How many phase encoding steps are collected every TR period in a spin echo pulse sequence?

In a spin echo pulse sequence, only one phase encoding step is typically collected during each repetition time (TR) period. This means that for each TR, only one slice or region of tissue is being excited and imaged based on the phase encoding gradient applied.

The structure of the spin echo sequence involves an initial 90-degree radiofrequency pulse followed by a series of echo times (TE) where the data is acquired. During this time, a phase encoding gradient is applied before the signal is acquired, but it is done once per TR. Subsequent phase encoding steps require additional TR periods, leading to the collection of multiple phase encoding steps over several repetitions, but in any single TR period, only one phase encoding step is captured. This highlights the systematic nature of how MRI sequences are designed to achieve spatial localization in imaging.