Here, the individual dipoles have all been moved to the origin. Notice that the 180 degree pulse rotates the dipoles around the Y-axis. This not only restores the Z-component to a larger value but reverses the order of the precession so that the echo has the highest possible XY-component.
<p>Previously, the graphic showed protons with different Larmor frequencies because of a magnetic field gradient, but remember that this desynchronization happens even if we try to apply a uniform magnetic field, and that we call this phenomenon T2.
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One large difference between gradient echo and spin echo is that spin echo reverses the Z-component losses, while gradient echo can only reverse the XY-component desynchronization. Interestingly, as a result, the spin echo sequence can be repeated many times with little decay of signal.
<a rel="nofollow" class="external text" href="https://www.youtube.com/user/overtone1000">Tyler Moore</a>
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