Magnetic Resonance Imaging (MRI) - Chest
What is MRI of the Chest? Back to Top
Magnetic resonance imaging (MRI)
is a non-invasive way of viewing organs, soft tissues, bone and
virtually all other internal body structures. MRI uses radio waves
passed through a powerful magnetic field to produce clear and detailed
pictures of the chest, providing information on the heart and lungs
that cannot be otherwise obtained from an x-ray, ultrasound, or
computed tomography (CT) scan.
What are some common uses of the procedure? Back to Top
Magnetic resonance
imaging (MRI) of the chest gives very detailed pictures of the
structures within the chest cavity, including the heart and lungs, from
almost any angle. Abnormal growths, including cancer of the lungs
or other tissues, may be detected and their size, extent, and degree of
spread determined for staging purposes. MRI can often distinguish
between tumors, other lesions, and normal tissues.
MRI may reveal diseased heart valves at a stage when surgery may be easier and safer to perform.
An
MRI exam displays lymph nodes and blood vessels. It is a noninvasive
method of showing whether blood flow is normal or reduced.
A
special form of MRI called magnetic resonance angiography (MRA) can
demonstrate narrowing of the coronary arteries or an actual blockage by
fatty deposits or a blood clot, which often precedes clinical heart
disease. MRA may also demonstrate an abnormal ballooning out of the
wall of an artery (aneurysm) or a torn inner lining of an artery
(dissection).
Disorders of the ribs and sternum may be assessed by chest MRI. An MRI exam can detect breast cancer at an early stage. MRI can provide visual guidance when doing an interventional procedure or taking a tissue sample (biopsy). MRI of the chest may clarify findings from a previous x-ray exam or computed tomography (CT) scan.
How should I prepare for the MRI? Back to Top
The strong magnetic field used
in magnetic resonance imaging (MRI) of the chest can exert a force on
metallic objects that contain iron. Examples include artificial heart
valves, intrauterine devices (IUD's), vascular access ports, metal
plates, pins, screws, surgical staples, prostheses and medication
pumps. If you have an implanted heart pacemaker you should not have an
MRI exam. In most cases, metal objects used in orthopedic surgery pose
no risk. A recently placed artificial joint may, however, mean that
another imaging procedure should replace MRI. If there is any question,
an x-ray may be taken to detect the presence of any metal object. An
x-ray is indicated if there might be shrapnel or a bullet in your body.
MRI may be a problem for sheet metal workers and others who are exposed
to small metal fragments. You will be asked to remove all metal objects
such as jewelry, watch, eyeglasses and hairpins. You should take out
any removable dental work. Pens, eyeglasses or pocketknives can become
dangerous projectiles when the magnet is activated. Red dyes used in
tattoos may contain iron, but this is rarely a problem.
Some MRI procedures use oral or injected contrast material to help
obtain clearer images. For this reason the radiologist or technologist
will ask if you have an allergy of any kind such as hay fever, hives,
allergic asthma, or allergy to food or drugs. The radiologist should
know if you have any serious health problems. Some conditions, such as
kidney disease and sickle cell anemia, may rule out a contrast MRI
study.
People who tend to be nervous when in a confined space
(claustrophobic) may become very uncomfortable if examined in an
enclosed MRI unit and have trouble lying still. A sedative may relieve
anxiety in this circumstance. Another option is to use an open MRI
unit, if available, which is less confining than a conventional unit.
Rules about eating and drinking before the exam vary at different
MRI facilities. Unless told otherwise, you may follow your usual
routine. Medications can be taken as usual. When you arrive at the MRI
unit you may receive a lightweight hospital gown to wear, or you may be
allowed to wear your own clothing if it is loose-fitting and has no
metal fasteners. You should inform MRI staff if you might be pregnant.
What does the equipment look like? Back to Top
The traditional MRI unit is a
large cylinder-shaped tube surrounded by a circular magnet. The patient
is asked to lie on a wheeled bed that is moved into the magnet. Other
MRI units are designed so that the magnet will not completely surround
the patient; some of them are open on all sides. These units are
especially helpful for examining patients who fear being in a closed
space and those who are very obese, but may not provide the same image
quality.
How does the procedure work? Back to Top
Magnetic resonance imaging (MRI)
does not depend on radiation, as do conventional x-ray examinations and
computed tomography (CT) scans. Instead, radio waves are directed at
protons which are most abundant in water molecules (they are the "H" in
H20). When a strong magnetic field is generated, the protons change
their position, producing signals that are processed by a computer
program to form images. MRI images show differences in water content
between various tissues. The images are very sharp and detailed, and
may be viewed in any plane or from any direction. MRI is especially
suited to detecting disorders that increase the amount of fluid, such
as tumors, infection and inflammation.
A typical chest MRI exam includes two to six imaging sequences, each
of which produces sectional views, or slices, through the structures
within the chest. The slices are about one-quarter-inch apart and are
presented in different planes: left to right, front to back, upper to
lower. In addition to these two-dimensional slices, the computer is
able to produce three-dimensional images of various tissues. The images
may be stored in a computer and viewed on screen, or they may be stored
on photographic film or videotape. Unlike conventional x-ray exams, MRI
scans can see through the bones to image the underlying soft tissues.
How is the MRI procedure performed? Back to Top
The MRI scanner is in a
special room that is shielded from outside magnetic fields. You will
lie on your back on a narrow table that slides inside a tunnel-like
tube. Your chest may be secured with straps to help you remain still,
and bolsters may be used to maintain correct positioning. A device
called a coil will be placed over your chest or wrapped around it. If
contrast material is needed, the technologist will inject it, usually
through an intravenous line in your arm. Additional images are obtained
after the injection. Each set of images takes two to 15 minutes to
collect; the complete exam takes about an hour or sometimes longer.
What will I experience during the procedure? Back to Top
Magnetic resonance
imaging (MRI) of the chest is a pain-free procedure. Your chest may
feel slightly warm, but this is a normal and harmless sensation. The
scanner is air-conditioned, well lit, and you can talk to the
technologist. Sedation is available for patients who become anxious
from feeling closed in, but probably fewer than one in twenty require
it. Because excessive movement can blur the images, you may be sedated
to help you remain still as the images are recorded. If contrast
material is injected, you may feel coolness and flushing for a minute
or two.
The MRI scanner produces loud thumping and humming noises. You can
request earplugs to reduce the noise. Some scanners have recorded music
or a built-in television screen to help pass the time. Many MRI centers
permit a relative or friend to remain near the patient during the exam,
or a parent if the patient is a child. The technologist who operates
the scanner will observe you from an adjacent room throughout the exam.
You will be able to speak with the technologist at any time using a
two-way intercom installed in the MRI unit. It is important to remain
perfectly still when images are being recorded, but you may relax
between imaging sequences. If you have not been sedated, no recovery
period is necessary. You may resume your usual activities immediately
after the exam.
Who interprets the results and how do I get them? Back to Top
An hour or more
is needed to develop the images, which will be interpreted by a
radiologist. The radiologist may give you preliminary results of the
exam shortly after it is completed. The final results usually are
available to your primary care physician within one to two days. Some
MRI facilities are able to transmit a diagnostic report along with some
of the images over the Internet.
What are the benefits vs. risks? Back to Top
Benefits - Magnetic resonance
imaging (MRI) of the chest is more informative than other imaging
procedures for differentiating soft tissues. It provides very clear and
detailed views of internal organs of the chest without obstruction by
bone. Tissues are imaged from multiple viewpoints.
- MRI is a totally noninvasive imaging technique that does not require exposure to radiation.
- MRI
of the chest can help to diagnose abnormal growths and provides the
information needed to stage them. Tumors may be distinguished from
other lesions and from normal tissues.
- Blood flow may be assessed without risking the side effects of angiography.
- The
contrast material used in some chest MRI exams does not contain iodine
and so is very unlikely to produce an allergic reaction.
Risks
- Although there is no reason to believe that magnetic
resonance imaging (MRI) harms the fetus, the effects of a strong
magnetic field are not well understood. For this reason pregnant women
usually are advised not to have an MRI exam.
- If sedation is
used there are risks of excessive sedation. The technologist monitors
the patient's vital signs to minimize this risk.
- Patients
have been injured when failing to remove metal objects from their
clothing or when such objects were left in the room by a previous
patient.
- There is a very slight risk of an allergic reaction
if contrast material is injected. Such reactions usually are mild and
easily controlled by medication. There also is a very small risk of
skin infection at the site of injection.
What are the limitations of MRI of the Chest? Back to Top
Magnetic resonance
imaging (MRI) of the chest is a costly procedure that takes more time
than an x-ray or CT study. High-quality images are assured only if the
patient is able to remain perfectly still while the images are being
recorded. Patients who are anxious or confused may find it difficult to
lie still. Although the strong magnetic field is not harmful in itself,
medical devices that contain metal may malfunction or cause problems
during an MRI exam.
Patients who are claustrophobic may have to be sedated during an MRI
study, or else examined in one of the newer open MRI units. A person
who is very obese may not fit into the opening of a conventional MRI
machine.
MRI generally is not recommended for patients who have been acutely
injured. This is because traction devices and life support equipment
must be kept away from the area to be imaged. Furthermore, the results
will not be immediately available as is often necessary in trauma
situations. Back to Top |
