Magnetic field gradients aren't the only way to make an echo. A similar effect can be achieved by using RF pulses.
As before, an initial 90 degree pulse rotates the protons away from equilibrium. As they become desynchronized, the measurable signal diminishes. Later, a 180 degree pulse rotates the protons around the Y-axis. Instead of speeding up the slow protons and slowing down the fast ones, the 180 degree pulse reverses the order. The slow ones have been placed in front and the fast ones have been placed in the rear. Once they catch up, the echo effect is seen again. This is hard to discern in this graphic, but the next displays how the 180 degree pulse accomplishes this a bit more clearly.
<a rel="nofollow" class="external text" href="https://www.youtube.com/user/overtone1000">Tyler Moore</a>
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<bdo lang="en" dir="ltr"><a href="//commons.wikimedia.org/wiki/Commons:YouTube_files" title="Commons:YouTube files">YouTube</a>: <a rel="nofollow" class="external text" href="https://www.youtube.com/watch?v=vMh11VtUA5o"><bdi lang="">Spin Echo   Dispersed (expand the description below for a detailed explanation)</bdi></a> – View/save archived versions on <a rel="nofollow" class="external text" href="https://web.archive.org/web/*/https://www.youtube.com/watch?v=vMh11VtUA5o">archive.org</a> and <a rel="nofollow" class="external text" href="https://archive.today/https://www.youtube.com/watch?v=vMh11VtUA5o">archive.today</a></bdo>