What occurs if the TR is very short in a spin echo pulse sequence?

In a spin echo pulse sequence, if the repetition time (TR) is very short, the result is decreased signal intensity. This occurs because a short TR does not allow for adequate relaxation of the longitudinal magnetization of the spins between consecutive excitations. When the TR is too short, the spins do not have enough time to fully recover before they are excited again, which leads to a lower overall signal being detected by the coil.

Essentially, the longitudinal magnetization (M₁) needs time to recover after each radiofrequency pulse. A short TR results in less complete recovery of the magnetization, and therefore, the resulting signal intensity in the image is diminished.

This explanation helps clarify why decreased signal intensity is the outcome of a very short TR in a spin echo pulse sequence, illustrating the importance of selecting an appropriate TR to optimize image quality and signal intensity.