, which are similar to conventional coils used in transcranial magnetic stimulation (TMS), lacks focality and selectivity. In this study, we introduce a novel design and development of a flexible miniaturized coil (mcoil) – two 15 mm diameter coils in a figure-of-eight configuration – for more precise and controlled targeting of peripheral nerves during PMS. Hybrid electromagnetic (EM) and electrophysiological simulations were executed (i) to characterize the EM exposure induced by the mcoil in the arm and along the median nerve of the neuro-functionalized Virtual Population (ViP) human body model Yoon-sun, and (ii) to estimate the threshold current required to induce magnetic radial nerve stimulation. The radial nerves of 11 healthy subjects were stimulated with the mcoil for validation purposes, whereby two electrodes were used to measure sensory nerve action potentials (SNAPs), as well as SNAP conduction velocities. Then, the magnetically induced SNAPs were compared to those induced by standard electrical stimulation. We found that the numerical estimates of SNAP latencies and waveforms agree well with measurements within the uncertainty limits. For the specific mcoil geometry, stimulation was elicited by an induced electric field threshold of 39 V/m underneath the mcoil, where the mcoil was fed with a current of 70 A. It was further found that stimulation was initiated in the median nerve at about 7 mm from the center of the mcoil.

The scientific and technical impact of the study can be summarized as:

  • A novel, flexible miniaturized coil for more precise and controlled targeting of peripheral nerves during PMS was developed
  • Hybrid EM-electrophysiological simulations performed on the ViP Yoon-Sun anatomical model were successful at predicting thresholds for radial nerve stimulation by PMS induced with the novel coil design
  • The computational framework was validated by findings from clinical investigations that confirm the predicted thresholds for stimulation and the principal characteristics of the recorded SNAPs
  • Validation of the EM and electrophysiological models supports the design development of miniaturized coils based on flexible circuits for further improvement of stimulation focality and selectivity