# Electric Field Characteristics of Rotating Permanent Magnet Stimulation

## TL;DR

This study examines how rotating permanent magnets generate electric fields in the brain, finding that the strength is much lower than traditional methods and may affect clinical trial results.

## Contribution

The study provides a detailed characterization of E-field patterns and strengths from rotating magnet configurations for neurostimulation.

## Key findings

- E-field strength on the head surface ranged from 0.0092 to 0.52 V/m.
- E-field strength is significantly lower than conventional transcranial magnetic stimulation.
- Rotational frequency impacts E-field strength, potentially affecting clinical trial outcomes.

## Abstract

Neurostimulation devices that use rotating permanent magnets are being explored for their potential therapeutic benefits in patients with psychiatric and neurological disorders. This study aims to characterize the electric field (E-field) for ten configurations of rotating magnets using finite element analysis and phantom measurements. Various configurations were modeled, including single or multiple magnets, and bipolar or multipolar magnets, rotated at 10, 13.3, and 350 revolutions per second (rps). E-field strengths were also measured using a hollow sphere (r=9.2 cm) filled with a 0.9% sodium chloride solution and with a dipole probe. The E-field spatial distribution is determined by the magnets’ dimensions, number of poles, direction of the magnetization, and axis of rotation, while the E-field strength is determined by the magnets’ rotational frequency and magnetic field strength. The induced E-field strength on the surface of the head ranged between 0.0092 and 0.52 V/m. In the range of rotational frequencies applied, the induced E-field strengths were approximately an order or two of magnitude lower than those delivered by conventional transcranial magnetic stimulation. The impact of rotational frequency on E-field strength represents a confound in clinical trials that seek to tailor rotational frequency to individual neural oscillations. This factor could explain some of the variability observed in clinical trial outcomes.