Transcranial Magnetic Stimulation is a non-invasive clinical therapy used to treat depression and migraine, and shows further promise as treatment for Parkinson's disease, Alzheimer's disease, and other neurological disorders. However, it is yet unclear as to how anatomical differences may affect stimulation from this treatment. We use finite element analysis to model and analyze the results of Transcranial Magnetic Stimulation in various head models. A number of heterogeneous head models have been developed using MRI data of real patients, including healthy individuals as well as patients of Parkinson's disease. Simulations of Transcranial Magnetic Stimulation performed on 22 anatomically different models highlight the differences in induced stimulation. A standard Figure of 8 coil is used with frequency 2.5 kHz, placed 5 mm above the head. We compare cortical stimulation, volume of brain tissue stimulated, specificity, and maximum E-field induced in the brain for models ranging from ages 20 to 60. Results show that stimulation varies drastically between patients of the same age and health status depending upon brain-scalp distance, which is not necessarily a linear progression with age.
Patients with advanced Parkinson's disease (PD) often receive deep brain stimulation (DBS) treatment, in which conductive leads are surgically implanted in the brain. While DBS treats tremor and rigidity, patients often continue to suffer from speech and swallowing impairments. There is preliminary evidence that transcranial magnetic stimulation (TMS) of the cortex may be beneficial for these symptoms. However, the potential electromagnetic interactions of the strong magnetic fields from TMS on the conductive leads is unknown, and the combination therapy has not been approved for use. In this article, we report an experimental study of the safety of combining DBS and TMS. We fabricated an anatomically accurate head and brain phantom with electrical conductivities matching cerebrospinal fluid and averaged conductivity of gray and white matter. Induced current on an implanted DBS probe in the brain phantom was measured. Our results show that TMS will induce current values in the range or higher than typical DBS stimulation current. Thus, the combination of TMS/DBS treatment might cause over-stimulation in the brain when stimulated directly over the DBS lead with 100% TMS current intensity.
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