resonance, in acoustics: see vibration.

resonance, in chemistry: see chemical bond.

paramagnetic resonance: see magnetic resonance.

nuclear magnetic resonance: see magnetic resonance.

magnetic resonance imaging (MRI), noninvasive diagnostic technique that uses nuclear magnetic resonance to produce cross-sectional images of organs and other internal body structures. The patient lies inside a large, hollow cylinder containing a strong electromagnet, which causes the nuclei of certain atoms in the body (especially those of hydrogen) to align magnetically. The patient is then subjected to radio waves, which cause the aligned nuclei to "flip"; when the radio waves are withdrawn the nuclei return to their original positions, emitting radio waves that are then detected by a receiver and translated into a two-dimensional picture by computer. Unhampered by bone and capable of producing images in a variety of planes, MRI is used in the diagnosis of brain tumors and disorders, spinal disorders, multiple sclerosis, and cardiovascular disease. The procedure is considered to be without risk, but the scanner may interfere with pacemakers, hearing aids, or other mechanical devices. Although the images are similar in many ways to those of CAT scans, they are obtained without X rays or other radiation, and generally provide more contrast between normal and abnormal tissue.

magnetic resonance, in physics and chemistry, phenomenon produced by simultaneously applying a steady magnetic field and electromagnetic radiation (usually radio waves) to a sample of atoms and then adjusting the frequency of the radiation and the strength of the magnetic field to produce absorption of the radiation. The resonance refers to the enhancement of the absorption that occurs when the correct combination of field and frequency is reached. The procedure is analogous to tuning a radio dial exactly to a desired station.

Several distinct kinds of magnetic resonance exist. In cyclotron resonance the magnetic field is adjusted so that the frequency of revolution of a charged particle around the field lines is exactly equal to the frequency of the radiation. This principle is used to produce beams of energetic particles in particle accelerators.

Other magnetic resonance phenomena depend on the fact that both the proton and electron exhibit intrinsic spin about their own axes and thus act like microscopic magnets. Electron paramagnetic resonance (EPR) arises from unpaired electron spins in liquids or solid crystals. Because of their own magnetism, the spins line up with the external magnetic field. For a given magnetic field the spins can be made to "flip" to the opposite direction when they absorb radiation at a corresponding "resonant" frequency. From the point of view of quantum mechanics, the spin flips can be considered as transitions between states that become separated in energy when the magnetic field is applied. The effect is related to the splitting of spectral lines when an atom is subjected to a magnetic field (see spectrum; Zeeman effect).

Nuclear magnetic resonance (NMR) is analogous to EPR; however NMR is produced by the much smaller magnetism associated with unpaired nuclear spins. The NMR resonant frequency (usually that of protons in complex molecules) is slightly shifted by interactions with nearby atoms in the sample, thus providing information about the chemical structure of organic molecules and other materials. NMR is now extensively employed in medicine, although the use of the word "nuclear" is avoided, the preferred name being magnetic resonance imaging (MRI). The technique provides high-quality cross-sectional images of internal organs and structures. Paul Lauterbur, an American physicist, and Peter Mansfield, a British physicist, shared the 2003 Nobel Prize in Physiology or Medicine for pioneering contributions that later led to the application of magnetic resonance in medical imaging.

Magnetic resonance can also occur without an external magnetic field from interactions of the electron and nuclear spins; such resonance produces the fine and hyperfine structure of atomic spectra.

electron paramagnetic resonance: see magnetic resonance.

In physics, the relatively large selective response of an object or a system that vibrates in step with an externally applied vibration. Acoustical resonance is the vibration induced in a string of a given pitch when a note of the same pitch is produced nearby, in the sound box of an instrument such as a guitar, or in the mouth or nasal cavity when speaking. Mechanical resonance, such as that produced in a bridge by wind or by marching soldiers, can eventually produce wide swings great enough to cause the bridge's destruction. Resonance in frequency-sensitive electrical circuits makes it possible for certain communication devices to accept signals of some frequencies while rejecting others. Magnetic resonance occurs when electrons or atomic nuclei respond to the application of magnetic fields by emitting or absorbing electromagnetic radiation. Seealso nuclear magnetic resonance.

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Selective absorption of very high-frequency radio waves by certain atomic nuclei subjected to a strong stationary magnetic field. Nuclei that have at least one unpaired proton or neutron act like tiny magnets. When a strong magnetic field acts on such nuclei, it sets them into precession. When the natural frequency of the precessing nuclear magnets corresponds to the frequency of a weak external radio wave striking the material, energy is absorbed by the nuclei at a frequency called the resonant frequency. NMR is used to study the molecular structure of various solids and liquids. Magnetic resonance imaging, or MRI, is a version of NMR used in medicine to view soft tissues of the human body in a hazard-free, noninvasive way.

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Absorption or emission of electromagnetic radiation by electrons or atomic nuclei in response to certain magnetic fields. The principles of magnetic resonance are used to study the atomic and nuclear properties of matter; two common laboratory techniques are nuclear magnetic resonance and electron spin resonance. In medicine, magnetic resonance imaging is used to produce images of human tissue.

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or electron paramagnetic resonance (EPR)

Technique of spectroscopic analysis (see spectroscopy) used to identify paramagnetic substances (see paramagnetism) and investigate the nature of the bonding within molecules by identifying unpaired electrons and their interaction with their immediate surroundings. Unpaired electrons, because of their spin, behave like tiny magnets and can be lined up in an applied magnetic field; energy applied by alternating microwave radiation is absorbed when its frequency coincides with that of precession of the electron magnets in the sample. The graph or spectrum of radiation absorbed as the field changes gives information valuable in chemistry, biology, and medicine.

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