Test Overview
Magnetic resonance imaging (MRI) is a test that uses a magnetic field and pulses of radio
wave energy to take pictures of the head. In many cases, MRI gives information
that cannot be seen on an
X-ray,
ultrasound, or
computed tomography (CT) scan.
For an MRI
of the head, you lie with your head inside a special machine (scanner) that has
a strong magnet. The MRI can show tissue damage or disease, such as infection,
inflammation, or a tumor. Information from an MRI can be saved and stored on a
computer for more study. Photographs or films of certain views can also be
made. See MRI images of a person who has had a
stroke or a
seizure.
In some cases, a dye (contrast material) may be used during the MRI to show
pictures of structures more clearly. The dye may help show blood flow, look for
some types of tumors, and show areas of inflammation.
MRI of the
head may be used to look for the cause of headaches. For more information, see:
Headaches: Should I Have Imaging tests to Find Out What's Causing My Headaches?
See pictures of
a standard MRI machine and an
open MRI machine.
Why It Is Done
Magnetic resonance imaging (MRI) of the
head is done to:
- Look for the cause of
headaches.
- Help diagnose a
stroke or blood vessel problems in the head. Problems
with blood vessels may include an
aneurysm or abnormal twisted blood vessels that are
present at birth (this is called an arteriovenous [AV]
malformation).
- Check blood flow or blood clots to the brain. MRI
can show bleeding in or around the brain.
- Check symptoms of a known
or suspected
head injury.
- Check symptoms such as change
in consciousness, confusion, or abnormal movements. These symptoms may be
caused by brain diseases, such as
Huntington's disease,
multiple sclerosis (MS),
Parkinson's disease, or
Alzheimer's disease. See an
MRI showing MS in the brain.
- Check for "water on the brain"
(hydrocephaly).
- Look for tumors,
infections, an
abscess, or conditions of the brain or brain stem,
such as
encephalitis or
meningitis.
- Check the eyes, the nerves
from the eyes to the brain (optic nerves), the ears, and the nerves
from the ears to the brain (auditory nerves).
- Look for
problems of the
pituitary gland.
- Investigate or follow a
finding seen on another test.
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 have iodine.
If you know that you are allergic to the contrast material used for the MRI, tell your doctor before having another test.
- Have any other health conditions,
such as kidney problems or
sickle cell disease, that may prevent you from having
an MRI using contrast material.
- Are or might be
pregnant.
- 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:
- o Heart and blood vessel devices such as a coronary artery stent, pacemaker, ICD (implantable cardioverter-defibrillator), or metal heart valve.
- o Metal pins, clips, or metal parts in your body, including artificial limbs and dental work or braces.
- o Any other implanted medical device, such as a medicine infusion pump or a cochlear implant.
- o Cosmetic metal implants, such as in your ears, or tattooed eyeliner.
- Have an
intrauterine device (IUD) in place. An IUD may prevent
you from having the MRI test done.
- Become very nervous in small,
tight spaces. You need to lie very still inside the MRI magnet. You may need
medicine to help you relax.
- Wear any medicine patches. The MRI may cause a burn at the patch
site.
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.
You will need to sign a consent form that says you
understand the risks of an 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?).
How It Is Done
A magnetic resonance imaging (MRI) test
is done by an MRI technologist. The pictures are read by a
radiologist. But some other types of doctors (such as
a
neurologist or
neurosurgeon) can also read an MRI scan of the
head.
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. If you have
had an accident or you work around metal, there is a chance that you have metal
pieces in your head, eyes, skin, or spine. An X-ray may be taken before the MRI
to see if you can have the test.
You may need to take off some of
your clothes. You will be given a gown to wear during the test. If you keep
your clothes on, 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. Your head, chest, and
arms may be held with straps to help you lie still. The table will slide into
the space with the magnet. A device called a coil may be placed over or wrapped
around your head. Some MRI machines (called open MRI) are now made so that the
magnet is not around your whole body. See pictures of a standard MRI machine and an open MRI machine.
Some people feel nervous (claustrophobic)
inside the MRI magnet. If this keeps you from lying still, you can be given a
medicine (sedative) to help you relax. 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
pictures are taken. This is normal. 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. The technologist will watch you through a window. You will be able to
talk through a speaker.
If
contrast material is needed, the technologist will put
it in an
intravenous (IV) line in a vein in your arm or hand.
The material may be given over 1 to 2 minutes. Then more MRI scans are
done.
An MRI test usually takes 30 to 60 minutes but can take as
long as 2 hours.
How It Feels
You will not 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 your head. 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, implantable cardioverter-defibrillators (ICDs),
artificial limbs, and other medical devices that contain iron. The magnet will
stop a watch that is close to the magnet.
Metal pieces in the eyes
can damage the
retina. If you might have metal pieces in your 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 small chance of an allergic reaction
if
contrast material is used during the MRI. But most
reactions are mild and can be treated with 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 that uses a magnetic field and pulses of radio
wave energy to take pictures of the head.
The
radiologist may tell you some of the results of the
MRI right after the test. Full results are sent to your doctor or specialist in
1 to 2 days.
Magnetic resonance imaging (MRI) of the headNormal: | All structures of the head—the brain, its vessels,
spaces, nerves, and surrounding structures—are normal. |
|---|
No abnormal growths, such as tumors, in or around the
brain are present. |
No bleeding, abnormal blood vessels (AV malformations),
abnormal pockets of fluid, blockage in the flow of blood, or bulges in the
blood vessels (aneurysm) are present. |
No signs of infection or inflammatory disease, such as
encephalitis or
meningitis, are present. |
Abnormal: | Tumors in the brain or in areas outside the brain, such
as an
acoustic neuroma, are present. |
|---|
Bleeding or swelling (edema) in or around the brain is
present. |
Areas of infection or inflammatory disease, such as
encephalitis or meningitis, are present. |
Abnormal areas in the brain may mean that certain
diseases, such as
Huntington's disease,
multiple sclerosis,
Parkinson's disease, or
Alzheimer's disease, are present. |
Bulges or weak areas (aneurysms) or abnormal blood
vessels (such as an AV malformation) 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:
- Being pregnant. An MRI test is not usually done
during pregnancy even though the strong magnetic field is not known to be
harmful.
- Having medical devices with metal. The MRI magnet may
cause these devices to not work right or to have problems during an MRI scan.
This test may not be done if you have:
- Being unable to lie still during the
test.
What To Think About
- Sometimes your MRI results may be different from earlier test
results because you were tested at a different medical center or you had a
different kind of test.
- Several special MRI methods have been
developed to look at the brain.
- Magnetic resonance spectroscopy
shows changes in brain chemistry that may occur in certain areas of the brain.
These changes may help show diseases that affect the brain.
- Magnetic resonance angiography (MRA) can be used to study
blood vessels. Dye may be injected into the blood vessels so blood flow can be
seen clearly. MRA can also be used to show the speed and direction of blood
flow. For more information, see the topic
Magnetic Resonance Angiogram (MRA).
- Diffusion-perfusion imaging shows the water content and
character of the brain. This method can give an early diagnosis of a
stroke and may help predict the outcome of stroke. It
can also be used to find tumors or inflammation of the brain.
References
Other Works Consulted
- Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis:
Saunders.
- Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.
Credits
| By | Healthwise Staff |
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| Primary Medical Reviewer | Kathleen Romito, MD - Family Medicine |
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| Specialist Medical Reviewer | Paul D. Traughber, MD - Diagnostic Radiology |
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| Last Revised | December 8, 2010 |
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Seton is proud to have four hospitals – the only hospitals in Central Texas - that have earned the Magnet designation, the highest award for nursing excellence given by the American Nurses Association.