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The body is largely composed of water molecules which each contain two hydrogen nuclei or protons. When a person goes inside the powerful magnetic field of the scanner, these protons align with the direction of the field.* U( J; y/ M" X+ Q3 i, I6 o) l+ y
$ S( s8 h2 C) | `; N) Y5 [; j) sA radio frequency electromagnetic field is then briefly turned on, causing the protons to alter their alignment relative to the field. When this field is turned off the protons return to the original magnetization alignment. These alignment changes create a signal which can be detected by the scanner. The frequency of the emitted signal depends on the strength of the magnetic field. The position of protons in the body can be determined by applying additional magnetic fields during the scan which allows an image of the body to be built up. These are created by turning gradients coils on and off which creates the knocking sounds heard during an MR scan.
! j4 ?1 g( I% O. _! _8 @--- from Wikipedia http://en.wikipedia.org/wiki/Magnetic_resonance_imaging |
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