Magnetic resonance imaging; Nuclear magnetic resonance (NMR) imaging
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 the test is performed
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 the test
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 the test will 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 the test is 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
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 others.
For specific information about why the test is performed and normal and abnormal results, please see the specific MRI topics:
by Sharon M. Smith, M.D.
All ArmMed Media material is provided for information only and is neither advice nor a substitute for proper medical care. Consult a qualified healthcare professional who understands your particular history for individual concerns.