Associate Professor of Biomedical Engineering University of Memphis, United States
Introduction: Transcranial magnetic stimulation (TMS) is a non-invasive brain-stimulating method that has proven to be effective in treating mental disorders such as depression and obsessive-compulsive disorder. TMS is the application of magnetic fields to specific brain areas to induce an electric current. These magnetic fields can be high or low intensity, with excitatory or inhibitory effects, respectively. Depression is defined by low brain activity in the left dorsolateral prefrontal cortex (DLPFC) and hyperactivity in the right DLPFC which causes other associated disorders such as general anxiety. High-intensity repetitive transcranial magnetic stimulation (rTMS) to the left hemisphere DLPFC is a Food and Drug Administration (FDA) approved treatment for drug-resistant depression and is the most used version of TMS in clinical trials. Although protocols exist for determining the ideal coil location for targeting the DLPFC, the location varies depending on the individual brain anatomy and the location method followed; a probabilistic location based on the 5-cm rule, a location based on a coordinate EEG system, or a location based on a form of MRI. Since the coil location can vary from one patient to another, the orientation of the coil could affect the efficacy of the TMS through the E-field distribution and the volume of activated regions. In this study, the effects of coil orientation on maximum E-field and volume of activation on four DLPFC coil locations were observed and compared.
Materials and
Methods: SimNIBS 4.0.1 was used to simulate TMS at four commonly used locations reported in the literature. These four locations were chosen because they represent different techniques for TMS coil placement: the F3 location on the 10-10 EEG system, the 5-cm rule (5 cm towards the DLPFC from the patient’s motor hotspot – location in motor cortex that produces maximal motor-evoked potentials (MEPs) when stimulated) and using activation imaging such as MRI to find a location that yields wanted behavior. The Magstim 70 mm coil was chosen due to its popularity. The stimulation current (dI/dt) was set to 1.5x10^6 A/s and the coil was placed directly in contact with the scalp (0 mm coil-skin distance). TMS trials were run for three different coil orientations for each location: Nz, FCz, and Oz. The Nz orientation is the default orientation commonly used in TMS where the coil is pointing toward the nose, the FCz orientation is approximately a 60-degree clockwise rotation from Nz, and the Oz orientation is a 180-degree rotation from Nz [8]. For each TMS trial, the resulting maximum electric field (E-field, V/m) and the volume of activation (〖mm〗^3) were reported.
Results, Conclusions, and Discussions: The resulting E-field distributions on the cortical surface for different coil orientations placed at the same location were markedly different (Figure 1). The maximum E-field and volume of activation results were similar for the Nz and Oz orientations; however, the FCz orientation produced a lower maximum E-field and a higher volume of activation than both Nz and Oz. The maximum E-field values displayed a percent difference range of 5% to 14% between FCz and Nz and 6% to 13% between FCz and Oz. The volume of activation values showed a percent difference ranging from 15% to 57% between FCz and Nz and 14% to 58% between FCz and Oz (see Figure 2). Simulation results show that the extent of the coil orientation effect on maximum E-field and volume of activation varies based on coil location. Specifically, changing the coil orientation has different effects in the same region of the same brain model. A 60-degree rotation to the coil could increase the volume of activated brain regions by up to 58% which can lead to significantly different TMS results. Most TMS clinics use previously mentioned methods such as MRI to determine the coil location unique to the patient’s brain anatomy. Variation in the percent differences from one location to another shows that using a default location for all patients might not produce maximal results. To accommodate the varying stimulation locations, our study results suggest that coil orientation should be customized for each patient based on the DLPFC stimulation site and the intended target brain region.
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