Magnetic resonance imaging (MRI) is a non-invasive way to take pictures of the body.Unlike x-rays and computed tomographic (CT) scans, which use radiation, MRI uses powerful magnets and radio waves. The MRI scanner contains the magnet. The magnetic field produced by an MRI is about 10 thousand times greater than the earth's.
The magnetic field forces hydrogen atoms in the body to line up in a certain way (similar to how the needle on a compass moves when you hold it near a magnet). When radio waves are sent toward the lined-up hydrogen atoms, they bounce back, and a computer records the signal. Different types of tissues send back different signals. For example, healthy tissue sends back a slightly different signal than cancerous tissue.
Single MRI images are called slices. The images can be stored on a computer or printed on film.
Memorial Hermann imaging Centers offer cutting-edge MRI technology, including:
With twice the strength of a conventional magnet, 3.0T MRI produces images with extreme detail and clarity while revealing fine changes in body structures of physiological processes. It can often pick up early chemical changes that other tests may not find. Our new 3T MRI services feature the Philips InteraAchieva 3.0T MR system and the Siemens Magnetom Verio. Both machines allow for more precise diagnosis, better pre-surgical plannign and enhanced patient care.
3T MRI is used for these applications:- Breast biopsy
- Breast MRI
- Neurological
- Musculoskeletal
- Whole body imaging
Magnetic resonance imaging; Nuclear magnetic resonance (NMR) imaging You may be asked to wear a hospital gown or clothing without metal fasteners (such as sweatpants and a t-shirt).
You
will be asked to lie on a narrow table, which slides into the middle of
the MRI machine. If you have a fear of confined spaces
(claustrophobia), tell your doctor before the exam. You may be
prescribed a mild sedative, or your doctor may recommend an "open" MRI,
in which the machine is not as close to the body.
Small devices,
called coils, may be placed around the head, arm, or leg, or other
areas to be studied. These devices help send and receive the radio
waves, and improve the quality of the images.
Certain exams
require that a special dye (contrast) be given before the test. The dye
is usually given through an intravenous line (IV) in your hand or
forearm. The contrast helps the radiologist see certain areas more
clearly.
During the MRI, the person who operates the machine will
watch you from a room next door. Several sets of images are usually
need, each taking from 2 to 15 minutes. Depending on the areas being
studied and type of equipment, the exam may take 1 hour or longer.
An
MRI can be performed immediately after other imaging studies. Depending
on the area of interest, the patient may be asked to fast for 4 - 6
hours prior to the scan. Other preparations are usually not needed.
The strong magnetic fields created during an MRI can interfere with certain implants, particularly pacemakers. Persons with cardiac pacemakers can not receive an MRI and should not enter an MRI area.
If you have any of the following metallic objects in your body, you should not get an MRI:
- Inner ear (cochlear) implants
- Brain aneurysm clips
- Certain artificial heart valves
- Older vascular stents
- Recently placed artificial joints
You will be asked to sign a consent form that says you do not have any of these items in your body.
Before
an MRI, sheet metal workers or any person that may have been exposed to
small metal fragments should receive a skull x-ray to check for metal
in the eyes.
Because of the strong magnets, certain metallic objects are not allowed into the room.
- Items such as jewelry, watches, credit cards, and hearing aids can be damaged.
- Pins, hairpins, metal zippers, and similar metallic items can distort the images.
- Removable dental work should be taken out just prior to the scan.
- When
the MRI magnet is turned on, pens, pocketknives, and eyeglasses may fly
across the room. This can be dangerous, so such items are not allowed
into the scanner area.
An
MRI exam causes no pain. Some people may become anxious when inside the
scanner. If you have difficulty lying still or are very anxious, you
may be given a mild sedative. Excessive movement can blur MRI images
and cause errors.
The table may be hard or cold, but you can
request a blanket or pillow. The machine produces loud thumping and
humming noises when turned on. Ear plugs are usually given to help
reduce the noise.
An intercom in the scanner allows you to speak
to the person operating the exam at any time. Some MRIs have
televisions and special headphones that you can use to help the time
pass.
There is no recovery time, unless sedation was necessary.
After an MRI scan, you can resume your normal diet, activity, and
medications.
Combining MRIs with other imaging methods can often help the doctor make a more definitive diagnosis.
MRI
images taken after a special dye (contrast) is delivered into the body
may provide additional information about the blood vessels.
An
MRA, or magnetic resonance angiogram, is a form of magnetic resonance
imaging, which creates three-dimensional pictures of blood vessels. It
is often used when traditional angiography cannot be done.
There
is no ionizing radiation involved in MRI, and there have been no
documented significant side effects of the magnetic fields and radio
waves used on the human body to date.
The most common type of
contrast (dye) used is gadolinium. It is very safe. Allergic reactions
to the substance rarely occur. The person operating the machine will
monitor your heart rate and breathing as needed.
MRI is usually not recommended for acute trauma situations, because traction and life-support equipment cannot safely enter the scanner area and the exam can take quite a bit of time.
People
have been harmed in MRI machines when they did not remove metal objects
from their clothes or when metal objects were left in the room by
others.
For more information, see the specific MRI topics: