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When Is A Proton Just
Like a Dreidle?
When it wobbles. (Also called precesses)
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Just as a spinning top wobbles about its axis so do spinning protons wobble, or precess, about the axis of the external Bø field. The frequency of the precession is directly proportional to the strength of the magnetic field and is defined by the Larmor Equation:
Where wø (omega zerio) is known as either the precessional, Larmor or resonance frequency. Gamma is the gyromagnetic ratio and is a constant unique to every atom. At the magnetic field strengths used in clinical MRI systems, .05 to 2 Tesla, the resonance frequency of hydrogen ranges from 2.13 MHz to 85 MHz.
 Spinning protons wobble or precess.
The resonance frequency is proportional to Bø.
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Apply an RF Pulse
on Resonance
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 RF energy is absorbed.
An observer in the surrounding laboratory will see Mø spiral down to the XY plane (or even to the -Z axis) |
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 An observer riding on the Mø vector sees the external world rotating about him. Mø then seems to tip alpha° towards the Y' axis. |
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If an electromagnetic radio frequency (RF) pulse is applied at the resonance (Larmor, precession, wobble) frequency, then the protons can absorb that energy. At the quantum level, a single proton jumps to a higher energy state. At the macro or classical level, to an observer in the external laboratory frame of reference, the magnetization vector, Mø, (roughly 6 million billion protons) spirals down towards the XY plane. If you could somehow jump aboard Mø, just like a merry-go-round, the laboratory would be rotating around you. In this rotating frame of reference, Mø would seem to smoothly tip down. The tip angle, alpha, is a function of the strength and duration of the RF pulse.
Laboratory Frame: The viewpoint of an observer in the laboratory. The laboratory is stationary, the protons are spinning.
Rotating Frame: The viewpoint of an observer riding along on the protons. The protons appear stationary, the laboratory is rotating..
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