| Functional MR Imaging (fMRI) - Brain
What is fMRI of the Brain? Back to Top
Magnetic resonance
imaging (MRI) uses radio waves and a strong magnetic field rather than
x-rays to provide clear and detailed pictures of internal organs and
tissues. Functional magnetic resonance imaging (fMRI) is a relatively
new procedure that uses MR imaging to measure the quick, tiny metabolic
changes that take place in an active part of the brain. Physicians know
the general areas of the brain where speech, sensation, memory and
other functions occur. However, the exact locations vary from
individual to individual. Injuries and disease, such as stroke or brain
tumor, can even cause functions to shift to other parts of the brain.
fMRI not only helps radiologists look closely at the anatomy of the
brain, but can help them determine precisely which part of the brain is
handling critical functions such as thought, speech, movement and
sensation. This information can be critical to planning surgery,
radiation therapy, treatment for stroke or other interventions to treat
brain disorders.
What are some common uses of fMRI? Back to Top
fMRI is
becoming the diagnostic method of choice for learning how a normal,
diseased or injured brain is working, as well as for assessing the
potential risks of surgery or other invasive treatment of the brain.
The term "Functional MRI" (with a capital F) can also include other MRI
techniques that are sensitive to physiologic changes (such as changes
in water motion), whereas fMRI with a lower case f usually refers to
the mapping of brain activity using MRI. As a group, these "Functional
MRI" techniques appear to provide the most sensitive method currently
available for identifying, investigating and monitoring brain tumors,
strokes and certain chronic disorders of the nervous system such as
multiple sclerosis. In addition, these methods appear to provide a
useful means of documenting some brain abnormalities related to
dementia or seizures.
In routine practice, fMRI studies are often used in planning brain
surgery, since they can help physicians monitor normal brain function
as well as any disturbed brain function. While research is still
ongoing, it appears that fMRI can also help assess the effects of
stroke, trauma or degenerative disease (such as Alzheimer's) on brain
function.
How should I prepare for the procedure? Back to Top
Since
fMRI uses an MRI device, the standard preparations for an MRI procedure
are necessary. For example, because the strong magnetic field used for
MRI will pull on any ferromagnetic metal object implanted in the body,
MRI staff will ask whether you have a heart pacemaker (or artificial
heart valve), implanted port, infusion catheter (brand names
Port-o-cath, Infusaport, Lifeport), intrauterine device (IUD) or any
metal plates, pins, screws or surgical staples in your body. In most
cases, surgical staples, plates, pins and screws pose no risk during
MRI. Red dyes used in tattoos and permanent eyeliner may contain
metallic iron oxide and could heat up during MRI, however this is rare.
You will be asked if you have ever had a bullet or shrapnel in your
body, or ever worked with metal. If there is any question of metal
fragments, you may be asked to have an x-ray that will detect any such
metal objects. Tooth fillings are not affected by the magnetic field,
but they may distort images of the facial area or brain, so the
radiologist should be made aware of them. The same is true of braces,
which may make it hard to "tune" the MRI unit to your body. You will be
asked to remove anything that might degrade MRI images of the head,
including hairpins, jewelry, eyeglasses, hearing aids and any removable
dental work.
The radiologist or an assistant may ask about drug allergies and
whether head surgery has been done in the past. If you might be
pregnant, this should be mentioned. Less than one in 20 patients who
undergo MRI in an enclosed unit may feel confined or claustrophobic.
What does the MRI equipment look like? Back to Top
The
conventional MRI unit is a cylindrical magnet in which the patient must
lie totally still for several seconds at a time, and consequently may
feel closed-in or truly claustrophobic. However, new patient-friendly
designs are rapidly coming into routine use. The "short-bore" systems
are wider and shorter and do not fully enclose the patient. Some newer
units are open on all sides; however, the image quality may vary.
How does the procedure work? Back to Top
MRI uses radio
waves and a strong magnetic field rather than x-rays to provide clear
and detailed pictures of internal organs and tissues. fMRI uses this
technology to identify regions of the brain where blood vessels are
expanding, chemical changes are taking place or extra oxygen is being
delivered—all signs that this part of the brain is currently processing
information and giving commands to the body.
In fMRI, the patient performs a particular task while the imaging is
taking place. The metabolism in the area of the brain responsible for
this task will increase and the signal in the MR image will change. By
performing specific tasks that correspond to different functions, it is
possible to locate the corresponding area of the brain that governs the
function. This information can then be incorporated into a surgical
planner to help a surgeon avoid these areas.
How is the procedure performed? Back to Top
The patient lies
on a sliding table, with his or her head in a brace designed to help
hold the head still. This brace may include a mask created especially
for the individual. As the examination proceeds, the patient will be
asked to perform a number of small tasks, such as tapping the thumb of
one hand against each of the fingers of that hand, or rubbing a block
of sandpaper, or answering simple questions. The patient is able to
communicate with the radiologist or technologist throughout the exam.
Also, many MRI centers allow a friend or, if a child is being examined,
a parent into the room.
Depending on how many images are needed, the exam will generally
take 15 to 45 minutes, although a very detailed study may take longer.
You will be asked not to move during the actual imaging process, but
between sequences some movement is allowed. Patients are generally
required to remain still for only a few minutes at a time.
When the exam is over the patient is asked to wait until the images are examined to determine if more images are needed.
What will I experience during the MRI procedure? Back to Top
MRI
causes no pain, but there may be discomfort from being closed in or
from the need to keep your head very still. You may notice a warm
feeling in the area under examination; this is normal, but if it
bothers you the radiologist or technologist should be told. The loud
tapping or knocking noises heard at certain phases of imaging can be
bothersome to some people. However, in fMRI exams, the actual imaging
is done in a series of very short bursts, so the noise will not last
long.
Who interprets the results and how do I get them? Back to Top
A
radiologist, who is a physician experienced in MRI and other radiology
examinations, will analyze the images and send a signed report with his
or her interpretation to the patient's referring physician. The patient
receives the results from the referring physician who ordered the test.
New technology also allows for distribution of diagnostic reports and
referral images over the Internet at many facilities.
What are the benefits vs. risks? Back to Top
Benefits
- Functional MRI can identify the location of normal brain
function in order to allow surgeons to attempt to avoid these areas
during brain surgery.
- Functional MR images can
enable detection of a stroke at a very early stage, so physicians can
initiate effective treatments earlier.
- fMRI studies
can help physicians monitor the growth and function of brain tumors and
guide the planning of radiation therapy or surgical treatment.
- fMRI images of the brain and other head structures are clearer and more detailed than images obtained with other methods.
- Exposure to radiation is avoided.
- fMRI enables the detection of abnormalities that might be obscured by bone tissue with other imaging methods.
Risks
- An undetected metal implant may be affected by the strong magnetic field.
- fMRI
is generally avoided in the first 12 weeks of pregnancy. Doctors
usually use other methods of imaging, such as ultrasound, on pregnant
women, unless there is a strong medical reason for using fMRI.
- See the MRI Safety page for additional information.
What are the limitations of fMRI of the Brain? Back to Top
Functional
MRI is still evolving and improving. While it appears to be as accurate
in finding the location of brain activity as any other method, overall
there is less experience with fMRI than with many other MRI techniques.
In most cases the exam is safe for patients with metal implants, with
the exception of a few types of implants, so patients should inform the
technician of an implant prior to the test. MRI is often more expensive
than CT scanning. Back to Top |
