Cardiac MRI (MRI of the Heart, Great Vessels and Adjacent Structures)
What is a Cardiac MRI? Back to Top
MRI (magnetic resonance imaging) uses
radiofrequency 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.
What are some common uses of Cardiac MRI? Back to Top
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.
How should I prepare for the procedure? Back to Top
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. Fewer than one in 20
patients will require medication.
What does the MRI equipment look like? Back to Top
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.
Examples of the MRI equipment that may be used are shown at the top of this page.
How does the procedure work? Back to Top
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, radiofrequency waves are directed at protons, part
of the nuclei of hydrogen atoms, in a strong magnetic field. The
protons are first "excited" and then "relaxed," causing them to emit
radiofrequency 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.
How is the procedure performed? Back to Top
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 by 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 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 will be asked to wait until the images are examined to determine if more images are needed.
What will I experience during the MRI procedure? Back to Top
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 material 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.
Who interprets the results and how do I get them? Back to Top
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.
What are the benefits vs. risks? Back to Top
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.
What are the limitations of a Cardiac MRI? Back to Top
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. Back to Top |
