Magnetic resonance imaging (MRI), a painless testing procedure, takes pictures of the body, much like regular X-ray, using a strong magnetic field. MRI provides detailed pictures of brain, nerve tissues, cartilage and ligaments from multiple planes without obstruction by overlying bone. Nearly 90 percent of all MRI scans are for cranial or spine disorders. However, doctors also use MRI to determine injuries to complex joints like the shoulder and the knee.
Most doctors prefer the MRI for detecting brain disorders. MRI is useful in brain and neurological disorders because it can clearly show types of nerve tissue. It provides clear pictures of the brainstem and posterior brain, which are difficult to view on CT scan. Doctors also use it to diagnose demyelinating disorders (disorders such as multiple sclerosis that cause destruction of the myelin sheath of the nerve). MRI is sometimes used to avoid the dangers of angiography or of repeated exposure to radiation.
The MRI scanner sends the signals to a computer, which manages the information and creates a 3-D image of the scanned tissue. The image then prints on photographic film or videotape. MRI scans can penetrate bone and provide clear, detailed picture of tissues and muscles. In preparation for the MRI, the patient lies on a narrow table that slides inside a large tunnel-like tube. The scanner then surrounds the patient with a magnetic field. A radiologist, who specializes in MRIs, examines the film or computer for abnormalities.
People who are claustrophobic, agitated, or disturbed by the loud noise may be given an antianxiety medication before the examination. Sometimes hospitals use open MRI machines that are less noisy and not as confining as the closed models but have other limitations.
The MRI technique operates on the principle that the most abundant atom in the body is hydrogen, which is present in every water molecule. When placed in a powerful magnetic field, such as that of an MRI machine, the nuclei of these hydrogen atoms line up in one direction, just as compass needles point to the poles of the earth's magnetic field. When energy from radio waves is directed into the field of the body part that is being examined, the nuclei are temporarily moved out of alignment. When the radio waves stop, the nuclei return to their alignment, giving off their own energy in the process. The machine's computers record the duration and intensity of these signal changes and convert the data into information that produces an image showing the internal structure of the examined part. An MRI costs about $1,000 or more per examination, and the machines themselves are prohibitively expensive for small hospitals or rural areas.
Last updated: 11-Nov-02