Free MRSO Static Magnetic Field (B0) Bioeffects and Exposure Limits Questions and Answers — Questions and Answers
Question 1: What is the primary bioeffect associated with the magneto-hydrodynamic effect caused by the static magnetic field (B0)?
- Peripheral nerve stimulation
- An increase in T-wave amplitude on an ECG (Correct answer)
- Auditory damage from acoustic noise
- Tissue heating due to RF absorption
Correct answer: An increase in T-wave amplitude on an ECG
The magneto-hydrodynamic effect is the induction of a voltage in flowing conductive fluids, like blood, within a static magnetic field. This is most prominently observed on an electrocardiogram (ECG) as an elevation or peaking of the T-wave, which can sometimes be mistaken for a cardiac abnormality but is a reversible physiological effect.
Question 2: According to the U.S. Food and Drug Administration (FDA), what is the static magnetic field (B0) limit for clinical imaging of adults, children, and infants older than one month?
- 1.5 Tesla
- 3.0 Tesla
- 8.0 Tesla (Correct answer)
- 4.0 Tesla
Correct answer: 8.0 Tesla
The FDA considers MRI systems with a static magnetic field strength of 8.0 Tesla or less to be a non-significant risk device for clinical use in patients over one month of age. Higher field strengths require an Investigational Device Exemption (IDE) for clinical applications.
Question 3: The translational force on a ferromagnetic object is greatest where the...
- static magnetic field (B0) is highest.
- spatial gradient of the magnetic field is maximal. (Correct answer)
- radiofrequency (RF) field is most uniform.
- time-varying gradient fields are switched fastest.
Correct answer: spatial gradient of the magnetic field is maximal.
The translational force, which pulls ferromagnetic objects into the scanner, is a product of the object's magnetic properties, the main magnetic field strength (B0), and the spatial gradient of the magnetic field (dB/dz). This force is strongest where the rate of change of the magnetic field is highest, which is typically near the ends of the magnet bore.
Question 4: What is the primary physiological mechanism responsible for the sensations of vertigo, dizziness, or nystagmus when moving within a strong static magnetic field?
- Stimulation of the auditory nerve by Lorentz forces.
- Induced currents in the cerebral cortex.
- Interaction with the vestibular system's endolymph. (Correct answer)
- Temporary reduction in cerebral blood flow.
Correct answer: Interaction with the vestibular system's endolymph.
These sensations are caused by the interaction of the static magnetic field with the ionic fluids (endolymph) within the semicircular canals of the vestibular system in the inner ear. The induced Lorentz forces stimulate the sensory hair cells, creating a false sensation of motion.
Question 5: The International Commission on Non-Ionizing Radiation Protection (ICNIRP) guidelines for occupational exposure to static magnetic fields recommend a time-weighted average (TWA) exposure limit over an 8-hour workday of:
- 50 mT
- 100 mT
- 200 mT (Correct answer)
- 500 mT
Correct answer: 200 mT
ICNIRP guidelines aim to protect workers from the acute effects of magnetic field exposure, such as vertigo. For occupational exposure during a standard 8-hour workday, the recommended time-weighted average limit for the whole body is 200 mT (milliTesla).
Question 6: During a magnet quench, the rapid boil-off of liquid helium poses several immediate hazards. Which of the following is the MOST critical and immediate life-threatening hazard inside the magnet room?
- The loud noise of the event
- Frostbite from contact with cold gas
- Asphyxiation due to oxygen displacement (Correct answer)
- Projectile risk as the field collapses
Correct answer: Asphyxiation due to oxygen displacement
A quench rapidly converts liquid helium into a massive volume of helium gas, which displaces oxygen in the enclosed magnet room. This can lead to rapid asphyxiation, which is the most immediate and life-threatening danger to anyone trapped in the room.
Free MRSO Static Magnetic Field (B0) Bioeffects and Exposure Limits Questions and Answers
What is the primary bioeffect associated with the magneto-hydrodynamic effect caused by the static magnetic field (B0)?