Magnetic Resonance Imaging (MRI)
Definition:
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, called artifacts, 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 - 6 hours prior to the scan.
MRI also should not be used for people with metallic objects in their bodies, such as:
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.
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.
Adapted from the National Library of Medicine (http://medlineplus.gov).