Transcript SAFETY ASPECTS OF MAGNETIC RESONANCE IMAGING

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.
2011 update: some updates added to terminology, MR conditional
devices, and contrast with renal failure. JMoon
Magnetic Resonance
One 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
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
Static magnetic field
Measured in Gauss or Tesla (10,000G equivalent to 1T)
1.5 T is 30,000 x the strength of earth’s magnetic field
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
Static Magnetic Field
Biological effects
Magnetohydrodynamic effect – augmented T wave
Outside field
0.5 Tesla
1.5T
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
Static Magnetic Field
Pregnancy
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
Static Magnetic Field
Mechanical effects
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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.
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Effect on ferromagnetic implants
- electro-mechanical eg pacemakers
- biomedical eg valves, stents
What is typically ferromagnetic?
EQUIPMENT
• Oxygen cylinders
• Wheelchairs
• Trolleys
• IV stands
• Monitoring equipment
• Ventilators
PERSONAL ITEMS (leave outside)
Keys, pens
Bleeps
Mobile phones
Coins
Stethoscopes
Scissors
It is easy to forget objects, particularly when
responding to an emergency!
Remember also – magnetic strips will be wiped (credit cards)
Warnings to minimise accidents?
OUTSIDE THE SCANNER ROOM
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Signage
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Barrier cords to scanner room
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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
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greater hazard because there is no gradual warning
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field strength increases hundreds of gauss in a few inches
Never be complacent – accidents do happen!
Oxygen cylinder
Infusion pump
Floor buffer
“Contraindications” to CMR*
Implants & metal
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Electromechanical implants
Cerebral aneurysm clips
Metallic foreign body in the eye
Shrapnel, bullets (in critical area)
Ocular implants (containing metal)
Swan-Ganz
• Pacemakers /ICD’s
• 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.
Implants
Cerebral clips
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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
Pacemakers
Highly specific guidelines are required
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General rule has been “absolute contra-indication”
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All 3 electromagnetic fields effect pacemaker
function
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Non-pacemaker dependent patients have been
scanned in specialist centres in exceptional cases
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Future: new pacemaker/lead designs may be MRI safe
**2011 update: MR-conditional Medtronic pacemaker and leads now available
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
What can be safely scanned at 1.5T?
Lists of approved implants are available
BIOMEDICAL IMPLANTS
OTHER
CARDIAC
• sternal wires
• heart valves & annuloplasty rings
• coronary stents
• epicardial wires (cut short and taped)
• other vascular stents, coils and filters
• orthopaedic implants
• contraceptive devices
• dental implants
•If unsure, best to check at – check: www.mrisafety.com
• occlusion devices
• catheters (without guidewire)
Effects of implants on imaging - signal void / local distortion
Safety Checklist – comprehensive but concise
Essential to know
• Cardiac pacemaker?
• Previous neurosurgery?
• Implants or metal in the body?
• Pregnant (prudent approach)?
• Drug patch with foil backing?
Important to know
• 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
Preparation of patient
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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
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
Gradient fields – Auditory damage
Hearing protection mandatory above 90dB
time averaged for:
• patients
• staff remaining in the scanner room
• relatives accompanying children or patients
Radiofrequency (RF) fields
Thermogenic effects - health & safety concern
Physiological tissue heating response
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most of the transmitted RF power is transferred into heat
within the patient’s tissue
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all MR systems have safety thresholds to avoid
dangerous levels
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Patients with compromised thermoregularory systems are
at greatest risk
Specific Absorption Rate (SAR)
SAR is the RF power absorbed per unit mass of tissue
(expressed in W/kg)
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complex function of numerous variables
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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
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SAR increases 9 fold from 0.5T to 1.5T
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
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
RF Burn from non-Carbon Electrode
Other Issues
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Contrast agents
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Critically ill patients / resuscitation
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Monitoring and equipment related hazards
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Quench of superconducting magnets
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Claustrophobia
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 1:100,000
Contraindications
•GFR<30ms/minute
• pregnancy (risk versus benefit ratio)
• breast feeding mothers
• infants < 2 years
Caution
• asthmatics, history of previous reactions
Critically ill patients
AICU and High Dependency Patients
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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
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
Resuscitation
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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
Monitoring
Equipment must be MR Safe or MR Conditional
• 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 Conditional
The object may or may not be safe for the patient undergoing an
MR procedure or an individual in the MR environment, depending
on the specific conditions that are present.
Ie can be used if certain device specific conditions met.
See http://www.mrisafety.com/list.asp for definitions
2011: ‘MR conditional’ replaces ‘MR compatible’ as terminology
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 760:1
- should vent to the outside, but this can fail
- pressure build up
- asphyxiation/frostbite
Elective Quenching
The magnet should only be quenched in two
situations:•
If someone is trapped to the scanner by a
ferromagnetic object and is injured and/or
distressed (eg O2 cylinder, piece of equipment)
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If there is a fire in the immediate vicinity on
order to reduce risk to the Fire Brigade
Action to be taken in the event of
a Quench
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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
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
Factors to reduce anxiety
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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
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
This could be you!
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/