Basic MR Safety (Magnetic Fields, Peripheral Nerve Stimulation, etc)
1Department of Clinical Radiology, University of Münster, Muenster, Germany
Synopsis
Magnetic resonance techniques are considered to be not harmful. The three electromagnetic fields used for MR - static magnetic field, switched gradient fields, and radio frequency field - interact with human tissue, but also with other materials exposed to these fields. The physical interactions with human tissue do not cause irreversible physiological effects, as long as certain limits are not exceeded. Concerning foreign material (e.g. implants), the physical effects of the applied fields may cause severe hazards for patients, staff, and material, if MR examinations are not performed properly.
Objective
Information about the potential dangers· of the static magnetic field B0 (physiological effects: interactions with cells and tissues; physical effects: interactions with ferromagnetic objects: force and torque; requirements for MR safe implants and devices),
· of the low frequency (= audio frequency) switched gradient fields (physiological effects: induction of current pulses in nerves, peripheral nerve stimulation, possible cardiac nerve stimulation; noise; physical effects: induction, effects on implants; requirements for MR safe implants and devices),
· of the radio frequency field B1 (physiological effects: warming (RF absorption, SAR), burns caused by current loops with skin-skin contact; physical effects: 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).
Risks associated with the static field B0
Up to now there is no scientific evidence that static magnetic fields (B0) within the range typically used for MR produce permanent bioeffects that could lead to health problems [1]. 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 causes 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. The translational force depends on the spatial gradient of B0 and thus is largest near the entrance of the MR tunnel. The torque acting to align the long axis of a ferromagnetic object with the field lines is largest at the isocenter of 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', which 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.Risks associated with rapidly switched magnetic fields (gradient fields)
The switched field gradients (Gx, Gy, Gz), necessary to provide spatial information, are active during scanning only. A set of three small magnetic fields, linearly increasing in three orthogonal directions, is generated, which modifies the static field. Switching frequencies are in the order of some 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.Risks associated with the pulsed radiofrequency field B1
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. 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 [8]. 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 excitation coil. Especially at bad connections sparking may occur, which in the extreme case may ignite inflammable material [8]. Sparks can also be generated from carbon rods, as used for external fixation (Fig.1). 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 [10]) refer to burns [11].Risks associated with the cryogenic system
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.Safety limits
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 interactions 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 these limit values based on physiology further scanner parameters must be controlled and new limiting values for physical interactions with implants 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].Conclusion
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.Acknowledgements
No acknowledgement found.References
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