HOW I DO CMR SCANNING SAFELY

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HOW I DO CMR SCANNING SAFELY

• Elisabeth Burman
• Research Sister
• Royal Brompton Hospital, London UK

This presentation is posted for members of scmr
as an educational guide it represents the views
and practices of the author, and not necessarily
those of SCMR.
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Magnetic ResonanceOne of the safest imaging
modalities but there are still safety issues

• Advantages
• Non-invasive
• No ionizing radiation
• Repeat studies present no problem
• Fewer ethical problems imaging volunteers
• Why are there safety issues?
• Potential for accidents and even fatalities
• Damage to equipment / cost

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Safety issues centre around the 3
electromagnetic fields and issues indirectly
related
Static magnetic field biological
effects mechanical effects Gradient
Field induced currents (PNS) auditory
damage RF Field RF power deposition Burns
Other issues contrast agents critically
ill patients/resuscitation monitoring
quench claustrophobia
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Static magnetic field
Measured in Gauss or Tesla (10,000G equivalent to
1T)
1.5 T is 30,000 x the strength of earths
magnetic field
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Static Magnetic Fields
A superconducting magnet is always switched on!
1. Biological effects (potential risk) -
exposure to static magnetic fields of up to 4T
are not thought to be harmful Biological
effects relevant to clinical imaging - distorted
ECG (magnetohydrodynamic effect) - consider
prudency with pregnancy 2. Mechanical effects
(very real risk) - translational or attractive
forces on metallic objects when brought into the
field
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Static Magnetic FieldBiological effects
Magnetohydrodynamic effect augmented T wave
Outside field
0.5 Tesla
1.5T
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Magnetohydrodynamic effect seen as augmentation
of T-wave

• Caused by the effect of the static magnetic field
  on moving blood (systole) as a conducting fluid.
• The gradient and RF fields also affect the
  configuration of the ECG
• Morphological ECG changes are therefore difficult
  to detect and diagnose, but rhythm is usually
  recognised
• Any concern regarding rhythm, remove patient from
  scanner and perform 12 lead ECG

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Static Magnetic FieldPregnancy

• Patients
• 1st trimester prudent to avoid CMR where
  possible
• 2nd and 3rd trimester decision made on a risk
  versus benefit determination. For example if it
  avoids the patient being subjected to x-rays.
• Health Care Workers
• May enter MR scanning room regardless of
  trimester
• Should not remain in the room when scanner is
  operational, avoiding exposure to gradient and
  radiofrequency fields

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Static Magnetic FieldMechanical effects

• Projectile or missile effect
• - the attractive forces exerted by the static
  magnetic field present the greatest potential
  for patient injury
• - objects will be pulled out of hands, pockets
  etc, and fly into magnet which has caused injury
  and death.
• Effect on ferromagnetic implants
• - electro-mechanical eg pacemakers
• - biomedical eg valves, stents

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What is typically ferromagnetic?

• EQUIPMENT PERSONAL ITEMS (leave outside)
• Oxygen cylinders Keys, pens
• Wheelchairs Bleeps
• Trolleys Mobile phones
• IV stands Coins
• Monitoring equipment Stethoscopes
• Ventilators Scissors

It is easy to forget objects, particularly when
responding to an emergency! Remember also
magnetic strips will be wiped (credit cards)
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Warnings to minimise accidents?

• OUTSIDE THE SCANNER ROOM
• Signage
• Barrier cords to scanner room
• Restricted access - swipe card / coded entry
  system
• INSIDE THE SCANNER ROOM (fringe field 5 Gauss
  line)
• Older systems - passively shielded
• - extensive fringe field gave a warning
• Newer systems - actively shielded
• - small fringe field
• greater hazard because there is no gradual
  warning
• field strength increases hundreds of gauss in a
  few inches

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Never be complacent accidents do happen!
Infusion pump
Oxygen cylinder
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Floor buffer
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Contraindications to CMR
Implants metal
Electromechanical implants

• Cerebral aneurysm clips
• Metallic foreign body in the eye
• Shrapnel, bullets (in critical area)
• Ocular implants (containing metal)
• Swan-Ganz

• Pacemakers /ICDs
• Pacing wires
• Cochlear implants
• Neurostimulators
• Hydrocephalus shunts

Any device electrically or mechanically activated
In some circumstances MR has been performed
despite contraindications, especially in
specialist centres. Work is
in progress to make some devices safe. For
example, some cerebral aneurysm clips may now be
scanned.
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Implants

• Cerebral clips
• modern clips are considered safe (titanium,
  elgiloy)
• older clips (martensic steel is highly ferrous)
• obtain operation notes with serial number of clip
• radiologist to consent patient
• seek advice from specialist centre
• Foreign bodies
• Maybe situated near vascular or nervous tissue
• If in doubt x-ray

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PacemakersHighly specific guidelines are required

• General rule has been absolute
  contra-indication
• All 3 electromagnetic fields effect pacemaker
  function
• Non-pacemaker dependent patients have been
  scanned in specialist centres in exceptional
  cases
• Future new pacemaker/lead designs may be MRI
  safe

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Potential effects of MRI on pacemakers

• excessive heating / induced currents in the
  pacemaker lead
• temporary or permanent modification of pacemaker
  function
• inappropriate sensing or triggering of the device
• movement of the pulse generator or lead
• pacing the heart

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What can be safely scanned at 1.5T?Lists of
approved implants are available
BIOMEDICAL IMPLANTS

• OTHER
• orthopaedic implants
• contraceptive devices
• dental implants

• CARDIAC
• sternal wires
• heart valves annuloplasty rings
• coronary stents
• epicardial wires (cut short and taped)
• other vascular stents, coils and filters
• If unsure, best to check at check
  www.mrisafety.com
• occlusion devices
• catheters (without guidewire)

Effects of implants on imaging - signal void /
local distortion
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Safety Checklist comprehensive but concise
Important to know
Essential to know

• Cardiac pacemaker?
• Previous neurosurgery?
• Implants or metal in the body?
• Pregnant (prudent approach)?
• Drug patch with foil backing?

• Previous heart surgery?
• Diabetic or epileptic?
• Asthmatic or allergies?
• (in relation to contrast)
• Tattoos or permanent eye liner
  (iron
  oxide)?

Essential preparation

• Removal of accessories - watch, jewellery (except
  wedding rings), body piercing rings, hearing
  aids, glasses, false teeth, artificial limbs and
  prostheses
• Removal of clothes containing metal eg zips, bras

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Preparation of patient

• Remove watches / jewellery except gold wedding
  rings
• Remove hearing aids, false teeth, glasses,
  prostheses
• Remove all clothes except socks and underpants
• Patient gowns - no pockets
• - no metallic fastenings
• - ¾ length sleeves for IV access
• - wrap round for easy chest access
• Brass changing room keys
• Screen all accompanying personnel
• Check any suspicious item with small bar magnet

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Gradient fields - Induced Currents

• Gradient fields induce an electric field and thus
  a current in the patient, potentially this can be
  of sufficient intensity in modern systems to
  produce a physiological response
• - peripheral nerve stimulation (PNS)
• - cardiac stimulation is not considered possible

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Gradient fields Auditory damage

• Hearing protection mandatory above 90dB
• time averaged for
• patients
• staff remaining in the scanner room
• relatives accompanying children or patients

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Radiofrequency (RF) fields

• Thermogenic effects - health safety concern
• Physiological tissue heating response
• most of the transmitted RF power is transferred
  into heat
• within the patients tissue
• all MR systems have safety thresholds to avoid
• dangerous levels
• Patients with compromised thermoregularory
  systems are
• at greatest risk

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Specific Absorption Rate (SAR)

• SAR is the RF power absorbed per unit mass of
  tissue
• (expressed in W/kg)
• complex function of numerous variables
• calculated by software from the average forward
  power passing into the RF transmitter coil and
  the body mass situated in the RF transmitting
  field
• Therefore an accurate patient weight is vital
• SAR increases 9 fold from 0.5T to 1.5T

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Radiofrequency (RF) fields

• Potential for burns
• 1, 2, 3 burns have occurred in the past in
  patients undergoing MRI
• This is a result of excessive heat developing in
  the devices or objects
• ECG system is often the culprit
• Interventional MRI poses greater risk

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Prevention of burns

• Electrodes - carbon fibre studs
• - placed close together
• ECG leads - carbon fibre
• - fibre optic
• - high impedance
• - short as possible (plaited if necessary)
• All conductive leads should be placed in a linear
  fashion coming out of bore of scanner
• Avoid crossed limbs where possible

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RF Burn from non-Carbon Electrode
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Other Issues

• Contrast agents
• Critically ill patients / resuscitation
• Monitoring and equipment related hazards
• Quench of superconducting magnets
• Claustrophobia

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Contrast agents

• all gadolinium chelates
• overall patient tolerance high
• incidence of adverse reactions very low

• Possible reactions-
• headache, nausea and vomiting, pain if
  extravasation occurs
• anaphylaxis 1100,000

• Contraindications
• pregnancy (risk versus benefit ratio)
• breast feeding mothers
• infants lt 2 years
• Caution
• asthmatics, history of previous reactions

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Critically ill patients
AICU and High Dependency Patients

• Assess patient the day before
• Lengthen and prime IV lines on ward
• Replace electrodes - carbon studs
• Lengthen chest drains
• Check intubation circuits for metal
• Educate nursing staff in preparing patient
• Ensure status of patient has not changed since
  referral
• Screen all accompanying personnel

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Sedated patients

• Departmental sedation protocol with anaesthetic
  approval
• Nil by mouth for 6 hrs (food milk) 2 hrs water
• Medical examination
• Informed consent
• Tipping trolley, piped oxygen, full resuscitation
• equipment
• Responsible adult to collect from department and
  remain with for 24 hours

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Resuscitation

• Alert cardiac arrest team
• Call for help
• BLS only can be performed in scanner room
• Keep MR Compatible tipping trolley adjacent to
  scanner
• Patient must be moved to designated resuscitation
  area as quickly as possible
• Appoint non-clinical person to prevent access to
  scanner room
• Practice regular cardiac arrest scenarios to
  identify potential problems

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Monitoring
Equipment must be MR Safe or MR Compatible

• MR Safe
• Device when used in the MR environment, has been
  demonstrated to present no additional risk to the
  patient BUT may affect the quality of diagnostic
  information
• MR Compatible
• Device has been demonstrated to neither
  significantly affect the quality of the
  diagnostic information or to be operationally
  affected by the scanner in use

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Quench
Cryogens maintain the magnetic field -
helium QUENCH the liquid helium boils off and
becomes a gas
Causes - physical - human error
(accidental) - intervention (elective)
Effects - ratio of gaseous to liquid helium
7601 - should vent to the outside, but this
can fail - pressure build up -
asphyxiation/frostbite
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Elective Quenching

• The magnet should only be quenched in two
  situations-
• If someone is trapped to the scanner and is
  injured and/or distressed
• If there is a fire in the immediate vicinity on
  order to reduce risk to the Fire Brigade

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Action to be taken in the event of a Quench

• Evacuate the room as quickly as possible
• Ensure the door is kept open during evacuation
• Close door after evacuation
• If trapped in room stay close to floor level
• Seek the advice of a senior physicist immediately
• Call scanner engineer

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Claustrophobia

• Affects 5-10 of patients
• Causes
• restrictive dimensions of the interior of the
  magnet
• duration of the examination
• gradient coil induced noises
• the ambient condition within the imaging bore

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Factors to reduce anxiety

• education and explanation
• trial visit to the department
• maintaining physical and verbal contact
• presence of a relative or friend
• use of a mirror, prism glasses
• good communication system
• alarm
• good light and ventilation
• music
• pleasant thoughts

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Ensuring Safe Practice

• Safety Checklist comprehensive but concise
• Restricted access to magnetic field area
• MR Compatible equipment monitors, wheelchairs,
  syringe pumps, stethoscopes
• Awareness of medication common to patient area
  contrast agents, stress agents
• Staff Induction
• Education and training of staff of all
  disciplines
• Risk Assessments
• Policies and Procedures cardiac arrest, quench

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This could be you!
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References

• Shellock F.G. and Kanal E. (2001)
• - Magnetic Resonance Bioeffects, Safety, and
  Patient
• Management
• Health Protection Agency (HPA)
• (was National Radiological Protection Board
  -NRPB)
• International Electrical Committee (IEC)
• Medical and Healthcare products Regulatory
  Agency
• (MRHA) (was Medical Devices Agency - MDA)
• http//www.mrisafety.com (Shellock and Kanal)
• http//www.imrser.org/
• http//www.magneticresonancesafetytesting.com/