Test Overview
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 tumour. 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 tumours, and show areas of
inflammation or infection. The contrast material may be put in a vein (IV) in your arm or directly into your knee.
Meniscus Tear: Should I Have a Diagnostic Test (MRI or Arthroscopy)?
See pictures of a
standard MRI machine and an
open MRI machine.
Why It Is Done
Magnetic resonance imaging (MRI) of the
knee is done to:
- Check for the cause of unexplained knee pain or
the knee giving out for no reason.
- Find problems in the knee joint,
such as
arthritis, bone tumours, or infection, or damaged
cartilage,
meniscus, ligaments, or
tendons.
- Find out if a knee
arthroscopy is needed.
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 doctor
and the MRI technologist if you:
- Are allergic to any medicines. The contrast
material used for MRI does not contain iodine. If you know that you are allergic to the contrast material used for the MRI, tell your doctor before having another test.
- Are or might be pregnant.
- Have metal screws in
your knee from a past knee surgery.
- Have any metal implanted in your body. This helps your doctor know if the test is safe for you. Tell your doctor if you have:
- Heart and blood vessel devices such as a coronary artery stent, pacemaker, ICD (implantable cardioverter-defibrillator), or metal heart valve.
- Metal pins, clips, or metal parts in your body, including artificial limbs and dental work or braces.
- Any other implanted medical device, such as a medicine infusion pump or a cochlear implant.
- Cosmetic metal implants, such as in your ears, or tattooed eyeliner.
- Had recent surgery on a blood vessel. In some cases, you may
not be able to have the MRI test.
- Have an
intrauterine device (IUD) in place. An IUD may prevent
you from having the MRI test done.
- Become very nervous in confined
spaces. You need to lie very still inside the MRI magnet, so you may need
medicine to help you relax. Or you may be able 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 doctor about any concerns you have regarding the need for the test, its
risks, how it will be done, or what the results will mean. 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. But 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, jewellery, 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. See pictures of a
standard MRI machine and an
open MRI machine.
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. But 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 usually put it in through an IV in your arm or hand.
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. But 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 doctor if you are wearing a
patch.
There is a slight chance of an
allergic reaction if contrast material is used during
the MRI. But most reactions are mild and can be treated using medicine.
Contrast material that contains gadolinium may cause a serious problem (called
nephrogenic systemic fibrosis) in people with
kidney failure. If you have decreased kidney function
or serious kidney disease, tell your doctor before having an MRI scan.
There also is a slight risk of an infection at the IV site if contrast
material was used.
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
doctor in 1 to 2 days.
Magnetic resonance imaging (MRI) of the knee | Normal: | The
ligaments,
meniscus,
tendons, bones, and joints look normal in size, shape,
and location. |
|---|
No growths, such as tumours, are
present. |
No broken bones (fractures),
extra fluid, or loose bodies are present. |
No signs of inflammation or infection in
bones, joints, or soft tissues are present. |
| Abnormal: | Bones show an injury or a fracture. The MRI
also may show a collection of fluid, which could mean an infection is
present. |
|---|
Ligament or meniscus tears are
present. |
Tendon tears or thickening is present,
indicating a remote tear, past surgery, or repeated stress. |
Growths, such as tumours, are
present. |
Changes common to
arthritis are present. |
What Affects the Test
Reasons you may not be able to
have the test or why the results may not be helpful 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 medicine infusion pump. The MRI magnet may cause problems with
these devices.
- If you are not able to remain still during the
test.
- Obesity. A person who is very
overweight may not fit into the opening of some standard MRI machines.
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 topic
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 topic
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.
References
Other Works Consulted
- Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis:
Saunders.
- Fischbach FT, Dunning MB III, eds. (2009).
Manual of Laboratory and Diagnostic Tests, 8th ed.
Philadelphia: Lippincott Williams and Wilkins.
- Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.
Credits
| By | Healthwise Staff |
|---|
| Primary Medical Reviewer | Kathleen Romito, MD - Family Medicine |
|---|
| Primary Medical Reviewer | Donald Sproule, MD, CM, CCFP, FCFP - Family Medicine |
|---|
| Specialist Medical Reviewer | Paul D. Traughber, MD - Diagnostic Radiology |
|---|
| Last Revised | February 7, 2011 |
|---|
