The Effect Of Offresonance Pulses On Net Magnetization

In the acquisition of a simple 1D spectrum, our goal is to excite all of the spins of a certain type (e.g., 1H) in the sample, regardless of chemical shift, at the same time. This requires a radio frequency pulse of very high power and short duration. The frequency of the pulse is adjusted to correspond to the resonance frequency at the center of the spectral window, so that it will be close to the resonance frequency of all of the spins in the sample.

8.2.1 On-Resonance Pulses

For a spin whose chemical shift is exactly at the center of the spectral window, we call the pulse an "on-resonance" pulse because the pulse (or "carrier") frequency is exactly equal to the resonant frequency (precession frequency or Larmor frequency vo) of the spin. During the pulse, we can use the vector model to show the B1 field (the pulse) as stationary in the rotating frame of reference, because the x' and y' axes are rotating about the z axis at exactly the frequency of the pulse. The position of the B1 field in the x'-y' plane depends on the phase of the pulse, which is just the place in the sine function (0-360°) where the radio frequency oscillation starts at the beginning of the pulse. This can be controlled by the spectrometer and is written into the pulse sequence by the user:


Phase shift


B1 vector

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