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Archived Issues of Radiology Rounds
MGH Department of Radiology Website
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MRI Safety
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| Note: Updated information available (June 2007) |
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- MRI is generally very safe and adverse reactions to contrast agents are extremely rare
- Recent reports suggest an association between gadolinium contrast administration and
nephrogenic systemic fibrosis in patient with moderate to severe renal insufficiency
- Cardiac pacemakers, implanted cardiac defibrillators, otic/inner ear/cochlear implants, and metal
framents in the eye contraindicate MRI
- Loose ferromagnetic objects can become dangerous missiles in an MRI room
- In order to ensure patient safety, all implants that contain metal must be verified as safe before
an MRI procedure can be performed
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MRI
is generally regarded as a very safe imaging modality since it does not
use ionizing radiation and because toxic reactions to MRI contrast
agents are very rare. However, there are recent reports of an
association between the use of gadolinium contrast agents in patients
with moderated to severe renal insufficency and the development of a
newly discovered disease, nephrogenic systemic fibrosis, a debilitating
or even fatal disease that affects the skin and other organs. Further
details on this condition may be found on the PCOI Website,
http://oi/pcoi/frontpage_frames.asp.
There are no data that shows that the high
magnetic fields or the radiofrequency pulses used cause any adverse
effects to the human body. However, the powerful magnets (10,000 –
30,000 times that of the earth’s magnetic field) used in clinical MRI
scanners can be hazardous because of the behavior of metals in a
magnetic field. Ferromagnetic metals can become missiles that fly
towards the magnet with high speed, which can cause serious injury or
even death. Other metals develop an electrical charge or current when
moved in a magnetic field with accompanying heat that can burn.
No loose metallic objects of any kind are allowed in the MRI room at
any time because the MRI magnet is always left on to maintain the
stability required for high quality images. Furthermore, the magnetic
field is not confined within the bore of the magnet, but surrounds the
magnet as well. The magnetic field is, however, confined within the MRI
room by specially constructed shielding walls.
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Access
to the MRI environment is strictly controlled. In the Emergency
Department, a red light comes on whenever someone enters the area.
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For
safety, no one entering the MRI room may carry anything metallic,
including beeper, cell phone, and credit cards etc. (which have a
magnetic strip that will be erased). In addition, everyone must also
remove hearing aids, eyeglasses, dentures, hairpins, barrettes,
jewelry, watch, and steel-toed boots/shoes. Note that these rules apply
to a parent or family member who wishes to accompany a patient into the
MRI room. Patients with any type of body piercing, regardless of
specific location, must remove the jewelry in order to avoid skin
burns. At this time, the only permitted metallic object allowed in the
MRI room is a wedding band although this rule may change for patients
who have an MRI in the new 3 Tesla scanner.
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Surgical Implants and Prostheses
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Many
patients have metal-containing implants, some of which are safe in the
MRI environment while others are not. Therefore, any metal-containing
foreign body is a concern. In order to avoid accidental injury to
patients, MGH has three-step safety check system that is designed to
identify the individuals who have metallic implants or foreign bodies
and prevent them from entering the MRI room until it has been
established that it is safe to do so. The MRI requisition form has
check boxes for special considerations that may contraindicate MRI, the
patient will be asked to complete an extensive questionnaire before
entering the MRI room, and the technologists will also verify that
there are no contraindications.
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Common “MRI Alert” Items Found via Patient Questionnaire
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Intravascular stent
Wire sutures or surgical staples
Artificial joint replacements
Body piercing jewelry
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MRI
is strictly contraindicated for patients with pacemakers or otic (inner
ear or cochlear) implants because these devices contain permanent
magnets that can malfunction or cause injury when exposed to the high
magnetic fields in an MRI room. Permanent magnets are also used to hold
some prostheses, such as some orbital prostheses and dental implants,
in place. These externally applied prostheses must be removed prior to
the MRI procedure in order to prevent injury to the patient.
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Another
absolute contraindication for MRI is metal that has lodged in the eye
or orbit. Therefore, patients are asked if they have worked in a
machine shop or have had metal removed from an eye. If medical
attention was needed to remove the metal fragment an x-ray examination
of the orbits/eyes will be obtained to determine if there is still any
residual metal fragments.
The
risk of injury from other medical implants depends on the tissue into
which it is implanted, the type of metal, and the time since it was
placed. For example, aneurysm clips or heart valves can cause injury if
they are ferromagnetic. However, most devices of this type implanted
after 1996 are made of MRI compatible material. Nevertheless, to ensure
safety, a radiologist or radiology technologist must know the make,
model, and serial number of the implant, which is normally given to the
patient at the time of implantation. This information is essential in
order to verify the safety of every single implant before a patient,
visitor, or employee enters the MRI room. In some cases, implants have
been verified as safe for a 1.5T but not for a 3T MRI scanner.
Other
kinds of implants such as joint replacements or sternotomy wires do not
pose any danger once granulation tissue has developed around them
although they will cause localized distortion of MR images. In addition
to implants, extensive tattoos and some kinds of make-up contain enough
metal particles to have the potential to cause heat injury although
this is rarely a problem.
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Relative Contraindications for MRI
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| Absolute Contraindication |
Pacemaker
Otic implant
Metal in eye or orbit
Implanted cardiac defibrilator
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| Likely Contraindication |
Heart valve or aneurysm clip installed before 1996
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| Possible Contraindication |
Heart valve or aneurysm clip installed after 1996
Any type of prosthesis
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| Usually Allowable 6-8 weeks after implantation |
Passive implants, weakly ferromagnetic (e.g. coils, filters, and stents; metal sutures or staples)
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| Usually Allowable immediately after implantation |
Passive
implants, non ferromagnetic (e.g. bone/joint pins, screws, or rods;
coils, filters, and stents; metal sutures or staples)
Rigidly fixed passive implants, weakly ferromagnetic (e.g. bone/joint pins, screws, or rods)
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| Caution |
Tattoos |
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Pregnancy and Breast Feeding
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The Patient Experience
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There
are no known risks for undergoing MRI scanning during pregnancy.
However, late effects on the fetus may be unrealized since MR scanning
has been widely available for only approximately 15 years. All pregnant
patients will be asked to sign a consent form. Although gadolinium
based contrast agents have not been found to cause any harm to the
fetus, these have not been FDA approved for use in pregnancy and are
not used as a general precaution. All pregnant patients should consult
with an obstetrician prior to undergoing an MRI examination. If
gadolinium contrast agent is used in a mother who is breast feeding her
baby, she will be advised to abstain from breast-feeding for 24 hours
because gadolinium passes into the milk and to express milk and discard
it. In anticipation of the study, the mother may wish to obtain
enough milk to feed the infant during the 24-hour period following the
exam.
Sedation and Anesthesia
Some
patients, such as pediatric patients or those with dementia or extreme
claustrophobia, may be unable to remain still for an MRI examination.
In these cases, sedation may be required and, with prior arrangements,
can be administered. For conscious sedation, a specialized nurse will
be present to monitor the patient. If general anesthesia is necessary,
an anesthesiologist will be present throughout the procedure to monitor
the patient. In both instances, the patients will go to a recovery room
after the MRI procedure.
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Before
an MRI, patients may continue to take medications as normal. Usually
there are no any restrictions on food and drink. Before entering the
MRI room, all patients must be wearing a hospital gown that has ties,
not snaps, and remove anything metallic, including jewelry, dentures,
hearing aids, and glasses. Once in the MRI room, the patient will be
asked to lie down on the patient table and, if a contrast agent is to
be used, a technologist will insert an IV line in the patient’s arm.
All patients are given earplugs or head phones to muffle the loud
knocking noises made by the MR scanner during an examination.
Immediately before the scan, patients are slid into position into the
MRI tube. Although the tube is narrow, patients are able to see out by
looking in a mirror, which reduces claustrophobic feelings. The
technologist can observe the patient at all times during the procedure
and will talk to the patient after each set of MR images.
The
total scanning time is usually about 45 minutes, broken into several
short segments. During this time, the patient must lie still. The
technologist will let the patient know how long each segment will last
and, if necessary, ask the patient to hold her breath for up to 30
seconds. During the scan, the patient should not experience any
sensation except for the sound of the scanner and the motion of the
bed. As a precaution, all patients are given a squeeze ball alarm and
instructed to alert the technologist if they experience any abnormal
sensations.
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Scheduling
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Further Information
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MRI
is performed at Mass General West Imaging in Waltham, Mass General
Imaging in Chelsea, and the Charlestown Navy Yard, as well as in the
Yawkey Center and the Ellison building on the main MGH campus. If
conscious sedation or anesthesia is required, the MRI must be performed
on the main campus. Appointments at all locations can be scheduled by
calling 4-XRAY (617-724-9729) or through the web-based Radiology Order
Entry system, http://mghroe
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For further questions, please call Sherry Piskadlo
, MRI Operations Manager, at 617-724-4220 and from the web at http://www.mrisafety.com
A brochure, entitled A Patient Guide to an MRI scan, is available at http://www.massgeneralimaging.org and copies of the printed version may be obtained by contacting Kristen Dean at 617-724-4902. In addition, information on preparing your child for MRI, including a recording of the sound of an MRI scanner, is available online at http://www.mghpedirad.org and a booklet My MRI, written by Emily Sumner when she was 7 years old, can be found online at http://www.massgeneral.org.
This article provided useful information about the appropriate use of imaging studies:
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References
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Kirchin, MA and Runge, VM. (2003) Contrast agents for magnetic resonance imaging: safety update. Top Magn Reson Imaging 14: 35
Shellock, FG and Crues, JV. (2004) MR procedures: biologic effects, safety, and patient care. Radiology 232: 52
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