Field of gradients
fields with gradients
MHRA and ICNIRP Limitations on patient exposure
80% of median perceptual threshold is the standard operating mode.
100% of the median operating level is the controlled mode.
There is no research operating mode available, however a limit of 120% of the median perception threshold is recommended.
1. voltages and currents induced
These time-varying gradient fields induce eddying currents in conductive tissues and cause stimulation e.g.:
Stimulation of peripheral nerves
contraction of muscles without conscious thought
breathing problems
Ventricular fibrillation For static fields greater than 3T:
Light bursts on the retina
dizziness and nausea
Taste sensation that is metallic
Devices may also be impacted by the induced voltages:
Pacemakers for the heart
Cochlear implants
ECG observers
2. Sound Absorption
The fast-switching magnetic fields in gradient echo sequences make loud booms.
Increased noise level brought on by:
Higher field strength
Greater amplitudes of gradients
machine limit