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Magnetic resonance imaging (MRI) makes cross-sectional images by measuring changes in the body's natural magnetic field as parts of the body are exposed to strong magnets and various radio frequencies. Using these techniques, a doctor can examine the structure and appearance of internal organs. Like computed tomography (CT) scanning, MRI uses a computer to construct images from information recorded by the scanner. In MRI, however, the information is not provided by X-rays. Instead, the person being examined is placed inside a powerful magnet, which arranges the nuclei of some of the hydrogen atoms in the body in a precise pattern (like iron shavings around a magnet). A pulse of radio waves is then passed through the person's body, moving the nuclei of the aligned hydrogen atoms briefly out of alignment. The nuclei then return to their original pattern, emitting radio signals as they do so. Different tissues such as tumors emit a more or less intense signal. These signals are detected by the machine and analyzed by the computer. The information is then used by the computer to construct an image. MRI can sometimes produce more sensitive three-dimensional images than CT.
MRI technology includes magnetic resonance angiography (MRA) and magnetic resonance spectroscopy (MRS). MRA, like other types of angiography, is used to evaluate blood flow but does not use dyes or radioactive tracers. MRS is different from conventional MRI in that MRS uses a continuous band of radio waves to excite hydrogen atoms in a variety of chemical compounds other than water. The compounds absorb and emit radio energy at certain frequencies (or spectra) that can be used to identify them. A color image is created by assigning a color to each distinct spectral emission. MRS is used to produce color images of brain function and to identify the chemical composition of diseased tissue.
Functional magnetic resonance imaging (fMRI) — also called "brain mapping" — uses the same MRI scanner hardware to provide noninvasive images of the brain's activity, and to detect changes resulting from biological function. Functional MRI tracks blood flow in the brain. The more active an area of the brain is, the more blood flows to it. Functional MRI enables doctors to take a series of images in quick succession and to analyze the differences between them. It also allows doctors to identify the parts of the brain that "light up" (are active) when a person is asked to perform specific tasks or is exposed to certain stimuli.
The Procedure
Because the MRI scanner creates a very strong magnetic field, you cannot carry or wear any metal objects (such as jewelry, eyeglasses or hair clips) during the examination. Make sure you let your doctor know if you have any metal implants (such as artificial joints, plates, screws or clips), metal attachments (such as braces) or electrical devices (such as a hearing aid), which could be affected by the magnet. It is especially important to tell your doctor if you have a pacemaker because the magnet can make the pacemaker stop working. Although MRI is not known to pose any health risks to a fetus, tell your doctor before having the procedure if you are (or could be) pregnant.
For an MRI, you lie on your back on a narrow padded table with a cushion under your knees. You are given a signal button to alert the technician if you begin to feel uncomfortable during the test. (If you don't like confined spaces, ask your doctor for a sedative before the test.) Because the scanner is very noisy, you are given earplugs or headphones to help block the noise. You may be given a contrast medium through a needle inserted into a vein. You must lie still during the MRI, and you may be asked to hold your breath occasionally. In closed MRI, the table slides into a narrow tunnel inside the scanner. In open MRI, the scanner is quieter and less confining. MRI is painless, and the test can last from 20 minutes to 90 minutes or longer.