Jody Culham Brain and Mind Institute Department of Psychology Western University http://www.fmri4newbies.com/ fMRI Physics in a Nutshell Understanding WTF your MR physicist is talking about Last Update: September 14, 2017 Last Course: Psychology 9223, F2017

1D: MRI Safety

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

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 Westchester NY, 2001 Source: http://www.mrireview.com/docs/mrideath.pdf

Very Serious Risk Source: http://www.fmrib.ox.ac.uk/%7Epeterj/safety_docs/fda_primer.html

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 actively shielded magnet

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!”

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 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

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

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