Prior to a 90° excitation pulse no transverse magnetization exists and no signal is recieved. After the 90° pulse, the created transverse magnetization starts its precession and a signal oscillating with the Larmor frequency can be oserved. This signal (also called the free induction decay (FID)) decays with the relaxation time T2* due to  the spin-spin relaxation (T2) and a potentially inhomogeneous magnetic field in the sample. When at time TE/2 a 180° refocussing pulse is applied, the dephasing due to inhomogeneities is reversed and an spin echo forms at the echo time (TE). The echo again decays with T2* (dotted lines). The signal decay due to T2 cannot be reversed (dashed line). Tx: transmitted signal, Rx: recieved signal
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FbrG