Immediately after a 90 degree RF pulse, the individual magnetic moments are all located in the transverse plane ; more important. However , they are coherent ( pointig in the same direction at nearly the same frequency). In other words , all protons have the same rotating phase.
Because the FID (free induction decay) is the sum of voltage from many protons at each spatial location,the protons must precess at the sane frequency and with the same alignment so that their magnetization work together. As the time goes on ,following the RF excitation pulse the individual magnetic moments begin to become spread out in the transverse plane. One cause of this spread is due to small local variation in the otherwise static magnetic field resulting out of random interaction from the magnetic moments of nearby protons. It is the spreading out that causes the vector sum to decrease. With time ,therefore, the individual spins
Because the FID (free induction decay) is the sum of voltage from many protons at each spatial location,the protons must precess at the sane frequency and with the same alignment so that their magnetization work together. As the time goes on ,following the RF excitation pulse the individual magnetic moments begin to become spread out in the transverse plane. One cause of this spread is due to small local variation in the otherwise static magnetic field resulting out of random interaction from the magnetic moments of nearby protons. It is the spreading out that causes the vector sum to decrease. With time ,therefore, the individual spins
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