The NMR signal observed following an initial excitation pulse decays with time due to both spin relaxation and any "inhomogeneous" effects which cause different spins in the sample to precess at different rates. The first of these, relaxation, leads to an irreversible loss of magnetisation. However, the inhomogeneous dephasing can be removed by applying a 180° "inversion" pulse that inverts the magnetisation vectors. Examples of inhomogeneous effects include a magnetic field gradient and a distribution of chemical shifts. If the inversion pulse is applied after a period "t" of dephasing, the inhomogeneous evolution will rephase to form an echo at time 2"t". In simple cases, the intensity of the echo relative to the initial signal is given by "e–2t/T2" where "T"2 is the time constant for spin-spin relaxation. The echo time (TE) is the time between the excitation pulse and the peak of the signal.