title: Occupational exposure to static and time-varying gradient magnetic fields in MR units.
abstract: OBJECTIVE
  To assess exposure of MR staff to static and time-varying gradient magnetic fields by direct measurement and questionnaire and to compare these to the action values and exposure limits stated in published guidance.
  
  
  METHODS
  Instantaneous and time-averaged exposure to static magnetic fields were measured on clinical and research MR scanners. A questionnaire was sent to 100 clinical MR centers to estimate the average static field exposure. Instantaneous measurements of the time-varying magnetic field were conducted on a 1.5 T conventional and a 0.6 T open-bore MR system.
  
  
  RESULTS
  The 24-hour time-averaged static field exposure was measured to be 1.8 +/- 1 mT and the questionnaire produced a 24-hour time-average of 2.3 mT. The maximum static field strength recorded during patient set up was 554 mT. Time-varying gradient field exposure was found to exceed the action values up to 30 cm from the bore entrance or magnet rim.
  
  
  CONCLUSIONS
  The time-varying gradient field exposure limit is the only limit to be exceeded. The time-averaged static field exposure to staff was found to be 100 times lower than the recommended guidelines.
authors.affiliation: Department of Medical Physics and Clinical Engineering, Oxford Radcliffe Hospitals Trust, Oxford, UK. jill.bradley@orh.nhs.uk
authors.forename: Jill K
authors.initials: JK
authors.lastname: Bradley
authors.forename: Matilda
authors.initials: M
authors.lastname: Nyekiova
authors.forename: David L
authors.initials: DL
authors.lastname: Price
authors.forename: Lindsay D'jon
authors.initials: LD
authors.lastname: Lopez
authors.forename: Therese
authors.initials: T
authors.lastname: Crawley
mesh_terms: D001822:Body Burden; D004574:Electromagnetic Fields; D006282:Health Personnel / Q000706:statistics & numerical data*; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000706:statistics & numerical data*; D016273:Occupational Exposure / Q000032:analysis* / Q000706:statistics & numerical data*; D011829:Radiation Dosage; D011834:Radiation Monitoring / Q000706:statistics & numerical data*; D012062:Relative Biological Effectiveness; D018570:Risk Assessment; D006113:United Kingdom