MR Safety

Dr William Holmes

Effects of MR

• Direct biological effects to tissues of the body by, 1. Static magnetic field (ie large superconducting magnet)

2. Magnetic gradients (only when scans are running)

3. RF fields (only when scans are running)

• Indirect effects on metal

1. Strong attractive force on metal object/implants.

2. Heating effects on implants

1. Direct effects of static field

• Many studies have been performed on fruit flies to primates.

• No long term effects shown at clinical field levels (up to 4T).

• At fields >2T rapidly turning one’s head can result in ‘magnetophosphenes’ due to torque on retinal cone cells, also metallic taste and dizziness

Important, do not rapidly move your head in proximity to magnet bore. The concern is you may get dizzy, fall over and then bang your head.

Courtesy Prof. A. Geim

2. Direct effects of magnetic gradients

• These can induce electrical currents in the peripheral nervous system, causing twitching and pain.

• Fast sequences such as EPI, turbos and diffusion weighted gradients can be the worst.

Important, avoid being in the magnet room when scans are being run.

Your can generally tell scans are running by the loud clicking sound.

If you do need to be in the magnet room during scanning, then wear ear protection!!.

• People vary markedly in terms of their sensitivity to such gradient effects.

Important

Seek advice from staff before entering magnet room when scans are running. Especially the first time!

2. Direct effects of magnetic gradients

3. Direct effects of Radiofrequency (RF)

RF radiation can cause an heating effect on biological tissue.

Metal implants can concentrate RF energy and cause local burns

Non-MR compatible pulse oxymeter used on baby on ward

Transponder implant.

Important

1. Check list for implants

2. Avoid being in Magnet room when scans are run. Especially close to bore!

3. Ask staff before entering Magnet room during scanning.

Aneurysm clips

Implants

Implant RF ID chips

Pacemakers

Aneuysm clip

Transponders

Metal plates or pins

Vascular clamp, coil or stent

Infusion pump

Electronic implant (Neurostimulator)

Metal from accident or military service (bullet)

Artificial joints, limbs

Metal fragments in the eye

Important,

On check list identify ANY implant you may have. Ask staff for advice.

You will be asked to fill out and sign an,

“MRI safety checklist form”

It is your responsibility to read and respond accurately.

If you answer “yes” to any question, discuss with staff member.

“Missile” effect of magnetic field

IMPORTANT

7.0 Tesla is a VERY VERY strong magnetic field!!!

http://www.youtube.com/watch?v=vIQaGt_fkkw

Watch the youtube video showing a pair of scissors being pulled into a 1.5Tesla magnet.

Imagine a 7tesla magnet!!!

“Missile” effect of magnetic field

IMPORTANT

• Never take metal object into Magnet room.

• If you need to take an object into Magnet room, ask staff to check the object and authorise it to enter the Magnet room.

• The red/white line marks safe distance limit

Missile injuries in MR units caused by:

• Hairpins

• Earrings

• Pens

• Paperclips

• Cigarette lighters

• Pocket knives

• Surgical scissors/clamps

• Wrenches/screwdrivers

• ID Badges

• Radiation badges

• Needles

• Oxygen tanks

• IV poles/pumps

• Step stools

• Floor buffers

• Wheelchairs

• etc. etc. etc.

MRI: Reported Adverse Incidents

• 17 pacemaker/defibrillator deaths• 1 death due to implant migration/rotation• 1 death due to ferromagnetic missile impact• 1 blinding• >200 cases of burns• >100 impact injuries? Are these reports the tip of the iceberg?

17/11/05

Metal in eye

• One case of a blinding due to fragment rotation/migration

• If metal in eye they must be screened first with a plain film x-ray (detects metal down to 0.1x0.1x0.1.mm)

Safety measures in GEMRIC

• Warning signs posted on all approaches to high field area

• ‘Controlled area’ accessed only through coded self-locking doors

• Extensive staff safety checklist• Emergency policies eg

firesuper-conducting magnet quenchheart attack etc.

Emergencies

• Cardiac arrest: patient must be removed from Controlled area before crash team can have access to them

• Fire: Firemen must be taken through checklist before entering magnet room.

• Quench of super-conducting magnet

Super-conducting quenches• If the quench pipe is blocked there can be a

blowback of helium into the magnet room. Helium is not toxic but can displace oxygen causing asphyxiation

• In the event of a quench the magnet suite must be evacuated immediately!.