title: Visualization of the electric field evoked by transcranial electric stimulation during a craniotomy using the finite element method.
abstract: BACKGROUND
  Transcranial 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.
  
  
  METHODS
  We 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.
  
  
  RESULTS
  The 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.
  
  
  CONCLUSIONS
  So far no report in the literature the electric field during intraoperative tMEP using a 3-dimensional realistic head model.
  
  
  CONCLUSIONS
  Our 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.
authors.affiliation: Departments of Neurosurgery, School of Medicine, Keio University, Tokyo, Japan. Electronic address: tomy0807@hotmail.com.
authors.forename: Ryosuke
authors.initials: R
authors.lastname: Tomio
authors.affiliation: Departments of Neurosurgery, School of Medicine, Keio University, Tokyo, Japan.
authors.forename: Takenori
authors.initials: T
authors.lastname: Akiyama
authors.affiliation: Departments of Neurosurgery, School of Medicine, Keio University, Tokyo, Japan.
authors.forename: Tomo
authors.initials: T
authors.lastname: Horikoshi
authors.affiliation: Departments of Neurosurgery, School of Medicine, Keio University, Tokyo, Japan.
authors.forename: Takayuki
authors.initials: T
authors.lastname: Ohira
authors.affiliation: Departments of Neurosurgery, School of Medicine, Keio University, Tokyo, Japan.
authors.forename: Kazunari
authors.initials: K
authors.lastname: Yoshida
mesh_terms: D001921:Brain / Q000502:physiology*; D002555:Cerebrospinal Fluid / Q000502:physiology; D003198:Computer Simulation; D003399:Craniotomy / Q000379:methods*; D055590:Electromagnetic Phenomena; D020342:Finite Element Analysis; D006801:Humans; D008954:Models, Biological; D009929:Organ Size; D012886:Skull / Q000502:physiology; D065908:Transcranial Direct Current Stimulation / Q000379:methods*
keywords: Finite element method; Frontotemporal craniotomy; Neurosurgery; Transcranial electric stimulation; Transcranial motor evoked potential