Immediately after the 90 degree RF pulse in a spin echo pulse sequence, the spins are described as?

Immediately following a 90-degree RF pulse in a spin echo pulse sequence, the spins are indeed in the transverse plane and phase coherent.

When the RF pulse is applied at 90 degrees, the net magnetization vector, which is primarily oriented along the z-axis (longitudinal direction), is flipped into the transverse plane (xy-plane). In this transverse state, the magnetic moments of the individual spins are aligned such that they precess in phase with one another, thereby maintaining coherence. This means they have a uniform phase relationship, leading to the generation of a detectable transverse magnetization signal.

As the spins begin to relax and dephase after the initial pulse, they can experience variations in magnetic fields, causing some spins to fall out of phase, but directly after the pulse, they collectively maintain their phase relationship, making them phase coherent in the transverse plane.

Understanding this setup is crucial for interpreting results in imaging sequences, as it sets the stage for how the spins evolve over time and how signals are produced in subsequent echoes.