title: The motor-evoked potential threshold evaluated by tractography and electrical stimulation.
abstract: OBJECTIVE
  To validate the corticospinal tract (CST) illustrated by diffusion tensor imaging, the authors used tractography-integrated neuronavigation and direct fiber stimulation with monopolar electric currents.
  
  
  METHODS
  Forty patients with brain lesions adjacent to the CST were studied. During the operation, the motor responses (motor evoked potential [MEP]) elicited at the hand by the cortical stimulation to the hand motor area were continuously monitored, maintaining the consistent stimulus intensity (mean 15.1 +/- 2.21 mA). During lesion resection, direct fiber stimulation was applied to elicit MEP (referred to as fiber MEP) to identify the CST functionally. The threshold intensity for the fiber MEP was determined by searching for the best stimulus point and changing the stimulus intensity. The minimum distance between the resection border and illustrated CST was measured on postoperative isotropic images.
  
  
  RESULTS
  Direct fiber stimulation demonstrated that tractography accurately reflected anatomical CST functioning. There were strong correlations between stimulus intensity for the fiber MEP and the distance between the CST and the stimulus points. The results indicate that the minimum stimulus intensity of 20, 15, 10, and 5 mA had stimulus points approximately 16, 13.2, 9.6, and 4.8 mm from the CST, respectively. The convergent calculation formulated 1.8 mA as the electrical threshold of the CST for the fiber MEP, which was much smaller than that of the hand motor area.
  
  
  CONCLUSIONS
  The investigators found that diffusion tensor imaging-based tractography is a reliable way to map the white matter connections in the entire brain in clinical and basic neuroscience applications. By combining these techniques, investigating the cortical-subcortical connections in the human CNS could contribute to elucidating the neural networks of the human brain and shed light on higher brain functions.
authors.affiliation: Department of Neurosurgery, Faculty of Medicine, The University of Tokyo, Japan. kamady-k@umin.ac.jp
authors.forename: Kyousuke
authors.initials: K
authors.lastname: Kamada
authors.forename: Tomoki
authors.initials: T
authors.lastname: Todo
authors.forename: Takahiro
authors.initials: T
authors.lastname: Ota
authors.forename: Kenji
authors.initials: K
authors.lastname: Ino
authors.forename: Yoshitaka
authors.initials: Y
authors.lastname: Masutani
authors.forename: Shigeki
authors.initials: S
authors.lastname: Aoki
authors.forename: Fumiya
authors.initials: F
authors.lastname: Takeuchi
authors.forename: Kensuke
authors.initials: K
authors.lastname: Kawai
authors.forename: Nobuhito
authors.initials: N
authors.lastname: Saito
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D001931:Brain Mapping / Q000379:methods*; D001932:Brain Neoplasms / Q000473:pathology / Q000503:physiopathology / Q000601:surgery; D002540:Cerebral Cortex / Q000473:pathology / Q000503:physiopathology*; D002648:Child; D038524:Diffusion Magnetic Resonance Imaging / Q000379:methods*; D004558:Electric Stimulation; D055724:Electrophysiological Phenomena / Q000502:physiology; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D038361:Neuronavigation / Q000379:methods*; D009493:Neurosurgery / Q000295:instrumentation / Q000379:methods; D011712:Pyramidal Tracts / Q000473:pathology / Q000503:physiopathology*; D055815:Young Adult