High Magnetic Field Science and Its Application in the United States Current Status and Future Directions (2013) / Chapter Skim
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Appendix F: MRI - Safety and Potential Health Effects
Appendix F: MRI - Safety and Potential Health Effects
Pages 196-206
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From page 196... ...
The committee presents a short history of the health effects controversy followed by physiological and perception observations and then a synopsis of the physics behind these experimental observations of effects of static, radiofrequency fields, and switched magnetic field gradients. Scope of the Problem Investigations of health effects from magnetic field exposures have been placed into three categories: static field effects, switched gradient effects, and radio-frequency (RF)
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From page 197... ...
. An exception is the physiological effect of increased bone healing from the induced electric fields associated with exposure of bone fractures to oscil lating magnetic fields of low amplitude (Bassett et al., 1974)
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From page 198... ...
But because the MRI and fMRI procedure involves use of rapidly changing magnetic fields to acquire spatial information and RF oscillating fields to achieve the resonance condition, the evaluation of safety of MRI and spectroscopy must include the slew rate of magnetic field changes and the RF power. These three aspects are discussed below.
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From page 199... ...
Magnetic Field Effects on Vestibular Apparati of Fish, Birds, and Mammals A wide variety of experimental observations implicate the vestibular appa ratus for the variety of symptoms and signs manifested by animals and human subjects in high magnetic fields as well as in the fringe fields of high-field magnets, where forces can cause small but physiologically significant relative tissue motion. Observed symptoms and signs include avoidance by animals of high fields and field gradients (Weiss et al., 1992; Houpt et al., 2007)
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From page 200... ...
carbonate particles (10-100 μm) embedded in a gelatin platform move hair cells in response to gravity and linear motion in the utrical and saccule; and the motion of fluid in response to sound pressure waves stimulates hair cells lining the chan nels of the cochlea.
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From page 201... ...
. Turning Torque on Macromolecules and Large Molecular Assemblages In addition to high magnetic field effects from susceptibility differences between tissues, there is an important effect of high fields on molecules or tissue elements with large susceptibility differences between major and minor axes of molecular assemblages (e.g., retinal rods, chloroplasts, platelets)
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From page 202... ...
Effects of High Magnetic Fields on Nerve Conduction Speed The Lorentz effect of force on moving ions in a magnetic field can slow nerve conduction velocity. The motion of sodium and potassium ions during nerve con duction can be pictured as small current loops along the axis of the conducting nerve fiber.
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From page 203... ...
Specific Absorbed Power and Rapidly Changing Gradients Major safety issues that underpin limiting guidelines for magnetic resonance imaging and spectroscopy of human subjects are the RF heating effects, whose metric is the specific absorbed power. A second limitation on the pulse sequences is the magnitude of induced electric fields from rapidly changing magnetic fields.
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From page 204... ...
The average electric field is E/√2, so that the absorbed power, SAR, by a mass of tissue is as follows: SAR = |E|2 s/2r where |E| is the magnitude of the E-field, s is the conductivity, and r is the density. An increase in field leads to an increase in the magnetic resonance frequency for a given spin.
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From page 205... ...
1991. Physiological effects of fast-oscillating magnetic field gradients.
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From page 206... ...
1983. Cardiovascular alterations in Ma caca monkeys exposed to stationary magnetic fields: Experimental observations and theoretical analysis.
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