MRI scan
MRI stands for magnetic resonance imaging. It is a relatively
new technology that allows imaging of the interior of the body without using X-rays or
other types of ionizing radiation. An MRI scan is capable of showing fine detail of
different tissues. Its use is rapidly increasing while the use of standard X-rays is
decreasing.
General Information about a MRI Scan
MRI is a non-invasive procedure that uses powerful magnets and
radio waves to construct pictures of the body.
Unlike conventional radiography and Computed Tomographic (CT) imaging, which make use
of potentially harmful radiation (X-rays), MRI imaging is based on the magnetic properties
of atoms. A powerful magnet generates a magnetic field roughly 10,000 times stronger than
the natural background magnetism from the earth. A very small percentage of hydrogen atoms
within a human body will align with this field.
When focused radio wave pulses are broadcast towards the aligned hydrogen atoms in
tissues of interest, they will return a signal. The subtle differences in that signal from
various body tissues enables MRI to differentiate organs, and potentially contrast benign
and malignant tissue.
Any imaging plane (or "slice") can be projected, stored in a computer, or
printed on film. MRI can easily be performed through clothing and bones. However, certain
types of metal in the area of interest can cause significant errors in the reconstructed
images.
How is an MRI preformed
Since MRI makes use of radio waves very close in frequency to
those of ordinary FM radio stations, the scanner must be located within a specially
shielded room to avoid outside interference. The patient will be asked to lie on a narrow
table which slides into a large tunnel-like tube within the scanner.
In addition, small devices may be placed around the head, arm, or leg, or adjacent to
other areas to be studied. These are special body coils which send and receive the radio
wave pulses, and are designed to improve the quality of the images. If contrast is to be
administered, an IV will be placed, usually in a small vein of the hand or forearm. A
technologist will operate the machine and observe you during the entire study from an
adjacent room.
Several sets of images are usually required, each taking from 2 to 15 minutes. A
complete scan, depending on the organs studied, sequences performed, and need for contrast
enhancement may take up to one hour or more. Newer scanners with more powerful magnets
utilizing updated software and advanced sequences may complete the process in less time.
How to prepare for an MRI
No preparatory tests, diets, or medications are usually needed.
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 to 6 hours prior to the scan.
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.
- Pens, pocketknives, and eyeglasses can become dangerous projectiles when the magnet is
activated and should not accompany the patient into the scanner area.
Because the strong magnetic fields can displace or disrupt the action of implanted
metallic objects, people with cardiac pacemakers cannot be scanned and should not enter
the MRI area.
MRI also should not be used for people with metallic objects in their bodies, such as:
- Inner ear (cochlear) implants
- Brain aneurysm clips
- Some artificial heart valves
- Older vascular stents
- Recently placed artificial joints
Sheet metal workers, or persons with similar potential exposure to small metal
fragments, will first be screened for metal shards within the eyes with X-rays of the
skull.
The patient will be asked to sign a consent form confirming that none of the above
issues apply before the study will be performed.
A hospital gown may be recommended, or the patient may be allowed to wear
"sweats" or similar clothing without metal fasteners.
How will an MRI feel?
There is no pain. The magnetic field and radio waves are not
felt. The primary possible discomfort is the claustrophobic feeling that some experience
from being inside the scanner. The table may be hard or cold, but you can request a
blanket or pillow. The machine produces loud thumping and humming noises during normal
operation. Ear plugs are usually given to the patient to reduce the noise.
A technologist observes the patient during the entire procedure and may be spoken to
through an intercom in the scanner. Some MRI scanners are equipped with televisions and
special headphones to help the examination time pass.
Excessive movement can blur MRI images and cause certain artifacts. If the patient has
difficulty lying still or is very anxious, an oral or intravenous sedative may be given.
There is no recovery time, unless sedation was necessary. After an MRI scan, you can
resume normal diet, activity, and medications.
Why is the MRI Scan performed
An MRI can provide additional imaging information for the
clinician based upon its superior tissue contrast resolution. Combined with other imaging
methods, a more definitive diagnosis can be given in the work up of a patient's disease.
Sequences performed with intravenous contrast may provide additional data about the
blood vessels within masses.
An MRA, or magnetic resonance angiogram, is a special type of MR that creates
three-dimensional reconstructions of vessels containing flowing blood and is often
utilized when conventional angiography cannot be performed due to renal failure or other
contraindications.
What the risks are with an MRI
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 MR intravenous contrast agent, gadolinium, is very
safe, and although there have been documented allergic reactions to it, this is extremely
rare. If sedation is used, there are associated risks of over-sedation. The technologist
monitors the patient's vital signs, including heart rate and respiration as needed.
However, because the effects of strong magnetic fields on a fetus are not well
documented at this time, pregnant women are usually advised to avoid MRI scans.
MRI is usually not recommended for acute trauma
situations, because traction
and life-support equipment cannot safely enter the scanner area and scan times are
relatively lengthy.
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 other |