Spin Echo Sequence

The spin echo pulse sequence is the most commonly used pulse sequence. The pulse sequence timing can be adjusted to give T1-weighted, Proton or spin density, and T2-weighted images. Dual echo and multiecho sequences can be used to obtain both proton density and T2-weighted images simultaneously.
The two variables of interest in spin echo sequences is the repetition time (TR) and the echo time (TE). All spin echo sequences include a slice selective 90 degree pulse followed by one or more 180 degree refocusing pulses as shown in the following diagram.

Dual echo spin echo sequence

In this diagram, RF is the radio frequency pulse, a slice selective 90 degree pulse followed by two 180 degree refocusing pulses. GS, GP, and GF are the slice selective, phase encoding, and frequency encoding gradients, respectively. "Echo" represents the signal received from the slice of interest in the body. A short TR an short TE will give a T1-weighted image, a long TR and short TE (first echo) will give a proton density image, and a long TR and long TE (second echo) will give a T2-weighted image.
An example of the changes in the net magnetization vector for a spin echo sequence with one echo is shown below. After 1/2 of the TE time, dephasing of individual spins in the volume occurs as represented by the black and red arrows. The 180 degree RF pulse flips these spins to the opposite side in the x-y plane so that after another 1/2 TE, they refocus and are detected.