Level 1 MR Safety - E. Jackson
CRCPD Meeting – Level 1 MR Safety, 2013
Safety Aspects and Challenges of Magnetic
Resonance in Radioactive Materials
Environments
Edward F. Jackson, PhD
Department of Imaging Physics
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Introduction
• Over the past three decades, MRI has clearly been demonstrated to be
one of the most powerful imaging modalities and, if all appropriate
safeguards are in place, one of the safest.
• However, there are very real dangers associated with diagnostic MRI
facilities.
• The objectives of this presentations are to
– provide a background on safety issues associated with each of the magnetic
fields utilized in MRI,
– provide an introduction to other safety issues, e.g., MR compatible/safe devices,
safety screening procedures, cryogen gas issues, MR contrast agents,
– discuss MR facility safety systems.
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Level 1 MR Safety - E. Jackson
Before we delve into MR
safety….
….an intro to how MRI works
Introduction or MR101
Basic ingredients to form an MR image:
– Non-zero magnetic moment nuclei
Nuclear
– Static magnetic field, Bo
Magnetic
– Radiofrequency field, B1
Resonance
– Magnetic field gradients, Gx,y,z
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Level 1 MR Safety - E. Jackson
MR Scanner Block Diagram
X-Gradient
Amplifier
Y-Gradient
Amplifier
Z-Gradient
Waveform
Generators
Amplifier
RF
Amplifier
Transcoupler
Shim
Control
ADCs
Host Computer
Image
Processor
Console
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Nuclear Magnetic Resonance
• Only nuclei with non-zero magnetic moments can be
imaged.
• A non-zero magnetic moment occurs for odd numbers of
protons and/or neutrons.
• Most commonly, the nuclei have spin quantum numbers of
1/2, e.g., 1H, or 3/2, e.g., 23Na.
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Level 1 MR Safety - E. Jackson
Nuclear Magnetic Resonance
Properties of some common nuclei with potential use in MR studies
Nucleus
1
H
C
19
F
23
Na
31
P
13
Spin
1/2
1/2
1/2
3/2
1/2
Natural
Isotopic
Sensitivity
Gyromagnetic Abundance Relative to 1H
(%)*
ratio (MHz/T)
(%)
42.57
99.98
100
10.71
1.11
1.6
40.06
100
83.4
11.26
100
9.3
17.24
100
6.6
*At constant field for equal number of nuclei.
Note: The steady state magnetization depends on the square of , the gyromagnetic ratio
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Nuclear Magnetic Resonance
• Without an external magnetic field, the magnetic moments in a
sample, or patient, are randomly distributed, i.e., there is no net
magnetization.
• In an externally applied field, protons, which have a spin quantum
number of 1/2, have two allowed states: parallel or antiparallel to the
applied magnetic field.
|1/2 -1/2>
|1/2 -1/2>
|1/2 1/2>
B= 0
Zeeman Splitting
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4
B = Bo
|1/2 1/2>
Level 1 MR Safety - E. Jackson
Nuclear Magnetic Resonance
Bo
N0-
Mo
N0+
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Small net excess of proton spins aligned
with B0 yields net magnetization, M0.
Nuclear Magnetic Resonance
• To generate time-dependent transverse magnetization that can be
detected, transitions between the two allowed energy states (for a
spin 1/2 nucleus) must be induced.

• The transitions result from the application
of a time-varying
magnetic field with energy equal to the difference in the energy
levels, i.e.,
ΔE =   =  Bo
where  is Planck’s constant,  is the gyromagnetic ratio, and Bo is
the static magnetic field strength.
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Level 1 MR Safety - E. Jackson
Nuclear Magnetic Resonance
• The Larmor frequency is given by:
  Bo
• For protons in a 1.5 T field, the Larmor frequency is ~64 MHz.
• The radiofrequency field is commonly known as the B1 field and,
in addition to being applied at the Larmor frequency, must be
applied perpendicular to the static field in order to induce the
desired transitions => there is both a resonant frequency and a B1
field polarization requirement.
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Nuclear Magnetic Resonance
z
Mo
z
90o Pulsex
Mxy
y
x
y
x
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Level 1 MR Safety - E. Jackson
Free Induction Decay
Relative Signal Intensity
1
0.5
0
0
100
200
300
400
500
-0.5
-1
V (t )  0 M 0  0
Time (ms)
N  0
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Spatial Orientation
– The MR signals must be
encoded in 3 dimensions
– By convention:
• z - slice selection
• x - frequency-encoded
• y - phase-encoded
y
x
z
– Note: The slice, frequency
and phase encoding may be
on any axis
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Image compliments of Carl Keener, PhD
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Level 1 MR Safety - E. Jackson
Slice Selection
– Gradient along z-axis modifies B0 to produce a range of Larmor
frequencies which vary with position.
– Max amplitudes ~10 - 50 mT/m
 0    B0
 z    ( B0  zGz )
Gz
-z
z
0
Image compliments of Carl Keener, PhD
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Slice Thickness
• Slice thickness is determined by
– Gradient strength
 Gz   z
– Bandwidth of RF pulse (B1)
    z
    ( B0  zGz ) 
z 
z2

  Gz
G2
G1

z1
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Level 1 MR Safety - E. Jackson
Slice Position
Slice position is determined by the transmit “center frequency”.
f0 - 1000Hz
f0
f0 + 1000Hz
 - 1000Hz
 0
 + 1000Hz
Gz
z
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Image compliments of Carl Keener, PhD
Frequency Encoding
To encode the spatial information in the x-axis, another gradient (Gx)
is used:
-x
0
x
x =  (Bo+ x Gx) <== one-to-one correspondence between frequency
and position
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Image compliments of Carl Keener, PhD
Level 1 MR Safety - E. Jackson
Frequency Encoding
When the gradient is on:
– frequencies vary along x-axis
The signal is sampled while the
x-gradient is on
– frequency of signal encodes
position along x-axis
-x
0
x
Image compliments of Carl Keener, PhD
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Phase Encoding
To encode the spatial information in the y-axis, a 3rd gradient
(Gy) is used
+ y
0
- y
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Image compliments of Carl Keener, PhD
Level 1 MR Safety - E. Jackson
Spin Echo Imaging Sequence
TE
900
TR
1800
900
1800
RF
...
...
...
...
...
Gslice
Gphase
Gfreq
Signal
k-space
2nd echo
1st echo
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Reconstructing the Image
image has been phaseencoded in one direction
gradient applied before
frequency-encoding &
sampling
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image has been frequencyencoded in other dimension
gradient applied during
sampling
Image compliments of Carl Keener, PhD
Level 1 MR Safety - E. Jackson
Image Formation
|2D FFT|
Re[s(t,n)]
S(,)
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Now….
….back to MR safety
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Im[s(t,n)]
Level 1 MR Safety - E. Jackson
Levels of MR Safety Training
• Safety training divided into two levels:
– Level 1
•
•
•
•
maintain one’s own safety in a limited set of conditions
limited experience / responsibility in the MR environment
REQUIRED minimum level of training in order to work in MR environment
Examples: nurses, (non-MR) technologists, state inspectors, non-MR
facilities staff who must enter the MR scan rooms, etc.
– Level 2
•
•
•
•
highly trained and experienced personnel
work constantly in the MR environment
level of training qualifies staff to train/oversee others
Examples: MR technologist, MR radiologist, MR physicist, etc.
• This presentation for Level 1 training
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Kanal, Barkovich, et al., JMRI 37:501-530, 2013
Levels of MR Safety Training
It’s important to remember that:
– Level 1 MR Personnel can work in the Zone IV
(MR scan room) environment, but only a Level
2 MR Personnel can approve the entry of other
individuals or devices into Zone IV.
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Level 1 MR Safety - E. Jackson
Introduction
Each of the magnetic fields used in MR imaging can be a source of
safety concerns:
– Static B0 field (always on!)
• Physiological effects, projectile motion, medical device
displacement and/or interference with normal operation
– Radiofrequency B1 field (on during scanning)
• Tissue heating, heating of conductors, interference with
patient monitoring equipment
– Gradient fields (on during scanning)
• Peripheral nerve stimulation, excessive sound pressure
levels, interference with patient monitoring equipment
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Introduction
Other safety issues in MR imaging include:
– contrast agents
– pregnant patients, technologists, and nursing staff
– effects of MR scanners on patient monitoring equipment
– cryogen gases
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Level 1 MR Safety - E. Jackson
MR Site Safety Issues / Access Control
• ACR Guidance Document on MR Safe Practices: 2013
– Available at www.acr.org
– Kanal, Barkovich, et al., J Magn Reson Imaging 37:501-530, 2013
– Recommends facility design with respect to the four zones, with
increasing security / access control requirements.
– Updated version of original white paper (2002), supplement (2004), and
revision (2007).
• Another good source of information on practical clinical MR safety is
F.G. Shellock and J.V. Crues, MR Procedures: Biologic Effects, Safety,
and Patient Care, Radiology 232:635-652, 2004.
• Other MR safety references are provided at the end of this presentation.
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2013
MR Site Safety Issues / Access Control
ACR White Paper on Safety Zone Concept
• Zone I
Zone I
2007
– Open access
• Zone II
– Preparation and holding
• Zone III
Zone II
Zone II
– Carefully controlled by MR facility
personnel. May be partially within
5 G exclusion zone.
• Zone IV
Zone III
– Actual scan room. No admittance
w/o documented training and
screening.
Zone IV
•
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Access to any space contained in the
5 G (0.5 mT) fringe field should be
carefully controlled and that space must
be posted.
Level 1 MR Safety - E. Jackson
An MD Anderson 3.0T MR Facility
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An MD Anderson 3.0T MR Facility
Note that the 5G fringe
field (“exclusion zone”) is
completely contained in the
MR scan room (Zone IV).
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Level 1 MR Safety - E. Jackson
Static Field Safety Issues
The primary safety concern from the Bo field is prevention of injury
from ferrous objects becoming projectiles.
– Examples of objects that have found their way into the bores of MR
scanner magnets: hairpins, stethoscopes, forceps, oxygen cylinders,
vacuum cleaners, electrical filter panels, floor buffers, a fork lift tine.
– Facilities must carefully limit scan room entry to authorized personnel who
clearly understand the dangers associated with such powerful magnets.
Preferred siting is a single access door within clear view of the MR
technologist.
– Every individual (patients, employees, visitors) entering the scan room
must first be screened.
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Static Field Safety Issues
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Level 1 MR Safety - E. Jackson
Static Field Safety Issues
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Static Field Safety Issues
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Level 1 MR Safety - E. Jackson
Static Field Safety Issues
MR Scanner Fields – How Strong?
• Magnetic Field Units:
–
–
–
–
–
–
–
Gauss (G) and Tesla (T)
1 T = 10,000 G
Earth’s magnetic field is ~0.5 G (0.05 mT)
Posted exclusion zone (pacemakers/neurostimulators): 5 G (0.5 mT)
1.5T MR scanner
~30,000 times Earth’s field
3.0T MR scanner
~90,000 times Earth’s field
~3,000 – 9,000 times the 5 G exclusion zone field strength
• Force of attraction at 1.5T:
– Near bore opening: can easily be ≥25-50x the weight of the object
– Translational force, Fz, is proportional to B0  dB0/dz
– Torque = –m H sin , where m is the magnetic moment, H is the magnetic field
density, and  is the angle between the magnetic moment and the field
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Level 1 MR Safety - E. Jackson
MR Fringe Fields – 1.5T Espree
Field Strength vs Distance (1.5T Espree)
1600
5G
Radial
5
1200
Field Strength (mT)
Field Strength (mT)
1400
Field Strength vs Distance (1.5T Espree)
6
5G
Axial
1000
800
600
400
4
3
2
200
0
y = 258.97x-4.3457
R 2 = 0.999
y = 65.63x-4.9078
R 2 = 0.9931
1
0
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
0.0
1.0
2.0
3.0
Distance (m)
Axial
4.0
5.0
6.0
7.0
Distance (m)
Axial
Radial
Radial
Data extracted from shielded configuration
Drawing 400-0000005597-2005-MR02-FBH-01 OR122
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Static Field Safety Issues
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8.0
Level 1 MR Safety - E. Jackson
REMEMBER!
The magnet is
NEVER off!
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Static Field Safety Issues
A second concern is the movement/displacement of certain implanted
medical devices and/or metal fragments (screening is essential!):
– 5 G exclusion zone must be posted for persons with pacemakers
and neurostimulators.
– Pacemakers, neurostimulators, cochlear implants, and aneurysm
clips are exclusion criteria for MR scanning in many MR centers.
– Some ferrous temporary or permanent medical devices are
exclusion criteria for patient scans.
– Some body piercings are ferromagnetic (and even if they are not
they can cause significant artifacts and, potentially, local heating!)
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Level 1 MR Safety - E. Jackson
Static Field Safety Issues
MR safety of implanted medical devices and other metal objects:
– Extensive lists summarizing the safety of a wide range of medical
devices and metal objects (bullets, etc.) are available in the literature
and, particularly, in F.G. Shellock and E. Kanal, Magnetic
Resonance. Bioeffects, Safety, and Patient Management, LippincottRaven Publishers, 1996. This inexpensive paperback book (and its
successors) are highly recommended for anyone involved with the
support of a MR imaging facility.
– Information can also be obtained from the FDA and at
www.mrisafety.com (maintained by Shellock et al.)
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Safety Reference
Shellock & Kanal, pp.179-213
Magnetic Resonance. Bioeffects,
Safety, and Patient Management.
2nd Edition, Lippincott-Raven, 1996
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Level 1 MR Safety - E. Jackson
MDACC Employee Screening Form
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Static Field Safety Issues
• Note that even non-ferrous metal materials, e.g., aluminum, are
affected by such strong magnetic fields when they are moved
through the field.
• Due to magnetic field forces secondary to currents induced in the
metal due to its motion in the static magnetic field (Lenz’s Law).
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Level 1 MR Safety - E. Jackson
Labeling of items that may go in Zone IV
MR Safe
NOTE:
Items in Zone III (control room)
that may be taken into Zone IV
(procedure room) must be
appropriately labeled in order to
minimize the potential for an MR
accident.
MR Conditional
Not MR Safe
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MR Compatibility
• Any tool or device that is brought into Zone IV
(the MR scan room) must be known/labeled to be
MR safe or must be tested by Level 2 MR
Personnel before it can be taken into the MR
scan room.
• If there is ANY doubt about device compatibility,
or anything dealing with MR safety or the MR
environment, ASK!
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Level 1 MR Safety - E. Jackson
Static Field Safety Issues
NEVER assume nearby unlabeled objects are MR safe!!!
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A Brave New World…
Brave new world  potential dangers
Interventional MR Systems
MR–PET Systems
1.5T Supercon Magnet
3.0T Supercon Magnet
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Level 1 MR Safety - E. Jackson
A Brave New World…
Brave new world  potential dangers
MR Systems in Radiation Therapy
Univ Alberta
0.6 T / 6 MV Linac
Viewray
0.35T / 3 x 60Co
Philips / Elekta
1.5 T / Linac
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All Supercon Magnets
A Brave New World…
Appropriate Caution Necessary
• Each of these new types of facilities is an impressive technological
advancement that promises to improve patient care and quality of
life.
• Each of these new types of facilities, however, brings individuals
without prior MR safety training and experience into the MR
environment, e.g., state inspectors, cleaning staff, non-MR
radiologists, technologists, and physicists, etc.
• As such, MR safety training and screening of these non-MR
personnel is absolutely essential to protect these individuals, and
others in the MR suite with them, from potential harm or even
death.
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Level 1 MR Safety - E. Jackson
MR Safety
Beyond being screened, what does this mean to non-MR
workers who need to work (make measurements, inspect
equipment, etc.) in the MR scan room (Zone IV)?
–
Different equipment might be needed (plastic levels, cloth
measuring tape, etc.).
–
Don’t take ANYTHING into the scan room without first
having a Level 2 MR Personnel check it!
–
Stop at the door to the MR scanner room. Check all pockets
and take NOTHING in the room that isn’t absolutely necessary
and, if necessary, checked first.
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MR Safety
If “something bad” happens, what do I do?
–
Unless the situation is life threatening, i.e., someone is pinned
against the magnet, do NOTHING but report the incident.
–
Do NOT try to remove a ferromagnetic object that has
inadvertently been attracted to the magnet. Danger to you
and/or others, and the equipment, might result.
–
If the situation is, in fact, life threatening, a Level II MR
Personnel, who should be in the facility monitoring your
activities anyway, should activate emergency procedures.
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Level 1 MR Safety - E. Jackson
Static Field Safety Issues
Physiological concerns:
– There have been no documented permanent deleterious effects
resulting from MR scanning.
– Temporary effects typical all arise from the induced voltages in
tissues due to the motion of charged substances through the
strong magnetic field (v  dB/dt; typically for B > 3T):
• Magnetophosphenes
- “flashes of light”
• Vestibular function
- “feeling of vertigo”
• Taste perversions
- “metallic taste”
• Altered ECG waveforms - elevated T-wave (even at 1.5T)
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Static Field Safety Issues
With regard to any permanent deleterious physiological effects from
the static field, Shellock and Kanal1 report:
“…static magnetic fields up to 2 T produce no substantial harmful
bioeffects, including no alterations of cell growth and morphology,
DNA structure and gene expression, pre- and postnatal reproduction
and development, visual functions, nerve bioelectric activity, animal
behavior, visual response to photic stimulation, cardiovascular
dynamics, hematologic indices, physiologic regulation and circadian
rhythms, or immune responsiveness.”
Similar findings have been reported in higher field strengths (≥4T).
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Level 1 MR Safety - E. Jackson
Static Field Safety Issues
FDA Guidelines (7/2003):
MR systems with main static magnetic fields greater than 8
Tesla (4 Tesla for infants <1 mo) are considered significant risk
devices.
http://www.fda.gov/downloads/MedicalDevices/DeviceRegulationandGuidance/GuidanceDocuments/ucm072688.pdf
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Radiofrequency Field Safety Issues
• At 1.5 T, the Larmor frequency is ~ 64 MHz => good penetration and
possible source of tissue heating.
• Tissue heating is primarily due to magnetic induction with a negligible
electric field contribution.
• Measure: the specific absorption rate (SAR) - W/kg.
• The heating of the tissue is greatest at the periphery and minimal at the
center of the body.
Head equivalent phantom scans demonstrate significant changes in
temperature during an MR only occur less than 4 cm from the edge and
do not exceed 1-2oC for 1.0 and 2.5 W/kg scans for 30 minutes1.
• Tissues that are poorly perfused, such as the orbits, require particular
attention.
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Level 1 MR Safety - E. Jackson
Radiofrequency Field Safety Issues
F.G. Shellock and E. Kanal, Magnetic Resonance. Bioeffects, Safety, and Patient Management,
Lippincott-Raven Publishers, 1996
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Radiofrequency Field Safety Issues
The RF power required in MRI studies scales as the:
– square of the static magnetic field for a given flip angle
– square of the flip angle at a given static magnetic field
– size of the patient
– duty cycle of the RF pulses (# pulses per unit time)
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Level 1 MR Safety - E. Jackson
Radiofrequency Field Safety Issues
• Considerable care must be taken to insure that no unnecessary
conductors (including jewelry or other removable metal objects) are in
the magnet bore during scanning.
• All necessary conductors, e.g., surface coil leads and ECG leads,
should be padded away from the patient, should not be allowed to
loop, and should, to the extent possible, travel down the center of the
magnet bore.
• First, second, and even third degree burns due to poorly placed ECG
leads or other non-MR compatible conductive devices have been
reported.
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Radiofrequency Field Safety Issues
FDA Guidelines (7/2003):
MR systems performing studies where the specific absorption rates
are greater than
a. 4 W/kg averaged over the whole body for any period of 15 min; or
b. 3 W/kg averaged over the head for any period of 10 min; or
c. 8 W/kg in any gram of tissue in the head or torso, or 12 W/kg in
any gram of tissue in the extremities, for any period of 5 min;
are considered significant risk devices.
http://www.fda.gov/downloads/MedicalDevices/DeviceRegulationandGuidance/GuidanceDocuments/ucm072688.pdf
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Level 1 MR Safety - E. Jackson
Gradient Field Safety Issues
Two concerns arising from the time-varying gradient magnetic fields:
– Induced voltages from the time-varying magnetic fields can
produce nerve stimulation, and can distort waveforms on patient
monitoring equipment.
– Auditory sound pressure levels produced by the rapidly switched
gradient coils (due to the interaction of the gradient and static
field coils) can be excessive. These levels can be up to 100 dBA
at isocenter during fast scan techniques1. Hearing protection
should be used by patients (and others near the magnet bore)
during such scans.
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Gradient Field Safety Issues
FDA Guidelines (7/2003):
MR systems producing scans with time rates of change of
gradient fields (dB/dt) sufficient to produce severe discomfort or
painful nerve stimulation are considered significant risk devices.
Sequences producing peak unweighted sound pressure levels
greater than 140 dB or A-weighted rms sound pressure levels
greater than 99 dBA with hearing protection in place require an
IDE.
http://www.fda.gov/downloads/MedicalDevices/DeviceRegulationandGuidance/GuidanceDocuments/ucm072688.pdf
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Level 1 MR Safety - E. Jackson
Emergency Procedures/Systems
Superconducting Magnet Field “Quench”
– Recall that superconducting magnets are ALWAYS “on”. The static magnetic
field is never off. Even if there is absolutely no noise being generated by the
scanner, always assume the magnetic field is on.
– In a life threatening emergency situation, e.g., someone pinned against the
magnet or a ferrous object entering the bore along with the patient, the
superconducting magnet can be “quenched”.
– A quench is a sudden loss of superconductivity typically secondary to an
increase in temperature in one section of the superconducting wiring that
forms the main magnet.
– During a quench, the static field strength is quickly lost (~20 sec - 1 min).
– During the quench, virtually all of the liquid helium (4O K), which keeps the
magnet in a superconducting state, is converted to very, very cold helium gas.
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Emergency Procedures/Systems
Life safety systems
– Cryogen exhaust system – to provide a path for hundreds of liters of
liquid helium, once converted to gas in a quench situation, to exit the
facility. (~1000 m3 of gas; scan room is ~100 m3)
– Emergency room exhaust system in case cryogen exhaust system
fails.
– Patient intercoms, “panic” switches, etc.
– Emergency supercon magnet “rundown” systems => QUENCH.
– Emergency power off (EPO) systems.
– Patient monitoring equipment – must be MR compatible.
– Anesthesia systems (MR compatible equipment AND cylinders)
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Level 1 MR Safety - E. Jackson
Emergency Procedures/Systems
Emergency Systems
– “Table Stop” buttons
• Located on intercom (at operator console) and magnet housing.
• Used to quickly stop the motion of the scan table.
– Emergency room exhaust system (“Purge Fan”) in case the
cryogen exhaust system fails fully or partially
• Switches located near operator console and in the scan room (near
main door) for all clinical MR facilities except the CABIR facility.
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Emergency Procedures/Systems
Emergency Systems (cont.)
– Emergency power off (EPO) systems
• Switches located near operator console, in scan room, and in
equipment room.
• Used in case of scanner-related electrical problems (arcing, smoke,
etc.).
• Shuts down power to the MR system (including the computers).
– Emergency supercon magnet “rundown” systems
• Switches located near operator console and in scan room.
• Quenches superconducting magnet.
• Very expensive recovery – to be used only in life threatening
situations, e.g., individual pinned by ferrous object.
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Level 1 MR Safety - E. Jackson
CABIR 3.0T MR Facility
Cryogen exhaust stack
Emergency run
down (quench) unit
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MR and Pregnancy
Pregnant Healthcare Workers
– In 1990, Shellock and Kanal conducted a survey of all female MR
technologists and nurses at most clinical MR facilities in the U.S.1
– Five categories analyzed: spontaneous abortion rate, preterm
delivery (<39 weeks), low birth rate (<5.5 lbs), infertility (>11 mo to
conceive), and gender of offspring.
– Data indicated there were no statistically significant changes in the
five areas studied for MR workers relative to other workers.
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Level 1 MR Safety - E. Jackson
MR and Pregnancy
Pregnant Healthcare Workers (continued)
– Shellock/Kanal recommendation1: Pregnant healthcare workers
be permitted to continue performing MR procedures, to enter the
scan room, and attend to the patient. However, the worker should
avoid remaining in the scan room during actual operation of the
system. (Based on conservative position, not any demonstrated
adverse effects.)
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Concluding Remarks
• It takes constant vigilance on the part of every employee who works in
or near the MR facilities to make the facilities safe for our patients,
employees, and visitors.
• Before entering Zone IV of ANY MR facility, employ a “time-out” at
the door entering the MR scan room!
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Level 1 MR Safety - E. Jackson
REMEMBER!
The magnet is
NEVER off!
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References
1.
2.
3.
4.
5.
6.
F.G. Shellock and E. Kanal, Magnetic Resonance. Bioeffects, Safety, and Patient
Management, 2nd edition, Lippincott-Raven Publishers, New York, 1996.
http://www.mrisafety.com – “the list” of implants and devices
http://www.acr.org - white papers on MR safety (2002), update (2004), revision
(2007), and current (2013)
F.G. Shellock and J.V. Crues, MR Procedures: Biologic Effects, Safety, and
Patient Care, Radiology 232:635-652, 2004.
E. Kanal, ed., Practical MR Safety Considerations for Physicians, Physicists,
and Technologists, RSNA 2001 Syllabus
FDA website guidance document:
http://www.fda.gov/downloads/MedicalDevices/DeviceRegulationandGuidance/
GuidanceDocuments/ucm072688.pdf
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