HOW I DO CMR SCANNING SAFELY

clappergappawpawΠολεοδομικά Έργα

16 Νοε 2013 (πριν από 3 χρόνια και 9 μήνες)

57 εμφανίσεις

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

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


Safety issues centre around the 3 electromagnetic fields
and issues indirectly related


Static magnetic field

1.5 T is 30,000 x the strength of earth’s magnetic field

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

Static Magnetic Fields

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


A superconducting magnet is always switched on!

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


1
st

trimester



prudent to avoid CMR where possible


2
nd

and 3
rd

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




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

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)

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

Never be complacent


accidents do happen!

Oxygen cylinder

Infusion pump

Floor buffer


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 implants



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


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




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

**
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

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)

BIOMEDICAL IMPLANTS


OTHER






orthopaedic implants



contraceptive devices



dental implants





Effects of implants on imaging
-

signal void / local distortion

Safety Checklist


comprehensive 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

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

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

Hearing protection mandatory
above 90dB

time averaged for:




patients



staff remaining in the scanner room



relatives accompanying children or patients

Gradient fields


Auditory damage

Radiofrequency (RF) fields

Thermogenic effects
-

health & safety concern


Physiological tissue heating response





most of the transmitted RF power is transferred into heat


within the patient’s tissue





all MR systems have safety thresholds to avoid


dangerous levels





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)





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



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




Contrast agents





Critically ill patients / resuscitation




Monitoring and equipment related hazards





Quench of superconducting magnets





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


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

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


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


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

Equipment must be MR Safe or MR Conditional

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 O
2

cylinder, piece of equipment
)





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


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


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/