WHAT IS A SPIN ECHO?
Spin Echo is a medical and science term referring to an MRI Process.

The most common pulse sequence used in MR imaging is based of the detection of a spin or Hahn echo. It uses 90� radio frequency pulses to excite the magnetization and one or more 180� pulses to refocus the spins to generate signal echoes named spin echoes.

In nuclear magnetic resonance, spin echo refers to the refocusing of precessing nuclear spin magnetisation by a 180� pulse of resonant radiofrequency.

The NMR signal observed following an initial excitation pulse (orange in diagram) decays with time due to both spin-spin relaxation and any inhomogeneous effects which cause different spins to precess at different rates e.g. a distribution of chemical shifts or magnetic field gradients. Relaxation leads to an irreversible loss of magnetisation (decoherence), but the inhomogeneous dephasing can be reversed by applying a 180� or inversion pulse (blue in diagram) that inverts the magnetisation vectors. If the inversion pulse is applied after a period T of dephasing, the inhomogeneous evolution will rephase to form an echo at time 2T. The intensity of the echo relative to the initial signal is given by exp( − 2T / T2) where T2 is the time constant for spin-spin relaxation.