Electric field depth–focality tradeoff in transcranial magnetic stimulation: Simulation comparison of 50 coil designs

Abstract

Background Various transcranial magnetic stimulation (TMS) coil designs are available or have been proposed. However, key coil characteristics such as electric field focality and attenuation in depth have not been adequately compared. Knowledge of the coil focality and depth characteristics can help TMS researchers and clinicians with coil selection and interpretation of TMS studies. Objective To quantify the electric field focality and depth of penetration of various TMS coils. Methods The electric field distributions induced by 50 TMS coils were simulated in a spherical human head model using the finite element method. For each coil design, we quantified the electric field penetration by the half-value depth, d1/2, and focality by the tangential spread, S1/2, defined as the half-value volume (V1/2) divided by the half-value depth, S1/2 = V1/2/d1/2. Results The 50 TMS coils exhibit a wide range of electric field focality and depth, but all followed a depth–focality tradeoff: coils with larger half-value depth cannot be as focal as more superficial coils. The ranges of achievable d1/2 are similar between coils producing circular and figure-8 electric field patterns, ranging 1.0–3.5 cm and 0.9–3.4 cm, respectively. However, figure-8 field coils are more focal, having S1/2 as low as 5 cm2 compared to 34 cm2 for circular field coils. Conclusions For any coil design, the ability to directly stimulate deeper brain structures is obtained at the expense of inducing wider electrical field spread. Novel coil designs should be benchmarked against comparison coils with consistent metrics such as d1/2 and S1/2.

The dorsal premotor cortex exerts a powerful and specific inhibitory effect on the ipsilateral corticofacial system: a dual-coil transcranial magnetic stimulation study

“…Indeed other authors have found corticocortical connectivity to M1 from the supplementary motor area (Arai et al 2012), which is not far from the dorsal premotor region over which we applied the condTMS. However, the coils that we used for condTMS were considerably smaller (35 mm of outer diameter) than conventional coils, thereby assuring focality of stimulation (Deng et al 2013).…”

Resting-state networks link invasive and noninvasive brain stimulation across diverse psychiatric and neurological diseases

“…115), and modeling the volume of tissue affected by DBS is nontrivial (116,121). Similarly, regions of interest (ROIs) representing sites of noninvasive brain stimulation often had to be approximated based on clinical descriptions and scalp landmarks and used a relatively simple model of tissue activation (44,122,123). We hope that this article and others like it will encourage the use of neuronavigation in future TMS clinical trials, improving our ability to relate brain-stimulation sites to brain networks.…”

Transcranial Magnetic Stimulation-coil design with improved focality

“…Most published research either compares different coil geometries or the effects of anatomical variation, but previous studies have not been able to utilize a broad range of subjects to confirm the potential differences in the stimulation site of different coils. 13,21,23 This paper introduces a new coil design, compares its results with the Figure-8 coil and also discusses the effect of anatomical variation by using 50 head models.…”

“…QBC geometry, without the additional set of smaller coils, is based on Eaton et al and highlighted in Deng et al as a 50mm V-coil. 21,22 There are equal number of windings in both the bigger and smaller coils as in the Figure-8 coil, and left and right coils have current flowing in the same direction at the point where the windings are closest, allowing for summation of field intensities. The reason for adding the smaller coils on top of larger coils in the QBC is to increase the magnetic vector potential over the target stimulation site, which is decreased when the coils are angled upwards.…”