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5. Static magnetic field

5.1 Definition

Static magnetic field (B0) is dependent on the field strength of the magnet.

The SI unit used for measuring magnetic field strength is the tesla, its symbol is T. An alternative unit of measurement is the Gauss. 1tesla is equal to 10,000 Gauss.

A magnetic field of one tesla (1T) is approximately 30,000 times as powerful as the Earth's magnetic field.

5.1 MR controlled access area

Please refer to the MHRA Guidance Document 2 page 24 for an example layout of an MRI Unit.

An MR controlled access area is characterised by the MHRA as follows: A locally defined area of such a size to contain the MR environment. Access to this area should be restricted and controlled by suitable control methods (eg keypad entry) with suitable warning signs displayed at all entrances.2

5.2 MR environment

The MR environment is defined by the MHRA as ‘the three dimensional volume of space surrounding the MR magnet that contains both the Faraday shielded volume and the 0.5MT field contour (5 Gauss (G) line). This volume is the region in which an item might pose a hazard from exposure to the electromagnetic fields produced by the MR equipment and accessories’ .2 Note: the MR environment can include aspects of the technical room and console area

  • Owing to the hazards of the static magnetic field described in this section, resuscitation of patients should take place outside the MR environment. Local rules should outline specific procedures to reflect this.

5.3  Fringe field

Every MR scanner has an affiliated fringe field. The extent of this fringe field is dependent upon the static magnetic field strength (B0), type of shielding (active, passive cladding or whole room shielding) and whether the magnet has an open or closed design.

  • Fringe field plot diagrams should be displayed in every MR control room highlighting the 0.5mT (5 Gauss line) and the 3mT (30 Gauss line).

5.4  Projectile zone

Some MR units, for example those performing interventional MRI, may wish to define a projectile zone within the MR environment.

A projectile zone is ‘a locally defined volume containing the full extent of the 3 mT magnetic field contour, or other appropriate measure, around the MRI scanner.‘2

5.5  Biological effects
The interaction of the static magnetic field (B0) with the body and its functions may result in the creation of electrical potentials, currents generated by body movements and the possible displacement of naturally generated currents within the body by (B0).  Electrical potentials and related effects during physical movements within static magnetic field gradients may induce sensations of vertigo, nausea, phosphenes and a metallic taste in the mouth. Public Health England, PHE, (formerly The Health Protection Agency HPA)15 offers the following advice regarding the movement of patients and volunteers in the static field:

  • The biological effects most likely to occur are the production of vertigo-like sensations and these acute effects are associated with movement in the static field.  The sensitivity to these effects varies considerably between individuals.  Patients and volunteers should be moved slowly into the scanner, to avoid the possibility of vertigo and nausea.’

Further recommendations guidance and exposure limits relating to bioeffects of Bo can be viewed in the following publications:

Public Health England  MRI Procedures: Protection of Patients and Volunteers 2008 15
https://www.gov.uk/government/publications/magnetic-resonance-imaging-mr...

International Commission on Non- Ionising Radiation (ICNIRP) Guidelines on Limits of Exposure to Static Magnetic Fields 2009 16
http://www.icnirp.org/cms/upload/publications/ICNIRPstatgdl.pdf

Amendment to the ICNIRP ‘Statement on Medical Magnetic Resonance (MR) Procedures Protection of Patients’ 2009 17
http://www.icnirp.org/cms/upload/publications/ICNIRPMR2009.pdf

5.6 Projectile and attractive forces

The potential hazard of the projectile effect of ferro-magnetic material in a strong magnetic field must be taken very seriously. The HPA reports that serious incidents have occurred including a patient fatality when items such as ferromagnetic oxygen cylinders have inadvertently been brought into the scan room. 15

  • Extreme caution must be employed, and strict screening protocols in place and adhered to, to ensure ferromagnetic items do not enter the MR environment. Loose metallic objects can reach considerable velocities.

​5.7 Labelling of equipment

All equipment used or stored within the MR environment should be clearly labelled as one of the following as defined by the American Society for Testing and Materials.18


 

 

 

 

 

MR safe – defined as ‘an item that poses no known hazards resulting from exposure to any MR environment. MR safe items are composed of materials that are electrically nonconductive, non metallic, and nonmagnetic’

MR conditional – defined as ‘an item with demonstrated safety in the MR environment within defined conditions. At a minimum, address the conditions of the static magnetic field, the switched gradient magnetic field and the radiofrequency fields. Additional conditions, including specific configurations of the item, may be required.’

MHRA further advises:
Descriptions of MR CONDITIONAL should specify information such as the maximum magnetic field in which the device was tested, the magnitude and location of the maximum spatial gradient, the maximum rate of change of the gradient field, and radio-frequency fields tolerated in terms of RF interference, RF heating and type of transmit mode.2

MR unsafe – defined as ‘an item which poses unacceptable risks to the patient, medical staff or other persons within the MR environment.’

  • All staff should be conversant with the labelling system and understand the conditions for use. Further information on the requirements regarding labelling of equipment can be viewed in the MHRA guidelines.2
  • Departments should take into account the different field strengths when labelling equipment. For example, if a department is equipped with two MR units of 1.5T and 3T only equipment which is safe in both units should be labelled as MR safe.
  • Clear guidance for use should be marked on equipment that is MR conditional.

5.7.1 Ancillary equipment

  • Caution should be exercised when assessing, purchasing and authorising equipment for patient support in the MR environment. Equipment must be assessed for MR compatibility and clearly labelled as indicated in section 5.7. Departments should ensure that an up to date inventory is kept of all equipment for use in the MR environment.

5.7.2 Other items

The MHRA makes the following recommendations regarding other items that may be used in the MR Unit (eg patient comfort and immobilisation aids)
‘Many items such as consumables cannot be reasonably labelled. Sites should have processes in place to ensure that these items are safe.’2

5.8 Procurement

The MR responsible person and/or the MR safety expert should be involved in all procurement decisions relating to items for use in the MR environment.

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