What happens to FID immediately after it is generated in a spin echo pulse sequence?

In a spin echo pulse sequence, the free induction decay (FID) signal is a result of the coherent precession of magnetic moments after the application of a radiofrequency pulse. Immediately after FID is generated, the spins begin to lose coherence due to interactions with their environment, a process known as dephasing. This dephasing occurs because different spins precess at slightly different frequencies, causing the signal to spread out and diminish over time.

Phase encoding is an essential part of the imaging process in MRI. It involves manipulating the phase of the spins to spatially encode the MRI signal before the echo is formed. The dephasing of the spins during FID is critical for preparing the system to generate a coherent echo signal later on in the sequence. Without this initial dephasing, the signal would not help encode spatial information necessary for image reconstruction.

While other options discuss processes like amplification, demodulation, or calibration, which do occur in MRI signal processing, they happen at different stages and are not the immediate actions taken after FID generation. The immediate effect is the natural dephasing of the spins, which is crucial for the subsequent formation of the echo in the sequence.