title: Magnetoencephalography for clinical pediatrics: the effect of head positioning on measurement of somatosensory-evoked fields.
abstract: OBJECTIVE
  At present, whole-head MEG systems are designed to accommodate adult heads, thereby introducing a technical issue unique to pediatric MEG. It is known that magnetic field strength decreases as a function of 1/distance(2). For pediatric patients, we questioned whether re-positioning the head to minimize the distance between the expected source location and the MEG sensor array would significantly improve source measurement.
  
  
  METHODS
  Somatosensory-evoked fields (SEFs) were recorded in 17 children (mean=4.96 years) with their head placed centrally in the MEG, and then re-positioned laterally to reduce the distance between the cortical source and sensors. Equivalent current dipole (ECD) source models were evaluated for changes in residual variance (RV), signal-to-noise ratio (SNR), moment (strength), and location.
  
  
  RESULTS
  Re-positioning the head closer to the sensors resulted in a significant shift in the mediolateral dipole coordinate location, accompanied by a significant increase in the SNR, decrease in the dipole RV, and a reduction in size of ECD confidence volumes.
  
  
  CONCLUSIONS
  We conclude that for clinical pediatric measurement of the SEF, repositioning of the head to minimize the distance between the expected SEF source location and the sensor array will significantly improve SEF source measurement and concomitant ECD source modeling.
  
  
  CONCLUSIONS
  These issues are relevant to all pediatric MEG settings involving healthy or clinical populations and underscores the need for future development of a MEG helmet specifically designed for pediatric populations.
authors.affiliation: Department of Diagnostic Imaging, Hospital for Sick Children, 555 University Avenue, Toronto, Ont., Canada M5G 1X8; Department of Medical Imaging, University of Toronto, Ont., Canada. Electronic address: william.gaetz@sickkids.ca.
authors.forename: W
authors.initials: W
authors.lastname: Gaetz
authors.affiliation: Division of Neurology, Hospital for Sick Children, Toronto, Ont., Canada; Department of Paediatrics, University of Toronto, Ont., Canada.
authors.forename: H
authors.initials: H
authors.lastname: Otsubo
authors.affiliation: Division of Neurology, Hospital for Sick Children, Toronto, Ont., Canada; Department of Paediatrics, University of Toronto, Ont., Canada.
authors.forename: E W
authors.initials: EW
authors.lastname: Pang
mesh_terms: D000293:Adolescent; D000367:Age Factors; D001931:Brain Mapping; D002648:Child; D002675:Child, Preschool; D004558:Electric Stimulation / Q000379:methods; D004566:Electrodes; D004569:Electroencephalography / Q000379:methods; D005073:Evoked Potentials, Somatosensory / Q000502:physiology*; D005260:Female; D006257:Head / Q000502:physiology*; D006801:Humans; D007223:Infant; D015225:Magnetoencephalography; D008297:Male; D008475:Median Nerve; D008954:Models, Biological; D010372:Pediatrics