MR Angiography (MRA)
What is MR Angiography? Back to Top
Magnetic resonance
imaging (MRI) is a method of producing extremely detailed pictures of
body tissues and organs without the need for x-rays. The
electromagnetic energy that is released when exposing a patient to
radiofrequency waves in a strong magnetic field is measured and
analyzed by a computer, which forms two- or three-dimensional images
that may be viewed on a TV monitor.
MR angiography (MRA) is an MRI study of the blood vessels. It
utilizes MRI technology to detect, diagnose and aid the treatment of
heart disorders, stroke, and blood vessel diseases. MRA provides
detailed images of blood vessels without using any contrast material,
although a special form of contrast material is often given to make the
MRI images even clearer. The procedure is painless, and the magnetic
field is not known to cause tissue damage of any kind.
What are some common uses of the MRA procedure? Back to Top
Many
patients with arterial disease now have it treated in the radiology
department rather than undergoing surgery in an operating room. MRA is
a very useful way of finding problems with blood vessels and
determining how to best to treat those problems.
The carotid
arteries in the neck that conduct blood to the brain are a common site
of atherosclerosis, which may severely narrow or block off an artery,
reducing blood flow to the brain and even causing a stroke. If an
ultrasound study shows that such disease is present, many surgeons will
perform the necessary operation after confirmation with MRA, dispensing
with the need for catheter angiography.
MRA has found wide use
in checking patients for diseased intracranial (in the head) arteries,
so that only those with positive findings will need to undergo a more
invasive catheter study.
MRA is also used to detect disease in the aorta and in blood vessels supplying the kidneys, lungs and legs.
Patients
with a family history of arterial aneurysm, a ballooning out of a
segment of the vessel wall, can be screened with MRA to see if they
have a similar disorder that has not produced symptoms. If an aneurysm
is found, it may be eliminated surgically, possibly avoiding serious or
fatal bleeding.
How should I prepare for the procedure? Back to Top
The
magnetic field used for MRA will pull on any iron-containing object in
the body, such as a heart pacemaker, intrauterine device, vascular
access port, metal plate, pins, screws or staples. You will be given a
questionnaire to answer regarding these issues. The radiologist or
technologist should know about any such item and also whether you have
ever had a bullet in your body, whether you ever worked with metals, or
if you have had a joint replacement. If there is any question, an x-ray
can be taken to detect metal objects. The radiologist should also know
if you have fillings in your teeth, which could distort images of the
facial region or brain. Braces make it harder to properly adjust the
MRI unit. You will be asked to remove hairpins, jewelry, eyeglasses,
hearing aids and any dental work that can be taken out. Some wigs
contain metal and must be removed. Red dyes used in tattoos and
permanent eyeliner may contain metallic iron, but this is rarely a
problem. You should report any drug allergies to the radiologist or
technologist and should mention if there's any possibility that you
might be pregnant.
You can eat normally before the exam
(unless told differently), but a young child should not eat or drink
for about four hours if they will receive a sedative. The rules vary at
different MRI facilities, so be sure to check with your medical center
about eating and drinking before the exam. Medications may be taken as
usual. Some patients will feel uncomfortably confined (claustrophobic)
when enclosed in an MRI unit. If necessary, you will be given a
sedative to help put you at ease, though probably fewer than one in
every 20 patients will need this. You will wear a lightweight medical
gown for the exam.
What does the MRI equipment look like? Back to Top
The
traditional MRI unit is a large tube surrounded by a circular magnet,
in which the patient lies without moving for several seconds at a time.
The patient is placed on a wheeled bed that is moved into the magnet.
In recent years, patient-friendly units have been designed, and
examination in such units is becoming increasingly available. These
machines are both shorter and wider than a conventional MRI unit and do
not fully enclose the patient. Some of the newer C-shaped units are
even open on all sides and are thus very attractive to patients who
tend to be claustrophobic. A drawback is that image quality is not as
consistently good.
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
Exposing
the patient to radio waves in a strong magnetic field generates data
that are used by a computer to create images of tissue slices that may
be viewed in any plane or from any direction. The magnetic field lines
up atomic particles in the tissues called protons, which are then spun
by a beam of radiofrequency waves and produce signals that are picked
up by a receiver in the imager. It is these signals that are processed
by the computer to produce images. The resulting images are very sharp
and detailed and are thus able to demonstrate tiny changes from the
normal pattern that are caused by disease or injury. Special settings
are used to image various structures, such as arteries in the case of
MRA.
How is the procedure performed? Back to Top
The
patient is placed on a special table and positioned inside the opening
of the MRI unit. A typical exam consists of two to six imaging
sequences, each taking two to 15 minutes. Each sequence provides a
specific image orientation and a specified degree of image clarity or
contrast. Depending on the type of exam being done, the total time
needed can range from 10 to 60 minutes, not counting the time needed to
change clothing, have an IV put in and answer questions. When contrast
material is needed, a substance called gadolinium is given by IV
injection during one of the imaging sequences. It highlights blood
vessels, making them stand out from surrounding tissues.
The
radiologist and technologist leave the examining room during the actual
imaging process, but the patient can communicate with them at any time
using an intercom. Some centers permit a friend to stay nearby, or a
parent if a child is being examined. When the exam is completed you
will be asked to wait to make sure that more images are not needed.
What will I experience during the MRA procedure? Back to Top
The
technologist will make you as comfortable as possible, but at times the
magnet may be within a few inches of your face. For those who become
very uncomfortable when enclosed in a small space, a mild sedative is
nearly always effective. You may notice a warm feeling in the area
being studied. This is normal, but do not hesitate to report it if it
bothers you. If you receive a contrast material injection, there may be
some local discomfort at the IV site. The loud tapping or knocking
noises that are heard during certain parts of the exam disturb some
patients; earplugs may help.
Who interprets the results and how do I get them? Back to Top
A
radiologist experienced in MRI will analyze the results and send a
report to your physician, along with an interpretation of the findings.
Your physician in turn will discuss the MRA findings with you. Some
centers now send diagnostic reports and images over the Internet,
speeding up the process.
What are the benefits vs. risks? Back to Top
Benefits
- Detailed images of blood vessels and blood flow
are obtained without having to insert a catheter directly into the area
of interest, so that there is no risk of damaging an artery.
- The procedure itself and the time needed to recover are shorter than after a traditional catheter angiogram.
- MRA is less costly than catheter angiography.
- There is no exposure to x-rays during an MRI study.
- Even
without using contrast material, MRA can provide high-quality images of
many blood vessels, making it very useful for patients prone to
allergic reactions.
- As with catheter-based angiography
or CT angiography, it frequently is possible to defer surgery after
getting the results of an MRA study. If surgery remains necessary, it
can be performed more accurately.
Risks
There are no definite side effects from any type of MRI study,
including MR angiography. Claustrophobia may be a problem, however.
When it is severe and not relieved by giving a sedative, an alternative
imaging method may have to be tried. If a metal implant is present but
goes undetected, it may be affected by the strong magnetic field to
which the patient is exposed. In addition, if the implant is close to
the examination site it may be hard to get high-quality images.
MRI
is generally avoided during the first three months of pregnancy.
Ultrasound is preferred at this time, unless the woman might have a
very serious condition that is best detected with MRA. The effects of
MRI on the fetus, if any, remain to be determined. The general rule for
MRI and other diagnostic studies in pregnancy is that they should be
avoided unless there is substantial risk from missing the correct
diagnosis because the procedure is not done. Women who are
breast-feeding should inform the radiologist and ask how to proceed.
They may pump breast milk before the exam for use until the gadolinium
contrast material has cleared from the body.
What are the limitations of MR Angiography? Back to Top
MRA
does not image calcium, as does CT angiography. The procedure should be
avoided in any patient with a pacemaker, implanted neurostimulator,
metallic ear implant or metallic object within the eye socket. It
should also be avoided if there is a bullet fragment or if the patient
has a port for delivering insulin or chemotherapy. For patients who are
very claustrophobic, adequate nursing staff must be on hand to monitor
sedation.
The clearness of MRA images does not yet match those
obtained with conventional angiography. MRI of small vessels, in
particular, may not be adequate for diagnosis and treatment planning.
Sometimes it may be difficult to separate images of arteries from veins
with MRA. Back to Top |
