Magnetic coil design considerations for functional magnetic stimulation

Abstract

Our studies have demonstrated effective stimulation of the bladder, bowel, and expiratory muscles in patients with spinal cord injury using functional magnetic stimulation. However, one limitation of the magnetic coils (MC) is related to their inability to specifically stimulate the target tissue without activation of surrounding tissue. The primary goal of this study was to determine the governing parameters in the MC design, such as coil configuration, diameter, and number of turns in one loop of the coil. By varying these parameters, our approach was to design, construct, and evaluate the induced electric field distributions of two sets of novel MC's. Based on the slinky coil design, the first set of coils was constructed to compare their abilities in generating induced electric fields for focal nerve excitation. The second set of coils was built to determine the effect that changes in two parameters, coil diameter and number of turns in one loop, had on field penetration. The results showed that the slinky coil design produced more focalized stimulation when compared to the planar round coils. The primary-to-secondary peak ratios of the induced electric field from slinky 1 to 5 were 1.00, 2.20, 2.85, 2.62, and 3.54. We also determined that coils with larger diameters had better penetration than those with smaller diameters. Coils with less number of turns in one loop had higher initial field strengths; when compared to coils that had more turns per loop, initial field strengths remained higher as distance from the coil increased. In our attempt to customize MC design according to each functional magnetic stimulation application and patients of different sizes, the parameters of MC explored in this study may facilitate designing an optimal MC for a certain clinical application.

Magnetic Nerve Stimulation: Field Focality and Depth of Penetration

“…However, the unique design proposed in this study for the core geometry adapts and controls the flux lines produced by the coil and consequently improves its performance. Further, these findings agree with Lin et al [10] regarding the superior performance of slinky-type coils compared to Figure-8 and circular coils. In the proposed coils the introduction of the ferromagnetic material coupled with the inclusion of a tightly wound third set of windings with its own power supply, give greater flexibility for generating a more focussed and deeper field.…”

“…A novel coil, the "slinky" coil, was suggested by Ren et al [9] and Lin et al [10]. Their results show that the magnetic field is stronger and more focused when compared to the circular or butterfly coils.…”

Latency of compound muscle action potentials of the anal sphincter after magnetic sacral stimulation

“…1,12,14,19,20,22 Preliminary LMSS measurements in these patients may be used to test the integrity of the distal motor pathway at both sides.…”

Weak electric fields detectability in a noisy neural network

“…Some studies have proposed SR as a key mechanism in optimizing the performance of specific functional electric stimulation devices, such as hearing implants (Morse and Roper 2000;Zeng et al 2000). Beside this specific application, the functional electric stimulation technique covers a huge class of application fields (Russell et al 1999;Lin et al 2000;Fraccalvieri et al 2015). Transcranial electrical stimulation (TES) with weak currents is actively investigated to treat a range of neurological and psychiatric disorders (Fregni et al 2006;Liebetanz et al 2006), and there is evidence that low-intensity TES can affect ongoing brain activity (Kirov et al 2009;Iliopoulos et al 2014).…”