MRI Safety & Site Planning Lawrance Yip Senior Radiographer, DR, QMH 16 June 2007
Outline Safety-oriented architectural design Safety of fringe magnetic field Cryogen safety Safety considerations for upgrading 1.5T MRI system to 3T
Magnetic Resonance Imaging
Safe MRI Practices Safety-oriented architectural and interior design Screening of all patients, accompanying personnel, attending health care professionals and house-keeping staff who enter the controlled access volume Monitoring of the patients being examined MR safety education for all staff Clear lines of responsibility Regular review of safety practices
MRI Site Planning Technical considerations for the environment of MRI site Structural loading Avoid vehicle and lift ferromagnetic influences Avoid vibration and noise sensitive areas Avoid electrical main supplies to the building Feasibility to control access to regions within 5 Gauss line Safe area for the outlet of quench vent pipe Easy access of service vehicle and cryogen supply
Safety-oriented Floor Layout MRI site is conceptually divided into four zones Zone I Freely accessible to the general public Zone II Interface between the publicly accessible, uncontrolled Zone I and the strictly controlled Zones III & IV Patient preparation area ACR Safe MR Practices 2007
Safety-oriented Floor Layout Zone III All access to Zone III should be strictly restricted Free access by unscreened non-mr personnel or ferromagnetic objects / equipment can result in serious injury and death ACR Safe MR Practices 2007
Safety-oriented Floor Layout Zone IV MR scanner magnet room Zone IV should always located within Zone III MR technologists would be able to directly observe and control, via line of sight or via CCTV, the entrances or access corridors to Zone IV from their normal positions when stationed at the operator console ACR Safe MR Practices 2007
Safety-oriented Floor Layout Many multi-modality radiology facilities may combine patient waiting areas for patients undergoing different types of imaging examinations Safety challenges for unscreened patients inadvertently enter Zone III & IV with a contraindicated implant or a ferromagnetic object The practice of shared patient waiting areas between MR and other imaging modalities is discouraged from the point of view of MR safety
Inappropriate Design of MRI Suite Patient Waiting Area Magnet Room Control Room
Safety-oriented Floor Layout
Safety-oriented Floor Layout Zone II Zone I Zone II Zone III Zone IV Zone III Zone IV
Emergency Resuscitation In case of medical emergency within Zone IV (Magnet Room), MR personnel should immediate initiate CPR while the patient is emergently removed from Zone IV to a predetermined magnetically safe location in Zone II or III Quenching the magnet is not advised for medical emergency
Emergency Resuscitation Emergency patient table unlatching and undocking mechanism MRI compatible stretcher
Magnetic Fringe Field Magnetic Field Strength Tesla (T) Gauss (G) 1 Tesla = 10,000G Hand-held Gaussmeter
Magnetic Fringe Field Magnetic field is three dimensional and extends into space above and below the magnet as well as to the surrounding space at the same level Objects including biomedical implants & other equipment within this 3D space can be affected by the magnetic field or can affect the magnetic field Proximity Limits
Fringe Field Plot Top View Vertical View
Magnet Moving Metal Sensitivity Line Plot
Two Magnet Site Layout For two magnets installation, interaction can occur between magnetic fields To avoid magnetic field interaction, the 3 Gauss line of each magnet must not intersect
Magnetic Shielding Magnetic shielding is used to limit the fringe field around the magnet Limit the size of 5- Gauss exclusion zone Methods of magnetic shielding Active Shielding Passive Shielding
Active Shielding The interaction of the magnetic coils and shielding coils results in effective shielding of active shield magnets
Passive Shielding Consists of iron plates in the magnet room walls, floor & ceiling Special consideration should be given when selecting a magnet site location due to the expense and effort required to provide passive shielding Designing a magnetic shield requires a comprehensive computer analysis which predicts the effect the shield will have on the homogeneity of the magnetic field
5 Gauss Exclusion Zone 3-dimensional volume Confine within the magnet room (Zone IV) for the sake of easy control access
5-Gauss Exclusion Zone Outside Magnet Room 5-Gauss Exclusion Zone Magnet Room Control Room
Blooming of Magnetic Field Active shield magnets Potential for the active shielding coils that restrain the magnetic field to fail independently from the primary magnetic coils The magnetic field will be bloomed or expanded to a much larger volume It is necessary to consider the size of the unrestrained magnetic field during site planning
Magnetic Contamination Gradual buildup of permanent magnetic field in ferrous materials near the magnet including steel column, steel reinforcing in concrete and steel conduits due to long term exposure to high-strength magnetic field of MRI system Unlikely to develop magnetic field at the level of Zone III & IV May interfere with operation of other modalities if there were to be sited in the former magnet room
Cryogen Safety Superconducting magnets for mid- to high-field MRI systems Liquid Helium is the most commonly used cryogen in MR environment In case of quenching, the cryogenic liquid will rapidly boil off and expand into a gaseous state The liquid / gas ratio for Helium is 1 / 760
Cryogen Vent Pathway If superconducting magnet quenches, the escaping cryogenic gases are ducted outside the building to an unoccupied discharge area via the quench vent pipe Obstruction, inappropriate pipe materials, insufficient pipe caliber / length, or faulty connections in the length of quench vent pathway may cause failure between the magnet and the point of discharge
Cryogen Safety Potential safety concerns: Asphyxiation is possible as cryogenic gas replace oxygenated air Frostbite and cryo-burn may occur due to the exceedingly low temperature of the cryogen Hyperbaric pressure considerations within magnet room (Zone IV) can rarely exist in the unlikely event of a quench in which some of the cryogenic gases escape into the magnet room
Cryogen Vent Pathway To install the quench vent pipe according to the technical specifications of individual MRI system To inspect cryogen vent systems at least annually Following any quench of a superconducting magnet, to conduct a thorough inspection of the cryogen vent system
Cryogen Safety Failure of cryogen vent pipe may lead to considerable quantities of cryogenic gas being inadvertently discharged into the magnet room (Zone IV) The thermal expansion of the cryogens can positively pressurize the magnet room and displace oxygenated air within the magnet room Oxygen sensor and alarm system shall be available to alert the MR personnel for low oxygen level within the magnet room
Design Considerations Door of Magnet Room Swing in towards magnet room The positive pressure builds up within the magnet room due to leakage of cryogenic gases may push a closed door against the stops and entrap persons inside the magnet room until the pressure inside the magnet room is equalized with that outside the room
Design Considerations Door of Magnet Room Swing away from magnet room In a severe positivepressure situation, unlatching an outwardswing door may permit the door to burst open with tremendous pressure, potentially injuring person opening the door
Design Elements for Hyperbaric Pressure Considerations All magnet rooms shall be provided with an emergency exhaust pathway to vent the cryogenic gas away from the magnet room An additional form of passive pressure relief / pressure equalization system shall be available to minimize the risk of hyperbaric pressure
Technical Considerations for upgrading 1.5T MRI System to 3T Increased structural loading for 3T MRI system
Technical Considerations for upgrading 1.5T MRI System to 3T Increased structural loading for 3T MRI system Expanded fringe fields? Interference with other modalities in close vicinity 5-Gauss exclusion zone may be outside the magnet room (Zone IV) Bird Eye View 5.0m Magnet 2.48m 3.0T & 1.5T 4.0m 5 Gauss line for 3T MRI system 2.8m 5 Gauss line for 1.5T MRI system
Technical Considerations for upgrading 1.5T MRI System to 3T Increased structural loading Expanded fringe fields? Interference with other modalities in close vicinity 5-Gauss exclusion zone may be outside the magnet room (Zone IV) Blooming of magnetic field
Technical Considerations for upgrading 1.5T MRI System to 3T Increased structural loading Expanded fringe fields? Interference with other modalities 5-Gauss exclusion zone may be outside the magnet room (Zone IV) Blooming of magnetic field Vibration sensitivity & Moving metal sensitivity
Technical Considerations for upgrading 1.5T MRI System to 3T Increased structural loading Expanded fringe fields? Interference with other modalities 5-Gauss exclusion zone may be outside the magnet room (Zone IV) Blooming of magnetic field Vibration sensitivity & moving metal sensitivity Field strength limits for MR conditional equipment Physiological monitor, infusion pump or portable ventilator
Conclusion Safety-oriented architectural design is important for implementation of MR safety procedures and policy Although quenching is a rare event, cryogen safety shall be seriously considered Upgrade of 1.5T MRI system to 3T is not a simple equipment replacement Safety awareness of MRI personnel is crucial
Conclusion There is escalating pressure on MRI Unit to perform ever-increasing no. & range of examinations in shorter time Without good site design, incorporating safety considerations from very beginning, clear operational procedures, well-trained staff and cooperation from other health-care professionals this pressure may lead to system failure which will sooner or later result in an accident or adverse event