For MRI, a person lies on a motorized table that is moved into the narrow interior of a large tubular scanner, which produces a strong magnetic field. Normally, protons (positively charged parts of an atom) in tissues are in no particular arrangement. But when protons are surrounded by a strong magnetic field, as in an MRI scanner, they line up with the magnetic field. Then, the scanner emits a pulse of radio waves, which momentarily knocks the protons out of line. As the protons line up with the magnetic field again, they release energy (called signals). The strength of the signal varies by tissue. The MRI scanner records these signals. A computer is used to analyze the signals and produce images.
Magnetic Resonance Imaging (MRI)
Magnetic Resonance Imaging (MRI)
Examiners can change how various tissues appear on a scan by varying the radio wave pulses, the strength and direction of the magnetic field, and other factors. For example, fat tissue appears dark on one type of scan and bright on another. These different scans provide complementary information, so more than one is often obtained.
A contrast agent containing gadolinium (a paramagnetic contrast agent) may be injected into a vein or a joint. Gadolinium agents change the magnetic field in a way that makes images clearer.
Before the test, people remove most or all of their clothing and are given a gown that has no buttons, snaps, zippers, or other metal on it to wear. All metal objects (such as keys, jewelry, and cell phones) and other objects that could be affected by the magnetic field (such as credit cards and watches) should be left outside the MRI scanning room. People must lie still when images are taken and may have to hold their breath at times. Because the scanner makes loud banging noises, people may be given headphones or ear plugs to wear. A scan may take 20 to 60 minutes. After the test, people can resume their usual activities immediately.
