Transcranial Magnetic Stimulation is an emerging non-invasive treatment for depression, Parkinson’s disease, and a variety of other neurological disorders. Many Parkinson’s patients receive the treatment known as Deep Brain Stimulation, but often require additional therapy for speech and swallowing impairment. Transcranial Magnetic Stimulation has been explored as a possible treatment by stimulating the mouth motor area of the brain. We have calculated induced electric field, magnetic field, and temperature distributions in the brain using finite element analysis and anatomically realistic heterogeneous head models fitted with Deep Brain Stimulation leads. A Figure of 8 coil, current of 5000 A, and frequency of 2.5 kHz are used as simulation parameters. Results suggest that Deep Brain Stimulation leads cause surrounding tissues to experience slightly increased E-field (ΔEmax=30 V/m), but not exceeding the nominal values induced in brain tissue by Transcranial Magnetic Stimulation without leads (215 V/m). The maximum temperature in the brain tissues surrounding leads did not change significantly from the normal human body temperature of 37 °C. Therefore, we ascertain that Transcranial Magnetic Stimulation in the mouth motor area may stimulate brain tissue surrounding Deep Brain Stimulation leads, but will not cause tissue damage.

Transcranial Magnetic Stimulation (TMS) is a non-invasive neuromodulation technique which utilizes external time-varying magnetic fields to induce electric fields within the conductive tissues of the brain, thus downregulating or upregulating targeted regions. TMS is currently FDA-approved for treatment of depression, but shows promise for treating Parkinson’s disease (PD), Schizophrenia, Obsessive Compulsive disorder, and a variety of other neurological conditions.1 A primary concern for clinicians when considering TMS therapy is the risk of tissue damage and seizure from over-stimulation. In addition, for PD and Essential Tremor (ET) patients, a widely used treatment for tremor control is Deep Brain Stimulation (DBS), in which one (unilateral) or two (bilateral) leads are placed inside the brain to deliver current directly to the globus pallidus internus (GPi) or subthalamic nucleus (STN).2,3 These leads are comprised of four electrodes, insulating material, and a wire running through the center of the lead body. DBS is a reliable and successful treatment for controlling tremor; however, PD patients often suffer from other symptoms, such as difficulty with speech and swallowing, which are caused by dysfunction in the mouth motor area of the brain, which is not accessible to DBS leads.4,5 In these cases, supplementary TMS treatment to the mouth motor area can be a significant benefit to the patient’s quality of life. It has been unclear in the past, however, whether the E-field induced from TMS would stimulate GPi and STN due to the conductive material within the lead. DBS provides sufficient stimulation to these regions, therefore additional stimulation from TMS may be hazardous to the patient’s health. Therefore stimulation in tissues surrounding DBS lead in the presence of TMS must be considered. Limited work has been done to study these effects, with results indicating that tissue damage may occur.6,7 However, we posit that the models used in these studies did not possess the complexity needed to replicate the human brain, and that simulation with a heterogeneous head model with accurate parameters may give dissimilar results. We use Sim4Life, a finite element analysis software developed by Zurich MedTech, to perform such simulations. Additionally, while various coil shapes and designs have been proposed in the past for increased focality and optimization, we use a model of the commercial, FDA-approved Figure-of-8 coil to simulate a more realistic scenario.8 

Protocol for the desired results requires accurate, anatomically realistic head models and quasi-static solvers suitable for low frequency stimulation parameters. This study used the “Duke” model, which has been developed by Zurich MedTech from MRI data of a real patient. This model includes full heterogeneity, including a variety of brain tissues such as grey matter, white matter, cerebrospinal fluid, and thalamus, with best estimates of density, conductivity, and permittivity parameters obtained by Zurich MedTech.8 A frequency of 2500 Hz was used for a Figure-of-8 coil, which was operated at currents ranging from 1000 to 5000 Amps and situated near the mouth motor area of the brain. A Deep Brain Stimulation probe model was developed and simulated as “off”, with no current running through the lead. Figure 1 shows a visualization of the setup and a comparison of the full and simplified DBS probe models. We model conductors in the probe as Perfect Electrical Conductors in E&M simulations and as pure platinum in thermal simulations.

FIG. 1.

(a) Figure-of-8 coil positioned 5 mm away from model’s head, by mouth motor area. (b) DBS probe visibly propagating through grey matter. (c) Full probe model with insulators (blue) and conductors (pink). (d) Simplified probe model in which conductors and insulators are placed further apart for simulation ease. (e) Wire and wire insulation (green), which is included in both full and simple probe models.

FIG. 1.

(a) Figure-of-8 coil positioned 5 mm away from model’s head, by mouth motor area. (b) DBS probe visibly propagating through grey matter. (c) Full probe model with insulators (blue) and conductors (pink). (d) Simplified probe model in which conductors and insulators are placed further apart for simulation ease. (e) Wire and wire insulation (green), which is included in both full and simple probe models.

Close modal

Vector Potential, decoupled from the E-field, is calculated using the Biot-Savart Law, and Maxwell’s equations are used to calculate H-field, B-field, and E-field induced in the tissues similar to our previous publications.8–10 Previous work has cited charge density as a means of determining the occurrence of tissue damage.6 However, current density is generally computed using time and pulse durations. Our electromagnetic simulation is quasi-static, and we do not use discrete pulses but rather a continuous source of current. Therefore, rather than using current density, we will instead consider tissue damage as a function of heat.

We use the Transient Thermal Simulation allows to compute time-varying temperature distribution in tissue. This solver uses the Pennes Bio-Heat Equation, finds heat generation and heat transfer in the time domain. Table I gives our referenced thermal values for probe materials.7,11,12

TABLE I.

Referenced values for probe materials used for simulations.

MaterialMass Density(kg/m3)Specific HeatCapacity, c(J/kg/K)Thermal Conductivity(W/m/K)Electrical Conductivity(S/m)Relative Permittivity
Conductor (Platinum) 21,450 133 72 N/A for PEC N/A for PEC 
Insulator 1780 1200 0.2 
MaterialMass Density(kg/m3)Specific HeatCapacity, c(J/kg/K)Thermal Conductivity(W/m/K)Electrical Conductivity(S/m)Relative Permittivity
Conductor (Platinum) 21,450 133 72 N/A for PEC N/A for PEC 
Insulator 1780 1200 0.2 

We compute magnetic field, induced electric field and temperature in brain tissue surrounding DBS lead to determine if presence of DBS lead will cause health issues in the patient. Excessive E-field will lead to over-stimulation and consequently adverse effects such as motor contractions, tingling, and mood changes, while excessive heat in brain tissue will cause tissue damage. Figure 2 shows major results from simulations run using 5000 Amps. Increased E-field and temperature can be observed in mouth motor area due to placement of the Figure-of-8 TMS coil. We position the DBS lead along the z-axis, from cortex to basal ganglia, as is the case for patients with PD. Figure 2c shows a sagittal slice in close proximity to DBS lead, and stimulation in tissues surrounding DBS lead can be observed. E-field and time-varying temperature have been calculated and shown in Figures 3 and 4. Note that in Figure 3, the maximum E-field induced in brain tissue is a linear function of the current value in the coils. While the presence of DBS lead increased E-field induced by TMS in the surrounding tissue, the values were smaller than the E-field induced in the mouth motor region of the brain. Therefore, in our simulations the presence of DBS lead does not cause excessive stimulation in surrounding tissue. Additionally, because PD patients receive electrical stimulation in the basal ganglia from DBS, clinicians must be certain that no additional stimulation is provided to basal ganglia area from TMS. Figure 3 shows that the E-field induced in basal ganglia tissue of the model is too low to cause stimulation. Finally, Figure 4 shows time-varying temperature distribution in brain tissue. We first allowed the brain model 5000 seconds to reach steady-state, then applied TMS from 5000 seconds to 6000 seconds, and finally we allowed a second rest period to observe heat diffusion. Total simulation time was 10000 seconds. Maximum temperature in the model’s brain in all cases remained below 37.35 °C. Because the overall typical body temperature is 37 °C, this variation can be considered negligible, especially when considering heat diffusion in the body over time. Furthermore, while tissue surrounding DBS lead can be seen to increase slightly in the presence of TMS, these temperatures remain below 37.15 °C in all cases. Simulations were performed on the model of a healthy patient with normal vasculature and blood flow; therefore, the effect of decreased vasculature and blood flow is not shown here.

FIG. 2.

(a), (b) Induced E-field on grey matter surface; scale: 0 – 213 V/m. (c) E-field through coronal slice; scale: 0 – 50 V/m. (d) Temperature of grey matter surface at 5900 sec; scale: 37 – 37.4 °C.

FIG. 2.

(a), (b) Induced E-field on grey matter surface; scale: 0 – 213 V/m. (c) E-field through coronal slice; scale: 0 – 50 V/m. (d) Temperature of grey matter surface at 5900 sec; scale: 37 – 37.4 °C.

Close modal
FIG. 3.

Maximum values for E-field in motor mouth (MM) area, brain tissue surrounding DBS lead, and tissue in basal ganglia (BG) area for various current values.

FIG. 3.

Maximum values for E-field in motor mouth (MM) area, brain tissue surrounding DBS lead, and tissue in basal ganglia (BG) area for various current values.

Close modal
FIG. 4.

Time-varying temperature distribution in brain tissues during rest stage (1000 sec – 5000 sec), TMS (5000 sec – 6000 sec), and second rest stage (6000 sec to 10000 sec). Solid lines show Mouth Motor area tissue temperature while dashed lines show temperature in tissue surrounding DBS lead.

FIG. 4.

Time-varying temperature distribution in brain tissues during rest stage (1000 sec – 5000 sec), TMS (5000 sec – 6000 sec), and second rest stage (6000 sec to 10000 sec). Solid lines show Mouth Motor area tissue temperature while dashed lines show temperature in tissue surrounding DBS lead.

Close modal

In the recent past, studies have analyzed, either computationally or using simple physical models, the effect of TMS on brain tissue. However, very few of these studies have replicated the presence of Deep Brain Stimulation leads in a brain which receives TMS treatment. Those that have done so have used simple spherical head models and non-commercial circular coil shapes. We use a complex, heterogeneous, anatomically correct head model with a commercial double coil design, as well as a DBS lead modeled using standard, commercial-grade DBS leads. We use the Finite Element Analysis software, Sim4Life, to solve highly refined grids for electric field and time-varying temperature distribution in brain tissue. Analysis of these simulations suggest that while the presence of DBS leads may slightly increase the induced electric field in surrounding tissues, no overstimulation or overheating occurs. Therefore, our computations indicate that while DBS is switched off, Parkinson’s patients may safely receive TMS treatment in the motor mouth area for treating speech and swallowing impairment.

We are grateful to Dr. Deepak Kumbhare for helpful comments.

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