MR Safety

Bioeffects

Static Magnetic Fields Varying Magnetic Fields RF Exposure, Acoustic Noise, SAR Implants Missile Effect Pregnancy Pediatric

MR-Related Injuries

Missile Effect/Projectile injury Dislodged ferromagnetic implants Burns Failure of internal/external devices Failure of patient support systems Acoustic injury Contrast agent adverse effects Cryogen handling related injuries

Relative Proportions

Reported 2005, injuries occurred over 10 year span

Reported Deaths 9 deaths out of 389 reported events

3 due to pacemaker failure

2 due to insulin pump failure

Remaining 4 due to Projectile Implant disturbance Asphyxiation (due to cryogenic mishap during

install) Escaped He gas displaces O2

Static Fields No deleterious bioeffects have been shown for typical

clinical magnet strengths

ECG traces have shown enhanced T wave amplitude

Very high magnet strengths (4 T +) anecdotal reports Dizziness/nausea Disorientation Metallic taste When moving through field

Mild sensory effects Visual sensations (magnetophosphenes)

Bioeffects Studied

Alterations in cell growth/morphology DNA structure & gene expression Pre- & post-natal reproduction & development Visual functions Animal Behavior Visual response to photic stimulation Nerve bioelectric activity Cardiovascular dynamics, hematological indices Physiological regulation & circadian rhythms Immune response

Study Results

Often contradictory

Cannot be reliably reproduced

Data based on animal models Cannot be readily extrapolated to humans

Varying Fields Gradient switching

Rapidly oscillating magnetic fields Induces current in conductors Produces loud banging

Earplugs necessary

Very high gradient fields Magnetophosphenes Peripheral nerve stimulation Most likely with EPI sequences

Most common effect is heating Epicardial or retained intracardiac pacer wires More often heating attributable to RF fields

RF Fields Major cause of tissue heating

Ferromagnetic implants esp. susceptible

Temperatures < 42° C body does a good job of self regulation FDA limits core temperature rise to 1° C

Specific Absorption Rate (SAR) Watts/Kg Measured by MR

Enter correct patient’s weight! FDA limits

ECG gating studies requires MRI compatible leads Otherwise skin burns could result

Limiting SAR SAR depends on:

[Magnetic field strength]² [Flip angle]² Size of patient Duty cycle of RF pulses

Minimize heat deposition by: Controlling flip angle ↑ repetition time ↓ # of sections ↑ echo spacing in FSE ↓ # echoes in FSE ↓ refocusing flip angle in FSE

To reduce SAR:

Decrease Power & Duty Cycle of RF Pulses

Issue Parameter Variables Specified ValueStatic magnetic Field

Magnetic Field B0Maximum strength 3.0 T

Inadvertent exposure

Maximum 0.0005 T

Changing magnetic Field Axial gradients

Τ > 120μs < 20 T/s12μs < Τ < 120μs < 2400/Τ T/s

Τ < 12μs < 200T/sTransverse gradients <3x axial gradients

System < 6 T/sRF Power Deposition

Temperature

Core of body < 18º CMaximum head < 38º CMaximum trunk < 39º CMaximum extremities < 40º C

SAR

Whole body (average) < 4W/kgHead (average) < 3W/kgHead or torso per gram

< 8W/kg

Extremities per gram < 12W/kgAcoustic noise Levels

Peak pressure 200 PaAverage pressure 105 dBA

FDA Limits

Projectiles

Wheelchair

Office Chair

Firearms

O2 Tank

Bed

Floor Polisher

Other Metal Objects

Screening Options

Written screening form

Verbal screening

Screening devices Hand-held magnets Metal detectors CAUTION

Device sensitivity may not guarantee no metal present

Portable Item Labels

MR SafeWholly nonmetallic itemsComposition known

Not MR SafeClearly ferromagnetic

MR ConditionalNo/negligible attractive forces observed

Screening Form

Form Cont’d

Implants & Prostheses

Intracranial Vascular clips Intravascular coils, filters, stents Extra-cranial vascular clips Vascular access ports Heart valves Dental devices & materials Penile implants Otologic implants Ocular implants Intra-occular ferrous foreign bodies Bullets, pellets & shrapnel Orthopedic implants, materials & devices Halo vest & other similar externally applied devices

Electrically, Magnetically, orMechanically-Activated or Electrically Conductive Implants

Cardiac pacemakers May be possible to scan assuming:

Non-dependent pacer patient Clinically fit to scan Radiologist, cardiologist, rep from pacer company present Or Newer MR safer pacemaker implanted

Cochlear implants Tissue expanders Ocular prostheses Dental implants Neurostimulators Bone growth stimulators Implantable cardiac defibrillators Implantable drug infusion pumps

Safety Zones

Zone 1General Public

Zone 2Interface between public & strictly controlled access areas

Zone 3Access strictly controlled by MR personnelSerious injury could result from interaction between ferromagnetic materials & static/time-varying magnetic fields

Zone 4Magnet roomWithin Zone 3

Personnel Definitions

Non-MR personnel Patients Visitors Facility staff

Not undergone formal training in MR Safety in previous 12 months

Level 1 personnel Passed minimal safety education efforts MR department office staff Patient aides

Level 2 personnel More extensively trained/educated in broader aspects of MR safety issues MR techs Radiologists Radiology department nursing staff

Level 1 & 2 may move about in all zones

Quench Superconducting magnet suddenly becomes resistive

Wires formerly offering no resistance to large currents suddenly have resistance

Rapidly heat

Liquid He quickly boils Converts to gaseous state Rapidly expands

Typical cryostat volume ~1500 Liters Produces 1 million liters of gas

Rapid expansion causes large ↑ in volume Large ↑ in pressure

May have to break control room window to allow entry if venting fails

Must be vented to outside

Magnet Quench

Vented Inside

Case in which stack was unable to handle amount of venting gases during a quench while patient was being scanned

Room filled with cloud of escaped gases & condensed water vapors

Pressure increased to the point where door could not be opened

After breaking glass, patient was removed

Damaged stack

Separated ceiling & walls

Installation Planning

Before Installing Is floor strong enough? What is the delivery path for magnet? Evaluate possible electromagnetic interference

Moving metal-cars, elevators Evaluate vibrations Examine fringe field extent vs. planned controlled

areas Within 5 gauss line, posted & controlled access Within 1 gauss line exclude sensitive equipment

CT scanners CRTs

If not possible, add magnetic shielding Consider adding Ferrous metal detectors Sound penetration into adjacent areas

RF Shielding

Walls, floors & ceilings covered with copper sheeting

Door perimeter lined with continuous pneumatic RF sealStainless steel frame/hinges

View window includes wire mesh

FDA recommends limiting personnel access to 5 gauss line

Fringe Fields

Active ShieldingFringe field: Any field outside magnet boreActive shielding ensures fringe field contained w/in magnet room

1 documented case fringe field fatality (torque on ferromagnetic aneurism clip)

Pregnancy To date no deleterious effects seen in fetus

Single Shot FSE sequences can be used

Pregnant healthcare workers No restrictions Should not remain in magnet room however

Pregnant patients No restrictions on scans

Many facilities prefer to delay scan until after 1st trimester Written consent Review on case-by-case basis

Use of contrast agents should not routinely be used Agents do cross placental barrier & enter amniotic fluid Ingested by fetus, passed via urinary tract & re-ingested

ACR White PaperEstablish, Implement, & Maintain Current MR Safety Policies & Procedures1. All clinical & research MR sites, irrespective of magnet format / field strength, including

installations for diagnostic, research, interventional, and/or surgical applications, should maintain MR safety policies.

2. These PAP should also be reviewed concurrently w/ introduction of any significant changes in safety parameters of MR environment of site (e.g., adding faster / stronger gradient capabilities or higher RF duty cycle studies) & updated as needed. In this review process, national & international standards & recommendations should be taken into consideration prior to establishing local guidelines, PAP.

3. Each site will name an MR medical director whose responsibilities will include ensuring that MR safe practice guidelines are established & maintained as current & appropriate for the site. It is the responsibility of site’s administration to ensure that PAP that result from these guidelines are implemented & adhered to at all times by all of the site’s personnel.

4. Procedures should be in place to ensure that any & all adverse events, MR safety incidents, or “near incidents” that occur in the MR site are reported to the medical director in a timely fashion (e.g., within 24 hours or 1 business day of their occurrence) & used in continuous quality improvement efforts. It should be stressed that the FDA states that it is incumbent upon the sites to also report adverse events & incidents to them via their MedWatch program. The ACR supports this requirement & feels that it is in the ultimate best interest of all MR practitioners to create & maintain this consolidated database of such events to help us all learn about them & how to better avoid them in future

FDA Recommendation

‘If non-ionizing imaging (like sonography) is sub-optimal, or if the information to be gained by MR would have required more invasive testing (like radiography, CT, angiography to name a few), MRI is acceptable.’

Pediatric

Special consideration mainly due to: Sedation Monitoring

EndMRI Safety