Spin echo sequence diagrams
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In the first figure, 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 magnetisation vector for a spin echo sequence with one echo is shown in the second figure. 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.