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MRI Safety
Hazards, Risks and
Side Effects
 
  • Hazards, Risks and
    Side Effects
 
 
 
Radiology Safety Open this link in a new window
'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).Open this link in a new window

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).Open this link in a new window

 
Further Reading:
  Basics:
MRI Noise in Utero Not Harmful for Baby's Ears
Tuesday, 28 September 2010   by www.medgadget.com    
  News & More:
Noise from Magnetic Resonance Imaging Can Have Short-Term Impact on Hearing
Thursday, 22 February 2018   by www.diagnosticimaging.com    
Echo Planar Imaging at 4 Tesla With Minimum Acoustic Noise(.pdf)
   by www.bnl.gov    
TOSHIBA SHOWCASES PATIENT-FRIENDLY 3T MR SYSTEM
Sunday, 29 November 2009   by medical.toshiba.com    
Absorbed Dose 

This dose means the RF power absorbed per unit of mass of an object, and is measured in watts per kilogram (W/kg).
The absorbed dose is dependent on the duty cycle and transmitter-coil type and increases with field strength, radio frequency power and and body size.
The specific absorption rate (SAR) describes the potential for heating of the patient's tissue due to the application of the RF energy necessary to produce the MR signal.

See also Specific Absorption Rate, MRI Safety, and MRI Risks.

• View the DATABASE results for 'Absorbed Dose' (2).Open this link in a new window

 
Further Reading:
  Basics:
Commission delays electromagnetic fields legislation
Monday, 29 October 2007   by cordis.europa.eu:80    
Physics of MRI Safety
   by www.aapm.org    
  News & More:
SED Guidance
Saturday, 1 January 2022   by www.mriphysics.scot.nhs.uk    
Specific Absorption Rate and Specific Energy Dose: Comparison of 1.5-T versus 3.0-T Fetal MRI
Tuesday, 7 April 2020   by pubs.rsna.org    
Evaluation of Absorbed Dose by MRI Read-Out
Saturday, 18 November 2017   by www.jstage.jst.go.jp    
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).Open this link in a new window

 
Further Reading:
  Basics:
Claustrophobia in Magnetic Resonance Imaging: An Analysis of Causes, Impacts and Solutions
Friday, 19 January 2024   by www.diagnosticimaging.com    
Open MRI scanners reduce anxiety in patients
Thursday, 8 September 2011   by www.mtbeurope.info    
  News & More:
CSU study explores MRI distress and patient experience
Thursday, 7 May 2020   by www.portnews.com.au    
Lego Open MRI aims to reduce anxiety
Thursday, 21 February 2019   by www.inhealthgroup.com    
VR application helps patients overcome MRI anxiety, claustrophobia
Tuesday, 4 December 2018   by www.radiologybusiness.com    
Women Suffer from Claustrophobia More Often Than Men
Saturday, 25 July 2015   by www.youthhealthmag.com    
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.
mri safety guidance
MRI Safety Guidance
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).Open this link in a new window

 
Further Reading:
  Basics:
A Primer on Medical Device Interactions with Magnetic Resonance Imaging Systems
   by govpulse.us    
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.
mri safety guidance
MRI Safety Guidance
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).Open this link in a new window

 
Further Reading:
  News & More:
Measuring magnetic force field distributions in microfluidic devices: Experimental and numerical approaches
Saturday, 2 December 2023   by analyticalsciencejournals.onlinelibrary.wiley.com    
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).Open this link in a new window

 
Further Reading:
  Basics:
Electrical eddy currents in the human body: MRI scans and medical implants
   by www.phy.olemiss.edu    
  News & More:
NERVE STIMULATORS
Tuesday, 18 January 2005   by www.health.adelaide.edu.au    
Conductivity tensor mapping of the human brain using diffusion tensor MRI
   by www.pnas.org    
  Safety Rules top
Radiology Safety Open this link in a new window
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