Here are the documents referred to in my earlier e-mail. When our MSL500
was installed with substantial stray magnetic fields we enquired of the
British National Radiological Protection Board (NRPB) what protective
action we should take. This is the information given to us then. There
may of course be more up to date information available and I am sure that
there also comparable US standards.
The following is taken from a letter from NRPB dated 5th Feb 1991:
Static Magnetic Fields
Until further information becomes available, exposure should be such as to
avoid any possible adverse effect on mental alertness or on cardiac
function and aortic blood flow. This possibility will be avoided by an
adequate margin of safety if the short-term exposure of static magnetic
flux density to the head and trunk does not exceed 2 tesla (T). The limbs
may be exposed up to 4 tesla. Short exposure periods totalling less than
15 minutes may be used and a number of such short exposures in any day is
permissible, providing reasonable intervals of about one hour occur between
exposures.
Eight hour exposure periods should result in the operator not being exposed
to a field greater than 1 mT.
Notices should be posted where appropriate to warn staff of the presence of
the magnetic field and of the possibility that it may affect the
functioning of cardiac pacemakers and other electronic equipment.
Loose metallic objects should not be permitted in the vicinity of the
field. Metal surgical implants and rings on the hands, etc. could result
in injuries and heating effects.
Consideration is currently being given to having the vicinity of the
equipment classified as a controlled area with the boundary set at one (1)
mT.
Although the magnetic field between the pole pieces is not inconsiderable,
the field is highly localised and rapidly diminishes the distance. It
would be good practice to ensure that the operator minimises the time spent
adjacent to the magnet and if a sample handling tool could also be
utilised, it would result in the exposure being kept to a minimum.
It is not necessary to provide medical surveillance for the operator as
exposure to the trunk and head will be minimal.
* * * * *
There is also and NRPB document available from Her Majesty's Stationery
Office dated Jan. 1984 (ISBN 0 85951 218 5) concerning clinical imaging
(MRI) whose text I append below in which some equations have not come out.
I can send the file as a Microsoft word document that should contain the
equations to anyone who requests it. Text follows:
National Radiological Protection Board
Advice on acceptable limits of exposure to nuclear magnetic resonance
clinical imaging
The Board recommends that the following conditions be fulfilled during the
operation of nuclear magnetic resonance clinical imaging equipment in the
UK.
1 Exposure conditions
Exposures should be kept within the following limits.
Static magnetic fields For those exposed to the imaging process, the
static magnetic field should not exceed 2.5 tesla to the whole or a
substantial portion of the body.
Staff operating equipment should not be exposed for prolonged periods to
more than 0.02 tesla to the whole body or 0.2 tesla to the arms or hands.
=46or short periods totalling less than 15 minutes, these limits may be
increased to 0.2 tesla for the whole body and 2 tesla to the arms and
hands. A number of such short exposures in any day is permissible,
providing reasonable intervals of about 1 hour occur between exposures.
Time varying magnetic fields (excluding radiofrequency fields) For periods
of magnetic flux density change exceeding 10 ms, exposure should be
restricted to rms (root mean square) rates of change of less than 20 tesla
s-1 for all persons. For periods of change less than 10 ms the
relationship should be observed where is the rms value of the rate of
change of magnetic flux density in any part of the body in tesla s-1 and t
is the duration of the change of the magnetic field in seconds. For
sinusoidally varying magnetic fields or other continuously varying periodic
fields the duration of the change can be considered to be half period of
the waveform.
Radiofrequency fields Exposure should be such as to avoid any significant
rise in the temperature of the sensitive tissues of the body. Acceptable
exposures should not result in a rise in body temperature of more than 1=83C
as shown by skin and rectal temperature or more than 1=83C in any mass of
tissue not exceeding 1 gram in the body. This may be ensured by limiting
the mean specific absorption rate in the whole body to 0.4 W kg-1 and the
specific absorption rate in any mass of tissue not exceeding 1 gram to 4 W
kg-1. These limits apply to volunteers, patients and staff exposures. The
Board has made more general proposals for the limitation of exposures to
radiofrequency and microwave radiations1 in the form of a consultative
document.
2 Supervision of exposed persons
Patients should be exposed only with the approval of a registered medical
practitioner who should be satisfied wither that the exposure is likely to
contribute to the treatment of the patient or that it is part of a research
project which has been approved by a local research ethical committee and
in which the patients consent to take part after being fully informed.
Volunteers participating in experimental trials of NMR imaging techniques
should be medically assessed and pronounced suitable candidates before
exposure. There is no reason automatically to exclude those volunteers
with a history of epilepsy or cardiac disease but this should be judged at
the time of examination. Volunteers should be freely consenting and fully
informed. The guidance in paragraph 3.2.3 of the WHO Technical Report 6112
regarding exposure to ionising radiation is equally applicable to NMR
exposures. Volunteers who are exposed frequently should be checked at
regular intervals for evidence of abnormal changes in electrocardiogram
recordings.
Although there is no evidence to suggest that the embryo is sensitive to
magnetic and radiofrequency fields at the intensities encountered in NMR
imaging, it might be prudent to exclude pregnant women during the first
three months of pregnancy. There is no need to exclude women who will
subsequently undergo abortion.
Persons fitted with cardiac pacemakers should not be exposed unless
corrective procedures are available and they have been made aware of the
potential hazard before being exposed.
The exposure of individuals with large metallic implants, such as hip
prostheses, should be stopped immediately if discomfort is experienced
around the site of the implant. Intra-cranial metallic clips, particularly
those used to treat aneurysms, may present a hazard if the clips are made
of magnetic materials. Patients fitted with such clips should be excluded.
Appropriate resuscitation equipment should be available during imaging and
a registered medical practitioner trained in the techniques of
resuscitation should be available at short notice, although there is no
need for the practitioner to be present during the exposure.
Other precautions Notices should be posted where appropriate to warn
passers-by of the presence of magnetic and radiofrequency fields and of the
possibility that these may affect the functioning of cardiac pacemakers and
other electronic equipment. Loose metallic objects should not be permitted
in the vicinity of strong magnetic fields.
The Board acknowledges the help of an advisory group3 in formulating these
recommendations.
References
1 Proposals for the health protection of workers and members of the
public against the dangers of extra low frequency, radiofrequency and
microwave radiations: A consultative document. Chilton, National
Radiological Protection Board (1982).
2 Use of ionising radiation and radionuclides on human beings for
medical research, training and non-medical purposes. Report of a WHO
Expert Committee, Technical Report Series 611. Geneva, World Health
Organisation (1977).
3 NRPB ad hoc Advisory Group on Nuclear Magnetic Resonance Clinical
Imaging. Revised guidance on acceptable limits of exposure during nuclear
magnetic resonance clinical imaging, Br. J. Radiol., 56, 974 (1983).
National Radiological Protection Board
Chilton, Didcot, Oxon OX11 0RQ
January 1984.
* * * * * * *
If there is a demand I will post these documents on my WWW page.
Best wishes from a pleasantly warm St Andrews by the Sea,
=46rank Riddell
=46rank Riddell, School of Chemistry, University of St Andrews,
St Andrews, Fife, KY16 9ST, Scotland, UK.
Tel: + 44 (0) 1334 - 463815 Fax: + 44 (0) 1334 - 463808
E-mail: fgr@st-andrews.ac.uk
WWW: http://www.st-and.ac.uk/~www_sc/personal/fgr