(SE) 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 (SE).
In the pulse sequence timing diagram, the simplest form of a spin echo sequence is illustrated.
The 90° excitation pulse rotates the longitudinal magnetization (Mz) into the xy-plane and the dephasing of the transverse magnetization (Mxy) starts.
The following application of a 180° refocusing pulse (rotates the magnetization in the x-plane) generates signal echoes. The purpose of the 180° pulse is to rephase the spins, causing them to regain coherence and thereby to recover transverse magnetization, producing a spin echo.
The recovery of the z-magnetization occurs with the T1 relaxation time and typically at a much slower rate than the T2-decay, because in general T1 is greater than T2 for living tissues and is in the range of 100-2000 ms.
The SE pulse sequence was devised in the early days of NMR days by Carr and Purcell and exists now in many forms: the multi echo pulse sequence using single or multislice acquisition, the fast spin echo (FSE/TSE) pulse sequence, echo planar imaging (EPI) pulse sequence and the gradient and spin echo (GRASE) pulse sequence;; all are basically spin echo sequences.
In the simplest form of SE imaging, the pulse sequence has to be repeated as many times as the image has lines.
Contrast values:
PD weighted: Short TE (20 ms) and long TR.
T1 weighted: Short TE (10-20 ms) and short TR (300-600 ms)
T2 weighted: Long TE (greater than 60 ms) and long TR (greater than 1600 ms)
With spin echo imaging no T2* occurs, caused by the 180° refocusing pulse. For this reason, spin echo sequences are more robust against e.g., susceptibility artifacts than gradient echo sequences.

See also Pulse Sequence Timing Diagram to find a description of the components.

(DE - dual / double echo) Dual echo sequences include images with different weightings and / or echo times and are used to obtain both, proton density and T2 weighted images or in phase and out of phase gradient echo images, simultaneously without increasing the measurement time.

(MSE) A spin echo technique with a flip angle over 90°.

See Spin Echo Sequence and Fast Spin Echo.

(MEMP) Sequence with a multislice and multi echo acquisition in one TR.

See also Multi Echo Imaging, Multiple Echo Imaging and Fast Spin Echo.

(PSSE) Partial saturation sequence in which the signal is detected as a spin echo. Even though a spin echo is used, there will not necessarily be a significant contribution of the T2 relaxation time to image contrast, unless the echo time, TE, is on the order of or longer than T2.

(VEMP) MR imaging spin echo pulse sequence in which signals for multiple variable echoes are collected.