Cardiac MRI (MRI of the Heart, Great Vessels and Adjacent Structures)
What is a Cardiac MRI?
MRI (magnetic resonance imaging) uses radio waves and a strong magnetic
field rather than x-rays to provide remarkably clear and detailed pictures
of internal organs and tissues. The procedure is valuable in diagnosing
a broad range of conditions in all parts of the body, including heart and
vascular disease, stroke, cancer, and joint and musculoskeletal disorders.
MRI is unique in that it can also create detailed images of blood vessels
without the use of contrast material (although there is a trend toward
the use of special non-iodinated MRI-contrast material, for example, Gadolinium).
MRI requires specialized equipment and expertise and allows evaluation
of some body structures that may not be as visible with other imaging methods.
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What are some common uses of Cardiac MRI?
Magnetic resonance imaging is becoming very important in the initial diagnosis
and subsequent management of coronary heart disease. MRI can help physicians
to look closely at the structures and function of the heart and major vessels
quickly and thoroughly, without the risks associated with traditional,
more invasive procedures. Using MRI, physicians can examine the size and
thickness of the chambers of the heart, and determine the extent of damage
caused by a heart attack or progressive heart disease.
After a heart attack, for example, an MRI examination can help the cardiologist
understand how well the heart is pumping, whether the flow of blood is
blocked in any chamber or major vessel, whether the heart muscles are damaged,
or whether the lining of the heart is swelling. This is critical knowledge
needed to administer prompt and effective treatment.
MRI can also detect the buildup of plaque and blockages in the blood vessels,
making it an invaluable tool for detecting and evaluating coronary artery
disease. Recently, specialists in MRI have demonstrated its potential for
showing not only the structure, but also the function of the heart muscles,
valves and vessels. Using MRI, they have created movie-like images of the
beating heart that doctors can use to diagnose a variety of cardiovascular
problems. More and more, MRI is being used as part of the traditional cardiac
stress test to help physicians with earlier diagnosis and treatment of
heart disease and to assess the patient’s recovery after treatment.
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How should I prepare for the procedure?
Because the strong magnetic field used for MRI will pull on any iron-containing
object in the body, MRI staff will ask whether you have a heart pacemaker
or implanted defibrillator, implanted port, infusion catheter (often referred
to by brand names such as Port-o-cath, Infusaport or Lifeport), intrauterine
device (IUD), or any metal plates, pins, screws or surgical staples in
your body. In most cases, surgical staples, plates, pins and screws pose
no risk during MRI. Red dyes used in tattoos and permanent eyeliner may
contain metallic iron oxide and could heat up during MRI; however, this
is rare. You will be asked if you have ever had a bullet or shrapnel in
your body, or ever worked with metal. If there is any question of metal
fragments, you may be asked to have an x-ray that will detect any metal
objects.
The radiologist or technologist may ask about drug allergies and whether
you have undergone any surgery in the past. If you are or might be pregnant,
mention it to the MRI staff. Some patients who undergo MRI in an enclosed
unit may feel confined or claustrophobic. If you are not easily reassured,
a sedative may be administered. Less than one in 20 patients will require
medication.
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What does the MRI equipment look like?
The conventional MRI unit is a closed cylindrical magnet in which the patient
must lie totally still for several seconds at a time, and consequently
may feel closed-in or truly claustrophobic. However, new patient-friendly
designs are rapidly coming into routine use. The "short-bore" systems
are wider and shorter and do not fully enclose the patient, while other
units are open on all sides. However, image quality may vary between machines.
Your radiologist will recommend the use of equipment best suited for your
procedure.
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How does the procedure work?
MRI is a unique imaging method because, unlike the usual radiographs (x-rays),
radioisotope studies, and even computed tomography (CT) scanning, it does
not rely on radiation. Instead, radio waves are directed at protons, the
nuclei of hydrogen atoms, in a strong magnetic field. The protons are first "excited" and
then "relaxed," causing them to emit radio signals that can be
computer-processed to form an image. In the body, protons are most abundant
in the hydrogen atoms of water—the "H" of H2O—so
that an MRI shows differences in the water content and distribution in
various body tissues. Even different types of tissue within the same organ,
such as the muscle and vessels of the heart, can be easily distinguished.
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How is the procedure performed?
The patient is placed on a sliding table and positioned comfortably for
the MRI examination. Then the radiologist and technologist leave the room
and the individual MRI sequences are performed. The patient is able to
communicate with the technologist at any time using an intercom. Also,
many MRI centers allow a friend or, if a child is being examined, a parent
to stay in the room.
Depending on how many images are needed, the exam will generally take from
15 to 45 minutes, although a very detailed study may take longer. You will
be asked not to move during the actual imaging process.
For examination of the heart, contrast material may be used to enhance the
visibility of the heart’s chambers and major vessels. A small needle
connected to an intravenous line is placed in an arm or hand vein. A saline
solution will drip through the intravenous line to prevent clotting until
the contrast material is injected, about two-thirds of the way through
the exam.
When the exam is over the patient is asked to wait until the images are
examined to determine if more images are needed.
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What will I experience during the MRI procedure?
MRI causes no pain, but some patients find it uncomfortable to remain still
during the examination. Others experience a sense of being closed in, though
the more open construction of many new MRI systems has helped reduce that
reaction. You may notice a warm feeling in the area under examination;
this is normal, but tell the radiologist or technologist if it bothers
you.
If a contrast injection is used, there may be discomfort at the injection
site, and you may have a cool sensation at the site during the injection.
The loud tapping or knocking noises heard at certain phases of the imaging
exam disturb some patients; earplugs may help.
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Who interprets the results and how do I get them?
A radiologist, who is a physician experienced in MRI and other radiology
examinations, will analyze the images and send a signed report with his
or her interpretation to your primary care physician. Your physician will
discuss the findings with you. At some facilities, new technology also
allows for distribution of diagnostic reports and referral images over
the Internet.
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What are the benefits vs. risks?
Benefits
- Images of soft-tissue structures such as the heart and major vessels
are clearer and more detailed than with other imaging methods.
- The detail of MRI makes it an invaluable tool in early detection and
evaluation of coronary disease.
- Even without the use of contrast material, MRI often shows sufficient
detail of the heart to be valuable in diagnosis and treatment planning.
- When it is used, MRI contrast material is less likely to produce an
allergic reaction than the iodine-based materials used for conventional
x-rays and CT scanning, and does not contain the radioisotopes used
in nuclear medicine exams.
- MRI enables the detection of abnormalities that might be obscured
by bone tissue with other imaging methods.
- MRI provides a fast, noninvasive and often less expensive alternative
to other techniques of cardiac diagnosis.
- MRI can help physicians evaluate the function as well as the structure
of the heart muscles and valves.
- MRI does not require exposure to radiation or the introduction of
radioisotopes to the body.
Risks
- An undetected metal implant may be affected by the strong magnetic
field.
- MRI is generally avoided in the first 12 weeks of pregnancy. Doctors
usually use other methods of imaging—such as ultrasound—on
pregnant women, unless there is a strong medical reason to conduct
an MRI exam.
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What are the limitations of a Cardiac MRI?
In most cases the exam is safe for patients with metal implants, with the
exception of a few types of implants, so patients should inform the technologist
of an implant prior to the test. The exam must be used cautiously in early
pregnancy.
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