In a spin echo sequence, if 120 phase encoding steps are selected, how many times is the TR repeated?

In a spin echo sequence, the repetition time (TR) is the time interval between successive excitations of the same slice. When using phase encoding, each phase encoding step corresponds to a unique imaging angle for capturing data from the sample being imaged.

If 120 phase encoding steps are selected, it means that the sequence will need to complete a full data collection for all 120 steps to create a single complete image. Each of these steps occurs after a separate TR period. Therefore, to acquire the complete dataset required for the image reconstruction while utilizing these 120 phase encoding steps, the TR is repeated 120 times, once for each phase encoding step.

Thus, the correct answer reflects that the TR is repeated in alignment with the number of phase encoding steps, which ensures that sufficient data is collected to create a high-quality image.