Construction and Evaluation of Rodent-Specific rTMS Coils

Abstract

Rodent models of transcranial magnetic stimulation (TMS) play a crucial role in aiding the understanding of the cellular and molecular mechanisms underlying TMS induced plasticity. Rodent-specific TMS have previously been used to deliver focal stimulation at the cost of stimulus intensity (12 mT). Here we describe two novel TMS coils designed to deliver repetitive TMS (rTMS) at greater stimulation intensities whilst maintaining spatial resolution. Two circular coils (8 mm outer diameter) were constructed with either an air or pure iron-core. Peak magnetic field strength for the air and iron-cores were 90 and 120 mT, respectively, with the iron-core coil exhibiting less focality. Coil temperature and magnetic field stability for the two coils undergoing rTMS, were similar at 1 Hz but varied at 10 Hz. Finite element modeling of 10 Hz rTMS with the iron-core in a simplified rat brain model suggests a peak electric field of 85 and 12.7 V/m, within the skull and the brain, respectively. Delivering 10 Hz rTMS to the motor cortex of anaesthetized rats with the iron-core coil significantly increased motor evoked potential amplitudes immediately after stimulation (n = 4). Our results suggest these novel coils generate modest magnetic and electric fields, capable of altering cortical excitability and provide an alternative method to investigate the mechanisms underlying rTMS-induced plasticity in an experimental setting.

Keywords: electric field; magnetic field; motor evoked potentials; rTMS; rodent models.

FIGURE 1

Schematic diagrams of coils and waveforms. Commercial 70 mm round coil over a rat brain (A). Rodent-specific 8 mm round coil placed over the rat brain (B). Birdseye (top) and side on views (bottom) of the novel air-core coil (left) and iron-core coils (right) (C). Diagram of the input coil voltage (top) and resulting magnetic field output as measured by a hall device (bottom) (D).

FIGURE 2

Characterization of coil properties. Magnetic field decay in the z (A) and xy (B) axes where 0 is the center of the coils, shows the iron-core coil produced a greater peak magnetic field (119.05 mT) than the air-core coil (89.50 mT) with a trade-off of focality. Half-maximum field occurred at ~1.2 mm_{z axis,} ~3.5 mm_{xy axis} (air-core) and ~2 mm_{z axis}, ~4 mm_{xy axis} (iron-core). Changes in the iron-core coil temperature during 600 pulses of 1 and 10 Hz rTMS (C) shows tolerable changes in temperature (≤Δ5°C) at both frequencies. 10 Hz stimulation with the air-core coil resulted in a large temperature change (~Δ17.5°C). Magnetic field stability (D) shows the iron-core coil shows high stability at both 1 and 10 Hz stimulation. Magnetic field stability for the air-core coil at 10 Hz significantly decreased (^∗p < 0.001) at 10 Hz.

FIGURE 3

Finite element modeling of the iron-core coil. The magnitude of the magnetic field (mT) and magnetic flux density in the xy plane (A). The arrows represent the direction of the current density separated in 15 bins. The induced current density within the brain, shown by normalized arrows separated into 12 equal bins for the xy grid and 4 in the z direction (B). Electric field magnitude (V/m) in a coronal slice of the ellipsoids representing the skull and brain below the coil windings (C). The inset shows an enlarged view of the electric field at the brain and skull interface. The simulated electric field strength within the skull and brain as a function of depth (D). The inset shows electric field strength with the brain domain on a different y-axis scale.

FIGURE 4

Finite element modeling of the Magventure BC-65HO butterfly coil. The induced electric field (V/m) in a coronal slice of the ellipsoid model (A). The simulated electric field strength within the skull and brain as a function of depth (B).

FIGURE 5

Characterization of motor evoked potentials (MEP’s) before and after 10 Hz rTMS to the anaesthetized rat motor cortex with the iron-core coil. Raw electromyography (EMG) traces of sham (top) and active (bottom) rTMS (A). Log₁₀ transformation of MEP ratios (post stimulation/baseline) recorded in the right forepaw after 3 min of Sham or 10 Hz rTMS to the left motor cortex. rTMS significantly increased MEP ratios relative to sham stimulation (^∗p < 0.05) (B).