Magnetic nerve stimulation: field focality and depth of penetration

Al-Mutawaly, Nafia; Bruin, Hubert de; Findlay, D. L.

doi:10.1109/iembs.2001.1019082

Cited by 6 publications

(8 citation statements)

References 9 publications

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“…Other designs for deep brain TMS coils have been proposed, such as the stretched C-shaped ferromagnetic core coil, 3,68,97 circular crown coil, 3 and large halo coil. 98 The halo coil has been shown to elicit lower limb and trunk muscle contractions in a cynomolgus monkey, suggesting that this coil design is capable of directly stimulating relatively deep brain regions.…”

Section: Introductionmentioning

confidence: 99%

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

2013

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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.

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Section: Introductionmentioning

confidence: 99%

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

2013

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“…Based on analysis and simulations, it was suggested that a C-shaped ferromagnetic core coil with a wide opening angle (Figure 1(b)) can suppress the surface field and, consequently, could reduce scalp stimulation (Davey, 2008; Davey and Riehl, 2006; Al-Mutawaly et al, 2001). It is unclear whether the addition of ferromagnetic cores is practical as they might enter magnetic saturation in the field range needed for dTMS.…”

Section: Introductionmentioning

confidence: 99%

Coil design considerations for deep transcranial magnetic stimulation

Deng

Lisanby

Peterchev

2014

Clinical Neurophysiology

245

155

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Objectives To explore the field characteristics and design tradeoffs of coils for deep transcranial magnetic stimulation (dTMS). Methods We simulated parametrically two dTMS coil designs on a spherical head model using the finite element method, and compare them with five commercial TMS coils, including two that are FDA approved for the treatment of depression (ferromagnetic-core figure-8 and H1 coil). Results Smaller coils have a focality advantage over larger coils; however, this advantage diminishes with increasing target depth. Smaller coils have the disadvantage of producing stronger field in the superficial cortex and requiring more energy. When the coil dimensions are large relative to the head size, the electric field decay in depth becomes linear, indicating that, at best, the electric field attenuation is directly proportional to the depth of the target. Ferromagnetic cores improve electrical efficiency for targeting superficial brain areas; however magnetic saturation reduces the effectiveness of the core for deeper targets, especially for highly focal coils. Distancing winding segments from the head, as in the H1 coil, increases the required stimulation energy. Conclusions Among standard commercial coils, the double cone coil offers high energy efficiency and balance between stimulated volume and superficial field strength. Direct TMS of targets at depths of ~ 4 cm or more results in superficial stimulation strength that exceeds the upper limit in current rTMS safety guidelines. Approaching depths of ~ 6 cm is almost certainly unsafe considering the excessive superficial stimulation strength and activated brain volume. Significance Coil design limitations and tradeoffs are important for rational and safe exploration of dTMS.

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“…In other words, it denotes the maximum distance beneath the coil that the electric field can exceed neuron excitation threshold. Studies have shown that the threshold approximately meets the equation (5) and the variables are defined in TABLE.I [10][11]:…”

Section: Evaluation Of Stimulation Depthmentioning

confidence: 99%

“…But efficiency is not much concerned in practice. Although quite amount of work [5][6][7][8] has been done to investigate the stimulation effect both from TMS devices and biological-effect aspects, there has no unified evaluation criterion for TMS devices, which is of great importance to assess the stimulation effect quantitively instead of vague expression such as µbetter ¶ or µworse ¶. In this paper, preliminary work has been done to propose several indexes which can be used as design guidelines for TMS devices and performance comparison as well.…”

Section: Introductionmentioning

confidence: 99%

Study on performance evaluation system for transcranial magnetic stimulation devices

Liang

et al. 2013

2013 35th Annual International Conference of the IEEE Engineering in Medicine and Biology Society (EMBC)

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Transcranial magnetic stimulation devices have been widely used for clinic and research purposes and their performance such as stimulation strength, depth and focality are of great concern from various studies. However, there is no specific evaluation index nowadays. TMS devices could only be appraised qualitatively with accurate parameters, thus, a preliminary evaluation work for TMS devices is proposed in this paper. Practical indexes are proposed in order to give a clear image of the TMS device performance. This work may provide a design guideline for TMS device, performance evaluation and comparison as well.

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Magnetic nerve stimulation: field focality and depth of penetration

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Cited by 6 publications

References 9 publications

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

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

Coil design considerations for deep transcranial magnetic stimulation

Study on performance evaluation system for transcranial magnetic stimulation devices

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