MRI Safety Flashcards
(23 cards)
What are the six main sources of hazards associated with MRI?
- Static field
- Gradient fields
- Radiofrequency
- Cryogens
- Noise
- Contrast agents
At what field strength does magnetic media feel a force?
3 mT
At what field strength do computers and x-ray tubes start to be affected?
1 mT
At what field strength do pacemakers become affected?
0.5 mT
What are the hazards associated with the static magnetic field?
Fringe fields
Translational forces (F prop to d) - projectiles
Torque forces - alignment with B
Lenz law (force opposing motion)
Where is force due to the static field strongest?
Near the edge of the bore
What are the biological effects of the static magnetic field?
Movement induces electrical current in conductors in spatially variant field - nerve and muscle cell conduction
Vertigo/dizziness
Metallic taste
Magnetophosphenes
What are the hazards associated with the gradient field?
Magnetophosphenes
PNS
Discomfort
Leg movement
Fibrillation
What hazards are associated with the RF field?
Heat deposition due to dielectric heating from molecular vibration
How does the body compensate for RF heating?
Dilation of blood vessels
How is RF heating quantified?
SAR (W/kg)
What does SAR depend on?
Sequence
Number of slices
TR
Flip angle
Patient size
Conductivity (metals heat up quicker)
RF amplitude
What is a consequence of RF heating on patients?
Heating and contact burns
What are controls around MR safety?
Controlled areas
Authorised personnel
Local rules
Labelling
Screening
What is quenching?
Destroys magnetic field, taking ~30 s to 10 minutes, by releasing cryogen
When should quenching be used?
When a person is being crushed reusulting in life or limb emergency or if there is a fire
What hazards are associated with cryogens?
Leaks
Cold burns
Frostbite
Asphyxiation
What is the risk of using MRI contrast agents?
Nephrogenic systemic fibrosis (NSF) - rare fatal disease resulting in skin thickening, painful joints and organ fibrosis (lungs, liver, muscles, heart)
What is NSF caused by?
Gadolinium escaping from the chelate over time - more likely with linear than macrocyclic
What are the three MR labels?
MR Safe
MR Conditional
MR Unsafe
What does MR Safe mean
The device poses no risk to the patient in the MR environment, but image quality may be effected
What does MR Conditional mean?
The device poses no additional risk to the patient when introduced to the MR environment under specified conditions
What does MR Unsafe mean?
The device may not be introduced into the MR environment as it poses significant risk to the patient and/or staff
