Information about the potential dangers

· of the static magnetic field B₀ (interactions with cells and tissues, interactions with ferromag­netic objects: force and torque; requirements for MR safe implants and devices),

· of the low frequency (= audio frequency) switched gradient fields (induction of current pulses in nerves, peripheral nerve stimulation, possible cardiac nerve stimulation; noise; effects on implants),

· of the radio frequency field B₁ (interaction with tissue: warming (RF absorption, SAR), burns caused by current loops with skin-skin contact; interaction with metals and conducting material: induction, heating, sparking, burns at implant-tissue contact surfaces; requirements for MR safe implants and devices),

· of the cryo system (loss of cooling/quench, handling of cryogenic agents).

Up to now there is no scientific evidence that static magnetic (B₀) fields within the range typically used for MR produce permanent bioeffects that could lead to health problems [1]. Only transient effects causing slight indispositions of persons are possible, usually related to movements in inhomogeneous parts of the field. However, in spite of the low interaction with body tissue, the static field B₀ represents the hazard of most concern: As B₀ is commonly produced by a superconducting coil, it is always switched on. Ferromagnetic objects may be accelerated towards the magnet. Persons lying in the scanner or standing near the bore opening may be injured. Ferromagnetic implants may be dislocated and damage surrounding tissue. Fatal outcomes of these interactions have been reported [2,3]. While B₀ is strongest in the scanner bore, the field extends with significant strength several meters around the scanner. A field strength of 0.5 mT defines the border of the 'controlled access area'. This area must be blocked to the general public [4], as impairment of active implants, e.g. pacemakers, cannot be excluded even in the fringe field of an MR scanner. The switched gradient system (G_x, G_y, G_z), necessary to provide spatial information, is active during scanning only. A set of three orthogonal small magnetic fields are generated, which modify the static field. Switching frequencies are in the order of 100 Hz to several kHz, i.e. in the range of audible frequencies.
Concerning safety, two effects are of relevance. The first is peripheral nerve stimulation. Its occurrence depends on gradient steepness and switching time. The exact function depends on the model applied [5], and people are differently susceptible to stimulation [6]. Peripheral nerve stimulation is not by itself dangerous, but it is taken as last noticeable limit before the possible generation of stimulation in vital nerves, e.g. cardiac nerves, which must be avoided at any case.
The second effect is noise production. Noise levels of 99 dB(A) may be reached, sometimes even more, and hearing damage is possible [7]. If implants are present, gradient switching may induce current pulses, which interfere with the function of the implant electronics.The radio frequency field B₁ has a significant power only inside or adjacent to the excitation coil. In most cases the body coil is used for excitation, so that the RF field stretches over a significant portion of the body. The main concern is heating due to eddy currents, which can be rather high especially in the presence of metallic implants. The danger of heating hazards is commonly un­derestimated. Most MR accidents reported in the FDA collection of reports on adverse events (the Manufacturer And User facility Device Experience, MAUDE [8]) refer to burns [9]. While overall warming of the body is limited to acceptable levels by limiting RF absorption (the RF Specific Absorption Rate (SAR) must be below 4 W/kg body weight), heat release at skin-skin contacts in loops formed by arms or legs may cause severe burns at the contact point. Even second or third degree burns have been reported [10]. In metallic implants the current is higher than in surrounding tissue. At crossover points of the current into or out of the implant the local current density in the tissue may be so high that burns are possible. Similar effects may also occur in wires outside the tissue, but inside the range of the excitation coil. Especially at bad connections sparking may occur, which in the extreme case may ignite inflammable material [11]. Sparks can also be generated from carbon rods, as used for external fixation (Fig.1).In addition to the electromagnetic fields cryogens - usually liquid Helium (LHe) - used in super­conducting magnets must be considered. Cryogens pose a risk only in case of a quench, which in most sites never happens. During an accidental or deliberate quench of the magnetic field superconductivity is disrupted and the current crosses over to a conducting copper matrix, where the energy of the current is converted to heat. This causes the liquid helium to immediately evaporate, which in the end causes a 700-fold volume increase compared to liquid He. This requires the venting off of typically 700 m³ gaseous He within a couple of seconds. Usually quench lines are designed to handle this amount of gas. However, imperfectly maintained quench lines may be blocked. In this case the gas will evaporate into the scanner room ('in-room quench'), which in most cases is far smaller than 700 m³, creating a severe overpressure. This has happened a couple of times, and severe damage to buildings is reported. Therefore, careful maintenance of the cryo system and the quench lines is man­datory to prevent the danger of an in-room quench.To cope with MR hazards, limits of fields and field changes have been determined in international standards, especially for the specific absorption rate (SAR) to prevent tissue injury due to excessive warming, and for slew rate and slope of switched gradients to prevent nerve stimulation. While these limits provide adequate safety against unwanted physiological reactions, they do not describe adequate limitations for interaction of these fields with implants, especially active implants. To be able to provide conditions that allow safe MR examinations also in the presence of implants, in addition to limit values based on physiology further scanner parameters must be controlled and new limiting values must be defined. This is accomplished by the introduction of a specific implant scanning option (Fixed Parameter Option: Basic, FPO:B) in the international MR safety standard [4] and technical specifications for Implants [12].To prevent accidents and damages, everybody working with an MR scanner must be informed about possible risks originating from the electromagnetic fields and the cryogens of the MR system.