The MRI Environment

Abstract

The examination room which contains the magnet assembly and patient table is enclosed within a RF shield (or Faraday cage) to prevent environmental electrical noise from interfering with the MR signal. The main magnet also generates a magnetic fringe field that is always present and extends well beyond the outside the magnet. This can create an extremely hazardous environment unless control measures are put in place. The general public are excluded from entering the fringe field above the 0.5 mT threshold as values above this may adversely affect the operation of cardiac pacemakers. A controlled area is defined to fully enclose the 0.5 mT magnetic fringe field contour. Access to this area is restricted to authorised staff with appropriate training. Patients and visitors should not enter unless they have first been screened for the presence of pacemakers and other contraindicated implants. Access is also restricted to prevent accidents caused by ferromagnetic objects being attracted by the magnet and becoming projectiles. Specific items should be identified as being safe, unsafe or conditional using an international labelling system. Most commercially available magnets use active shielding to reduce the extent of the fringe field. Additionally the examination room may be passively shielded using steel plate to prevent the fringe field from extending outside the controlled area.