1. Jody Culham
Brain and Mind Institute
Department of Psychology
Western University
fMRI Physics in a Nutshell Understanding WTF your MR physicist is talking about
Last Update: September 14, 2017
Last Course: Psychology 9223, F2017

2. 1D: MRI Safety

3. Magnetic Fields main magnetic field is very strong
BUT static magnetic fields are less of a concern than changing magnetic fields
moving quickly through a magnetic field, especially the head, is a BAD idea -- like doing whole brain TMS on yourself
some people experience dizziness, nausea, metallic tastes
BUT these were also reported in 45% of subjects when the magnet was OFF!
typical consent form phrasing: “no known risks”
you can never prove anything is safe, only that something is unsafe

4. Magnet Safety: Big Things
Source:
Source:
flying_objects.html
“Large ferromagnetic objects that were reported as having been drawn into the MR equipment include a defibrillator, a wheelchair, a respirator, ankle weights, an IV pole, a tool box, sand bags containing metal filings, a vacuum cleaner, and mop buckets.”
-Chaljub et al., (2001) AJR

5. Very Serious Risk Westchester NY, 2001
Source:

6. Very Serious Risk
Source:

7. Magnet Safety: Little Things
Aneurysm clips can be pulled off vessels, leading to death
Flying things can kill people.
Even in less severe incidents, they can fly into the magnet and damage it or require an expensive shutdown.

8. Subject Safety
Anyone going near the magnet – subjects, staff and visitors – must be thoroughly screened:
Subjects must have no metal in their bodies:
pacemaker
aneurysm clips
metal implants (e.g., cochlear implants)
interuterine devices (IUDs)
some dental work (but fillings are okay)
Subjects must remove metal from their bodies
jewellery, watch, piercings
coins, etc.
wallet
any metal that may distort the field (e.g., underwire bra)
Females must not be pregnant or at risk of conceiving
Some institutions even require pregancy tests for any female, every session
Subjects must be given ear plugs (acoustic noise can reach 120 dB)
This subject was wearing a hair band with a ~2 mm copper clamp. Left: with hair band. Right: without.
Source: Jorge Jovicich

9. Fall-off of Magnetic Field
actively shielded magnet

10. Magnet Safety
Principal Investigators should be sure all lab members are aware of hazards.
Make sure that anyone who is about to enter the magnet room has been filled out consent and screening forms (subjects, lab members, visitors).
Remove all metal, coins, credit cards etc. as soon as you enter the magnet area.
Think! Train yourself to mini-screen yourself every time you approach the threshold of the magnet room.
Do not enter the magnet room with any tools (e.g., scissors). Use only magnet-friendly tools in the toolbox in the magnet room.
Think!
Do the “Metal Macarena!”

11. Specific Absorption Rate (SAR)
excess energy heats body tissues
if body heats faster than natural cooling, temperature rises
Specific Absorption Rate (SAR) = amount of heat absorbed by body
magnets have SAR limits to prevent overheating
limited to 1 degree rise in core body temperature
depends on body size, geometry, thermoregulation
depends on pulse sequences (e.g., larger flip angles = greater SAR)

12. Other safety issues fire safety quenching burns
always give subjects a panic button
make sure that subject can be evacuated quickly if needed
have an MR-compatible fire extinguisher available
operator must know safety protocols
quenching
rapid decrease in magnetic field strength
helium boils off and can fill room (displacing oxygen)
can occur spontaneously
only voluntarily initiated in extreme situations
burns
do not loop any wires or cables
do not place electrodes on subjects’ skin

13. Other safety issues claustrophobia peripheral nerve stimulation
subject screening
peripheral nerve stimulation
rapid switching of gradients can lead to generation of currents in the body that stimulate the nerves (e.g., twitching)
manufacturers limit rate of gradient switching to avoid problems
acoustic noise
without ear protection, could cause hearing loss
soundproofing
earplugs
headphones

14. See links on www.fmri4newbies.com