Magnetic resonance imaging (MRI) is a test
done with a large machine that uses a magnetic field and pulses of radio wave
energy to make pictures of the knee. Muscles,
ligaments,
cartilage, and other joint structures are often best
seen with an MRI. In many cases MRI gives information about structures in the
body that cannot be seen as well with an
X-ray,
ultrasound, or
CT scan.
For an MRI test, you are placed
inside the magnet so that your knee is inside the strong magnetic field. MRI
can find changes in the structure of organs or other tissues. It also can find
tissue damage or disease, such as infection or a tumor. Pictures from an MRI
scan are digital images that can be saved and stored on a computer for further
study. The images also can be reviewed remotely, such as in a clinic or an
operating room. Photographs or films of selected pictures can also be made. See
MRI images of the knee.
In some cases, a
contrast material may be used during the MRI scan to
show certain structures more clearly in the pictures. The contrast material may
be used to check blood flow, find some types of tumors, and show areas of
inflammation or infection. The contrast material may be put in a vein (IV) in your arm.
MRI may also find a bone fracture when X-rays and other
tests do not give a clear answer. MRI is done more commonly than other tests to
check for certain bone and joint problems.
How To Prepare
Before your MRI test, tell your health
professional and the MRI technologist if you:
Are allergic to any medicines. The contrast
material used for MRI does not contain iodine. If you have a known allergy to
the contrast material used for MRI, tell your health professional before having
the test. Sometimes the benefits of having this test may outweigh the
risks.
Are or might be pregnant.
Have metal screws in
your knee from a past knee surgery.
Have a pacemaker, artificial
limb, any metal pins or metal parts in your body (especially in the eyes),
metal heart valves, metal clips in your brain, metal implants in your ear,
tattooed eyeliner, or any other implanted or prosthetic medical device (such as
a medicine infusion pump). If you have had an accident or you work around
metal, there is a possibility that you have metal fragments in your head, eyes,
skin, or spine. An X-ray may be taken before the MRI to see if you can have the
test.
Had recent surgery on a blood vessel. In some cases you may
not be able to have the MRI test.
Become very nervous in confined
spaces. You need to lie very still inside the MRI magnet, so you may need to
have the test done with open MRI equipment. It is not as confining as standard
MRI machines. You may need medicine to help you relax.
Have any
other health conditions, such as kidney problems or
sickle cell anemia, that may prevent you from having
an MRI using contrast material.
Wear any medicine patches. The MRI may cause a burn at the patch
site.
You may need to sign a consent form that says you
understand the risks of a knee MRI and agree to have the test done.Talk to your
health professional about any concerns you have regarding the need for the
test, its risks, how it will be done, or what the results will indicate. To
help you understand the importance of this test, fill out the
medical test information form(What is a PDF document?)
.
You may need to arrange for someone to
drive you home after the test, if you are given a medicine (sedative) to help you relax.
How It Is Done
A magnetic resonance imaging (MRI) test
is usually done by an MRI technologist. The resulting pictures are usually
interpreted by a
radiologist. However, some other types of doctors,
such as an
orthopedic surgeon, can also interpret a knee MRI
scan.
You will need to remove all metal objects (such as hearing
aids, dentures, jewelry, watches, and hairpins) from your body because these
objects may be attracted to the powerful magnet used for the test.
You will need to take off all or most of your clothes, depending on which
area is examined (you may be allowed to keep on your underwear if it is not in
the way). You will be given a gown to use during the test. If you are allowed
to keep some of your clothes on, you should empty your pockets of any coins and
cards (such as credit cards or ATM cards) with scanner strips on them because
the MRI magnet may erase the information on the cards.
During the
test, you will lie on your back on a table that is part of the MRI scanner. The
table will slide into the space that contains the magnet. A device called a
coil may be placed over or wrapped around the area to be scanned.
Some people feel nervous (claustrophobic) inside the MRI magnet. If
feeling nervous keeps you from lying still, you can be given a medicine
(sedative) to help you relax. Some MRI machines (called open MRI) are made so
that the magnet does not enclose your entire body. Open MRI machines may be
helpful if you are claustrophobic.
Inside the scanner, you will
hear a fan and feel air moving. You may also hear tapping or snapping noises as
the MRI scans are taken. You may be given earplugs or headphones with music to
reduce the noise. It is very important to hold completely still while the scan
is being done. You may be asked to hold your breath for short periods of
time.
During the test, you may be alone in the scanner room.
However, the technologist will watch you through a window. You will be able to
talk with the technologist through a two-way intercom..
If
contrast material is needed, the
technologist will put it in an IV in your arm. The
injection may be given over 1 to 2 minutes.
An MRI test usually
takes 30 to 60 minutes but can take as long as 2 hours.
How It Feels
You won't have pain from the magnetic
field or radio waves used for the MRI test. The table you lie on may feel hard
and the room may be cool. You may be tired or sore from lying in one position
for a long time.
If a contrast material is used, you may feel some
coolness and flushing as it is put into your IV.
In rare cases,
you may feel:
A tingling feeling in the mouth if you have
metal dental fillings.
Warmth in the area being examined. This is
normal. Tell the technologist if you have nausea, vomiting, headache,
dizziness, pain, burning, or breathing problems.
Risks
There are no known harmful effects from the
strong magnetic field used for MRI. However, the magnet is very powerful. The
magnet may affect pacemakers, artificial limbs, and other medical devices that
contain iron. The magnet will stop a watch that is close to the magnet. Any
loose metal object has the risk of causing damage if it gets pulled toward the
strong magnet.
Metal parts in the eyes can damage the
retina. If you may have metal fragments in the eye, an
X-ray of the eyes may be done before the MRI. If metal is found, the MRI will
not be done.
Iron pigments in tattoos or tattooed eyeliner can
cause skin or eye irritation.
An MRI can cause a burn with some
medicine patches. Be sure to tell your health professional if you are wearing a
patch.
There is a slight chance of an
allergic reaction if contrast material is used during
the MRI. However, most reactions are mild and can be treated using medicine.
There also is a slight risk of an infection at the IV site.
Results
Magnetic resonance imaging (MRI) is a test done with a large machine that uses a magnetic
field and pulses of radio wave energy to make pictures of the knee.
The
radiologist may discuss preliminary results of the MRI
with you right after the test. Complete results are usually available for your
health professional in 1 to 2 days.
Factors that can interfere with
your test or the accuracy of the results include:
Metal devices in your leg from previous
surgery. These may make the MRI pictures blurry and prevent your doctor from
seeing what is wrong with your knee.
Pregnancy. An MRI test usually is not done during
pregnancy.
Medical devices that use electronics, such as a
pacemaker or medication infusion pump. The MRI magnet may cause problems with
these devices.
Inability to remain still during the
test.
Obesity. A person who is very
overweight may not fit into the opening of some standard MRI machines.
Many modern medical devices that do not use electronics,
such as heart valves, stents, or clips, can be safely placed in most MRI
machines. But some newer MRI machines have stronger magnets. The safety of MRI
scans with these stronger MRI magnets in people with medical devices is not
completely known.
What To Think About
Sometimes your MRI test results may be different than those from
CT,
ultrasound, or
X-ray tests because the MRI scan is more
specific.
In some people, a knee MRI has given enough information about the
knee joint that those people do not need an
arthroscopy. For more information, see the medical
test
Arthroscopy.
Contrast material put
directly in the knee (arthrogram) may be done in people who cannot have an MRI
or where MRI is not available. An arthrogram may also be done in people with
total knee joint replacements to check for loose parts. For more information,
see the medical test
Arthrogram.
Open MRI machines are now made
so that the magnet does not completely surround you. Open MRI is useful for
people who are claustrophobic or obese. But these machines are not available
everywhere. Also, these machines may not be able to do all the studies needed
to check for problems.
Contrast material that contains gadolinium may cause a
serious skin problem (called nephrogenic fibrosing dermopathy) in people with
kidney failure. Tell your doctor if you have serious
kidney disease before having an MRI scan.
This information does not replace the advice of a doctor. Healthwise disclaims any warranty or liability for your use of this information. Your use of this information means that you agree to the Terms of Use. How this information was developed to help you make better health decisions.
