| In MRI, the individual is placed inside a hollow, magnetic tube. Individuals with any metal in their body (implants, pacemakers, metal not removed following an accident or surgery) should inform the doctor and technician before the procedure begins, because some types of metal in the body prevent this procedure from being done.
Once in the MRI tube, the body is exposed to short, pulsed radio waves. In a magnetic field, hydrogen molecules in the body line up parallel to each other. When knocked out of alignment by the strong pulse of radio waves, hydrogen nuclei produce a detectable radio signal as they fall back into alignment. Magnetic coils in the machine detect these signals. They are changed by a computer into an image based on the strength of the signal produced by the different types of tissue. Because the concentration of hydrogen varies in different soft tissues (muscle, fat, fibrous tissue), MRI can differentiate between them. Tissues that contain little or no hydrogen (such as bone) appear as darker images.
In some cases, the contrast of MRI images can be enhanced by means of paramagnetic contrast agents, a substance that can be temporarily magnetized by placing it in a magnetic field. The contrast agent, injected into a vein, is taken up selectively by tissues with more blood vessels. During the MRI procedure, these tissues emit radio waves more readily, increasing the image contrast.
For the individual, most MRI procedures involve lying on a movable table, which is inserted into the opening of the magnet. The part of the body being examined needs to be in the middle of the tube. Although confining, the opening is not overly constrictive. Mild sedation is available for individuals uncomfortable in this position (claustrophobic), as are view mirrors, music, and voice communication with the attending technologists. The procedure may take as little as a few minutes or up to an hour and is characterized by the various rapid, tapping sounds of the MRI scanner acquiring the image. Other than these sounds, the individual perceives nothing, and merely has to lie still until the attendants complete the procedure.
Some newer models of MRI scanners, called open MRI scanners, are less claustrophobic than the normal scanner, but they are not always able to capture the detailed images provided by traditional scanners. If an individual is having a head MRI, he or she may be asked to wear a special magnetic coil around the head. The coil does not touch the individual, and the individual can see through it. |
Source: Medical Disability Advisor