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Electromagnetic Fields in Biological Systems
6.3.1.3.1 Static Magnetic Fields
One of the basic interaction mechanisms of static magnetic fields is generation of forces,
in particular,
• Translational forces within gradient fields on diamagnetic, paramagnetic, or
ferromagnetic molecules (e.g., hemoglobin, collagen, and fibrin), particles (e.g.,
magnetite), and parts (such as metallic implants)
• Rotational or twisting forces on magnetic dipoles until alignment with the magnetic field vector
• Electrodynamic forces on moving electrical charged particles leading to potential differences perpendicular to movement (Hall voltage) or increased blood flow
resistance
In addition, on a microscopic level, static magnetic fields can alter electron spin states
and hence influence the speed and balance of chemical reactions and/or the transit time
of free radicals, which modifies their destructive potential.
Electric flow potentials induced by static magnetic fields can change the T wave of
electrocardiograms (associated with ventricular contraction). This can be observed
above 100 mT (Tenforde et al. 1983). In higher static magnetic fields due to the contributions of electric voltages across the aorta and other blood vessels, recorded electrographs
(ECGs) could become even uninterpretable. Induced electric field strengths may also
cause current densities, for example, 100 mA/m² at the atrial sinus at 5 T (Kinuichi
et al. 1996). They may increase up to 220 mA/m² at 11 T. However, this still remains well
below the cardiac fibrillation threshold of 40–50 A/m² (Irnich 1994).
Electric potentials may also be induced in objects moving through static magnetic
fields. Because high static magnetic fields are generated by electric currents flowing in
supraconducting coils, they cannot be simply switched on and off on demand but persist
after imaging. Therefore, people close to the patient tunnel such as medical staff, service
engineers, and room cleaners may move through static fields. Also patients are moving
through the field when moved into or out of the magnet.
The impact of movement has been studied by numerical simulation. It was demonstrated that induced electric current densities increase with velocity of the movement
and the amplitude of the magnetic gradient. The ICNIRP’s basic restriction for workers
(40 mA/m²) can be exceeded in the vicinity of devices with 1.5 T or more even if motion
is slow such as 1 m/s. Due to different external field profiles, no strict scaling law exists.
However, restrictions are expected to be met at a compliance distance of 1 m to the magnet (Crozier and Chadwick 2007; Crozier and Liu 2005).
Experimental investigations revealed that transient sensory effects such as vertigo,
nausea, metallic taste, and even magnetophosphenes (flickering light sensations) may be
experienced during movement through gradient static fields, although with considerable
interpersonally varying sensitivity. Threshold for motion-induced magnetophosphenes
in sensitive people was determined to be 1 T/s for movements longer than 1 second or in
field gradients of 1 T²/m for postural sways (ICNIRP 2009b).
In addition, MRI scanners present a unique set of safety risks for a health care facility.
In the space surrounding an MRI scanner due to attractive forces of magnetic gradient fields, ferromagnetic objects may be accelerated and become dangerous projectiles