MR Imaging (MRI) - Head 
What is MRI of the Head? Back to Top
Magnetic resonance imaging (MRI) uses
radio waves and a strong magnetic field rather than x-rays to provide
remarkably clear and detailed pictures of internal organs and tissues.
This technique has proved very helpful to radiologists in diagnosing
tumors of the brain as well as disorders of the eyes and the inner ear.
It requires specialized equipment and expertise and allows evaluation
of some body structures that may not be as visible with other imaging
methods.
What are some common uses of the MRI procedure? Back to Top
MRI is the most
sensitive exam for brain tumors, strokes and certain chronic disorders
of the nervous system such as multiple sclerosis. In addition, it is a
useful means of documenting brain abnormalities in patients with
dementia and it is commonly used for patients with disease of the
pituitary gland. MRI can detect tiny areas of tissue abnormality in
patients with disease of the eyes or the inner ear.
How should I prepare for the procedure? Back to Top
Because the strong
magnetic field used for MRI will pull on any metal object implanted in
the body, MRI staff will ask whether you have a prosthetic hip, 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. Tattoos and permanent eyeliner may also create a problem.
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 usually are not affected by the magnetic
field, but they may distort images of the facial area or brain, so the
radiologist should be 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 technologist may ask about drug allergies and
whether head surgery has been done in the past. If you might be
pregnant, this should be mentioned. Some patients who undergo MRI of
the head in an enclosed unit may feel confined or claustrophobic. If
you are not easily reassured, a sedative may be administered. Roughly
one in 20 patients will require medication.
What does the MRI equipment look like? Back to Top
The conventional MRI unit
is a closed 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. New,
more powerful magnets that allow highly detailed images of the brain
are also coming into use nowadays. For the patient, such magnets (e.g.
3 Tesla systems) look and feel similar to standard magnets in use today.
Examples of the radiography equipment that may be used are shown at the top of this page.
How does the procedure work? Back to Top
MRI is a unique imaging method
because, unlike the usual radiographs (x-rays), radioisotope studies
and even CT scanning, it does not rely on radiation. Instead, radio
waves are directed at protons, the nuclei of hydrogen atoms, in a
strong magnetic field. The protons are first "excited" and then
"relaxed," emitting radio signals, which can be computer-processed to
form an image. In the body, protons are most abundant in the hydrogen
atoms of water—the "H" of H2O—so that an MRI image shows differences in
the water content and distribution in various body tissues. Even
different types of tissue within the same organ, such as the gray and
white matter of the brain, can easily be distinguished. Typically an
MRI exam consists of two to six imaging sequences, each lasting two to
15 minutes. Each sequence has its own degree of contrast and shows a
cross section of the head in one of several planes (right to left,
front to back, upper to lower).
How is the procedure performed? Back to Top
The patient is placed on a
sliding table and a radio antenna device called a surface coil is
positioned around the upper part of the head. After positioning the
patient with the head inside the MRI gantry, the radiologist and
technologist leave the room and the individual MRI sequences are
performed. The patient is able to communicate with the radiologist or
technologist at any time using an intercom. 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.
The patient 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 seconds at a time.
Some patients will require an injection of a contrast material to
enhance the visibility of certain tissues or blood vessels. A small
needle connected to an intravenous line is placed in an arm or hand
vein. A saline solution will drip through the intravenous line to
prevent clotting until the contrast material is injected about
two-thirds of the way through the exam.
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 a feeling of claustrophobia from being closed-in
or from the need to remain 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. If a contrast injection is
needed, there may be discomfort at the injection site, and you may have
a cool sensation at the site during the injection. Most bothersome to
many patients are the loud tapping or knocking noises heard at certain
phases of imaging. Ear plugs may help. When the knocking and the slight
vibration that may accompany it stops, you can reposition your arms.
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 personal physician. The personal
physician's office will inform the patient on how to obtain the
results. New technology also allows for distribution of diagnostic
reports and referral images over the Internet at some facilities.
What are the benefits vs. risks? Back to Top
Benefits
- Images of the brain and other head structures are clearer and more detailed than with other imaging methods.
- MRI
contrast material is less likely to produce an allergic reaction than
the iodine-based materials used for conventional x-rays and CT
scanning.
- Exposure to radiation is avoided.
- MRI enables the detection of abnormalities that might be obscured by bone tissue with other imaging methods.
- A
variant called MR angiography (MRA) provides detailed images of blood
vessels in the brain—oftentimes without the need for contrast material.
The risk of an allergic reaction from MRA contrast is extremely low and
kidney damage does not occur.
- New MRI systems can depict brain function, and in this way detect a stroke at a very early stage.
Risks
- An undetected metal implant may be affected by the strong magnetic field.
- MRI
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.
What are the limitations of MRI of the Head? Back to Top
Bone is better
imaged by conventional x-rays, and CT is preferred for patients with
severe bleeding, acute trauma or who because of their medical condition
are unable to tolerate an MR scan procedure. MRI may not always
distinguish between tumor tissue and edema fluid and does not detect
calcium when this is present within a tumor. In most cases the exam is
safe for patients with metal implants but there are a few exceptions,
so patients should inform the technician of an implant prior to the
test. The exam must be used cautiously in early pregnancy. MRI often
costs more than CT scanning. Back to Top |
