Magnet Safety
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4.1.1 Shielding
Most of the newer NMR magnet systems are actively shielded. The following must be
understood when installing or working with such a shielded magnet:
• The active shielding of the superconducting coil reduces the stray magnetic fields, and
therefore its effects.
• Nevertheless, the magnetic field gradient is much stronger compared to non-shielded
magnets, hence the distance interval between various stray field contour lines (for
instance distance between 0.5 mT (5 Gauss) and 5 mT (50 Gauss) is much smaller, and
caution must be taken to avoid bringing ferromagnetic objects close to the magnet.
• In spite of the active shielding, the stray magnetic field directly above and directly below
the magnet is very high and the attractive forces on ferromagnetic objects are very strong!
4.1.2 Electronic, Electrical and Mechanical Medical Implants
The following must be understood concerning the effects on electronic, electrical and
mechanical medical implants and devices:
• The operation of electronic, electrical or mechanical medical implants, such as cardiac
pacemakers, biostimulators, and neurostimulators may be affected or even stopped in the
presence of either static or changing magnetic fields.
• Not all pacemakers respond in the same way or at the same field strength if exposed to
fields above 5 gauss.
4.1.3 Surgical Implants and Prosthetic Devices
The following must be understood concerning the effects on surgical implants and prosthetic
devices:
• Besides electronic, electrical, and mechanical medical implants, other medical surgical
implants such as aneurysm clips, surgical clips or prostheses, may contain ferromagnetic
materials and therefore would be subject to strong attractive forces near to the NMR
magnet system. This could result in injury or death.
• Additionally, in the vicinity of rapidly changing fields (e.g. pulsed gradient fields) eddy
currents may be induced in the implant, hence resulting in heat generation and possibly
create a life-threatening situation.
4.1.4 Operation of Equipment
The operation of equipment may be directly affected by the presence of strong magnetic
fields.
• Items such as watches, tape recorders and cameras may be magnetized and irreparably
damaged if exposed to fields above 1 mT (10 Gauss).
• Information encoded magnetically on credit cards and magnetic tapes may be irreversibly
corrupted.
• Electrical transformers may become magnetically saturated in fields above 5 mT (50
Gauss). The safety characteristics of equipment may also be affected.
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