title: Induced magnetic field gradients and forces in the human head in MRI.
abstract: OBJECTIVE
  To map the induced magnetic field gradients and estimate the magnetic force in the human head during magnetic resonance imaging at 4 Tesla (T).
  
  
  METHODS
  The magnetic field distribution in the human head was measured using two gradient-echo experiments with different echo times. The phase of the complex image ratio removed the wrapping artifact, characteristic of phase images, and was used to map the magnetic field distribution and calculate the accurate maps of the magnetic field gradients in the human head.
  
  
  RESULTS
  The time-independent gradient fields induced by air/tissue interfaces in the head can be 50 times larger than those resulting from the magnetic field inhomogeneity of the MRI magnet. However, the associated magnetic force in the brain is by far smaller than the gravitational force.
  
  
  CONCLUSIONS
  The induced gradient fields increase the magnetic force on tissues. However, even for tissue components with large magnetic susceptibility such as iron-containing proteins, this force is negligible compared with the gravitational force. Therefore, this study suggests that static and uniform magnetic fields do not have a significant risk for the tissues in the head.
authors.affiliation: Medical Department, Brookhaven National Laboratory, Upton, New York 11973, USA. tomasi@bnl.gov
authors.forename: Dardo G
authors.initials: DG
authors.lastname: Tomasi
authors.forename: Ruiliang
authors.initials: R
authors.lastname: Wang
mesh_terms: D001921:Brain / Q000503:physiopathology* / Q000528:radiation effects*; D003198:Computer Simulation; D004574:Electromagnetic Fields; D004735:Energy Transfer; D006257:Head / Q000502:physiology* / Q000528:radiation effects*; D006801:Humans; D008279:Magnetic Resonance Imaging; D008954:Models, Biological; D008959:Models, Neurological; D011829:Radiation Dosage; D011874:Radiometry / Q000379:methods*; D013314:Stress, Mechanical