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| | | MRI Safety Hazards, Risks and Side Effects | | | | | 'Safety' in MRI News (69) and in MRI Resources (43) | |
| | | Quenching | | |
A quench is the rapid helium evaporation and the loss of superconductivity of the current-carrying coil that may occur unexpectedly, or from pressing the emergency button in a superconducting magnet. As the superconductive magnet becomes resistive, heat will be released that can result in boiling of liquid helium in the cryostat. This may present a hazard if not properly planned for.
The evaporated coolant requires emergency venting systems to protect patients and operators. Quenching can cause total magnet failure and cannot be stopped. MRI systems are designed such that all of the escaping cryogenic gas is directed out of the building ( quench pipe through the roof or the wall). In the event of a burst of the tank (possible in the case of an accident) or a blockage of the pipes, the helium gas will be forced into the scanner room, giving rise to a large white cloud of chilled gas. Under such circumstances it is essential that the scanner room is evacuated, also caused by the displacement of oxygen, which under extreme conditions could lead to asphyxiation. The force of quenching can be strong enough to destroy the walls of the scanner room or the MRI equipment. | | | • View the DATABASE results for 'Quenching' (5).
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| | | Acoustic Noise | | | Vibrations of the gradient coil support structure create sound waves. These are caused by the interactions of the magnetic field created by pulses of the current through the gradient coil with the main magnetic field in a manner similar to a loudspeaker coil. The sounds made by the scanner vary in volume and tone with the type of procedure being performed.
Sound pressure is reported on a logarithmic scale called sound-pressure level, expressed in decibel (dB) referenced to the weakest audible 1 000 Hz sound pressure of 2 * 10 -5 pascal (20 micropascal). Sound level meters contain filters that simulate the ear's frequency response. The most commonly used filter provides what is called 'A' weighting, with the letter 'A' appended to the dB units, i.e. dBA.
MRI system noise levels increase with field strength.
Disposable earplugs and/or headphones for the patient are recommended in high-field systems. Noise-canceling systems and special earphones are available, and active acoustic control systems were developed, e.g. softtone, pianissimo. A sequence with low noise gradient pulses is also called 'whisper sequence'.
See also Phon and Decibel. | | | • View the DATABASE results for 'Acoustic Noise' (9).
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| | | | | Claustrophobia | | | A psychological reaction to being confined in a relatively small area.
This is a very real psychological danger for some individuals during the MRI procedure. A small percentage of patients is claustrophobic and cannot tolerate the confined space within a closed MRI magnet. Claustrophobia, panic attacks and other psychological stress situations have been reported in about 1-4% of cases as a reason to interrupt the MRI examination. Principally short and wide open MRI devices are advantageous because the percentage of claustrophobic incidents drops significantly.
Detailed explanation of the MRI procedure, careful attention and special equipment (mirrors to look outside the machine, emergency bells) help to reduce claustrophobia significantly. The majority of claustrophobic patients will be sufficiently relaxed with orally or intravenous sedatives. See also Open MRI. | | | • View the DATABASE results for 'Claustrophobia' (16).
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| | | Cardiac Risks | | | During the MRI scan an augmentation of T waves is observed at fields used in standard imaging but this possible MRI side effect is completely reversible upon removal from the magnet. A field strength dependent increase in the amplitude of the ECG in rats has been observed during exposure to high homogeneous stationary magnetic fields, but this side effect is not transferable to standard imaging situations for humans.
The minimum level at which augmentation can be observed is 0.3 T and increases by higher field strength.
An augmentation in T-wave amplitude can occur instantaneously and is immediately reversible after exposure to the magnetic field ceased. There should be no abnormalities in the ECG in the later follow-up. Augmentation of the signal amplitude in the T-wave segment may result from superimposed electrical potential.
No circulatory alterations coincide with the ECG changes. Therefore, no biological risks are believed to be associated with them.
For more MRI safety information see also Contraindications
and MRI Risks. | | | • View the DATABASE results for 'Cardiac Risks' (2).
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| | | Magnetohydrodynamic Effect | | | This effect is an additional electrical charge generated by ions in blood (loaded particles) moving perpendicular to the magnetic field.
At 1.5 T, no significant changes are expected; at 6.0 T a 10% blood pressure change is expected.
A blood pressure increase is predicted theoretically for a field of 10 T. This is claimed to be caused by interaction of induced electrical potentials and currents within a solution, e.g. blood, and an electrical volume force causing a retardation in the direction opposite to the fluid flow. This decrease in blood flow-velocity must be compensated for by an elevation in pressure.
Static magnetic field gradients of 0.01 T/cm (100 G/cm) make no significant difference in the membrane transport processes. The influence of a static magnetic field upon erythrocytes is not sufficient to provoke sedimentation, as long as there is a normal blood circulation.
The magnetohydrodynamic effect which results from a voltage occurring across a vessel in a magnetic field, is irrelevant at the field strengths used. | | | • View the DATABASE results for 'Magnetohydrodynamic Effect' (3).
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| | | Nerve Conductivity | | | Rapid echo planar imaging and high-performance MRI gradient systems create fast-switching magnetic fields that can stimulate muscle and nerve tissues produced by either changing the electrical resistance or the potential of the excitation. There are apparently no effects on the conduction of impulses in the nerve fiber up to field strength of 0.1 T. A preliminary study has indicated neurological effects by exposition to a whole body imager at 4.0 T. Theoretical examinations argue that field strengths of 24 T are required to produce a 10% reduction of nerve impulse conduction velocity.
Nerve stimulations during MRI scans can be induced by very rapid changes of the magnetic field. This stimulation may occur for example during diffusion weighted sequences or diffusion tensor imaging and can result in muscle contractions caused by effecting motor nerves. The so-called magnetic phosphenes are attributed to magnetic field variations and may occur in a threshold field change of between 2 and 5 T/s. Phosphenes are stimulations of the optic nerve or the retina, producing a flashing light sensation in the eyes. They seem not to cause any damage in the eye or the nerve.
Varying magnetic fields are also used to stimulate bone-healing in non-unions and pseudarthroses. The reasons why pulsed magnetic fields support bone-healing are not completely understood. The mean threshold levels for various stimulations are 3600 T/s for the heart, 900 T/s for the respiratory system, and 60 T/s for the peripheral nerves.
Guidelines in the United States limit switching rates at a factor of three below the mean threshold for peripheral nerve stimulation. In the event that changes in nerve conductivity happens, the MRI scan parameters should be adjusted to reduce dB/dt for nerve stimulation. | | | • View the DATABASE results for 'Nerve Conductivity' (2).
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