Deep brain stimulation (DBS) is an intervention used for several neurological conditions such as Parkinson’s disease. To evaluate the clinical response in relation to anatomical location, electric field simulation using the finite element method is commonly used. The models presented in different studies are varying in complexity and this study aims to evaluate the effect of including anisotropy in the tissue model using homogenous tissue with varying level of anisotropy both parallel and perpendicular to the DBS lead. As a benchmark, data from one patient was included and simulations was performed in zona incerta (Zi) and the internal capsule (IC). The parameters investigated were impedance, volume within the 0.2 V/mm isosurface, radial and longitudinal expansion as well as visual representation of the isosurface. The investigations show that both the impedance and volume are increasing with increasing anisotropy together with the electric field isosurface in the principal direction of the anisotropy. When comparing different stimulation modes, current control (CC) stimulation had a steeper increase with increasing anisotropy for all parameters compared to voltage control (VC) stimulation. This could be due to a joint effect of the anisotropy and the increasing impedance. The result from the patient simulations are in the anisotropy range where simulations from the homogenous models starts to have a higher slope for all parameters. This indicates that including anisotropy in computer models will be of importance in areas of high anisotropy.

Deep brain stimulation (DBS) is a technique for disruption of pathological activity in the brain by the use of chronically implanted electrodes in the central parts of the brain. DBS is used in e.g. Parkinson’s disease, essential tremor and dystonia and is under investigation for severe cases of Tourette syndrome and obsessive-compulsive disorder. While DBS is nowadays a well-established technique, optimal targeting of brain structures is still not fully known. One way to investigate this is to perform finite element method (FEM) simulations of the activated tissue around the active DBS electrode contacts. The FEM simulations can then be compared with the clinical outcomes of symptom improvement and potential detrimental side effects in the patients. A gratis open access software package for patient-specific FEM simulations based on pre-complied modules is presented. It is based on tissue classification from preoperative magnetic resonance imaging. In the first part, ELMA, the tissue is classified into grey matter, white matter, blood and cerebrospinal fluid. Electric conductivity is assigned based of tissue type and is then used in the second part, DBSim, where patient-specific FEM simulations of the electric field around the DBS electrodes are performed. These simulations can then be used to estimate the volume of tissue activated directly from the electric field magnitude thresholds for different axon diameters or be exported for direct simulation of axon activation in other software. The software package is available for free download at https://liu.se/en/article/ne-downloads   

The autocorrelation of laser speckles from coherent near infrared light is used for noninvasive estimates of relative changes in blood perfusion in techniques such as laser Doppler flowmetry (LDF) and diffuse correlation spectroscopy (DCS). In this study, a 2D array of single photon avalanche diodes (SPADs) was used to combine the strengths of multiple detectors in LDF with high light sensitivity in DCS. The system was tested on milk phantoms with varying detector fiber diameter (200 and 600 μm), source‐detector fiber separation (4.6‐10.2 mm), fiber‐SPAD distance (2.5‐36.5 mm), contiguous measurement time per repetition for the autocorrelation (1‐33 ms) and temperature (15.6‐46.7°C). An in vivo blood occlusion test was also performed. The multipixel approach improved signal‐to‐noise ratio (SNR) and, in our setup, the use of a multimode detector fiber was beneficial for SNR. In conclusion, the multipixel system works, but improvements and further studies regarding, for example, the data acquisition and optimal settings are still 

Deep brain stimulation (DBS) is an established technique for reduction of symptoms in movement disorders. Finite element method (FEM) simulations of the electric field magnitude (EF) are useful for estimating the affected tissue around the DBS lead and this can help optimize the therapy. This paper describes how patient-specific FEM models can be set up with the aid of the Matlab-based in-house software tool ELMA. Electrode placement is determined from two coordinates in postoperative medical imaging and electric conductivity is assigned from preoperative magnetic resonance imaging (MRI) and patient-specific DBS data. Simulations are performed using the equation for steady currents in Comsol Multiphysics (CM). The simulated EF is superimposed on the preoperative MRI for evaluation of affected structures. The method is demonstrated with patient-specific simulations in the zona incerta and a globus pallidus example containing cysts with higher conductive which causes considerable distortion of the EF. The improved software modules and precise lead positioning simplifies and reduces the time for DBS EF modelling and simulation.

Deep Brain Stimulation (DBS) is an established technique for symptomatic treatment of some neurological disorders such as Parkinson’s disease and essential tremor. Finite Element Method (FEM) simulations of the electric field magnitude (EF) can be used to estimate which structures in the brain that are affected by DBS, where an EF of 0.2 V/mm has been estimated to affect the structures when using a pulse width of 60 µs [1, 2]. We are developing a Matlab-based application named ELMA [3] for tissue classification and electric conductivity assignment to use in the FEM software Comsol Multiphysics (CM). Preoperative MR images are used to classify tissue into grey matter, white matter, blood and cerebrospinal fluid and this classification is then used to assign the electric conductivity for the FEM simulations. We are also developing a CM application for easy electrode placement and simulations of the EF. It is currently containing the Medtronic 3389 lead but will be expanded with other lead types such as segmented leads for directional stimulation. Both applications will be free and available for download at

https://liu.se/en/research/dbs

The CM application does require a CM license at the moment though.

References

[1]          J. D. Johansson, F. Alonso, and K. Wårdell, "Modelling details for electric field simulations of deep brain stimulation," in World Congress on Medical Physics & Biomedical Engineering, Prague, 2018, vol. 68, no. 1: Springer, pp. 645-648.

[2]          M. Åström, E. Diczfalusy, H. Martens, and K. Wårdell, "Relationship between Neural Activation and Electric Field Distribution during Deep Brain Stimulation," (in English), IEEE T Bio-Med Eng, vol. 62, no. 2, pp. 664-672, Feb 2015.

[3]          J. D. Johansson, F. Alonso, and K. Wårdell, "Patient-Specific Simulations of Deep Brain Stimulation Electric Field with Aid of In-house Software ELMA " in the 41st International Engineering in Medicine and Biology Conference, Berlin, 2019.

Continuous measurement of local brain oxygen saturation (SO2) can be used to monitor the status of brain trauma patients in the neurocritical care unit. Currently, micro-oxygen-electrodes are considered as the “gold standard” in measuring cerebral oxygen pressure (pO2), which is closely related to SO2 through the oxygen dissociation curve (ODC) of hemoglobin, but with the drawback of slow in response time. The present study suggests estimation of SO2 in brain tissue using diffuse reflectance spectroscopy (DRS) for finding an analytical relation between measured spectra and the SO2 for different blood concentrations. The P3 diffusion approximation is used to generate a set of spectra simulating brain tissue for various levels of blood concentrations in order to estimate SO2. The algorithm is evaluated on optical phantoms mimicking white brain matter (blood volume of 0.5–2%) where pO2 and temperature is controlled and on clinical data collected during brain surgery. The suggested method is capable of estimating the blood fraction and oxygen saturation changes from the spectroscopic signal and the hemoglobin absorption profile.

Introduction

The electric field (EF) around the active deep brain stimulation (DBS) contact is of interest for optimizing the therapeutic effect. We have previously developed a method for simulation and visualization of the EF. The aim of the project is to improve the software for quick and user friendly simulations.  

Methods

The ELMA software for brain model creation has been improved by adding quick ROI selection and transformation to an electrical conductivity map based on tissue classification through multiple slices of the preoperative MRI. These data are used as input for Comsol Multiphysics simulations of the EF. Two points along the position of the lead, as seen in the postoperative images, are used for correct placement in the brain model. Multiple DBS lead models are pre-programmed. The active contact and amplitude are user-selected.

Results

After a simulation the result is visualized with a user defined isolevel or isosurface superimposed on the patients preoperative MRI. An example is shown in Fig. 1. The 3389 lead is places in zona inserta (Zi) and contact 1 activated with 2 and 4 V respectively. An isolevel of 0.2 V/mm is used corresponding to a ~ 3-4 µm axon diameter when using a pulse length of 60 µs. More examples will be presented at the meeting.

Conclusion

The software for patient-specific simulations of EF around DBS electrodes has been improved for quicker simulations and more DBS leads. As a next step user friendly Apps will be implemented.