When is the phase encoding gradient activated in a spin echo pulse sequence?

In a spin echo pulse sequence, the phase encoding gradient is activated immediately following the 90-degree RF pulse. This is a crucial step because the activation of the phase encoding gradient is what introduces phase shifts to the spins in the selected slice after they have been tipped into the transverse plane by the 90-degree pulse.

When the 90-degree pulse is applied, the magnetization is tilted into the transverse plane, and the unsynchronized precession of spins begins to occur. It is after this pulse that the phase encoding gradient is applied, which modifies the phase of the spins in the region of interest. This gradient determines the spatial frequency encoding in the phase direction, allowing for the image to be constructed in that dimension.

After the phase encoding gradient is turned off, a 180-degree RF pulse is applied, which primarily serves to refocus the dephasing spins and creates the echo signal. Thus, understanding the timing of the phase encoding gradient in relation to the RF pulses is essential for proper sequence execution and image acquisition.