What is a key characteristic of a spin-echo sequence?

A spin-echo sequence is characterized primarily by its use of a combination of a 90-degree radiofrequency (RF) pulse followed by a 180-degree RF pulse. This two-pulse sequence is essential because the initial 90-degree pulse tips the longitudinal magnetization into the transverse plane, creating the initial signal. The subsequent 180-degree pulse serves to refocus the spins, which helps to form an echo at a specific time, known as the echo time (TE).

The use of the 180-degree pulse is fundamental in spin-echo sequences as it corrects for dephasing that may occur due to magnetic field inhomogeneities or differences in precession frequencies among the spins. By re-focusing the spins, a cleaner and more defined signal is obtained, which is instrumental in producing high-quality images.

While enhancing T1 contrast and producing echoes are aspects of MRI sequences, they do not specifically define the spin-echo sequence as prominently as the sequence of the two pulses does. Additionally, while multiple pulse sequences exist, the fundamental characteristic of the spin-echo sequence lies in its particular application of the 90-degree followed by a 180-degree pulse.