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Self Shielding
 
Magnetic shielding by attaching a high permeability yoke to the magnet (passive shielding) or by incorporating additional magnetic field-generating coils designed to reduce the external field (active shielding).

See also Magnetic shielding, Active Shielding and Passive Shielding.
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Active Shielding
 
Magnetic shielding through the use of secondary shielding coils designed to produce a magnetic field that cancels the field from primary coils in regions where it is not desired. These coils may be inside the magnet cryostat. Active shielding can be applied to the main magnet or to the gradient magnetic fields.

See also Magnetic Shielding, Self Shielding and Room Shielding.
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Passive Shielding
 
Magnetic shielding through the use of high permeability material. The iron provides a return path for the stray field lines of magnetic flux and so significantly decreases the flux away from the magnet.
Passive shielding (see also Faraday cage) significantly eases the problems of siting a MR imager in a confined space. Ferromagnetic objects are less prone to being attracted to the magnet, ancillary electronic equipment, credit cards and computer disks can be brought closer to the magnet and the MRI safety limit for pacemaker wearers (the 5 gauss line = 0.5 mT) is reduced from, typically, 10 m to 2 m from the magnet. A passive shield for a whole-body MRI magnet weights many tons. An alternative method of controlling stray field is active shielding.

See also Active Shielding, Magnetic Shielding, Self Shielding and Room Shielding.
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Further Reading:
  Basics:
Faraday's Law
   by hyperphysics.phy-astr.gsu.edu    
  News & More:
Magnetic Sensitivity of MRI Systems to External Iron: The Design Process
   by www.integratedsoft.com    
MRI Resources 
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MRI SafetyMRI Resource Directory:
 - Safety -
 
There are different types of contraindications that would prevent a person from being examined with an MRI scanner. MRI systems use strong magnetic fields that attract any ferromagnetic objects with enormous force. Caused by the potential risk of heating, produced from the radio frequency pulses during the MRI procedure, metallic objects like wires, foreign bodies and other implants needs to be checked for compatibility. High field MRI requires particular safety precautions. In addition, any device or MRI equipment that enters the magnet room has to be MR compatible. MRI examinations are safe and harmless, if these MRI risks are observed and regulations are followed.

Safety concerns in magnetic resonance imaging include:
•
the magnetic field strength;
•
possible 'missile effects' caused by magnetic forces;
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the potential for heating of body tissue due to the application of the radio frequency energy;
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the effects on implanted active devices such as cardiac pacemakers or insulin pumps;
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magnetic torque effects on indwelling metal (clips, etc.);
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the audible acoustic noise;
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danger due to cryogenic liquids;
•
the application of contrast medium;
mri safety guidance
MRI Safety Guidance
It is important to remember when working around a superconducting magnet that the magnetic field is always on. Under usual working conditions the field is never turned off. Attention must be paid to keep all ferromagnetic items at an adequate distance from the magnet. Ferromagnetic objects which came accidentally under the influence of these strong magnets can injure or kill individuals in or nearby the magnet, or can seriously damage every hardware, the magnet itself, the cooling system, etc.. See MRI resources Accidents.
The doors leading to a magnet room should be closed at all times except when entering or exiting the room. Every person working in or entering the magnet room or adjacent rooms with a magnetic field has to be instructed about the dangers. This should include the patient, intensive-care staff, and maintenance-, service- and cleaning personnel, etc..
The 5 Gauss limit defines the 'safe' level of static magnetic field exposure. The value of the absorbed dose is fixed by the authorities to avoid heating of the patient's tissue and is defined by the specific absorption rate. Leads or wires that are used in the magnet bore during imaging procedures, should not form large-radius wire loops. Leg-to-leg and leg-to-arm skin contact should be prevented in order to avoid the risk of burning due to the generation of high current loops if the legs or arms are allowed to touch. The patient's skin should not be in contact with the inner bore of the magnet.
The outflow from cryogens like liquid helium is improbable during normal operation and not a real danger for patients.
The safety of MRI contrast agents is tested in drug trials and they have a high compatibility with very few side effects. The variations of the side effects and possible contraindications are similar to X-ray contrast medium, but very rare. In general, an adverse reaction increases with the quantity of the MRI contrast medium and also with the osmolarity of the compound.

See also 5 Gauss Fringe Field, 5 Gauss Line, Cardiac Risks, Cardiac Stent, dB/dt, Legal Requirements, Low Field MRI, Magnetohydrodynamic Effect, MR Compatibility, MR Guided Interventions, Claustrophobia, MRI Risks and Shielding.
Radiology-tip.comradRadiation Safety,  Ionizing Radiation
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Medical-Ultrasound-Imaging.comUltrasound Safety,  Absorbed Dose
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• View the NEWS results for 'MRI Safety' (13).Open this link in a new window.
 
Further Reading:
  Basics:
MRI Safety
2001   by www.fda.gov    
What MRI Sequences Produce the Highest Specific Absorption Rate (SAR), and Is There Something We Should Be Doing to Reduce the SAR During Standard Examinations?
Thursday, 16 April 2015   by www.ajronline.org    
Contrast Agents: Safety Profile
   by www.clinical-mri.com    
  News & More:
How safe is 7T MRI for patients with neurosurgical implants?
Thursday, 17 November 2022   by healthimaging.com    
Newer Heart Devices Safe During MRI
Monday, 23 August 2004   by www.hospimedica.com    
Study: Face Masks Unsafe in MRI Machines
Wednesday, 13 July 2022   by www.laboratoryequipment.com    
COVID-19: Attention shifts to MRI infection control
Thursday, 9 July 2020   by https://www.auntminnieeurope.com/index.aspx?sec=ser§sub=def§pag=dis§ItemID=619012    
FDA Releases New Guidance On Establishing Safety, Compatibility Of Passive Implants In MR Environments
Tuesday, 16 December 2014   by www.meddeviceonline.com    
Modern Implantable Heart Devices Safe For Use In MRI Scans
Wednesday, 16 March 2005   by www.sciencedaily.com    
MRI Resources 
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Magnetic Shielding
 
Means to confine the region of strong magnetic field surrounding a magnet; most commonly the use of material with high permeability (passive shielding) or by employing secondary counteracting coils outside of the primary coils (active shielding). The high permeability material can be employed in the form of a yoke immediately surrounding the magnet (self-shielding) or installed in the walls of a room as full or partial room-shielding. Unlike shielding ionizing radiation, for example, magnetic shielding can only be accomplished by forcing the unavoidable magnetic return flux through more confined areas or structures, not by absorbing it.

See also Radio Frequency Shielding Radio Frequency Shielding, and Faraday cage.

See also the related poll result: 'Most outages of your scanning system are caused by failure of'
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• View the DATABASE results for 'Magnetic Shielding' (11).Open this link in a new window

 
Further Reading:
  Basics:
Faraday's Law
   by hyperphysics.phy-astr.gsu.edu    
  News & More:
Magnetic Sensitivity of MRI Systems to External Iron: The Design Process
   by www.integratedsoft.com    
MRI Resources 
MR Myelography - Mobile MRI - Absorption and Emission - Online Books - Pediatric and Fetal MRI - MRI Training Courses
 
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