Spin Precession
Are the individual nuclei still precessing after a 180°-pulse?
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YES. Individual spins continue to precess.
Remember that as long as a static magnetic field is present (even the earth's magnetic field), nuclear spins are always precessing. This remains true even after they have been subjected to RF-pulses. An RF-pulse of any value will change the orientation of magnetic moments for all the spins in the sample, but the spins will continue to precess.
The source of confusion likely comes from the fact that we often talk about the net magnetization (M) before and after a 180°-pulse. If M starts out in equilibrium, perfectly aligned with Bo, it does not precess (though the individual spins that compose M are continuously precessing). This is similar to a gyroscope or top that does not precess when directly vertical. If this perfectly aligned M is now perfectly inverted by a 180°-RF pulse, the inverted M likewise will not precess (though the individual spins that compose M continue to precess).
Remember that as long as a static magnetic field is present (even the earth's magnetic field), nuclear spins are always precessing. This remains true even after they have been subjected to RF-pulses. An RF-pulse of any value will change the orientation of magnetic moments for all the spins in the sample, but the spins will continue to precess.
The source of confusion likely comes from the fact that we often talk about the net magnetization (M) before and after a 180°-pulse. If M starts out in equilibrium, perfectly aligned with Bo, it does not precess (though the individual spins that compose M are continuously precessing). This is similar to a gyroscope or top that does not precess when directly vertical. If this perfectly aligned M is now perfectly inverted by a 180°-RF pulse, the inverted M likewise will not precess (though the individual spins that compose M continue to precess).
References
Elster AD, Burdette JH. Questions and Answers in MRI, 2nd Ed. St. Louis: Mosby, 2001, pp. 22-24.
Levitt MH. Spin Dynamics. Basics of Nuclear Magnetic Resonance, 2nd Ed. Chichester, UK: 2012, pp 231-258.
Elster AD, Burdette JH. Questions and Answers in MRI, 2nd Ed. St. Louis: Mosby, 2001, pp. 22-24.
Levitt MH. Spin Dynamics. Basics of Nuclear Magnetic Resonance, 2nd Ed. Chichester, UK: 2012, pp 231-258.
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