1 Abstract-Models and simulations are commonly used to study deep brain stimulation (DBS). Simulated stimulation fields are often defined and visualized by electric field isolevels or volumes of tissue activated (VTA). The aim of the present study was to evaluate the relationship between stimulation field strength as defined by the electric potential, V, the electric field, E, and the divergence of the electric field , and neural activation. Axon cable models were developed and coupled to finite element DBS models in 3D. Field thresholds (VT, ET, and VT) were derived at the location of activation for various stimulation amplitudes (1 to 5 V), pulse widths (30 to 120 µs), and axon diameters (2.0 to 7.5 µm). Results showed that thresholds for VT and VT were highly dependent on the stimulation amplitude while ET, were approximately independent of the amplitude for large axons. The activation field strength thresholds presented in this study may be used in future studies to approximate the VTA during model-based investigations of DBS without the need of computational axon models.

Patient-specific DBS electric field simulations in the GPi as visualized on proton-density MR scans can be implemented in patients with GTS. Visualization of electric fields together with stereotactic thin-slice MRI can provide further support when predicting anatomical structures possibly influenced by DBS in this complex disorder.

Background: The mechanism by which deep brain stimulation of the nucleus subthalamicus improves Parkinson's disease symptoms remains unclear. In a previous perioperative study, we showed that there might be alterations of neurotransmitter levels in the globus pallidum interna during deep brain stimulation of the nucleus subthalamicus. Aim: In this study, we examined whether deep brain stimulation of the nucleus subthalamicus and levodopa infusion interact and affect the levels of neurotransmitters. Methods: Five patients with advanced Parkinson's disease took part in the study. During subthalamic nucleus surgery, microdialysis catheters were inserted bilaterally in the globus pallidum interna and unilaterally in the right putamen. A study protocol was set up and was followed for 3 days. Levodopa infusion with and without concomitant bilateral deep brain stimulation of the nucleus subthalamicus was also carried out. Results: The putaminal dopamine levels increased during deep brain stimulation of the nucleus subthalamicus. In addition, an increase of gamma amino buturic acid concentrations in the globus pallidum interna during deep brain stimulation of the nucleus subthalamicus and during levodopa infusion was found. Conclusions: These findings provide evidence that the subthalamic nucleus has a direct action on the substantia nigra pars compacta, and that deep brain stimulation of the nucleus subthalamicus might indirectly release putaminal dopamine. There is also evidence that deep brain stimulation of the nucleus subthalamicus interferes with levodopa therapy resulting in higher levels of levodopa in the brain, explaining why it is possible to decrease levodopa medication after deep brain stimulation surgery.

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Deep brain stimulation (DBS) is widely used for reduction of symptoms caused by movement disorders. In this chapter a patient-specific finite element method for modeling and simulation of DBS electric parameters is presented. The individual’s stereotactic preoperative MR-batch of images is used as input to the model in order to classify tissue type and allot electrical conductivity for cerebrospinal fluid, blood and grey as well as white matter. With patient-specific positioning of the DBS electrodes the method allows for investigation of the relative electric field changes in relation to anatomy and DBS-settings. Examples of visualization of the patient-specific electric entities together with the surrounding anatomy are given. The use of the method is exemplified on patients with Parkinson’s disease. Future applications including multiphysics simulations and applicability for new DBS targets and symptoms are discussed