Safety Aspects of Magnetic Resonance Imaging455

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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 guideit 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 issuesAdvantages

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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Static magnetic field biological effectsmechanical effects

Gradient Field induced currents (PNS)auditory damage

RF Field RF power depositionBurns

Otherissues contrast agents

critically ill patients/resuscitationmonitoring

quench

claustrophobia

Safety issues centre around the 3 electromagnetic fields

and issues indirectly related


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Static magnetic field

1.5 T is 30,000 x the strength of earths magnetic field

Measured in Gauss or Tesla (10,000G equivalent to 1T)


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Static Magnetic Fields

1. Biological effects (potential risk)

- exposure to static magnetic fields of up to 4T are not thoughtto be harmful

Biological effects relevant to clinical imaging- distorted ECG (magnetohydrodynamic effect)

- consider prudency with pregnancy

2. Mechanical effects (very real risk)

- translational or attractiveforces on metallic objects whenbrought into the field

A superconducting magnet is always switched on!


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Static Magnetic Field

Biological effects

Magnetohydrodynamic effectaugmented 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 configurationof the ECG

Morphological ECG changes are therefore difficult todetect and diagnose, but rhythm is usually recognised

Any concern regarding rhythm, remove patient fromscanner and perform 12 lead ECG


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Pregnancy

Patients 1st trimesterprudent to avoid CMR where possible

2nd and 3rd trimesterdecision made on a risk versusbenefit 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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Mechanical effects

Projectile or missile effect- the attractive forces exerted by the static magneticfield present the greatest potential for patient injury

- objects will be pulled out of hands, pockets etc, and fly intomagnet 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 alsomagnetic 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 field5 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 complacentaccidents do happen

Oxygen cylinder Infusion pump


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Floor buffer


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Cerebral aneurysm clips

Metallic foreign body in the eye Shrapnel, bullets (in critical area)

Ocular implants (containing metal)

Swan-Ganz

Contraindications to CMR*

Implants & metal Electromechanical implan

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 doubtx-ray


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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

CARDIAC

sternal wires heart valves & annuloplasty rings

coronary stents epicardial wires (cut short and taped)

othervascular stents, coils and filters

If unsure, best to check atcheck: www.mrisafety.com

occlusion devices

catheters (without guidewire)

BIOMEDICAL IMPLANTSOTHER

orthopaedic implants contraceptive devices

dental implants

Effects of implants on imaging -signal void / local distortion


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Safety Checklistcomprehensive but concise

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

Important to know

Previous heart surgery?

Diabetic or epileptic?

Asthmatic or allergies?

(in relation to contrast)

Tattoos or permanent eye liner(iron oxide)?

Essential to know

Cardiac pacemaker? Previous neurosurgery?

Implants or metal in the body?

Pregnant (prudent approach)?

Drug patch with foil backing?

Essential preparation


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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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Hearing protection mandatory above 90dB

time averaged for:

patients staff remaining in the scanner room

relatives accompanying children or patients

Gradient fieldsAuditory damage


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Radiofrequency (RF) fields

Thermogenic effects - health & safety concerPhysiological 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 forwardpower passing into the RF transmitter coil and thebody mass situated in the RF transmitting fieldTherefore 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 fashioncoming 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 1:100,000

Contraindications pregnancy (risk versus benefit ratio)

breast feeding mothers infants < 2 years

Caution asthmatics, history of previous reactions


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Critically ill 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

AICU and High Dependency Patients


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Sedated patients

Departmental sedation protocol with anaestheticapproval

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 remainwith 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 asquickly as possible

Appoint non-clinical person to prevent access to scannerroom

Practice regular cardiac arrest scenarios to identifypotential problems


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Monitoring

MR Safe

Device when used in the MR environment, has beendemonstrated 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 beoperationally affected by the scanner in use

Equipment must be MR Safe or MR Compatible


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Elective Quenching

The magnet should only be quenched in twosituations:-

If someone is trapped to the scanner and isinjured and/or distressed

If there is a fire in the immediate vicinity onorder 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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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 Checklistcomprehensive but concise

Restricted access to magnetic field area

MR Compatible equipmentmonitors, wheelchairs,syringe pumps, stethoscopes

Awareness of medication common to patient areacontrast agents, stress agents

Staff Induction

Education and training of staff of all disciplines

Risk Assessments

Policies and Procedurescardiac arrest, quench

This co ld be o !


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This could be you!

References


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References

Shellock F.G. and Kanal E. (2001)

- Magnetic Resonance Bioeffects, Safety, and PatientManagement

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/

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Safety Aspects of Magnetic Resonance Imaging455

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