Figure-eight Coil Research Articles

Immediate Effect of Joint Mobilization on Corticospinal Excitability in Individuals with Functional Ankle Instability

Individuals with functional ankle instability (FAI) often present with problems in neuromuscular control and balance performance. Joint mobilization is a common intervention for ankle joint dysfunction, but its effect on corticospinal excitability (an important component of the neuromuscular control) of the ankle musculature has not been discussed in individuals with functional ankle instability. PURPOSE: To examine the immediate effect of joint mobilization on corticospinal excitability in individuals with FAI. METHODS: This is a single blind, randomized controlled trial. We recruited individuals with self-reported ankle instability (CAIT score≦27), randomly assigned into the control group (CG, n=15, 7F8M, 27.94±6.56 y), or the mobilization group (MG, n=15, 10M5F, 26.48±4.82 y). Single pulse transcranial magnetic stimulation (TMS) was performed using Magstim 220 (Magstim Company, Whitland, UK) and a figure-eight coil (70 mm), and the active motor threshold (AMT), motor evoked potential (MEP) and the cortical silent period were recorded for the fibularis longus (FL) and soleus (SOL) muscles. One session of mobilization for the talocrual joint and the tibiofibular joints was performed for the MG group, and the AMT, MEP, and cortical silent period were compared between the two groups using two-way repeated measures MANOVA. The significance level was set at 0.05. RESULTS: One-session joint mobilization significantly increased the MEP of the FL (350.77± 180.19 uV to 420.95± 201.06 uV vs. 288.42± 102.49 uV to 302.58± 116.40 uV; p = 0.029). No other significant changes were detected between the two groups following the intervention. CONCLUSION: Single session of joint mobilization improved the corticospinal excitability which might have a beneficial effect on the neuromuscular control of the fibularis longus muscle in individuals with functional ankle instability.

Efficacy of repetitive transcranial stimulation with H-coil for treatment of intractable neuropathic pain in lower extremities

Objectives In our previous study, repetitive transcranial magnetic stimulation (rTMS) using figure-8 shaped coil could relieve neuropathic pain (NP) less effectively in the lower extremities than the upper ones. It may be because the primary cortex (M1) of lower extremities locate in the deeper regions. H-coil is a novel development that can stimulate deep neuronal regions effectively. In this study, we compared the clinical effects for NP patients in their lower extremities between rTMS with H-coil, rTMS with figure-8 shaped coil and sham stimulation. Methods This was a randomized, double-blind, three-way crossover trial. 17 NP patients in their lower extremities received three types of stimulations for 5 consecutive days with 17 days follow-up. In each rTMS session, 5-Hz rTMS to M1 corresponding to the painful lower extremity was administered. Outcome measures were visual analogue scale (VAS). Results H-coil rTMS, compared with the sham and figure-8 coil, showed significant pain improvement soon and one hour after rTMS in VAS ( p Conclusions & key message Our findings demonstrated that rTMS with H-coil could be tolerable and provide good pain relief in NP patients in their lower extremities.

除細動を目的とした 8 の字コイルの提案

除細動を目的とした 8 の字コイルの提案

Effectiveness and safety of high dose transcranial magnetic stimulation in schizophrenia with refractory negative symptoms: a randomized controlled study

To evaluate the efficacy and safety of high dose transcranial magnetic stimulation (rTMS) in patients with schizophrenia with refractory negative symptoms. From January 2013 to April 2014 at our institute, 70 hospitalized patients of schizophrenia, according to the diagnostic criteria of Diagnostic and Statistical Manual of Mental Disorders-4th Edition (DSM-IV), aged from 18 to 45 were randomly divided into study group (n = 33) and control group (n = 37). Both kinds and dosages of antipsychotics were preserved as before. All patients were treated with 10 Hz rTMS. rTMS was delivered to the left dorsolateral prefrontal cortex (DLPFC) with a figure-eight solid core coil at 100% motor threshold, 2 times daily for 10 days within 2 weeks. Sham stimulation was used in control group. In both groups, Positive and Negative Syndrome Scale (PANSS) and Treatment Emergent Symptom Scale (TESS) were used to evaluate the efficacy and safety before treatment, at week 1 and week 2, and subjective visual analogue scale (VAS) score was checked after each rTMS session. PANSS was performed 3 times at followup visits of week 4, week 8 and week 12. Compared with before treatment the total score of PANSS and the score of negative symptoms at week 2 declined in study group (q = 3.780, 4.258, P < 0.05), especially the factors of blunted affect, emotional withdrawal and passive/apathetic social withdrawal (q = 3.829, 4.089, 4.072, P < 0.05). At the follow-up of week 4, week 8 and week 12 after treatment, none of the above factors got significant changes in study group (P > 0.05). After 2 weeks' treatment, the effective rates were 43.75% and 11.43% in study group and control group, respectively, and there was a significant difference between two groups (X2 = 8.888, P =0.003). The incidence of headache in study group was higher than that in control group(37.50% vs 8.57%, χ2 = 8.051, P = 0.005). The highest score of pain was (49 ± 14) in study group, which occurred after the first rTMS treatment. Along with the treatment, the score gradually become lower, and the lowest was (11 ± 5) after 20th treatment. Among schizophrenia patients with refractory negative symptoms, 10 Hz rTMS applied 2 times daily within 2 weeks is effective and safe, especially, may improve blunted affect, emotional withdrawal and passive/apathetic social withdrawal.

Randomized crossover sham‐controlled clinical trial of targeted low‐frequency transcranial magnetic stimulation comparing a figure‐8 and a round coil to treat refractory neocortical epilepsy

Determine the efficacy and side effects of low-frequency repetitive transcranial magnetic stimulation (rTMS) to treat refractory neocortical epilepsy and study differences in effect between a figure-8 and round coil type. This single-center randomized sham-controlled crossover trial (NCT01745952 on ClinicalTrials.gov) included 11 patients with well-defined focal epilepsy. rTMS (0.5 Hz) was targeted to the focus during three treatment conditions consisting of 1,500 stimulations/day for 10 weekdays at 90% of resting motor threshold (rMT) followed by a 10-week observation period. Patients were randomized for the order in which the figure-8, round, and sham coil were used. Outcome assessors and patients were blinded to the type of coil used. The primary outcome measure was the percentage of seizure reduction after active rTMS treatment. Other outcome measures were responder rate, quality of life, and side effects. There was no difference between a figure-8 and round coil. None of the patients achieved an overall 50% seizure reduction. One patient responded during 1 month after treatment with either active coil, followed by a significant increase in seizure frequency. Another patient had a fourfold increase in seizure frequency during rTMS treatment. This study provides evidence that rTMS is on average not effective for reducing seizure frequency. No difference in effectiveness between the different coil types was observed. It can, however, exacerbate seizures during treatment and lead to a rebound in seizure frequency after an initial reduction.

Investigating the cortical regions involved in MEP modulation in tDCS.

Transcranial magnetic stimulation (TMS) is used in several studies to evaluate cortical excitability changes induced by transcranial direct current stimulation (tDCS) of the primary motor cortex. Interpretation of these results, however, is hindered by the very different spatial distribution of the electric field (E-field) induced by the two techniques and by the different target neurons that they might act upon. In this study we used the finite element method to calculate the E-field distribution induced by TMS and tDCS in a realistically shaped model of a human head. A model of a commercially available figure-8 coil was placed over a position above the identified hand knob (HK) region. We also modeled two configurations of bipolar tDCS montages with one of the electrodes placed over the HK and a return electrode over the contralateral orbital region. The electrodes over the HK were either rectangular in shape, with an area of 35 cm2 or cylindrical with an area of π cm2 (1 cm radius). To compare the E-field distribution in TMS and the two tDCS models, average values of the E-field's magnitude as well as the polar and azimuthal angle were investigated in the HK region and premotor areas. The results show that both techniques induce fields with different magnitudes and directions in the HK: the field in tDCS is predominantly perpendicular to the cortical surface, contrary to what happens in TMS where the field is mostly parallel to it. In the premotor areas, the magnitude of the E-field induced in TMS was well below the accepted threshold for MEP generation, 100 V/m. In tDCS, the magnitude of the field in these areas was comparable to that induced at the HK with a significant component perpendicular to the cortical surface. These results indicate that tDCS and TMS target preferentially different neuronal structures at the HK. Besides, they show that premotor areas may play a role in the tDCS-induced after effects on motor cortex excitability.

Subject-specific optimization of channel currents for multichannel transcranial magnetic stimulation.

The goal of this work is to develop a focal transcranial magnetic stimulation (TMS) system using a multichannel coil array for high-resolution neuromodulation. We proposed a novel spatially-distributed stimulation strategy to significantly improve the focality of TMS. Computer simulations were conducted to evaluate the proposed approach and test the merits of multichannel TMS. Three different multichannel coil arrays were modeled in addition to a conventional figure-8 coil for comparison. Simulations were performed on finite element head models of six subjects constructed from anatomical MR images via an automated pipeline. Multichannel TMS arrays exhibited significantly more focal induced electric field magnitudes compared to the figure-8 coil. Additionally, electrical steering of stimulation sites without physical movement of the coil array was demonstrated.

Subject-Specific Multiscale Modeling to Investigate Effects of Transcranial Magnetic Stimulation.

Transcranial magnetic stimulation (TMS) is an effective intervention in noninvasive neuromodulation used to treat a number of neurophysiological disorders. Predicting the spatial extent to which neural tissue is affected by TMS remains a challenge. The goal of this study was to develop a computational model to predict specific locations of neural tissue that are activated during TMS. Using this approach, we assessed the effects of changing TMS coil orientation and waveform. We integrated novel techniques to develop a subject-specific computational model, which contains three main components: 1) a figure-8 coil (Magstim, Magstim Company Limited, Carmarthenshire, UK); 2) an electromagnetic, time-dependent, nonhomogeneous, finite element model of the whole head; and 3) an adaptation of a previously published pyramidal cell neuron model. We then used our modeling approach to quantify the spatial extent of affected neural tissue for changes in TMS coil rotation and waveform. We found that our model shows more detailed predictions than previously published models, which underestimate the spatial extent of neural activation. Our results suggest that fortuitous sites of neural activation occur for all tested coil orientations. Additionally, our model predictions show that excitability of individual neural elements changes with a coil rotation of ±15°. Our results indicate that the extent of neuromodulation is more widespread than previous published models suggest. Additionally, both specific locations in cortex and the extent of stimulation in cortex depend on coil orientation to within ±15° at a minimum. Lastly, through computational means, we are able to provide insight into the effects of TMS at a cellular level, which is currently unachievable by imaging modalities.

Characteristics of bowl-shaped coils for transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) has recently been used as a method for the treatment of neurological and psychiatric diseases. Daily TMS sessions can provide continuous therapeutic effectiveness, and the installation of TMS systems at patients' homes has been proposed. A figure-eight coil, which is normally used for TMS therapy, induces a highly localized electric field; however, it is challenging to achieve accurate coil positioning above the targeted brain area using this coil. In this paper, a bowl-shaped coil for stimulating a localized but wider area of the brain is proposed. The coil's electromagnetic characteristics were analyzed using finite element methods, and the analysis showed that the bowl-shaped coil induced electric fields in a wider area of the brain model than a figure-eight coil. The expanded distribution of the electric field led to greater robustness of the coil to the coil-positioning error. To improve the efficiency of the coil, the relationship between individual coil design parameters and the resulting coil characteristics was numerically analyzed. It was concluded that lengthening the outer spherical radius and narrowing the width of the coil were effective methods for obtaining a more effective and more uniform distribution of the electric field.

Basic study on the influence of inhibition induced by the magnetic stimulation on the peripheral nerve

The purpose of this study is to analyze the inhibition mechanism of magnetic stimulation on motor function. A magnetic stimulator with a flat figure-eight coil was used to stimulate the peripheral nerve of the antebrachium. The intensity of magnetic stimulation was 0.8 T, and the stimulation frequency was 1 Hz. The amplitudes of the motor-evoked potentials (MEPs) at the abductor pollicis brevis muscle and first dorsal interosseous muscle were used to evaluate the effects of magnetic stimulation. The effects of magnetic stimulation were evaluated by analyzing the MEP amplitude before and after magnetic stimulation to the primary motor cortex. The results showed that MEP amplitude after magnetic stimulation compared with before magnetic stimulation decreased. Because there were individual differences in MEP amplitude induced by magnetic stimulation, the MEP amplitude after stimulation was normalized by the amplitude of each participant before stimulation. The MEP amplitude after stimulation decreased by approximately 58% (p &amp;lt; 0.01) on average compared with before stimulation. Previous studies suggested that magnetic stimulation to the primary motor cortex induced an increase or a decrease in MEP amplitude. Furthermore, previous studies have shown that the alteration in MEP amplitude was induced by cortical excitability based on magnetic stimulation. The results of this study showed that MEP amplitude decreased following magnetic stimulation to the peripheral nerve. We suggest that the decrease in MEP amplitude found in this study was obtained via the feedback from a peripheral nerve through an afferent nerve to the brain. This study suggests that peripheral excitement by magnetic stimulation of the peripheral nerve may control the central nervous system via afferent feedback.

Transcranial Magnetic Stimulation Changes Response Selectivity of Neurons in the Visual Cortex.

Transcranial magnetic stimulation (TMS) is used to selectively alter neuronal activity of specific regions in the cerebral cortex. TMS is reported to induce either transient disruption or enhancement of different neural functions. However, its effects on tuning properties of sensory neurons have not been studied quantitatively. Here, we use specific TMS application parameters to determine how they may alter tuning characteristics (orientation, spatial frequency, and contrast sensitivity) of single neurons in the cat's visual cortex. Single unit spikes were recorded with tungsten microelectrodes from the visual cortex of anesthetized and paralyzed cats (12 males). Repetitive TMS (4 Hz, 4 s) was delivered with a 70 mm figure-8 coil. We quantified basic tuning parameters of individual neurons for each pre- and post-TMS condition. The statistical significance of changes for each tuning parameter between the two conditions was evaluated with a Wilcoxon signed-rank test. We generally find long-lasting suppression which persists well beyond the stimulation period. Pre- and post-TMS orientation tuning curves show constant peak values. However, strong suppression at non-preferred orientations tends to narrow the widths of tuning curves. Spatial frequency tuning exhibits an asymmetric change in overall shape, which results in an emphasis on higher frequencies. Contrast tuning curves show nonlinear changes consistent with a gain control mechanism. These findings suggest that TMS causes extended interruption of the balance between sub-cortical and intra-cortical inputs.

Effects of pulsed magnetic stimulation on isolated crayfish heart

ABSTRACTCardiac muscular contraction of the neurogenic heart that could be excited by pulsed magnetic stimulation (PMS) was investigated using preparation of the isolated crayfish heart. When a figure-eight magnetic coil was set over the isolated heart, cardiac contraction induced by a single PMS was not observed. Cardiac arrest occurred immediately after repetitive PMS and persisted for dozens of seconds depending on the number of stimuli. We concluded that PMS caused neuronal modulation in the neuronal network in the cardiac ganglion.

Eccentric figure-eight coils for transcranial magnetic stimulation.

Previously we proposed an eccentric figure-eight coil that can cause threshold stimulation in the brain at lower driving currents. In this study, we performed numerical simulations and magnetic stimulations to healthy subjects for evaluating the advantages of the eccentric coil. The simulations were performed using a simplified spherical brain model and a realistic human brain model. We found that the eccentric coil required a driving current intensity of approximately 18% less than that required by the concentric coil to cause comparable eddy current densities within the brain. The eddy current localization of the eccentric coil was slightly higher than that of the concentric coil. A prototype eccentric coil was designed and fabricated. Instead of winding a wire around a bobbin, we cut eccentric-spiral slits on the insulator cases, and a wire was woven through the slits. The coils were used to deliver magnetic stimulation to healthy subjects; among our results, we found that the current slew rate corresponding to motor threshold values for the concentric and eccentric coils were 86 and 78 A/µs, respectively. The results indicate that the eccentric coil consistently requires a lower driving current to reach the motor threshold than the concentric coil. Future development of compact magnetic stimulators will enable the treatment of some intractable neurological diseases at home.

FDTD-based Transcranial Magnetic Stimulation model applied to specific neurodegenerative disorders

Non-invasive treatment of neurodegenerative diseases is particularly challenging in Western countries, where the population age is increasing. In this work, magnetic propagation in human head is modelled by Finite-Difference Time-Domain (FDTD) method, taking into account specific characteristics of Transcranial Magnetic Stimulation (TMS) in neurodegenerative diseases. It uses a realistic high-resolution three-dimensional human head mesh. The numerical method is applied to the analysis of magnetic radiation distribution in the brain using two realistic magnetic source models: a circular coil and a figure-8 coil commonly employed in TMS. The complete model was applied to the study of magnetic stimulation in Alzheimer and Parkinson Diseases (AD, PD). The results show the electrical field distribution when magnetic stimulation is supplied to those brain areas of specific interest for each particular disease. Thereby the current approach entails a high potential for the establishment of the current underdeveloped TMS dosimetry in its emerging application to AD and PD.

Optimization of multiple coils immersed in a conducting liquid for half-hemisphere or whole-brain deep transcranial magnetic stimulation: a simulation study.

Transcranial magnetic stimulation (TMS) was proposed in 1985. Nevertheless, its wider use in the treatment of several neurologic diseases has been hindered by its inability to stimulate deep-brain regions. This is mainly due to the physical limiting effect arising from the presence of surface discontinuities, particularly between the scalp and air. Here, we present the optimization of a system of large multiple coils for whole-brain and half-hemisphere deep TMS, termed orthogonal configuration. COMSOL(®)-based simulations show that the system is capable of reaching the very center of a spherical brain phantom with 58% induction relative to surface maximum. Such penetration capability surpasses to the best of our knowledge that of existing state of the art TMS systems. This induction capability strongly relies on the immersion of the stimulating coils and part of the head of the patient in a conducting liquid (e.g. simple saline solution). We show the impact of the presence of this surrounding conducting liquid by comparing the performance of our system with and without such liquid. In addition, we also compare the performance of the proposed coil with that of a circular coil, a figure-eight coil, and the H-coil. Finally, in addition to its whole-brain stimulation capability (e.g. potentially useful for prophylaxis of epileptic patients) the system is also able to stimulate mainly one brain hemisphere, which may be useful in stroke rehabilitation, among other applications.

Design of a dynamic transcranial magnetic stimulation coil system.

To study the brain activity at the whole-head range, transcranial magnetic stimulation (TMS) researchers need to investigate brain activity over the whole head at multiple locations. In the past, this has been accomplished with multiple single TMS coils that achieve quasi whole-head array stimulation. However, these designs have low resolution and are difficult to position and control over the skull. In this study, we propose a new dynamic whole-head TMS mesh coil system. This system was constructed using several sagittal and coronal directional wires. Using both simulation and real experimental data, we show that by varying the current direction and strength of each wire, this new coil system can form both circular coils or figure-eight coils that have the same features as traditional TMS coils. Further, our new system is superior to current coil systems because stimulation parameters such as size, type, location, and timing of stimulation can be dynamically controlled within a single experiment.

P643: Safety and characterization of a novel multi-channel TMS stimulator

Background: Currently available TMS stimulators have a single channel operating a single coil. Objective: To outline and present physical and physiological benefits of a novel convenient multi-channel stimulator, comprising five channels, where the stimulation parameters of each channel are independently controllable. Methods: Simultaneous and sequential operation of various channels was tested in healthy volunteers. Paired pulses schemes with various inter-stimulus intervals (ISIs) were studied for the hand APB and the leg AH muscles. Energy consumption and coil heating rates with simultaneous operation of 4 channels was compared to a single figure-8 coil. Results: Repetitive operation of separate channels with different stimulation parameters is demonstrated. The operations of various channels can be combined simultaneously or sequentially to induce multiple pulses with ISIs of ms resolution. A universal pattern of inhibition and facilitation as a function of ISI was found, with some dependence on coils configurations and on pulse widths. A strong dependence of the induced inhibition on the relative orientation of the conditioning and test pulses was discovered. The ability of this method to induce inhibition in shallow brain region but not in deeper region, thus focusing the effect in the deep brain region, is demonstrated. A significant saving in energy consumption and a reduction in coil heating were demonstrated for several channels operated simultaneously compared to a standard single channel figure-8 coil. Conclusions: The multi-channel stimulator enables the synchronized induction of different excitability modulations to different brain regions using different stimulation patterns in various channels. Multiple pulses operation with coils with various depth profiles can increase the focality of TMS effect in deep brain regions.

Continuous theta-burst stimulation combined with occupational therapy for upper limb hemiparesis after stroke: a preliminary study

The purpose of this study was to assess the safety, feasibility and efficacy of continuous theta-burst stimulation (cTBS) combined with intensive occupational therapy (OT) for upper limb hemiparesis after stroke. Ten patients with history of stroke and upper limb hemiparesis (age 62.0±11.1years, time since stroke 95.7±70.2months, mean±SD) were studied. Each patient received 13 sessions, each comprising 160s of cTBS applied to the skull on the area of the non-lesional hemisphere (using a 70-mm figure-8 coil, three pulse bursts at 50Hz, repeated every 200ms, i.e., 5Hz, with total stimulation of 2,400 pulses), followed by intensive OT (comprising 120-min one-to-one training and 120-min self-training) during 15-day hospitalization. The motor function of the affected upper limb was evaluated by Fugl-Meyer Assessment (FMA) and Wolf Motor Function Test (WMFT) on the days of admission and discharge. All patients completed the 15-day protocol without any adverse effects. Treatment significantly increased the FMA score (from 46.6±8.7 to 51.6±8.2 points, p<0.01) and shortened the log performance time of WMFT (from 2.5±1.1 to 2.2±1.2s, p<0.01). The 15-day protocol of cTBS combined with intensive OT is a safe and potentially useful therapeutic modality for upper limb hemiparesis after stroke.

A two-site pilot randomized 3 day trial of high dose left prefrontal repetitive transcranial magnetic stimulation (rTMS) for suicidal inpatients.

Suicide attempts and completed suicides are common, yet there are no proven acute medication or device treatments for treating a suicidal crisis. Repeated daily left prefrontal repetitive transcranial magnetic stimulation (rTMS) for 4-6 weeks is a new FDA-approved treatment for acute depression. Some open-label rTMS studies have found rapid reductions in suicidality. This study tests whether a high dose of rTMS to suicidal inpatients is feasible and safe, and also whether this higher dosing might rapidly improve suicidal thinking. This prospective, 2-site, randomized, active sham-controlled (1:1 randomization) design incorporated 9 sessions of rTMS over 3 days as adjunctive to usual inpatient suicidality treatment. The setting was two inpatient military hospital wards (one VA, the other DOD). Research staff screened approximately 377 inpatients, yielding 41 adults admitted for suicidal crisis. Because of the funding source, all patients also had either post-traumatic stress disorder, mild traumatic brain injury, or both. Repetitive TMS (rTMS) was delivered to the left prefrontal cortex with a figure-eight solid core coil at 120% motor threshold, 10 Hertz (Hz), 5 second (s) train duration, 10 s intertrain interval for 30 minutes (6000 pulses) 3 times daily for 3 days (total 9 sessions; 54,000 stimuli). Sham rTMS used a similar coil that contained a metal insert blocking the magnetic field and utilized electrodes on the scalp, which delivered a matched somatosensory sensation. Primary outcomes were the daily change in severity of suicidal thinking as measured by the Beck Scale of Suicidal Ideation (SSI) administered at baseline and then daily, as well as subjective visual analog scale measures before and after each TMS session. Mixed model repeated measures (MMRM) analysis was performed on modified intent to treat (mITT) and completer populations. This intense schedule of rTMS with suicidal inpatients was feasible and safe. Minimal side effects occurred, none differing by arm, and the 3-day retention rate was 88%. No one died of suicide within the 6 month followup. From the mITT analyses, SSI scores declined rapidly over the 3 days for both groups (sham change -15.3 points, active change -15.4 points), with a trend for more rapid decline on the first day with active rTMS (sham change -6.4 points, active -10.7 points, P = 0.12). This decline was more pronounced in the completers subgroup [sham change -5.9 (95% CI: -10.1, -1.7), active -13 points (95% CI: -18.7, -7.4); P = 0.054]. Subjective ratings of 'being bothered by thoughts of suicide' declined non-significantly more with active rTMS than with sham at the end of 9 sessions of treatment in the mITT analysis [sham change -31.9 (95% CI: -41.7, -22.0), active change -42.5 (95% CI: -53.8, -31.2); P = 0.17]. There was a significant decrease in the completers sample [sham change -24.9 (95% CI: -34.4, -15.3), active change -43.8 (95% CI: -57.2, -30.3); P = 0.028]. Delivering high doses of left prefrontal rTMS over three days (54,000 stimuli) to suicidal inpatients is possible and safe, with few side effects and no worsening of suicidal thinking. The suggestions of a rapid anti-suicide effect (day 1 SSI data, Visual Analogue Scale data over the 3 days) need to be tested for replication in a larger sample. ClinicalTrials.gov Identifier: NCT01212848, TMS for suicidal ideation.

Coil design considerations for deep transcranial magnetic stimulation

ObjectivesTo explore the field characteristics and design tradeoffs of coils for deep transcranial magnetic stimulation (dTMS). MethodsWe 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). ResultsSmaller 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. ConclusionsAmong 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 ∼4cm or more results in superficial stimulation strength that exceeds the upper limit in current rTMS safety guidelines. Approaching depths of ∼6cm is almost certainly unsafe considering the excessive superficial stimulation strength and activated brain volume. SignificanceCoil design limitations and tradeoffs are important for rational and safe exploration of dTMS.