Appendix 4
MAGNETIC RESONANCE IMAGING AND NUCLEAR MAGNETIC RESONANCE
Magnetic Resonance Imaging (MRI) and Nuclear Magnetic Resonance (NMR) is used extensively
throughout the University in the analysis of chemicals, geological specimens and imaging biological
tissues. The general principle of both MRI and NMR is the fact that certain nuclei possess a
property called spin i.e. the net angular momentum of the constituent nucleons. This spin produces
a magnetic moment which has random orientations, but will be aligned by an externally supplied
magnetic field. By then applying radiofrequency energy, the nuclei undergo spin flips to generate a
net moment in the x-y plane. Measurement of the resonant frequency of the resulting precessing
magnetic moment or the spatial location of this phenomenon enables analysis of the tissue or
substance.
The various forms of EMF therefore pose differing risks, which must be risk assessed. The most
significant occupational health risks relate to the Static Magnetic Field. Departments planning to use
MRI or NMR equipment must inform the University Safety Office at the earliest opportunity prior to
installation. In general, the use of the MRI is expected to conform to the Medicines and Healthcare
Products Regulatory Agency’s ‘Safety Guidelines for Magnetic Resonance Imaging Equipment in
Clinical Use’.
1
Static magnetic fields
In the UK, the HPA has responsibility for providing advice on exposure restrictions for
electromagnetic fields. In the case of static magnetic fields, current guidance is the adoption
of the International Commission on Non-Ionizing Radiation Protection (ICNIRP) exposure
limits. These limits guard against the possible health effects of exposure to static magnetic
fields, such as vertigo, nausea, cardiac arrhythmia and impaired mental function. The ICNIRP
exposure limits for static magnetic fields, for occupational exposure, are:
Quantity
Exposure Limit (Tesla)
Exposure of head and trunk
2 T1
Exposure of limbs
8T
1
Exposure up to 8 T can be permitted if the environment is controlled and appropriate
work practices are implemented to control movement-induced effects
Whole body exposure to static magnetic fields below 2.5 T is unlikely to have any adverse
effect on health. However, static magnetic fields above 2.5 T must be accompanied by a risk
assessment to assess potential exposure against these exposure limits. Risk assessments
for potential whole body exposure to static magnetic fields above 4 T should be forwarded to
the University Safety Office.
The greatest risk associated with static magnetic fields is the potential effect on implants
either in terms of ferromagnetic objects or medical electronic devices. Ferromagnetic implants
are susceptible to forces and torques within the static magnetic fields. Static fields have also
been shown to affect the operation of medical electronic devices, such as cardiac
pacemakers. Again, ICNIRP advises that the possible effects on implants are unlikely at static
magnetic fields below 0.5 mT (or 5 Gauss).
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The other significant risk associated with static magnetic fields is the potential damage from
projectile ferromagnetic items being drawn to the magnet bore. Not only does this have a very
serious financial implication, it can result in serious injury to someone trapped or struck by the
object. It can also result in a ‘quench’ and sudden release of the cryogenic gases.
Therefore, equipment where individual exposure is above 0.5mT must:
2

Be notified to the University Safety Office.

An outline plan of the static magnetic field must be produced and physically measured by
the contracted installer. If the installer is unable to conduct these measurements, then the
Safety Office must be consulted.

The location of the 0.5mT line must be demarcated in all potential axes.

Routine access into the 0.5mT area must be restricted to named and suitably trained
individuals only. FMRIB annual training provides a suitable standard.

Safe systems of work must be documented and implemented. These must outline the
systems for screening individuals, entry restrictions for non-authorised persons, approval
systems for magnetic safe/compatible equipment, emergency procedures for dealing with
a projectile injury or quench.

Appropriate warning signs must be displayed, indicating restricted access and the
potential risk to implants or medical devices. Safety signs are available from the
University Safety Office.
Time-varying gradients
In addition, to the static magnetic fields, MRI scanners use rapidly changing magnetic timevarying fields. This can induce electric currents that interfere with the normal function of that
tissue, e.g. nerve tissue and muscle tissue. The risks are predominantly associated with
patients undergoing an MRI scan and less so the operator.
Significantly, the rapid changing of the magnetic field results in acoustic noise. Noise levels
can be hazardous and so appropriate measures (e.g. hearing protection) should be
implemented in line with University Policy Statement S1/06).
3
Radiofrequency
The main risks associated with radiofrequency fields are thermal heating, leading to induced
current burns and contact burns. At all frequencies, energy will be absorbed by the relevant
tissue, which can cause eye lesions and cataracts, temporary sterility in men and foetal
developmental problems. Again the risks are predominantly associated with patients
undergoing an MRI scan. Indeed, manufacturers should be consulted about the potential
exposure of radiofrequency fields to operators. However, operators must be aware of the
risks when setting up equipment so as to avoid potential burns from equipment.
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