MRI Safety

Jody Culham

Department of Psychology

University of Western Ontario

Last Update: November 29, 2008
fMRI Course, Louvain, Belgium

http://www.fmri4newbies.com/

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!

• consent form lingo: “no known risks”

– you can never prove anything is safe, only that something is

unsafe

Magnet Safety: Big Things

Source:

www.howstuffworks.com

Source:

http://www.simplyphysics.com/

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

Very Serious Risk

Source: http://www.mrireview.com/docs/mrideath.pdf

Westchester NY, 2001

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.

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

Fall-off of Magnetic Field

Very Serious Risk

Source:

http://www.fmrib.ox.ac.uk/%7Epeterj/safety_docs/fda_primer.html

Magnet Safety

1. Principal Investigators should be sure all lab members are aware of

hazards.

2. Make sure that anyone who is about to enter the magnet room has

been filled out consent and screening forms (subjects, lab members,
visitors).

3. Remove all metal, coins, credit cards etc. as soon as you enter the

magnet area.

4. Think! Train yourself to mini-screen yourself every time you approach

the magnet room.

5. Do not enter the magnet room with any tools (e.g., scissors). Use only

magnet-friendly tools in the toolbox in the magnet room.

Do the “Metal
Macarena!”

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)

Other safety issues

• fire safety

– 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

Other safety issues

• claustrophobia

– subject screening

• acoustic noise

– soundproofing
– earplugs
– headphones