Transcript    Safety Background  The MRI scanner is a very large and powerful magnet  Most clinical scanners are 1.5 - 3

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 Safety Background
 The MRI scanner is a very large and powerful magnet
 Most clinical scanners are 1.5 - 3 Tesla scanners
 3 Tesla = 30,000 gauss
 Earths magnetic field ~ 0.5 gauss
Magnetic Resonance Imaging (MRI) is one of the most powerful medical
diagnostic technologies, combining strong magnetic fields with powerful
computer image processing.
The core of an MRI machine is a very high-strength magnet arranged to surround most of the
patient's body. This magnet is usually wrapped with coils of copper wire, and when electricity
flows through the wire a powerful magnetic field is produced.
Significance
• In normal MRI machines, the magnets are superconducting---
their wires have zero resistance to the flow of electric current.
The only way to achieve superconductivity is to make the
magnets ultra-cold by submerging them in liquid helium: at
about -453 °F, the magnet assembly is barely 5°F above
absolute zero. Often, this liquid helium is surrounded by an
insulating blanket of liquid nitrogen.
Because superconducting wires continually conduct and
recalculated the electric current without resistance, the
magnetic field is present 24 hours a day; whether the MRI
machine is being used or not.
Considerations
• Safe operation of an MRI machine requires managing
three main categories of potential safety issues, all
addressed by MRI safety training programs:
1. Medical/biological effects of high-strength magnets
2. Area/vicinity effects of high-strength magnets
3. Cryogenic (ultra-cold) gases
Biological magnetic issues
• There's no evidence indicating biological side effects from
the magnetic fields of an MRI scan, and MRI machines
don't utilize any ionizing radiation that can damage cells
or DNA.
Pregnant patients and technicians are generally advised to
avoid MRI during the early months of pregnancy, due to a
lack of research on potential magnetic effects on the
developing fetus. MRI operators are trained on pregnancy
safety guidelines.
Magnetic object safety
• The main potential danger from MRI machines comes from the
interaction of the magnetic field with metallic objects or particles.
Any metal inside the patient (such as medical devices or shrapnel)
can cause severe injuries.
Metallic objects in the vicinity of an MRI room can be attracted by
the "fringe" magnetic field---causing them to be sucked into the MRI
machine at high speed, posing a severe danger to patients. MRI
training videos demonstrate this threat.
MRI safety training outlines the methods of screening for potentially
hazardous metal in a patient's body, and for preventing the entry of
any ferromagnetic objects into the MRI room.
Potential Projectiles:
Examples
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Cell phone
Keys
Glasses
Hair pins / barrettes
Jewelry
Safety pins
Paper clips
Coins
Pens
Pocket knife
Nail clippers
Steel-toed boots / shoes
Tools
Clipboards
No loose metallic objects should be taken into the Scan room!
Remember, the magnet is ALWAYS on!
Even when the MRI Scanner is not in use,
the magnet is on. Ferromagnetic objects
should NEVER be taken into the Scan
Room.
Cryogenic safety
• MRI operators are trained in the safe management of cryogenic
equipment and gases. In the event of a rapid boil-off of the
liquid nitrogen or liquid helium surrounding the magnet (a
"quench"), a failure of the emergency venting system can cause
a life-threatening oxygen shortage in the MRI room.
MRI safety training includes preparation for unplanned and
emergency quench conditions with vent malfunction.
Additionally, operators are trained in emergency shutdown
procedures short of a damaging quench---which is typically
only initiated if a patient's life is threatened by a foreign
metallic object.
Hearing Protection in the MR
 A characteristic of the switching gradient fields is the
production of acoustic noise. When the alternating lowfrequency currents flow through the gradient coils, which are
immersed in the high static magnetic field , forces are exerted
on the gradient coils that move like a loudspeaker coil and
generate sound waves. The level of this acoustic noise at the
location of the subject or volunteer can reach an unacceptable
and even dangerous level.
 All personnel (MRI Technician, research group member,
volunteer subject, family member, etc.) are required to wear
hearing protection in the magnet room while the scanning
procedure is being performed.
Additional training issues
 Operators may also be trained in responding to medical
emergencies during an MRI scan; fire and evacuation
procedures; and hearing protection and communication
during the often-noisy scanning process.
Safety Issue of
Static Magnetic Field
 All non-MR personnel & study subjects entering
 the MRI scanner room must be screened.
 ACR recommends that non-emergent
 patients should be screened by two separate
 individuals.
 Consider using plain-film radiographic study
 to confirm the absence of metal fragments in
 critical parts of the body.
Safety Issue of
Static Magnetic Field Zones
Zone 1: Reception area
Zone 2: Screening interview area
Zone 3: Control area, access
restricted with key locks, passkey
systems etc.
Zone 4: MR scanner room
MR personnel must have safety
training,
non-MR personnel cannot have
independent
access to Zone 3 or 4.
Biological Effect of
Radiofrequency Field (1)
Radiofrequency energy deposited in the
body during an MR examination will be
converted into heat.
‘Specific Absorption Rate’ (SAR) is
defined
as the average energy dissipated in the
body per unit mass and time.
This heat gain is countered by heat loss
through sweat glands and cutaneous blood
vessels.
Thermal Safety of Subject
• Resonant circuitry can result in heating of the
tips of wires or leads to temperature in excess of
90 deg C within a few seconds
• All unnecessary electrically conductive
materials should be removed before imaging.
• All attached leads should be covered with
cold compress or ice pack.
• Avoid any large conductive loops, including
limbs(do not cross arms or legs in the MR, this
forms a loop as well).
• Care should be taken to place thermal
insulation between the subject and electrically
conductive material.
Onlyitems marked “MR Safe” are permitted
in the magnet room. If there is a question
concerning the MR safety aspect of any
item—do not allow it to be brought into the
magnet room!
Magnet Room Acceptability
Labels
MR SAFE
an item which poses no known hazards in all MR
environments
MR UNSAFE
an item which is known to pose hazards in all MR
environments.
MHRA DB2007(03) December 2007
MR CONDITIONAL
An item which has been demonstrated to pose no
known hazards in a specified MR environment with
specified conditions of use. Field conditions that define
the specified MR environment include field strength,
spatial gradient, dB/dt (time rate of change of the
magnetic field), radio frequency (RF) fields, and specific
absorption rate (SAR). Additional conditions, including
specific configurations of the item, may be required.
The End