MR Spin Echo
In nuclear magnetic resonance, spin echo refers to the refocusing of precessing nuclear spin magnetisation by a 180° pulse of resonant radiofrequency.
Press Reset to rephase vectors.
You can think it as running rabbits and turtules. When 180 pulse is applied at 1/2 time to echo, all runners reverse their direction. Those who are slow are now ahead of faster ones. Assuming all maintain their speed, at time to echo all are approximately at starting position.