Computational NeuroscienceVisualization of the electric field evoked by transcranial electric stimulation during a craniotomy using the finite element method

- Influences of craniotomy in intraoperative tMEP monitoring were numerically estimated.
- The electric field in the brain radiates out from the cortex just below the electrodes.
- Intensity of the electric field in the brain is most affected by thickness of the CSF layer.
- When the CSF layer is thick, CSF decrease has a major impact in the electric field.
- Bone deletion has larger effect in a case that skull has direct contact with the brain.

BackgroundTranscranial MEP (tMEP) monitoring is more readily performed than cortical MEP (cMEP), however, tMEP is considered as less accurate than cMEP. The craniotomy procedure and changes in CSF levels must affect current spread. These changes can impair the accuracy. The aim of this study was to investigate the influence of skull deformation and cerebrospinal fluid (CSF) decrease on tMEP monitoring during frontotemporal craniotomy.MethodsWe used the finite element method to visualize the electric field in the brain, which was generated by transcranial electric stimulation, using realistic 3-dimensional head models developed from T1-weighted images. Surfaces of 5 layers of the head were separated as accurately as possible. We created 3 brain types and 5 craniotomy models.ResultsThe electric field in the brain radiates out from the cortex just below the electrodes. When the CSF layer is thick, a decrease in CSF volume and depression of CSF surface level during the craniotomy has a major impact on the electric field. When the CSF layer is thin and the distance between the skull and brain is short, the craniotomy has a larger effect on the electric field than the CSF decrease.Comparison with existing methodSo far no report in the literature the electric field during intraoperative tMEP using a 3-dimensional realistic head model.ConclusionOur main finding was that the intensity of the electric field in the brain is most affected by changes in the thickness and volume of the CSF layer.

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