Understanding Nuclear Magnetic Resonance
Atomic Nuclei, Physics, NMR Spectroscopy, and MRI Medical Imaging
Oct 9, 2009
Understanding nuclear magnetic resonance builds on the physics concepts of resonance and nuclear spin.
Resonance
In physics resonance occurs when an oscillating motion has a driving force that periodically pushes at the same point in the oscillatory cycle. The driving force and the oscillatory motion have the same frequency, so the effect of small driving forces is magnified. If the driving force is at a different frequency, the effect of the driving force cancels out.
Pushing a swing is a good example. Pushing at the same point in the swing's motion magnifies the effect and it swings higher. Random pushes at different points in the swing's motion can cancel out. The swing does not go very high,
Elementary Particles and Nuclear Spin
Physicists measure the amount of spinning motion by an object's angular momentum. More rapidly spinning objects have more angular momentum. Physicists call the angular momentum of elementary particles the spin.
Protons, electrons, atomic nuclei, and other elementary particles have an intrinsic spin as if they were spinning on their axes. Because these particles are so small, physicists don't really know if they are spinning on their axes, as Earth spins on its axis, but they have measured spin properties suggesting they do.
A bar magnet is what physicists call a magnetic dipole. When atomic nuclei, or other charged particles, spin they become like a magnetic dipole, essentially a very miniscule bar magnet.
Precession
Watch a top spin and notice how it wobbles. This wobble is caused by Earth's gravitational field acting on a spinning mass and is called precession. Magnetic fields act on spinning charges the same way. So a nucleus, or elementary particle, possessing spin will wobble or precess in a similar way when placed in a magnetic field. A nucleus in a magnetic field precesses at a frequency, called the Larmor frequency, which is proportional to the magnetic field.
Nuclear Magnetic Resonance
When an atom is in a constant magnetic field, it nucleus precesses at the Larmor frequency. If in addition to the constant magnetic field, there is also a weaker perpendicular magnetic field that varies at the same frequency as the Larmor frequency for the nucleus, then the nucleus resonates.
This nuclear magnetic resonance (NMR) causes the spin of the nucleus to flip. The analogy for Earth spinning on its axis would be if Earth flipped so that the north and south poles interchanged and Earth were suddenly spinning in the opposite direction. Just as it would take considerable energy to flip Earth's spinning motion, the nucleus absorbs and emits energy as it flips its spin. With the correct electronic equipment physicists can measure the spectrum of absorbed and emitted energy.
NMR Spectroscopy
One common application of nuclear magnetic resonance is NMR spectroscopy. Physicists and chemists study the NMR spectrum produced by a sample of material and deduce the properties of the nuclei in the sample. This tells them what elements are in the sample.
MRI Medical Imaging
A better known application of nuclear magnetic resonance is magnetic resonance imaging (MRI). MRI uses the nuclear magnetic resonance effect of the hydrogen atoms in the human body. Computer analysis of the resonance data produces an internal image of the patients body. MRI produces diagnostic medical imaging that neither harms the body in any way nor requires surgeons to cut open or enter the body.
Nuclear magnetic resonance is an interesting phenomenon taking place in the nuclei of atoms that also has very important applications.
Further Reading
Serway, R.A., Moses, C.J., and Moyer, C.A., Modern Physics 3rd ed., Thomson, 2005.
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