Magnetic
Resonance Imaging (MRI) of the Heart
(MRI
Scan of the Heart, Cardiac MRI)
What
is magnetic resonance imaging (MRI) of the heart?
Magnetic
resonance imaging (MRI) is a diagnostic procedure
that uses a combination of a large magnet, radiofrequencies,
and a computer to produce detailed images of organs
and structures within the body.
How
does MRI work?
The
MRI machine is a large, cylindrical (tube-shaped)
machine that creates a strong magnetic field around
the patient. This magnetic field, along with a
radiofrequency, alters the hydrogen atoms'
natural alignment in the body. Computers are then
used to form two-dimensional (2D) images of the
heart's structure based on the activity of
the hydrogen atoms. Cross-sectional views can
be obtained to reveal further details. MRI does
not use radiation, as do x-rays or computed tomography
(CT scans).
A magnetic
field is created and pulses of radio waves are
sent from a scanner. The radio waves knock the
nuclei of the atoms in the body out of their normal
position. As the nuclei realign back into proper
position, they send out radio signals. These signals
are received by a computer that analyzes and converts
them into an image of the part of the body being
examined. This image appears on a viewing monitor.
Some MRI machines look like narrow tunnels, while
others are more open.
MRI
may be used instead of a CT scan in situations
where organs or soft tissue are being studied,
because with MRI scanning bones do not obscure
the images of organs and soft tissues, as does
CT scanning.
Other
related procedures that may be used to assess
the heart include resting or exercise electrocardiogram
(ECG), Holter monitor, signal-averaged ECG, cardiac
catheterization, chest x-ray, computed tomography
(CT scan) of the chest, electrophysiological studies,
myocardial perfusion scans, radionuclide angiography,
and ultrafast CT scan. Please see these procedures
for additional information.
MRI
of the heart may be performed for further evaluation
of signs or symptoms that may suggest:
- atherosclerosis -
a gradual clogging of the arteries over many
years by fatty materials and other substances
in the blood stream
- cardiomyopathy -
an enlargement of the heart due to thickening
or weakening of the heart muscle
- congenital heart disease
- defects in one or more heart structures
that occur during formation of the fetus, such
as a ventricular septal defect (hole in the
wall between the two lower chambers of the heart)
- congestive heart failure
- a condition in which the heart
muscle has become weakened to an extent that
blood cannot be pumped efficiently, causing
buildup (congestion) in the blood vessels, lungs,
feet, ankles, and other parts of the body
- aneurysm -
a dilation of a part of the heart muscle or
the aorta (the large artery that carries oxygenated
blood out of the heart to the rest of the body),
which may cause weakness of the tissue at the
site of the aneurysm
- valvular heart disease
- malfunction of one or more of
the heart valves that may cause an obstruction
of the blood flow within the heart
- cardiac tumor -
a tumor of the heart that may occur on the outside
surface of the heart, within one or more chambers
of the heart (intracavitary), or within the
muscle tissue of the heart
There
may be other reasons for your physician to recommend
an MRI of the heart.
Because
radiation is not used, there is no risk of exposure
to radiation during an MRI procedure.
Due
to the use of the strong magnet, MRI cannot be
performed on patients with implanted pacemakers,
intracranial aneurysm clips, cochlear implants,
certain prosthetic devices, implanted drug infusion
pumps, neurostimulators, bone-growth stimulators,
certain intrauterine contraceptive devices, or
any other type of iron-based metal implants. MRI
is also contraindicated in the presence of internal
metallic objects such as bullets or shrapnel,
as well as surgical clips, pins, plates, screws,
metal sutures, or wire mesh.
If you
are pregnant or suspect that you may be pregnant,
you should notify your physician. Due to the potential
for a harmful increase in the temperature of the
amniotic fluid, MRI is not advised for pregnant
patients.
MRI
generally is not advised for patients with epilepsy.
If contrast
dye is used, there is a risk for allergic reaction
to the dye. Patients who are allergic to or sensitive
to medications, contrast dye, iodine, or shellfish
should notify the radiologist or technologist.
MRI
contrast may have an effect on other conditions
such as allergies, asthma, anemia, hypotension
(low blood pressure), and sickle cell disease.
There
may be other risks depending upon your specific
medical condition. Be sure to discuss any concerns
with your physician prior to the procedure.
- Your physician will
explain the procedure to you and offer you the
opportunity to ask any questions that you might
have about the procedure.
- If your procedure involves
the use of contrast dye, you will be asked to
sign a consent form that gives permission to
do the procedure. Read the form carefully and
ask questions if something is not clear.
- Notify the technologist
if you have ever had a reaction to any contrast
dye, or if you are allergic to iodine or seafood.
- Generally, there is
no special restriction on diet or activity prior
to an MRI procedure.
- Notify the technologist
if you are pregnant or suspect you may be pregnant.
- Before the examination,
it is extremely important that you inform the
technologist if any of the following apply to
you:
- you are claustrophobic
and think that you will be unable to lie
still while inside the scanning machine,
in which case you may be given a sedative
- you have a pacemaker
or have had heart valves replaced
- you have any type
of implanted pump, such as an insulin pump
- you have metal
plates, pins, metal implants, surgical staples,
or aneurysm clips
- you have any metallic
fragments anywhere in the body
- you have permanent
eye liner
- you are pregnant
or suspect you may be pregnant
- you ever had a
bullet wound
- you have ever worked
with metal (e.g., a metal grinder or welder)
- you have any body
piercing
- you have an intrauterine
device (IUD)
- Sedative medication
may be given if you have claustrophobia and/or
anxiety that would make it difficult for you
to remain still during the procedure.
- Based upon your medical
condition, your physician may request other
specific preparation.
MRI
may be performed on an outpatient basis or as
part of your stay in a hospital. Procedures may
vary depending on your condition and your physician’s
practices.
Generally,
an MRI follows this process:
- You will be asked to
remove any clothing, jewelry, eyeglasses,
hearing aids, hairpins, removable dental work,
or other objects that may interfere with the
procedure.
- If you are asked to
remove clothing, you will be given a gown to
wear.
- If you are to have
a procedure done with contrast, an intravenous
(IV) line will be started in the hand or arm
for injection of the contrast dye.
- You will lie on a scan
table that slides into a large circular opening
of the scanning machine. Pillows and straps
may be used to prevent movement during the procedure.
- The technologist will
be in another room where the scanner controls
are located. However, you will be in constant
sight of the technologist through a window.
Speakers inside the scanner will enable the
technologist to communicate with and hear you.
You will have a call button so that you can
let the technologist know if you have any problems
during the procedure. The technologist will
be watching you at all times and will be in
constant communication.
- A surface coil may
be placed over the area to be examined.
- You will be given earplugs
or a headset to wear to help block out the noise
from the scanner. Some headsets may provide
music for you to listen to.
- During the scanning
process, a clicking noise will sound as the
magnetic field is created and pulses of radio
waves are sent from the scanner.
- It will be important
for you to remain very still during the examination,
as any movement could cause distortion and affect
the quality of the scan.
- At intervals, you may
be instructed to hold your breath, or to not
breathe, for a few seconds. You will then be
told when you can breathe. You should not have
to hold your breath for longer than a few seconds.
- If contrast dye is
used for your procedure, you may feel some effects
when the dye is injected into the IV line. These
effects include coolness or discomfort at the
IV site, and should only last for a few moments.
- You should notify the
technologist if you feel any breathing difficulties,
sweating, numbness, or heart palpitations.
- Once the scan has been
completed, the table will slide out of the scanner
and you will be assisted off the table.
- If an IV line was inserted
for contrast administration, the line will be
removed.
While
the MRI procedure itself causes no pain, having
to lie still for the length of the procedure might
cause some discomfort or pain, particularly in
the case of a recent injury or invasive procedure
such as surgery. The technologist will use all
possible comfort measures and complete the procedure
as quickly as possible to minimize any discomfort
or pain.
On occasion,
some patients with metal fillings in their teeth
may experience some slight tingling of the teeth
during the procedure.
You
should move slowly when getting up from the scanner
table to avoid any dizziness or lightheadedness
from lying flat for the length of the procedure.
If any
sedatives were taken for the procedure, you may
be required to rest until the sedatives have worn
off. You will also need to avoid driving.
If contrast
dye is used during your procedure, you may be
monitored for a period of time for any side effects
or reactions to the contrast dye, such as itching,
swelling, rash, or difficulty breathing.
If you
notice any pain, redness, and/or swelling at the
IV site after you return home following your procedure,
you should notify your physician as this could
indicate an infection or other type of reaction.
Otherwise,
there is no special type of care required after
a MRI scan of the heart. You may resume your usual
diet and activities, unless your physician advises
you differently.
Your
physician may give you additional or alternate
instructions after the procedure, depending on
your particular situation.
|
