What changes from one TR period to the next in a spin echo pulse sequence?

In a spin echo pulse sequence, the phase encoding gradient is the key factor that changes from one repetition time (TR) period to the next. This gradient is applied after the initial 90-degree radiofrequency pulse and before the 180-degree refocusing pulse. Its function is to assign a specific phase to the spins in the region of interest, which allows for spatial localization of the signal.

As the phase encoding gradient is varied for each TR, it enables the acquisition of data at different phase positions, leading to the construction of the image during the Fourier transform process. By systematically varying this gradient across multiple TRs, the MRI system can sample different areas in the k-space, which ultimately contributes to the image's contrast and resolution.

In contrast, the slice encoding gradient and readout gradient remain constant during the acquisition of data for each slice, while frequency encoding primarily deals with the frequency of the received signals from the spins within the slice that has already been selected. Thus, it is the variability of the phase encoding gradient across TRs that is instrumental to the imaging process in a spin echo sequence.